An extended 3D discrete-continuous model and its application on single- and bi-crystal micropillars

NASA Astrophysics Data System (ADS)

Huang, Minsheng; Liang, Shuang; Li, Zhenhuan

2017-04-01

A 3D discrete-continuous model (3D DCM), which couples the 3D discrete dislocation dynamics (3D DDD) and finite element method (FEM), is extended in this study. New schemes for two key information transfers between DDD and FEM, i.e. plastic-strain distribution from DDD to FEM and stress transfer from FEM to DDD, are suggested. The plastic strain induced by moving dislocation segments is distributed to an elementary spheroid (ellipsoid or sphere) via a specific new distribution function. The influence of various interfaces (such as free surfaces and grain boundaries (GBs)) on the plastic-strain distribution is specially considered. By these treatments, the deformation fields can be solved accurately even for dislocations on slip planes severely inclined to the FE mesh, with no spurious stress concentration points produced. In addition, a stress correction by singular and non-singular theoretical solutions within a cut-off sphere is introduced to calculate the stress on the dislocations accurately. By these schemes, the present DCM becomes less sensitive to the FE mesh and more numerically efficient, which can also consider the interaction between neighboring dislocations appropriately even though they reside in the same FE mesh. Furthermore, the present DCM has been employed to model the compression of single-crystal and bi-crystal micropillars with rigid and dislocation-absorbed GBs. The influence of internal GB on the jerky stress-strain response and deformation mode is studied in detail to shed more light on these important micro-plastic problems.

Laser shock microforming of aluminum foil with fs laser

NASA Astrophysics Data System (ADS)

Ye, Yunxia; Feng, Yayun; Xuan, Ting; Hua, Xijun; Hua, Yinqun

2014-12-01

Laser shock microforming of Aluminum(Al) foil through fs laser has been researched in this paper. The influences of confining layer, clamping method and impact times on induced dent depths were investigated experimentally. Microstructure of fs laser shock forming Al foil was observed through Transmission electron microscopy (TEM). Under the condition of tightly clamping, the dent depths increase with impact times and finally tend to saturating. Another new confining layer, the main component of which is polypropylene, was applied and the confining effect of it is better because of its higher impedance. TEM results show that dislocation is one of the main deformation mechanisms of fs laser shock forming Al foil. Specially, most of dislocations exist in the form of short and discrete dislocation lines. Parallel straight dislocation slip line also were observed. We analyzed that these unique dislocation arrangements are due to fs laser-induced ultra high strain rate.

Cross-slip in face-centered cubic metals: a general Escaig stress-dependent activation energy line tension model

NASA Astrophysics Data System (ADS)

Malka-Markovitz, Alon; Mordehai, Dan

2018-02-01

Cross-slip is a dislocation mechanism by which screw dislocations can change their glide plane. This thermally activated mechanism is an important mechanism in plasticity and understanding the energy barrier for cross-slip is essential to construct reliable cross-slip rules in dislocation models. In this work, we employ a line tension model for cross-slip of screw dislocations in face-centred cubic (FCC) metals in order to calculate the energy barrier under Escaig stresses. The analysis shows that the activation energy is proportional to the stacking fault energy, the unstressed dissociation width and a typical length for cross-slip along the dislocation line. Linearisation of the interaction forces between the partial dislocations yields that this typical length is related to the dislocation length that bows towards constriction during cross-slip. We show that the application of Escaig stresses on both the primary and the cross-slip planes varies the typical length for cross-slip and we propose a stress-dependent closed form expression for the activation energy for cross-slip in a large range of stresses. This analysis results in a stress-dependent activation volume, corresponding to the typical volume surrounding the stressed dislocation at constriction. The expression proposed here is shown to be in agreement with previous models, and to capture qualitatively the essentials found in atomistic simulations. The activation energy function can be easily implemented in dislocation dynamics simulations, owing to its simplicity and universality.

Symposium on Continuum Models and Discrete Systems (6th) Held in Dijon, France on June 26 - 29, 1989

DTIC Science & Technology

1986-01-01

regard the I’ Burgers’ vector and the dislocation density tensor as measures of defectiveness. This practice can be given a systematic flavour . To begin...behaviour aims at describing the two main mechanisms of deformation, namely plastic slip of two granules over one another and changes of microstructure as

Role Of Impurities On Deformation Of HCP Crystal: A Multi-Scale Approach

NASA Astrophysics Data System (ADS)

Bhatia, Mehul Anoopkumar

Commercially pure (CP) and extra low interstitial (ELI) grade Ti-alloys present excellent corrosion resistance, lightweight, and formability making them attractive materials for expanded use in transportation and medical applications. However, the strength and toughness of CP titanium are affected by relatively small variations in their impurity/solute content (IC), e.g., O, Al, and V. This increase in strength is due to the fact that the solute either increases the critical stress required for the prismatic slip systems ({10- 10}) or activates another slip system ((0001), {10-11}). In particular, solute additions such as O can effectively strengthen the alloy but with an attendant loss in ductility by changing the behavior from wavy (cross slip) to planar nature. In order to understand the underlying behavior of strengthening by solutes, it is important to understand the atomic scale mechanism. This dissertation aims to address this knowledge gap through a synergistic combination of density functional theory (DFT) and molecular dynamics. Further, due to the long-range strain fields of the dislocations and the periodicity of the DFT simulation cells, it is difficult to apply ab initio simulations to study the dislocation core structure. To alleviate this issue we developed a multiscale quantum mechanics/molecular mechanics approach (QM/MM) to study the dislocation core. We use the developed QM/MM method to study the pipe diffusion along a prismatic edge dislocation core. Complementary to the atomistic simulations, the Semi-discrete Variational Peierls-Nabarro model (SVPN) was also used to analyze the dislocation core structure and mobility. The chemical interaction between the solute/impurity and the dislocation core is captured by the so-called generalized stacking fault energy (GSFE) surface which was determined from DFT-VASP calculations. By taking the chemical interaction into consideration the SVPN model can predict the dislocation core structure and mobility in the presence and absence of the solute/impurity and thus reveal the effect of impurity/solute on the softening/hardening behavior in alpha-Ti. Finally, to study the interaction of the dislocation core with other planar defects such as grain boundaries (GB), we develop an automated method to theoretically generate GBs in HCP type materials.

Mechanical annealing under low-amplitude cyclic loading in micropillars

NASA Astrophysics Data System (ADS)

Cui, Yi-nan; Liu, Zhan-li; Wang, Zhang-jie; Zhuang, Zhuo

2016-04-01

Mechanical annealing has been demonstrated to be an effective method for decreasing the overall dislocation density in submicron single crystal. However, simultaneously significant shape change always unexpectedly happens under extremely high monotonic loading to drive the pre-existing dislocations out of the free surfaces. In the present work, through in situ TEM experiments it is found that cyclic loading with low stress amplitude can drive most dislocations out of the submicron sample with virtually little change of the shape. The underlying dislocation mechanism is revealed by carrying out discrete dislocation dynamic (DDD) simulations. The simulation results indicate that the dislocation density decreases within cycles, while the accumulated plastic strain is small. By comparing the evolution of dislocation junction under monotonic, cyclic and relaxation deformation, the cumulative irreversible slip is found to be the key factor of promoting junction destruction and dislocation annihilation at free surface under low-amplitude cyclic loading condition. By introducing this mechanics into dislocation density evolution equations, the critical conditions for mechanical annealing under cyclic and monotonic loadings are discussed. Low-amplitude cyclic loading which strengthens the single crystal without seriously disturbing the structure has the potential applications in the manufacture of defect-free nano-devices.

Dislocation mechanism based model for stage II fatigue crack propagation rate

NASA Technical Reports Server (NTRS)

Mazumdar, P. K.

1986-01-01

Repeated plastic deformation, which of course depends on dislocation mechanism, at or near the crack tip leads to the fatigue crack propagation. By involving the theory of thermally activated flow and the cumulative plastic strain criterion, an effort is made here to model the stage II fatigue crack propagation rate in terms of the dislocation mechanism. The model, therefore, provides capability to ascertain: (1) the dislocation mechanism (and hence the near crack tip microstructures) assisting the crack growth, (2) the relative resistance of dislocation mechanisms to the crack growth, and (3) the fracture surface characteristics and its interpretation in terms of the dislocation mechanism. The local microstructure predicted for the room temperature crack growth in copper by this model is in good agreement with the experimental results taken from the literature. With regard to the relative stability of such dislocation mechanisms as the cross-slip and the dislocation intersection, the model suggests an enhancement of crack growth rate with an ease of cross-slip which in general promotes dislocation cell formation and is common in material which has high stacking fault energy (produces wavy slips). Cross-slip apparently enhances crack growth rate by promoting slip irreversibility and fracture surface brittleness to a greater degree.

Modeling of abnormal mechanical properties of nickel-based single crystal superalloy by three-dimensional discrete dislocation dynamics

NASA Astrophysics Data System (ADS)

Yang, Hui; Li, Zhenhuan; Huang, Minsheng

2014-12-01

Unlike common single crystals, the nickel-based single crystal superalloy shows surprisingly anomalous flow strength (i.e. with the increase of temperature, the yield strength first increases to a peak value and then decreases) and tension-compression (TC) asymmetry. A comprehensive three-dimensional discrete dislocation dynamics (3D-DDD) procedure was developed to model these abnormal mechanical properties. For this purpose, a series of complicated dynamic evolution details of Kear-Wilsdorf (KW) locks, which are closely related to the flow strength anomaly and TC asymmetry, were incorporated into this 3D-DDD framework. Moreover, the activation of the cubic slip system, which is the origin of the decrease in yield strength with increasing temperature at relatively high temperatures, was especially taken into account by introducing a competition criterion between the unlocking of the KW locks and the activation of the cubic slip system. To test our framework, a series of 3D-DDD simulations were performed on a representative volume cell model with a cuboidal Ni3Al precipitate phase embedded in a nickel matrix. Results show that the present 3D-DDD procedure can successfully capture the dynamic evolution of KW locks, the flow strength anomaly and TC asymmetry. Then, the underlying dislocation mechanisms leading to these abnormal mechanical responses were investigated and discussed in detail. Finally, a cyclic deformation of the nickel-based single crystal superalloy was modeled by using the present DDD model, with a special focus on the influence of KW locks on the Bauschinger effect and cyclic softening.

Solute softening and defect generation during prismatic slip in magnesium alloys

NASA Astrophysics Data System (ADS)

Yi, Peng; Cammarata, Robert C.; Falk, Michael L.

2017-12-01

Temperature and solute effects on prismatic slip of 〈a〉 dislocations in Mg are studied using molecular dynamics simulation. Prismatic slip is controlled by the low mobility screw dislocation. The screw dislocation glides on the prismatic plane through alternating cross-slip between the basal plane and the prismatic plane. In doing so, it exhibits a locking-unlocking mechanism at low temperatures and a more continuous wavy propagation at high temperatures. The dislocation dissociates into partials on the basal plane and the constriction formation of the partials is identified to be the rate-limiting process for unlocking. In addition, the diffusion of partials on the basal plane enables the formation of jogs and superjogs for prismatic slip, which lead to the generation of vacancies and dislocation loops. Solute softening in Mg alloys was observed in the presence of both Al and Y solute. The softening in prismatic slip is found to be due to solute pinning on the basal plane, instead of the relative energy change of the screw dislocation on the basal and prismatic planes, as has been hypothesized.

Pyramidal dislocation induced strain relaxation in hexagonal structured InGaN/AlGaN/GaN multilayer

NASA Astrophysics Data System (ADS)

Yan, P. F.; Du, K.; Sui, M. L.

2012-10-01

Due to the special dislocation slip systems in hexagonal lattice, dislocation dominated deformations in hexagonal structured multilayers are significantly different from that in cubic structured systems. In this work, we have studied the strain relaxation mechanism in hexagonal structured InGaN/AlGaN/GaN multilayers with transmission electron microscopy. Due to lattice mismatch, the strain relaxation was found initiated with the formation of pyramidal dislocations. Such dislocations locally lie at only one preferential slip direction in the hexagonal lattice. This preferential slip causes a shear stress along the basal planes and consequently leads to dissociation of pyramidal dislocations and operation of the basal plane slip system. The compressive InGaN layers and "weak" AlGaN/InGaN interfaces stimulate the dissociation of pyramidal dislocations at the interfaces. These results enhance the understanding of interactions between dislocations and layer interfaces and shed new lights on deformation mechanism in hexagonal-lattice multilayers.

Sequential slip transfer of mixed-character dislocations across Σ3 coherent twin boundary in FCC metals: a concurrent atomistic-continuum study

DOE PAGES

Xu, Shuozhi; Xiong, Liming; Chen, Youping; ...

2016-01-29

Sequential slip transfer across grain boundaries (GB) has an important role in size-dependent propagation of plastic deformation in polycrystalline metals. For example, the Hall–Petch effect, which states that a smaller average grain size results in a higher yield stress, can be rationalised in terms of dislocation pile-ups against GBs. In spite of extensive studies in modelling individual phases and grains using atomistic simulations, well-accepted criteria of slip transfer across GBs are still lacking, as well as models of predicting irreversible GB structure evolution. Slip transfer is inherently multiscale since both the atomic structure of the boundary and the long-range fieldsmore » of the dislocation pile-up come into play. In this work, concurrent atomistic-continuum simulations are performed to study sequential slip transfer of a series of curved dislocations from a given pile-up on Σ3 coherent twin boundary (CTB) in Cu and Al, with dominant leading screw character at the site of interaction. A Frank-Read source is employed to nucleate dislocations continuously. It is found that subject to a shear stress of 1.2 GPa, screw dislocations transfer into the twinned grain in Cu, but glide on the twin boundary plane in Al. Moreover, four dislocation/CTB interaction modes are identified in Al, which are affected by (1) applied shear stress, (2) dislocation line length, and (3) dislocation line curvature. Our results elucidate the discrepancies between atomistic simulations and experimental observations of dislocation-GB reactions and highlight the importance of directly modeling sequential dislocation slip transfer reactions using fully 3D models.« less

Sequential slip transfer of mixed-character dislocations across Σ3 coherent twin boundary in FCC metals: a concurrent atomistic-continuum study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Shuozhi; Xiong, Liming; Chen, Youping

Sequential slip transfer across grain boundaries (GB) has an important role in size-dependent propagation of plastic deformation in polycrystalline metals. For example, the Hall–Petch effect, which states that a smaller average grain size results in a higher yield stress, can be rationalised in terms of dislocation pile-ups against GBs. In spite of extensive studies in modelling individual phases and grains using atomistic simulations, well-accepted criteria of slip transfer across GBs are still lacking, as well as models of predicting irreversible GB structure evolution. Slip transfer is inherently multiscale since both the atomic structure of the boundary and the long-range fieldsmore » of the dislocation pile-up come into play. In this work, concurrent atomistic-continuum simulations are performed to study sequential slip transfer of a series of curved dislocations from a given pile-up on Σ3 coherent twin boundary (CTB) in Cu and Al, with dominant leading screw character at the site of interaction. A Frank-Read source is employed to nucleate dislocations continuously. It is found that subject to a shear stress of 1.2 GPa, screw dislocations transfer into the twinned grain in Cu, but glide on the twin boundary plane in Al. Moreover, four dislocation/CTB interaction modes are identified in Al, which are affected by (1) applied shear stress, (2) dislocation line length, and (3) dislocation line curvature. Our results elucidate the discrepancies between atomistic simulations and experimental observations of dislocation-GB reactions and highlight the importance of directly modeling sequential dislocation slip transfer reactions using fully 3D models.« less

Novel Cross-Slip Mechanism of Pyramidal Screw Dislocations in Magnesium.

PubMed

Itakura, Mitsuhiro; Kaburaki, Hideo; Yamaguchi, Masatake; Tsuru, Tomohito

2016-06-03

Compared to cubic metals, whose primary slip mode includes twelve equivalent systems, the lower crystalline symmetry of hexagonal close-packed metals results in a reduced number of equivalent primary slips and anisotropy in plasticity, leading to brittleness at the ambient temperature. At higher temperatures, the ductility of hexagonal close-packed metals improves owing to the activation of secondary ⟨c+a⟩ pyramidal slip systems. Thus, understanding the fundamental properties of corresponding dislocations is essential for the improvement of ductility at the ambient temperature. Here, we present the results of large-scale ab initio calculations for ⟨c+a⟩ pyramidal screw dislocations in magnesium and show that their slip behavior is a stark counterexample to the conventional wisdom that a slip plane is determined by the stacking fault plane of dislocations. A stacking fault between dissociated partial dislocations can assume a nonplanar shape with a negligible energy cost and can migrate normal to its plane by a local shuffling of atoms. Partial dislocations dissociated on a {21[over ¯]1[over ¯]2} plane "slither" in the {011[over ¯]1} plane, dragging the stacking fault with them in response to an applied shear stress. This finding resolves the apparent discrepancy that both {21[over ¯]1[over ¯]2} and {011[over ¯]1} slip traces are observed in experiments while ab initio calculations indicate that dislocations preferably dissociate in the {21[over ¯]1[over ¯]2} planes.

Temperature-dependent plastic hysteresis in highly confined polycrystalline Nb films

NASA Astrophysics Data System (ADS)

Waheed, S.; Hao, R.; Zheng, Z.; Wheeler, J. M.; Michler, J.; Balint, D. S.; Giuliani, F.

2018-02-01

In this study, the effect of temperature on the cyclic deformation behaviour of a confined polycrystalline Nb film is investigated. Micropillars encapsulating a thin niobium interlayer are deformed under cyclic axial compression at different test temperatures. A distinct plastic hysteresis is observed for samples tested at elevated temperatures, whereas negligible plastic hysteresis is observed for samples tested at room temperature. These results are interpreted using planar discrete dislocation plasticity incorporating slip transmission across grain boundaries. The effect of temperature-dependent grain boundary energy and dislocation mobility on dislocation penetration and, consequently, the size of plastic hysteresis is simulated to correlate with the experimental results. It is found that the decrease in grain boundary energy barrier caused by the increase in temperature does not lead to any appreciable change in the cyclic response. However, dislocation mobility significantly affects the size of plastic hysteresis, with high mobilities leading to a larger hysteresis. Therefore, it is postulated that the experimental observations are predominantly caused by an increase in dislocation mobility as the temperature is increased above the critical temperature of body-centred cubic niobium.

Effect of solute atoms on dislocation motion in Mg: An electronic structure perspective

PubMed Central

Tsuru, T.; Chrzan, D. C.

2015-01-01

Solution strengthening is a well-known approach to tailoring the mechanical properties of structural alloys. Ultimately, the properties of the dislocation/solute interaction are rooted in the electronic structure of the alloy. Accordingly, we compute the electronic structure associated with, and the energy barriers to dislocation cross-slip. The energy barriers so obtained can be used in the development of multiscale models for dislocation mediated plasticity. The computed electronic structure can be used to identify substitutional solutes likely to interact strongly with the dislocation. Using the example of a-type screw dislocations in Mg, we compute accurately the Peierls barrier to prismatic plane slip and argue that Y, Ca, Ti, and Zr should interact strongly with the studied dislocation, and thereby decrease the dislocation slip anisotropy in the alloy. PMID:25740411

Water redistribution in experimentally deformed natural milky quartz single crystals—Implications for H2O-weakening processes

NASA Astrophysics Data System (ADS)

Stünitz, H.; Thust, A.; Heilbronner, R.; Behrens, H.; Kilian, R.; Tarantola, A.; Fitz Gerald, J. D.

2017-02-01

Natural quartz single crystals were experimentally deformed in two orientations: (1) ⊥ to one prism plane and (2) in O+ orientation at 900 and 1000°C, 1.0 and 1.5 GPa, and strain rates of 1 × 10-6 s-1. In addition, hydrostatic and annealing experiments were performed. The starting material was milky quartz, which consisted of dry quartz with a large number of fluid inclusions of variable size up to several 100 µm. During pressurization fluid inclusions decrepitated producing much smaller fluid inclusions. Deformation on the sample scale is anisotropic due to dislocation glide on selected slip systems and inhomogeneous due to an inhomogeneous distribution of fluid inclusions. Dislocation glide is accompanied by minor dynamic recovery. Strongly deformed regions show a pointed broad absorption band in the 3400 cm-1 region consisting of a superposition of bands of molecular H2O and three discrete absorption bands (at 3367, 3400, and 3434 cm-1). In addition, there is a discrete absorption band at 3585 cm-1, which only occurs in deformed regions and reduces or disappears after annealing, so that this band appears to be associated with dislocations. H2O weakening in inclusion-bearing natural quartz crystals is assigned to the H2O-assisted dislocation generation and multiplication. Processes in these crystals represent recycling of H2O between fluid inclusions, cracking and crack healing, incorporation of structurally bound H in dislocations, release of H2O from dislocations during recovery, and dislocation generation at very small fluid inclusions. The H2O weakening by this process is of disequilibrium nature because it depends on the amount of H2O available.

Dislocation evolution in 316 L stainless steel during multiaxial ratchetting deformation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dong Yawei; Kang Guozheng, E-mail: guozhengkang@yahoo.com.cn; Liu Yujie

2012-03-15

Dislocation patterns and their evolutions in 316 L stainless steel during the multiaxial ratchetting deformation were observed by transmission electron microscopy (TEM). The microscopic observations indicate that the dislocation evolution presented during the multiaxial ratchetting with four kinds of multiaxial loading paths is similar to that in the uniaxial case [G. Z. Kang et al., Mater Sci Eng A 527 (2010) 5952]. That is, dislocation networks and dislocation tangles are formed quickly by the multiple-slip and cross-slip of dislocation activated by applied multiaxial stress; and then polarized patterns such as dislocation walls and elongated incipient dislocation cells are formed atmore » the last stage of multiaxial ratchetting. The dislocation patterns evolve more quickly from the modes at low dislocation density to the ones at high density during the multiaxial ratchetting than that in the uniaxial case, and some traces of multiple-slip are observed in the multiaxial ones. The dislocation evolution during the multiaxial ratchetting deformation is summarized by comparing the observed dislocation patterns with those presented in the multiaxial strain-controlled and symmetrical stress-controlled cyclic tests. The multiaxial ratchetting of 316 L stainless steel can be microscopically and qualitatively explained by the observed evolution of dislocation patterns. - Highlights: Black-Right-Pointing-Pointer Dislocation patterns change from lines and nets to tangles, walls and cells. Black-Right-Pointing-Pointer Dislocation patterns evolve quicker in the multiaxial case. Black-Right-Pointing-Pointer Aligned dislocation arrays and some traces of multiple slips are observed. Black-Right-Pointing-Pointer Heterogeneous dislocation patterns result in the multiaxial ratchetting.« less

Simulation of uniaxial deformation of hexagonal crystals (Mg, Be)

NASA Astrophysics Data System (ADS)

Vlasova, A. M.; Kesarev, A. G.

2017-12-01

Molecular dynamics (MD) simulations were performed for the nanocompression loading of nanocrystalline magnesium and beryllium modeled by an interatomic potential of the embedded atom method (EAM). It is shown that the main deformation modes are prismatic slip and twinning for magnesium, and only prismatic slip for beryllium. The formation of stable configurations of dislocation grids in magnesium and beryllium was observed. Dislocation networks are formed in the habit plane of the twin in a magnesium nanocrystall. Some dislocation reactions are suggested to explain the appearance of such networks. Shockley partial dislocations in a beryllium nanocrystall form grids in the slip plane. A strong anisotropy between slip systems was observed, which is in agreement with experimental data.

The influence of anisotropy on the core structure of Shockley partial dislocations within FCC materials

NASA Astrophysics Data System (ADS)

Szajewski, B. A.; Hunter, A.; Luscher, D. J.; Beyerlein, I. J.

2018-01-01

Both theoretical and numerical models of dislocations often necessitate the assumption of elastic isotropy to retain analytical tractability in addition to reducing computational load. As dislocation based models evolve towards physically realistic material descriptions, the assumption of elastic isotropy becomes increasingly worthy of examination. We present an analytical dislocation model for calculating the full dissociated core structure of dislocations within anisotropic face centered cubic (FCC) crystals as a function of the degree of material elastic anisotropy, two misfit energy densities on the γ-surface ({γ }{{isf}}, {γ }{{usf}}) and the remaining elastic constants. Our solution is independent of any additional features of the γ-surface. Towards this pursuit, we first demonstrate that the dependence of the anisotropic elasticity tensor on the orientation of the dislocation line within the FCC crystalline lattice is small and may be reasonably neglected for typical materials. With this approximation, explicit analytic solutions for the anisotropic elasticity tensor {B} for both nominally edge and screw dislocations within an FCC crystalline lattice are devised, and employed towards defining a set of effective isotropic elastic constants which reproduce fully anisotropic results, however do not retain the bulk modulus. Conversely, Hill averaged elastic constants which both retain the bulk modulus and reasonably approximate the dislocation core structure are employed within subsequent numerical calculations. We examine a wide range of materials within this study, and the features of each partial dislocation core are sufficiently localized that application of discrete linear elasticity accurately describes the separation of each partial dislocation core. In addition, the local features (the partial dislocation core distribution) are well described by a Peierls-Nabarro dislocation model. We develop a model for the displacement profile which depends upon two disparate dislocation length scales which describe the core structure; (i) the equilibrium stacking fault width between two Shockley partial dislocations, R eq and (ii) the maximum slip gradient, χ, of each Shockley partial dislocation. We demonstrate excellent agreement between our own analytic predictions, numerical calculations, and R eq computed directly by both ab-initio and molecular statics methods found elsewhere within the literature. The results suggest that understanding of various plastic mechanisms, e.g., cross-slip and nucleation may be augmented with the inclusion of elastic anisotropy.

Kinematic assumptions and their consequences on the structure of field equations in continuum dislocation theory

NASA Astrophysics Data System (ADS)

Silbermann, C. B.; Ihlemann, J.

2016-03-01

Continuum Dislocation Theory (CDT) relates gradients of plastic deformation in crystals with the presence of geometrically necessary dislocations. Therefore, the dislocation tensor is introduced as an additional thermodynamic state variable which reflects tensorial properties of dislocation ensembles. Moreover, the CDT captures both the strain energy from the macroscopic deformation of the crystal and the elastic energy of the dislocation network, as well as the dissipation of energy due to dislocation motion. The present contribution deals with the geometrically linear CDT. More precise, the focus is on the role of dislocation kinematics for single and multi-slip and its consequences on the field equations. Thereby, the number of active slip systems plays a crucial role since it restricts the degrees of freedom of plastic deformation. Special attention is put on the definition of proper, well-defined invariants of the dislocation tensor in order to avoid any spurious dependence of the resulting field equations on the coordinate system. It is shown how a slip system based approach can be in accordance with the tensor nature of the involved quantities. At first, only dislocation glide in one active slip system of the crystal is allowed. Then, the special case of two orthogonal (interacting) slip systems is considered and the governing field equations are presented. In addition, the structure and symmetry of the backstress tensor is investigated from the viewpoint of thermodynamical consistency. The results will again be used in order to facilitate the set of field equations and to prepare for a robust numerical implementation.

Hydrogen-induced strain localisation in oxygen-free copper in the initial stage of plastic deformation

NASA Astrophysics Data System (ADS)

Yagodzinskyy, Yuriy; Malitckii, Evgenii; Tuomisto, Filip; Hänninen, Hannu

2018-03-01

Single crystals of oxygen-free copper oriented to easy glide of dislocations were tensile tested in order to study the hydrogen effects on the strain localisation in the form of slip bands appearing on the polished specimen surface under tensile straining. It was found that hydrogen increases the plastic flow stress in Stage I of deformation. The dislocation slip localisation in the form of slip bands was observed and analysed using an online optical monitoring system and atomic force microscopy. The fine structure of the slip bands observed with AFM shows that they consist of a number of dislocation slip offsets which spacing in the presence of hydrogen is markedly reduced as compared to that in the hydrogen-free specimens. The tensile tests and AFM observations were accompanied with positron annihilation lifetime measurements showing that straining of pure copper in the presence of hydrogen results in free volume generation in the form of vacancy complexes. Hydrogen-enhanced free-volume generation is discussed in terms of hydrogen interactions with edge dislocation dipoles forming in double cross-slip of screw dislocations in the initial stage of plastic deformation of pure copper.

Coupled crystal orientation-size effects on the strength of nano crystals

PubMed Central

Yuan, Rui; Beyerlein, Irene J.; Zhou, Caizhi

2016-01-01

We study the combined effects of grain size and texture on the strength of nanocrystalline copper (Cu) and nickel (Ni) using a crystal-plasticity based mechanics model. Within the model, slip occurs in discrete slip events exclusively by individual dislocations emitted statistically from the grain boundaries. We show that a Hall-Petch relationship emerges in both initially texture and non-textured materials and our values are in agreement with experimental measurements from numerous studies. We find that the Hall-Petch slope increases with texture strength, indicating that preferred orientations intensify the enhancements in strength that accompany grain size reductions. These findings reveal that texture is too influential to be neglected when analyzing and engineering grain size effects for increasing nanomaterial strength. PMID:27185364

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

NASA Astrophysics Data System (ADS)

Weikusat, Ilka; Kuiper, Ernst-Jan N.; Pennock, Gill M.; Kipfstuhl, Sepp; Drury, Martyn R.

2017-09-01

Ice has a very high plastic anisotropy with easy dislocation glide on basal planes, while glide on non-basal planes is much harder. Basal glide involves dislocations with the Burgers vector b = 〈a〉, while glide on non-basal planes can involve dislocations with b = 〈a〉, b = [c], and b = 〈c + a〉. During the natural ductile flow of polar ice sheets, most of the deformation is expected to occur by basal slip accommodated by other processes, including non-basal slip and grain boundary processes. However, the importance of different accommodating processes is controversial. The recent application of micro-diffraction analysis methods to ice, such as X-ray Laue diffraction and electron backscattered diffraction (EBSD), has demonstrated that subgrain boundaries indicative of non-basal slip are present in naturally deformed ice, although so far the available data sets are limited. In this study we present an analysis of a large number of subgrain boundaries in ice core samples from one depth level from two deep ice cores from Antarctica (EPICA-DML deep ice core at 656 m of depth) and Greenland (NEEM deep ice core at 719 m of depth). EBSD provides information for the characterization of subgrain boundary types and on the dislocations that are likely to be present along the boundary. EBSD analyses, in combination with light microscopy measurements, are presented and interpreted in terms of the dislocation slip systems. The most common subgrain boundaries are indicative of basal 〈a〉 slip with an almost equal occurrence of subgrain boundaries indicative of prism [c] or 〈c + a〉 slip on prism and/or pyramidal planes. A few subgrain boundaries are indicative of prism 〈a〉 slip or slip of 〈a〉 screw dislocations on the basal plane. In addition to these classical polygonization processes that involve the recovery of dislocations into boundaries, alternative mechanisms are discussed for the formation of subgrain boundaries that are not related to the crystallography of the host grain.The finding that subgrain boundaries indicative of non-basal slip are as frequent as those indicating basal slip is surprising. Our evidence of frequent non-basal slip in naturally deformed polar ice core samples has important implications for discussions on ice about plasticity descriptions, rate-controlling processes which accommodate basal glide, and anisotropic ice flow descriptions of large ice masses with the wider perspective of sea level evolution.

FAST TRACK COMMUNICATION High rate straining of tantalum and copper

NASA Astrophysics Data System (ADS)

Armstrong, R. W.; Zerilli, F. J.

2010-12-01

High strain rate measurements reported recently for several tantalum and copper crystal/polycrystal materials are shown to follow dislocation mechanics-based constitutive relations, first at lower strain rates, for dislocation velocity control of the imposed plastic deformations and, then at higher rates, transitioning to nano-scale dislocation generation control by twinning or slip. For copper, there is the possibility of added-on slip dislocation displacements to be accounted for from the newly generated dislocations.

Dwell fatigue in two Ti alloys: An integrated crystal plasticity and discrete dislocation study

NASA Astrophysics Data System (ADS)

Zheng, Zebang; Balint, Daniel S.; Dunne, Fionn P. E.

2016-11-01

It is a well known and important problem in the aircraft engine industry that alloy Ti-6242 shows a significant reduction in fatigue life, termed dwell debit, if a stress dwell is included in the fatigue cycle, whereas Ti-6246 does not; the mechanistic explanation for the differing dwell debit of these alloys has remained elusive for decades. In this work, crystal plasticity modelling has been utilised to extract the thermal activation energies for pinned dislocation escape for both Ti alloys based on independent experimental data. This then allows the markedly different cold creep responses of the two alloys to be captured accurately and demonstrates why the observed near-identical rate sensitivity under non-dwell loading is entirely consistent with the dwell behaviour. The activation energies determined are then utilised within a recently developed thermally-activated discrete dislocation plasticity model to predict the strain rate sensitivities of the two alloys associated with nano-indentation into basal and prism planes. It is shown that Ti-6242 experiences a strong crystallographic orientation-dependent rate sensitivity while Ti-6246 does not which is shown to agree with recently published independent measurements; the dependence of rate sensitivity on indentation slip plane is also well captured. The thermally-activated discrete dislocation plasticity model shows that the incorporation of a stress dwell in fatigue loading leads to remarkable stress redistribution from soft to hard grains in the classical cold dwell fatigue rogue grain combination in alloy Ti-6242, but that no such load shedding occurs in alloy Ti-6246. The key property controlling the behaviour is the time constant of the thermal activation process relative to that of the loading. This work provides the first mechanistic basis to explain why alloy Ti-6242 shows a dwell debit but Ti-6246 does not.

Documentation of programs that compute 1) static tilts for a spatially variable slip distribution, and 2) quasi-static tilts produced by an expanding dislocation loop with a spatially variable slip distribution

USGS Publications Warehouse

McHugh, Stuart

1976-01-01

The material in this report is concerned with the effects of a vertically oriented rectangular dislocation loop on the tilts observed at the free surface of an elastic half-space. Part I examines the effect of a spatially variable static strike-slip distribution across the slip surface. The tilt components as a function of distance parallel, or perpendicular, to the strike of the slip surface are displayed for different slip-versus-distance profiles. Part II examines the effect of spatially and temporally variable slip distributions across the dislocation loop on the quasi-static tilts at the free surface of an elastic half space. The model discussed in part II may be used to generate theoretical tilt versus time curves produced by creep events.

Mechanistic basis of temperature-dependent dwell fatigue in titanium alloys

NASA Astrophysics Data System (ADS)

Zheng, Zebang; Balint, Daniel S.; Dunne, Fionn P. E.

2017-10-01

The temperature-dependent dwell sensitivity of Ti-6242 and Ti-6246 alloys has been assessed over a temperature range from - 50∘ C to 390 °C using discrete dislocation plasticity which incorporates both thermal activation of dislocation escape from obstacles and slip transfer across grain boundaries. The worst-case load shedding in Ti-6242 alloy is found to be at or close to 120 °C under dwell fatigue loading, which diminishes and vanishes at temperatures lower than - 50∘ C or higher than 230 °C. Load shedding behaviour is predicted to occur in alloy Ti-6246 also but over a range of higher temperatures which are outside those relevant to in-service conditions. The key controlling dislocation mechanism with respect to load shedding in titanium alloys, and its temperature sensitivity, is shown to be the time constant associated with the thermal activation of dislocation escape from obstacles, with respect to the stress dwell time. The mechanistic basis of load shedding and dwell sensitivity in dwell fatigue loading is presented and discussed in the context of experimental observations.

Observations of Glide and Decomposition of a<101> Dislocations at High Temperatures in Ni-Al Single Crystals Deformed along the Hard Orientation

NASA Technical Reports Server (NTRS)

Srinivasan, R.; Daw, M. S.; Noebe, R. D.; Mills, M. J.

2003-01-01

Ni-44at.% Al and Ni-50at.% single crystals were tested in compression in the hard (001) orientations. The dislocation processes and deformation behavior were studied as a function of temperature, strain and strain rate. A slip transition in NiAl occurs from alpha(111) slip to non-alphaaaaaaaaaaa9111) slip at intermediate temperatures. In Ni-50at.% Al single crystal, only alpha(010) dislocations are observed above the slip transition temperature. In contrast, alpha(101)(101) glide has been observed to control deformation beyond the slip transition temperature in Ni-44at.%Al. alpha(101) dislocations are observed primarily along both (111) directions in the glide plane. High-resolution transmission electron microscopy observations show that the core of the alpha(101) dislocations along these directions is decomposed into two alpha(010) dislocations, separated by a distance of approximately 2nm. The temperature window of stability for these alpha(101) dislocations depends upon the strain rate. At a strain rate of 1.4 x 10(exp -4)/s, lpha(101) dislocations are observed between 800 and 1000K. Complete decomposition of a alpha(101) dislocations into alpha(010) dislocations occurs beyond 1000K, leading to alpha(010) climb as the deformation mode at higher temperature. At lower strain rates, decomposition of a alpha(101) dislocations has been observed to occur along the edge orientation at temperatures below 1000K. Embedded-atom method calculations and experimental results indicate that alpha(101) dislocation have a large Peieris stress at low temperature. Based on the present microstructural observations and a survey of the literature with respect to vacancy content and diffusion in NiAl, a model is proposed for alpha(101)(101) glide in Ni-44at.%Al, and for the observed yield strength versus temperature behavior of Ni-Al alloys at intermediate and high temperatures.

Nano-indentation used to study pyramidal slip in GaN single crystals

NASA Astrophysics Data System (ADS)

Krimsky, E.; Jones, K. A.; Tompkins, R. P.; Rotella, P.; Ligda, J.; Schuster, B. E.

2018-02-01

The nucleation and structure of dislocations created by the nano-indentation of GaN samples with dislocation densities ≈103, 106 or 109 ⊥/cm2 were studied in the interest of learning how dislocations can be created to relieve the mismatch strain in ternary nitride films grown on (0001) oriented binary nitride substrates. Using transmission electron microscopy and stress analyses to assist in interpreting the nano-indentation data, we determined that the pop-ins in the indenter load vs. penetration depth curves are created by an avalanche process at stresses well above the typical yield stress. The process begins by the homogeneous formation of a basal plane screw dislocation that triggers the formation of pyramidal and other basal plane dislocations that relieve the excess stored elastic energy. It appears that pyramidal slip can occur on either the {1122} or {0111} planes, as there is little resistance to the cross slip of screw dislocations.

Mechanism and energetics of dislocation cross-slip in hcp metals

NASA Astrophysics Data System (ADS)

Wu, Zhaoxuan; Curtin, W. A.

2016-10-01

Hexagonal close-packed (hcp) metals such as Mg, Ti, and Zr are lightweight and/or durable metals with critical structural applications in the automotive (Mg), aerospace (Ti), and nuclear (Zr) industries. The hcp structure, however, brings significant complications in the mechanisms of plastic deformation, strengthening, and ductility, and these complications pose significant challenges in advancing the science and engineering of these metals. In hcp metals, generalized plasticity requires the activation of slip on pyramidal planes, but the structure, motion, and cross-slip of the associated dislocations are not well established even though they determine ductility and influence strengthening. Here, atomistic simulations in Mg reveal the unusual mechanism of dislocation cross-slip between pyramidal I and II planes, which occurs by cross-slip of the individual partial dislocations. The energy barrier is controlled by a fundamental step/jog energy and the near-core energy difference between pyramidal dislocations. The near-core energy difference can be changed by nonglide stresses, leading to tension-compression asymmetry and even a switch in absolute stability from one glide plane to the other, both features observed experimentally in Mg, Ti, and their alloys. The unique cross-slip mechanism is governed by common features of the generalized stacking fault energy surfaces of hcp pyramidal planes and is thus expected to be generic to all hcp metals. An analytical model is developed to predict the cross-slip barrier as a function of the near-core energy difference and applied stresses and quantifies the controlling features of cross-slip and pyramidal I/II stability across the family of hcp metals.

Evidence for and implications of self-healing pulses of slip in earthquake rupture

USGS Publications Warehouse

Heaton, T.H.

1990-01-01

Dislocation time histories of models derived from waveforms of seven earthquakes are discussed. In each model, dislocation rise times (the duration of slip for a given point on the fault) are found to be short compared to the overall duration of the earthquake (??? 10%). However, in many crack-like numerical models of dynamic rupture, the slip duration at a given point is comparable to the overall duration of the rupture; i.e. slip at a given point continues until information is received that the rupture has stopped propagating. Alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are: (1) the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes; (2) earthquakes are composed of a sequence of small-dimension (short duration) events that are separated by locked regions (barriers); (3) rupture occurs in a narrow self-healing pulse of slip that travels along the fault surface. Evidence is discussed that suggests that slip durations are indeed short and that the self-healing slip-pulse model is the most appropriate explanation. A qualitative model is presented that produces self-healing slip pulses. The key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity. The model has the following features: high static strength of materials (kilobar range), low static stress drops (in the range of tens of bars), and relatively low frictional stress during slip (less than several hundreds of bars). It is suggested that the reason that the average dislocation scales with fault length is because large-amplitude slip pulses are difficult to stop and hence tend to propagate large distances. This model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes. It also qualitatively explains why the recurrence time for large earthquakes may be irregular. ?? 1990.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kang, Guozheng, E-mail: guozhengkang@home.swjtu.edu.cn; Dong, Yawei; Liu, Yujie

The uniaxial ratchetting of Ti–6Al–4V alloy with two phases (i.e., primary hexagonal close packed (HCP) α and secondary body-centered cubic (BCC) β phases) was investigated by macroscopic and microscopic experiments at room temperature. Firstly, the effects of cyclic softening/hardening feature, applied mean stress and stress amplitude on the uniaxial ratchetting of the alloy were discussed. The macroscopic investigation of Ti–6Al–4V alloy presents obvious strain-amplitude-dependent cyclic softening, as well as a three-staged evolution curve with regard to the ratchetting strain rate. The ratchetting depends greatly on the applied mean stress and stress amplitude while the ratchetting strain increases with the increasingmore » applied mean stress and stress amplitude. Then, the evolution of dislocation patterns and deformation twinning during the uniaxial ratchetting of two-phase Ti–6Al–4V alloy were observed using transmission electron microscopy (TEM). The microscopic observation shows that deformation twinning occurs in the primary α phase and its amount increases gradually during the uniaxial ratchetting. Simultaneously, the planar dislocation evolves from discrete lines to some dislocation nets and parallel lines with the increasing number of cycles. The deformation twinning in the primary α phase is one of main contributions to the uniaxial ratchetting of Ti–6Al–4V alloy, and should be considered in the construction of corresponding constitutive model. - Highlights: • A three-staged ratchetting occurs in the stress-controlled cyclic tests of Ti–6Al–4V alloy. • Dislocation patterns change from discrete lines to nets and parallel lines. • Deformation twinning occurs during the uniaxial ratchetting. • Both dislocation slipping and twinning are the causes of ratchetting.« less

Multiscale modeling of dislocation-precipitate interactions in Fe: From molecular dynamics to discrete dislocations.

PubMed

Lehtinen, Arttu; Granberg, Fredric; Laurson, Lasse; Nordlund, Kai; Alava, Mikko J

2016-01-01

The stress-driven motion of dislocations in crystalline solids, and thus the ensuing plastic deformation process, is greatly influenced by the presence or absence of various pointlike defects such as precipitates or solute atoms. These defects act as obstacles for dislocation motion and hence affect the mechanical properties of the material. Here we combine molecular dynamics studies with three-dimensional discrete dislocation dynamics simulations in order to model the interaction between different kinds of precipitates and a 1/2〈111〉{110} edge dislocation in BCC iron. We have implemented immobile spherical precipitates into the ParaDis discrete dislocation dynamics code, with the dislocations interacting with the precipitates via a Gaussian potential, generating a normal force acting on the dislocation segments. The parameters used in the discrete dislocation dynamics simulations for the precipitate potential, the dislocation mobility, shear modulus, and dislocation core energy are obtained from molecular dynamics simulations. We compare the critical stresses needed to unpin the dislocation from the precipitate in molecular dynamics and discrete dislocation dynamics simulations in order to fit the two methods together and discuss the variety of the relevant pinning and depinning mechanisms.

Mechanism and energetics of 〈c + a〉 dislocation cross-slip in hcp metals.

PubMed

Wu, Zhaoxuan; Curtin, W A

2016-10-04

Hexagonal close-packed (hcp) metals such as Mg, Ti, and Zr are lightweight and/or durable metals with critical structural applications in the automotive (Mg), aerospace (Ti), and nuclear (Zr) industries. The hcp structure, however, brings significant complications in the mechanisms of plastic deformation, strengthening, and ductility, and these complications pose significant challenges in advancing the science and engineering of these metals. In hcp metals, generalized plasticity requires the activation of slip on pyramidal planes, but the structure, motion, and cross-slip of the associated [Formula: see text] dislocations are not well established even though they determine ductility and influence strengthening. Here, atomistic simulations in Mg reveal the unusual mechanism of [Formula: see text] dislocation cross-slip between pyramidal I and II planes, which occurs by cross-slip of the individual partial dislocations. The energy barrier is controlled by a fundamental step/jog energy and the near-core energy difference between pyramidal [Formula: see text] dislocations. The near-core energy difference can be changed by nonglide stresses, leading to tension-compression asymmetry and even a switch in absolute stability from one glide plane to the other, both features observed experimentally in Mg, Ti, and their alloys. The unique cross-slip mechanism is governed by common features of the generalized stacking fault energy surfaces of hcp pyramidal planes and is thus expected to be generic to all hcp metals. An analytical model is developed to predict the cross-slip barrier as a function of the near-core energy difference and applied stresses and quantifies the controlling features of cross-slip and pyramidal I/II stability across the family of hcp metals.

Mechanism and energetics of 〈c + a〉 dislocation cross-slip in hcp metals

PubMed Central

Wu, Zhaoxuan; Curtin, W. A.

2016-01-01

Hexagonal close-packed (hcp) metals such as Mg, Ti, and Zr are lightweight and/or durable metals with critical structural applications in the automotive (Mg), aerospace (Ti), and nuclear (Zr) industries. The hcp structure, however, brings significant complications in the mechanisms of plastic deformation, strengthening, and ductility, and these complications pose significant challenges in advancing the science and engineering of these metals. In hcp metals, generalized plasticity requires the activation of slip on pyramidal planes, but the structure, motion, and cross-slip of the associated 〈c+a〉 dislocations are not well established even though they determine ductility and influence strengthening. Here, atomistic simulations in Mg reveal the unusual mechanism of 〈c+a〉 dislocation cross-slip between pyramidal I and II planes, which occurs by cross-slip of the individual partial dislocations. The energy barrier is controlled by a fundamental step/jog energy and the near-core energy difference between pyramidal 〈c+a〉 dislocations. The near-core energy difference can be changed by nonglide stresses, leading to tension–compression asymmetry and even a switch in absolute stability from one glide plane to the other, both features observed experimentally in Mg, Ti, and their alloys. The unique cross-slip mechanism is governed by common features of the generalized stacking fault energy surfaces of hcp pyramidal planes and is thus expected to be generic to all hcp metals. An analytical model is developed to predict the cross-slip barrier as a function of the near-core energy difference and applied stresses and quantifies the controlling features of cross-slip and pyramidal I/II stability across the family of hcp metals. PMID:27647908

Cascadia Slow Earthquakes: Strategies for Time Independent Inversion of Displacement Fields

NASA Astrophysics Data System (ADS)

Szeliga, W. M.; Melbourne, T. I.; Miller, M. M.; Santillan, V. M.

2004-12-01

Continuous observations using Global Positioning System geodesy (CGPS) have revealed periodic slow or silent earthquakes along the Cascadia subduction zone with a spectrum of timing and periodicity. These creep events perturb time series of GPS observations and yield coherent displacement fields that relate to the extent and magnitude of fault displacement. In this study, time independent inversions of the surface displacement fields that accompany eight slow earthquakes characterize slip distributions along the plate interface for each event. The inversions employed in this study utilize Okada's elastic dislocation model and a non- negative least squares approach. Methodologies for optimizing the slip distribution smoothing parameter for a particular station distribution have also been investigated, significantly reducing the number of possible slip distributions and the range of estimates for total moment release for each event. The discretized slip distribution calculated for multiple creep events identifies areas of the Cascadia plate interface where slip persistently recurs. The current hypothesis, that slow earthquakes are modulated by forced fluid flow, leads to the possibility that some regions of the Cascadia plate interface may display fault patches preferentially exploited by fluid flow. Thus, the identification of regions of the plate interface that repeatedly slip during slow events may yield important information regarding the identification of these fluid pathways.

Atomistic simulations of dislocation pileup: Grain boundaries interaction

DOE PAGES

Wang, Jian

2015-05-27

Here, using molecular dynamics (MD) simulations, we studied the dislocation pileup–grain boundary (GB) interactions. Two Σ11 asymmetrical tilt grain boundaries in Al are studied to explore the influence of orientation relationship and interface structure on dislocation activities at grain boundaries. To mimic the reality of a dislocation pileup in a coarse-grained polycrystalline, we optimized the dislocation population in MD simulations and developed a predict-correct method to create a dislocation pileup in MD simulations. MD simulations explored several kinetic processes of dislocations–GB reactions: grain boundary sliding, grain boundary migration, slip transmission, dislocation reflection, reconstruction of grain boundary, and the correlation ofmore » these kinetic processes with the available slip systems across the GB and atomic structures of the GB.« less

A dislocation density-based continuum model of the anisotropic shock response of single crystal α-cyclotrimethylene trinitramine

NASA Astrophysics Data System (ADS)

Luscher, D. J.; Addessio, F. L.; Cawkwell, M. J.; Ramos, K. J.

2017-01-01

We have developed a model for the finite deformation thermomechanical response of α-cyclotrimethylene trinitramine (RDX). Our model accounts for nonlinear thermoelastic lattice deformation through a free energy-based equation of state developed by Cawkwell et al. (2016) in combination with temperature and pressure dependent elastic constants, as well as dislocation-mediated plastic slip on a set of slip systems motivated by experimental observation. The kinetics of crystal plasticity are modeled using the Orowan equation relating slip rate to dislocation density and the dislocation velocity developed by Austin and McDowell (2011), which naturally accounts for transition from thermally activated to dislocation drag limited regimes. Evolution of dislocation density is specified in terms of local ordinary differential equations reflecting dislocation-dislocation interactions. This paper presents details of the theory and parameterization of the model, followed by discussion of simulations of flyer plate impact experiments. Impact conditions explored within this combined simulation and experimental effort span shock pressures ranging from 1 to 3 GPa for four crystallographic orientations and multiple specimen thicknesses. Simulation results generated using this model are shown to be in strong agreement with velocimetry measurements from the corresponding plate impact experiments. Finally, simulation results are used to motivate conclusions about the nature of dislocation-mediated plasticity in RDX.

The effect of aluminum alloying on strength properties and deformation mechanisms of the <123> Hadfield steel single crystals

NASA Astrophysics Data System (ADS)

Astafurova, E. G.; Tukeev, M. S.; Chumlyakov, Yu. I.

2007-10-01

The role of aluminum alloying on strength properties and deformation mechanisms (slip, twinning) of <123> single crystals of Hadfield steel under tensile loading at T = 300 K is demonstrated. It is found out that aluminum alloying suppresses twinning deformation in the <123> single crystals and, during slip, results in a dislocation structure change from a uniform dislocation distribution to a planar dislocation structure.

Physics-Based Crystal Plasticity Modeling of Single Crystal Niobium

NASA Astrophysics Data System (ADS)

Maiti, Tias

Crystal plasticity models based on thermally activated dislocation kinetics has been successful in predicting the deformation behavior of crystalline materials, particularly in face-centered cubic (fcc) metals. In body-centered cubic (bcc) metals success has been limited owing to ill-defined slip planes. The flow stress of a bcc metal is strongly dependent on temperature and orientation due to the non-planar splitting of a/2 screw dislocations. As a consequence of this, bcc metals show two unique deformation characteristics: (a) thermally-activated glide of screw dislocations--the motion of screw components with their non-planar core structure at the atomistic level occurs even at low stress through the nucleation (assisted by thermal activation) and lateral propagation of dislocation kink pairs; (b) break-down of the Schmid Law, where dislocation slip is driven only by the resolved shear stress. Since the split dislocation core has to constrict for a kink pair formation (and propagation), the non-planarity of bcc screw dislocation cores entails an influence of (shear) stress components acting on planes other than the primary glide plane on their mobility. Another consequence of the asymmetric core splitting on the glide plane is a direction-sensitive slip resistance, which is termed twinning/atwinning sense of shear and should be taken into account when developing constitutive models. Modeling thermally-activated flow including the above-mentioned non-Schmid effects in bcc metals has been the subject of much work, starting in the 1980s and gaining increased interest in recent times. The majority of these works focus on single crystal deformation of commonly used metals such as Iron (Fe), Molybdenum (Mo), and Tungsten (W), while very few published studies address deformation behavior in Niobium (Nb). Most of the work on Nb revolves around fitting parameters of phenomenological descriptions, which do not capture adequately the macroscopic multi-stage hardening behavior and evolution of crystallographic texture from a physical point of view. Therefore, we aim to develop a physics-based crystal plasticity model that can capture these effects as a function of grain orientations, microstructure parameters, and temperature. To achieve this goal, first, a new dilatational constitutive model is developed for simulating the deformation of non-compact geometries (foams or geometries with free surfaces) using the spectral method. The model has been used to mimic the void-growth behavior of a biaxially loaded plate with a circular inclusion. The results show that the proposed formulation provides a much better description of void-like behavior compared to the pure elastic behavior of voids. Using the developed dilatational framework, periodic boundary conditions arising from the spectral solver has been relaxed to study the tensile deformation behavior of dogbone-shaped Nb single crystals. Second, a dislocation density-based constitutive model with storage and recovery laws derived from Discrete Dislocation Dynamics (DDD) is implemented to model multi-stage strain hardening. The influence of pre-deformed dislocation content, dislocation interaction strengths and mean free path on stage II hardening is then simulated and compared with in-situ tensile experiments.

Microstructural comparison of the kinematics of discrete and continuum dislocations models

NASA Astrophysics Data System (ADS)

Sandfeld, Stefan; Po, Giacomo

2015-12-01

The Continuum Dislocation Dynamics (CDD) theory and the Discrete Dislocation Dynamics (DDD) method are compared based on concise mathematical formulations of the coarse graining of discrete data. A numerical tool for converting from a discrete to a continuum representation of a given dislocation configuration is developed, which allows to directly compare both simulation approaches based on continuum quantities (e.g. scalar density, geometrically necessary densities, mean curvature). Investigating the evolution of selected dislocation configurations within analytically given velocity fields for both DDD and CDD reveals that CDD contains a surprising number of important microstructural details.

Slip, twinning, and fracture at a grain boundary in the L1/sub 2/ ordered structure: A. sigma. = 9 tilt boundary

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yoo, M.H.; King, A.H.

The role of interaction between slip dislocations and a ..sigma.. = 9 tilt boundary in localized microplastic deformation, cleavage, or intergranular fracture in the L1/sub 2/ ordered structure has been analyzed by using the anisotropic elasticity theory of dislocations and fracture. Screw superpartials cross slip easily at the boundary onto the (11-bar1) and the (001) planes at low and high temperatures, respectively. Transmission of primary slip dislocations onto the conjugate slip system occurs with a certain degree of difficulty, which is eased by localized disordering. When the transmission is impeded, cleavage fracture on the (1-bar11) plane is predicted to occur,more » not intergranular fracture, unless a symmetric double pileup occurs simultaneously. Absorption (or emission) of superpartials occurs only when the boundary region is disordered. Slip initiation from pre-existing sources near the boundary can occur under the local stress concentration. Implications of the present result on the inherent brittleness of grain boundaries in Ni/sub 3/ Al and its improvement by boron segregation are discussed.« less

Plasticity mechanisms in HfN at elevated and room temperature.

PubMed

Vinson, Katherine; Yu, Xiao-Xiang; De Leon, Nicholas; Weinberger, Christopher R; Thompson, Gregory B

2016-10-06

HfN specimens deformed via four-point bend tests at room temperature and at 2300 °C (~0.7 T m ) showed increased plasticity response with temperature. Dynamic diffraction via transmission electron microscopy (TEM) revealed ⟨110⟩{111} as the primary slip system in both temperature regimes and ⟨110⟩{110} to be a secondary slip system activated at elevated temperature. Dislocation line lengths changed from a primarily linear to a curved morphology with increasing temperature suggestive of increased dislocation mobility being responsible for the brittle to ductile temperature transition. First principle generalized stacking fault energy calculations revealed an intrinsic stacking fault (ISF) along ⟨112⟩{111}, which is the partial dislocation direction for slip on these close packed planes. Though B1 structures, such as NaCl and HfC predominately slip on ⟨110⟩{110}, the ISF here is believed to facilitate slip on the {111} planes for this B1 HfN phase.

The impact of water on dislocation content and slip system activity in olivine constrained by HR-EBSD and visco-plastic self-consistent simulations

NASA Astrophysics Data System (ADS)

Wallis, D.; Hansen, L. N.; Tasaka, M.; Kumamoto, K. M.; Lloyd, G. E.; Parsons, A. J.; Kohlstedt, D. L.; Wilkinson, A. J.

2016-12-01

Changes in concentration of H+ ions in olivine have impacts on its rheological behaviour and therefore on tectonic processes involving mantle deformation. Deformation experiments on aggregates of wet olivine exhibit different evolution of crystal preferred orientations (CPO) and substructure from experiments on dry olivine, suggesting that elevated H+ concentrations impact activity of dislocation slip-systems. We use high angular-resolution electron backscatter diffraction (HR-EBSD) to map densities of different types of geometrically necessary dislocations (GND) in polycrystalline olivine deformed experimentally under wet and dry conditions and also in nature. HR-EBSD provides unprecedented angular resolution, resolving misorientations < 0.01°. We also employ visco-plastic self-consistent (VPSC) simulations to investigate changes in slip-system activity. HR-EBSD maps from experimental samples demonstrate that olivine deformed under hydrous conditions contains higher proportions of (001)[100] and (100)[001] edge dislocations than olivine deformed under anhydrous conditions. Furthermore, maps of wet olivine exhibit more polygonal subgrain boundaries indicative of enhanced recovery by dislocation climb. VPSC simulations with low critical resolved shear stresses for the (001)[100] and (100)[001] slip systems reproduce an unusual CPO with bimodal maxima of both [100] and [001] observed in wet olivine aggregates. Analysis of a mylonitic lherzolite xenolith from Lesotho reveals the same unusual CPO and similar proportions of dislocation types to `wet' experimental samples, supporting the applicability of these findings to natural deformation conditions. These results support suggestions that H+ impacts the flow properties of olivine by altering dislocation activity and climb, while also providing full quantification of GND content. In particular, the relative proportions of dislocation types may provide a basis for identifying olivine deformed under wet and dry conditions.

Lattice Rotation Patterns and Strain Gradient Effects in Face-Centered-Cubic Single Crystals Under Spherical Indentation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gao, Y. F.; Larson, B. C.; Lee, J. H.

Strain gradient effects are commonly modeled as the origin of the size dependence of material strength, such as the dependence of indentation hardness on contact depth and spherical indenter radius. However, studies on the microstructural comparisons of experiments and theories are limited. First, we have extended a strain gradient Mises-plasticity model to its crystal plasticity version and implemented a finite element method to simulate the load-displacement response and the lattice rotation field of Cu single crystals under spherical indentation. The strain gradient simulations demonstrate that the forming of distinct sectors of positive and negative angles in the lattice rotation fieldmore » is governed primarily by the slip geometry and crystallographic orientations, depending only weakly on strain gradient effects, although hardness depends strongly on strain gradients. Second, the lattice rotation simulations are compared quantitatively with micron resolution, three-dimensional X-ray microscopy (3DXM) measurements of the lattice rotation fields under 100mN force, 100 mu m radius spherical indentations in < 111 >, < 110 >, and < 001 > oriented Cu single crystals. Third, noting the limitation of continuum strain gradient crystal plasticity models, two-dimensional discrete dislocation simulation results suggest that the hardness in the nanocontact regime is governed synergistically by a combination of strain gradients and source-limited plasticity. However, the lattice rotation field in the discrete dislocation simulations is found to be insensitive to these two factors but to depend critically on dislocation obstacle densities and strengths.« less

A dislocation density-based continuum model of the anisotropic shock response of single crystal α-cyclotrimethylene trinitramine

DOE PAGES

Luscher, Darby Jon; Addessio, Francis L.; Cawkwell, Marc Jon; ...

2017-01-01

Here, we have developed a model for the finite deformation thermomechanical response of α-cyclotrimethylene trinitramine (RDX). Our model accounts for nonlinear thermoelastic lattice deformation through a free energy-based equation of state developed by Cawkwell et al. (2016) in combination with temperature and pressure dependent elastic constants, as well as dislocation-mediated plastic slip on a set of slip systems motivated by experimental observation. The kinetics of crystal plasticity are modeled using the Orowan equation relating slip rate to dislocation density and the dislocation velocity developed by Austin and McDowell (2011), which naturally accounts for transition from thermally activated to dislocation dragmore » limited regimes. Evolution of dislocation density is specified in terms of local ordinary differential equations reflecting dislocation–dislocation interactions. This paper presents details of the theory and parameterization of the model, followed by discussion of simulations of flyer plate impact experiments. Impact conditions explored within this combined simulation and experimental effort span shock pressures ranging from 1 to 3 GPa for four crystallographic orientations and multiple specimen thicknesses. Simulation results generated using this model are shown to be in strong agreement with velocimetry measurements from the corresponding plate impact experiments. Finally, simulation results are used to motivate conclusions about the nature of dislocation-mediated plasticity in RDX.« less

RETRACTED ARTICLE: Microstructure and strengthening mechanism of Ni3Al intermetallic compound

NASA Astrophysics Data System (ADS)

Oh, Chang-Sup; Han, Chang-Suk

2013-09-01

Structural studies have been performed on precipitation hardening found in Ni3Al-base ordered alloys using transmission electron microscopy. The γ' phase hardens appreciably by the fine precipitation of disordered γ. The strength of γ' increases over the temperature range of experiment by the precipitation of fine γ particles. The peak temperature where a maximum strength was obtained shifted to higher temperature. Superlattice dislocations dissociate into fourfold Shockley partial dislocations in a uniform supersaturated solid solution of the γ' phase. Dislocations are attracted into the disordered γ phase and dissociate further in the particles. At any stage of aging, dislocations cut through the particles and the Orowan bypassing process does not occur even in the overaged stage of this alloy system. When the applied stress is removed, the dislocations make cross slip into (010) plane, while those in γ precipitates remain on the (111) primary slip plane. The increase of high temperature strength in γ' containing γ precipitates is due to the restraint of cross slip of dislocations from (111) to (010) by the dispersion of disordered γ particles. The orientation dependence of strength is decreased by the fine precipitation of a disordered γ phase.

Computational modelling of mesoscale dislocation patterning and plastic deformation of single crystals

NASA Astrophysics Data System (ADS)

Xia, Shengxu; El-Azab, Anter

2015-07-01

We present a continuum dislocation dynamics model that predicts the formation of dislocation cell structure in single crystals at low strains. The model features a set of kinetic equations of the curl type that govern the space and time evolution of the dislocation density in the crystal. These kinetic equations are coupled to stress equilibrium and deformation kinematics using the eigenstrain approach. A custom finite element method has been developed to solve the coupled system of equations of dislocation kinetics and crystal mechanics. The results show that, in general, dislocations self-organize in patterns under their mutual interactions. However, the famous dislocation cell structure has been found to form only when cross slip is implemented in the model. Cross slip is also found to lower the yield point, increase the hardening rate, and sustain an increase in the dislocation density over the hardening regime. Analysis of the cell structure evolution reveals that the average cell size decreases with the applied stress, which is consistent with the similitude principle.

Atomistically determined phase-field modeling of dislocation dissociation, stacking fault formation, dislocation slip, and reactions in fcc systems

NASA Astrophysics Data System (ADS)

Rezaei Mianroodi, Jaber; Svendsen, Bob

2015-04-01

The purpose of the current work is the development of a phase field model for dislocation dissociation, slip and stacking fault formation in single crystals amenable to determination via atomistic or ab initio methods in the spirit of computational material design. The current approach is based in particular on periodic microelasticity (Wang and Jin, 2001; Bulatov and Cai, 2006; Wang and Li, 2010) to model the strongly non-local elastic interaction of dislocation lines via their (residual) strain fields. These strain fields depend in turn on phase fields which are used to parameterize the energy stored in dislocation lines and stacking faults. This energy storage is modeled here with the help of the "interface" energy concept and model of Cahn and Hilliard (1958) (see also Allen and Cahn, 1979; Wang and Li, 2010). In particular, the "homogeneous" part of this energy is related to the "rigid" (i.e., purely translational) part of the displacement of atoms across the slip plane, while the "gradient" part accounts for energy storage in those regions near the slip plane where atomic displacements deviate from being rigid, e.g., in the dislocation core. Via the attendant global energy scaling, the interface energy model facilitates an atomistic determination of the entire phase field energy as an optimal approximation of the (exact) atomistic energy; no adjustable parameters remain. For simplicity, an interatomic potential and molecular statics are employed for this purpose here; alternatively, ab initio (i.e., DFT-based) methods can be used. To illustrate the current approach, it is applied to determine the phase field free energy for fcc aluminum and copper. The identified models are then applied to modeling of dislocation dissociation, stacking fault formation, glide and dislocation reactions in these materials. As well, the tensile loading of a dislocation loop is considered. In the process, the current thermodynamic picture is compared with the classical mechanical one as based on the Peach-Köhler force.

Complex earthquake rupture and local tsunamis

USGS Publications Warehouse

Geist, E.L.

2002-01-01

In contrast to far-field tsunami amplitudes that are fairly well predicted by the seismic moment of subduction zone earthquakes, there exists significant variation in the scaling of local tsunami amplitude with respect to seismic moment. From a global catalog of tsunami runup observations this variability is greatest for the most frequently occuring tsunamigenic subduction zone earthquakes in the magnitude range of 7 < Mw < 8.5. Variability in local tsunami runup scaling can be ascribed to tsunami source parameters that are independent of seismic moment: variations in the water depth in the source region, the combination of higher slip and lower shear modulus at shallow depth, and rupture complexity in the form of heterogeneous slip distribution patterns. The focus of this study is on the effect that rupture complexity has on the local tsunami wave field. A wide range of slip distribution patterns are generated using a stochastic, self-affine source model that is consistent with the falloff of far-field seismic displacement spectra at high frequencies. The synthetic slip distributions generated by the stochastic source model are discretized and the vertical displacement fields from point source elastic dislocation expressions are superimposed to compute the coseismic vertical displacement field. For shallow subduction zone earthquakes it is demonstrated that self-affine irregularities of the slip distribution result in significant variations in local tsunami amplitude. The effects of rupture complexity are less pronounced for earthquakes at greater depth or along faults with steep dip angles. For a test region along the Pacific coast of central Mexico, peak nearshore tsunami amplitude is calculated for a large number (N = 100) of synthetic slip distribution patterns, all with identical seismic moment (Mw = 8.1). Analysis of the results indicates that for earthquakes of a fixed location, geometry, and seismic moment, peak nearshore tsunami amplitude can vary by a factor of 3 or more. These results indicate that there is substantially more variation in the local tsunami wave field derived from the inherent complexity subduction zone earthquakes than predicted by a simple elastic dislocation model. Probabilistic methods that take into account variability in earthquake rupture processes are likely to yield more accurate assessments of tsunami hazards.

Open dislocation of the proximal interphalangeal joint of the little finger subsequent to chronic radial collateral ligament injury: a case report of primary ligament reconstruction with a half-slip of the flexor digitorum superficialis: Case Report.

PubMed

Wada, Kazuma; Hibino, Naohito; Kondo, Kenji; Yoshioka, Shinji; Terai, Tomoya; Henmi, Tatsuhiko; Sairyo, Koichi

2015-01-01

Open dislocation of the proximal interphalangeal (PIP) joint is relatively rare. We report a case of a 32-year-old man who had open dislocation of the PIP joint of the little finger while playing American football. He had a history of chronic radial collateral ligament injury. We reconstructed the radial collateral ligament with a half-slip of the flexor digitorum superficialis tendon.

Influence of strain rate on the structure/property behavior of the alpha-2 alloy Ti-24.5Al-10.5Nb-1.5Mo

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gray, G.T. III; Hong, Sun Ig; Marquardt, B.J.

Preliminary dislocation g{center_dot}b analysis revealed that following room temperature deformation at low strain rate the majority of the dislocations are a-dislocations lying on basal planes, 2nd order pyramidal (a/2 + c) slip on [1211], and 1st order pyramidal a-slip on [1011]. Increasing the rate of deformation at room temperature to 6000 s{sup {minus}1} is seen to result in increased a-slip on prism planes and a decreased amount of basal slip. At high-strain-rates and elevated temperatures the substructure was seen to be generally similar to that observed following high-rate deformation at room temperature except for an increased amount of basal slipmore » and a somewhat higher incidence of 2nd order pyramidal slip. The defect generation and the rate sensitivity of Ti-24.5Al-10.5Nb-1.5Mo are discussed as a function of strain rate and temperature and contrasted to that observed in conventional titanium alloys and TiAl.« less

Insights on activation enthalpy for non-Schmid slip in body-centered cubic metals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hale, Lucas M.; Lim, Hojun; Zimmerman, Jonathan A.

2014-12-18

We use insights gained from atomistic simulation to develop an activation enthalpy model for dislocation slip in body-centered cubic iron. Furthermore, using a classical potential that predicts dislocation core stabilities consistent with ab initio predictions, we quantify the non-Schmid stress-dependent effects of slip. The kink-pair activation enthalpy is evaluated and a model is identified as a function of the general stress state. Thus, our model enlarges the applicability of the classic Kocks activation enthalpy model to materials with non-Schmid behavior.

Infrared absorption band in deformed qtz crystals analyzed by combining different microstructural methods

NASA Astrophysics Data System (ADS)

Stunitz, Holger; Thust, Anja; Behrens, Harald; Heilbronner, Renee; Kilian, Ruediger

2016-04-01

Natural single crystals of quartz have been experimentally deformed in two orientations: (1) normal to one prism-plane, (2) In O+ orientation at temperatures of 900 and 1000°C, pressures of 1.0 and 1.5 GPa, and strain rates of ~1 x 10-6s-1. The starting material is milky quartz, consisting of dry quartz (H2O contents of <150 H/106Si) with fluid inclusions (FI). During pressurization many FÍs decrepitate. Cracks heal and small neonate FÍs form, increasing the number of FÍs drastically. During subsequent deformation, the size of FÍs is further reduced (down to ~10 nm). Sample deformation occurs by dominant dislocation glide on selected slip systems, accompanied by some dynamic recovery. Strongly deformed regions show FTIR spectra with a pointed broad absorption band in the ~3400 cm-1 region as a superposition of molecular H2O bands and three discrete absorption bands (at 3367, 3400, and 3434 cm-1). In addition, there is a discrete absorption band at 3585 cm-1, which only occurs in deformed regions. The 3585 cm-1 band is reduced or even disappears after annealing. This band is polarized and represents structurally bound H, its H-content is estimated to be 1-3% of the total H2O-content and appears to be associated with dislocations. The H2O weakening effect in our FI-bearing natural quartz crystals is assigned to the processes of dislocation generation and multiplication at small FÍs. The deformation processes in these crystals represent a recycling of H2O between FÍs, dislocation generation at very small fluid inclusions, incorporation of structurally bound H into dislocation cores, and release of H2O from dislocations back into FÍs during recovery. Cracking and crack healing play an important role in the recycling process and imply a close interrelationship between brittle and crystal plastic deformation. The H2O weakening by this process is of a disequilibrium nature and thus depends on the amount of H2O available.

Water weakening in experimentally deformed milky quartz single crystals

NASA Astrophysics Data System (ADS)

Stunitz, H.; Thust, A.; Kilian, R.; Heilbronner, R.; Behrens, H.; Tarantola, A.; Fitz Gerald, J. D.

2015-12-01

Natural single crystals of quartz have been experimentally deformed in two orientations: (1) normal to one prism-plane, (2) In O+ orientation at temperatures of 900 and 1000°C, pressures of 1.0 and 1.5 GPa, and strain rates of ~1 x 10-6s-1. The starting material is milky quartz, consisting of dry quartz (H2O contents of <150 H/106Si) with fluid inclusions (FI). During pressurization many FI´s decrepitate. Cracks heal and small neonate FI´s form, increasing the number of FI´s drastically. During subsequent deformation, the size of FI´s is further reduced (down to ~10 nm). Sample deformation occurs by dominant dislocation glide on selected slip systems, accompanied by some dynamic recovery. Strongly deformed regions show FTIR spectra with a pointed broad absorption band in the ~3400 cm-1 region as a superposition of molecular H2O bands and three discrete absorption bands (at 3367, 3400, and 3434 cm-1). In addition, there is a discrete absorption band at 3585 cm-1, which only occurs in deformed regions. The 3585 cm-1 band is reduced or even disappears after annealing. This band is polarized and represents structurally bound H, its H-content is estimated to be 1-3% of the total H2O-content and appears to be associated with dislocations. The H2O weakening effect in our FI-bearing natural quartz crystals is assigned to the processes of dislocation generation and multiplication at small FI´s. The deformation processes in these crystals represent a recycling of H2O between FI´s, dislocation generation at very small fluid inclusions, incorporation of structurally bound H into dislocation cores, and release of H2O from dislocations back into FI´s during recovery. Cracking and crack healing play an important role in the recycling process and imply a close interrelationship between brittle and crystal plastic deformation. The H2O weakening by this process is of a disequilibrium nature and thus depends on the amount of H2O available.

Plastic Properties of MgSiO3 Post-Perovskite in the Lower Mantle : Do We Care about Twinning ?

NASA Astrophysics Data System (ADS)

Carrez, P.; Goryaeva, A.; Cordier, P.

2017-12-01

Plastic properties of post-perovskite MgSiO3 are believed to be one of the key issues for the understanding of seismic anisotropy at the bottom of the D'' layer. Unfortunately, results from high pressure deformation experiments have led to several conflicting interpretations regarding slip systems and dislocation activities. Whereas, plastic slip has attracted much more attention, twinning mechanism has not been addressed despite some experimental evidence on low-pressure analogues. Based on a hierarchical mechanical model of the emission of 1/6<110> partial dislocations, we present a twin nucleation model in MgSiO3 and CaIrO3 post-perovskite. Relying on first-principles calculations, we show that {110} twin wall formation resulting from the interaction of multiple twin dislocations occurs for twinning stress comparable to the easiest slip system in post-perovskite. Dislocations activities and twinning being competitive strain producing mechanism, twinning has to be considered in future interpretation of crystallographic preferred orientations in post-perovskite.

Sessile dislocations by reactions in NiAl severely deformed at room temperature

DOE PAGES

Geist, D.; Gammer, C.; Rentenberger, C.; ...

2015-02-05

B2 ordered NiAl is known for its poor room temperature (RT) ductility; failure occurs in a brittle like manner even in ductile single crystals deforming by single slip. In the present study NiAl was severely deformed at RT using the method of high pressure torsion (HPT) enabling the hitherto impossible investigation of multiple slip deformation. Methods of transmission electron microscopy were used to analyze the dislocations formed by the plastic deformation showing that as expected dislocations with Burgers vector a(100) carry the plasticity during HPT deformation at RT. In addition, we observe that they often form a(110) dislocations by dislocationmore » reactions; the a(110) dislocations are considered to be sessile based on calculations found in the literature. It is therefore concluded that the frequently encountered 3D dislocation networks containing sessile a(110) dislocations are pinned and lead to deformation-induced embrittlement. In spite of the severe deformation, the chemical order remains unchanged.« less

In Situ TEM Study of Interaction between Dislocations and a Single Nanotwin under Nanoindentation.

PubMed

Wang, Bo; Zhang, Zhenyu; Cui, Junfeng; Jiang, Nan; Lyu, Jilei; Chen, Guoxin; Wang, Jia; Liu, Zhiduo; Yu, Jinhong; Lin, Chengte; Ye, Fei; Guo, Dongming

2017-09-06

Nanotwinned (nt) materials exhibit excellent mechanical properties, and have been attracting much more attention of late. Nevertheless, the fundamental mechanism of interaction between dislocations and a single nanotwin is not understood. In this study, in situ transmission electron microscopy (TEM) nanoindentation is performed, on a specimen of a nickel (Ni) alloy containing a single nanotwin of 89 nm in thickness. The specimen is prepared using focused ion beam (FIB) technique from an nt surface, which is formed by a novel approach under indentation using a developed diamond panel with tips array. The stiffness of the specimen is ten times that of the pristine counterparts during loading. The ultrahigh stiffness is attributed to the generation of nanotwins and the impediment of the single twin to the dislocations. Two peak loads are induced by the activation of a new slip system and the penetration of dislocations over the single nanotwin, respectively. One slip band is parallel to the single nanotwin, indicating the slip of dislocations along the nanotwin. In situ TEM observation of nanoindentation reveals a new insight for the interaction between dislocations and a single nanotwin. This paves the way for design and preparation of high-performance nt surfaces of Ni alloys used for aircraft engines, gas turbines, turbocharger components, ducts, and absorbers.

Distribution of aseismic slip rate on the Hayward fault inferred from seismic and geodetic data

USGS Publications Warehouse

Schmidt, D.A.; Burgmann, R.; Nadeau, R.M.; d'Alessio, M.

2005-01-01

We solve for the slip rate distribution on the Hayward fault by performing a least squares inversion,of geodetic and seismic data sets. Our analysis focuses on the northern 60 km of the fault. Interferometric synthetic aperture radar (InSAR) data from 13 independent ERS interferograms are stacked to obtain range change rates from 1992 to 2000. Horizontal surface displacement rates at 141 bench marks are measured using GPS from 1994 to 2003. Surface creep observations and estimates of deep slip rates determined from characteristic repeating earthquake sequences are also incorporated in the inversion. The fault is discretized into 283 triangular dislocation elements that approximate the nonplanar attributes of the fault surface. South of the city of Hayward, a steeply, east dipping fault geometry accommodates the divergence of the surface trace and the microseismicity at depth. The inferred slip rate distribution is consistent with a fault that creeps aseismically at a rate of ???5 mm/yr to a depth of 4-6 km. The interferometric synthetic aperture radar (InSAR) data require an aseismic slip rate that approaches the geologic slip rate on the northernmost fault segment beneath Point Pinole, although the InSAR data might be complicated by a small dip-slip component at this location. A low slip rate patch of <1 mm/yr is inferred beneath San Leandro consistent with the source location of the 1868 earthquake. We calculate that the entire fault is accumulating a slip rate deficit equivalent to a Mw = 6.77 ?? 0.05 per century. However, this estimate of potential coseismic moment represents an upper bound because we do not know how much of the accumulated strain will be released through aseismic processes such as afterslip. Copyright 2005 by the American Geophysical Union.

Anisotropic frictional heating and defect generation in cyclotrimethylene-trinitramine molecular crystals

NASA Astrophysics Data System (ADS)

Rajak, Pankaj; Mishra, Ankit; Sheng, Chunyang; Tiwari, Subodh; Kalia, Rajiv K.; Nakano, Aiichiro; Vashishta, Priya

2018-05-01

Anisotropic frictional response and corresponding heating in cyclotrimethylene-trinitramine molecular crystals are studied using molecular dynamics simulations. The nature of damage and temperature rise due to frictional forces is monitored along different sliding directions on the primary slip plane, (010), and on non-slip planes, (100) and (001). Correlations between the friction coefficient, deformation, and frictional heating are established. We find that the friction coefficients on slip planes are smaller than those on non-slip planes. In response to sliding on a slip plane, the crystal deforms easily via dislocation generation and shows less heating. On non-slip planes, due to the inability of the crystal to deform via dislocation generation, a large damage zone is formed just below the contact area, accompanied by the change in the molecular ring conformation from chair to boat/half-boat. This in turn leads to a large temperature rise below the contact area.

Three-dimensional formulation of dislocation climb

NASA Astrophysics Data System (ADS)

Gu, Yejun; Xiang, Yang; Quek, Siu Sin; Srolovitz, David J.

2015-10-01

We derive a Green's function formulation for the climb of curved dislocations and multiple dislocations in three-dimensions. In this new dislocation climb formulation, the dislocation climb velocity is determined from the Peach-Koehler force on dislocations through vacancy diffusion in a non-local manner. The long-range contribution to the dislocation climb velocity is associated with vacancy diffusion rather than from the climb component of the well-known, long-range elastic effects captured in the Peach-Koehler force. Both long-range effects are important in determining the climb velocity of dislocations. Analytical and numerical examples show that the widely used local climb formula, based on straight infinite dislocations, is not generally applicable, except for a small set of special cases. We also present a numerical discretization method of this Green's function formulation appropriate for implementation in discrete dislocation dynamics (DDD) simulations. In DDD implementations, the long-range Peach-Koehler force is calculated as is commonly done, then a linear system is solved for the climb velocity using these forces. This is also done within the same order of computational cost as existing discrete dislocation dynamics methods.

EBSD in Antarctic and Greenland Ice

NASA Astrophysics Data System (ADS)

Weikusat, Ilka; Kuiper, Ernst-Jan; Pennock, Gill; Sepp, Kipfstuhl; Drury, Martyn

2017-04-01

Ice, particularly the extensive amounts found in the polar ice sheets, impacts directly on the global climate by changing the albedo and indirectly by supplying an enormous water reservoir that affects sea level change. The discharge of material into the oceans is partly controlled by the melt excess over snow accumulation, partly by the dynamic flow of ice. In addition to sliding over bedrock, an ice body deforms gravitationally under its own weight. In order to improve our description of this flow, ice microstructure studies are needed that elucidate the dominant deformation and recrystallization mechanisms involved. Deformation of hexagonal ice is highly anisotropic: ice is easily sheared in the basal plane and is about two orders of magnitude harder parallel to the c-axis. As dislocation creep is the dominant deformation mechanism in polar ice this strong anisotropy needs to be understood in terms of dislocation activity. The high anisotropy of the ice crystal is usually ascribed to a particular behaviour of dislocations in ice, namely the extension of dislocations into partials on the basal plane. Analysis of EBSD data can help our understanding of dislocation activity by characterizing subgrain boundary types thus providing a tool for comprehensive dislocation characterization in polar ice. Cryo-EBSD microstructure in combination with light microscopy measurements from ice core material from Antarctica (EPICA-DML deep ice core) and Greenland (NEEM deep ice core) are presented and interpreted regarding substructure identification and characterization. We examined one depth for each ice core (EDML: 656 m, NEEM: 719 m) to obtain the first comparison of slip system activity from the two ice sheets. The subgrain boundary to grain boundary threshold misorientation was taken to be 3-5° (Weikusat et al. 2011). EBSD analyses suggest that a large portion of edge dislocations with slip systems basal gliding on the basal plane were indeed involved in forming subgrain boundaries. However, an almost equal number of tilt subgrain boundaries were measured, involving dislocations gliding on non-basal planes (prism or prism slip). A few subgrain boundaries involving prism edge dislocation glide, as well as boundaries involving basal twist dislocation slip, were also identified. The finding that subgrain boundaries built up by dislocations gliding on non-basal planes are as frequent as those originating from basal plane slip is surprising and has impact on the discussion on rate-controlling processes for the ice flow descriptions of large ice masses with respect to sea-level evolution. Weikusat, I.; Miyamoto, A.; Faria, S. H.; Kipfstuhl, S.; Azuma, N. & Hondoh, T.: Subgrain boundaries in Antarctic ice quantified by X-ray Laue diffraction J. Glaciol., 2011, 57, 85-94

Dislocation Processes and Frictional Stability of Faults

NASA Astrophysics Data System (ADS)

Toy, V. G.; Mitchell, T. M.; Druiventak, A.

2011-12-01

The rate dependence of frictional processes in faults in quartzofeldspathic crust is proposed to change at c. 300°C, because above this temperature asperity deformation can be accommodated by crystal plastic processes. As a consequence, the real fault contact area increases and the fault velocity strengthens. Conversely, faults at lower temperatures are velocity weakening and therefore prone to earthquake slip. We have investigated whether dislocation processes are important around faults in quartzites on seismic timescales, by inducing fault slip on a saw cut surface in novaculite blocks. Deformation was carried out at 450°C and 600°C in a Griggs apparatus. Slip rates of 8.3 x 10-7s-1 allowed total slip, u, of 0.5mm to be achieved in c. 10 minutes. Failure occurred at peak differential stresses of ~1.7 GPa and 1.4 GPa respectively, followed by significant weakening. Structures of the novaculite within and surrounding the fault surface were examined using EBSD, FIB-SEM and TEM to elucidate changes to their dislocation substructure. In the sample deformed at 450°C, a ~50μm thick layer of amorphous / non-crystalline silica was developed on the saw-cut surface during deformation. Rare clasts of the wall rock are preserved within this material. The surrounding sample is mostly composed of equant quartz grains of 5-10μm diameter that lack a preferred orientation, contain very few intercrystalline dislocations, and are divided by organised high angle grain boundaries. After deformation, most quartz grains within the sample retain their starting microstructure. However, within ~10μm of the sliding surface, dislocations are more common, and these are arranged into elongated, tangled zones (subgrain boundaries?). Microfractures are also observed. These microstructures are characteristic of deformation accommodated by low temperature plasticity. Our preliminary observations suggest that dislocation processes may be able to accommodate some deformation around fault surfaces, at least at the slightly sub-seismic deformation rates of these experiments. Furthermore, once sliding initiated on the saw cut surface, an amorphous material was generated. We hypothesise that this could have been due to a breakdown of the crystal structure by a combination of cataclasis and generation of excessive dislocation densities. There would also have been a slight increase in temperature around the sliding surface during and after fault slip, which may have aided the focussing of dislocation processes around the sliding surface.

A molecular dynamics study of tilt grain boundary resistance to slip and heat transfer in nanocrystalline silicon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Xiang; Chen, Youping; Xiong, Liming

2014-12-28

We present a molecular dynamics study of grain boundary (GB) resistance to dislocation-mediated slip transfer and phonon-mediated heat transfer in nanocrystalline silicon bicrystal. Three most stable 〈110〉 tilt GBs in silicon are investigated. Under mechanical loading, the nucleation and growth of hexagonal-shaped shuffle dislocation loops are reproduced. The resistances of different GBs to slip transfer are quantified through their constitutive responses. Results show that the Σ3 coherent twin boundary (CTB) in silicon exhibits significantly higher resistance to dislocation motion than the Σ9 GB in glide symmetry and the Σ19 GB in mirror symmetry. The distinct GB strengths are explained bymore » the atomistic details of the dislocation-GB interaction. Under thermal loading, based on a thermostat-induced heat pulse model, the resistances of the GBs to transient heat conduction in ballistic-diffusive regime are characterized. In contrast to the trend found in the dislocation-GB interaction in bicrystal models with different GBs, the resistances of the same three GBs to heat transfer are strikingly different. The strongest dislocation barrier Σ3 CTB is almost transparent to heat conduction, while the dislocation-permeable Σ9 and Σ19 GBs exhibit larger resistance to heat transfer. In addition, simulation results suggest that the GB thermal resistance not only depends on the GB energy but also on the detailed atomic structure along the GBs.« less

The effect of interstitial carbon on the mechanical properties and dislocation substructure evolution in Fe 40.4Ni 11.3Mn 34.8Al 7.5Cr 6 high entropy alloys

DOE PAGES

Wang, Zhangwei; Baker, Ian; Cai, Zhonghou; ...

2016-09-01

A systematic study of the effects of up to 1.1 at. % carbon on the mechanical properties and evolution of the dislocation substructure in a series of a high entropy alloys (HEA) based on Fe 40.4Ni 11.3Mn 34.8Al 7.5Cr 6 is presented. Transmission electron microscopy (TEM), synchrotron X-ray diffraction (XRD) and atom probe tomography (APT) were used to show that all the alloys are single-phase f.c.c. random solid solutions. The lattice constant, determined from synchrotron XRD measurements, increases linearly with increasing carbon concentration, which leads to a linear relationship between the yield strength and the carbon concentration. The dislocation substructures,more » as determined by a TEM, show a transition from wavy slip to planar slip and, at higher strains, and from cell-forming structure (dislocations cells, cell blocks and dense dislocation walls) to non-cell forming structure (Taylor lattice, microbands and domain boundaries) with the addition of carbon, features related to the increase in lattice friction stress. The stacking fault energy (measured via weak-beam imaging of the separation of dislocation partials) decreases with increasing carbon content, which also contributes to the transition from wavy slip to planar slip. The formation of non-cell forming structure induced by carbon leads to a high degree of strain hardening and a substantial increase in the ultimate tensile strength. In conclusion, the consequent postponement of necking due to the high strain hardening, along with the plasticity accommodation arising from the formation of microbands and domain boundaries, result in an increase of ductility due to the carbon addition.« less

Non-Invasive Optical Characterization of Defects in Gallium Arsenide.

NASA Astrophysics Data System (ADS)

Cao, Xuezhong

This work is concerned with the development of a non-invasive comprehensive defect analysis system based on computer-assisted near infrared (NIR) microscopy. Focus was placed on the development of software for quantitative image analysis, contrast enhancement, automated defects density counting, and two-dimensional defect density mapping. Bright field, dark field, phase contrast, and polarized light imaging modes were explored for the analysis of striations, precipitates, decorated and undecorated dislocations, surface and subsurface damage, and local residual strain in GaAs wafers. The origin of the contrast associated with defect image formation in NIR microscopy was analyzed. The local change in the index of refraction about a defect was modelled as a mini-lens. This model can explain reversal of image contrast for dislocations in heavily doped n-type GaAs during defocusing. Defect structures in GaAs crystals grown by the conventional liquid encapsulated Czochralski (LEC) method are found to differ significantly from those grown by the horizontal Bridgman (HB) or vertical gradient freeze (VGF) method. Dislocation densities in HB and VGF GaAs are one to two orders of magnitude lower compared to those in conventional LEC GaAs. The dislocations in HB and VGF GaAs remain predominantly on the {111}/<1 |10> primary slip system and tend to form small-angle subboundaries. Much more complicated dislocation structures are found in conventional LEC GaAs. Dislocation loops, dipoles, and helices were observed, indicating strong interaction between dislocations and point defects in these materials. Precipitates were observed in bulk GaAs grown by the LEC, HB, and VGF methods. Precipitation was found to occur predominantly along dislocation lines, however, discrete particles were also observed in dislocation-free regions of the GaAs matrix. The size of discrete precipitates is much smaller than that of the precipitates along dislocations. Quenching after high temperature annealing at 1150^ circC was found effective in dissolving the precipitates but glide dislocations are generated during the quenching process. STEM/EDX analysis showed that the precipitates are essentially pure arsenic in both undoped and doped GaAs. NIR phase contrast transmission microscopy was found to be very sensitive in detecting surface and subsurface damage on commercial GaAs wafers. Wafers from a number of GaAs manufacturers were examined. It was shown that some GaAs wafers exhibit perfect surface quality, but in many instances they exhibit, to various extents, subsurface damage. Computer-assisted NIR transmission microscopy in a variety of modes is found to be a rapid and non-invasive technique suitable for wafer characterization in a fabline environment. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.) (Abstract shortened by UMI.).

Irradiation defect dispersions and effective dislocation mobility in strained ferritic grains: A statistical analysis based on 3D dislocation dynamics simulations

NASA Astrophysics Data System (ADS)

Li, Y.; Robertson, C.

2018-06-01

The influence of irradiation defect dispersions on plastic strain spreading is investigated by means of three-dimensional dislocation dynamics (DD) simulations, accounting for thermally activated slip and cross-slip mechanisms in Fe-2.5%Cr grains. The defect-induced evolutions of the effective screw dislocation mobility are evaluated by means of statistical comparisons, for various defect number density and defect size cases. Each comparison is systematically associated with a quantitative Defect-Induced Apparent Straining Temperature shift (or «ΔDIAT»), calculated without any adjustable parameters. In the investigated cases, the ΔDIAT level associated with a given defect dispersion closely replicates the measured ductile to brittle transition temperature shift (ΔDBTT) due to the same, actual defect dispersion. The results are further analyzed in terms of dislocation-based plasticity mechanisms and their possible relations with the dose-dependent changes of the ductile to brittle transition temperature.

Atomically informed nonlocal semi-discrete variational Peierls-Nabarro model for planar core dislocations

PubMed Central

Liu, Guisen; Cheng, Xi; Wang, Jian; Chen, Kaiguo; Shen, Yao

2017-01-01

Prediction of Peierls stress associated with dislocation glide is of fundamental concern in understanding and designing the plasticity and mechanical properties of crystalline materials. Here, we develop a nonlocal semi-discrete variational Peierls-Nabarro (SVPN) model by incorporating the nonlocal atomic interactions into the semi-discrete variational Peierls framework. The nonlocal kernel is simplified by limiting the nonlocal atomic interaction in the nearest neighbor region, and the nonlocal coefficient is directly computed from the dislocation core structure. Our model is capable of accurately predicting the displacement profile, and the Peierls stress, of planar-extended core dislocations in face-centered cubic structures. Our model could be extended to study more complicated planar-extended core dislocations, such as <110> {111} dislocations in Al-based and Ti-based intermetallic compounds. PMID:28252102

Plastic deformation of silicon dendritic web ribbons during the growth

NASA Technical Reports Server (NTRS)

Cheng, L. J.; Dumas, K. A.; Su, B. M.; Leipold, M. H.

1984-01-01

The distribution of slip dislocations in silicon dendritic web ribbons due to plastic deformation during the cooling phase of the growth was studied. The results show the existence of two distinguishable stress regions across the ribbon formed during the plastic deformation stage, namely, shear stress at the ribbon edges and tensile stress at the middle. In addition, slip dislocations caused by shear stress near the edges appear to originate at the twin plane.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Tianyi; Tan, Lizhen; Lu, Zizhe

Instrumented nanoindentation was used in this paper to investigate the hardness, elastic modulus, and creep behavior of an austenitic Fe-20Cr-25Ni model alloy at room temperature, with the indented grain orientation being the variant. The samples indented close to the {111} surfaces exhibited the highest hardness and modulus. However, nanoindentation creep tests showed the greatest tendency for creep in the {111} indented samples, compared with the samples indented close to the {001} and {101} surfaces. Scanning electron microscopy and cross-sectional transmission electron microscopy revealed slip bands and dislocations in all samples. The slip band patterns on the indented surfaces were influencedmore » by the grain orientations. Deformation twinning was observed only under the {001} indented surfaces. Finally, microstructural analysis and molecular dynamics modeling correlated the anisotropic nanoindentation-creep behavior with the different dislocation substructures formed during indentation, which resulted from the dislocation reactions of certain active slip systems that are determined by the indented grain orientations.« less

Mobile application MDDCS for modeling the expansion dynamics of a dislocation loop in FCC metals

NASA Astrophysics Data System (ADS)

Kirilyuk, Vasiliy; Petelin, Alexander; Eliseev, Andrey

2017-11-01

A mobile version of the software package Dynamic Dislocation of Crystallographic Slip (MDDCS) designed for modeling the expansion dynamics of dislocation loops and formation of a crystallographic slip zone in FCC-metals is examined. The paper describes the possibilities for using MDDCS, the application interface, and the database scheme. The software has a simple and intuitive interface and does not require special training. The user can set the initial parameters of the experiment, carry out computational experiments, export parameters and results of the experiment into separate text files, and display the experiment results on the device screen.

The role of strain hardening in the transition from dislocation-mediated to frictional deformation of marbles within the Karakoram Fault Zone, NW India

NASA Astrophysics Data System (ADS)

Wallis, David; Lloyd, Geoffrey E.; Hansen, Lars N.

2018-02-01

The onset of frictional failure and potentially seismogenic deformation in carbonate rocks undergoing exhumation within fault zones depends on hardening processes that reduce the efficiency of aseismic dislocation-mediated deformation as temperature decreases. However, few techniques are available for quantitative analysis of dislocation slip system activity and hardening in natural tectonites. Electron backscatter diffraction maps of crystal orientations offer one such approach via determination of Schmid factors, if the palaeostress conditions can be inferred and the critical resolved shear stresses of slip systems are constrained. We analyse calcite marbles deformed in simple shear within the Karakoram Fault Zone, NW India, to quantify changes in slip system activity as the rocks cooled during exhumation. Microstructural evidence demonstrates that between ∼300 °C and 200-250 °C the dominant deformation mechanisms transitioned from dislocation-mediated flow to twinning and frictional failure. However, Schmid factor analysis, considering critical resolved shear stresses for yield of undeformed single crystals, indicates that the fraction of grains with sufficient resolved shear stress for glide apparently increased with decreasing temperature. Misorientation analysis and previous experimental data indicate that strain-dependent work hardening is responsible for this apparent inconsistency and promoted the transition from dislocation-mediated flow to frictional, and potentially seismogenic, deformation.

Study on Dynamic Strain Aging and Low-Cycle Fatigue of Stainless Steel in Ultra-Supercritical Unit

NASA Astrophysics Data System (ADS)

Hongwei, Zhou; Yizhu, He; Jizu, Lv; Sixian, Rao

Dynamic strain aging (DSA) and low-cycle fatigue (LCF) behavior of TP347H stainless steel in ultra-supercritical unit were investigated at 550-650 °C. All the LCF tests were carried out under a fully-reversed, total axial strain control mode at the total strain amplitude from ±0.2% to ±1.0%. The effects of DSA in cyclic stress response, microstructure evolution and fatigue fracture surfaces and fatigue life were investigated in detail. The results show that DSA occurs during tensile, which is manifested as serrated flow in tensile stress-strain curves. The apparent activation energy for appearing of serrations in tensile stress-strain curves was 270 kJ/mol. Pipe diffusion of substitutional solutes such as Cr and Nb along the dislocation core, and strong interactions between segregated solutes and dislocations are considered as the mechanism of DSA. DSA partly restricts dislocation cross-slip, and dislocation cross-slip and planar-slip happen simultaneously during LCF. A lot of planar structures form, which is due to dislocation gliding on the special plane. This localized deformation structures result in many crack initiation sites. Meanwhile, DSA hardening increases cyclic stress response, accelerating crack propagation, which reduces high temperature strain fatigue life of steel.

TEM study on relationship between stacking faults and non-basal dislocations in Mg

NASA Astrophysics Data System (ADS)

Zhang, Dalong; Jiang, Lin; Schoenung, Julie M.; Mahajan, Subhash; Lavernia, Enrique J.

2015-12-01

Recent interest in the study of stacking faults and non-basal slip in Mg alloys is partly based on the argument that these phenomena positively influence mechanical behaviour. Inspection of the published literature, however, reveals that there is a lack of fundamental information on the mechanisms that govern the formation of stacking faults, especially I1-type stacking faults (I1 faults). Moreover, controversial and sometimes contradictory mechanisms have been proposed concerning the interactions between stacking faults and dislocations. Therefore, we describe a fundamental transmission electron microscope investigation on Mg 2.5 at. % Y (Mg-2.5Y) processed via hot isostatic pressing (HIP) and extrusion at 623 K. In the as-HIPed Mg-2.5Y, many and dislocations, together with some dislocations were documented, but no stacking faults were observed. In contrast, in the as-extruded Mg-2.5Y, a relatively high density of stacking faults and some non-basal dislocations were documented. Specifically, there were three different cases for the configurations of observed stacking faults. Case (I): pure I2 faults; Case (II): mixture of I1 faults and non-basal dislocations having component, together with basal dislocations; Case (III): mixture of predominant I2 faults and rare I1 faults, together with jog-like dislocation configuration. By comparing the differences in extended defect configurations, we propose three distinct stacking fault formation mechanisms for each case in the context of slip activity and point defect generation during extrusion. Furthermore, we discuss the role of stacking faults on deformation mechanisms in the context of dynamic interactions between stacking faults and non-basal slip.

Anisotropic frictional heat dissipation in cyclotrimethylene trinitramine

NASA Astrophysics Data System (ADS)

Rajak, Pankaj; Kalia, Rajiv; Nakano, Aiichiro; Vashishta, Priya

Anisotropic frictional response and corresponding heat dissipation from different crystallographic planes of RDX crystal is studied using molecular dynamics simulations. The effect of frictional force on the nature of damage and system temperature is monitored along different directions on primary slip plane, (010), of RDX and on non-slip planes, (100) and (001). The correlation between the friction coefficient, deformation and the frictional heating in these system is determined. It is observed that friction coefficients on slip planes are smaller than those of non-slip planes. In response to friction on slip plane, RDX crystal deforms via dislocation formation and shows less heating. On non-slip planes due to the inability of the system to deform by dislocation formation, large temperature rise is observed in the system just below the contact area of two surfaces. Frictional sliding on non-slip planes also lead to the formation of damage zone just below the contact area of two surfaces due to the change in RDX ring conformation from chair to boat/half-boat. This research is supported by the AFOSR Grant: FA9550-16- 1-0042.

AFM study of the plastic deformation behavior of poly-synthetically-twinned (PST) titanium aluminide crystals

NASA Astrophysics Data System (ADS)

Chen, Yali

The plastic deformation behavior of PST TiAl crystals was investigated using AFM techniques to reveal the effects of lamellar structure on the deform mechanisms of two-phase TiAl materials. PST crystals with a nominal composition of Ti52Al48 (atomic percent) were grown by the floating zone method and at various orientations deformed in compression at room temperature. Atomic Force Microscopy (AFM) was employed to investigate the deformation structure on the free surfaces. The deformation of the PST crystals is highly anisotropic and the deformation mechanism changes dramatically with sample orientation. When the angle between the loading axis and the lamellar interfaces is below 20°, the gamma lamellae deform by dislocation slip and twinning on planes oblique to the lamellar interfaces, but the Burgers vectors or the resultant shear vectors are parallel to the lamellar interfaces inside each lamella. When the angle is between 20° and 80° the gamma phase deforms by shear on planes parallel to the lamellar interfaces. Some domains deform by a combination of ordinary dislocation slip and twinning. In the domains where twinning cannot be activated, slip occurs by ordinary dislocations or superdislocations. When the loading axis is nearly perpendicular to the lamellar interfaces ordinary dislocation slip and twinning on slip planes inclined with the lamellar interfaces are dominant and the shear is trans-lamellar. The three deformation modes are termed as A, B and N type deformation modes respectively. In the A type mode the alpha2 lamellae concomitantly deform by prismatic slip. In the other two modes, the alpha2 phase does not deform and acts as strong obstacles to the transfer of deformation. Abundant misfit dislocations are emitted from the lamellar interfaces which is beneficial for the plastic deformation. On the other hand, the lamellar interfaces strongly impede trans-lamellar deformation and channel the deformation inside each lamella. The inhomogeneous coherency stresses at the lamellar interfaces also lead to heterogeneous deformation of PST crystals. The deformation behavior of the lamellar grains produces remarkable strain incompatibility in lamellar polycrystals and deteriorates the deformability.

Full characterization of dislocations in ion-irradiated polycrystalline UO2

NASA Astrophysics Data System (ADS)

Onofri, C.; Legros, M.; Léchelle, J.; Palancher, H.; Baumier, C.; Bachelet, C.; Sabathier, C.

2017-10-01

In order to fully characterize the dislocation loops and lines features (Burgers vectors, habit/slip planes, interstitial or vacancy type) induced by irradiation in UO2, polycrystalline thin foils were irradiated with 4 MeV Au or 390 keV Xe ions at different temperatures (25, 600 and 800 °C) and fluences (0.5 and 1 × 1015 ions/cm2), and further analyzed using TEM. In all the cases, this study, performed on a large number of dislocation loops (diameter ranging from 10 to 80 nm) and for the first time on several dislocation lines, reveals unfaulted prismatic dislocation loops with an interstitial nature and Burgers vectors only along the <110>-type directions. Almost 60% of the studied loops are purely prismatic type and lie on {110} habit planes perpendicular to the Burgers vector directions. The others lie on the {110} or {111} planes, which are neither perpendicular to the Burgers vectors, nor contain them. About 87% of the dislocation lines, formed by loop overlapping as fluence increases, are edge or mixed type in the <100>{100} slip systems, as those induced under mechanical load.

Visualization and quantification of deformation processes controlling the mechanical response of alloys in aggressive environments

DOE Office of Scientific and Technical Information (OSTI.GOV)

Robertson, Ian M.

The overall objective of this program was to develop the technique of electron tomography for studies of defects and to couple it with real time dynamic experiments such that four-dimensional (time and three spatial dimensions) characterization of dislocation interactions with defects is feasible and apply it to discovery of the fundamental unit processes of dislocation-defect interactions in metallic systems. Strategies to overcome the restrictions normally associated with electron tomography and to make it practical within the constraints of conducting a dynamic experiment in the transmission electron microscope were developed. These methods were used to determine the mechanism controlling the transfermore » of slip across grain boundaries in FCC and HCP metals, dislocation precipitate interactions in Al alloys, and dislocation-dislocation interactions in HCP Ti. In addition, preliminary investigations of slip transfer across cube-on-cube and incoherent twin interfaces in a multi-layered system, thermal stability of grains in nanongrained Ni and Fe, and on corrosion of Fe films were conducted.« less

The Role of Twinning Deformation on the Hardening Response of Polycrystalline Magnesium from Discrete Dislocation Dynamics Simulations

DTIC Science & Technology

2015-01-01

polycrystalline magnesium (Mg) was studied using three-dimensional discrete dislocation dynamics ( DDD ). A systematic interaction model between dislocations...and f1012g tension twin boundaries (TBs) was proposed and introduced into the DDD framework. In addition, a nominal grain boundary (GB) model based...dynamics ( DDD ). A systematic interaction model between dislocations and f10 12g tension twin boundaries (TBs) was proposed and introduced into the DDD

How the Slip Distribution Complexities Control the Tsunami Scenarios: a Sensitivity Analysis for the Hellenic and Calabrian Subduction Interfaces.

NASA Astrophysics Data System (ADS)

Scala, A.; Murphy, S.; Herrero, A.; Maesano, F. E.; Lorito, S.; Romano, F.; Tiberti, M. M.; Tonini, R.; Volpe, M.; Basili, R.

2017-12-01

Recent giant tsunamigenic earthquakes (Sumatra 2004, Chile 2010, Tohoku 2011) have confirmed that the complexity of seismic slip distributions may play a fundamental role in the generation and the amplitude of the tsunami waves. In particular, big patches of large slip on the shallower part of the subduction zones, as well as slow rupture propagation within low rigidity areas, can contribute to increase the tsunamigenic potential thus generating devastating coastal inundation. In the Mediterranean Sea, some subduction structures can be identified, such as the Hellenic Arc at the boundary between the African and Aegean plates, and the Calabrian Arc between the European and African plates. We have modelled these areas using discretized high-resolution 3D fault geometries with realistic variability of the strike and dip angles. In particular, the latter geometries have been constrained from the analysis of a dense network of seismic reflection profiles and the seismicity of the areas. To study the influence of different rigidity conditions, we compare the tsunami scenarios deriving from homogeneous slip to those obtained from depth-dependent slip distributions at different magnitudes. These depth-dependent slip distributions are obtained by imposing a variability with depth of both shear modulus and seismic rate, and the conservation of the dislocation over the whole subduction zone. Furthermore, we generate along the Hellenic and Calabrian arc subduction interfaces an ensemble of stochastic slip distributions using a composite source model technique. To mimic either single or multiple asperity source models, the distribution of sub-events whose sum produces the stochastic slip, are distributed based on a PDF, defined as the combination of either one or more Gaussian functions. Tsunami scenarios are then generated from this ensemble in order to address how the position of the main patch of slip can affect the tsunami amplitude along the coast.

A detailed investigation of the strain hardening response of aluminum alloyed Hadfield steel

NASA Astrophysics Data System (ADS)

Canadinc, Demircan

The unusual strain hardening response exhibited by Hadfield steel single and polycrystals under tensile loading was investigated. Hadfield steel, which deforms plastically through the competing mechanisms slip and twinning, was alloyed with aluminum in order to suppress twinning and study the role of slip only. To avoid complications due to a grained structure, only single crystals of the aluminum alloyed Hadfield steel were considered at the initial stage of the current study. As a result of alloying with aluminum, twinning was suppressed; however a significant increase in the strain hardening response was also present. A detailed microstructural analysis showed the presence of high-density dislocation walls that evolve in volume fraction due to plastic deformation and interaction with slip systems. The very high strain hardening rates exhibited by the aluminum alloyed Hadfield steel single crystals was attributed to the blockage of glide dislocations by the high-density dislocation walls. A crystal plasticity model was proposed, that accounts for the volume fraction evolution and rotation of the dense dislocation walls, as well as their interaction with the active slip systems. The novelty of the model lies in the simplicity of the constitutive equations that define the strain hardening, and the fact that it is based on experimental data regarding the microstructure. The success of the model was tested by its application to different crystallographic orientations, and finally the polycrystals of the aluminum alloyed Hadfield steel. Meanwhile, the capability of the model to predict texture was also observed through the rotation of the loading axis in single crystals. The ability of the model to capture the polycrystalline deformation response provides a venue for its utilization in other alloys that exhibit dislocation sheet structures.

Observations of cleavage steps, slip traces and dislocation hollow cores on cleaved ?100? faces of ?-arginine phosphate monohydrate single crystals by atomic force microscopy

NASA Astrophysics Data System (ADS)

Sangwal, K.; Torrent-Burgués, J.; Sanz, F.; Servat, J.

1997-03-01

The results of an atomic force microscopy study of the nature of cleavage steps, observation of slip traces and formation of hollow cores at the centres of dislocations on the {100} faces of L-arginine phosphate monohydrate (LAP) single crystals grown from aqueous solutions are described and discussed. It was observed that: (1) most of the cleavage steps and all the slip traces are of elementary height, a = 1.085 nm; (2) the origin of a cleavage step may or may not have a hollow core; and (3) close to its origin, the curvature of a cleavage step may be positive or negative or may change from positive to negative. The results suggest that slip traces observed on the cleaved surfaces of LAP are formed during the cleavage process while the rounding and the rearrangement of elementary cleavage steps take place immediately after the occurrence of cleavage. Analysis of the results also shows that the dislocations responsible for the origin of hollow cores always represent a stress field state corresponding to a trapped solution of different local interface supersaturations.

Lubrication of dislocation glide in MgO by hydrous defects

NASA Astrophysics Data System (ADS)

Skelton, Richard; Walker, Andrew M.

2018-02-01

Water-related defects, principally in the form of protonated cation vacancies, are potentially able to weaken minerals under high-stress or low-temperature conditions by reducing the Peierls stress required to initiate dislocation glide. In this study, we use the Peierls-Nabarro (PN) model to determine the effect of protonated Mg vacancies on the 1/2<110>{110} and 1/2<110>{100} slip systems in MgO. This PN model is parameterized using generalized stacking fault energies calculated using plane-wave density functional theory, with and without protonated Mg vacancies present at the glide plane. It found that these defects increase dislocation core widths and reduce the Peierls stress over the entire pressure range 0-125 GPa. Furthermore, 1/2<110>{110} slip is found to be more sensitive to the presence of protonated vacancies which increases in the pressure at which {100} becomes the easy glide plane for 1/2<110> screw dislocations. These results demonstrate, for a simple mineral system, that water-related defects can alter the deformation behavior of minerals in the glide-creep regime by reducing the stress required to move dislocations by glide. (Mg, Fe)O is the most anisotropic mineral in the Earth's lower mantle, so the differential sensitivity of the major slip systems in MgO to hydrous defects has potential implications for the interpretation of the seismic anisotropy in this region.

The effect of interstitial carbon on the mechanical properties and dislocation substructure evolution in Fe 40.4 Ni 11.3 Mn 34.8 Al 7.5 Cr 6 high entropy alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Zhangwei; Baker, Ian; Cai, Zhonghou

2016-11-01

A systematic study of the effects of up to 1.1 at. % carbon on the mechanical properties and evolution of the dislocation substructure in a series of a high entropy alloys (HEA) based on Fe40.4Ni11.3Mn34.8Al7.5Cr6 is presented. Transmission electron microscopy (TEM), synchrotron X-ray diffraction (XRD) and atom probe tomography (APT) were used to show that all the alloys are single-phase f.c.c. random solid solutions. The lattice constant, determined from synchrotron XRD measurements, increases linearly with increasing carbon concentration, which leads to a linear relationship between the yield strength and the carbon concentration. The dislocation substructures, as determined by a TEM,more » show a transition from wavy slip to planar slip and, at higher strains, and from cell-forming structure (dislocations cells, cell blocks and dense dislocation walls) to non-cell forming structure (Taylor lattice, microbands and domain boundaries) with the addition of carbon, features related to the increase in lattice friction stress. The stacking fault energy (measured via weak-beam imaging of the separation of dislocation partials) decreases with increasing carbon content, which also contributes to the transition from wavy slip to planar slip. The formation of non-cell forming structure induced by carbon leads to a high degree of strain hardening and a substantial increase in the ultimate tensile strength. The consequent postponement of necking due to the high strain hardening, along with the plasticity accommodation arising from the formation of microbands and domain boundaries, result in an increase of ductility due to the carbon addition. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.« less

The effect of grain orientation on nanoindentation behavior of model austenitic alloy Fe-20Cr-25Ni

DOE PAGES

Chen, Tianyi; Tan, Lizhen; Lu, Zizhe; ...

2017-07-26

Instrumented nanoindentation was used in this paper to investigate the hardness, elastic modulus, and creep behavior of an austenitic Fe-20Cr-25Ni model alloy at room temperature, with the indented grain orientation being the variant. The samples indented close to the {111} surfaces exhibited the highest hardness and modulus. However, nanoindentation creep tests showed the greatest tendency for creep in the {111} indented samples, compared with the samples indented close to the {001} and {101} surfaces. Scanning electron microscopy and cross-sectional transmission electron microscopy revealed slip bands and dislocations in all samples. The slip band patterns on the indented surfaces were influencedmore » by the grain orientations. Deformation twinning was observed only under the {001} indented surfaces. Finally, microstructural analysis and molecular dynamics modeling correlated the anisotropic nanoindentation-creep behavior with the different dislocation substructures formed during indentation, which resulted from the dislocation reactions of certain active slip systems that are determined by the indented grain orientations.« less

Interacting effects of strengthening and twin boundary migration in nanotwinned materials

NASA Astrophysics Data System (ADS)

Joshi, Kartikey; Joshi, Shailendra P.

Twin boundaries play a governing role in the mechanical characteristics of nanotwinned materials. They act as yield strengthening agents by offering resistance to non-coplanar dislocation slip. Twin boundary migration may cause yield softening while also enhancing the strain hardening response. In this work, we investigate the interaction between strengthening and twin boundary migration mechanisms by developing a length-scale dependent crystal plasticity framework for face-centered-cubic nanotwinned materials. The crystal plasticity model incorporates strengthening mechanisms due to dislocation pile-up via slip and slip-rate gradients and twin boundary migration via source-based twin partial nucleation and lattice dislocation-twin boundary interaction. The coupled effect of the load orientation and initial twin size on the speed of twin boundary is discussed and an expression for the same is proposed in terms of relevant material parameters. The efficacy of finite element simulations and the analytical expression in predicting evolution of nanotwinned microstructures comprising size and spatial distributions of twins is demonstrated.

Orientation Dependence of the Deformation Microstructure of Ta-4%W after Cold-Rolling

NASA Astrophysics Data System (ADS)

Zhang, J.; Ma, G. Q.; Godfrey, A.; Shu, D. Y.; Chen, Q.; Wu, G. L.

2017-07-01

One of the common features of deformed face-centered cubic metals with medium to high stacking fault energy is the formation of geometrically necessary dislocation boundaries. The dislocation boundary arrangements in refractory metals with body-centered cubic crystal structure are, however, less well known. To address this issue a Ta-4%W alloy was cold rolled up to 70% in thickness in the present work. The resulting deformation microstructures were characterized by electron back-scattering diffraction and the dislocation boundary arrangements in each grain were revealed using sample-frame misorientation axis maps calculated using an in-house code. The maps were used to analyze the slip pattern of individual grains after rolling, revealing an orientation dependence of the slip pattern.

Experimental Deformation of Olivine Crystals at Mantle P and T: Evidences for a Pressure-Induced Slip Transition and Implications for Upper-Mantle Seismic Anisotropy and Low Viscosity Zone

NASA Astrophysics Data System (ADS)

Raterron, P.; Chen, J.; Geenen, T.; Girard, J.

2009-04-01

Recent developments in high-pressure deformation devices coupled with synchrotron radiation allow investigating the rheology of mantle minerals and aggregates at the extreme pressure (P) and temperature (T) of their natural occurrence in the Earth. This is particularly true in the case of olivine, which rheology has been recently investigated in the Deformation-DIA apparatus (D-DIA, see Wang et al., 2003, Rev. Scientific Instr., 74, 3002) at upper-mantle P and T conditions. Olivine deforms by dislocation creep in the shallow upper-mantle, as revealed by the seismic velocity anisotropy observed in this region. The attenuation of seismic anisotropy at depth greater than 200 km is interpreted as a pressure-induced change in olivine main deformation mechanism. It was first attributed to a transition from dislocation creep to diffusion creep (Karato and Wu, 1993, Science, 260, 771). This interpretation has been challenged by deformation data obtained at high pressure (P > 3 GPa) in the dislocation creep regime (Couvy et al., 2004, EJM, 16, 877; Raterron et al., 2007, Am. Miner., 92, 1436; Raterron et al., 2009, PEPI, 72, 74), which support a second interpretation: a transition in olivine dominant dislocation slip, from [100] slip at low P to [001] slip at high P (e.g., Mainprice et al., 2005, Nature, 433, 731). Such a P -induced [100]/[001] slip transition is also supported by recent theoretical studies based on first-principle calculations of olivine dislocation slips (Durinck et al., 2005, PCM, 32, 646; Durinck et al., 2007, Eur. J. Mineral., 19, 631). In order to further constrain the effect of pressure on olivine slip system activities, deformation experiments were carried out in poor water condition at P > 5 GPa and T =1400˚ C, on pure forsterite (Fo100) and San Carlos olivine crystals, using the D-DIA at the X17B2 beamline of the NSLS (Upton, NY, USA). Crystals were oriented in order to active either [100] slip alone or [001] slip alone in (010) plane, or both [100](001) and [001](100) systems together. Constant applied stress < 300 MPa and specimen strain rates were monitored in situ using time-resolved X-ray diffraction and radiography, respectively. Run products were investigated by transmission electron microscopy (TEM) in order to verify the actual activation of the tested dislocation slip systems. The obtained data were compared with rheological data previously obtained at comparable T and conditions, but at room P (Darot and Gueguen, 1981, JGR, 86, 6219; Bai et al., 1991, JGR, 96, 2441), resulting in creep power laws which quantify the effect of P on olivine rheology. The new data confirm the occurrence of a P -induced [100]/[001] slip transition, and suggest that [001](010) system dominates olivine deformation in the deep upper mantle. Extrapolation of the obtained rheological laws to natural condition along upper-mantle geotherms, shows that the [100] / [001] slip transition should occur in the Earth at ~ 200 km depth, thus can explain the attenuation of seismic anisotropy in the deep upper mantle. The obtained rheological laws were also integrated into a straightforward olivine aggregate model, then extrapolated to mantle condition using a 2-D geodynamic modeling application (Van den Berg et al., 1993, Geophys. J. International, 115, 62), which is the simplest approach to investigate upper-mantle steady-state deformation. In the application, the velocity of the lower boundary (the transition-zone boundary at 410-km depth) was set to 0, while that at the Earth's surface was set to 2 cm/year. Results from this modeling suggest that the combine activity of [100] and [001] slips in olivine aggregates may significantly decrease mantle viscosity below the oceanic lithosphere, thus, may contribute to the low viscosity zone (LVZ) required in plate tectonics to decouple rigid plates from the more ductile asthenophere underneath.

Resolved shear stress intensity coefficient and fatigue crack growth in large crystals

NASA Technical Reports Server (NTRS)

Chen, QI; Liu, Hao-Wen

1988-01-01

Fatigue crack growth in large grain Al alloy was studied. Fatigue crack growth is caused primarily by shear decohesion due to dislocation motion in the crack tip region. The crack paths in the large crystals are very irregular and zigzag. The crack planes are often inclined to the loading axis both in the inplane direction and the thickness direction. The stress intensity factors of such inclined cracks are approximated from the two dimensional finite element calculations. The plastic deformation in a large crystal is highly anisotropic, and dislocation motion in such crystals are driven by the resolved shear stress. The resolved shear stress intensity coefficient in a crack solid, RSSIC, is defined, and the coefficients for the slip systems at a crack tip are evaluated from the calculated stress intensity factors. The orientations of the crack planes are closely related to the slip planes with the high RSSIC values. If a single slip system has a much higher RSSIC than all the others, the crack will follow the slip plane, and the slip plane becomes the crack plane. If two or more slip systems have a high RSSIC, the crack plane is the result of the decohesion processes on these active slip planes.

Peierls-Nabarro modeling of dislocations in UO2

NASA Astrophysics Data System (ADS)

Skelton, Richard; Walker, Andrew M.

2017-11-01

Under conditions of high stress or low temperature, glide of dislocations plays an important role in the deformation of UO2. In this paper, the Peierls-Nabarro model is used to calculate the core widths and Peierls stresses of ½<110> edge and screw dislocations gliding on {100}, {110}, and {111}. The energy of the inelastic displacement field in the dislocation core is parameterized using generalized stacking fault energies, which are calculated atomistically using interatomic potentials. We use seven different interatomic potential models, representing the variety of different models available for UO2. The different models broadly agree on the relative order of the strengths of the different slip systems, with the 1/2<110>{100} edge dislocation predicted to be the weakest slip system and 1/2<110>{110} the strongest. However, the calculated Peierls stresses depend strongly on the interatomic potential used, with values ranging between 2.7 and 12.9 GPa for glide of 1/2<110>{100} edge dislocations, 16.4-32.3 GPa for 1/2<110>{110} edge dislocations, and 6.8-13.6 GPa for 1/2<110>{111} edge dislocations. The glide of 1/2<110> screw dislocations in UO2 is also found to depend on the interatomic potential used, with some models predicting similar Peierls stresses for glide on {100} and {111}, while others predict a unique easy glide direction. Comparison with previous fully atomistic calculations show that the Peierls-Nabarro model can accurately predict dislocation properties in UO2.

The relationship between strain geometry and geometrically necessary dislocations

NASA Astrophysics Data System (ADS)

Hansen, Lars; Wallis, David

2016-04-01

The kinematics of past deformations are often a primary goal in structural analyses of strained rocks. Details of the strain geometry, in particular, can help distinguish hypotheses about large-scale tectonic phenomena. Microstructural indicators of strain geometry have been heavily utilized to investigate large-scale kinematics. However, many of the existing techniques require structures for which the initial morphology is known, and those structures must undergo the same deformation as imposed macroscopically. Many deformed rocks do not exhibit such convenient features, and therefore the strain geometry is often difficult (if not impossible) to ascertain. Alternatively, crystallographic textures contain information about the strain geometry, but the influence of strain geometry can be difficult to separate from other environmental factors that might affect slip system activity and therefore the textural evolution. Here we explore the ability for geometrically necessary dislocations to record information about the deformation geometry. It is well known that crystallographic slip due to the motion of dislocations yields macroscopic plastic strain, and the mathematics are established to relate dislocation glide on multiple slip systems to the strain tensor of a crystal. This theoretical description generally assumes that dislocations propagate across the entire crystal. However, at any point during the deformation, dislocations are present that have not fully transected the crystal, existing either as free dislocations or as dislocations organized into substructures like subgrain boundaries. These dislocations can remain in the lattice after deformation if the crystal is quenched sufficiently fast, and we hypothesize that this residual dislocation population can be linked to the plastic strain geometry in a quantitative manner. To test this hypothesis, we use high-resolution electron backscatter diffraction to measure lattice curvatures in experimentally deformed single crystals and aggregates of olivine for which the strain geometry is known. Tested geometries include constrictional strain, flattening strain, and plane strain. We use measured lattice curvatures to calculate the densities and spatial distributions of geometrically necessary dislocations. Dislocation densities are calculated for each of the major dislocation types in olivine. These densities are then used to estimate the plastic strain geometry under the assumption that the population of geometrically necessary dislocations accurately represents the relative activity of different dislocations during deformation. Our initial results demonstrate compelling relationships between the imposed strain geometry and the calculated plastic strain geometry. In addition, the calculated plastic strain geometry is linked to the distribution of crystallographic orientations, giving insight into the nature of plastic anisotropy in textured olivine aggregates. We present this technique as a new microstructural tool for assessing the kinematic history of deformed rocks.

Parallel Performance of Linear Solvers and Preconditioners

DTIC Science & Technology

2014-01-01

are produced by a discrete dislocation dynamics ( DDD ) simulation and change with each timestep of the DDD simulation as the dislocation structure...evolves. However, the coefficient—or stiffness matrix— remains constant during the DDD simulation and some expensive matrix factorizations only occur once...discrete dislocation dynamics ( DDD ) simulations. This can be achieved by coupling a DDD simulator for bulk material (Arsenlis et al., 2007) to a

Damage Evaluation for Ti Alloys in Creep based on Incompatibility Field Measurement via EBSD Technique and Micro-Pillar Experiments Toward Identification of Roles of Dislocation Substructures on Fatigue Crack Initiation

DTIC Science & Technology

2011-02-07

Reproduction of a slip band with a PSB -ladder-like internal structure is attempted assuming initial conditions with and without corresponding strain...into heat at the PSB region, the present study extensively examined possible transition mechanisms toward the growth of grooves thereabout and that...arrangements even with the same dislocation density. (2)A slip band-like region having a substructure mimicking PSB ladder is demonstrated to be

Cyclic Deformation Microstructure in Heavily Cold-Drawn Austenitic Stainless Steel

NASA Astrophysics Data System (ADS)

Xie, Xingfei; Ning, Dong; Sun, Jian

2018-04-01

Cyclic deformation microstructure of the heavily cold-drawn austenitic stainless steel is significantly influenced by the spacing between mechanical twins introduced by prior cold drawing. Well-developed dislocation cells form between mechanical twins with the spacing larger than about 800 nm. Persistent slip band (PSB)-like structure with ladders takes place between mechanical twins spacing from 300 to 800 nm. Few dislocations occur between neighboring mechanical twins with spacing less than about 100 nm. Pre-existing mechanical twins and deformation bands segment austenitic grains, facilitating multi-slip and consequently suppressing PSB formation.

Cyclic Deformation Microstructure in Heavily Cold-Drawn Austenitic Stainless Steel

NASA Astrophysics Data System (ADS)

Xie, Xingfei; Ning, Dong; Sun, Jian

2018-07-01

Cyclic deformation microstructure of the heavily cold-drawn austenitic stainless steel is significantly influenced by the spacing between mechanical twins introduced by prior cold drawing. Well-developed dislocation cells form between mechanical twins with the spacing larger than about 800 nm. Persistent slip band (PSB)-like structure with ladders takes place between mechanical twins spacing from 300 to 800 nm. Few dislocations occur between neighboring mechanical twins with spacing less than about 100 nm. Pre-existing mechanical twins and deformation bands segment austenitic grains, facilitating multi-slip and consequently suppressing PSB formation.

Modeling defects and plasticity in MgSiO3 post-perovskite: Part 2-screw and edge [100] dislocations.

PubMed

Goryaeva, Alexandra M; Carrez, Philippe; Cordier, Patrick

In this study, we propose a full atomistic study of [100] dislocations in MgSiO 3 post-perovskite based on the pairwise potential parameterized by Oganov et al. (Phys Earth Planet Inter 122:277-288, 2000) for MgSiO 3 perovskite. We model screw dislocations to identify planes where they glide easier. We show that despite a small tendency to core spreading in {011}, [100] screw dislocations glide very easily (Peierls stress of 1 GPa) in (010) where only Mg-O bonds are to be sheared. Crossing the Si-layers results in a higher lattice friction as shown by the Peierls stress of [100](001): 17.5 GPa. Glide of [100] screw dislocations in {011} appears also to be highly unfavorable. Whatever the planes, (010), (001) or {011}, edge dislocations are characterized by a wider core (of the order of 2 b ). Contrary to screw character, they bear negligible lattice friction (0.1 GPa) for each slip system. The layered structure of post-perovskite results in a drastic reduction in lattice friction opposed to the easiest slip systems compared to perovskite.

Quantifying the effect of hydrogen on dislocation dynamics: A three-dimensional discrete dislocation dynamics framework

NASA Astrophysics Data System (ADS)

Gu, Yejun; El-Awady, Jaafar A.

2018-03-01

We present a new framework to quantify the effect of hydrogen on dislocations using large scale three-dimensional (3D) discrete dislocation dynamics (DDD) simulations. In this model, the first order elastic interaction energy associated with the hydrogen-induced volume change is accounted for. The three-dimensional stress tensor induced by hydrogen concentration, which is in equilibrium with respect to the dislocation stress field, is derived using the Eshelby inclusion model, while the hydrogen bulk diffusion is treated as a continuum process. This newly developed framework is utilized to quantify the effect of different hydrogen concentrations on the dynamics of a glide dislocation in the absence of an applied stress field as well as on the spacing between dislocations in an array of parallel edge dislocations. A shielding effect is observed for materials having a large hydrogen diffusion coefficient, with the shield effect leading to the homogenization of the shrinkage process leading to the glide loop maintaining its circular shape, as well as resulting in a decrease in dislocation separation distances in the array of parallel edge dislocations. On the other hand, for materials having a small hydrogen diffusion coefficient, the high hydrogen concentrations around the edge characters of the dislocations act to pin them. Higher stresses are required to be able to unpin the dislocations from the hydrogen clouds surrounding them. Finally, this new framework can open the door for further large scale studies on the effect of hydrogen on the different aspects of dislocation-mediated plasticity in metals. With minor modifications of the current formulations, the framework can also be extended to account for general inclusion-induced stress field in discrete dislocation dynamics simulations.

Dislocation structure in textured zirconium tensile-deformed along rolling and transverse directions determined by X-ray diffraction line profile analysis

NASA Astrophysics Data System (ADS)

Fan, Zhijian; Jóni, Bertalan; Xie, Lei; Ribárik, Gábor; Ungár, Tamás

2018-04-01

Specimens of cold-rolled zirconium were tensile-deformed along the rolling (RD) and the transverse (TD) directions. The stress-strain curves revealed a strong texture dependence. High resolution X-ray line profile analysis was used to determine the prevailing active slip-systems in the specimens with different textures. The reflections in the X-ray diffraction patterns were separated into two groups. One group corresponds to the major and the other group to the random texture component, respectively. The dislocation densities, the subgrain size and the prevailing active slip-systems were evaluated by using the convolutional multiple whole profile (CMWP) procedure. These microstructure parameters were evaluated separately in the two groups of reflections corresponding to the two different texture components. Significant differences were found in both, the evolution of dislocation densities and the development of the fractions of and type slip systems in the RD and TD specimens during tensile deformation. The differences between the RD and TD stress-strain curves are discussed in terms of the differences of the microstructure evolution.

Suppression of Twinning and Phase Transformation in an Ultrafine Grained 2 GPa Strong Metastable Austenitic Steel: Experiment and Simulation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shen, Yongfeng; Jia, Nan; Wang, Y. D.

2015-07-17

An ultrafine-grained 304 austenitic 18 wt.%Cr-8 wt.%Ni stainless steel with a grain size of ~270 nm was synthesized by accumulative rolling (67 % total reduction) and annealing (550 °C, 150s). Uniaxial tensile testing at room temperature reveals an extremely high yield strength of 1890 ± 50MPa and a tensile strength of 2050 ± 30MPa, while the elongation reaches 6 ± 1%. Experimental characterization on samples with different grain sizes between 270 nm and 35 μm indicates that both, deformation twinning and martensitic phase transformation are significantly retarded with increasing grain refinement. A crystal plasticity finite element model incorporating a constitutivemore » law reflecting the grain size-controlled dislocation slip and deformation twinning captures the micromechanical behavior of the steels with different grain sizes. Comparison of simulation and experiment shows that the deformation of ultrafine-grained 304 steels is dominated by the slip of partial dislocations, whereas for coarse-grained steels dislocation slip, twinning and martensite formation jointly contribute to the shape change.« less

Peierls–Nabarro stresses of dislocations in monoclinic cyclotetramethylene tetranitramine (β-HMX)

NASA Astrophysics Data System (ADS)

Pal, Anirban; Picu, Catalin R.

2018-06-01

HMX (cyclotetramethylene tetranitramine) is an energetic material which releases substantial amounts of energy upon decomposition. The role of defects and deformation in causing reaction initiation was discussed in the literature but remains insufficiently understood. In this work, we identify, using computational methods, the slip systems which are potentially active in β-HMX and rank them in terms of their propensity for slip. To this end, we develop first a tentative ranking based on the degree of steric hindrance associated with slip. This is quantified using a geometric analog of the γ-surface. Further, we use atomistic models to compute the Peierls–Nabarro (PN) stress for the motion of dislocations in the slip systems with smallest degree of steric hindrance. A complex mechanical behavior is observed, including strong slip asymmetry, twinning and cleavage. The five systems with the lowest PN stress are (011)[01\\bar{1}], (011)[100], (101)[010], (101)[10\\bar{1}] and (021)[100]. We conclude that the material has enough slip systems available for supporting a generalized state of plastic strain provided the twinning system (101)[10\\bar{1}] is taken into consideration and that the resolved shear stress is at least 260 MPa.

A FFT-based formulation for efficient mechanical fields computation in isotropic and anisotropic periodic discrete dislocation dynamics

NASA Astrophysics Data System (ADS)

Bertin, N.; Upadhyay, M. V.; Pradalier, C.; Capolungo, L.

2015-09-01

In this paper, we propose a novel full-field approach based on the fast Fourier transform (FFT) technique to compute mechanical fields in periodic discrete dislocation dynamics (DDD) simulations for anisotropic materials: the DDD-FFT approach. By coupling the FFT-based approach to the discrete continuous model, the present approach benefits from the high computational efficiency of the FFT algorithm, while allowing for a discrete representation of dislocation lines. It is demonstrated that the computational time associated with the new DDD-FFT approach is significantly lower than that of current DDD approaches when large number of dislocation segments are involved for isotropic and anisotropic elasticity, respectively. Furthermore, for fine Fourier grids, the treatment of anisotropic elasticity comes at a similar computational cost to that of isotropic simulation. Thus, the proposed approach paves the way towards achieving scale transition from DDD to mesoscale plasticity, especially due to the method’s ability to incorporate inhomogeneous elasticity.

Gravity and gravity gradient changes caused by a point dislocation

NASA Astrophysics Data System (ADS)

Huang, Jian-Liang; Li, Hui; Li, Rui-Hao

1995-02-01

In this paper we studied gravitational potential, gravity and its gradient changes, which are caused by a point dislocation, and gave the concise mathematical deduction with definite physical implication in dealing with the singular integral at a seismic source. We also analysed the features of the fields of gravity and gravity gradient, gravity-vertical-displacement gradient. The conclusions are: (1) Gravity and gravity gradient changes are very small with the change of vertical position; (2) Gravity change is much greater than the gravity gradient change which is not so distinct; (3) The gravity change due to redistribution of mass accounts for 10 50 percent of the total gravity change caused by dislocation. The signs (positive or negative) of total gravity change and vertical displacement are opposite each other at the same point for strike slip and dip slip; (4) Gravity-vertical-displacement-gradient is not constant; it manifests a variety of patterns for different dislocation models; (5) Gravity-vertical-displacement-gradient is approximately equal to apparent gravity-vertical-displacement-gradient.

The Earth isn't flat: The (large) influence of topography on geodetic fault slip imaging.

NASA Astrophysics Data System (ADS)

Thompson, T. B.; Meade, B. J.

2017-12-01

While earthquakes both occur near and generate steep topography, most geodetic slip inversions assume that the Earth's surface is flat. We have developed a new boundary element tool, Tectosaur, with the capability to study fault and earthquake problems including complex fault system geometries, topography, material property contrasts, and millions of elements. Using Tectosaur, we study the model error induced by neglecting topography in both idealized synthetic fault models and for the cases of the MW=7.3 Landers and MW=8.0 Wenchuan earthquakes. Near the steepest topography, we find the use of flat Earth dislocation models may induce errors of more than 100% in the inferred slip magnitude and rake. In particular, neglecting topographic effects leads to an inferred shallow slip deficit. Thus, we propose that the shallow slip deficit observed in several earthquakes may be an artefact resulting from the systematic use of elastic dislocation models assuming a flat Earth. Finally, using this study as an example, we emphasize the dangerous potential for forward model errors to be amplified by an order of magnitude in inverse problems.

Influence of dislocations and second phases on the magnetostrictive behavior of iron-gallium and other iron alloy single crystals

NASA Astrophysics Data System (ADS)

Saha, Biswadeep

Rare-earth-free Fe-Ga magnetostrictive alloys exhibit an excellent combination of large low-field magnetostriction, strength, ductility, wide operating temperature range, and low cost. Various observations in these and other alpha-Fe-based magnetostrictive alloys suggest that lattice strain modulations that are influenced by solute elements, near neighbor atomic environments around Fe atoms, coherent and incoherent precipitates, and structural defects such as dislocations likely play an important role in their magnetostrictive behavior. In the first part, the effect of dislocations on the magnetostriction of Fe-Ga single crystals was examined. The [001]- and [126]-oriented Fe-20 at.% Ga single crystal samples were deformed in a controlled way to introduce dislocation arrays with two different array geometries. Magnetostriction values showed a much lower decrease after deformation for the case of a [001]-oriented crystal, where eight different slip systems were operative and consequently eight different sets of dislocation arrays are expected. A drastic drop in magnetostriction measured along the sample axis is observed in the sample subjected to a small strain by deformation of a [126]-oriented crystal during which slip occurred on only one slip system. The nature of strain modulation introduced in this case was spatially asymmetric. The [126] deformation was accompanied by an acoustic emission during the formation of slip band. Transmission electron microscopy was carried out to examine the nature of dislocation distribution. The results show that the nature of strain modulation introduced by the dislocation arrays has a strong influence on the magnetostrictive behavior of magnetostrictive alloys. In the second part of this research, the effect of Mo addition to Fe was examined in detail. Addition of Mo to Fe increased the magnetostriction (3/2)lambda100 Fe very rapidly to 137 ppm at 10 at.% Mo, the highest value observed in these alloys. Further Mo additions decreased the magnetostriction. Magnetization data show a drastic drop in magnetization to 63 emu/gm for Fe-20 at.% Mo from 176 emu/gm for Fe-10 at.% Mo suggesting the formation large amounts of nonmagnetic second phase and reduction in total Fe content of the alloy. The drop in magnetostriction at higher Mo contents is associated with the formation of a second phase.

An acoustic emission study of plastic deformation in polycrystalline aluminium

NASA Technical Reports Server (NTRS)

Bill, R. C.; Frederick, J. R.; Felbeck, D. K.

1979-01-01

Acoustic emission experiments were performed on polycrystalline and single crystal 99.99% aluminum while undergoing tensile deformation. It was found that acoustic emission counts as a function of grain size showed a maximum value at a particular grain size. Furthermore, the slip area associated with this particular grain size corresponded to the threshold level of detectability of single dislocation slip events. The rate of decline in acoustic emission activity as grain size is increased beyond the peak value suggests that grain boundary associated dislocation sources are giving rise to the bulk of the detected acoustic emissions.

Inversion for slip distribution using teleseismic P waveforms: North Palm Springs, Borah Peak, and Michoacan earthquakes

USGS Publications Warehouse

Mendoza, C.; Hartzell, S.H.

1988-01-01

We have inverted the teleseismic P waveforms recorded by stations of the Global Digital Seismograph Network for the 8 July 1986 North Palm Springs, California, the 28 October 1983 Borah Peak, Idaho, and the 19 September 1985 Michoacan, Mexico, earthquakes to recover the distribution of slip on each of the faults using a point-by-point inversion method with smoothing and positivity constraints. Results of the inversion indicate that the Global digital Seismograph Network data are useful for deriving fault dislocation models for moderate to large events. However, a wide range of frequencies is necessary to infer the distribution of slip on the earthquake fault. Although the long-period waveforms define the size (dimensions and seismic moment) of the earthquake, data at shorter period provide additional constraints on the variation of slip on the fault. Dislocation models obtained for all three earthquakes are consistent with a heterogeneous rupture process where failure is controlled largely by the size and location of high-strength asperity regions. -from Authors

Conjugated π electron engineering of generalized stacking fault in graphene and h-BN.

PubMed

Ouyang, Bin; Chen, Cheng; Song, J

2018-03-02

Generalized-stacking-fault energy (GSFE) serves as an important metric that prescribes dislocation behaviors in materials. In this paper, utilizing first-principle calculations and chemical bonding analysis, we studied the behaviors of generalized stacking fault in graphene and h-BN. It has been shown that the π bond formation plays a critical role in the existence of metastable stacking fault (MSF) in graphene and h-BN lattice along certain slip directions. Chemical functionalization was then proposed as an effective means to engineer the π bond, and subsequently MSF along dislocation slips within graphene and h-BN. Taking hydrogenation as a representative functionalization method, we demonstrated that, with the preferential adsorption of hydrogen along the slip line, π electrons along the slip would be saturated by adsorbed hydrogen atoms, leading to the moderation or elimination of MSF. Our study elucidates the atomic mechanism of MSF formation in graphene-like materials, and more generally, provides important insights towards predictive tuning of mechanic properties in two-dimensional nanomaterials.

Conjugated π electron engineering of generalized stacking fault in graphene and h-BN

NASA Astrophysics Data System (ADS)

Ouyang, Bin; Chen, Cheng; Song, J.

2018-03-01

Generalized-stacking-fault energy (GSFE) serves as an important metric that prescribes dislocation behaviors in materials. In this paper, utilizing first-principle calculations and chemical bonding analysis, we studied the behaviors of generalized stacking fault in graphene and h-BN. It has been shown that the π bond formation plays a critical role in the existence of metastable stacking fault (MSF) in graphene and h-BN lattice along certain slip directions. Chemical functionalization was then proposed as an effective means to engineer the π bond, and subsequently MSF along dislocation slips within graphene and h-BN. Taking hydrogenation as a representative functionalization method, we demonstrated that, with the preferential adsorption of hydrogen along the slip line, π electrons along the slip would be saturated by adsorbed hydrogen atoms, leading to the moderation or elimination of MSF. Our study elucidates the atomic mechanism of MSF formation in graphene-like materials, and more generally, provides important insights towards predictive tuning of mechanic properties in two-dimensional nanomaterials.

"A Chance Child": Jill Paton Walsh and the Re-Invention of the Time Slip Story

ERIC Educational Resources Information Center

Hall, Linda Marian

2011-01-01

In this study of Jill Paton Walsh's one time-slip novel, I attempt to show how she reinvents the genre by giving as much prominence to the dislocated present as she does to the sufferings of children caught up in the horrors of the Industrial Revolution. Where previous time-slip authors had concentrated on the past, she addresses clearly unwelcome…

Dislocation gliding and cross-hatch morphology formation in AIII-BV epitaxial heterostructures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kovalskiy, V. A., E-mail: kovalva@iptm.ru; Vergeles, P. S.; Eremenko, V. G.

2014-12-08

An approach for understanding the origin of cross-hatch pattern (CHP) on the surface of lattice mismatched GaMnAs/InGaAs samples grown on GaAs (001) substrates is developed. It is argued that the motion of threading dislocations in the (111) slip planes during the relaxation of InGaAs buffer layer is more complicated process and its features are similar to the ones of dislocation half-loops gliding in plastically deformed crystals. The heterostructures were characterized by atomic force microscopy and electron beam induced current (EBIC). Detailed EBIC experiments revealed contrast features, which cannot be accounted for by the electrical activity of misfit dislocations at themore » buffer/substrate interface. We attribute these features to specific extended defects (EDs) generated by moving threading dislocations in the partially relaxed InGaAs layers. We believe that the core topology, surface reconstruction, and elastic strains from these EDs accommodated in slip planes play an important role in the CHP formation. The study of such electrically active EDs will allow further understanding of degradation and changes in characteristics of quantum devices based on strained heterostructures.« less

Evaluating the influence of stress on the dislocation creep flow law for quartz

NASA Astrophysics Data System (ADS)

Tokle, L.; Hirth, G.

2017-12-01

Due to the abundance of quartz in the continental crust, quartz rheology is fundamental to our understanding of many geodynamic processes. Microstructures in many naturally deformed quartzites deformed at ductile conditions, indicate that dislocation creep is a common deformation mechanism in quartz at crustal conditions. The dislocation creep flow laws for quartz were constructed based on deformation experiments on aggregates at temperatures from 900 to 1100°C and strain rates of 10-5-10-6 s-1. Hirth et al. (2001) point out that these flow laws underestimate sample strengths for experiments conducted below 900°C; yet samples deformed as low as 700°C exhibit dislocation creep microstructures. To address this discrepancy, we compared 14 different studies on experimentally deformed wet quartzite aggregates ranging in temperature from 700 to 1100°C. Our analysis shows that two clear trends develop, one with a power-law stress exponent of n = 4 and the other, at a higher stress, with a stress exponent of n = 3. This change suggests a transition in the rate-limiting process; further, the conditions where the transition in stress exponent occurs correlate well with changes in quartz c-axis fabrics in general shear experiments. At low stresses, quartz fabrics are defined by a Y-max, indicating prism slip, while at higher stresses quartz fabrics are defined by basal slip. Our interpretation is that the c-axis fabrics represent the easy slip system in quartz and hypothesize that basal slip is rate-limiting at low stresses while prism is rate-limiting at high stresses. A change in the stress exponent has significant consequences for our understanding of high stress tectonic environments, such as the brittle-ductile transition and sediment rheology in a subducting slab.

Ligamentous and capsular injuries to the metacarpophalangeal joints of the hand.

PubMed

Shah, Smiresh Suresh; Techy, Fernando; Mejia, Alfonso; Gonzalez, Mark H

2012-01-01

The mechanism of dorsal dislocation of the metacarpophalangeal (MCP) joint is with forced hyperextension of the joint and the main structure injured is the volar plate. A simple dislocation can be reduced by closed means whereas a complex dislocation cannot. Care must be taken not to put traction across the joint, which may cause the volar plate to slip into the joint, converting a simple dislocation into a complex dislocation. Volar dislocations are rare and mainly treated nonoperatively. Sagittal band injuries can be treated with extension splinting or surgical management with direct repair or reconstruction. A locked MCP joint can usually be treated with closed manipulation. This article discusses these injuries and management options.

Exact solution of the generalized Peierls equation for arbitrary n-fold screw dislocation

NASA Astrophysics Data System (ADS)

Wang, Shaofeng; Hu, Xiangsheng

2018-05-01

The exact solution of the generalized Peierls equation is presented and proved for arbitrary n-fold screw dislocation. The displacement field, stress field and the energy of the n-fold dislocation are also evaluated explicitly. It is found that the solution defined on each individual fold is given by the tail cut from the original Peierls solution. In viewpoint of energetics, a screw dislocation has a tendency to spread the distribution on all possible slip planes which are contained in the dislocation line zone. Based on the exact solution, the approximated solution of the improved Peierls equation is proposed for the modified γ-surface.

Radiology of adolescent slipped capital femoral epiphysis: measurement of epiphyseal angles and diagnosis.

PubMed

Gekeler, Jörg

2007-10-01

AIMS OF DIAGNOSTIC RADIOGRAPHY: Visualization of the proximal femur in two clearly defined projections. Radiologic and morphological diagnosis of slipped capital femoral epiphysis. Evaluation of the stability of the femoral epiphysis: chronic slippage or acute interruption of continuity between the femoral epiphysis and the femoral neck metaphysis. Radiometric measurement of the spatial deformity of the femoral epiphysis. Measurement of the projected epiphyseal angle on the radiograph as the basis for possible conversion into anatomically correct angles at the proximal femur. Preoperative planning of therapeutic surgical procedures. Idiopathic hip pain in the growing child or adolescent. Referred pain to the knee or thigh. Unusual gait pattern with external rotation deformity of the leg, limping that favors one leg or limping due to leg length discrepancy. Abnormal sonography, CT or MRI findings. Eventful history including minor injury or genuine trauma. Symptoms and uncommon physical constitution: obesity, exceptional longitudinal growth of the extremities, and absence of secondary sex characteristics. Indications for Radiographic Imaging of the Hip Joint in Two Planes None. Standard positioning of the patient or the affected extremity. First standard radiograph: proximal femur in anteroposterior projection. Position of the leg with the patella directed anteriorly. Contraction of the external rotators at the hip joint is compensated by elevation of the hip until the leg is in the neutral position. Second standard radiograph: axial view of the proximal femur in anteroposterior projection. Leg flexed to 90 degrees at the hip and in 45 degrees abduction. Thigh position parallel to the longitudinal axis of the table (zero rotation). Early signs of incipient or imminent femoral epiphyseolysis: --Disintegration, widening and blurred margins of the epiphyseal plate. --Increasing loss of height of the femoral epiphysis due to incipient dislocation. --The tangent to the lateral femoral neck intersects only slightly with the femoral head or runs tangential to the epiphysis. --Important second radiograph in axial projection: incipient slippage is seen early here. Comparison with the contralateral side. Chronic slipped capital femoral epiphysis in adolescents: --Advanced epiphyseal dislocation visible in both planes. The tangent to the lateral femoral neck no longer intersects with the dislocated femoral epiphysis. In some cases, varus deformity of the femoral neck and periosteal elevation at the borders of the medial femoral neck. --Epiphyseal dislocation even more apparent in the axial view. Acute slipped capital femoral epiphysis in adolescents: --Complete interruption of continuity between epiphysis and metaphysis. --Widened gap between epiphysis and metaphysis. --Cystic irregularities of the metaphysis. --In most cases, substantial dislocation between epiphysis and metaphysis. --"Acute on chronic slip": specific type of acute epiphyseal dislocation subsequent to chronic epiphyseolysis. In addition to signs of acute separation, secondary symptoms of chronic epiphyseolysis such as femoral neck arcuation and spur formation at the head-neck junction. --Dynamic fluoroscopy may be indicated to confirm acute dislocation. Defined axes are marked on the radiograph: anatomic axis of the femur, femoral neck axis, and so-called epiphyseal axis (perpendicular to the base of the epiphysis). Measurement of the projected epiphysis-diaphysis angle (ED' angle) on the anteroposterior radiograph and the projected epiphyseal torsion angle (ET' angle) on the axial radiograph. For slight to moderate slippage, the difference between the epiphyseal dislocation angle obtained from the radiographs (as projected in two planes) compared with the anatomic, i.e., real dislocation angle at the proximal femur is generally relatively minor. Conversion of the projected angle to the real angle is not essential in these cases (if in doubt, see Table 1). For more severe dislocations, the differences between the projected and real angles are far more apparent. Table 1 facilitates conversion of the epiphyseal dislocation angles taken from the radiograph into anatomically correct dislocation angles at the proximal femur. Conversion to real angles, especially for preoperative planning of complex corrective surgery, is indicated for more severe deformities of the femoral epiphysis. Conversion into real (anatomic) angles is indicated for exact prognostic evaluation of prearthrotic deformities.

A Viscoelastic earthquake simulator with application to the San Francisco Bay region

USGS Publications Warehouse

Pollitz, Fred F.

2009-01-01

Earthquake simulation on synthetic fault networks carries great potential for characterizing the statistical patterns of earthquake occurrence. I present an earthquake simulator based on elastic dislocation theory. It accounts for the effects of interseismic tectonic loading, static stress steps at the time of earthquakes, and postearthquake stress readjustment through viscoelastic relaxation of the lower crust and mantle. Earthquake rupture initiation and termination are determined with a Coulomb failure stress criterion and the static cascade model. The simulator is applied to interacting multifault systems: one, a synthetic two-fault network, and the other, a fault network representative of the San Francisco Bay region. The faults are discretized both along strike and along dip and can accommodate both strike slip and dip slip. Stress and seismicity functions are evaluated over 30,000 yr trial time periods, resulting in a detailed statistical characterization of the fault systems. Seismicity functions such as the coefficient of variation and a- and b-values exhibit systematic patterns with respect to simple model parameters. This suggests that reliable estimation of the controlling parameters of an earthquake simulator is a prerequisite to the interpretation of its output in terms of seismic hazard.

Effects of torsional deformation on the microstructures and mechanical properties of a CoCrFeNiMo0.15 high-entropy alloy

NASA Astrophysics Data System (ADS)

Wu, Wenqian; Guo, Lin; Liu, Bin; Ni, Song; Liu, Yong; Song, Min

2017-12-01

The effects of torsional deformation on the microstructures and mechanical properties of a CoCrFeNiMo0.15 high-entropy alloy have been investigated. The torsional deformation generates a gradient microstructure distribution due to the gradient torsional strain. Both dislocation activity and deformation twinning dominated the torsional deformation process. With increasing the torsional equivalent strain, the microstructural evolution can be described as follows: (1) formation of pile-up dislocations parallel to the trace of {1 1 1}-type slip planes; (2) formation of Taylor lattices; (3) formation of highly dense dislocation walls; (3) formation of microbands and deformation twins. The extremely high deformation strain (strained to fracture) results in the activation of wavy slip. The tensile strength is very sensitive to the torsional deformation, and increases significantly with increasing the torsional angle.

Mechanism of slip and twinning

NASA Technical Reports Server (NTRS)

Rastani, Mansur

1992-01-01

The objectives are to: (1) demonstrate the mechanisms of deformation in body centered cubic (BCC), face centered cubic (FCC), and hexagonal close-packed (HCP)-structure metals and alloys and in some ceramics as well; (2) examine the deformed microstructures (slip lines and twin boundaries) in different grains of metallic and ceramic specimens; and (3) study visually the deformed macrostructure (slip and twin bands) of metals and alloys. Some of the topics covered include: deformation behavior of materials, mechanisms of plastic deformation, slip bands, twin bands, ductile failure, intergranular fracture, shear failure, slip planes, crystal deformation, and dislocations in ceramics.

Solute effect on basal and prismatic slip systems of Mg.

PubMed

Moitra, Amitava; Kim, Seong-Gon; Horstemeyer, M F

2014-11-05

In an effort to design novel magnesium (Mg) alloys with high ductility, we present a first principles data based on the Density Functional Theory (DFT). The DFT was employed to calculate the generalized stacking fault energy curves, which can be used in the generalized Peierls-Nabarro (PN) model to study the energetics of basal slip and prismatic slip in Mg with and without solutes to calculate continuum scale dislocation core widths, stacking fault widths and Peierls stresses. The generalized stacking fault energy curves for pure Mg agreed well with other DFT calculations. Solute effects on these curves were calculated for nine alloying elements, namely Al, Ca, Ce, Gd, Li, Si, Sn, Zn and Zr, which allowed the strength and ductility to be qualitatively estimated based on the basal dislocation properties. Based on our multiscale methodology, a suggestion has been made to improve Mg formability.

Comparison of dislocation content measured with transmission electron microscopy and micro-Laue diffraction based streak analysis

DOE PAGES

Zhang, C.; Balachandran, S.; Eisenlohr, P.; ...

2017-10-04

The subsurface dislocation content in a Ti-5Al-2.5Sn (wt%) uniaxial tension sample deformed at ambient temperature was characterized by peak streak analysis of micro-Laue diffraction patterns collected non-destructively by differential aperture X-raymicroscopy, and with focused ion beam transmission electron microscopy of material in the same volume. This comparison reveals that micro-Laue diffraction streak analysis based on an edge dislocation assumption can accurately identify the dominant dislocation slip system history (Burgers vector and plane observed by TEM), despite the fact that dislocations have predominantly screw character. As a result, other dislocations identified by TEM were not convincingly discernible from the peak streakmore » analysis.« less

Comparison of dislocation content measured with transmission electron microscopy and micro-Laue diffraction based streak analysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, C.; Balachandran, S.; Eisenlohr, P.

The subsurface dislocation content in a Ti-5Al-2.5Sn (wt%) uniaxial tension sample deformed at ambient temperature was characterized by peak streak analysis of micro-Laue diffraction patterns collected non-destructively by differential aperture X-raymicroscopy, and with focused ion beam transmission electron microscopy of material in the same volume. This comparison reveals that micro-Laue diffraction streak analysis based on an edge dislocation assumption can accurately identify the dominant dislocation slip system history (Burgers vector and plane observed by TEM), despite the fact that dislocations have predominantly screw character. As a result, other dislocations identified by TEM were not convincingly discernible from the peak streakmore » analysis.« less

The impact of model prediction error in designing geodetic networks for crustal deformation applications

NASA Astrophysics Data System (ADS)

Murray, J. R.

2017-12-01

Earth surface displacements measured at Global Navigation Satellite System (GNSS) sites record crustal deformation due, for example, to slip on faults underground. A primary objective in designing geodetic networks to study crustal deformation is to maximize the ability to recover parameters of interest like fault slip. Given Green's functions (GFs) relating observed displacement to motion on buried dislocations representing a fault, one can use various methods to estimate spatially variable slip. However, assumptions embodied in the GFs, e.g., use of a simplified elastic structure, introduce spatially correlated model prediction errors (MPE) not reflected in measurement uncertainties (Duputel et al., 2014). In theory, selection algorithms should incorporate inter-site correlations to identify measurement locations that give unique information. I assess the impact of MPE on site selection by expanding existing methods (Klein et al., 2017; Reeves and Zhe, 1999) to incorporate this effect. Reeves and Zhe's algorithm sequentially adds or removes a predetermined number of data according to a criterion that minimizes the sum of squared errors (SSE) on parameter estimates. Adapting this method to GNSS network design, Klein et al. select new sites that maximize model resolution, using trade-off curves to determine when additional resolution gain is small. Their analysis uses uncorrelated data errors and GFs for a uniform elastic half space. I compare results using GFs for spatially variable strike slip on a discretized dislocation in a uniform elastic half space, a layered elastic half space, and a layered half space with inclusion of MPE. I define an objective criterion to terminate the algorithm once the next site removal would increase SSE more than the expected incremental SSE increase if all sites had equal impact. Using a grid of candidate sites with 8 km spacing, I find the relative value of the selected sites (defined by the percent increase in SSE that further removal of each site would cause) is more uniform when MPE is included. However, the number and distribution of selected sites depends primarily on site location relative to the fault. For this test case, inclusion of MPE has minimal practical impact; I will investigate whether these findings hold for more densely spaced candidate grids and dipping faults.

Phase slip process and charge density wave dynamics in a one dimensional conductor

NASA Astrophysics Data System (ADS)

Habiballah, N.; Zouadi, M.; Arbaoui, A.; Qjani, M.; Dumas, J.

In this paper, we study the phase slip effect on the charge density wave (CDW) dynamics in a one-dimensional conductor in the weak pinning limit. A considerable enhancement of JCDW is observed in the presence of phase slips. In addition, a spatial dependence of the CDW current density JCDW is also studied showing that a decrease of JCDW with distance from the current contact occurs. The results are discussed in terms the relationship between additional phase slips and the mobility of phase dislocations nucleated at electrical contacts.

Size effects on plasticity and fatigue microstructure evolution in FCC single crystals

NASA Astrophysics Data System (ADS)

El-Awady, Jaafar Abbas

In aircraft structures and engines, fatigue damage is manifest in the progressive emergence of distributed surface cracks near locations of high stress concentrations. At the present time, reliable methods for prediction of fatigue crack initiation are not available, because the phenomenon starts at the atomic scale. Initiation of fatigue cracks is associated with the formation of Persistent slip bands (PSBs), which start at certain critical conditions inside metals with specific microstructure dimensions. The main objective of this research is to develop predictive computational capabilities for plasticity and fatigue damage evolution in finite volumes. In that attempt, a dislocation dynamics model that incorporates the influence of free and internal interfaces on dislocation motion is presented. The model is based on a self-consistent formulation of 3-D Parametric Dislocation Dynamics (PDD) with the Boundary Element method (BEM) to describe dislocation motion, and hence microscopic plastic flow in finite volumes. The developed computer models are bench-marked by detailed comparisons with the experimental data, developed at the Wright-Patterson Air Force Lab (WP-AFRL), by three dimensional large scale simulations of compression loading on micro-scale samples of FCC single crystals. These simulation results provide an understanding of plastic deformation of micron-size single crystals. The plastic flow characteristics as well as the stress-strain behavior of simulated micropillars are shown to be in general agreement with experimental observations. New size scaling aspects of plastic flow and work-hardening are identified through the use of these simulations. The flow strength versus the diameter of the micropillar follows a power law with an exponent equal to -0.69. A stronger correlation is observed between the flow strength and the average length of activated dislocation sources. This relationship is again a power law, with an exponent -0.85. Simulation results with and without the activation of cross-slip are compared. Discontinuous hardening is observed when cross-slip is included. Experimentally-observed size effects on plastic flow and work- hardening are consistent with a "weakest-link activation mechanism". In addition, the variations and periodicity of dislocation activation are analyzed using the Fast Fourier Transform (FFT). We then present models of localized plastic deformation inside Persistent Slip Band channels. We investigate the interaction between screw dislocations as they pass one another inside channel walls in copper. The model shows the mechanisms of dislocation bowing, dipole formation and binding, and dipole destruction as screw dislocations pass one another. The mechanism of (dipole passing) is assessed and interpreted in terms of the fatigue saturation stress. We also present results for the effects of the wall dipole structure on the dipole passing mechanism. The edge dislocation dipolar walls is seen to have an effect on the passing stress as well. It is shown that the passing stress in the middle of the channel is reduced by 11 to 23% depending on the initial configuration of the screw dislocations with respect to one another. Finally, from large scale simulations of the expansion process of the edge dipoles from the walls in the channel the screw dislocations in the PSB channels may not meet "symmetrically", i.e. precisely in the center of the channel but preferably a little on one or the other side. For this configuration the passing stress will be lowered which is in agreement to experimental observations.

Ratcheting Strain and Microstructure Evolution of AZ31B Magnesium Alloy under a Tensile-Tensile Cyclic Loading

PubMed Central

Wang, Denghui; Wang, Wenxian; Zhou, Jun; He, Xiuli; Dong, Peng; Zhang, Hongxia; Sun, Liyong

2018-01-01

In this paper, studies were conducted to investigate the deformation behavior and microstructure change in a hot-rolled AZ31B magnesium alloy during a tensile-tensile cyclic loading. The relationship between ratcheting effect and microstructure change was discussed. The ratcheting effect in the material during current tensile-tensile fatigue loading exceeds the material’s fatigue limit and the development of ratcheting strain in the material experienced three stages: initial sharp increase stage (Stage I); steady stage (Stage II); and final abrupt increase stage (Stage III). Microstructure changes in Stage I and Stage II are mainly caused by activation of basal slip system. The Extra Geometrically Necessary Dislocations (GNDs) were also calculated to discuss the relationship between the dislocation caused by the basal slip system and the ratcheting strain during the cyclic loading. In Stage III, both the basal slip and the {11−20} twins are found active during the crack propagation. The fatigue crack initiation in the AZ31B magnesium alloy is found due to the basal slip and the {11−20} tensile twins. PMID:29597278

Ratcheting Strain and Microstructure Evolution of AZ31B Magnesium Alloy under a Tensile-Tensile Cyclic Loading.

PubMed

Yan, Zhifeng; Wang, Denghui; Wang, Wenxian; Zhou, Jun; He, Xiuli; Dong, Peng; Zhang, Hongxia; Sun, Liyong

2018-03-28

In this paper, studies were conducted to investigate the deformation behavior and microstructure change in a hot-rolled AZ31B magnesium alloy during a tensile-tensile cyclic loading. The relationship between ratcheting effect and microstructure change was discussed. The ratcheting effect in the material during current tensile-tensile fatigue loading exceeds the material's fatigue limit and the development of ratcheting strain in the material experienced three stages: initial sharp increase stage (Stage I); steady stage (Stage II); and final abrupt increase stage (Stage III). Microstructure changes in Stage I and Stage II are mainly caused by activation of basal slip system. The Extra Geometrically Necessary Dislocations (GNDs) were also calculated to discuss the relationship between the dislocation caused by the basal slip system and the ratcheting strain during the cyclic loading. In Stage III, both the basal slip and the {11-20} twins are found active during the crack propagation. The fatigue crack initiation in the AZ31B magnesium alloy is found due to the basal slip and the {11-20} tensile twins.

Dislocation onset and nearly axial glide in carbon nanotubes under torsion

NASA Astrophysics Data System (ADS)

Zhang, D.-B.; James, R. D.; Dumitricǎ, T.

2009-02-01

The torsional plastic response of single-walled carbon nanotubes is studied with tight-binding objective molecular dynamics. In contrast with plasticity under elongation and bending, a torsionally deformed carbon nanotube can slip along a nearly axial helical path, which introduces a distinct (+1,-1) change in wrapping indexes. The low energy realization occurs without loss in mass via nucleation of a 5-7-7-5 dislocation dipole, followed by glide of 5-7 kinks. The possibility of nearly axial glide is supported by the obtained dependence of the plasticity onset on chirality and handedness and by the presented calculations showing the energetic advantage of the slip path and of the initial glide steps.

Compositionally graded relaxed AlGaN buffers on semipolar GaN for mid-ultraviolet emission

DOE Office of Scientific and Technical Information (OSTI.GOV)

Young, Erin C.; Wu Feng; Haeger, Daniel A.

In this Letter, we report on the growth and properties of relaxed, compositionally graded Al{sub x}Ga{sub 1-x}N buffer layers on freestanding semipolar (2021) GaN substrates. Continuous and step compositional grades with Al concentrations up to x = 0.61 have been achieved, with emission wavelengths in the mid-ultraviolet region as low as 265 nm. Coherency stresses were relaxed progressively throughout the grades by misfit dislocation generation via primary (basal) slip and secondary (non-basal) slip systems. Threading dislocation densities in the final layers of the grades were less than 10{sup 6}/cm{sup 2} as confirmed by plan-view transmission electron microscopy and cathodoluminescence studies.

Compositionally graded relaxed AlGaN buffers on semipolar GaN for mid-ultraviolet emission

NASA Astrophysics Data System (ADS)

Young, Erin C.; Wu, Feng; Romanov, Alexey E.; Haeger, Daniel A.; Nakamura, Shuji; Denbaars, Steven P.; Cohen, Daniel A.; Speck, James S.

2012-10-01

In this Letter, we report on the growth and properties of relaxed, compositionally graded AlxGa1 - xN buffer layers on freestanding semipolar (202¯1) GaN substrates. Continuous and step compositional grades with Al concentrations up to x = 0.61 have been achieved, with emission wavelengths in the mid-ultraviolet region as low as 265 nm. Coherency stresses were relaxed progressively throughout the grades by misfit dislocation generation via primary (basal) slip and secondary (non-basal) slip systems. Threading dislocation densities in the final layers of the grades were less than 106/cm2 as confirmed by plan-view transmission electron microscopy and cathodoluminescence studies.

3D discrete dislocation dynamics study of creep behavior in Ni-base single crystal superalloys by a combined dislocation climb and vacancy diffusion model

NASA Astrophysics Data System (ADS)

Gao, Siwen; Fivel, Marc; Ma, Anxin; Hartmaier, Alexander

2017-05-01

A three-dimensional (3D) discrete dislocation dynamics (DDD) creep model is developed to investigate creep behavior under uniaxial tensile stress along the crystallographic [001] direction in Ni-base single crystal superalloys, which takes explicitly account of dislocation glide, climb and vacancy diffusion, but neglects phase transformation like rafting of γ‧ precipitates. The vacancy diffusion model takes internal stresses by dislocations and mismatch strains into account and it is coupled to the dislocation dynamics model in a numerically efficient way. This model is helpful for understanding the fundamental creep mechanisms in superalloys and clarifying the effects of dislocation glide and climb on creep deformation. In cases where the precipitate cutting rarely occurs, e.g. due to the high anti-phase boundary energy and the lack of superdislocations, the dislocation glide in the γ matrix and the dislocation climb along the γ/γ‧ interface dominate plastic deformation. The simulation results show that a high temperature or a high stress both promote dislocation motion and multiplication, so as to cause a large creep strain. Dislocation climb accelerated by high temperature only produces a small plastic strain, but relaxes the hardening caused by the filling γ channels and lets dislocations further glide and multiply. The strongest variation of vacancy concentration occurs in the horizontal channels, where more mixed dislocations exit and tend to climb. The increasing internal stresses due to the increasing dislocation density are easily overcome by dislocations under a high external stress that leads to a long-term dislocation glide accompanied by multiplication.

Singularity-free dislocation dynamics with strain gradient elasticity

NASA Astrophysics Data System (ADS)

Po, Giacomo; Lazar, Markus; Seif, Dariush; Ghoniem, Nasr

2014-08-01

The singular nature of the elastic fields produced by dislocations presents conceptual challenges and computational difficulties in the implementation of discrete dislocation-based models of plasticity. In the context of classical elasticity, attempts to regularize the elastic fields of discrete dislocations encounter intrinsic difficulties. On the other hand, in gradient elasticity, the issue of singularity can be removed at the outset and smooth elastic fields of dislocations are available. In this work we consider theoretical and numerical aspects of the non-singular theory of discrete dislocation loops in gradient elasticity of Helmholtz type, with interest in its applications to three dimensional dislocation dynamics (DD) simulations. The gradient solution is developed and compared to its singular and non-singular counterparts in classical elasticity using the unified framework of eigenstrain theory. The fundamental equations of curved dislocation theory are given as non-singular line integrals suitable for numerical implementation using fast one-dimensional quadrature. These include expressions for the interaction energy between two dislocation loops and the line integral form of the generalized solid angle associated with dislocations having a spread core. The single characteristic length scale of Helmholtz elasticity is determined from independent molecular statics (MS) calculations. The gradient solution is implemented numerically within our variational formulation of DD, with several examples illustrating the viability of the non-singular solution. The displacement field around a dislocation loop is shown to be smooth, and the loop self-energy non-divergent, as expected from atomic configurations of crystalline materials. The loop nucleation energy barrier and its dependence on the applied shear stress are computed and shown to be in good agreement with atomistic calculations. DD simulations of Lome-Cottrell junctions in Al show that the strength of the junction and its configuration are easily obtained, without ad-hoc regularization of the singular fields. Numerical convergence studies related to the implementation of the non-singular theory in DD are presented.

Continuum dislocation-density based models for the dynamic shock response of single-crystal and polycrystalline materials

NASA Astrophysics Data System (ADS)

Luscher, Darby

2017-06-01

The dynamic thermomechanical responses of polycrystalline materials under shock loading are often dominated by the interaction of defects and interfaces. For example, polymer-bonded explosives (PBX) can initiate under weak shock impacts whose energy, if distributed homogeneously throughout the material, translates to temperature increases that are insufficient to drive the rapid chemistry observed. In such cases, heterogeneous thermomechanical interactions at the mesoscale (i.e. between single-crystal and macroscale) lead to the formation of localized hot spots. Within metals, a prescribed deformation associated with a shock wave may be accommodated by crystallographic slip, provided a sufficient population of mobile dislocations is available. However, if the deformation rate is large enough, there may be an insufficient number of freely mobile dislocations. In these cases, additional dislocations may be nucleated, or alternate mechanisms (e.g. twinning, damage) activated in order to accommodate the deformation. Direct numerical simulation at the mesoscale offers insight into these physical processes that can be invaluable to the development of macroscale constitutive theories, if the mesoscale models adequately represent the anisotropic nonlinear thermomechanical response of individual crystals and their interfaces. This talk will briefly outline a continuum mesoscale modeling framework founded upon local and nonlocal variations of dislocation-density based crystal plasticity theory. The nonlocal theory couples continuum dislocation transport with the local theory. In the latter, dislocation transport is modeled by enforcing dislocation conservation at a slip-system level through the solution of advection-diffusion equations. The configuration of geometrically necessary dislocation density gives rise to a back-stress that inhibits or accentuates the flow of dislocations. Development of the local theory and application to modeling the explosive molecular crystal RDX and polycrystalline PBX will be discussed. The talk will also emphasize recent implementation of the coupled nonlocal model into a 3D shock hydrocode and simulation results for the dynamic response of polycrystalline copper in two and three dimensions.

Deformation response of cube-on-cube and non-coherent twin interfaces in AgCu eutectic after dynamic plastic compression

DOE Office of Scientific and Technical Information (OSTI.GOV)

Eftink, Benjamin P.; Mara, Nathan Allan; Kingstedt, Owen T.

For this research, Split-Hopkinson pressure bar dynamic compression experiments were conducted to determine the defect/interface interaction dependence on interface type, bilayer thickness and interface orientation with respect to the loading direction in the Ag-Cu eutectic system. Specifically, the deformation microstructure in alloys with either a cube-on-cube orientation relationship with {111} Ag||{111} Cu interface habit planes or a twin orientation relationship with {more » $$\\overline{3}13$$} Ag||{$$\\overline{1}12$$} Cu interface habit planes and with bilayer thicknesses of 500 nm, 1.1 µm and 2.2 µm were probed using TEM. The deformation was carried by dislocation slip and in certain conditions, deformation twinning. The twinning response was dependent on loading orientation with respect to the interface plane, bilayer thickness, and interface type. Twinning was only observed when loading at orientations away from the growth direction and decreased in prevalence with decreasing bilayer thickness. Twinning in Cu was dependent on twinning partial dislocations being transmitted from Ag, which only occurred for cube-on-cube interfaces. Lastly, dislocation slip and deformation twin transfer across the interfaces is discussed in terms of the slip transfer conditions developed for grain boundaries in FCC alloys.« less

Deformation response of cube-on-cube and non-coherent twin interfaces in AgCu eutectic after dynamic plastic compression

DOE PAGES

Eftink, Benjamin P.; Mara, Nathan Allan; Kingstedt, Owen T.; ...

2017-12-02

For this research, Split-Hopkinson pressure bar dynamic compression experiments were conducted to determine the defect/interface interaction dependence on interface type, bilayer thickness and interface orientation with respect to the loading direction in the Ag-Cu eutectic system. Specifically, the deformation microstructure in alloys with either a cube-on-cube orientation relationship with {111} Ag||{111} Cu interface habit planes or a twin orientation relationship with {more » $$\\overline{3}13$$} Ag||{$$\\overline{1}12$$} Cu interface habit planes and with bilayer thicknesses of 500 nm, 1.1 µm and 2.2 µm were probed using TEM. The deformation was carried by dislocation slip and in certain conditions, deformation twinning. The twinning response was dependent on loading orientation with respect to the interface plane, bilayer thickness, and interface type. Twinning was only observed when loading at orientations away from the growth direction and decreased in prevalence with decreasing bilayer thickness. Twinning in Cu was dependent on twinning partial dislocations being transmitted from Ag, which only occurred for cube-on-cube interfaces. Lastly, dislocation slip and deformation twin transfer across the interfaces is discussed in terms of the slip transfer conditions developed for grain boundaries in FCC alloys.« less

A hierarchical dislocation-grain boundary interaction model based on 3D discrete dislocation dynamics and molecular dynamics

NASA Astrophysics Data System (ADS)

Gao, Yuan; Zhuang, Zhuo; You, XiaoChuan

2011-04-01

We develop a new hierarchical dislocation-grain boundary (GB) interaction model to predict the mechanical behavior of polycrystalline metals at micro and submicro scales by coupling 3D Discrete Dislocation Dynamics (DDD) simulation with the Molecular Dynamics (MD) simulation. At the microscales, the DDD simulations are responsible for capturing the evolution of dislocation structures; at the nanoscales, the MD simulations are responsible for obtaining the GB energy and ISF energy which are then transferred hierarchically to the DDD level. In the present model, four kinds of dislocation-GB interactions, i.e. transmission, absorption, re-emission and reflection, are all considered. By this methodology, the compression of a Cu micro-sized bi-crystal pillar is studied. We investigate the characteristic mechanical behavior of the bi-crystal compared with that of the single-crystal. Moreover, the comparison between the present penetrable model of GB and the conventional impenetrable model also shows the accuracy and efficiency of the present model.

Measuring strain and rotation fields at the dislocation core in graphene

NASA Astrophysics Data System (ADS)

Bonilla, L. L.; Carpio, A.; Gong, C.; Warner, J. H.

2015-10-01

Strain fields, dislocations, and defects may be used to control electronic properties of graphene. By using advanced imaging techniques with high-resolution transmission electron microscopes, we have measured the strain and rotation fields about dislocations in monolayer graphene with single-atom sensitivity. These fields differ qualitatively from those given by conventional linear elasticity. However, atom positions calculated from two-dimensional (2D) discrete elasticity and three-dimensional discrete periodized Föppl-von Kármán equations (dpFvKEs) yield fields close to experiments when determined by geometric phase analysis. 2D theories produce symmetric fields whereas those from experiments exhibit asymmetries. Numerical solutions of dpFvKEs provide strain and rotation fields of dislocation dipoles and pairs that also exhibit asymmetries and, compared with experiments, may yield information on out-of-plane displacements of atoms. While discrete theories need to be solved numerically, analytical formulas for strains and rotation about dislocations can be obtained from 2D Mindlin's hyperstress theory. These formulas are very useful for fitting experimental data and provide a template to ascertain the importance of nonlinear and nonplanar effects. Measuring the parameters of this theory, we find two characteristic lengths between three and four times the lattice spacings that control dilatation and rotation about a dislocation. At larger distances from the dislocation core, the elastic fields decay to those of conventional elasticity. Our results may be relevant for strain engineering in graphene and other 2D materials of current interest.

Mechanisms of strain accommodation in plastically-deformed zircon under simple shear deformation conditions during amphibolite-facies metamorphism

NASA Astrophysics Data System (ADS)

Kovaleva, Elizaveta; Klötzli, Urs; Wheeler, John; Habler, Gerlinde

2018-02-01

This study documents the strain accommodation mechanisms in zircon under amphibolite-facies metamorphic conditions in simple shear. Microstructural data from undeformed, fractured and crystal-plastically deformed zircon crystals are described in the context of the host shear zone, and evaluated in the light of zircon elastic anisotropy. Our work challenges the existing model of zircon evolution and shows previously undescribed rheological characteristics for this important accessory mineral. Crystal-plastically deformed zircon grains have axis oriented parallel to the foliation plane, with the majority of deformed grains having axis parallel to the lineation. Zircon accommodates strain by a network of stepped low-angle boundaries, formed by switching between tilt dislocations with the slip systems <100>{010} and < 1 bar 10>{110} and rotation axis [001], twist dislocations with the rotation axis [001], and tilt dislocations with the slip system <100>{001} and rotation axis [010]. The slip system < 1 bar 10>{110} is newly described for zircon. Most misorientation axes in plastically-deformed zircon grains are parallel to the XY plane of the sample and have [001] crystallographic direction. Such behaviour of strained zircon lattice is caused by elastic anisotropy that has a direct geometric control on the rheology, deformation mechanisms and dominant slip systems in zircon. Young's modulus and P wave velocity have highest values parallel to zircon [001] axis, indicating that zircon is elastically strong along this direction. Poisson ratio and Shear modulus demonstrate that zircon is also most resistant to shearing along [001]. Thus, [001] axis is the most common rotation axis in zircon. The described zircon behaviour is important to take into account during structural and geochronological investigations of (poly)metamorphic terrains. Geometry of dislocations in zircon may help reconstructing the geometry of the host shear zone(s), large-scale stresses in the crust, and, possibly, the timing of deformation, if the isotopic systems of deformed zircon were reset.

Size effects under homogeneous deformation of single crystals: A discrete dislocation analysis

NASA Astrophysics Data System (ADS)

Guruprasad, P. J.; Benzerga, A. A.

Mechanism-based discrete dislocation plasticity is used to investigate the effect of size on micron scale crystal plasticity under conditions of macroscopically homogeneous deformation. Long-range interactions among dislocations are naturally incorporated through elasticity. Constitutive rules are used which account for key short-range dislocation interactions. These include junction formation and dynamic source and obstacle creation. Two-dimensional calculations are carried out which can handle high dislocation densities and large strains up to 0.1. The focus is laid on the effect of dimensional constraints on plastic flow and hardening processes. Specimen dimensions ranging from hundreds of nanometers to tens of microns are considered. Our findings show a strong size-dependence of flow strength and work-hardening rate at the micron scale. Taylor-like hardening is shown to be insufficient as a rationale for the flow stress scaling with specimen dimensions. The predicted size effect is associated with the emergence, at sufficient resolution, of a signed dislocation density. Heuristic correlations between macroscopic flow stress and macroscopic measures of dislocation density are sought. Most accurate among those is a correlation based on two state variables: the total dislocation density and an effective, scale-dependent measure of signed density.

Characteristics of dislocation structure in creep deformed lamellar tial alloy within primary regime

NASA Astrophysics Data System (ADS)

Cho, H. S.; Nam, Soo W.

1999-06-01

In this investigation, dislocations of a lamellar TiAl alloy are analyzed after creeping in the primary range at 800°C/200MPa in order to interpret their mobility It was found that the dislocation density in γ-laths decreased as the creep deformation proceeds within primary creep regime Schmid factor analysis suggests that the creep deformation in the early stage of the primary creep regime is controlled by the gliding of some of the initial dislocations which have a high enough Schmid factor As the creep deformation progressed, those dislocations with high Schmid factors slip preferentially to be annihilated at the α-γ interface For further continuous deformation, dislocation generation is required, and for this, α-phase is transformed to γ-phase in order to generate new dislocations A slow dislocation generation process by phase transformation of α-phase compared with the absorbing rate to sinks is responsible for the decreasing dislocation density as the creep strain increases

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Lehao

A nanocrystalline surface layer was produced on Mg-3 wt.% Li-6 wt.% Al alloy by means of surface mechanical attrition treatment (SMAT). Microstructure features of various sections were systematically characterized by transmission electron microscopy. The results indicate that grain refinement induced by SMAT is dominated mainly by dislocation slip. Twinning is active at the early stage of grain refinement process when the grain size is large. The dislocation-dominated deformation mechanism is attributed to the change of c/a ratio due to the alloying of Li in Mg matrix and the suppression of twinning due to grain refinement. Nanoindentation results show that themore » hardness of the surface is enhanced by SMAT. - Highlights: •Nanocrystalline surface layer was produced on Mg-3 wt.%Li-6 wt.%Al alloy by SMAT. •Grain refinement induced by SMAT is dominated mainly by dislocation slip. •Twinning is active at the early stage of grain refinement process. •The hardness of the surface was enhanced by SMAT.« less

Hardening mechanisms in olivine single crystal deformed at 1090 °C: an electron tomography study

NASA Astrophysics Data System (ADS)

Mussi, Alexandre; Cordier, Patrick; Demouchy, Sylvie; Hue, Benoit

2017-11-01

The dislocation microstructures in a single crystal of olivine deformed experimentally in uniaxial compression at 1090 °C and under a confining pressure of 300 MPa, have been investigated by transmission electron tomography in order to better understand deformation mechanisms at the microscale relevant for lithospheric mantle deformations. Investigation by electron tomography reveals microstructures, which are more complex than previously described, composed of ? and ? dislocations commonly exhibiting 3D configurations. Numerous mechanisms such as climb, cross-slip, double cross-slip as well as interactions like junction formations and collinear annihilations are the source of this complexity. The diversity observed advocates for microscale deformation of olivine significantly less simple than classic dislocation creep reported in metals or ice close to melting temperature. Deciphering mechanism of hardening in olivine at temperatures where ionic diffusion is slow and is then expected to play very little role is crucial to better understand and thus model deformation at larger scale and at temperatures (900-1100 °C) highly relevant for the lithospheric mantle.

Coupled DDD-FEM modeling on the mechanical behavior of microlayered metallic multilayer film at elevated temperature

NASA Astrophysics Data System (ADS)

Huang, Minsheng; Li, Zhenhuan

2015-12-01

To investigate the mechanical behavior of the microlayered metallic thin films (MMMFs) at elevated temperature, an enhanced discrete-continuous model (DCM), which couples rather than superposes the two-dimensional climb/glide-enabled discrete dislocation dynamics (2D-DDD) with the linearly elastic finite element method (FEM), is developed in this study. In the present coupling scheme, two especial treatments are made. One is to solve how the plastic strain captured by the DDD module is transferred properly to the FEM module as an eigen-strain; the other is to answer how the stress field computationally obtained by the FEM module is transferred accurately to the DDD module to drive those discrete dislocations moving correctly. With these two especial treatments, the interactions between adjacent dislocations and between dislocation pile-ups and inter-phase boundaries (IBs), which are crucial to the strengthening effect in MMMFs, are carefully taken into account. After verified by comparing the computationally predicted results with the theoretical solutions for a dislocation residing in a homogeneous material and nearby a bi-material interface, this 2D-DDD/FEM coupling scheme is used to model the tensile mechanical behaviors of MMMFs at elevated temperature. The strengthening mechanism of MMMFs and the layer thickness effect are studied in detail, with special attentions to the influence of dislocation climb on them.

Dislocation nucleation from symmetric tilt grain boundaries in body-centered cubic vanadium

NASA Astrophysics Data System (ADS)

Xu, Shuozhi; Su, Yanqing

2018-05-01

We perform molecular dynamics (MD) simulations with two interatomic potentials to study dislocation nucleation from six symmetric tilt grain boundaries (GB) using bicrystal models in body-centered cubic vanadium. The influences of the misorientation angle are explored in the context of activated slip systems, critical resolved shear stress (CRSS), and GB energy. It is found that for four GBs, the activated slip systems are not those with the highest Schmid factor, i.e., the Schmid law breaks down. For all misorientation angles, the bicrystal is associated with a lower CRSS than their single crystalline counterparts. Moreover, the GB energy decreases in compressive loading at the yield point with respect to the undeformed configuration, in contrast to tensile loading.

The complexity of non-Schmid behavior in the CuZnAl shape memory alloy

NASA Astrophysics Data System (ADS)

Alkan, S.; Ojha, A.; Sehitoglu, H.

2018-05-01

The paper addresses one of the most important yet overlooked phenomenon in shape memory research- the plastic slip response. We show that the slip response is highly crystal orientation dependent and we demonstrate the precise reasons behind such complex response. The fractional dislocations on <111> {112} or <111> {011} systems can be activated depending on the sample orientation and solutions are derived for the variations in disregistries and dislocation core spreadings. This leads to the calculation of critical resolved shear stress in close agreement with experimental trends. The results show considerable dependence of the flow behavior on the non-Schmid stress components and the proposed yield criterion captures the role of stress tensor components.

In-situ neutron diffraction study on the tension-compression fatigue behavior of a twinning induced plasticity steel

DOE PAGES

Xie, Qingge; Liang, Jiangtao; Stoica, Alexandru Dan; ...

2017-05-17

Grain orientation dependent behavior during tension-compression type of fatigue loading in a TWIP steel was studied using in-situ neutron diffraction. Orientation zones with dominant behavior of (1) twinning-de-twinning, (2) twinning-re-twinning followed by twinning-de-twinning, (3) twinning followed by dislocation slip and (4) dislocation slip were identified. Jumps of the orientation density were evidenced in neutron diffraction peaks which explains the macroscopic asymmetric behavior. The asymmetric behavior in early stage of fatigue loading is mainly due to small volume fraction of twins in comparison with that at later stage. As a result, easy activation of the de-twin makes the macroscopically unloading behaviormore » nonlinear.« less

In-situ neutron diffraction study on the tension-compression fatigue behavior of a twinning induced plasticity steel

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xie, Qingge; Liang, Jiangtao; Stoica, Alexandru Dan

Grain orientation dependent behavior during tension-compression type of fatigue loading in a TWIP steel was studied using in-situ neutron diffraction. Orientation zones with dominant behavior of (1) twinning-de-twinning, (2) twinning-re-twinning followed by twinning-de-twinning, (3) twinning followed by dislocation slip and (4) dislocation slip were identified. Jumps of the orientation density were evidenced in neutron diffraction peaks which explains the macroscopic asymmetric behavior. The asymmetric behavior in early stage of fatigue loading is mainly due to small volume fraction of twins in comparison with that at later stage. As a result, easy activation of the de-twin makes the macroscopically unloading behaviormore » nonlinear.« less

Morphology of single Shockley-type stacking faults generated by recombination enhanced dislocation glide in 4H-SiC

NASA Astrophysics Data System (ADS)

Matsuhata, Hirofumi; Sekiguchi, Takashi

2018-04-01

Morphology of single Shockley-type stacking faults (SFs) generated by recombination enhanced dislocation glide (REDG) in 4H-SiC are discussed and analysed. A complete set of the 12 different dissociated states of basal-plane dislocation loops is obtained using the crystallographic space group operations. From this set, six different double rhombic-shaped SFs are derived. These tables indicate the rules that connect shapes of SFs with the locations of partial dislocations having different core structures, the positions of slip planes in a unit cell, and the Burgers vectors of partial dislocations. We applied these tables for the analysis of SFs generated by the REDG effect reported in the past articles. Shapes, growing process of SFs and perfect dislocations for origins of SFs were well analysed systematically.

Internal stresses, dislocation mobility and ductility

NASA Astrophysics Data System (ADS)

Saada, G.

1991-06-01

The description of plastic deformation must take into account individual mechanisms and heterogeneity of plastic strain. Influence of dislocation interaction with forest dislocations and of cross slip are connected with the organization of dipole walls. The latter are described and their development is explained as a consequence of edge effects. Applications are discussed. La description de la déformation plastique doit prendre en compte les interactions individuelles des dislocations et l'hétérogénéité à grande échelle de la déformation plastique. Les interactions des dislocations mobiles avec la forêt de dislocations, le glissement dévié, ont pour effet la création de parois dipolaires. Celles-ci sont décrites et leur développement est appliqué à partir des effets de bord.

Discrete Dislocation Dynamics Simulations of Twin Size-Effects in Magnesium

DTIC Science & Technology

2015-01-01

deformation induced softening. Over the past two decades, discrete dislocation dynamics ( DDD ) has been one of the most efficient methods to capture...14] and intermittent behavior [15] of the FCC and BCC materials. More recently, DDD simulations of Mg investigated a number of important effects...plays an important and sometimes dominant role in the mechanical behavior of both single crystals and polycrystals. As a result, such DDD simulations

Comparison of dislocation density tensor fields derived from discrete dislocation dynamics and crystal plasticity simulations of torsion

DOE PAGES

Jones, Reese E.; Zimmerman, Jonathan A.; Po, Giacomo; ...

2016-02-01

Accurate simulation of the plastic deformation of ductile metals is important to the design of structures and components to performance and failure criteria. Many techniques exist that address the length scales relevant to deformation processes, including dislocation dynamics (DD), which models the interaction and evolution of discrete dislocation line segments, and crystal plasticity (CP), which incorporates the crystalline nature and restricted motion of dislocations into a higher scale continuous field framework. While these two methods are conceptually related, there have been only nominal efforts focused at the global material response that use DD-generated information to enhance the fidelity of CPmore » models. To ascertain to what degree the predictions of CP are consistent with those of DD, we compare their global and microstructural response in a number of deformation modes. After using nominally homogeneous compression and shear deformation dislocation dynamics simulations to calibrate crystal plasticity ow rule parameters, we compare not only the system-level stress-strain response of prismatic wires in torsion but also the resulting geometrically necessary dislocation density fields. To establish a connection between explicit description of dislocations and the continuum assumed with crystal plasticity simulations we ascertain the minimum length-scale at which meaningful dislocation density fields appear. Furthermore, our results show that, for the case of torsion, that the two material models can produce comparable spatial dislocation density distributions.« less

Spontaneous Athermal Cross-Slip Nucleation at Screw Dislocation Intersections in FCC Metals and L1(2) Intermetallics Investigated via Atomistic Simulations

DTIC Science & Technology

2013-01-01

LAMMPS [12], developed at Sandia National Labora- tory. The simulation cell is a rectangular parallelepiped having the x-axis oriented along the [1 1 0...cross-slip during deformation. Acknowledgements The authors acknowledge use of the 3d molecular dynamics code, LAMMPS , which was developed at Sandia

Automated identification and indexing of dislocations in crystal interfaces

DOE PAGES

Stukowski, Alexander; Bulatov, Vasily V.; Arsenlis, Athanasios

2012-10-31

Here, we present a computational method for identifying partial and interfacial dislocations in atomistic models of crystals with defects. Our automated algorithm is based on a discrete Burgers circuit integral over the elastic displacement field and is not limited to specific lattices or dislocation types. Dislocations in grain boundaries and other interfaces are identified by mapping atomic bonds from the dislocated interface to an ideal template configuration of the coherent interface to reveal incompatible displacements induced by dislocations and to determine their Burgers vectors. Additionally, the algorithm generates a continuous line representation of each dislocation segment in the crystal andmore » also identifies dislocation junctions.« less

Discrete dislocation plasticity analysis of loading rate-dependent static friction.

PubMed

Song, H; Deshpande, V S; Van der Giessen, E

2016-08-01

From a microscopic point of view, the frictional force associated with the relative sliding of rough surfaces originates from deformation of the material in contact, by adhesion in the contact interface or both. We know that plastic deformation at the size scale of micrometres is not only dependent on the size of the contact, but also on the rate of deformation. Moreover, depending on its physical origin, adhesion can also be size and rate dependent, albeit different from plasticity. We present a two-dimensional model that incorporates both discrete dislocation plasticity inside a face-centred cubic crystal and adhesion in the interface to understand the rate dependence of friction caused by micrometre-size asperities. The friction strength is the outcome of the competition between adhesion and discrete dislocation plasticity. As a function of contact size, the friction strength contains two plateaus: at small contact length [Formula: see text], the onset of sliding is fully controlled by adhesion while for large contact length [Formula: see text], the friction strength approaches the size-independent plastic shear yield strength. The transition regime at intermediate contact size is a result of partial de-cohesion and size-dependent dislocation plasticity, and is determined by dislocation properties, interfacial properties as well as by the loading rate.

Hydrogenated vacancies lock dislocations in aluminium

PubMed Central

Xie, Degang; Li, Suzhi; Li, Meng; Wang, Zhangjie; Gumbsch, Peter; Sun, Jun; Ma, Evan; Li, Ju; Shan, Zhiwei

2016-01-01

Due to its high diffusivity, hydrogen is often considered a weak inhibitor or even a promoter of dislocation movements in metals and alloys. By quantitative mechanical tests in an environmental transmission electron microscope, here we demonstrate that after exposing aluminium to hydrogen, mobile dislocations can lose mobility, with activating stress more than doubled. On degassing, the locked dislocations can be reactivated under cyclic loading to move in a stick-slip manner. However, relocking the dislocations thereafter requires a surprisingly long waiting time of ∼103 s, much longer than that expected from hydrogen interstitial diffusion. Both the observed slow relocking and strong locking strength can be attributed to superabundant hydrogenated vacancies, verified by our atomistic calculations. Vacancies therefore could be a key plastic flow localization agent as well as damage agent in hydrogen environment. PMID:27808099

Consideration of the oxide particle-dislocation interaction in 9Cr-ODS steel

NASA Astrophysics Data System (ADS)

Ijiri, Yuta; Oono, N.; Ukai, S.; Yu, Hao; Ohtsuka, S.; Abe, Y.; Matsukawa, Y.

2017-05-01

The interaction between oxide particles and dislocations in a 9Cr-ODS ferritic steel is investigated by both static and in situ TEM observation under dynamic straining conditions and room temperature. The measured obstacle strength (?) of the oxide particles was no greater than 0.80 and the average was 0.63. The dislocation loops around some coarsened particles were also observed. The calculated obstacle strength by a stress formula of the Orowan interaction is nearly equaled to the average experimental value. Not only cross-slip system but also the Orowan interaction should be considered as the main interaction mechanism between oxide particles and dislocation in 9CrODS ferritic steel.

Crack Tip Dislocation Nucleation in FCC Solids

NASA Astrophysics Data System (ADS)

Knap, J.; Sieradzki, K.

1999-02-01

We present results of molecular dynamic simulations aimed at examining crack tip dislocation emission in fcc solids. The results are analyzed in terms of recent continuum formulations of this problem. In mode II, Au, Pd, and Pt displayed a new unanticipated mechanism of crack tip dislocation emission involving the creation of a pair of Shockley partials on a slip plane one plane below the crack plane. In mode I, for all the materials examined, Rice's continuum formulation [J. Mech. Phys. Solids 40, 239 (1992)] underestimated the stress intensity for dislocation emission by almost a factor of 2. Surface stress corrections to the emission criterion brought the agreement between continuum predictions and simulations to within 20%.

Characterization of dislocation structures and deformation mechanisms in as-grown and deformed directionally solidified NiAl–Mo composites

DOE PAGES

Kwon, J.; Bowers, M. L.; Brandes, M. C.; ...

2015-02-26

In this paper, directionally solidified (DS) NiAl–Mo eutectic composites were strained to plastic strain values ranging from 0% to 12% to investigate the origin of the previously observed stochastic versus deterministic mechanical behaviors of Mo-alloy micropillars in terms of the development of dislocation structures at different pre-strain levels. The DS composites consist of long, [1 0 0] single-crystal Mo-alloy fibers with approximately square cross-sections embedded in a [1 0 0] single-crystal NiAl matrix. Scanning transmission electron microscopy (STEM) and computational stress state analysis were conducted for the current study. STEM of the as-grown samples (without pre-straining) reveal no dislocations inmore » the investigated Mo-alloy fibers. In the NiAl matrix, on the other hand, a(1 0 0)-type dislocations exist in two orthogonal orientations: along the [1 0 0] Mo fiber axis, and wrapped around the fiber axis. They presumably form to accommodate the different thermal contractions of the two phases during cool down after eutectic solidification. At intermediate pre-strain levels (4–8%), a/2(1 1 1)-type dislocations are present in the Mo-alloy fibers and the pre-existing dislocations in the NiAl matrix seem to be swept toward the interphase boundary. Some of the dislocations in the Mo-alloy fibers appear to be transformed from a(1 0 0)-type dislocations present in the NiAl matrix. Subsequently, the transformed dislocations in the fibers propagate through the NiAl matrix as a(1 1 1) dislocations and aid in initiating additional slip bands in adjacent fibers. Thereafter, co-deformation presumably occurs by (1 1 1) slip in both phases. With a further increase in the pre-strain level (>10%), multiple a/2(1 1 1)-type dislocations are observed in many locations in the Mo-alloy fibers. Interactions between these systems upon subsequent deformation could lead to stable junctions and persistent dislocation sources. Finally, the transition from stochastic to deterministic, bulk-like behavior in sub-micron Mo-alloy pillars may therefore be related to an increasing number of multiple a(1 1 1) dislocation systems within the Mo fibers with increasing pre-strain, considering that the bulk-like behavior is governed by the forest hardening of these junctions.« less

On the role of fluids in stick-slip dynamics of saturated granular fault gouge using a coupled computational fluid dynamics-discrete element approach

NASA Astrophysics Data System (ADS)

Dorostkar, Omid; Guyer, Robert A.; Johnson, Paul A.; Marone, Chris; Carmeliet, Jan

2017-05-01

The presence of fault gouge has considerable influence on slip properties of tectonic faults and the physics of earthquake rupture. The presence of fluids within faults also plays a significant role in faulting and earthquake processes. In this paper, we present 3-D discrete element simulations of dry and fluid-saturated granular fault gouge and analyze the effect of fluids on stick-slip behavior. Fluid flow is modeled using computational fluid dynamics based on the Navier-Stokes equations for an incompressible fluid and modified to take into account the presence of particles. Analysis of a long time train of slip events shows that the (1) drop in shear stress, (2) compaction of granular layer, and (3) the kinetic energy release during slip all increase in magnitude in the presence of an incompressible fluid, compared to dry conditions. We also observe that on average, the recurrence interval between slip events is longer for fluid-saturated granular fault gouge compared to the dry case. This observation is consistent with the occurrence of larger events in the presence of fluid. It is found that the increase in kinetic energy during slip events for saturated conditions can be attributed to the increased fluid flow during slip. Our observations emphasize the important role that fluid flow and fluid-particle interactions play in tectonic fault zones and show in particular how discrete element method (DEM) models can help understand the hydromechanical processes that dictate fault slip.

Dislocation mediated alignment during metal nanoparticle coalescence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lange, A. P.; Samanta, A.; Majidi, H.

2016-09-13

Dislocation mediated alignment processes during gold nanoparticle coalescence were studied at low and high temperatures using molecular dynamics simulations and transmission electron microscopy. Particles underwent rigid body rotations immediately following attachment in both low temperature (500 K) simulated coalescence events and low temperature (~315 K) transmission electron microscopy beam heating experiments. In many low temperature simulations, some degree of misorientation between particles remained after rigid body rotations, which was accommodated by grain boundary dislocation nodes. These dislocations were either sessile and remained at the interface for the duration of the simulation or dissociated and cross-slipped through the adjacent particles, leadingmore » to improved co-alignment. Minimal rigid body rotations were observed during or immediately following attachment in high temperature (1100 K) simulations, which is attributed to enhanced diffusion at the particles' interface. However, rotation was eventually induced by {111} slip on planes parallel to the neck groove. These deformation modes led to the formation of single and multi-fold twins whose structures depended on the initial orientation of the particles. The driving force for {111} slip is attributed to high surface stresses near the intersection of low energy {111} facets in the neck region. The details of this twinning process were examined in detail using simulated trajectories, and the results reveal possible mechanisms for the nucleation and propagation of Shockley partials on consecutive planes. Deformation twinning was also observed in-situ using transmission electron microscopy, which resulted in the co-alignment of a set of the particles' {111} planes across their grain boundary and an increase in their dihedral angle. As a result, this constitutes the first detailed experimental observation of deformation twinning during nanoparticle coalescence, validating simulation results presented here and elsewhere.« less

On the role of fluids in stick-slip dynamics of saturated granular fault gouge using a coupled computational fluid dynamics-discrete element approach: STICK-SLIP IN SATURATED FAULT GOUGE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dorostkar, Omid; Guyer, Robert A.; Johnson, Paul A.

The presence of fault gouge has considerable influence on slip properties of tectonic faults and the physics of earthquake rupture. The presence of fluids within faults also plays a significant role in faulting and earthquake processes. In this study, we present 3-D discrete element simulations of dry and fluid-saturated granular fault gouge and analyze the effect of fluids on stick-slip behavior. Fluid flow is modeled using computational fluid dynamics based on the Navier-Stokes equations for an incompressible fluid and modified to take into account the presence of particles. Analysis of a long time train of slip events shows that themore » (1) drop in shear stress, (2) compaction of granular layer, and (3) the kinetic energy release during slip all increase in magnitude in the presence of an incompressible fluid, compared to dry conditions. We also observe that on average, the recurrence interval between slip events is longer for fluid-saturated granular fault gouge compared to the dry case. This observation is consistent with the occurrence of larger events in the presence of fluid. It is found that the increase in kinetic energy during slip events for saturated conditions can be attributed to the increased fluid flow during slip. Finally, our observations emphasize the important role that fluid flow and fluid-particle interactions play in tectonic fault zones and show in particular how discrete element method (DEM) models can help understand the hydromechanical processes that dictate fault slip.« less

On the role of fluids in stick-slip dynamics of saturated granular fault gouge using a coupled computational fluid dynamics-discrete element approach: STICK-SLIP IN SATURATED FAULT GOUGE

DOE PAGES

Dorostkar, Omid; Guyer, Robert A.; Johnson, Paul A.; ...

2017-05-01

The presence of fault gouge has considerable influence on slip properties of tectonic faults and the physics of earthquake rupture. The presence of fluids within faults also plays a significant role in faulting and earthquake processes. In this study, we present 3-D discrete element simulations of dry and fluid-saturated granular fault gouge and analyze the effect of fluids on stick-slip behavior. Fluid flow is modeled using computational fluid dynamics based on the Navier-Stokes equations for an incompressible fluid and modified to take into account the presence of particles. Analysis of a long time train of slip events shows that themore » (1) drop in shear stress, (2) compaction of granular layer, and (3) the kinetic energy release during slip all increase in magnitude in the presence of an incompressible fluid, compared to dry conditions. We also observe that on average, the recurrence interval between slip events is longer for fluid-saturated granular fault gouge compared to the dry case. This observation is consistent with the occurrence of larger events in the presence of fluid. It is found that the increase in kinetic energy during slip events for saturated conditions can be attributed to the increased fluid flow during slip. Finally, our observations emphasize the important role that fluid flow and fluid-particle interactions play in tectonic fault zones and show in particular how discrete element method (DEM) models can help understand the hydromechanical processes that dictate fault slip.« less

Pressure Dependence of the Peierls Stress in Aluminum

NASA Astrophysics Data System (ADS)

Dang, Khanh; Spearot, Douglas

2018-03-01

The effect of pressure applied normal to the {111} slip plane on the Peierls stress in Al is studied via atomistic simulations. Edge, screw, 30°, and 60° straight dislocations are created using the Volterra displacement fields for isotropic elasticity. For each dislocation character angle, the Peierls stress is calculated based on the change in the internal energy, which is an invariant measure of the dislocation driving force. It is found that the Peierls stress for dislocations under zero pressure is in general agreement with previous results. For screw and 60° dislocations, the Peierls stress versus pressure relationship has maximum values associated with stacking fault widths that are multiples of the Peierls period. For the edge dislocation, the Peierls stress decreases with increasing pressure from tension to compression. Compared with the Mendelev potential, the Peierls stress calculated from the Mishin potential is more sensitive to changes in pressure.

Identical activation volumes of dislocation mobility in the [100](010) and [001](010) slip systems in natural olivine

NASA Astrophysics Data System (ADS)

Wang, Lin; Blaha, Stephan; Kawazoe, Takaaki; Miyajima, Nobuyoshi; Katsura, Tomoo

2017-03-01

Dislocation recovery experiments were performed on predeformed olivine single crystals at pressures of 2, 7 and 12 GPa and a constant temperature of 1650 K to determine the pressure dependence of the annihilation rate constants for [100](010) edge dislocation (a dislocation) and [001](010) screw dislocation (c dislocation). The constants of both types of dislocations are comparable within 0.3 orders of magnitude. The activation volumes of a and c dislocations are small and identical within error: 2.7 ± 0.2 and 2.5 ± 0.9 cm3/mol, respectively. These values are slightly larger and smaller than those of Si lattice and grain-boundary diffusions in olivine, respectively. The small and identical activation volumes for the a and c dislocations suggest that the pressure-induced fabric transition is unlikely in the asthenosphere. The decrease in seismic anisotropy with depth down in the asthenosphere may be caused by the fabric transition from A type or B type to AG type with decreasing stress with depth.

The Fundamentals of Dislocation Transport of Hydrogen in BCC Iron.

DTIC Science & Technology

1984-10-01

4.2.1. Single Crystal Material 24 4.2.2. Polycrystalline Material 25 4.3. Single Crystal Orientation Determination 25 4.4. Straining Permeation Test 27...Test 45 4.6. Supersaturation Study 47 S. RESULTS AND DISCUSSION 50 5.1. Single Crystal Orientation Determination 50 5.1.1. Slip System Determination 58...Orientation 162 Determination B.1. Dislocation Line Direction Determination 162 B.2. Burgers Vector Determination 164

Mechanism of underthrusting in southwest Japan: A model of convergent plate interactions

NASA Technical Reports Server (NTRS)

Fitch, T. J.; Scholz, C. H.

1971-01-01

An elastic rebound mechanism consistent with underthrusting at the time of the magnitude 8.2 Nankaido earthquake of 21 December, 1946 accounts for a reversal in sense between seismic and preseismic changes in elevation throughout a large portion of southwest Japan. It is shown that the seismic movements during the earthquake were generated by slip on a complex thrust fault that is inferred to intersect the surface near the base of the continental slope. The preseismic deformation is explained by strain accumulation equivalent to a virtual dislocation with the same orientation as the fault surface but a sense opposite to that of the real dislocation that occurred at the time of the earthquake. Adjustments by slip along the fault surface and extensions of that surface can account for postseismic movements.

Recovery of strain-hardening rate in Ni-Si alloys.

PubMed

Yang, C L; Zhang, Z J; Cai, T; Zhang, P; Zhang, Z F

2015-10-21

In this study, the recovery of strain-hardening rate (RSHR) was discovered for the first time in polycrystalline materials (Ni-Si alloys) that have only dislocation activities during tensile test. Detailed microstructure characterizations show that the activation of dislocations in the secondary slip systems during tensile deformation is the major reason for this RSHR. By taking into account other metals that also exhibit RSHR during tension, a more general mechanism for the RSHR was proposed, i.e. the occurrence of a sharp decrease of dislocation mean free path (Λ) during plastic deformation, caused by either planar defects or linear defects.

Recovery of strain-hardening rate in Ni-Si alloys

PubMed Central

Yang, C. L.; Zhang, Z. J.; Cai, T.; Zhang, P.; Zhang, Z. F.

2015-01-01

In this study, the recovery of strain-hardening rate (RSHR) was discovered for the first time in polycrystalline materials (Ni-Si alloys) that have only dislocation activities during tensile test. Detailed microstructure characterizations show that the activation of dislocations in the secondary slip systems during tensile deformation is the major reason for this RSHR. By taking into account other metals that also exhibit RSHR during tension, a more general mechanism for the RSHR was proposed, i.e. the occurrence of a sharp decrease of dislocation mean free path (Λ) during plastic deformation, caused by either planar defects or linear defects. PMID:26487419

Recovery of strain-hardening rate in Ni-Si alloys

NASA Astrophysics Data System (ADS)

Yang, C. L.; Zhang, Z. J.; Cai, T.; Zhang, P.; Zhang, Z. F.

2015-10-01

In this study, the recovery of strain-hardening rate (RSHR) was discovered for the first time in polycrystalline materials (Ni-Si alloys) that have only dislocation activities during tensile test. Detailed microstructure characterizations show that the activation of dislocations in the secondary slip systems during tensile deformation is the major reason for this RSHR. By taking into account other metals that also exhibit RSHR during tension, a more general mechanism for the RSHR was proposed, i.e. the occurrence of a sharp decrease of dislocation mean free path (Λ) during plastic deformation, caused by either planar defects or linear defects.

Coseismic and postseismic slip distribution of the 2003 Mw = 6.5 Chengkung earthquake in eastern Taiwan: Elastic modeling from inversion of GPS data

NASA Astrophysics Data System (ADS)

Cheng, Li-Wei; Lee, Jian-Cheng; Hu, Jyr-Ching; Chen, Horng-Yue

2009-03-01

The Chengkung earthquake with ML = 6.6 occurred in eastern Taiwan at 12:38 local time on December 10th 2003. Based on the main shock relocation and aftershock distribution, the Chengkung earthquake occurred along the previously recognized N20°E trending Chihshang fault. This event did not cause human loss, but significant cracks developed at the ground surface and damaged some buildings. After 1951 Taitung earthquake, there was no larger ML > 6 earthquake occurred in this region until the Chengkung earthquake. As a result, the Chengkung earthquake is a good opportunity to study the seismogenic structure of the Chihshang fault. The coseismic displacements recorded by GPS show a fan-shaped distribution with maximal displacement of about 30 cm near the epicenter. The aftershocks of the Chengkung earthquake revealing an apparent linear distribution helps us to construct the clear fault geometry of the Chihshang fault. In this study, we employ a half-space angular elastic dislocation model with GPS observations to figure out the slip distribution and seismological behavior of the Chengkung earthquake on the Chihshang fault. The elastic half-space dislocation model reveals that the Chengkung earthquake is a thrust event with minor left-lateral strike-slip component. The maximum coseismic slip is located around the depth of 20 km and up to 1.1 m. The slips are gradually decreased to less than 10 cm near the surface part of the Chihshang fault. The seismogenic fault plane, which is constructed by the delineation of the aftershocks, demonstrates that the Chihshang fault is a high-angle fault. However the fault plane changes to a flat plane at depth of 20 km. In addition, a significant part of the measured deformation across the surface fault zone for this earthquake can be attributed to postseismic creep. The postseismic elastic dislocation model shows that most afterslips are distributed to the upper level of the Chihshang fault. And most afterslips consist of both of dip- and left-lateral slip. The model results show that the Chihshang fault may be partially locked or damped near surface during coseismic slip. After the mainshock, the strain, which cumulated near the surface, was released by postseismic creep resulting in significant postseismic deformation.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hansen, Benjamin L; Bronkhorst, Curt; Beyerlein, Irene

The goal of this work is to formulate a constitutive model for the deformation of metals over a wide range of strain rates. Damage and failure of materials frequently occurs at a variety of deformation rates within the same sample. The present state of the art in single crystal constitutive models relies on thermally-activated models which are believed to become less reliable for problems exceeding strain rates of 10{sup 4} s{sup -1}. This talk presents work in which we extend the applicability of the single crystal model to the strain rate region where dislocation drag is believed to dominate. Themore » elastic model includes effects from volumetric change and pressure sensitive moduli. The plastic model transitions from the low-rate thermally-activated regime to the high-rate drag dominated regime. The direct use of dislocation density as a state parameter gives a measurable physical mechanism to strain hardening. Dislocation densities are separated according to type and given a systematic set of interactions rates adaptable by type. The form of the constitutive model is motivated by previously published dislocation dynamics work which articulated important behaviors unique to high-rate response in fcc systems. The proposed material model incorporates thermal coupling. The hardening model tracks the varying dislocation population with respect to each slip plane and computes the slip resistance based on those values. Comparisons can be made between the responses of single crystals and polycrystals at a variety of strain rates. The material model is fit to copper.« less

Hydrogenated vacancies lock dislocations in aluminium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xie, Degang; Li, Suzhi; Li, Meng

Due to its high diffusivity, hydrogen is often considered a weak inhibitor or even a promoter of dislocation movements in metals and alloys. By quantitative mechanical tests in an environmental transmission electron microscope, here we demonstrate that after exposing aluminium to hydrogen, mobile dislocations can lose mobility, with activating stress more than doubled. On degassing, the locked dislocations can be reactivated under cyclic loading to move in a stick-slip manner. However, relocking the dislocations thereafter requires a surprisingly long waiting time of ~10 3 s, much longer than that expected from hydrogen interstitial diffusion. Both the observed slow relocking andmore » strong locking strength can be attributed to superabundant hydrogenated vacancies, verified by our atomistic calculations. In conclusion, vacancies therefore could be a key plastic flow localization agent as well as damage agent in hydrogen environment.« less

Hydrogenated vacancies lock dislocations in aluminium

DOE PAGES

Xie, Degang; Li, Suzhi; Li, Meng; ...

2016-11-03

Due to its high diffusivity, hydrogen is often considered a weak inhibitor or even a promoter of dislocation movements in metals and alloys. By quantitative mechanical tests in an environmental transmission electron microscope, here we demonstrate that after exposing aluminium to hydrogen, mobile dislocations can lose mobility, with activating stress more than doubled. On degassing, the locked dislocations can be reactivated under cyclic loading to move in a stick-slip manner. However, relocking the dislocations thereafter requires a surprisingly long waiting time of ~10 3 s, much longer than that expected from hydrogen interstitial diffusion. Both the observed slow relocking andmore » strong locking strength can be attributed to superabundant hydrogenated vacancies, verified by our atomistic calculations. In conclusion, vacancies therefore could be a key plastic flow localization agent as well as damage agent in hydrogen environment.« less

Teleseismic body waves from dynamically rupturing shallow thrust faults: Are they opaque for surface-reflected phases?

USGS Publications Warehouse

Smith, D.E.; Aagaard, Brad T.; Heaton, T.H.

2005-01-01

We investigate whether a shallow-dipping thrust fault is prone to waveslip interactions via surface-reflected waves affecting the dynamic slip. If so, can these interactions create faults that are opaque to radiated energy? Furthermore, in this case of a shallow-dipping thrust fault, can incorrectly assuming a transparent fault while using dislocation theory lead to underestimates of seismic moment? Slip time histories are generated in three-dimensional dynamic rupture simulations while allowing for varying degrees of wave-slip interaction controlled by fault-friction models. Based on the slip time histories, P and SH seismograms are calculated for stations at teleseismic distances. The overburdening pressure caused by gravity eliminates mode I opening except at the tip of the fault near the surface; hence, mode I opening has no effect on the teleseismic signal. Normalizing by a Haskell-like traditional kinematic rupture, we find teleseismic peak-to-peak displacement amplitudes are approximately 1.0 for both P and SH waves, except for the unrealistic case of zero sliding friction. Zero sliding friction has peak-to-peak amplitudes of 1.6 for P and 2.0 for SH waves; the fault slip oscillates about its equilibrium value, resulting in a large nonzero (0.08 Hz) spectral peak not seen in other ruptures. These results indicate wave-slip interactions associated with surface-reflected phases in real earthquakes should have little to no effect on teleseismic motions. Thus, Haskell-like kinematic dislocation theory (transparent fault conditions) can be safety used to simulate teleseismic waveforms in the Earth.

Modeling the evolution of the lower crust with laboratory derived rheological laws under an intraplate strike slip fault

NASA Astrophysics Data System (ADS)

Zhang, X.; Sagiya, T.

2015-12-01

The earth's crust can be divided into the brittle upper crust and the ductile lower crust based on the deformation mechanism. Observations shows heterogeneities in the lower crust are associated with fault zones. One of the candidate mechanisms of strain concentration is shear heating in the lower crust, which is considered by theoretical studies for interplate faults [e.g. Thatcher & England 1998, Takeuchi & Fialko 2012]. On the other hand, almost no studies has been done for intraplate faults, which are generally much immature than interplate faults and characterized by their finite lengths and slow displacement rates. To understand the structural characteristics in the lower crust and its temporal evolution in a geological time scale, we conduct a 2-D numerical experiment on the intraplate strike slip fault. The lower crust is modeled as a 20km thick viscous layer overlain by rigid upper crust that has a steady relative motion across a vertical strike slip fault. Strain rate in the lower crust is assumed to be a sum of dislocation creep and diffusion creep components, each of which flows the experimental flow laws. The geothermal gradient is assumed to be 25K/km. We have tested different total velocity on the model. For intraplate fault, the total velocity is less than 1mm/yr, and for comparison, we use 30mm/yr for interplate faults. Results show that at a low slip rate condition, dislocation creep dominates in the shear zone near the intraplate fault's deeper extension while diffusion creep dominates outside the shear zone. This result is different from the case of interplate faults, where dislocation creep dominates the whole region. Because of the power law effect of dislocation creep, the effective viscosity in the shear zone under intraplate faults is much higher than that under the interplate fault, therefore, shear zone under intraplate faults will have a much higher viscosity and lower shear stress than the intraplate fault. Viscosity contract between inside and outside of the shear zone is smaller under an intraplate situation than in the interplate one, and smaller viscosity difference will result in a wider shear zone.

Crystal plasticity investigation of the microstructural factors influencing dislocation channeling in a model irradiated bcc material

DOE PAGES

Patra, Anirban; McDowell, David L.

2016-03-25

We use a continuum crystal plasticity framework to study the effect of microstructure and mesoscopic factors on dislocation channeling and flow localization in an irradiated model bcc alloy. For simulated dislocation channeling characteristics we correlate the dislocation and defect densities in the substructure, local Schmid factor, and stress triaxiality, in terms of their temporal and spatial evolution. A metric is introduced to assess the propensity for localization and is correlated to the grain-level Schmid factor. We also found that localization generally takes place in grains with a local Schmid factor in the range 0.42 or higher. Surface slip step heightsmore » are computed at free surfaces and compared to relevant experiments.« less

Computational issues in the simulation of two-dimensional discrete dislocation mechanics

NASA Astrophysics Data System (ADS)

Segurado, J.; LLorca, J.; Romero, I.

2007-06-01

The effect of the integration time step and the introduction of a cut-off velocity for the dislocation motion was analysed in discrete dislocation dynamics (DD) simulations of a single crystal microbeam. Two loading modes, bending and uniaxial tension, were examined. It was found that a longer integration time step led to a progressive increment of the oscillations in the numerical solution, which would eventually diverge. This problem could be corrected in the simulations carried out in bending by introducing a cut-off velocity for the dislocation motion. This strategy (long integration times and a cut-off velocity for the dislocation motion) did not recover, however, the solution computed with very short time steps in uniaxial tension: the dislocation density was overestimated and the dislocation patterns modified. The different response to the same numerical algorithm was explained in terms of the nature of the dislocations generated in each case: geometrically necessary in bending and statistically stored in tension. The evolution of the dislocation density in the former was controlled by the plastic curvature of the beam and was independent of the details of the simulations. On the contrary, the steady-state dislocation density in tension was determined by the balance between nucleation of dislocations and those which are annihilated or which exit the beam. Changes in the DD imposed by the cut-off velocity altered this equilibrium and the solution. These results point to the need for detailed analyses of the accuracy and stability of the dislocation dynamic simulations to ensure that the results obtained are not fundamentally affected by the numerical strategies used to solve this complex problem.

Slip and Twinning in the [ 1 ¯ $ /line{mathbf{1}} $ 49]-Oriented Single Crystals of a High-Entropy Alloy

NASA Astrophysics Data System (ADS)

Kireeva, I. V.; Chumlyakov, Yu. I.; Pobedennaya, Z. V.; Platonova, Yu. N.; Kuksgauzen, I. V.; Kuksgauzen, D. A.; Poklonov, V. V.; Karaman, I.; Sehitoglu, H.

2016-12-01

Using [ overline{1} 49] - oriented single crystals of an FCC Fe20Ni20Mn20Cr20Co20 (at.%) high-entropy alloy subjected to tensile deformation, the temperature dependence of critical resolved shear stresses τcr(T) and the deformation mechanism of slip and twinning are investigated in the early stages of deformation at ɛ ≤ 5% within the temperature interval T = 77-573 K. It is shown that τcr increases with decreasing the testing temperature and the τcr(T) temperature dependence is controlled by the slip of perfect dislocations a/2<110>. The early deformation stages ɛ ≤ 5% are associated with the development of planar slip by pileups of perfect dislocations a/2<110>, stacking faults and mechanical twins, which is observed in the temperature interval from 77 to 423 K. A comparison of the temperature dependence τcr(T) and the development of mechanical twinning is performed between the [ overline{1} 49] -oriented single crystals of the Fe20Ni20Mn20Cr20Co20 high-entropy alloy, the single crystals of the austenitic stainless steel, Fe - 18% Cr - 12% Ni - 2Mo (wt.%) without nitrogen atoms (Steel 316) and Hadfield steel, Fe - 13% Mn - (1-1.3)% C (wt.%).

A discrete dislocation dynamics model of creeping single crystals

NASA Astrophysics Data System (ADS)

Rajaguru, M.; Keralavarma, S. M.

2018-04-01

Failure by creep is a design limiting issue for metallic materials used in several high temperature applications. Current theoretical models of creep are phenomenological with little connection to the underlying microscopic mechanisms. In this paper, a bottom-up simulation framework based on the discrete dislocation dynamics method is presented for dislocation creep aided by the diffusion of vacancies, known to be the rate controlling mechanism at high temperature and stress levels. The time evolution of the creep strain and the dislocation microstructure in a periodic unit cell of a nominally infinite single crystal is simulated using the kinetic Monte Carlo method, together with approximate constitutive laws formulated for the rates of thermal activation of dislocations over local pinning obstacles. The deformation of the crystal due to dislocation glide between individual thermal activation events is simulated using a standard dislocation dynamics algorithm, extended to account for constant stress periodic boundary conditions. Steady state creep conditions are obtained in the simulations with the predicted creep rates as a function of stress and temperature in good agreement with experimentally reported values. Arrhenius scaling of the creep rates as a function of temperature and power-law scaling with the applied stress are also reproduced, with the values of the power-law exponents in the high stress regime in good agreement with experiments.

An Experimental Investigation of Intermittent Flow and Strain Burst Scaling Behavior in LiF Crystals During Microcompression Testing (Preprint)

DTIC Science & Technology

2010-01-01

or in more general terms, as a result of dislocation nucleation, motion, multiplication, and interaction). Nonetheless, state-of-the-art simulation ...computational power, together with under-developed physics within the simulation codes (i.e. cross-slip, climb, crystal rotations and patterning to...name a few), prevent realistic dislocation simulations over temporal and spatial domains that are readily accessible by experimental methods [9, 10

The Strength of Binary Junctions in Hexagonal Close-Packed Crystals

DTIC Science & Technology

2014-03-01

equilib- rium, on either slip plane, the dislocation on that plane intersects both triple points at the same angle with the junc- tion line, regardless...electronic properties of threading dislocations in wide band-gap gallium nitride (a wurtzite crystal structure consisting of two interpenetrating hcp...yield surface was composed of individual points , it pro- vided insight on the resistance of the lock to breaking as a result of the applied stresses. Via

Superplastic flow lubricates carbonate faults during earthquake slip

NASA Astrophysics Data System (ADS)

De Paola, Nicola; Holdsworth, Robert; Viti, Cecilia; Collettini, Cristiano; Faoro, Igor; Bullock, Rachael

2014-05-01

Tectonic earthquakes are hosted in the shallower portion of crustal fault zones, where fracturing and cataclasis are thought to be the dominant processes during frictional sliding. Aseismic shear in lower crust and lithospheric mantle shear zones is accomplished by crystal plasticity, including superplastic flow acting at low strain rates on ultrafine-grained rocks. Superplasticity has also been observed at high strain rates for a range of nano-phase alloys and ceramics, and could potentially occur in fine-grained geological materials, if deformed at high strain rates and temperatures. We performed a set of displacement-controlled experiments to explore whether superplastic flow can effectively weaken faults, and facilitate earthquake propagation. The experiments were performed on fine-grained synthetic gouges (63 < f < 93 μm) of undeformed, protolith carbonate rocks using a rotary shear apparatus, at target speed v = 1 ms-1, normal stresses σn = 12-18 MPa, displacements d from 0.009 to 1.46 m, room temperature and humidity conditions. Samples were recovered after each experiment to study the slip zone microstructures. The integration of experimental data and microstructural observations shows that during sliding at seismic velocity, brittle fracturing and cataclasis control shear localization and grain size reduction in the slip zone at relatively low temperatures (T ≤ 100 °C). Stress levels predicted by such behaviours match those measured during the experiments. As temperatures rise due to frictional heating (T ≥ 500 °C), dislocation creep mechanisms start to accommodate intragranular strain, and play a key role in producing nanoscale subgrains (< 200 nm) in the slip zone. At this stage, despite of the presence of nanoparticles in the slip zone and the attainment of seismic slip rates, the measured frictional strength of experimental faults still lies within Byerlee's range of values μ = 0.8. This suggests that the slip zone bulk strength at this stage is controlled by cataclastic frictional sliding rather than by dislocation creep or nanopowder lubrication mechanisms. When T ≥ 800 °C are attained, micro-textures diagnostic of diffusion-dominated grain boundary sliding are widespread within the slip zone, and suggest bulk superplastic flow. Flow stresses predicted by superplasticity constitutive laws at the slip zone temperatures, grain sizes and strain rates attained during the experiments match those we measured in the laboratory (μ = 0.16). We propose therefore that the activation of diffusion creep at high temperatures (T ≥ 800 °C) leads to slip zone-localised superplastic flow and that this causes the dynamic weakening of carbonate faults at seismic slip rates. Note, however, that both cataclasis and dislocation creep operating at lower temperatures, during the earlier stages of slip, are critical, precursory processes needed to produce the nanoscale grain sizes required to activate grainsize sensitive mechanisms during superplastic flow. Finally, the re-strengthening observed during the decelerating phase of deformation can be explained by the falling temperature "switching off" slip zone-localized superplasticity, leading to a return to frictional sliding. These results indicate that superplastic flow can effectively weaken faults, and facilitate earthquake propagation in the upper crust.

Investigation into nanoscratching mechanical response of AlCrCuFeNi high-entropy alloys using atomic simulations

NASA Astrophysics Data System (ADS)

Wang, Zining; Li, Jia; Fang, QiHong; Liu, Bin; Zhang, Liangchi

2017-09-01

The mechanical behaviors and deformation mechanisms of scratched AlCrCuFeNi high entropy alloys (HEAs) have been studied by molecular dynamics (MD) simulations, in terms of the scratching forces, atomic strain, atomic displacement, microstructural evolution and dislocation density. The results show that the larger tangential and normal forces and higher friction coefficient take place in AlCrCuFeNi HEA due to its outstanding strength and hardness, and high adhesion and fracture toughness over the pure metal materials. Moreover, the stacking fault energy (SFE) in HEA increases the probability to initiate dislocation and twinning, which is conducive to the formation of complex deformation modes. Compared to the single element metal workpieces, the segregation potency of solutes into twinning boundary (TB) is raised due to the decreasing segregation energy of TB, resulting in the stronger solute effects on improving twinning properties for HEA workpiece. The higher dislocation density and the more activated slipping planes lead to the outstanding plasticity of AlCrCuFeNi HEA. The solute atoms as barriers to hinder the motion of dislocation and the severe lattice distortion to suppress the free slipping of dislocation are significantly stronger obstacles to strengthen HEA. The excellent comprehensive scratching properties of the bulk AlCrCuFeNi HEAs are associated with the combined effects of multiple strengthening mechanisms, such as dislocation strengthening, deformation twinning strengthening as well as solute strengthening. This work provides a basis for further understanding and tailoring SFE in mechanical properties and deformation mechanism of HEAs, which maybe facilitate the design and preparation of new HEAs with high performance.

Ductile failure initiation and evolution in porous polycrystalline aggregates due to interfacial effects

NASA Astrophysics Data System (ADS)

Ashmawi, Waeil Muhammad Al-Anwar

New analytical and computational formulations have been developed for the investigation of micro structurally induced ductile failure mechanisms in porous polycrystalline aggregates with low and high (CSL) angle grain-boundaries (GBs). A multiple-slip rate-dependent crystalline constitutive formulation that is coupled to the evolution of mobile and immobile dislocation densities, a new internal porosity formulation for void nucleation and growth, and specialized computational schemes have been developed to obtain a detailed understanding of the multi-scale interrelated physical mechanisms that result in ductile failure in polycrystalline materials. Comprehensive transmission and pile-up mechanisms have also been introduced to investigate dislocation-density impedance and slip-rate incompatibility at the GBs. The interrelated effects of GB orientation, mobile and immobile dislocation densities, strain hardening, geometrical softening, localized plastic strains, and dislocation-density transmission and blockage on void growth, interaction, and coalescence have been studied. Criteria have been developed to identify and monitor the initiation and development of potential dislocation-density activity sites adjacent to GB regions. These interactions play an important role in the formation of GB pile-up and transmission regions. The effects of GB structure and orientation on ductile failure have been accounted for by the development of GB interfacial kinematic conditions that account for a multitude of dislocation-density interactions with GBs, such as full and partial transmission, impedance, blockage, and absorption. Pile-ups and transmission regions are identified and monitored as the deformation and failure evolve. These kinematic conditions are linked to the initiation and evolution of failure modes by the development of a new internal porosity evolution formulation that accounts for void nucleation and growth. The internal porosity relation is coupled with the proposed dislocation-density based crystalline constitutive formulation, the interfacial GB dislocation-density interaction models, and the specialized computational schemes to obtain detailed predictions of the behavior of aggregates with explicit voids that have different orientations and combinations of sizes, shapes, and spacings. Results from the present study indicate that material failure is a competition between different interrelated effects, such as stress triaxiality, accumulated plastic shear strain, temperature, dislocation density concentration, and grain and GB crystallographic orientations. For all void arrangements, as the void size is increased, specimen necking is diffuse and failure is concentrated in the ligament regions. Furthermore, there are more dislocation-density activity sites for potential transmission and pile-ups at the GBs. Failure is concentrated along the void peripheries and within intervoid ligaments. It has been shown that the evolution of the mobile dislocation density saturation curves, and their saturation rate are directly related to the aggregate response. Nucleation and growth for all void distributions have occurred in regions of maximum dislocation density and along preferred crystallographic orientations. Spatial distributions of porosity, accumulated plastic strains, and pressure have been obtained to further elucidate how these parameters evolve and affect void to void interaction in critical ligament and localized regions as a function of intervoid spacing and nominal strains. These failure predictions can be also used to identify intergranular and transgranular failure propagation. The present study underscores the importance of using dislocation-density based multiple-slip crystalline constitutive formulations and GB interfacial mechanisms that are consistent with experimental observations and results to accurately characterize the microstructural evolution of deformation and failure modes on a length scale that is commensurate with the material competition between the inherent strengthening and softening mechanisms of crystalline systems.

The response of creeping parts of the San Andreas fault to earthquakes on nearby faults: Two examples

USGS Publications Warehouse

Simpson, R.W.; Schulz, S.S.; Dietz, L.D.; Burford, R.O.

1988-01-01

Rates of shallow slip on creeping sections of the San Andreas fault have been perturbed on a number of occasions by earthquakes occurring on nearby faults. One example of such perturbations occurred during the 26 January 1986 magnitude 5.3 Tres Pinos earthquake located about 10 km southeast of Hollister, California. Seven creepmeters on the San Andreas fault showed creep steps either during or soon after the shock. Both left-lateral (LL) and right-lateral (RL) steps were observed. A rectangular dislocation in an elastic half-space was used to model the coseismic fault offset at the hypocenter. For a model based on the preliminary focal mechanism, the predicted changes in static shear stress on the plane of the San Andreas fault agreed in sense (LL or RL) with the observed slip directions at all seven meters; for a model based on a refined focal mechanism, six of the seven meters showed the correct sense of motion. Two possible explanations for such coseismic and postseismic steps are (1) that slip was triggered by the earthquake shaking or (2) that slip occurred in response to the changes in static stress fields accompanying the earthquake. In the Tres Pinos example, the observed steps may have been of both the triggered and responsive kinds. A second example is provided by the 2 May 1983 magnitude 6.7 Coalinga earthquake, which profoundly altered slip rates at five creepmeters on the San Andreas fault for a period of months to years. The XMM1 meter 9 km northwest of Parkfield, California recorded LL creep for more than a year after the event. To simulate the temporal behavior of the XMM1 meter and to view the stress perturbation provided by the Coalinga earthquake in the context of steady-state deformation on the San Andreas fault, a simple time-evolving dislocation model was constructed. The model was driven by a single long vertical dislocation below 15 km in depth, that was forced to slip at 35 mm/yr in a RL sense. A dislocation element placed in the seismogenic layer under XMM1 was given a finite breaking strength of sufficient magnitude to produce a Parkfield-like earthquake every 22 years. When stress changes equivalent to a Coalinga earthquake were superposed on the model running in a steady state mode, the effect was to make a segment under XMM1, that could slip in a linear viscous fashion, creep LL and to delay the onset of the next Parkfield-like earthquake by a year or more. If static stress changes imposed by earthquakes off the San Andreas can indeed advance or delay earthquakes on the San Andreas by months or years, then such changes must be considered in intermediate-term prediction efforts. ?? 1988 Birkha??user Verlag.

Role of copper in precipitation hardening of high-alloy Cr-Ni cast steels

NASA Astrophysics Data System (ADS)

Gajewski, Mirosław

2006-02-01

The mechanism of strengthening with second-phase particles that results from heat treatment, i.e., precipitate hardening, plays an important role in modern alloys. The strengthening effect of such particles can result from their coherence with the matrix, inhibition of dislocation slip, inhibition of grain boundary slip, as well as hampering recovery processes due to dislocation network pinning. The results of investigations into high-alloy Cr-Ni-Cu cast steels precipitate hardened with highly dispersed ɛ phase particles are presented within. The influence of heat treatment on changes in microstructure, mechanical properties, and morphology of fracture surfaces obtained under loading have been analyzed. It has been demonstrated that, with the appropriate selection of heat treatment parameters, it is possible to control the precipitation of the hardening ɛ phase and, thus, to change the final mechanical and functional properties.

A revised dislocation model of interseismic deformation of the Cascadia subduction zone

USGS Publications Warehouse

Wang, Kelin; Wells, Ray E.; Mazzotti, Stephane; Hyndman, Roy D.; Sagiya, Takeshi

2003-01-01

CAS3D‐2, a new three‐dimensional (3‐D) dislocation model, is developed to model interseismic deformation rates at the Cascadia subduction zone. The model is considered a snapshot description of the deformation field that changes with time. The effect of northward secular motion of the central and southern Cascadia forearc sliver is subtracted to obtain the effective convergence between the subducting plate and the forearc. Horizontal deformation data, including strain rates and surface velocities from Global Positioning System (GPS) measurements, provide primary geodetic constraints, but uplift rate data from tide gauges and leveling also provide important validations for the model. A locked zone, based on the results of previous thermal models constrained by heat flow observations, is located entirely offshore beneath the continental slope. Similar to previous dislocation models, an effective zone of downdip transition from locking to full slip is used, but the slip deficit rate is assumed to decrease exponentially with downdip distance. The exponential function resolves the problem of overpredicting coastal GPS velocities and underpredicting inland velocities by previous models that used a linear downdip transition. A wide effective transition zone (ETZ) partially accounts for stress relaxation in the mantle wedge that cannot be simulated by the elastic model. The pattern of coseismic deformation is expected to be different from that of interseismic deformation at present, 300 years after the last great subduction earthquake. The downdip transition from full rupture to no slip should take place over a much narrower zone.

Consistent lattice Boltzmann modeling of low-speed isothermal flows at finite Knudsen numbers in slip-flow regime: Application to plane boundaries

NASA Astrophysics Data System (ADS)

Silva, Goncalo; Semiao, Viriato

2017-07-01

The first nonequilibrium effect experienced by gaseous flows in contact with solid surfaces is the slip-flow regime. While the classical hydrodynamic description holds valid in bulk, at boundaries the fluid-wall interactions must consider slip. In comparison to the standard no-slip Dirichlet condition, the case of slip formulates as a Robin-type condition for the fluid tangential velocity. This makes its numerical modeling a challenging task, particularly in complex geometries. In this work, this issue is handled with the lattice Boltzmann method (LBM), motivated by the similarities between the closure relations of the reflection-type boundary schemes equipping the LBM equation and the slip velocity condition established by slip-flow theory. Based on this analogy, we derive, as central result, the structure of the LBM boundary closure relation that is consistent with the second-order slip velocity condition, applicable to planar walls. Subsequently, three tasks are performed. First, we clarify the limitations of existing slip velocity LBM schemes, based on discrete analogs of kinetic theory fluid-wall interaction models. Second, we present improved slip velocity LBM boundary schemes, constructed directly at discrete level, by extending the multireflection framework to the slip-flow regime. Here, two classes of slip velocity LBM boundary schemes are considered: (i) linear slip schemes, which are local but retain some calibration requirements and/or operation limitations, (ii) parabolic slip schemes, which use a two-point implementation but guarantee the consistent prescription of the intended slip velocity condition, at arbitrary plane wall discretizations, further dispensing any numerical calibration procedure. Third and final, we verify the improvements of our proposed slip velocity LBM boundary schemes against existing ones. The numerical tests evaluate the ability of the slip schemes to exactly accommodate the steady Poiseuille channel flow solution, over distinct wall slippage conditions, namely, no-slip, first-order slip, and second-order slip. The modeling of channel walls is discussed at both lattice-aligned and non-mesh-aligned configurations: the first case illustrates the numerical slip due to the incorrect modeling of slippage coefficients, whereas the second case adds the effect of spurious boundary layers created by the deficient accommodation of bulk solution. Finally, the slip-flow solutions predicted by LBM schemes are further evaluated for the Knudsen's paradox problem. As conclusion, this work establishes the parabolic accuracy of slip velocity schemes as the necessary condition for the consistent LBM modeling of the slip-flow regime.

Consistent lattice Boltzmann modeling of low-speed isothermal flows at finite Knudsen numbers in slip-flow regime: Application to plane boundaries.

PubMed

Silva, Goncalo; Semiao, Viriato

2017-07-01

The first nonequilibrium effect experienced by gaseous flows in contact with solid surfaces is the slip-flow regime. While the classical hydrodynamic description holds valid in bulk, at boundaries the fluid-wall interactions must consider slip. In comparison to the standard no-slip Dirichlet condition, the case of slip formulates as a Robin-type condition for the fluid tangential velocity. This makes its numerical modeling a challenging task, particularly in complex geometries. In this work, this issue is handled with the lattice Boltzmann method (LBM), motivated by the similarities between the closure relations of the reflection-type boundary schemes equipping the LBM equation and the slip velocity condition established by slip-flow theory. Based on this analogy, we derive, as central result, the structure of the LBM boundary closure relation that is consistent with the second-order slip velocity condition, applicable to planar walls. Subsequently, three tasks are performed. First, we clarify the limitations of existing slip velocity LBM schemes, based on discrete analogs of kinetic theory fluid-wall interaction models. Second, we present improved slip velocity LBM boundary schemes, constructed directly at discrete level, by extending the multireflection framework to the slip-flow regime. Here, two classes of slip velocity LBM boundary schemes are considered: (i) linear slip schemes, which are local but retain some calibration requirements and/or operation limitations, (ii) parabolic slip schemes, which use a two-point implementation but guarantee the consistent prescription of the intended slip velocity condition, at arbitrary plane wall discretizations, further dispensing any numerical calibration procedure. Third and final, we verify the improvements of our proposed slip velocity LBM boundary schemes against existing ones. The numerical tests evaluate the ability of the slip schemes to exactly accommodate the steady Poiseuille channel flow solution, over distinct wall slippage conditions, namely, no-slip, first-order slip, and second-order slip. The modeling of channel walls is discussed at both lattice-aligned and non-mesh-aligned configurations: the first case illustrates the numerical slip due to the incorrect modeling of slippage coefficients, whereas the second case adds the effect of spurious boundary layers created by the deficient accommodation of bulk solution. Finally, the slip-flow solutions predicted by LBM schemes are further evaluated for the Knudsen's paradox problem. As conclusion, this work establishes the parabolic accuracy of slip velocity schemes as the necessary condition for the consistent LBM modeling of the slip-flow regime.

On the development of a model predicting the recrystallization texture of aluminum using the Taylor model for rolling textures and the coincidence lattice site theory

NASA Astrophysics Data System (ADS)

T, Morimoto; F, Yoshida; A, Yanagida; J, Yanagimoto

2015-04-01

First, hardening model in f.c.c. metals was formulated with collinear interactions slips, Hirth slips and Lomer-Cottrell slips. Using the Taylor and the Sachs rolling texture prediction model, the residual dislocation densities of cold-rolled commercial pure aluminum were estimated. Then, coincidence site lattice grains were investigated from observed cold rolling texture. Finally, on the basis of oriented nucleation theory and coincidence site lattice theory, the recrystallization texture of commercial pure aluminum after low-temperature annealing was predicted.

Strain localization in <111> single crystals of Hadfield steel under compressive load

NASA Astrophysics Data System (ADS)

Astafurova, E. G.; Zakharova, G. G.; Melnikov, E. V.

2010-07-01

A study of strain localization under compression of <111> Hadfield steel single crystals at room temperature was done by light and transmission electron microscopy. At epsilon<1%, macro shear bands (MSB) form that have non-crystallographic and complex non-linear habit planes and are the results of the interaction of dislocation slip on conjugate slip planes. Mechanical twinning was experimentally found inside the MSB. After the stage of MSBs formation, deformation develops with high strain hardening coefficient and corresponds to interaction of slip and twinning inside as well as outside the MSBs.

The Microstructural Evolution of Fatigue Cracks in FCC Metals

NASA Astrophysics Data System (ADS)

Gross, David William

The microstructural evolution during fatigue crack propagation was investigated in a variety of planar and wavy slip FCC metals. The planar materials included Haynes 230, Nitronic 40, and 316 stainless steel, and the wavy materials included pure nickel and pure copper. Three different sets of experiments were performed to fully characterize the microstructural evolution. The first, performed on Haynes 230, mapped the strain field ahead a crack tip using digital image correlation and electron backscatter diffraction techniques. Focused ion beam (FIB) lift-out techniques were then utilized to extract transmission electron microscopy (TEM) samples at specific distances from the crack tip. TEM investigations compared the measured strain to the microstructure. Overall, the strain measured via DIC and EBSD was only weakly correlated to the density of planar slip bands in the microstructure. The second set of experiments concerned the dislocation structure around crack tips. This set of experiments was performed on all the materials. The microstructure at arrested fatigue cracks on the free surface was compared to the microstructure found beneath striations on the fracture surfaces by utilizing FIB micromachining to create site-specific TEM samples. The evolved microstructure depended on the slip type. Strong agreement was found between the crack tip microstructure at the free surface and the fracture surface. In the planar materials, the microstructure in the plastic zone consisted of bands of dislocations or deformation twins, before transitioning to a refined sub-grain microstructure near the crack flank. The sub-grain structure extended 300-500 nm away from the crack flank in all the planar slip materials studied. In contrast, the bulk structure in the wavy slip material consisted of dislocation cells and did not transition to a different microstructure as the crack tip was approached. The strain in wavy slip was highest near the crack tip, as the misorientations between the dislocation cells increased and the cell size decreased as the crack flank was approached. The final set of experiments involved reloading the arrested crack tips in monotonic tension. This was performed on both the Haynes 230 and 316 stainless steel. This technique exposed the fracture surface and location of the arrested crack tip away from the free surface, allowing for a sample to be extracted via FIB micromachining and TEM evaluation of the microstructure. This permitted the crack tip microstructure to be investigated without exposing the microstructure to crack closure or free surface effects. These experiments confirmed what was inferred from the earlier experiments, namely that the banded structure was a product of the crack tip plastic zone and the refined structure was a product of the strain associated with crack advance. Overall the microstructural complexity presented in this work was much higher than would be predicted by current models of fatigue crack propagation. It is recommended that future models attempt to simulate interactions between the dislocations emitted during fatigue crack growth and the pre-existing microstructure to more accurately simulate the processes occurring at the crack tip during crack growth.

Multiscale Modeling of Structurally-Graded Materials Using Discrete Dislocation Plasticity Models and Continuum Crystal Plasticity Models

NASA Technical Reports Server (NTRS)

Saether, Erik; Hochhalter, Jacob D.; Glaessgen, Edward H.

2012-01-01

A multiscale modeling methodology that combines the predictive capability of discrete dislocation plasticity and the computational efficiency of continuum crystal plasticity is developed. Single crystal configurations of different grain sizes modeled with periodic boundary conditions are analyzed using discrete dislocation plasticity (DD) to obtain grain size-dependent stress-strain predictions. These relationships are mapped into crystal plasticity parameters to develop a multiscale DD/CP model for continuum level simulations. A polycrystal model of a structurally-graded microstructure is developed, analyzed and used as a benchmark for comparison between the multiscale DD/CP model and the DD predictions. The multiscale DD/CP model follows the DD predictions closely up to an initial peak stress and then follows a strain hardening path that is parallel but somewhat offset from the DD predictions. The difference is believed to be from a combination of the strain rate in the DD simulation and the inability of the DD/CP model to represent non-monotonic material response.

The role of twinning deformation on the hardening response of polycrystalline magnesium from discrete dislocation dynamics simulations

DOE PAGES

Fan, Haidong; Aubry, Sylvie; Arsenlis, Athanasios; ...

2015-04-13

The mechanical response of micro-twinned polycrystalline magnesium was studied through three-dimensional discrete dislocation dynamics (DDD). A systematic interaction model between dislocations and (1012) tension twin boundaries (TBs) was proposed and introduced into the DDD framework. In addition, a nominal grain boundary (GB) model agreeing with experimental results was also introduced to mimic the GB’s barrier effect. The current simulation results show that TBs act as a strong obstacle to gliding dislocations, which contributes significantly to the hardening behavior of magnesium. On the other hand, the deformation accommodated by twinning plays a softening role. Therefore, the concave shape of the Mgmore » stress-strain curve results from the competition between dislocation-TB induced hardening and twinning deformation induced softening. At low strain levels, twinning deformation induced softening dominates and a decreasing hardening rate is observed in Stage-I. In Stage-II, both the hardening and softening effects decline, but twinning deformation induced softening declines faster, which leads to an increasing hardening rate.« less

Generalized continuum modeling of scale-dependent crystalline plasticity

NASA Astrophysics Data System (ADS)

Mayeur, Jason R.

The use of metallic material systems (e.g. pure metals, alloys, metal matrix composites) in a wide range of engineering applications from medical devices to electronic components to automobiles continues to motivate the development of improved constitutive models to meet increased performance demands while minimizing cost. Emerging technologies often incorporate materials in which the dominant microstructural features have characteristic dimensions reaching into the submicron and nanometer regime. Metals comprised of such fine microstructures often exhibit unique and size-dependent mechanical response, and classical approaches to constitutive model development at engineering (continuum) scales, being local in nature, are inadequate for describing such behavior. Therefore, traditional modeling frameworks must be augmented and/or reformulated to account for such phenomena. Crystal plasticity constitutive models have proven quite capable of capturing first-order microstructural effects such as grain orientation (elastic/plastic anisotropy), grain morphology, phase distribution, etc. on the deformation behavior of both single and polycrystals, yet suffer from the same limitations as other local continuum theories with regard to capturing scale-dependent mechanical response. This research is focused on the development, numerical implementation, and application of a generalized (nonlocal) theory of single crystal plasticity capable of describing the scale-dependent mechanical response of both single and polycrystalline metals that arises as a result of heterogeneous deformation. This research developed a dislocation-based theory of micropolar single crystal plasticity. The majority of nonlocal crystal plasticity theories are predicated on the connection between gradients of slip and geometrically necessary dislocations. Due to the diversity of existing nonlocal crystal plasticity theories, a review, summary, and comparison of representative model classes is presented in Chapter 2 from a unified dislocation-based perspective. The discussion of the continuum crystal plasticity theories is prefaced by a brief review of discrete dislocation plasticity, which facilitates the comparison of certain model aspects and also serves as a reference for latter segments of the research which make connection to this constitutive description. Chapter 2 has utility not only as a literature review, but also as a synthesis and analysis of competing and alternative nonlocal crystal plasticity modeling strategies from a common viewpoint. The micropolar theory of single crystal plasticity is presented in Chapter 3. Two different types of flow criteria are considered - the so-called single and multicriterion theories, and several variations of the dislocation-based strength models appropriate for each theory are presented and discussed. The numerical implementation of the two-dimensional version of the constitutive theory is given in Chapter 4. A user element subroutine for the implicit commercial finite element code Abaqus/Standard is developed and validated through the solution of initial-boundary value problems with closed-form solutions. Convergent behavior of the subroutine is also demonstrated for an initial-boundary value problem exhibiting strain localization. In Chapter 5, the models are employed to solve several standard initial-boundary value problems for heterogeneously deforming single crystals including simple shearing of a semi-infinite constrained thin film, pure bending of thin films, and simple shearing of a metal matrix composite with elastic inclusions. The simulation results are compared to those obtained from the solution of equivalent boundary value problems using discrete dislocation dynamics and alternative generalized crystal plasticity theories. Comparison and calibration with respect to the former provides guidance in the specification of non-traditional material parameters that arise in the model formulation and demonstrates its effectiveness at capturing the heterogeneous deformation fields and size-dependent mechanical behavior predicted by a finer scale constitutive description. Finally, in Chapter 6, the models are applied to simulate the deformation behavior of small polycrystalline ensembles. Several grain boundary constitutive descriptions are explored and the response characteristics are analyzed with respect to experimental observations as well as results obtained from discrete dislocation dynamics and alternative nonlocal crystal plasticity theories. Particular attention is focused on how the various grain boundary descriptions serve to either locally concentrate or diffuse deformation heterogeneity as a function of grain size.

Equivalent strike-slip earthquake cycles in half-space and lithosphere-asthenosphere earth models

USGS Publications Warehouse

Savage, J.C.

1990-01-01

By virtue of the images used in the dislocation solution, the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike-slip fault in an Earth model consisting of an elastic plate (lithosphere) overlying a viscoelastic half-space (asthenosphere) can be duplicated by prescribed slip on a vertical fault embedded in an elastic half-space. Inversion of 1973-1988 geodetic measurements of deformation across the segment of the San Andreas fault in the Transverse Ranges north of Los Angeles for the half-space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm/yr below 30 km. One equivalent lithosphere-asthenosphere model would have a 30-km thick lithosphere and an asthenosphere relaxation time greater than 33 years, but other models are possible. -from Author

Multiscale Computational Design Optimization of Copper-Strengthened Steel for High Cycle Fatigue

DTIC Science & Technology

2010-03-19

strain energy) and (3) modeling of a slip band (of PSB ladder underlying structure) and attendant crack initiation process. 15. SUBJECT TERMS 16...energy). (C) A modeling of a slip band (of PSB ladder underlying structure) and attendant crack initiation process. Major results obtained are...differentiate the morphology from others, e.g., vein and planar structures of dislocations. Results and Discussion for (C) (C-1) Modeling PSB For modeling

Understanding dislocation mechanics at the mesoscale using phase field dislocation dynamics

PubMed Central

Hunter, A.

2016-01-01

In this paper, we discuss the formulation, recent developments and findings obtained from a mesoscale mechanics technique called phase field dislocation dynamics (PFDD). We begin by presenting recent advancements made in modelling face-centred cubic materials, such as integration with atomic-scale simulations to account for partial dislocations. We discuss calculations that help in understanding grain size effects on transitions from full to partial dislocation-mediated slip behaviour and deformation twinning. Finally, we present recent extensions of the PFDD framework to alternative crystal structures, such as body-centred cubic metals, and two-phase materials, including free surfaces, voids and bi-metallic crystals. With several examples we demonstrate that the PFDD model is a powerful and versatile method that can bridge the length and time scales between atomistic and continuum-scale methods, providing a much needed understanding of deformation mechanisms in the mesoscale regime. PMID:27002063

Structure, Energetics, and Dynamics of Screw Dislocations in Even n-Alkane Crystals.

PubMed

Olson, Isabel A; Shtukenberg, Alexander G; Hakobyan, Gagik; Rohl, Andrew L; Raiteri, Paolo; Ward, Michael D; Kahr, Bart

2016-08-18

Spiral hillocks on n-alkane crystal surfaces were observed immediately after Frank recognized the importance of screw dislocations for crystal growth, yet their structures and energies in molecular crystals remain ill-defined. To illustrate the structural chemistry of screw dislocations that are responsible for plasticity in organic crystals and upon which the organic electronics and pharmaceutical industries depend, molecular dynamics was used to examine heterochiral dislocation pairs with Burgers vectors along [001] in n-hexane, n-octane, and n-decane crystals. The cores were anisotropic and elongated in the (110) slip plane, with significant local changes in molecular position, orientation, conformation, and energy. This detailed atomic level picture produced a distribution of strain consistent with linear elastic theory, giving confidence in the simulations. Dislocations with doubled Burgers vectors split into pairs with elementary displacements. These results suggest a pathway to understanding the mechanical properties and failure associated with elastic and plastic deformation in soft crystals.

Dislocation dynamics in non-convex domains using finite elements with embedded discontinuities

NASA Astrophysics Data System (ADS)

Romero, Ignacio; Segurado, Javier; LLorca, Javier

2008-04-01

The standard strategy developed by Van der Giessen and Needleman (1995 Modelling Simul. Mater. Sci. Eng. 3 689) to simulate dislocation dynamics in two-dimensional finite domains was modified to account for the effect of dislocations leaving the crystal through a free surface in the case of arbitrary non-convex domains. The new approach incorporates the displacement jumps across the slip segments of the dislocations that have exited the crystal within the finite element analysis carried out to compute the image stresses on the dislocations due to the finite boundaries. This is done in a simple computationally efficient way by embedding the discontinuities in the finite element solution, a strategy often used in the numerical simulation of crack propagation in solids. Two academic examples are presented to validate and demonstrate the extended model and its implementation within a finite element program is detailed in the appendix.

Review: mechanical behavior of metal/ceramic interfaces in nanolayered composites—experiments and modeling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Nan; Liu, Xiang-Yang

In this study, recent experimental and modeling studies in nanolayered metal/ceramic composites are reviewed, with focus on the mechanical behaviors of metal/nitrides interfaces. The experimental and modeling studies of the slip systems in bulk TiN are reviewed first. Then, the experimental studies of interfaces, including co-deformation mechanism by micropillar compression tests, in situ TEM straining tests for the dynamic process of the co-deformation, thickness-dependent fracture behavior, and interrelationship among the interfacial bonding, microstructure, and mechanical response, are reviewed for the specific material systems of Al/TiN and Cu/TiN multilayers at nanoscale. The modeling studies reviewed cover first-principles density functional theory-based modeling,more » atomistic molecular dynamics simulations, and mesoscale modeling of nanolayered composites using discrete dislocation dynamics. The phase transformation between zinc-blende and wurtzite AlN phases in Al/AlN multilayers at nanoscale is also reviewed. Finally, a summary and perspective of possible research directions and challenges are given.« less

Review: mechanical behavior of metal/ceramic interfaces in nanolayered composites—experiments and modeling

DOE PAGES

Li, Nan; Liu, Xiang-Yang

2017-11-03

In this study, recent experimental and modeling studies in nanolayered metal/ceramic composites are reviewed, with focus on the mechanical behaviors of metal/nitrides interfaces. The experimental and modeling studies of the slip systems in bulk TiN are reviewed first. Then, the experimental studies of interfaces, including co-deformation mechanism by micropillar compression tests, in situ TEM straining tests for the dynamic process of the co-deformation, thickness-dependent fracture behavior, and interrelationship among the interfacial bonding, microstructure, and mechanical response, are reviewed for the specific material systems of Al/TiN and Cu/TiN multilayers at nanoscale. The modeling studies reviewed cover first-principles density functional theory-based modeling,more » atomistic molecular dynamics simulations, and mesoscale modeling of nanolayered composites using discrete dislocation dynamics. The phase transformation between zinc-blende and wurtzite AlN phases in Al/AlN multilayers at nanoscale is also reviewed. Finally, a summary and perspective of possible research directions and challenges are given.« less

Pair Interaction of Dislocations in Two-Dimensional Crystals

NASA Astrophysics Data System (ADS)

Eisenmann, C.; Gasser, U.; Keim, P.; Maret, G.; von Grünberg, H. H.

2005-10-01

The pair interaction between crystal dislocations is systematically explored by analyzing particle trajectories of two-dimensional colloidal crystals measured by video microscopy. The resulting pair energies are compared to Monte Carlo data and to predictions derived from the standard Hamiltonian of the elastic theory of dislocations. Good agreement is found with respect to the distance and temperature dependence of the interaction potential, but not regarding the angle dependence where discrete lattice effects become important. Our results on the whole confirm that the dislocation Hamiltonian allows a quantitative understanding of the formation and interaction energies of dislocations in two-dimensional crystals.

Hydrogen diffusion in the elastic fields of dislocations in iron

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sivak, A. B., E-mail: Sivak-AB@nrcki.ru; Sivak, P. A.; Romanov, V. A.

2016-12-15

The effect of dislocation stress fields on the sink efficiency thereof is studied for hydrogen interstitial atoms at temperatures of 293 and 600 K and at a dislocation density of 3 × 10{sup 14} m{sup –2} in bcc iron crystal. Rectilinear full screw and edge dislocations in basic slip systems 〈111〉(110), 〈111〉(112), 〈100〉(100), and 〈100〉(110) are considered. Diffusion of defects is simulated by means of the object kinetic Monte Carlo method. The energy of interaction between defects and dislocations is calculated using the anisotropic theory of elasticity. The elastic fields of dislocations result in a less than 25% change ofmore » the sink efficiency as compared to the noninteracting linear sink efficiency at a room temperature. The elastic fields of edge dislocations increase the dislocation sink efficiency, whereas the elastic fields of screw dislocations either decrease this parameter (in the case of dislocations with the Burgers vector being 1/2〈111〉) or do not affect it (in the case of dislocations with the Burgers vector being 〈100〉). At temperatures above 600 K, the dislocations affect the behavior of hydrogen in bcc iron mainly owing to a high binding energy between the hydrogen atom and dislocation cores.« less

Deformation mechanism study of a hot rolled Zr-2.5Nb alloy by transmission electron microscopy. I. Dislocation microstructures in as-received state and at different plastic strains

DOE Office of Scientific and Technical Information (OSTI.GOV)

Long, Fei; Daymond, Mark R., E-mail: mark.daymond@queensu.ca; Yao, Zhongwen

Thin foil dog bone samples prepared from a hot rolled Zr-2.5Nb alloy have been deformed by tensile deformation to different plastic strains. The development of slip traces during loading was observed in situ through SEM, revealing that deformation starts preferentially in certain sets of grains during the elastic-plastic transition region. TEM characterization showed that sub-grain boundaries formed during hot rolling consisted of screw 〈a〉 dislocations or screw 〈c〉 and 〈a〉 dislocations. Prismatic 〈a〉 dislocations with large screw or edge components have been identified from the sample with 0.5% plastic strain. Basal 〈a〉 and pyramidal 〈c + a〉 dislocations were found in themore » sample that had been deformed with 1.5% plastic strain, implying that these dislocations require larger stresses to be activated.« less

Statistical study of ductility-dip cracking induced plastic deformation in polycrystalline laser 3D printed Ni-based superalloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Qian, Dan; Xue, Jiawei; Zhang, Anfeng

Ductility-dip cracking in Ni-based superalloy, resulting from heat treatment, is known to cause disastrous failure, but its mechanism is still not completely clear. A statistical study of the cracking behavior as a function of crystal orientation in a laser 3D-printed DL125L Ni-based superalloy polycrystal is investigated here using the synchrotron X-ray microdiffraction. The dislocation slip system in each of the forty crystal grains adjacent to the 300 μm long crack has been analyzed through Laue diffraction peak shapes. In all these grains, edge-type geometrically necessary dislocations (GNDs) dominate, and their dislocation line directions are almost parallel to the crack plane.more » Based on Schmid's law, the equivalent uniaxial tensile force direction is revealed normal to the trace of the crack. A qualitative mechanism is thus proposed. Thermal tensile stress perpendicular to the laser scanning direction is elevated due to a significant temperature gradient, and thus locations in the materials where the thermal stress exceeds the yield stress undergo plastic deformation mediated by GND activations. As the dislocations slip inside the crystal grains and pile up at the grain boundaries, local strain/stress keeps increasing, until the materials in these regions fail to sustain further deformation, leading to voids formation and cracks propagation.« less

Statistical study of ductility-dip cracking induced plastic deformation in polycrystalline laser 3D printed Ni-based superalloy

DOE PAGES

Qian, Dan; Xue, Jiawei; Zhang, Anfeng; ...

2017-06-06

Ductility-dip cracking in Ni-based superalloy, resulting from heat treatment, is known to cause disastrous failure, but its mechanism is still not completely clear. A statistical study of the cracking behavior as a function of crystal orientation in a laser 3D-printed DL125L Ni-based superalloy polycrystal is investigated here using the synchrotron X-ray microdiffraction. The dislocation slip system in each of the forty crystal grains adjacent to the 300 μm long crack has been analyzed through Laue diffraction peak shapes. In all these grains, edge-type geometrically necessary dislocations (GNDs) dominate, and their dislocation line directions are almost parallel to the crack plane.more » Based on Schmid's law, the equivalent uniaxial tensile force direction is revealed normal to the trace of the crack. A qualitative mechanism is thus proposed. Thermal tensile stress perpendicular to the laser scanning direction is elevated due to a significant temperature gradient, and thus locations in the materials where the thermal stress exceeds the yield stress undergo plastic deformation mediated by GND activations. As the dislocations slip inside the crystal grains and pile up at the grain boundaries, local strain/stress keeps increasing, until the materials in these regions fail to sustain further deformation, leading to voids formation and cracks propagation.« less

Chain Ends and the Ultimate Tensile Strength of Polyethylene Fibers

NASA Astrophysics Data System (ADS)

O'Connor, Thomas C.; Robbins, Mark O.

Determining the tensile yield mechanisms of oriented polymer fibers remains a challenging problem in polymer mechanics. By maximizing the alignment and crystallinity of polyethylene (PE) fibers, tensile strengths σ ~ 6 - 7 GPa have been achieved. While impressive, first-principal calculations predict carbon backbone bonds would allow strengths four times higher (σ ~ 20 GPa) before breaking. The reduction in strength is caused by crystal defects like chain ends, which allow fibers to yield by chain slip in addition to bond breaking. We use large scale molecular dynamics (MD) simulations to determine the tensile yield mechanism of orthorhombic PE crystals with finite chains spanning 102 -104 carbons in length. The yield stress σy saturates for long chains at ~ 6 . 3 GPa, agreeing well with experiments. Chains do not break but always yield by slip, after nucleation of 1D dislocations at chain ends. Dislocations are accurately described by a Frenkel-Kontorova model, parametrized by the mechanical properties of an ideal crystal. We compute a dislocation core size ξ = 25 . 24 Å and determine the high and low strain rate limits of σy. Our results suggest characterizing such 1D dislocations is an efficient method for predicting fiber strength. This research was performed within the Center for Materials in Extreme Dynamic Environments (CMEDE) under the Hopkins Extreme Materials Institute at Johns Hopkins University. Financial support was provided by Grant W911NF-12-2-0022.

Velocity statistics for interacting edge dislocations in one dimension from Dyson's Coulomb gas model.

PubMed

Jafarpour, Farshid; Angheluta, Luiza; Goldenfeld, Nigel

2013-10-01

The dynamics of edge dislocations with parallel Burgers vectors, moving in the same slip plane, is mapped onto Dyson's model of a two-dimensional Coulomb gas confined in one dimension. We show that the tail distribution of the velocity of dislocations is power law in form, as a consequence of the pair interaction of nearest neighbors in one dimension. In two dimensions, we show the presence of a pairing phase transition in a system of interacting dislocations with parallel Burgers vectors. The scaling exponent of the velocity distribution at effective temperatures well below this pairing transition temperature can be derived from the nearest-neighbor interaction, while near the transition temperature, the distribution deviates from the form predicted by the nearest-neighbor interaction, suggesting the presence of collective effects.

Strain rate dependent activation of slip systems in calcite marbles from Syros (Cyclades, Greece)

NASA Astrophysics Data System (ADS)

Rogowitz, Anna; Grasemann, Bernhard; Morales, Luiz F. G.; Huet, Benjamin; White, Joseph C.

2017-04-01

The activation of certain slip systems in calcite has been experimentally proven to be highly temperature dependent, but also the strain rate plays an important role on the activation of the dominant slip system. In this study, observations from a flanking structure (i.e. shear zone) that developed under lower greenschist-facies conditions, in an almost pure calcite marble (Syros Island, Greece) are presented. The shear zone is characterized by a strain gradient from the slightly deformed tips (γ ˜ 50) to the highly strained centre (γ up to 1000) while the host rock is moderately deformed (γ ˜ 3). During the shear zone development, the strain gradient coincided with a strain rate gradient with strain rate varying from 10-13 to 10-9 s-1. The studied outcrop thus represents the final state of a natural experiment and gives us a great opportunity to get natural constraints on strain rate dependent mechanical behaviour in a calcite marble. Detailed microstructural analyses have been performed via optical microscopy, electron microscopy, electron backscatter diffraction mapping and transmission electron microscopy, on samples from the highly strained shear zone and the host rock. The analyses show that the calcite microfabric varies depending on position within the shear zone, indicating activation of different deformation, recrystallization mechanisms and slip systems at different strain rates. Up to strain rates of ˜10-10 s-1 the marble deformed exclusively within the dislocation creep field, showing a change in recrystallization mechanism and dominant active slip system. While the marble preferentially recrystallized by grain boundary migration at relatively low strain rates (˜10-13 s-1), subgrain rotation recrystallization seems to be the dominant mechanism at higher strain rates (˜10-12 to 10-10 s-1). At higher strain rates (˜10-9 s-1), the recrystallization mechanism is bulging, resulting in the development of an extremely fine grained ultramylonite (average grain size ˜3 μm) accompanied by a switch in deformation mechanism from dislocation creep to a combined deformation by grain boundary sliding and dislocation activity. Constraints on dominant active slip system depending on deformation strain rate have been made by a combination of misorientation analyses and viscoplastic self-consistent modelling.

The key role of dislocation dissociation in the plastic behaviour of single crystal nickel-based superalloy with low stacking fault energy: Three-dimensional discrete dislocation dynamics modelling

NASA Astrophysics Data System (ADS)

Huang, Minsheng; Li, Zhenhuan

2013-12-01

To model the deformation of single crystal nickel based superalloys (SCNBS) with low stacking fault energy (SFE), three-dimensional discrete dislocation dynamics (3D-DDD) is extended by incorporating dislocation dissociation mechanism. The present 3D-DDD simulations show that, consistent with the existing TEM observation, the leading partial can enter the matrix channel efficiently while the trailing partial can hardly glide into it when the dislocation dissociation is taken into account. To determine whether the dislocation dissociation can occur or not, a critical percolation stress (CPS) based criterion is suggested. According to this CPS criterion, for SCNBS there exists a critical matrix channel width. When the channel width is lower than this critical value, the dislocation tends to dissociate into an extended configuration and vice versa. To clarify the influence of dislocation dissociation on CPS, the classical Orowan formula is improved by incorporating the SFE. Moreover, the present 3D-DDD simulations also show that the yielding stress of SCNBSs with low SFE may be overestimated up to 30% if the dislocation dissociation is ignored. With dislocation dissociation being considered, the size effect due to the width of γ matrix channel and the length of γ‧ precipitates on the stress-strain responses of SCNBS can be enhanced remarkably. In addition, due to the strong constraint effect by the two-phase microstructure in SCNBS, the configuration of formed junctions is quite different from that in single phase crystals such as Cu. The present results not only provide clear understanding of the two-phase microstructure levelled microplastic mechanisms in SCNBSs with low SFE, but also help to develop new continuum-levelled constitutive laws for SCNBSs.

A phenomenological creep model for nickel-base single crystal superalloys at intermediate temperatures

NASA Astrophysics Data System (ADS)

Gao, Siwen; Wollgramm, Philip; Eggeler, Gunther; Ma, Anxin; Schreuer, Jürgen; Hartmaier, Alexander

2018-07-01

For the purpose of good reproduction and prediction of creep deformation of nickel-base single crystal superalloys at intermediate temperatures, a phenomenological creep model is developed, which accounts for the typical γ/γ‧ microstructure and the individual thermally activated elementary deformation processes in different phases. The internal stresses from γ/γ‧ lattice mismatch and deformation heterogeneity are introduced through an efficient method. The strain hardening, the Orowan stress, the softening effect due to dislocation climb along γ/γ‧ interfaces and the formation of < 112> dislocation ribbons, and the Kear–Wilsdorf-lock effect as key factors in the main flow rules are formulated properly. By taking the cube slip in < 110> \\{100\\} slip systems and < 112> \\{111\\} twinning mechanisms into account, the creep behavior for [110] and [111] loading directions are well captured. Without specific interaction and evolution of dislocations, the simulations of this model achieve a good agreement with experimental creep results and reproduce temperature, stress and crystallographic orientation dependences. It can also be used as the constitutive relation at material points in finite element calculations with complex boundary conditions in various components of superalloys to predict creep behavior and local stress distributions.

Effect of strain-path change on the anisotropic mechanical properties of a commercially pure aluminum

NASA Astrophysics Data System (ADS)

Sun, P. L.; Huang, S. J.

2017-07-01

Samples of commercially pure aluminum were subjected to equal channel angular extrusion (ECAE) using a 90° square die by routes A and C, where the specimens are not rotated and are rotated 180° between extrusion passes, respectively. Qualitatively similar anisotropic responses under compressive loading along the three orthogonal directions of the ECAE billet are seen in both cases. The plastic anisotropy is related to the effect of strain-path change, namely that different slip activities are induced for specimens loaded along different directions with respect to the last ECAE pass. The anisotropic mechanical behavior is more evident in the sample deformed by route C. Considering the shear patterns imposed in each ECAE route, the characteristics of dislocations introduced in ECAE should affect the mechanical response in post-ECAE loading. It is suggested that during the ECAE process, dislocations on fewer slip systems are activated in route C than in route A, and therefore, a stronger plastic anisotropy results in this sample. The as-ECAE specimens were also heat treated to achieve a recovery-annealed state. The plastic anisotropy persists in the annealed specimens to slightly reduced extent, which can be ascribed to partial annihilation of preexisting dislocations.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Zhangwei; Baker, Ian; Guo, Wei

We investigated the effects of cold rolling followed by annealing on the mechanical properties and dislocation substructure evolution of undoped and 1.1 at. % carbon-doped Fe 40.4Ni 11.3Mn 34.8Al 7.5Cr 6 high entropy alloys (HEAs). X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT) were employed to characterize the microstructures. The as-cast HEAs were coarse-grained and single phase f.c.c., whereas the thermo-mechanical treatment caused recrystallization (to fine grain sizes) and precipitation (a B2 phase for the undoped HEA; and a B2 phase, and M 23C 6 and M 7C 3 carbides for the C-dopedmore » HEA). Carbon, which was found to have segregated to the grain boundaries using APT, retarded recrystallization. The reduction in grain size resulted in a sharp increase in strength, while the precipitation, which produced only a small increase in strength, probably accounted for the small decrease in ductility for both undoped and C-doped HEAs. For both undoped and C-doped HEAs, the smaller grain-sized material initially exhibited higher strain hardening than the coarse-grained material but showed a much lower strain hardening at large tensile strains. Wavy slip in the undoped HEAs and planar slip in C-doped HEAs were found at the early stages of deformation irrespective of grain size. At higher strains, dislocation cell structures formed in the 19 μm grain-sized undoped HEA, while microbands formed in the 23 μm grain-sized C-doped HEA. Conversely, localized dislocation clusters were found in both HEAs at the finest grain sizes (5 μm). The inhibition of grain subdivision by the grain boundaries and precipitates lead to the transformation from regular dislocation configurations consisting of dislocation-cells and microbands to irregular dislocation configurations consisting of localized dislocation clusters, which further account for the decrease in ductility. Our investigation of the formation mechanism and strain hardening of dislocation cells and microbands could benefit future structural material design.« less

A versatile atomic force microscope integrated with a scanning electron microscope.

PubMed

Kreith, J; Strunz, T; Fantner, E J; Fantner, G E; Cordill, M J

2017-05-01

A versatile atomic force microscope (AFM), which can be installed in a scanning electron microscope (SEM), is introduced. The flexible design of the instrument enables correlated analysis for different experimental configurations, such as AFM imaging directly after nanoindentation in vacuum. In order to demonstrate the capabilities of the specially designed AFM installed inside a SEM, slip steps emanating around nanoindents in single crystalline brass were examined. This example showcases how the combination of AFM and SEM imaging can be utilized for quantitative dislocation analysis through the measurement of the slip step heights without the hindrance of oxide formation. Finally, an in situ nanoindentation technique is introduced, illustrating the use of AFM imaging during indentation experiments to examine plastic deformation occurring under the indenter tip. The mechanical indentation data are correlated to the SEM and AFM images to estimate the number of dislocations emitted to the surface.

Ductile-brittle transition of thoriated chromium.

NASA Technical Reports Server (NTRS)

Wilcox, B. A.; Veigel, N. D.; Clauer, A. H.

1972-01-01

Unalloyed chromium and chromium containing approximately 3 wt % ThO2 were prepared from powder produced by a chemical vapor deposition process. When rolled to sheet and tested in tension, it was found that the thoriated material had a lower ductile-to-brittle transition temperature (DBTT) than unalloyed chromium. This ductilizing was evident both in the as-rolled condition and after the materials had been annealed for 1 hour at 1200 C. The improved ductility in thoriated chromium may be associated with several possible mechanisms: (1) particles may disperse slip, such that critical stress or strain concentrations for crack nucleation are more difficult to achieve; (2) particles may act as dislocation sources, thus providing mobile dislocations in this normally source-poor material, in a manner similar to prestraining; and (3) particles in grain boundaries may help to transmit slip across the boundaries, thus relieving stress concentrations and inhibiting crack nucleation.

In situ investigation of deformation mechanisms in magnesium-based metal matrix composites

NASA Astrophysics Data System (ADS)

Farkas, Gergely; Choe, Heeman; Máthis, Kristián; Száraz, Zoltán; Noh, Yoonsook; Trojanová, Zuzanka; Minárik, Peter

2015-07-01

We studied the effect of short fibers on the mechanical properties of a magnesium alloy. In particular, deformation mechanisms in a Mg-Al-Sr alloy reinforced with short alumina fibers were studied in situ using neutron diffraction and acoustic emission methods. The fibers' plane orientation with respect to the loading axis was found to be a key parameter, which influences the acting deformation processes, such as twinning or dislocation slip. Furthermore, the twinning activity was much more significant in samples with parallel fiber plane orientation, which was confirmed by both acoustic emission and electron backscattering diffraction results. Neutron diffraction was also used to assist in analyzing the acoustic emission and electron backscattering diffraction results. The simultaneous application of the two in situ methods, neutron diffraction and acoustic emission, was found to be beneficial for obtaining complementary datasets about the twinning and dislocation slip in the magnesium alloys and composites used in this study.

Development of Intergranular Residual Stress and Its Implication to Mechanical Behaviors at Elevated Temperatures in AL6XN Austenitic Stainless Steel

NASA Astrophysics Data System (ADS)

Hong, Yanyan; Li, Shilei; Li, Hongjia; Li, Jian; Sun, Guangai; Wang, Yan-Dong

2018-05-01

Neutron diffraction was used to investigate the residual lattice strains in AL6XN austenitic stainless steel subjected to tensile loading at different temperatures, revealing the development of large intergranular stresses after plastic deformation. Elastic-plastic self-consistent modeling was employed to simulate the micromechanical behavior at room temperature. The overall variations of the modeled lattice strains as a function of the sample direction with respect to the loading axis agree in general with the experimental values, indicating that dislocation slip is the main plastic deformation mode. At 300 °C, the serrated flow in the stress-strain curve and the great amount of slip bands indicate the appearance of dynamic strain aging. Except for promoting the local strain concentration, the long-range stress field caused by the planar slip bands near the grain boundaries is also attributed to the decrease in the experimental intergranular strains. An increase in the lattice strains localized at some specific specimen orientations for reflections at 600 °C may be explained by the segregation of solute atoms (Cr and Mo) at dislocation slip bands. The evolution of full-width at half-maximum demonstrates that the dynamic recovery indeed plays an important role in alleviating the local strain concentrations during tensile loading at 600 °C.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nguyen, Thao; Luscher, D. J.; Wilkerson, J. W.

We developed a framework for dislocation-based viscoplasticity and dynamic ductile failure to model high strain rate deformation and damage in single crystals. The rate-dependence of the crystal plasticity formulation is based on the physics of relativistic dislocation kinetics suited for extremely high strain rates. The damage evolution is based on the dynamics of void growth, which are governed by both micro-inertia as well as dislocation kinetics and dislocation substructure evolution. Furthermore, an averaging scheme is proposed in order to approximate the evolution of the dislocation substructure in both the macroscale as well as its spatial distribution at the microscale. Inmore » addition, a concept of a single equivalent dislocation density that effectively captures the collective influence of dislocation density on all active slip systems is proposed here. Together, these concepts and approximations enable the use of semi-analytic solutions for void growth dynamics developed in [J. Wilkerson and K. Ramesh. A dynamic void growth model governed by dislocation kinetics. J. Mech. Phys. Solids, 70:262–280, 2014.], which greatly reduce the computational overhead that would otherwise be required. The resulting homogenized framework has been implemented into a commercially available finite element package, and a validation study against a suite of direct numerical simulations was carried out.« less

A dislocation-based crystal plasticity framework for dynamic ductile failure of single crystals

NASA Astrophysics Data System (ADS)

Nguyen, Thao; Luscher, D. J.; Wilkerson, J. W.

2017-11-01

A framework for dislocation-based viscoplasticity and dynamic ductile failure has been developed to model high strain rate deformation and damage in single crystals. The rate-dependence of the crystal plasticity formulation is based on the physics of relativistic dislocation kinetics suited for extremely high strain rates. The damage evolution is based on the dynamics of void growth, which are governed by both micro-inertia as well as dislocation kinetics and dislocation substructure evolution. An averaging scheme is proposed in order to approximate the evolution of the dislocation substructure in both the macroscale as well as its spatial distribution at the microscale. Additionally, a concept of a single equivalent dislocation density that effectively captures the collective influence of dislocation density on all active slip systems is proposed here. Together, these concepts and approximations enable the use of semi-analytic solutions for void growth dynamics developed in (Wilkerson and Ramesh, 2014), which greatly reduce the computational overhead that would otherwise be required. The resulting homogenized framework has been implemented into a commercially available finite element package, and a validation study against a suite of direct numerical simulations was carried out.

The effect of crack blunting on the competition between dislocation nucleation and cleavage

NASA Astrophysics Data System (ADS)

Fischer, Lisa L.; Beltz, Glenn E.

2001-03-01

To better understand the ductile versus brittle fracture behavior of crystalline materials, attention should be directed towards physically realistic crack geometries. Currently, continuum models of ductile versus brittle behavior are typically based on the analysis of a pre-existing sharp crack in order to use analytical solutions for the stress fields around the crack tip. This paper examines the effects of crack blunting on the competition between dislocation nucleation and atomic decohesion using continuum methods. We accomplish this by assuming that the crack geometry is elliptical, which has the primary advantage that the stress fields are available in closed form. These stress field solutions are then used to calculate the thresholds for dislocation nucleation and atomic decohesion. A Peierls-type framework is used to obtain the thresholds for dislocation nucleation, in which the region of the slip plane ahead of the crack develops a distribution of slip discontinuity prior to nucleation. This slip distribution increases as the applied load is increased until an instability is reached and the governing integral equation can no longer be solved. These calculations are carried out for various crack tip geometries to ascertain the effects of crack tip blunting. The thresholds for atomic decohesion are calculated using a cohesive zone model, in which the region of the crack front develops a distribution of opening displacement prior to atomic decohesion. Again, loading of the elliptical crack tip eventually results in an instability, which marks the onset of crack advance. These calculations are carried out for various crack tip geometries. The results of these separate calculations are presented as the critical energy release rates versus the crack tip radius of curvature for a given crack length. The two threshold curves are compared simultaneously to determine which failure mode is energetically more likely at various crack tip curvatures. From these comparisons, four possible types of material fracture behavior are identified: intrinsically brittle, quasi-brittle, intrinsically ductile, and quasi-ductile. Finally, real material examples are discussed.

Dislocation Dissociation Strongly Influences on Frank—Read Source Nucleation and Microplasticy of Materials with Low Stacking Fault Energy

NASA Astrophysics Data System (ADS)

Huang, Min-Sheng; Zhu, Ya-Xin; Li, Zhen-Huan

2014-04-01

The influence of dislocation dissociation on the evolution of Frank—Read (F-R) sources is studied using a three-dimensional discrete dislocation dynamics simulation (3D-DDD). The classical Orowan nucleation stress and recently proposed Benzerga nucleation time models for F-R sources are improved. This work shows that it is necessary to introduce the dislocation dissociation scheme into 3D-DDD simulation, especially for simulations on micro-plasticity of small sized materials with low stacking fault energy.

Numerical Simulations of Slow Stick Slip Events with PFC, a DEM Based Code

NASA Astrophysics Data System (ADS)

Ye, S. H.; Young, R. P.

2017-12-01

Nonvolcanic tremors around subduction zone have become a fascinating subject in seismology in recent years. Previous studies have shown that the nonvolcanic tremor beneath western Shikoku is composed of low frequency seismic waves overlapping each other. This finding provides direct link between tremor and slow earthquakes. Slow stick slip events are considered to be laboratory scaled slow earthquakes. Slow stick slip events are traditionally studied with direct shear or double direct shear experiment setup, in which the sliding velocity can be controlled to model a range of fast and slow stick slips. In this study, a PFC* model based on double direct shear is presented, with a central block clamped by two side blocks. The gauge layers between the central and side blocks are modelled as discrete fracture networks with smooth joint bonds between pairs of discrete elements. In addition, a second model is presented in this study. This model consists of a cylindrical sample subjected to triaxial stress. Similar to the previous model, a weak gauge layer at a 45 degrees is added into the sample, on which shear slipping is allowed. Several different simulations are conducted on this sample. While the confining stress is maintained at the same level in different simulations, the axial loading rate (displacement rate) varies. By varying the displacement rate, a range of slipping behaviour, from stick slip to slow stick slip are observed based on the stress-strain relationship. Currently, the stick slip and slow stick slip events are strictly observed based on the stress-strain relationship. In the future, we hope to monitor the displacement and velocity of the balls surrounding the gauge layer as a function of time, so as to generate a synthetic seismogram. This will allow us to extract seismic waveforms and potentially simulate the tremor-like waves found around subduction zones. *Particle flow code, a discrete element method based numerical simulation code developed by Itasca Inc.

Simplex GPS and InSAR Inversion Software

NASA Technical Reports Server (NTRS)

Donnellan, Andrea; Parker, Jay W.; Lyzenga, Gregory A.; Pierce, Marlon E.

2012-01-01

Changes in the shape of the Earth's surface can be routinely measured with precisions better than centimeters. Processes below the surface often drive these changes and as a result, investigators require models with inversion methods to characterize the sources. Simplex inverts any combination of GPS (global positioning system), UAVSAR (uninhabited aerial vehicle synthetic aperture radar), and InSAR (interferometric synthetic aperture radar) data simultaneously for elastic response from fault and fluid motions. It can be used to solve for multiple faults and parameters, all of which can be specified or allowed to vary. The software can be used to study long-term tectonic motions and the faults responsible for those motions, or can be used to invert for co-seismic slip from earthquakes. Solutions involving estimation of fault motion and changes in fluid reservoirs such as magma or water are possible. Any arbitrary number of faults or parameters can be considered. Simplex specifically solves for any of location, geometry, fault slip, and expansion/contraction of a single or multiple faults. It inverts GPS and InSAR data for elastic dislocations in a half-space. Slip parameters include strike slip, dip slip, and tensile dislocations. It includes a map interface for both setting up the models and viewing the results. Results, including faults, and observed, computed, and residual displacements, are output in text format, a map interface, and can be exported to KML. The software interfaces with the QuakeTables database allowing a user to select existing fault parameters or data. Simplex can be accessed through the QuakeSim portal graphical user interface or run from a UNIX command line.

A diffraction based study of the deformation mechanisms in anomalously ductile B2 intermetallics

NASA Astrophysics Data System (ADS)

Mulay, Rupalee Prashant

For many decades, the brittle nature of most intermetallic compounds (e.g. NiAl) has been the limiting factor in their practical application. Many B2 (CsCl prototypical structure) intermetallics are known to exhibit slip on the <001>{110} slip mode, which provides only 3 independent slip systems and, hence, is unable to satisfy the von Mises (a.k.a. Taylor) criterion for polycrystalline ductility. As a result, inherent polycrystalline ductility is unexpected. Recent discovery of a number of ductile B2 intermetallics has raised questions about possible violation of the von Mises criterion by these alloys. These ductile intermetallic compounds are MR (metal (M) combined with a rare earth metal or group IV refractory metal (R)) alloys and are stoichiometric, ordered compounds. Single crystal slip trace analyses have only identified the presence of <100>{011} or <100>{010} slip systems. More than 100 other B2 MR compounds are known to exist and many of them have already been shown to be ductile (e.g., CuY, AgY, CuDy, CoZr, CoTi, etc.). Furthermore, these alloys exhibit a large Bauschinger effect. The present work uses several diffraction based techniques including electron back scattered diffraction (EBSD), X-ray diffraction (XRD) and in-situ neutron diffraction; in conjunction with scanning electron microscopy (SEM), transmission electron microscopy (TEM), mechanical testing, and crystal plasticity modeling, to elucidate the reason for ductility in select B2 alloys, explore the spread of this ductility over the B2 family, and understand the Bauschinger effect in these alloys. Several possible explanations (e.g., slip of <111> dislocations, strong texture, phase transformations and twinning) for the anomalous ductility were explored. An X-ray diffraction based analysis ruled out texture, phase purity and departure from order as explanations for the anomalous ductility in MR alloys. In-situ neutron diffraction and post deformation SEM, EBSD, and TEM were unable to detect any evidence for phase transformations in CoTi and CoZr. Also, post deformation characterization did not reveal any evidence of twinning. However, TEM based g·b analysis and EBSD based in-grain misorientation axis (IGMA) analysis showed that beyond a transition in the strain hardening behavior in CoTi, slip modes involving dislocations with <110> and <111> Burgers vectors are activated. The slip of such dislocations can reduce stress concentrations that could otherwise lead to premature fracture, thus providing a satisfying explanation for the anomalous ductility of CoTi and related compounds, like CoZr. Dislocation self-energy calculations accounting for elastic anisotropy suggest that the choice of slip direction in these alloys is mobility-, rather than source-, limited. The reach of this "ductilizing effect" over B2 alloys was explored by producing, characterizing, and testing a number of simple metal-rare earth metal compounds, namely MgY, MgNd and MgCe. MgR intermetallics with the B2 structure were found to be brittle and exhibit a cleavage type fracture indicating that the ductilizing effect is not as widespread as was initially thought. MgY and MgNd were found to primarily cleave along the {100} planes, while MgCe was found to cleave on the {111} planes. A large Bauschinger effect was observed in several of the anomalously ductile B2 compounds, such that the material actually begins to yield in the reverse direction on unloading. When only the primary slip mode <100>{011} is active in CoZr (prior to a transition in strain hardening), the buildup of intergranular stresses is large and is chiefly responsible for the observed Bauschinger effect. However, past the aforementioned transition in strain hardening, the effect of intergranular stresses diminishes. The results demonstrate that the activation of hard, secondary slip modes causes the internal strains to develop more uniformly among the grains, thus reducing the intergranular stresses which cause the Bauschinger effect. Crystal plasticity modeling, which accounts for the initial paucity of independent slip modes and allows for the activation of complementary hard slip modes, reproduces these trends in the Bauschinger effect and provides additional evidence that the experimental observations have correctly identified the cause of the anomalous ductility.

Fast-moving dislocations trigger flash weakening in carbonate-bearing faults during earthquakes.

PubMed

Spagnuolo, Elena; Plümper, Oliver; Violay, Marie; Cavallo, Andrea; Di Toro, Giulio

2015-11-10

Rupture fronts can cause fault displacement, reaching speeds up to several ms(-1) within a few milliseconds, at any distance away from the earthquake nucleation area. In the case of silicate-bearing rocks the abrupt slip acceleration results in melting at asperity contacts causing a large reduction in fault frictional strength (i.e., flash weakening). Flash weakening is also observed in experiments performed in carbonate-bearing rocks but evidence for melting is lacking. To unravel the micro-physical mechanisms associated with flash weakening in carbonates, experiments were conducted on pre-cut Carrara marble cylinders using a rotary shear apparatus at conditions relevant to earthquakes propagation. In the first 5 mm of slip the shear stress was reduced up to 30% and CO2 was released. Focused ion beam, scanning and transmission electron microscopy investigations of the slipping zones reveal the presence of calcite nanograins and amorphous carbon. We interpret the CO2 release, the formation of nanograins and amorphous carbon to be the result of a shock-like stress release associated with the migration of fast-moving dislocations. Amorphous carbon, given its low friction coefficient, is responsible for flash weakening and promotes the propagation of the seismic rupture in carbonate-bearing fault patches.

The effect of alloy composition on the mechanism of stress-corrosion cracking of titanium alloys in aqueous environments

NASA Technical Reports Server (NTRS)

Wood, R. A.; Boyd, J. D.; Williams, D. N.; Jaffee, R. I.

1972-01-01

A detailed study was made of the relation between the size distribution of Ti3Al particles in a Ti-8Al alloy and the tensile properties measured in air and in saltwater. The size distribution of Ti3Al was varied by isothermal aging for various times at temperatures in the range 770 to 970 K (930 to 1290 F). The aging kinetics were found to be relatively slow. Quantitative measurements of the particle coarsening rate at 920 K (1200 F) showed good agreement with the predicted behavior for coarsening controlled by matrix diffusion, and suggested that the specific free energy of the Ti3Al alpha interface in negligible small. In all cases, the Ti3Al particles were sheared by the glide dislocations. It was concluded that there is a definite correlation between the presence of deformable Ti3Al particles and an alloy's susceptibility to aqueous stress corrosion cracking. Furthermore, the appearance of the surface slip lines and the dislocation substructure in deformed specimens suggest that the specific effect of the Ti3Al particles is to cause a nonhomogeneous planar slip character and an enhanced chemical potential of the slip bands.

Dynamics of threading dislocations in porous heteroepitaxial GaN films

NASA Astrophysics Data System (ADS)

Gutkin, M. Yu.; Rzhavtsev, E. A.

2017-12-01

Behavior of threading dislocations in porous heteroepitaxial gallium nitride (GaN) films has been studied using computer simulation by the two-dimensional discrete dislocation dynamics approach. A computational scheme, where pores are modeled as cross sections of cylindrical cavities, elastically interacting with unidirectional parallel edge dislocations, which imitate threading dislocations, is used. Time dependences of coordinates and velocities of each dislocation from dislocation ensembles under investigation are obtained. Visualization of current structure of dislocation ensemble is performed in the form of a location map of dislocations at any time. It has been shown that the density of appearing dislocation structures significantly depends on the ratio of area of a pore cross section to area of the simulation region. In particular, increasing the portion of pores surface on the layer surface up to 2% should lead to about a 1.5-times decrease of the final density of threading dislocations, and increase of this portion up to 15% should lead to approximately a 4.5-times decrease of it.

Slip parameters on major thrusts at a convergent plate boundary: regional heterogeneity of potential slip distance at the shallow portion of the subducting plate

NASA Astrophysics Data System (ADS)

Mukoyoshi, Hideki; Kaneki, Shunya; Hirono, Tetsuro

2018-03-01

Understanding variations of slip distance along major thrust systems at convergent margins is an important issue for evaluation of near-trench slip and the potential generation of large tsunamis. We derived quantitative estimates of slip along ancient subduction fault systems by using the maturity of carbonaceous material (CM) of discrete slip zones as a proxy for temperature. We first obtained the Raman spectra of CM in ultracataclasite and pseudotachylyte layers in discrete slip zones at depths below the seafloor of 1-4 km and 2.5-5.5 km, respectively. By comparing the area-under-the-peak ratios of graphitic and disordered bands in those Raman spectra with spectra of experimentally heated CM from surrounding rocks, we determined that the ultracataclasite and pseudotachylyte layers had been heated to temperatures of up to 700 and 1300 °C, respectively. Numerical simulation of the thermal history of CM extracted from rocks near the two slip zones, taking into consideration these temperature constraints, indicated that slip distances in the ultracataclasite and pseudotachylyte layers were more than 3 and 7 m, respectively. Thus, potential distance of coseismic slip along the subduction-zone fault system could have regional variations even at shallow depth (≤ 5.5 km). The slip distances we determined probably represent minimum slips for subduction-zone thrusts and thus provide an important contribution to earthquake preparedness plans in coastal areas facing the Nankai and Sagami Troughs.

Use of fault striations and dislocation models to infer tectonic shear stress during the 1995 Hyogo-Ken Nanbu (Kobe) earthquake

USGS Publications Warehouse

Spudich, P.; Guatteri, Mariagiovanna; Otsuki, K.; Minagawa, J.

1998-01-01

Dislocation models of the 1995 Hyogo-ken Nanbu (Kobe) earthquake derived by Yoshida et al. (1996) show substantial changes in direction of slip with time at specific points on the Nojima and Rokko fault systems, as do striations we observed on exposures of the Nojima fault surface on Awaji Island. Spudich (1992) showed that the initial stress, that is, the shear traction on the fault before the earthquake origin time, can be derived at points on the fault where the slip rake rotates with time if slip velocity and stress change are known at these points. From Yoshida's slip model, we calculated dynamic stress changes on the ruptured fault surfaces. To estimate errors, we compared the slip velocities and dynamic stress changes of several published models of the earthquake. The differences between these models had an exponential distribution, not gaussian. We developed a Bayesian method to estimate the probability density function (PDF) of initial stress from the striations and from Yoshida's slip model. Striations near Toshima and Hirabayashi give initial stresses of about 13 and 7 MPa, respectively. We obtained initial stresses of about 7 to 17 MPa at depths of 2 to 10 km on a subset of points on the Nojima and Rokko fault systems. Our initial stresses and coseismic stress changes agree well with postearthquake stresses measured by hydrofracturing in deep boreholes near Hirabayashi and Ogura on Awaji Island. Our results indicate that the Nojima fault slipped at very low shear stress, and fractional stress drop was complete near the surface and about 32% below depths of 2 km. Our results at depth depend on the accuracy of the rake rotations in Yoshida's model, which are probably correct on the Nojima fault but debatable on the Rokko fault. Our results imply that curved or cross-cutting fault striations can be formed in a single earthquake, contradicting a common assumption of structural geology.

A dislocation-based crystal plasticity framework for dynamic ductile failure of single crystals

DOE PAGES

Nguyen, Thao; Luscher, D. J.; Wilkerson, J. W.

2017-08-02

We developed a framework for dislocation-based viscoplasticity and dynamic ductile failure to model high strain rate deformation and damage in single crystals. The rate-dependence of the crystal plasticity formulation is based on the physics of relativistic dislocation kinetics suited for extremely high strain rates. The damage evolution is based on the dynamics of void growth, which are governed by both micro-inertia as well as dislocation kinetics and dislocation substructure evolution. Furthermore, an averaging scheme is proposed in order to approximate the evolution of the dislocation substructure in both the macroscale as well as its spatial distribution at the microscale. Inmore » addition, a concept of a single equivalent dislocation density that effectively captures the collective influence of dislocation density on all active slip systems is proposed here. Together, these concepts and approximations enable the use of semi-analytic solutions for void growth dynamics developed in [J. Wilkerson and K. Ramesh. A dynamic void growth model governed by dislocation kinetics. J. Mech. Phys. Solids, 70:262–280, 2014.], which greatly reduce the computational overhead that would otherwise be required. The resulting homogenized framework has been implemented into a commercially available finite element package, and a validation study against a suite of direct numerical simulations was carried out.« less

Coupling continuum dislocation transport with crystal plasticity for application to shock loading conditions

DOE PAGES

Luscher, Darby Jon; Mayeur, Jason Rhea; Mourad, Hashem Mohamed; ...

2015-08-05

Here, we have developed a multi-physics modeling approach that couples continuum dislocation transport, nonlinear thermoelasticity, crystal plasticity, and consistent internal stress and deformation fields to simulate the single-crystal response of materials under extreme dynamic conditions. Dislocation transport is modeled by enforcing dislocation conservation at a slip-system level through the solution of advection-diffusion equations. Nonlinear thermoelasticity provides a thermodynamically consistent equation of state to relate stress (including pressure), temperature, energy densities, and dissipation. Crystal plasticity is coupled to dislocation transport via Orowan's expression where the constitutive description makes use of recent advances in dislocation velocity theories applicable under extreme loading conditions.more » The configuration of geometrically necessary dislocation density gives rise to an internal stress field that can either inhibit or accentuate the flow of dislocations. An internal strain field associated with the internal stress field contributes to the kinematic decomposition of the overall deformation. The paper describes each theoretical component of the framework, key aspects of the constitutive theory, and some details of a one-dimensional implementation. Results from single-crystal copper plate impact simulations are discussed in order to highlight the role of dislocation transport and pile-up in shock loading regimes. The main conclusions of the paper reinforce the utility of the modeling approach to shock problems.« less

Field Dislocation Mechanics for heterogeneous elastic materials: A numerical spectral approach

DOE Office of Scientific and Technical Information (OSTI.GOV)

Djaka, Komlan Senam; Villani, Aurelien; Taupin, Vincent

Spectral methods using Fast Fourier Transform (FFT) algorithms have recently seen a surge in interest in the mechanics of materials community. The present work addresses the critical question of determining accurate local mechanical fields using FFT methods without artificial fluctuations arising from materials and defects induced discontinuities. Precisely, this work introduces a numerical approach based on intrinsic discrete Fourier transforms for the simultaneous treatment of material discontinuities arising from the presence of dislocations and from elastic stiffness heterogeneities. To this end, the elasto-static equations of the field dislocation mechanics theory for periodic heterogeneous materials are numerically solved with FFT inmore » the case of dislocations in proximity of inclusions of varying stiffness. An optimal intrinsic discrete Fourier transform method is sought based on two distinct schemes. A centered finite difference scheme for differential rules are used for numerically solving the Poisson-type equation in the Fourier space, while centered finite differences on a rotated grid is chosen for the computation of the modified Fourier–Green’s operator associated with the Lippmann–Schwinger-type equation. By comparing different methods with analytical solutions for an edge dislocation in a composite material, it is found that the present spectral method is accurate, devoid of any numerical oscillation, and efficient even for an infinite phase elastic contrast like a hole embedded in a matrix containing a dislocation. The present FFT method is then used to simulate physical cases such as the elastic fields of dislocation dipoles located near the matrix/inclusion interface in a 2D composite material and the ones due to dislocation loop distributions surrounding cubic inclusions in 3D composite material. In these configurations, the spectral method allows investigating accurately the elastic interactions and image stresses due to dislocation fields in the presence of elastic inhomogeneities.« less

Field Dislocation Mechanics for heterogeneous elastic materials: A numerical spectral approach

DOE PAGES

Djaka, Komlan Senam; Villani, Aurelien; Taupin, Vincent; ...

2017-03-01

Spectral methods using Fast Fourier Transform (FFT) algorithms have recently seen a surge in interest in the mechanics of materials community. The present work addresses the critical question of determining accurate local mechanical fields using FFT methods without artificial fluctuations arising from materials and defects induced discontinuities. Precisely, this work introduces a numerical approach based on intrinsic discrete Fourier transforms for the simultaneous treatment of material discontinuities arising from the presence of dislocations and from elastic stiffness heterogeneities. To this end, the elasto-static equations of the field dislocation mechanics theory for periodic heterogeneous materials are numerically solved with FFT inmore » the case of dislocations in proximity of inclusions of varying stiffness. An optimal intrinsic discrete Fourier transform method is sought based on two distinct schemes. A centered finite difference scheme for differential rules are used for numerically solving the Poisson-type equation in the Fourier space, while centered finite differences on a rotated grid is chosen for the computation of the modified Fourier–Green’s operator associated with the Lippmann–Schwinger-type equation. By comparing different methods with analytical solutions for an edge dislocation in a composite material, it is found that the present spectral method is accurate, devoid of any numerical oscillation, and efficient even for an infinite phase elastic contrast like a hole embedded in a matrix containing a dislocation. The present FFT method is then used to simulate physical cases such as the elastic fields of dislocation dipoles located near the matrix/inclusion interface in a 2D composite material and the ones due to dislocation loop distributions surrounding cubic inclusions in 3D composite material. In these configurations, the spectral method allows investigating accurately the elastic interactions and image stresses due to dislocation fields in the presence of elastic inhomogeneities.« less

Hydrogen-assisted stable crack growth in iron-3 wt% silicon steel

DOE Office of Scientific and Technical Information (OSTI.GOV)

Marrow, T.J.; Prangnell, P.; Aindow, M.

1996-08-01

Observations of internal hydrogen cleavage in Fe-3Si are reported. Hydrogen-assisted stable crack growth (H-SCG) is associated with cleavage striations of a 300 nm spacing, observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). High resolution SEM revealed finer striations, previously undetected, with a spacing of approximately 30 nm. These were parallel to the coarser striations. Scanning tunneling microscopy (STM) also showed the fine striation spacing, and gave a striation height of approximately 15 nm. The crack front was not parallel to the striations. Transmission electron microscopy (TEM) of crack tip plastic zones showed {l_brace}112{r_brace} and {l_brace}110{r_brace} slip, withmore » a high dislocation density (around 10{sup 14}m{sup {minus}2}). The slip plane spacing was approximately 15--30 nm. Parallel arrays of high dislocation density were observed in the wake of the hydrogen cleavage crack. It is concluded that H-ScG in Fe-3Si occurs by periodic brittle cleavage on the {l_brace}001{r_brace} planes. This is preceded by dislocation emission. The coarse striations are produced by crack tip blunting and the fine striations by dislocations attracted by image forces to the fracture surface after cleavage. The effects of temperature, pressure and yield strength on the kinetics of H-SCG can be predicted using a model for diffusion of hydrogen through the plastic zone.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Guangming; Zhou, Zhangjian; Mo, Kun

An application of high-energy wide angle synchrotron X-ray diffraction to investigate the tensile deformation of 9Cr ferritic/martensitic (F/M) ODS steel is presented. With tensile loading and in-situ Xray exposure, the lattice strain development of matrix was determined. The lattice strain was found to decrease with increasing temperature, and the difference in Young's modulus of six different reflections at different temperatures reveals the temperature dependence of elastic anisotropy. The mean internal stress was calculated and compared with the applied stress, showing that the strengthening factor increased with increasing temperature, indicating that the oxide nanoparticles have a good strengthening impact at highmore » temperature. The dislocation density and character were also measured during tensile deformation. The dislocation density decreased with increasing of temperature due to the greater mobility of dislocation at high temperature. The dislocation character was determined by best-fit methods for different dislocation average contrasts with various levels of uncertainty. The results shows edge type dislocations dominate the plastic strain at room temperature (RT) and 300 C, while the screw type dislocations dominate at 600 C. The dominance of edge character in 9Cr F/M ODS steels at RT and 300 C is likely due to the pinning effect of nanoparticles for higher mobile edge dislocations when compared with screw dislocations, while the stronger screw type of dislocation structure at 600 C may be explained by the activated cross slip of screw segments.« less

The effects of intrinsic properties and defect structures on the indentation size effect in metals

NASA Astrophysics Data System (ADS)

Maughan, Michael R.; Leonard, Ariel A.; Stauffer, Douglas D.; Bahr, David F.

2017-08-01

The indentation size effect has been linked to the generation of geometrically necessary dislocations that may be impacted by intrinsic materials properties, such as stacking fault energy, and extrinsic defects, such as statistically stored dislocations. Nanoindentation was carried out at room temperature and elevated temperatures on four different metals in a variety of microstructural conditions. A size effect parameter was determined for each material set combining the effects of temperature and existing dislocation structure. Extrinsic defects, particularly dislocation density, dominate the size effect parameter over those due to intrinsic properties such as stacking fault energy. A multi-mechanism description using a series of mechanisms, rather than a single mechanism, is presented as a phenomenological explanation for the observed size effect in these materials. In this description, the size effect begins with a volume scale dominated by sparse sources, next is controlled by the ability of dislocations to cross-slip and multiply, and then finally at larger length scales work hardening and recovery dominate the effect.

Simulating subduction zone earthquakes using discrete element method: a window into elusive source processes

NASA Astrophysics Data System (ADS)

Blank, D. G.; Morgan, J.

2017-12-01

Large earthquakes that occur on convergent plate margin interfaces have the potential to cause widespread damage and loss of life. Recent observations reveal that a wide range of different slip behaviors take place along these megathrust faults, which demonstrate both their complexity, and our limited understanding of fault processes and their controls. Numerical modeling provides us with a useful tool that we can use to simulate earthquakes and related slip events, and to make direct observations and correlations among properties and parameters that might control them. Further analysis of these phenomena can lead to a more complete understanding of the underlying mechanisms that accompany the nucleation of large earthquakes, and what might trigger them. In this study, we use the discrete element method (DEM) to create numerical analogs to subduction megathrusts with heterogeneous fault friction. Displacement boundary conditions are applied in order to simulate tectonic loading, which in turn, induces slip along the fault. A wide range of slip behaviors are observed, ranging from creep to stick slip. We are able to characterize slip events by duration, stress drop, rupture area, and slip magnitude, and to correlate the relationships among these quantities. These characterizations allow us to develop a catalog of rupture events both spatially and temporally, for comparison with slip processes on natural faults.

Dislocation mechanisms in stressed crystals with surface effects

NASA Astrophysics Data System (ADS)

Wu, Chi-Chin; Crone, Joshua; Munday, Lynn; Discrete Dislocation Dynamics Team

2014-03-01

Understanding dislocation properties in stressed crystals is the key for important processes in materials science, including the strengthening of metals and the stress relaxation during the growth of hetero-epitaxial structures. Despite existing experimental approaches and theories, many dislocation mechanisms with surface effects still remain elusive in experiments. Even though discrete dislocation dynamics (DDD) simulations are commonly employed to study dislocations, few demonstrate sufficient computational capabilities for massive dislocations with the combined effects of surfaces and stresses. Utilizing the Army's newly developed FED3 code, a DDD computation code coupled with finite elements, this work presents several dislocation mechanisms near different types of surfaces in finite domains. Our simulation models include dislocations in a bended metallic cantilever beam, near voids in stressed metals, as well as threading and misfit dislocations in as-grown semiconductor epitaxial layers and their quantitative inter-correlations to stress relaxation and surface instability. Our studies provide not only detailed physics of individual dislocation mechanisms, but also important collective dislocation properties such as dislocation densities and strain-stress profiles and their interactions with surfaces.

Effects of pre-creep on the dislocations of 316LN Austenite stainless steel

NASA Astrophysics Data System (ADS)

Pei, Hai-xiang; Hui, Jun; Hua, Hou; Feng, Zai-xin; Xu, Xiao-long

2017-09-01

The 316LN Austenite stainless steels (316LNASS) were pre-creep treated, the evolution of microstructure were investigated. The samples were pre-creep at 593 K and from 500 to 2000 h at 873 K with a stress in the range of 20 to 150 MPa, Then the evolution of microstructure and precipitation were investigated by optical microscope (OM), and transmission electron microscope (TEM). The results show that the crystal surface slipping resulted in dislocations and original dislocations decomposition during the pre-creep process, and generate quadrilateral or hexagonal dislocation network was obviously. The sub-grain boundary gradually became narrow with the increasing of pre-creep treatment time and temperature. When the pre-creep temperature was 593 K and 873 K, dislocation network gradually disappear with the increasing of pre-creep time and load. When the pre-creep temperature was 873 K under 120 MPa, and the treatment time was 2000 h, the hexagonal dislocation network (HDN) would completely disappeared. When the pre-creep temperature was 593 K under 20 MPa, and the treatment time was 500 h, the quadrilateral dislocation network (QDN) would completely disappeared.

Single crystal plastic behavior of a single-phase, face-center-cubic-structured, equiatomic FeNiCrCo alloy

DOE PAGES

Wu, Zhenggang; Gao, Y. F.; Bei, Hongbin

2015-07-25

To understand the fundamental deformation mechanisms of compositionally complex alloys, single crystals of a multi-component equiatomic FeNiCoCr alloy with face-centered cubic (FCC) structure were grown for mechanical studies. Similarly to typical FCC pure metals, slip trace analyses indicate that dislocation slips take place on (1 1 1) planes along [11¯0] directions. The critical resolved shear stress (CRSS) obeys the Schmid law at both 77 and 293 K, and tension–compression asymmetry is not observed. Although this material slips in a normal FCC manner both at 293 and 77 K, compared to typical FCC metals the CRSS’s strong temperature dependence is abnormal.

Dislocation substructure of mantle-derived olivine as revealed by selective chemical etching and transmission electron microscopy

USGS Publications Warehouse

Kirby, S.H.; Wegner, M.W.

1978-01-01

Cleaved and mechanically polished surfaces of olivine from peridotite xenoliths from San Carlos, Arizona, were chemically etched using the techniques of Wegner and Christie (1974). Dislocation etch pits are produced on all surface orientations and they tend to be preferentially aligned along the traces of subgrain boundaries, which are approximately parallel to (100), (010), and (001). Shallow channels were also produced on (010) surfaces and represent dislocations near the surface that are etched out along their lengths. The dislocation etch channel loops are often concentric, and emanate from (100) subgrain boundaries, which suggests that dislocation sources are in the boundaries. Data on subgrain misorientation and dislocation line orientation and arguments based on subgrain boundary energy minimization are used to characterize the dislocation structures of the subgrain boundaries. (010) subgrain boundaries are of the twist type, composed of networks of [100] and [001] screw dislocations. Both (100) and (001) subgrain boundaries are tilt walls composed of arrays of edge dislocation with Burgers vectors b=[100] and [001], respectively. The inferred slip systems are {001} ???100???, {100} ???001???, and {010} ???100??? in order of diminishing importance. Exploratory transmission electron microscopy is in accord with these identifications. The flow stresses associated with the development of the subgrain structure are estimated from the densities of free dislocations and from the subgrain dimensions. Inferred stresses range from 35 to 75 bars using the free dislocation densities and 20 to 100 bars using the subgrain sizes. ?? 1978 Springer-Verlag.

Primary combination of phase-field and discrete dislocation dynamics methods for investigating athermal plastic deformation in various realistic Ni-base single crystal superalloy microstructures

NASA Astrophysics Data System (ADS)

Gao, Siwen; Rajendran, Mohan Kumar; Fivel, Marc; Ma, Anxin; Shchyglo, Oleg; Hartmaier, Alexander; Steinbach, Ingo

2015-10-01

Three-dimensional discrete dislocation dynamics (DDD) simulations in combination with the phase-field method are performed to investigate the influence of different realistic Ni-base single crystal superalloy microstructures with the same volume fraction of {γ\\prime} precipitates on plastic deformation at room temperature. The phase-field method is used to generate realistic microstructures as the boundary conditions for DDD simulations in which a constant high uniaxial tensile load is applied along different crystallographic directions. In addition, the lattice mismatch between the γ and {γ\\prime} phases is taken into account as a source of internal stresses. Due to the high antiphase boundary energy and the rare formation of superdislocations, precipitate cutting is not observed in the present simulations. Therefore, the plastic deformation is mainly caused by dislocation motion in γ matrix channels. From a comparison of the macroscopic mechanical response and the dislocation evolution for different microstructures in each loading direction, we found that, for a given {γ\\prime} phase volume fraction, the optimal microstructure should possess narrow and homogeneous γ matrix channels.

Invariant Deformation Element Model Interpretation to the Crystallography of Diffusional Body-Centered-Cube to Face-Centered-Cube Phase Transformations

NASA Astrophysics Data System (ADS)

Liu, Hongwei; Liu, Jiangwen; Su, Guangcai; Li, Weizhou; Zeng, Jianmin; Hu, Zhiliu

2012-10-01

The crystallography of body-centered-cube to face-centered cube (bcc-to-fcc) diffusion phase transformations in a duplex stainless steel and a Cu-Zn alloy, including long axis, orientation relationship (OR), habit plane (HP), and dislocation spacing, is successfully interpreted with one-step rotation from the Bain lattice relationship by applying a simplified invariant line (IL) analysis. It is proposed that the dislocation slipping direction in the matrix plays an important role in controlling the crystallography of precipitation.

Size and Crystallographic Orientation Effects on the Mechanical Behavior of 4H-SiC Micro-/nano-pillars

NASA Astrophysics Data System (ADS)

Guo, Xiaolei; Guo, Qiang; Li, Zhiqiang; Fan, Genlian; Xiong, Ding-Bang; Su, Yishi; Zhang, Jie; Tan, Zhanqiu; Guo, Cuiping; Zhang, Di

2018-02-01

Single crystalline 4H-SiC micro-/nano-pillars of various sizes and different crystallographic orientations were fabricated and tested by uniaxial compression. The pillars with zero shear stress resolved on the basal slip system were found to fracture in a brittle manner without showing significant size dependence, while the pillars with non-zero resolved shear stress showed a "smaller is stronger" behavior and a jerky plastic flow. These observations were interpreted by homogeneous dislocation nucleation and dislocation glide on the basal plane.

Unravelling the physics of size-dependent dislocation-mediated plasticity

NASA Astrophysics Data System (ADS)

El-Awady, Jaafar A.

2015-01-01

Size-affected dislocation-mediated plasticity is important in a wide range of materials and technologies. Here we develop a generalized size-dependent dislocation-based model that predicts strength as a function of crystal/grain size and the dislocation density. Three-dimensional (3D) discrete dislocation dynamics (DDD) simulations reveal the existence of a well-defined relationship between strength and dislocation microstructure at all length scales for both single crystals and polycrystalline materials. The results predict a transition from dislocation-source strengthening to forest-dominated strengthening at a size-dependent critical dislocation density. It is also shown that the Hall-Petch relationship can be physically interpreted by coupling with an appropriate kinetic equation of the evolution of the dislocation density in polycrystals. The model is shown to be in remarkable agreement with experiments. This work presents a micro-mechanistic framework to predict and interpret strength size-scale effects, and provides an avenue towards performing multiscale simulations without ad hoc assumptions.

Ultrasonic influence on evolution of disordered dislocation structures

NASA Astrophysics Data System (ADS)

Bachurin, D. V.; Murzaev, R. T.; Nazarov, A. A.

2017-12-01

Evolution of disordered dislocation structures under ultrasonic influence is studied in a model two-dimensional grain within the discrete-dislocation approach. Non-equilibrium grain boundary state is mimicked by a mesodefect located at the corners of the grain, stress field of which is described by that of a wedge junction disclination quadrupole. Significant rearrangement related to gliding of lattice dislocations towards the grain boundaries is found, which results in a noticeable reduction of internal stress fields and cancel of disclination quadrupole. The process of dislocation structure evolution passes through two stages: rapid and slow. The main dislocation rearrangement occurs during the first stage. Reduction of internal stress fields is associated with the number of dislocations entered into the grain boundaries. The change of misorientation angle due to lattice dislocations absorbed by the grain boundaries is evaluated. Amplitude of ultrasonic treatment significantly influences the relaxation of dislocation structure. Preliminary elastic relaxation of dislocation structure does not affect substantially the results of the following ultrasonic treatment. Substantial grain size dependence of relaxation of disordered dislocation systems is found. Simulation results are consistent with experimental data.

Unraveling cyclic deformation mechanisms of a rolled magnesium alloy using in situ neutron diffraction

DOE PAGES

Wu, Wei; An, Ke; Liaw, Peter K.

2014-12-23

In the current study, the deformation mechanisms of a rolled magnesium alloy were investigated under cyclic loading using real-time in situ neutron diffraction under a continuous-loading condition. The relationship between the macroscopic cyclic deformation behavior and the microscopic response at the grain level was established. The neutron diffraction results indicate that more and more grains are involved in the twinning and detwinning deformation process with the increase of fatigue cycles. The residual twins appear in the early fatigue life, which is responsible for the cyclic hardening behavior. The asymmetric shape of the hysteresis loop is attributed to the early exhaustionmore » of the detwinning process during compression, which leads to the activation of dislocation slips and rapid strain-hardening. The critical resolved shear stress for the activation of tensile twinning closely depends on the residual strain developed during cyclic loading. In the cycle before the sample fractured, the dislocation slips became active in tension, although the sample was not fully twinned. The increased dislocation density leads to the rise of the stress concentration at weak spots, which is believed to be the main reason for the fatigue failure. Furthermore, the deformation history greatly influences the deformation mechanisms of hexagonal-close-packed-structured magnesium alloy during cyclic loading.« less

Analysis of crystallographic preferred orientations of experimentally deformed Black Hills Quartzite

NASA Astrophysics Data System (ADS)

Kilian, Rüdiger; Heilbronner, Renée

2017-10-01

The crystallographic preferred orientations (textures) of three samples of Black Hills Quartzite (BHQ) deformed experimentally in the dislocation creep regimes 1, 2 and 3 (according to Hirth and Tullis, 1992) have been analyzed using electron backscatter diffraction (EBSD). All samples were deformed to relatively high strain at temperatures of 850 to 915 °C and are almost completely dynamically recrystallized. A texture transition from peripheral [c] axes in regime 1 to a central [c] maximum in regime 3 is observed. Separate pole figures are calculated for different grain sizes, aspect ratios and long-axis trends of grains, and high and low levels of intragranular deformation intensity as measured by the mean grain kernel average misorientation (gKAM). Misorientation relations are analyzed for grains of different texture components (named Y, B, R and σ grains, with reference to previously published prism, basal, rhomb and σ1 grains). Results show that regimes 1 and 3 correspond to clear end-member textures, with regime 2 being transitional. Texture strength and the development of a central [c]-axis maximum from a girdle distribution depend on deformation intensity at the grain scale and on the contribution of dislocation creep, which increases towards regime 3. Adding to this calculations of resolved shear stresses and misorientation analysis, it becomes clear that the peripheral [c]-axis maximum in regime 1 is not due to deformation by basal a slip. Instead, we interpret the texture transition as a result of different texture forming processes, one being more efficient at high stresses (nucleation or growth of grains with peripheral [c] axes), the other depending on strain (dislocation glide involving prism and rhomb a slip systems), and not as a result of temperature-dependent activity of different slip systems.

Evolution of the Deformation Behavior of Sn-Rich Solders during Cyclic Fatigue

NASA Astrophysics Data System (ADS)

Wentlent, Luke Arthur

Continuous developments in the electronics industry have provided a critical need for a quantitative, fundamental understanding of the behavior of SnAgCu (SAC) solders in both isothermal and thermal fatigue conditions. This study examines the damage behavior of Sn-based solders in a constant amplitude and variable amplitude environment. In addition, damage properties are correlated with crystal orientation and slip behavior. Select solder joints were continuously characterized and tested repeatedly in order to eliminate the joint to joint variation due to the anisotropy of beta-Sn. Characterization was partitioned into three different categories: effective properties and slip behavior, creep mechanisms and crystal morphology development, and atomic behavior and evolution. Active slip systems were correlated with measured properties. Characterization of the mechanical behavior was performed by the calculation and extrapolation of the elastic modulus, work, effective stiffness, Schmid factors, and time-dependent plasticity (creep). Electron microscopy based characterization methods included Scanning Electron Microscopy (SEM), Electron Backscattering Diffraction (EBSD), and Transmission Electron Microscopy (TEM). Testing showed a clear evolution of the steady-state creep mechanism when the cycling amplitudes were varied, from dislocation controlled to diffusion controlled creep. Dislocation behavior was examined and shown to evolve differently in single amplitude vs. variable amplitude testing. Finally, the mechanism of the recrystallization behavior of the beta-Sn was observed. This work fills a gap in the literature, providing a systematic study which identifies how the damage behavior in Sn-alloys depends upon the previous damage. A link is made between the observed creep behavior and the dislocation observations, providing a unified picture. Information developed in this work lays a stepping stone to future fundamental analyses as well as clarifying aspects of the mechanistic behavior of Sn and Sn-based alloys.

Displacement field for an edge dislocation in a layered half-space

USGS Publications Warehouse

Savage, J.C.

1998-01-01

The displacement field for an edge dislocation in an Earth model consisting of a layer welded to a half-space of different material is found in the form of a Fourier integral following the method given by Weeks et al. [1968]. There are four elementary solutions to be considered: the dislocation is either in the half-space or the layer and the Burgers vector is either parallel or perpendicular to the layer. A general two-dimensional solution for a dip-slip faulting or dike injection (arbitrary dip) can be constructed from a superposition of these elementary solutions. Surface deformations have been calculated for an edge dislocation located at the interface with Burgers vector inclined 0??, 30??, 60??, and 90?? to the interface for the case where the rigidity of the layer is half of that of the half-space and the Poisson ratios are the same. Those displacement fields have been compared to the displacement fields generated by similarly situated edge dislocations in a uniform half-space. The surface displacement field produced by the edge dislocation in the layered half-space is very similar to that produced by an edge dislocation at a different depth in a uniform half-space. In general, a low-modulus (high-modulus) layer causes the half-space equivalent dislocation to appear shallower (deeper) than the actual dislocation in the layered half-space.

The formation mechanisms of surface nanocrystallites in β-type biomedical TiNbZrFe alloy by surface mechanical attrition treatment

NASA Astrophysics Data System (ADS)

Jin, Lei; Cui, Wenfang; Song, Xiu; Zhou, Lian

2015-08-01

A nanostructured surface layer was successfully performed on a biomedical β-type TiNbZrFe alloy by surface mechanical attrition treatment (SMAT). The results reveal that the surface layer along the depth from treated surface to strain-free matrix could be divided into an outer nanocrystalline layer (0-30 μm), a high-density dislocation region (30-200 μm) and an inner region with low-density dislocations and twins (200-700 μm) when the surface was treated for 60 min. The microhardness of the surface layer is enhanced and increases with increasing treatment time. Although the {1 1 2} <1 1 1> twin coordinates the deformations with dislocations, this coordination only occurs in the low strain area and cannot affect the nanocrystalline formation. The self-nanocrystallization of TiNbZrFe alloy is mainly attributed to dislocation movements. First, the dislocations start to move and easily form dislocation bands along certain crystal directions; then, multiple slips of dislocations gradually form dislocation tangles; after that, high-density dislocation tangles increases, which divides primary grains into many small domain areas. As high strain energies accumulate on the interfaces among these areas, the lattice rotation can be driven between the adjacent small domain areas, finally resulting in a large number of nanocrystalline regions with low or large angle grain boundaries.

The effect of carbon on the microstructures, mechanical properties, and deformation mechanisms of thermo-mechanically treated Fe 40.4Ni 11.3Mn 34.8Al 7.5Cr 6 high entropy alloys

DOE PAGES

Wang, Zhangwei; Baker, Ian; Guo, Wei; ...

2017-03-01

We investigated the effects of cold rolling followed by annealing on the mechanical properties and dislocation substructure evolution of undoped and 1.1 at. % carbon-doped Fe 40.4Ni 11.3Mn 34.8Al 7.5Cr 6 high entropy alloys (HEAs). X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT) were employed to characterize the microstructures. The as-cast HEAs were coarse-grained and single phase f.c.c., whereas the thermo-mechanical treatment caused recrystallization (to fine grain sizes) and precipitation (a B2 phase for the undoped HEA; and a B2 phase, and M 23C 6 and M 7C 3 carbides for the C-dopedmore » HEA). Carbon, which was found to have segregated to the grain boundaries using APT, retarded recrystallization. The reduction in grain size resulted in a sharp increase in strength, while the precipitation, which produced only a small increase in strength, probably accounted for the small decrease in ductility for both undoped and C-doped HEAs. For both undoped and C-doped HEAs, the smaller grain-sized material initially exhibited higher strain hardening than the coarse-grained material but showed a much lower strain hardening at large tensile strains. Wavy slip in the undoped HEAs and planar slip in C-doped HEAs were found at the early stages of deformation irrespective of grain size. At higher strains, dislocation cell structures formed in the 19 μm grain-sized undoped HEA, while microbands formed in the 23 μm grain-sized C-doped HEA. Conversely, localized dislocation clusters were found in both HEAs at the finest grain sizes (5 μm). The inhibition of grain subdivision by the grain boundaries and precipitates lead to the transformation from regular dislocation configurations consisting of dislocation-cells and microbands to irregular dislocation configurations consisting of localized dislocation clusters, which further account for the decrease in ductility. Our investigation of the formation mechanism and strain hardening of dislocation cells and microbands could benefit future structural material design.« less

Modeling of Stick-Slip Behavior in Sheared Granular Fault Gouge Using the Combined Finite-Discrete Element Method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gao, Ke; Euser, Bryan J.; Rougier, Esteban

Sheared granular layers undergoing stick-slip behavior are broadly employed to study the physics and dynamics of earthquakes. In this paper, a two-dimensional implementation of the combined finite-discrete element method (FDEM), which merges the finite element method (FEM) and the discrete element method (DEM), is used to explicitly simulate a sheared granular fault system including both gouge and plate, and to investigate the influence of different normal loads on seismic moment, macroscopic friction coefficient, kinetic energy, gouge layer thickness, and recurrence time between slips. In the FDEM model, the deformation of plates and particles is simulated using the FEM formulation whilemore » particle-particle and particle-plate interactions are modeled using DEM-derived techniques. The simulated seismic moment distributions are generally consistent with those obtained from the laboratory experiments. In addition, the simulation results demonstrate that with increasing normal load, (i) the kinetic energy of the granular fault system increases; (ii) the gouge layer thickness shows a decreasing trend; and (iii) the macroscopic friction coefficient does not experience much change. Analyses of the slip events reveal that, as the normal load increases, more slip events with large kinetic energy release and longer recurrence time occur, and the magnitude of gouge layer thickness decrease also tends to be larger; while the macroscopic friction coefficient drop decreases. Finally, the simulations not only reveal the influence of normal loads on the dynamics of sheared granular fault gouge, but also demonstrate the capabilities of FDEM for studying stick-slip dynamic behavior of granular fault systems.« less

Modeling of Stick-Slip Behavior in Sheared Granular Fault Gouge Using the Combined Finite-Discrete Element Method

DOE PAGES

Gao, Ke; Euser, Bryan J.; Rougier, Esteban; ...

2018-06-20

Sheared granular layers undergoing stick-slip behavior are broadly employed to study the physics and dynamics of earthquakes. In this paper, a two-dimensional implementation of the combined finite-discrete element method (FDEM), which merges the finite element method (FEM) and the discrete element method (DEM), is used to explicitly simulate a sheared granular fault system including both gouge and plate, and to investigate the influence of different normal loads on seismic moment, macroscopic friction coefficient, kinetic energy, gouge layer thickness, and recurrence time between slips. In the FDEM model, the deformation of plates and particles is simulated using the FEM formulation whilemore » particle-particle and particle-plate interactions are modeled using DEM-derived techniques. The simulated seismic moment distributions are generally consistent with those obtained from the laboratory experiments. In addition, the simulation results demonstrate that with increasing normal load, (i) the kinetic energy of the granular fault system increases; (ii) the gouge layer thickness shows a decreasing trend; and (iii) the macroscopic friction coefficient does not experience much change. Analyses of the slip events reveal that, as the normal load increases, more slip events with large kinetic energy release and longer recurrence time occur, and the magnitude of gouge layer thickness decrease also tends to be larger; while the macroscopic friction coefficient drop decreases. Finally, the simulations not only reveal the influence of normal loads on the dynamics of sheared granular fault gouge, but also demonstrate the capabilities of FDEM for studying stick-slip dynamic behavior of granular fault systems.« less

Quantitative Assessment of Fatigue Damage Accumulation in Wavy Slip Metals from Acoustic Harmonic Generation

NASA Technical Reports Server (NTRS)

Cantrell, John H.

2006-01-01

A comprehensive, analytical treatment is presented of the microelastic-plastic nonlinearities resulting from the interaction of a stress perturbation with dislocation substructures (veins and persistent slip bands) and cracks that evolve during high-cycle fatigue of wavy slip metals. The nonlinear interaction is quantified by a material (acoustic) nonlinearity parameter beta extracted from acoustic harmonic generation measurements. The contribution to beta from the substructures is obtained from the analysis of Cantrell [Cantrell, J. H., 2004, Proc. R. Soc. London A, 460, 757]. The contribution to beta from cracks is obtained by applying the Paris law for crack propagation to the Nazarov-Sutin crack nonlinearity equation [Nazarov, V. E., and Sutin, A. M., 1997, J. Acoust. Soc. Am. 102, 3349]. The nonlinearity parameter resulting from the two contributions is predicted to increase monotonically by hundreds of percent during fatigue from the virgin state to fracture. The increase in beta during the first 80-90 percent of fatigue life is dominated by the evolution of dislocation substructures, while the last 10-20 percent is dominated by crack growth. The model is applied to the fatigue of aluminium alloy 2024-T4 in stress-controlled loading at 276MPa for which experimental data are reported. The agreement between theory and experiment is excellent.

Fast-moving dislocations trigger flash weakening in carbonate-bearing faults during earthquakes

PubMed Central

Spagnuolo, Elena; Plümper, Oliver; Violay, Marie; Cavallo, Andrea; Di Toro, Giulio

2015-01-01

Rupture fronts can cause fault displacement, reaching speeds up to several ms−1 within a few milliseconds, at any distance away from the earthquake nucleation area. In the case of silicate-bearing rocks the abrupt slip acceleration results in melting at asperity contacts causing a large reduction in fault frictional strength (i.e., flash weakening). Flash weakening is also observed in experiments performed in carbonate-bearing rocks but evidence for melting is lacking. To unravel the micro-physical mechanisms associated with flash weakening in carbonates, experiments were conducted on pre-cut Carrara marble cylinders using a rotary shear apparatus at conditions relevant to earthquakes propagation. In the first 5 mm of slip the shear stress was reduced up to 30% and CO2 was released. Focused ion beam, scanning and transmission electron microscopy investigations of the slipping zones reveal the presence of calcite nanograins and amorphous carbon. We interpret the CO2 release, the formation of nanograins and amorphous carbon to be the result of a shock-like stress release associated with the migration of fast-moving dislocations. Amorphous carbon, given its low friction coefficient, is responsible for flash weakening and promotes the propagation of the seismic rupture in carbonate-bearing fault patches. PMID:26552964

Effect of hydrogen on deformation structure and properties of CMSX-2 nickel-base single-crystal superalloy

NASA Technical Reports Server (NTRS)

Dollar, M.; Bernstein, I. M.; Walston, S.; Prinz, F.; Domnanovich, A.

1987-01-01

Material used in this study was a heat of the alloy CMSX-2. This nickel-based superalloy was provided in the form of oriented single crystals, solutionized for 3 hrs at 1315 C. It was then usually heat treated as follows: 1050 C/16h/air cool + 850 C/48h/air cool. The resulting microstructure is dominated by cuboidal, ordered gamma precipitates with a volume fraction of about 75% and an average size of 0.5 microns. In brief, the most compelling hydrogen induced-changes in deformation structure are: (1) enhanced dislocation accumulation in the gamma matrix; and (2) more extensive cross-slip of superdislocations in the gamma precipitates. The enhanced dislocation density in gamma acts to decrease the mean free path of a superdislocation, while easier cross slip hinders superdislocation movement by providing pinning points in the form of sessile jobs. Both processes contribute to the increase of flow stress and the notable work hardening that occurs prior to fracture.

Dislocation pinning effects induced by nano-precipitates during warm laser shock peening: Dislocation dynamic simulation and experiments

NASA Astrophysics Data System (ADS)

Liao, Yiliang; Ye, Chang; Gao, Huang; Kim, Bong-Joong; Suslov, Sergey; Stach, Eric A.; Cheng, Gary J.

2011-07-01

Warm laser shock peening (WLSP) is a new high strain rate surface strengthening process that has been demonstrated to significantly improve the fatigue performance of metallic components. This improvement is mainly due to the interaction of dislocations with highly dense nanoscale precipitates, which are generated by dynamic precipitation during the WLSP process. In this paper, the dislocation pinning effects induced by the nanoscale precipitates during WLSP are systematically studied. Aluminum alloy 6061 and AISI 4140 steel are selected as the materials with which to conduct WLSP experiments. Multiscale discrete dislocation dynamics (MDDD) simulation is conducted in order to investigate the interaction of dislocations and precipitates during the shock wave propagation. The evolution of dislocation structures during the shock wave propagation is studied. The dislocation structures after WLSP are characterized via transmission electron microscopy and are compared with the results of the MDDD simulation. The results show that nano-precipitates facilitate the generation of highly dense and uniformly distributed dislocation structures. The dislocation pinning effect is strongly affected by the density, size, and space distribution of nano-precipitates.

Surface dislocation nucleation controlled deformation of Au nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Roos, B.; Kapelle, B.; Volkert, C. A., E-mail: volkert@ump.gwdg.de

2014-11-17

We investigate deformation in high quality Au nanowires under both tension and bending using in-situ transmission electron microscopy. Defect evolution is investigated during: (1) tensile deformation of 〈110〉 oriented, initially defect-free, single crystal nanowires with cross-sectional widths between 30 and 300 nm, (2) bending deformation of the same wires, and (3) tensile deformation of wires containing coherent twin boundaries along their lengths. We observe the formation of twins and stacking faults in the single crystal wires under tension, and storage of full dislocations after bending of single crystal wires and after tension of twinned wires. The stress state dependence of themore » deformation morphology and the formation of stacking faults and twins are not features of bulk Au, where deformation is controlled by dislocation interactions. Instead, we attribute the deformation morphologies to the surface nucleation of either leading or trailing partial dislocations, depending on the Schmid factors, which move through and exit the wires producing stacking faults or full dislocation slip. The presence of obstacles such as neutral planes or twin boundaries hinder the egress of the freshly nucleated dislocations and allow trailing and leading partial dislocations to combine and to be stored as full dislocations in the wires. We infer that the twins and stacking faults often observed in nanoscale Au specimens are not a direct size effect but the result of a size and obstacle dependent transition from dislocation interaction controlled to dislocation nucleation controlled deformation.« less

Creep deformation in near-γ TiAl: Part 1. the influence of microstructure on creep deformation in Ti-49Al-1V

NASA Astrophysics Data System (ADS)

Worth, Brian D.; Jones, J. Wayne; Allison, John E.

1995-11-01

The influence of microstructure on creep deformation was examined in the near-y TiAl alloy Ti-49A1-1V. Specifically, microstructures with varying volume fractions of lamellar constituent were produced through thermomechanical processing. Creep studies were conducted on these various microstructures under constant load in air at temperatures between 760 °C and 870 °C and at stresses ranging from 50 to 200 MPa. Microstructure significantly influences the creep behavior of this alloy, with a fully lamellar microstructure yielding the highest creep resistance of the microstructures examined. Creep resistance is dependent on the volume fraction of lamellar constituent, with the lowest creep resistance observed at intermediate lamellar volume fractions. Examination of the creep deformation structure revealed planar slip of dislocations in the equiaxed y microstructure, while subboundary formation was observed in the duplex microstructure. The decrease in creep resistance of the duplex microstructure, compared with the equiaxed y microstructure, is attributed to an increase in dislocation mobility within the equiaxed y constituent, that results from partitioning of oxygen from the γ phase to the α2 phase. Dislocation motion in the fully lamellar microstructure was confined to the individual lamellae, with no evidence of shearing of γ/γ or γ/α2 interfaces. This suggests that the high creep resistance of the fully lamellar microstructure is a result of the fine spacing of the lamellar structure, which results in a decreased effective slip length for dislocation motion over that found in the duplex and equiaxed y microstructures.

Large-scale fault interactions at the termination of a subduction margin

NASA Astrophysics Data System (ADS)

Mouslopoulou, V.; Nicol, A., , Prof; Moreno, M.; Oncken, O.; Begg, J.; Kufner, S. K.

2017-12-01

Active subduction margins terminate against, and transfer their slip onto, plate-boundary transform faults. The manner in which plate motion is accommodated and partitioned across such kinematic transitions from thrust to strike-slip faulting over earthquake timescales, is poorly documented. The 2016 November 14th, Mw 7.8 Kaikoura Earthquake provides a rare snapshot of how seismic-slip may be accommodated at the tip of an active subduction margin. Analysis of uplift data collected using a range of techniques (field measurements, GPS, LiDAR) and published mapping coupled with 3D dislocation modelling indicates that earthquake-slip ruptured multiple faults with various orientations and slip mechanisms. Modelled and measured uplift patterns indicate that slip on the plate-interface was minor. Instead, a large offshore thrust fault, modelled to splay-off the plate-interface and to extend to the seafloor up to 15 km east of the South Island, appears to have released subduction-related strain and to have facilitated slip on numerous, strike-slip and oblique-slip faults on its hanging-wall. The Kaikoura earthquake suggests that these large splay-thrust faults provide a key mechanism in the transfer of plate motion at the termination of a subduction margin and represent an important seismic hazard.

Effect of Microstructural Interfaces on the Mechanical Response of Crystalline Metallic Materials

NASA Astrophysics Data System (ADS)

Aitken, Zachary H.

Advances in nano-scale mechanical testing have brought about progress in the understanding of physical phenomena in materials and a measure of control in the fabrication of novel materials. In contrast to bulk materials that display size-invariant mechanical properties, sub-micron metallic samples show a critical dependence on sample size. The strength of nano-scale single crystalline metals is well-described by a power-law function, sigma ∝ D-n, where D is a critical sample size and n is a experimentally-fit positive exponent. This relationship is attributed to source-driven plasticity and demonstrates a strengthening as the decreasing sample size begins to limit the size and number of dislocation sources. A full understanding of this size-dependence is complicated by the presence of microstructural features such as interfaces that can compete with the dominant dislocation-based deformation mechanisms. In this thesis, the effects of microstructural features such as grain boundaries and anisotropic crystallinity on nano-scale metals are investigated through uniaxial compression testing. We find that nano-sized Cu covered by a hard coating displays a Bauschinger effect and the emergence of this behavior can be explained through a simple dislocation-based analytic model. Al nano-pillars containing a single vertically-oriented coincident site lattice grain boundary are found to show similar deformation to single-crystalline nano-pillars with slip traces passing through the grain boundary. With increasing tilt angle of the grain boundary from the pillar axis, we observe a transition from dislocation-dominated deformation to grain boundary sliding. Crystallites are observed to shear along the grain boundary and molecular dynamics simulations reveal a mechanism of atomic migration that accommodates boundary sliding. We conclude with an analysis of the effects of inherent crystal anisotropy and alloying on the mechanical behavior of the Mg alloy, AZ31. Through comparison to pure Mg, we show that the size effect dominates the strength of samples below 10 microm, that differences in the size effect between hexagonal slip systems is due to the inherent crystal anisotropy, suggesting that the fundamental mechanism of the size effect in these slip systems is the same.

First-principles study of crystallographic slip modes in ω-Zr.

PubMed

Kumar, Anil; Kumar, M Arul; Beyerlein, Irene J

2017-08-21

We use first-principles density functional theory to study the preferred modes of slip in the high-pressure ω phase of Zr. The generalized stacking fault energy surfaces associated with shearing on nine distinct crystallographic slip modes in the hexagonal ω-Zr crystal are calculated, from which characteristics such as ideal shear stress, the dislocation Burgers vector, and possible accompanying atomic shuffles, are extracted. Comparison of energy barriers and ideal shear stresses suggests that the favorable modes are prismatic 〈c〉, prismatic-II [Formula: see text] and pyramidal-II 〈c + a〉, which are distinct from the ground state hexagonal close packed α phase of Zr. Operation of these three modes can accommodate any deformation state. The relative preferences among the identified slip modes are examined using a mean-field crystal plasticity model and comparing the calculated deformation texture with the measurement. Knowledge of the basic crystallographic modes of slip is critical to understanding and analyzing the plastic deformation behavior of ω-Zr or mixed α-ω phase-Zr.

High purith low defect FZ silicon

NASA Technical Reports Server (NTRS)

Kimura, H.; Robertson, G.

1985-01-01

The most common intrinsic defects in dislocation-free float zone (FZ) silicon crystals are the A- and B-type swirl defects. The mechanisms of their formation and annihilation have been extensively studied. Another type of defect in dislocation-free FZ crystals is referred to as a D-type defect. Concentrations of these defects can be minimized by optimizing the growth conditions, and the residual swirls can be reduced by the post-growth extrinsic gettering process. Czochralski (Cz) silicon wafers are known to exhibit higher resistance to slip and warpage due to thermal stress than do FZ wafers. The Cz crystals containing dislocations are more resistant to dislocation movement than dislocated FZ crystals because of the locking of dislocations by oxygen atoms present in the Cz crystals. Recently a transverse magnetic field was applied during the FZ growth of extrinsic silicon. Resultant flow patterns, as revealed by striation etching and spreading resistance in Ga-doped silicon crystals, indicate strong effects of the transverse magnetic field on the circulation within the melt. At fields of 5500 gauss, the fluid flow in the melt volume is so altered as to affect the morphology of the growing crystal.

Vitesses de glissement à long terme et dislocations cosismiques caractéristiques : clés du fonctionnement des failles actives et de l'aléa sismique

NASA Astrophysics Data System (ADS)

Tapponnier, Paul; Ryerson, Frederick James; Van der Woerd, Jerome; Mériaux, Anne-Sophie; Lasserre, Cécile

2001-11-01

Over periods of thousands of years, active faults tend to slip at constant rates. Pioneer studies of large Asian faults show that cosmogenic radionuclides ( 10Be, 26Al) provide an unparalleled tool to date surface features, whose offsets yield the longest records of recent cumulative movement. The technique is thus uniquely suited to determine long-term (10-100 ka) slip rates. Such rates, combined with coseismic slip-amounts, can give access to recurrence times of earthquakes of similar sizes. Landform dating - morphochronology - is therefore essential to understand fault-behaviour, evaluate seismic hazard, and build physical earthquake models. It is irreplaceable because long-term slip-rates on interacting faults need not coincide with GPS-derived, interseismic rates, and can be difficult to obtain from paleo-seismological trenching.

Stacking fault energies and slip in nanocrystalline metals.

PubMed

Van Swygenhoven, H; Derlet, P M; Frøseth, A G

2004-06-01

The search for deformation mechanisms in nanocrystalline metals has profited from the use of molecular dynamics calculations. These simulations have revealed two possible mechanisms; grain boundary accommodation, and intragranular slip involving dislocation emission and absorption at grain boundaries. But the precise nature of the slip mechanism is the subject of considerable debate, and the limitations of the simulation technique need to be taken into consideration. Here we show, using molecular dynamics simulations, that the nature of slip in nanocrystalline metals cannot be described in terms of the absolute value of the stacking fault energy-a correct interpretation requires the generalized stacking fault energy curve, involving both stable and unstable stacking fault energies. The molecular dynamics technique does not at present allow for the determination of rate-limiting processes, so the use of our calculations in the interpretation of experiments has to be undertaken with care.

Geodetic slip model of the 3 September 2016 Mw 5.8 Pawnee, Oklahoma, earthquake: Evidence for fault‐zone collapse

USGS Publications Warehouse

Pollitz, Fred; Wicks, Charles W.; Schoenball, Martin; Ellsworth, William L.; Murray, Mark

2017-01-01

The 3 September 2016 Mw 5.8 Pawnee earthquake in northern Oklahoma is the largest earthquake ever recorded in Oklahoma. The coseismic deformation was measured with both Interferometric Synthetic Aperture Radar and Global Positioning System (GPS), with measureable signals of order 1 cm and 1 mm, respectively. We derive a coseismic slip model from Sentinel‐1A and Radarsat 2 interferograms and GPS static offsets, dominated by distributed left‐lateral strike slip on a primary west‐northwest–east‐southeast‐trending subvertical plane, whereas strike slip is concentrated near the hypocenter (5.6 km depth), with maximum slip of ∼1  m located slightly east and down‐dip of the hypocenter. Based on systematic misfits of observed interferogram line‐of‐sight (LoS) displacements, with LoS based on shear‐dislocation models, a few decimeters of fault‐zone collapse are inferred in the hypocentral region where coseismic slip was the largest. This may represent the postseismic migration of large volumes of fluid away from the high‐slip areas, made possible by the creation of a temporary high‐permeability damage zone around the fault.

Quantifying the stress fields due to a delta-hydride precipitate in alpha-Zr matrix

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tummala, Hareesh; Capolungo, Laurent; Tome, Carlos N.

This report is a preliminary study on δ-hydride precipitate in zirconium alloy performed using 3D discrete dislocation dynamics simulations. The ability of dislocations in modifying the largely anisotropic stress fields developed by the hydride particle in a matrix phase is addressed for a specific dimension of the hydride. The influential role of probable dislocation nucleation at the hydride-matrix interface is reported. Dislocation nucleation around a hydride was found to decrease the shear stress (S 13) and also increase the normal stresses inside the hydride. We derive conclusions on the formation of stacks of hydrides in zirconium alloys. The contribution ofmore » mechanical fields due to dislocations was found to have a non-negligible effect on such process.« less

Effect of water on olivine single crystal plasticity, deformed under upper mantle condition

NASA Astrophysics Data System (ADS)

Girard, J.; Chen, J.; Raterron, P. C.; Holyoke, C. W.

2011-12-01

The earth upper mantle, mainly composed of olivine, is seismically anisotropic. Seismic anisotropy attenuation has been observed at 200km depth. Karato et al. (1992) attributed this attenuation to a transition between two deformation mechanisms, from dislocation creep above 200km to diffusion creep bellow 200km. This transition occurs because of hydroxyl concentration. Mainprice et al.(2005) predicted a change in LPO induced by a change of slip system, from [100] slip to [001] slip, though theoritical modeling. According to his study, pressure is the parameter inducing the slip system transition, which is responsible for the seismic anisotropy attenuation. Raterron et al. (2007) performed single crystal deformation experiment under anhydrous conditions and observe that slip system transition occurring around 8 GPa. However this pressure would correspond to 300km depth which doesn't match the seismic anisotropy attenuation depth, observed by seismologist. In this study, experiments have been performed to quantify the effects of water on olivine single crystals deformed using Deformation DIA press and synchrotron beam. Deformation was carried out in uniaxial compression along the so-called [110]c and [011]c crystallographic direction in order to activate [100](010) and [001](010) dislocation slip system respectively, at P ranging from 4 to 8GPa and T=1200°C. Both single crystals were loaded in the cell to directly compare their deformation in same condition of stress temperature and pressure. We used a sleeve (talc = enstatite + coesite + H2O) about the annulus of the single crystals as source of water in the assembly. Stress and specimen strain rates were calculated by in-situ X-ray diffraction and time resolved imaging, respectively. By direct comparison of single crystals strain rates, we observed that [110]c deform faster than [011]c bellow 5GPa. However above 6GPa [011]c deform faster. This revealed that [100](010) is the dominant slip system bellow 5GPa, and [001](010) becomes dominant above 6GPa. Thus, we observe a slip system switch over at about 5-6GPa in wet condition. This slip system switch over will result in change of lattice preferred orientation in olivine, from [100] slip to [001] slip, and therefore reduce seismic anisotropy attenuation down to 1.9% (Mainprice et al.,2005). According to our results, the slip system transition is induced by pressure, and water influences the pressure where the switch over occurs. These results not only can explain the depth where the seismic anisotropy attenuation (i.e. 200 km, corresponding to 6GPa) but also can help to understand the regional variation of the depth, as local hydroxyl contents in the mantle may varies significantly. TEM investigation and water content measurement of recovered specimens from the deformation experiments will also be discussed.

A FFT-based formulation for discrete dislocation dynamics in heterogeneous media

NASA Astrophysics Data System (ADS)

Bertin, N.; Capolungo, L.

2018-02-01

In this paper, an extension of the DDD-FFT approach presented in [1] is developed for heterogeneous elasticity. For such a purpose, an iterative spectral formulation in which convolutions are calculated in the Fourier space is developed to solve for the mechanical state associated with the discrete eigenstrain-based microstructural representation. With this, the heterogeneous DDD-FFT approach is capable of treating anisotropic and heterogeneous elasticity in a computationally efficient manner. In addition, a GPU implementation is presented to allow for further acceleration. As a first example, the approach is used to investigate the interaction between dislocations and second-phase particles, thereby demonstrating its ability to inherently incorporate image forces arising from elastic inhomogeneities.

Phase Transformation and Creep of Mg-Al-Ca Based Die-Cast Alloys

NASA Astrophysics Data System (ADS)

Suzuki, Akane; Saddock, Nicholas D.; Jones, J. Wayne; Pollock, Tresa M.

The microstructure and microstructural stability of die-cast AC53 (Mg-5Al-3Ca) and AXJ530 (Mg-5Al-3Ca-0.15Sr) have been investigated in detail by transmission electron microscopy (TEM). Both alloys have an as-cast microstructure of α-Mg with (Mg, Al)2Ca (dihexagonal C36) eutectic at grain boundaries. During aging at 573 K, the C36 phase transforms to Al2Ca (cubic Cl5) phase. These two phases have a crystallographic orientation relationship of (0001)C36//{111}C15 and [2110]C36//[011]C15, and the transformation from C36 to C15 occurs by a shear-assisted process. Despite this change in the phase constitution, the network structure of the intermetallic compound(s) surrounding α-Mg grains is fairly stable, morphologically, even after prolonged exposure at elevated temperature. In the α-Mg matrix phase, precipitation of Al2Ca was observed after aging for 360 ks at 573 K. The precipitates are disc-shaped with a habit plane of {111}C15//(0001)α. AXJ530 shows higher creep resistance than AC53. The dislocation substructure that evolved during creep deformation was investigated in both alloys, and the basal and non-basal slip of a-dislocation and other slip modes of a+c- dislocations were observed. The relationship between creep properties and microstructure is discussed.

A statistical analysis of the elastic distortion and dislocation density fields in deformed crystals

DOE PAGES

Mohamed, Mamdouh S.; Larson, Bennett C.; Tischler, Jonathan Z.; ...

2015-05-18

The statistical properties of the elastic distortion fields of dislocations in deforming crystals are investigated using the method of discrete dislocation dynamics to simulate dislocation structures and dislocation density evolution under tensile loading. Probability distribution functions (PDF) and pair correlation functions (PCF) of the simulated internal elastic strains and lattice rotations are generated for tensile strain levels up to 0.85%. The PDFs of simulated lattice rotation are compared with sub-micrometer resolution three-dimensional X-ray microscopy measurements of rotation magnitudes and deformation length scales in 1.0% and 2.3% compression strained Cu single crystals to explore the linkage between experiment and the theoreticalmore » analysis. The statistical properties of the deformation simulations are analyzed through determinations of the Nye and Kr ner dislocation density tensors. The significance of the magnitudes and the length scales of the elastic strain and the rotation parts of dislocation density tensors are demonstrated, and their relevance to understanding the fundamental aspects of deformation is discussed.« less

The Peierls stress of the moving [Formula: see text] screw dislocation in Ta.

PubMed

Liu, Ruiping; Wang, Shaofeng; Wu, Xiaozhi

2009-08-26

The Peierls stress of the moving [Formula: see text] screw dislocation with a planar and non-dissociated core structure in Ta has been calculated. The elastic strain energy which is associated with the discrete effect of the lattice and ignored in classical Peierls-Nabarro (P-N) theory has been taken into account in calculating the Peierls stress, and it can make the Peierls stress become smaller. The Peierls stress we obtain is very close to the experimental data. As shown in the numerical calculations and atomistic simulations, the core structure of the screw dislocation undergoes significant changes under the explicit stress before the screw dislocation moves. Moreover, the mechanism of the screw dislocation is revealed by our results and the experimental data that the screw dislocation retracts its extension in three {110} planes and transforms its dissociated core structure into a planar configuration. Therefore, the core structure of the moving [Formula: see text] screw dislocation in Ta is proposed to be planar.

A spectral approach for discrete dislocation dynamics simulations of nanoindentation

NASA Astrophysics Data System (ADS)

Bertin, Nicolas; Glavas, Vedran; Datta, Dibakar; Cai, Wei

2018-07-01

We present a spectral approach to perform nanoindentation simulations using three-dimensional nodal discrete dislocation dynamics. The method relies on a two step approach. First, the contact problem between an indenter of arbitrary shape and an isotropic elastic half-space is solved using a spectral iterative algorithm, and the contact pressure is fully determined on the half-space surface. The contact pressure is then used as a boundary condition of the spectral solver to determine the resulting stress field produced in the simulation volume. In both stages, the mechanical fields are decomposed into Fourier modes and are efficiently computed using fast Fourier transforms. To further improve the computational efficiency, the method is coupled with a subcycling integrator and a special approach is devised to approximate the displacement field associated with surface steps. As a benchmark, the method is used to compute the response of an elastic half-space using different types of indenter. An example of a dislocation dynamics nanoindentation simulation with complex initial microstructure is presented.

Stress evolution and associated microstructure during transient creep of olivine at 1000-1200 °C

NASA Astrophysics Data System (ADS)

Thieme, M.; Demouchy, S.; Mainprice, D.; Barou, F.; Cordier, P.

2018-05-01

We study the mechanical response and correlated microstructure of axial deformed fine-grained olivine aggregates as a function of incremental finite strains. Deformation experiments were conducted in uniaxial compression in an internally heated gas-medium deformation apparatus at temperatures of 1000 and 1200 °C, at strain rates of 10-6 s-1 to 10-5 s-1 and at confining pressure of 300 MPa. Sample volumes are around 1.2 cm3. Finite strains range from 0.1 to 8.6% and corresponding maximal (final) differential stresses range from 80 to 1073 MPa for deformation at 1000 °C and from 71 to 322 MPa for deformation at 1200 °C. At 1200 °C, samples approach steady state deformation after about 8% of strain. At 1000 °C, significant strain hardening leads to stresses exceeding the confining pressure by a factor of 3.5 with brittle deformation after 3% of strain. Deformed samples were characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). EBSD maps with step sizes as low as 50 nm were acquired without introducing analytical artifacts for the first time. The grain size of deformed samples ranges from 2.1 to 2.6 μm. Despite clear strain hardening, texture or microstructure do not change as a function of stress or finite strain. This observation is supported by a constant texture strength (J-index) and symmetry (BA-index), constant grain shape and aspect ratio, constant density of geometrically necessary dislocations, grain orientation spread, and constant subgrain boundary spacing and misorientation in between samples. TEM shows that all samples exhibit unambiguous dislocation activity but with a highly heterogeneous dislocation distribution. Olivine grains display evidence of [1 0 0] and [0 0 1] slip activity, but there is no evidence of interaction between the dislocations from the different slip systems. Several observations of grain boundaries acting as dislocation sources have been found. We find no confirmation of increasing dislocation densities as the cause for strain hardening during transient creep. This suggests other, yet not fully understood mechanisms affecting the strength of deformed olivine. These mechanisms could possibly involve grain boundaries. Such mechanisms are relevant for the deformation of uppermost mantle rocks, where the Si diffusion rate is too slow and dislocation glide must be accommodated in another way to fulfill the von Mises criterion.

Influence of misfit stresses on dislocation glide in single crystal superalloys: A three-dimensional discrete dislocation dynamics study

NASA Astrophysics Data System (ADS)

Gao, Siwen; Fivel, Marc; Ma, Anxin; Hartmaier, Alexander

2015-03-01

In the characteristic γ / γ ‧ microstructure of single crystal superalloys, misfit stresses occur due to a significant lattice mismatch of those two phases. The magnitude of this lattice mismatch depends on the chemical composition of both phases as well as on temperature. Furthermore, the lattice mismatch of γ and γ ‧ phases can be either positive or negative in sign. The internal stresses caused by such lattice mismatch play a decisive role for the micromechanical processes that lead to the observed macroscopic athermal deformation behavior of these high-temperature alloys. Three-dimensional discrete dislocation dynamics (DDD) simulations are applied to investigate dislocation glide in γ matrix channels and shearing of γ ‧ precipitates by superdislocations under externally applied uniaxial stresses, by fully taking into account internal misfit stresses. Misfit stress fields are calculated by the fast Fourier transformation (FFT) method and hybridized with DDD simulations. For external loading along the crystallographic [001] direction of the single crystal, it was found that the different internal stress states for negative and positive lattice mismatch result in non-uniform dislocation movement and different dislocation patterns in horizontal and vertical γ matrix channels. Furthermore, positive lattice mismatch produces a lower deformation rate than negative lattice mismatch under the same tensile loading, but for an increasing magnitude of lattice mismatch, the deformation resistance always diminishes. Hence, the best deformation performance is expected to result from alloys with either small positive, or even better, vanishing lattice mismatch between γ and γ ‧ phase.

Coseismic slip on the southern Cascadia megathrust implied by tsunami deposits in an Oregon lake and earthquake-triggered marine turbidites

NASA Astrophysics Data System (ADS)

Witter, Robert C.; Zhang, Yinglong; Wang, Kelin; Goldfinger, Chris; Priest, George R.; Allan, Jonathan C.

2012-10-01

We test hypothetical tsunami scenarios against a 4,600-year record of sandy deposits in a southern Oregon coastal lake that offer minimum inundation limits for prehistoric Cascadia tsunamis. Tsunami simulations constrain coseismic slip estimates for the southern Cascadia megathrust and contrast with slip deficits implied by earthquake recurrence intervals from turbidite paleoseismology. We model the tsunamigenic seafloor deformation using a three-dimensional elastic dislocation model and test three Cascadia earthquake rupture scenarios: slip partitioned to a splay fault; slip distributed symmetrically on the megathrust; and slip skewed seaward. Numerical tsunami simulations use the hydrodynamic finite element model, SELFE, that solves nonlinear shallow-water wave equations on unstructured grids. Our simulations of the 1700 Cascadia tsunami require >12-13 m of peak slip on the southern Cascadia megathrust offshore southern Oregon. The simulations account for tidal and shoreline variability and must crest the ˜6-m-high lake outlet to satisfy geological evidence of inundation. Accumulating this slip deficit requires ≥360-400 years at the plate convergence rate, exceeding the 330-year span of two earthquake cycles preceding 1700. Predecessors of the 1700 earthquake likely involved >8-9 m of coseismic slip accrued over >260 years. Simple slip budgets constrained by tsunami simulations allow an average of 5.2 m of slip per event for 11 additional earthquakes inferred from the southern Cascadia turbidite record. By comparison, slip deficits inferred from time intervals separating earthquake-triggered turbidites are poor predictors of coseismic slip because they meet geological constraints for only 4 out of 12 (˜33%) Cascadia tsunamis.

Slow wave contraction frequency plateaus in the small intestine are composed of discrete waves of interval increase associated with dislocations.

PubMed

Parsons, Sean P; Huizinga, Jan D

2018-06-03

What is the central question of this study? What is the nature of slow wave driven contraction frequency gradients in the small intestine? What is the main finding and its importance? Frequency plateaus are composed of discrete waves of increased interval, each wave associated with a contraction dislocation. Smooth frequency gradients are generated by localised neural modulation of wave frequency, leading to functionally important wave turbulence. Both patterns are emergent properties of a network of coupled oscillators, the interstitial cells of Cajal. A gut-wide network of interstitial cells of Cajal (ICC) generate electrical oscillations (slow waves) that orchestrate waves of muscle contraction. In the small intestine there is a gradient in slow wave frequency from high at the duodenum to low at the terminal ileum. Time-averaged measurements of frequency have suggested either a smooth or stepped (plateaued) gradient. We measured individual contraction intervals from diameter maps of the mouse small intestine to create interval maps (IMaps). IMaps showed that each frequency plateau was composed of discrete waves of increased interval. Each interval wave originated at a terminating contraction wave, a "dislocation", at the plateau's proximal boundary. In a model chain of coupled phase oscillators, interval wave frequency increased as coupling decreased or as the natural frequency gradient or noise increased. Injuring the intestine at a proximal point to destroy coupling, suppressed distal steps which then reappeared with gap junction block by carbenoxolone. This lent further support to our previous hypothesis that lines of dislocations were fixed by points of low coupling strength. Dislocations induced by electrical field pulses in the intestine and by equivalent phase shift in the model, were associated with interval waves. When the enteric nervous system was active, IMaps showed a chaotic, turbulent pattern of interval change with no frequency steps or plateaus. This probably resulted from local, stochastic release of neurotransmitters. Plateaus, dislocations, interval waves and wave turbulence arise from a dynamic interplay between natural frequency and coupling in the ICC network. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.

Investigation of the asymmetric misfit dislocation morphology in epitaxial layers with the zinc-blende structure

NASA Technical Reports Server (NTRS)

Fox, Bradley A.; Jesser, William A.

1990-01-01

The source of the asymmetry in the dislocation morphology exhibited in the epitaxial growth of compound semiconductors on (100) was investigated. A thickness wedge of p- and n-type GaAs(0.95)P(0.05) was grown on GaAs by metalorganic chemical vapor deposition, and the effect of misorientation on the resolved shear stress for each slip system was calculated and eliminated as the source of the asymmetry. Another potential source of asymmetry, the thickness gradient, was also eliminated. Results show that the substrate misorientation and the thickness gradient do not significantly contribute to the asymmetry and that the dominant contributor to the asymmetry of misfit dislocations in the (001) epitaxial interface can be attributed to the differences in the Peierls barriers between the two types of dilocations in GaAsP/GaAs.

Discrete Boltzmann Method with Maxwell-Type Boundary Condition for Slip Flow

NASA Astrophysics Data System (ADS)

Zhang, Yu-Dong; Xu, Ai-Guo; Zhang, Guang-Cai; Chen, Zhi-Hua

2018-01-01

The rarefied effect of gas flow in microchannel is significant and cannot be well described by traditional hydrodynamic models. It has been known that discrete Boltzmann model (DBM) has the potential to investigate flows in a relatively wider range of Knudsen number because of its intrinsic kinetic nature inherited from Boltzmann equation. It is crucial to have a proper kinetic boundary condition for DBM to capture the velocity slip and the flow characteristics in the Knudsen layer. In this paper, we present a DBM combined with Maxwell-type boundary condition model for slip flow. The tangential momentum accommodation coefficient is introduced to implement a gas-surface interaction model. Both the velocity slip and the Knudsen layer under various Knudsen numbers and accommodation coefficients can be well described. Two kinds of slip flows, including Couette flow and Poiseuille flow, are simulated to verify the model. To dynamically compare results from different models, the relation between the definition of Knudsen number in hard sphere model and that in BGK model is clarified. Support of National Natural Science Foundation of China under Grant Nos. 11475028, 11772064, and 11502117 Science Challenge Project under Grant Nos. JCKY2016212A501 and TZ2016002

Dislocations and deformation microstructure in a B2-ordered Al28Co20Cr11Fe15Ni26 high-entropy alloy

NASA Astrophysics Data System (ADS)

Feuerbacher, Michael

2016-07-01

High-entropy alloys are multicomponent metallic materials currently attracting high research interest. They display a unique combination of chemical disorder and crystalline long-range order, and due to their attractive properties are promising candidates for technological application. Many high-entropy alloys possess surprisingly high strength, occasionally in combination with high ductility and low density. The mechanisms effecting these attractive mechanical properties are not understood. This study addresses the deformation mechanism of a Al28Co20Cr11Fe15Ni26 high-entropy alloy, which is a two-phase material, consisting of a B2-ordered matrix and disordered body-centred inclusions. We quantitatively analyse the microstructure and dislocations in deformed samples by transmission-electron-microscopic methods including weak-beam imaging and convergent-beam electron diffraction. We find that the deformation process in the B2 phase is dominated by heterogeneous slip of screw dislocations gliding on planes. The dislocations are perfect superdislocations of the B2 lattice and show no dissociation. This indicates that the antiphase-boundary energy in the structure is very high, inhibiting spread of the dislocation core. Along with the observation of a widely extending strain field associated to the dislocations, our results provide a possible explanation for the high strength of this high-entropy alloy as a direct consequence of its dislocation structure.

A phase field dislocation dynamics model for a bicrystal interface system: An investigation into dislocation slip transmission across cube-on-cube interfaces

DOE PAGES

Zeng, Y.; Hunter, A.; Beyerlein, I. J.; ...

2015-09-14

In this study, we present a phase field dislocation dynamics formulation designed to treat a system comprised of two materials differing in moduli and lattice parameters that meet at a common interface. We apply the model to calculate the critical stress τ crit required to transmit a perfect dislocation across the bimaterial interface with a cube-on-cube orientation relationship. The calculation of τ crit accounts for the effects of: 1) the lattice mismatch (misfit or coherency stresses), 2) the elastic moduli mismatch (Koehler forces or image stresses), and 3) the formation of the residual dislocation in the interface. Our results showmore » that the value of τ crit associated with the transmission of a dislocation from material 1 to material 2 is not the same as that from material 2 to material 1. Dislocation transmission from the material with the lower shear modulus and larger lattice parameter tends to be easier than the reverse and this apparent asymmetry in τ crit generally increases with increases in either lattice or moduli mismatch or both. In efforts to clarify the roles of lattice and moduli mismatch, we construct an analytical model for τcrit based on the formation energy of the residual dislocation. We show that path dependence in this energetic barrier can explain the asymmetry seen in the calculated τ crit values.« less

Burgers Vector Analysis of Vertical Dislocations in Ge Crystals by Large-Angle Convergent Beam Electron Diffraction.

PubMed

Groiss, Heiko; Glaser, Martin; Marzegalli, Anna; Isa, Fabio; Isella, Giovanni; Miglio, Leo; Schäffler, Friedrich

2015-06-01

By transmission electron microscopy with extended Burgers vector analyses, we demonstrate the edge and screw character of vertical dislocations (VDs) in novel SiGe heterostructures. The investigated pillar-shaped Ge epilayers on prepatterned Si(001) substrates are an attempt to avoid the high defect densities of lattice mismatched heteroepitaxy. The Ge pillars are almost completely strain-relaxed and essentially defect-free, except for the rather unexpected VDs. We investigated both pillar-shaped and unstructured Ge epilayers grown either by molecular beam epitaxy or by chemical vapor deposition to derive a general picture of the underlying dislocation mechanisms. For the Burgers vector analysis we used a combination of dark field imaging and large-angle convergent beam electron diffraction (LACBED). With LACBED simulations we identify ideally suited zeroth and second order Laue zone Bragg lines for an unambiguous determination of the three-dimensional Burgers vectors. By analyzing dislocation reactions we confirm the origin of the observed types of VDs, which can be efficiently distinguished by LACBED. The screw type VDs are formed by a reaction of perfect 60° dislocations, whereas the edge types are sessile dislocations that can be formed by cross-slips and climbing processes. The understanding of these origins allows us to suggest strategies to avoid VDs.

Deformation behavior of HCP titanium alloy: Experiment and Crystal plasticity modeling

DOE PAGES

Wronski, M.; Arul Kumar, Mariyappan; Capolungo, Laurent; ...

2018-03-02

The deformation behavior of commercially pure titanium is studied using experiments and a crystal plasticity model. Compression tests along the rolling, transverse, and normal-directions, and tensile tests along the rolling and transverse directions are performed at room temperature to study the activation of slip and twinning in the hexagonal closed packed titanium. A detailed EBSD based statistical analysis of the microstructure is performed to develop statistics of both {10-12} tensile and {11-22} compression twins. A simple Monte Carlo (MC) twin variant selection criterion is proposed within the framework of the visco-plastic self-consistent (VPSC) model with a dislocation density (DD) basedmore » law used to describe dislocation hardening. In the model, plasticity is accommodated by prismatic, basal and pyramidal slip modes, and {10-12} tensile and {11-22} compression twinning modes. Thus, the VPSC-MC model successfully captures the experimentally observed activation of low Schmid factor twin variants for both tensile and compression twins modes. The model also predicts macroscopic stress-strain response, texture evolution and twin volume fraction that are in agreement with experimental observations.« less

Deformation behavior of HCP titanium alloy: Experiment and Crystal plasticity modeling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wronski, M.; Arul Kumar, Mariyappan; Capolungo, Laurent

The deformation behavior of commercially pure titanium is studied using experiments and a crystal plasticity model. Compression tests along the rolling, transverse, and normal-directions, and tensile tests along the rolling and transverse directions are performed at room temperature to study the activation of slip and twinning in the hexagonal closed packed titanium. A detailed EBSD based statistical analysis of the microstructure is performed to develop statistics of both {10-12} tensile and {11-22} compression twins. A simple Monte Carlo (MC) twin variant selection criterion is proposed within the framework of the visco-plastic self-consistent (VPSC) model with a dislocation density (DD) basedmore » law used to describe dislocation hardening. In the model, plasticity is accommodated by prismatic, basal and pyramidal slip modes, and {10-12} tensile and {11-22} compression twinning modes. Thus, the VPSC-MC model successfully captures the experimentally observed activation of low Schmid factor twin variants for both tensile and compression twins modes. The model also predicts macroscopic stress-strain response, texture evolution and twin volume fraction that are in agreement with experimental observations.« less

Fine tune of elasticity and plasticity of olivine by water: a key to reconcile discrepancy between mineral physics and seismology models

NASA Astrophysics Data System (ADS)

Chen, J.; Girard, J.

2012-12-01

Study of mechanical properties of mantle minerals has unveiled many mysteries of Earth's interior alluded through seismic events. However, some details of seismic models remain unexplained. For instance, magnitude of seismic discontinuity at 410 km depth in seismic models is significantly larger than that derived from elastic properties of dominant minerals at such depth. For another example, although the attenuation of seismic anisotropy in the upper mantle at about 200-220 km depth can be attributed to switchover of active dislocation slip system in the dominant mineral, olivine, the depth and its variation are discrepant from that derived from the pressure at which such switchover is observed in the deformation experiment of mineral plasticity study. We have investigated influence of water on elastic and plastic behaviors of olivine through equation of state and rheological creep experiments using synchrotron x-rays at the X17C and X17B2 beamlines of the NSLS. Results indicate water significantly weakens the mineral. Elastically, 0.4 wt% H2O in olivine results in a 5% reduction in bulk modulus (i.e. from 130 GPa for anhydrous sample to 123 GPa for hydrous sample). Plastically, structural H2O in olivine influences different dislocation slip system very differently, and therefore alters the pressure of active slip system switchover with respect to "dry" sample. Even 30 ppm H2O in weight may lower down the pressure for transition from [100](010) slip to [001](010) slip by 2 GPa (i.e. from 8 GPa in dry condition to 6 GPa in wet condition). Implications of these results will be discussed in this presentation. Together with previously reported results on elastic and plastic properties of mantle dominant minerals, we are able to reconcile the discrepancies between mineral physics and seismology models for the magnitude of 410 km discontinuity and for the attenuation of seismic anisotropy at about 200-220 km respectively.

Fabric transition with dislocation creep of a carbonate fault zone in the brittle regime

NASA Astrophysics Data System (ADS)

Kim, Sungshil; Ree, Jin-Han; Han, Raehee; Kim, Nahyeon; Jung, Haemyeong

2018-01-01

Fabric transition by a switch in the dominant slip system of minerals in the plastic regime can be induced by changes in temperature, strain rate, or water content. We propose here this fabric transition by frictional heating in seismogenic fault zones in the brittle regime. The Garam Thrust in the Taebaeksan Basin of South Korea has a hanging wall of Cambrian dolostone juxtaposed against a footwall of Ordovician limestone and records a minimum displacement of 120 m. In a 10 cm thick plastically deformed layer adjacent to the principal slip layer of the fault zone, the lattice preferred orientation of calcite grains suggests that the dominant slip system changes, approaching the principal slip layer, from r 〈02-21〉 and e-twinning, through r 〈02-21〉 and basal 〈a〉, to basal 〈a〉. This fabric transition requires a high temperature-gradient of 40 °C/cm, which we infer to result from frictional heating of the seismic fault zone. We suggest that fabric transition within a thin plastically deformed layer adjacent to the principal slip layer of a fault zone indicates an unusually steep temperature gradient and provides strong evidence of seismic slip.

First-principles study of crystallographic slip modes in ω-Zr

DOE PAGES

Kumar, Anil; Kumar, M. Arul; Beyerlein, Irene Jane

2017-08-21

We use first-principles density functional theory to study the preferred modes of slip in the high-pressure ω phase of Zr. The generalized stacking fault energy surfaces associated with shearing on nine distinct crystallographic slip modes in the hexagonal ω-Zr crystal are calculated, from which characteristics such as ideal shear stress, the dislocation Burgers vector, and possible accompanying atomic shuffles, are extracted. Comparison of energy barriers and ideal shear stresses suggests that the favorable modes are prismatic < c >, prismatic-II <101¯0> and pyramidal-II < c+a >, which are distinct from the ground state hexagonal close packed α phase of Zr.more » Operation of these three modes can accommodate any deformation state. The relative preferences among the identified slip modes are examined using a mean-field crystal plasticity model and comparing the calculated deformation texture with the measurement. In conclusion, knowledge of the basic crystallographic modes of slip is critical to understanding and analyzing the plastic deformation behavior of ω-Zr or mixed α-ω phase-Zr.« less

Crustal deformation in great California earthquake cycles

NASA Technical Reports Server (NTRS)

Li, Victor C.; Rice, James R.

1986-01-01

Periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model: A depth L (less than or similiar to H) extending downward from the Earth's surface at a transform boundary between uniform elastic lithospheric plates of thickness H is locked between earthquakes. It slips an amount consistent with remote plate velocity V sub pl after each lapse of earthquake cycle time T sub cy. Lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress. The plates are coupled at their base to a Maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions, compatible with plate velocity, are transmitted to the surface plates. The coupling is described approximately through a generalized Elsasser model. It is argued that the model gives a more realistic physical description of tectonic loading, including the time dependence of deep slip and crustal stress build up throughout the earthquake cycle, than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone.

Microstructural effects on the room and elevated temperature low cycle fatigue behavior of Waspaloy. M.S. Thesis Final Report

NASA Technical Reports Server (NTRS)

Lerch, B. A.

1982-01-01

Longitudinal specimens of Waspaloy containing either coarse grains with small gamma or fine grains with large gamma were tested in air at a frequency of 0.33 Hz or 0.50 Hz. The coarse grained structures exhibited planar slip on (III) planes and precipitate shearing at all temperatures. Cracks initiated by a Stage 1 mechanism and propagated by a striation forming mechanism. At 700 C and 800 C, cleavage and intergranular cracking were observed. Testing at 500 C, 700 C, and 800 C caused precipitation of grain boundary carbides. At 700 C, carbides precipitated on slip bands. The fine grained structures exhibited planar slip on (111) planes. Dislocations looped the large gamma precipitates. This structure led to stress saturation and propagation was observed. Increasing temperatures resulted in increased specimen oxidation for both heat treatments. Slip band and grain boundary oxidation were observed. At 800 C, oxidized grain boundaries were cracked by intersecting slip bands which resulted in intergranular failure. The fine specimens had crack initiation later in the fatigue life, but with more rapid propagation crack propagation.

Fluid-controlled faulting process in the Asal Rift, Djibouti, from 8 yr of radar interferometry observations

NASA Astrophysics Data System (ADS)

Doubre, Cécile; Peltzer, Gilles

2007-01-01

The deformation in the Asal Rift (Djibouti) is characterized by magmatic inflation, diking, distributed extension, fissure opening, and normal faulting. An 8 yr time line of surface displacement maps covering the rift, constructed using radar interferometry data acquired by the Canadian satellite Radarsat between 1997 and 2005, reveals the aseismic behavior of faults and its relation with bursts of microseismicity. The observed ground movements show the asymmetric subsidence of the inner floor of the rift with respect to the bordering shoulders accommodated by slip on three of the main active faults. Fault slip occurs both as steady creep and during sudden slip events accompanied by an increase in the seismicity rate around the slipping fault and the Fieale volcanic center. Slip distribution along fault strike shows triangular sections, a pattern not explained by simple elastic dislocation theory. These observations suggest that the Asal Rift faults are in a critical failure state and respond instantly to small pressure changes in fluid-filled fractures connected to the faults, reducing the effective normal stress on their locked section at depth.

Three-Dimensional Stress Fields and Slip Systems for Single Crystal Superalloy Notched Specimens

NASA Technical Reports Server (NTRS)

Magnan, Shannon M.; Throckmorton, David (Technical Monitor)

2002-01-01

Single crystal superalloys have become increasingly popular for turbine blade and vane applications due to their high strength, and creep and fatigue resistance at elevated temperatures. The crystallographic orientation of a single crystal material greatly affects its material properties, including elastic modulus, shear modulus, and ductility. These directional properties, along with the type of loading and temperature, dictate an anisotropic response in the yield strength, creep resistance, creep rupture ductility, fatigue resistance, etc. A significant amount of research has been conducted to determine the material properties in the <001> orientation, yet the material properties deviating from the <001> orientation have not been assessed for all cases. Based on the desired application and design criteria, a crystal orientation is selected to yield the maximum properties. Currently, single crystal manufacturing is able to control the primary crystallographic orientation within 15 of the target orientation, which is an acceptable deviation to meet both performance and cost guidelines; the secondary orientation is rarely specified. A common experiment is the standard load-controlled tensile test, in which specimens with different orientations can be loaded to observe the material response. The deformation behavior of single-crystal materials under tension and compression is known to be a function of not only material orientation, but also of varying microdeformation (i.e. dislocation) mechanisms. The underlying dislocation motion causes deformation via slip, and affects the activation of specific slip systems based on load and orientation. The slip can be analyzed by observing the visible traces left on the surface of the specimen from the slip activity within the single crystal material. The goal of this thesis was to predict the slip systems activated in three-dimensional stress fields of a notched tensile specimen, as a function of crystal orientation, using finite element analysis without addressing microstructural deformation mechanisms that govern their activation. Out of three orientations tested, the specimen with a [110] load orientation and a [001] growth direction had the lowest maximum resolved shear stress; this specimen orientation appears to be the best design candidate for a tensile application.

A new finite element code for the study of strain-localization under strike-slip faults

NASA Astrophysics Data System (ADS)

Rodríguez-González, J.; Montesi, L.

2016-12-01

Shear localization under strike-slip faults in ductile conditions remains a matter of debate. The rheology of rocks in the ductile regime is fundamentally strain-rate hardening, which complicates the understanding of the formation of narrow shear zones. Localized shear zones are present in a variety of scales, including kilometric structures at plate boundaries. To compensate for strain-rate hardening, shear zones must be weaker than their surroundings thanks to some weakening mechanism that works at multiple length scales. Mechanisms as shear heating or grain size reduction have been invoked to explain localization of deformation, but none of these mechanisms can work in scales that range from 1 to 1000 km. Layered fabric development has been suggested as a candidate to develop localized shear zones at multiple scales. To test this hypothesis, we have developed a new software that uses the Finite Element Method library deal.II written in C++. We solve the elasticity equations for elastic and Maxwell visco-elastic mediums. A key component required to study strain localization is adaptive mesh refinement. The code automatically identifies those regions in which the deformation is being localized and will increase the resolution. We benchmark the code and test its accuracy using analytical solutions of strike-slip deformation with different boundary conditions. We simulate the instantaneous deformation caused by two kinds of dislocations: a free fault subject to a far field traction and fault with an imposed displacement. We also simulate the visco-elastic relaxation following a strike-slip dislocation. We show that deal.II is a flexible library, suitable for different problems, which will prove useful to study the mechanisms that can lead to strain localization.

Dislocation-induced stress in polycrystalline materials: mesoscopic simulations in the dislocation density formalism

NASA Astrophysics Data System (ADS)

Berkov, D. V.; Gorn, N. L.

2018-06-01

In this paper we present a simple and effective numerical method which allows a fast Fourier transformation-based evaluation of stress generated by dislocations with arbitrary directions and Burgers vectors if the (site-dependent) dislocation density is known. Our method allows the evaluation of the dislocation stress using a rectangular grid with shape-anisotropic discretization cells without employing higher multipole moments of the dislocation interaction coefficients. Using the proposed method, we first simulate the stress created by relatively simple non-homogeneous distributions of vertical edge and so-called ‘mixed’ dislocations in a disk-shaped sample, which is necessary to understand the dislocation behavior in more complicated systems. The main part of our research is devoted to the stress distribution in polycrystalline layers with the dislocation density rapidly varying with the distance to the layer bottom. Considering GaN as a typical example of such systems, we investigate dislocation-induced stress for edge and mixed dislocations, having random orientations of Burgers vectors among crystal grains. We show that the rapid decay of the dislocation density leads to many highly non-trivial features of the stress distributions in such layers and study in detail the dependence of these features on the average grain size. Finally we develop an analytical approach which allows us to predict the evolution of the stress variance with the grain size and compare analytical predictions with numerical results.

Structure of screw dislocation core in Ta at high pressure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Shaofeng, E-mail: sfwang@cqu.edu.cn; Jiang, Na; Wang, Rui

2014-03-07

The core structure and Peierls stress of the 1/2 〈111〉(110) screw dislocation in Ta have been investigated theoretically using the modified Peierls–Nabarro theory that takes into account the discreteness effect of crystal. The lattice constants, the elastic properties, and the generalized-stacking-fault energy(γ-surface) under the different pressures have been calculated from the electron density functional theory. The core structure of dislocation is determined by the modified Peierls equation, and the Peierls stress is evaluated from the dislocation energy that varies periodically as dislocation moves. The results show the core width and Peierls stress in Ta are weakly dependent of the pressuremore » up to 100 GPa when the length and stress are measured separately by the Burgers vector b and shear modulus μ. This indicates that core structure is approximately scaling invariant for the screw dislocation in Ta. The scaled plasticity of Ta changes little in high pressure environment.« less

Free energy change of a dislocation due to a Cottrell atmosphere

NASA Astrophysics Data System (ADS)

Sills, R. B.; Cai, W.

2018-06-01

The free energy reduction of a dislocation due to a Cottrell atmosphere of solutes is computed using a continuum model. We show that the free energy change is composed of near-core and far-field components. The far-field component can be computed analytically using the linearized theory of solid solutions. Near the core the linearized theory is inaccurate, and the near-core component must be computed numerically. The influence of interactions between solutes in neighbouring lattice sites is also examined using the continuum model. We show that this model is able to reproduce atomistic calculations of the nickel-hydrogen system, predicting hydride formation on dislocations. The formation of these hydrides leads to dramatic reductions in the free energy. Finally, the influence of the free energy change on a dislocation's line tension is examined by computing the equilibrium shape of a dislocation shear loop and the activation stress for a Frank-Read source using discrete dislocation dynamics.

Source Process of the 2007 Niigata-ken Chuetsu-oki Earthquake Derived from Near-fault Strong Motion Data

NASA Astrophysics Data System (ADS)

Aoi, S.; Sekiguchi, H.; Morikawa, N.; Ozawa, T.; Kunugi, T.; Shirasaka, M.

2007-12-01

The 2007 Niigata-ken Chuetsu-oki earthquake occurred on July 16th, 2007, 10:13 JST. We performed a multi- time window linear waveform inversion analysis (Hartzell and Heaton, 1983) to estimate the rupture process from the near fault strong motion data of 14 stations from K-NET, KiK-net, F-net, JMA, and Niigata prefecture. The fault plane for the mainshock has not been clearly determined yet from the aftershock distribution, so that we performed two waveform inversions for north-west dipping fault (Model A) and south-east dipping fault (Model B). Their strike, dip, and rake are set to those of the moment tensor solutions by F-net. Fault plane model of 30 km length by 24 km width is set to cover aftershock distribution within 24 hours after the mainshock. Theoretical Green's functions were calculated by the discrete wavenumber method (Bouchon, 1981) and the R/T matrix method (Kennett, 1983) with the different stratified medium for each station based on the velocity structure including the information form the reflection survey and borehole logging data. Convolution of moving dislocation was introduced to represent the rupture propagation in an each subfault (Sekiguchi et al., 2002). The observed acceleration records were integrated into velocity except of F-net velocity data, and bandpass filtered between 0.1 and 1.0 Hz. We solved least-squared equation to obtain slip amount of each time window on each subfault to minimize squared residual of the waveform fitting between observed and synthetic waveforms. Both models provide moment magnitudes of 6.7. Regarding Model A, we obtained large slip in the south-west deeper part of the rupture starting point, which is close to Kashiwazaki-city. The second or third velocity pulses of observed velocity waveforms seem to be composed of slip from the asperity. Regarding Model B, we obtained large slip in the southwest shallower part of the rupture starting point, which is also close to Kashiwazaki-city. In both models, we found small slip near the rupture starting point, and largest slip at about ten kilometer in the south-west of the rupture starting point with the maximum slip of 2.3 and 2.5 m for Models A and B, respectively. The difference of the residual between observed and synthetic waveforms for both models is not significant, therefore it is difficult to conclude which fault plane is appropriate to explain. The estimated large-slip regions in the inverted source models with the Models A and B are located near the cross point of the two fault plane models, which should have similar radiation pattern. This situation may be one of the reasons why judgment of the fault plane orientation is such difficult. We need careful examinations not only strong motion data but also geodetic data to further explore the fault orientation and the source process of this earthquake.

The Microseismicity of Glacier Sliding

NASA Astrophysics Data System (ADS)

Walter, Fabian; Röösli, Claudia; Kissling, Edi

2017-04-01

Our understanding of glacier and ice sheet basal motion remains incomplete. The past decades have witnessed a shift away from initially proposed hard bed theories towards soft, till-laden beds, which deform and thus participate in basal motion. The theoretical treatment of deformable beds is subject to debate, yet our capability to predict ice sheet flow and ultimately sea level rise is contingent upon correct parameterization of basal motion (Ritz et al., 2015). Both hard and soft bed theories neglect frictional sliding across distinct basal fault planes and elastic deformation in response to sudden dislocation. Over recent years, this view has been repeatedly challenged as more and more studies report seismogenic faulting associated with basal sliding. For instance, large parts of the Whillans Ice Stream at Antarctica's Siple Coast move nearly exclusively during sudden sliding episodes (Wiens et al., 2008). This "stick-slip motion" is difficult to explain with traditional glacier sliding theories but more analogous to earthquake dislocation on tectonic faults. Although the Whillans Ice Stream motion may be an extreme example, there exists evidence for much smaller microseismic stick-slip events beneath the Greenland Ice Sheet and non-polar glaciers (Podolskiy and Walter, 2016). This raises the question how relevant and widespread the stick-slip phenomenon is and if it is necessary to include it into ice sheet models. Here we discuss recent seismic deployments, which focused on detection of stick-slip events beneath the Greenland Ice Sheet and European Alpine Glaciers. For all deployments, a considerable challenge lies in detection of stick-slip seismograms in the presence of a dominant background seismicity associated with surface crevassing. Nevertheless, automatic search algorithms and waveform characteristics provide important insights into temporal variation of stick-slip activity as well as information about fault plane geometry and co-seismic sliding direction. REFERENCES E.A. Podolskiy and F. Walter (2016). Cryo-seismology. Reviews of Geophysics. Ritz, C., Edwards, T. L., Durand, G., Payne, A. J., Peyaud, V., & Hindmarsh, R. C. (2015). Potential sea-level rise from Antarctic ice-sheet instability constrained by observations. Nature, 528(7580), 115-118. Wiens, D. A., Anandakrishnan, S., Winberry, J. P., & King, M. A. (2008). Simultaneous teleseismic and geodetic observations of the stick-slip motion of an Antarctic ice stream. Nature, 453(7196), 770-774.

A 3D dislocation dynamics analysis of the size effect on the strength of [1 1 1] LiF micropillars at 300K and 600K

NASA Astrophysics Data System (ADS)

Chang, Hyung-Jun; Segurado, Javier; Molina-Aldareguía, Jon M.; Soler, Rafael; LLorca, Javier

2016-03-01

The mechanical behavior in compression of [1 1 1] LiF micropillars with diameters in the range 0.5 μm to 2.0 μm was analyzed by means of discrete dislocation dynamics at ambient and elevated temperature. The dislocation velocity was obtained from the Peach-Koehler force acting on the dislocation segments from a thermally-activated model that accounted for the influence of temperature on the lattice resistance. A size effect of the type ‘smaller is stronger’ was predicted by the simulations, which was in quantitative agreement with previous experimental results by the authors [1]. The contribution of the different physical deformation mechanisms to the size effect (namely, nucleation of dislocations, dislocation exhaustion and forest hardening) could be ascertained from the simulations and the dominant deformation mode could be assessed as a function of the specimen size and temperature. These results shed light into the complex interaction among size, lattice resistance and dislocation mobility in the mechanical behavior of μm-sized single crystals.

Mechanical properties of β-HMX.

PubMed

Gallagher, Hugh G; Miller, John C; Sheen, David B; Sherwood, John N; Vrcelj, Ranko M

2015-01-01

For a full understanding of the mechanical properties of a material, it is essential to understand the defect structures and associated properties and microhardness indentation is a technique that can aid this understanding. The Vickers hardness on (010), {011} and {110} faces lay in the range of 304-363 MPa. The Knoop Hardnesses on the same faces lay in the range 314-482 MPa. From etching of three indented surfaces, the preferred slip planes have been identified as (001) and (101). For a dislocation glide, the most likely configuration for dislocation movement on the (001) planes is (001) [100] (|b| = 0.65 nm) and for the (101) plane as (101) [Formula: see text] (|b| = 1.084 nm) although (101) [010] (|b| = 1.105 nm) is possible. Tensile testing showed that at a stress value of 2.3 MPa primary twinning occurred and grew with increasing stress. When the stress was relaxed, the twins decreased in size, but did not disappear. The twinning shear strain was calculated to be 0.353 for the (101) twin plane. HMX is considered to be brittle, compared to other secondary explosives. Comparing HMX with a range of organic solids, the values for hardness numbers are similar to those of other brittle systems. Under the conditions developed beneath a pyramidal indenter, dislocation slip plays a major part in accommodating the local deformation stresses. Graphical abstractHMX undergoing tensile testing.

Replace with abstract title

NASA Astrophysics Data System (ADS)

Coho, Aleksander; Kioussis, Nicholas

2003-03-01

We use the semidiscrete variational generelized Peierls-Nabarro model to study the effect of Cu alloying on the dislocation properties of Al. First-principles density functional theory (DFT) is used to calculate the generalized-stacking-fault (GSF) energy surface when a <111> plane, on which one in four Al atoms has been replaced with a Cu atom, slips over a pure Al <111> plane. Various dislocation core properties (core width, energy, Peierls stress, dissociation tendency) are investigated and compared with the pure Al case. Cu alloying lowers the intrinsic stacking fault (ISF) energy, which makes dislocations more likely to dissociate into partials. We also try to understand the lowering of ISF energy in terms of Al-Cu and Al-Al bond formation and braking during shearing along the <112> direction. From the above we draw conclusions about the effects of Cu alloying on the mechanical properties of Al.

Collective behaviour of dislocations in a finite medium

NASA Astrophysics Data System (ADS)

Kooiman, M.; Hütter, M.; Geers, M. G. D.

2014-04-01

We derive the grand-canonical partition function of straight and parallel dislocation lines without making a priori assumptions on the temperature regime. Such a systematic derivation for dislocations has, to the best of our knowledge, not been carried out before, and several conflicting assumptions on the free energy of dislocations have been made in the literature. Dislocations have gained interest as they are the carriers of plastic deformation in crystalline materials and solid polymers, and they constitute a prototype system for two-dimensional Coulomb particles. Our microscopic starting level is the description of dislocations as used in the discrete dislocation dynamics (DDD) framework. The macroscopic level of interest is characterized by the temperature, the boundary deformation and the dislocation density profile. By integrating over state space, we obtain a field theoretic partition function, which is a functional integral of the Boltzmann weight over an auxiliary field. The Hamiltonian consists of a term quadratic in the field and an exponential of this field. The partition function is strongly non-local, and reduces in special cases to the sine-Gordon model. Moreover, we determine implicit expressions for the response functions and the dominant scaling regime for metals, namely the low-temperature regime.

Growth and dislocation studies of β-HMX.

PubMed

Gallagher, Hugh G; Sherwood, John N; Vrcelj, Ranko M

2014-01-01

The defect structure of organic materials is important as it plays a major role in their crystal growth properties. It also can play a subcritical role in "hot-spot" detonation processes of energetics and one such energetic is cyclotetramethylene-tetranitramine, in the commonly used beta form (β-HMX). The as-grown crystals grown by evaporation from acetone show prismatic, tabular and columnar habits, all with {011}, {110}, (010) and (101) faces. Etching on (010) surfaces revealed three different types of etch pits, two of which could be identified with either pure screw or pure edge dislocations, the third is shown to be an artifact of the twinning process that this material undergoes. Examination of the {011} and {110} surfaces show only one type of etch pit on each surface; however their natural asymmetry precludes the easy identification of their Burgers vector or dislocation type. Etching of cleaved {011} surfaces demonstrates that the etch pits can be associated with line dislocations. All dislocations appear randomly on the crystal surfaces and do not form alignments characteristic of mechanical deformation by dislocation slip. Crystals of β-HMX grown from acetone show good morphological agreement with that predicted by modelling, with three distinct crystal habits observed depending upon the supersaturation of the growth solution. Prismatic habit was favoured at low supersaturation, while tabular and columnar crystals were predominant at higher super saturations. The twin plane in β-HMX was identified as a (101) reflection plane. The low plasticity of β-HMX is shown by the lack of etch pit alignments corresponding to mechanically induced dislocation arrays. On untwinned {010} faces, two types of dislocations exist, pure edge dislocations with b = [010] and pure screw dislocations with b = [010]. On twinned (010) faces, a third dislocation type exists and it is proposed that these pits are associated with pure screw dislocations with b = [010]. Graphical abstractEtch pits on the twinned (010) face of β-HMX.

Discrete Dislocation Modeling of Fatigue

NASA Astrophysics Data System (ADS)

Needleman, Alan

2004-03-01

In joint work with V.S. Deshpande of Cambridge University and E. Van der Giessen of the University of Groningen a framework has been developed for the analysis of crack growth under cyclic loading conditions where plastic flow arises from the motion of large numbers of discrete dislocations and the fracture properties are embedded in a cohesive surface constitutive relation. The material model is independent of the presence of a crack and the only distinction between an analysis of monotonic crack growth and fatigue crack growth is that in fatigue the remote loading is specified to be an oscillating function of time. Thus, a basic question is: within this framework, do cracks grow at a lower driving force under cyclic loading than under monotonic loading, and if so, what features of fatigue crack growth emerge? Fatigue does emerge from the calculations as a consequence of the evolution of internal stresses associated with the irreversibility of the dislocation motion. A fatigue threshold, Paris law behavior, striations and the accelerated growth of short cracks are outcomes of the simulations. Also, scaling predictions obtained for the fatigue threshold and the fatigue crack growth rate are discussed.

Characterization and modeling of mechanical behavior of single crystal titanium deformed by split-Hopkinson pressure bar

DOE PAGES

Morrow, B. M.; Lebensohn, R. A.; Trujillo, C. P.; ...

2016-03-28

Single crystal titanium samples were dynamically loaded using split-Hopkinson pressure bar (SHPB) and the resulting microstructures were examined. Characterization of the twins and dislocations present in the microstructure was conducted to understand the pathway for observed mechanical behavior. Electron backscatter diffraction (EBSD) was used to measure textures and quantify twinning. Microstructures were profusely twinned after loading, and twin variants and corresponding textures were different as a function of initial orientation. Focused ion beam (FIB) foils were created to analyze dislocation content using transmission electron microscopy (TEM). Large amounts of dislocations were present, indicating that plasticity was achieved through slip andmore » twinning together. Viscoplastic self-consistent (VPSC) modeling was used to confirm the complex order of operations during deformation. The activation of different mechanisms was highly dependent upon crystal orientation. For [0001] and View the MathML source[101¯1]-oriented crystals, compressive twinning was observed, followed by secondary tensile twinning. Furthermore, dislocations though prevalent in the microstructure, contributed to final texture far less than twinning.« less

Dislocation mechanisms and 3D twin architectures generate exceptional strength-ductility-toughness combination in CrCoNi medium-entropy alloy

DOE PAGES

Zhang, Zijiao; Sheng, Hongwei; Wang, Zhangjie; ...

2017-02-20

Combinations of high strength and ductility are hard to attain in metals. Exceptions include materials exhibiting twinning-induced plasticity. To understand how the strength-ductility trade-off can be defeated, we apply in situ, and aberration-corrected scanning, transmission electron microscopy to examine deformation mechanisms in the medium-entropy alloy CrCoNi that exhibits one of the highest combinations of strength, ductility and toughness on record. Ab initio modelling suggests that it has negative stacking-fault energy at 0K and high propensity for twinning. With deformation we find that a three-dimensional (3D) hierarchical twin network forms from the activation of three twinning systems. This serves a dualmore » function: conventional twin-boundary (TB) strengthening from blockage of dislocations impinging on TBs, coupled with the 3D twin network which offers pathways for dislocation glide along, and cross-slip between, intersecting TB-matrix interfaces. The stable twin architecture is not disrupted by interfacial dislocation glide, serving as a continuous source of strength, ductility and toughness.« less

Dislocation mechanisms and 3D twin architectures generate exceptional strength-ductility-toughness combination in CrCoNi medium-entropy alloy

PubMed Central

Zhang, Zijiao; Sheng, Hongwei; Wang, Zhangjie; Gludovatz, Bernd; Zhang, Ze; George, Easo P.; Yu, Qian; Mao, Scott X.; Ritchie, Robert O.

2017-01-01

Combinations of high strength and ductility are hard to attain in metals. Exceptions include materials exhibiting twinning-induced plasticity. To understand how the strength-ductility trade-off can be defeated, we apply in situ, and aberration-corrected scanning, transmission electron microscopy to examine deformation mechanisms in the medium-entropy alloy CrCoNi that exhibits one of the highest combinations of strength, ductility and toughness on record. Ab initio modelling suggests that it has negative stacking-fault energy at 0K and high propensity for twinning. With deformation we find that a three-dimensional (3D) hierarchical twin network forms from the activation of three twinning systems. This serves a dual function: conventional twin-boundary (TB) strengthening from blockage of dislocations impinging on TBs, coupled with the 3D twin network which offers pathways for dislocation glide along, and cross-slip between, intersecting TB-matrix interfaces. The stable twin architecture is not disrupted by interfacial dislocation glide, serving as a continuous source of strength, ductility and toughness. PMID:28218267

Dislocation mechanisms and 3D twin architectures generate exceptional strength-ductility-toughness combination in CrCoNi medium-entropy alloy.

PubMed

Zhang, Zijiao; Sheng, Hongwei; Wang, Zhangjie; Gludovatz, Bernd; Zhang, Ze; George, Easo P; Yu, Qian; Mao, Scott X; Ritchie, Robert O

2017-02-20

Combinations of high strength and ductility are hard to attain in metals. Exceptions include materials exhibiting twinning-induced plasticity. To understand how the strength-ductility trade-off can be defeated, we apply in situ, and aberration-corrected scanning, transmission electron microscopy to examine deformation mechanisms in the medium-entropy alloy CrCoNi that exhibits one of the highest combinations of strength, ductility and toughness on record. Ab initio modelling suggests that it has negative stacking-fault energy at 0K and high propensity for twinning. With deformation we find that a three-dimensional (3D) hierarchical twin network forms from the activation of three twinning systems. This serves a dual function: conventional twin-boundary (TB) strengthening from blockage of dislocations impinging on TBs, coupled with the 3D twin network which offers pathways for dislocation glide along, and cross-slip between, intersecting TB-matrix interfaces. The stable twin architecture is not disrupted by interfacial dislocation glide, serving as a continuous source of strength, ductility and toughness.

Areas prone to slow slip events impede earthquake rupture propagation and promote afterslip.

PubMed

Rolandone, Frederique; Nocquet, Jean-Mathieu; Mothes, Patricia A; Jarrin, Paul; Vallée, Martin; Cubas, Nadaya; Hernandez, Stephen; Plain, Morgan; Vaca, Sandro; Font, Yvonne

2018-01-01

At subduction zones, transient aseismic slip occurs either as afterslip following a large earthquake or as episodic slow slip events during the interseismic period. Afterslip and slow slip events are usually considered as distinct processes occurring on separate fault areas governed by different frictional properties. Continuous GPS (Global Positioning System) measurements following the 2016 M w (moment magnitude) 7.8 Ecuador earthquake reveal that large and rapid afterslip developed at discrete areas of the megathrust that had previously hosted slow slip events. Regardless of whether they were locked or not before the earthquake, these areas appear to persistently release stress by aseismic slip throughout the earthquake cycle and outline the seismic rupture, an observation potentially leading to a better anticipation of future large earthquakes.

Areas prone to slow slip events impede earthquake rupture propagation and promote afterslip

PubMed Central

Rolandone, Frederique; Nocquet, Jean-Mathieu; Mothes, Patricia A.; Jarrin, Paul; Vallée, Martin; Cubas, Nadaya; Hernandez, Stephen; Plain, Morgan; Vaca, Sandro; Font, Yvonne

2018-01-01

At subduction zones, transient aseismic slip occurs either as afterslip following a large earthquake or as episodic slow slip events during the interseismic period. Afterslip and slow slip events are usually considered as distinct processes occurring on separate fault areas governed by different frictional properties. Continuous GPS (Global Positioning System) measurements following the 2016 Mw (moment magnitude) 7.8 Ecuador earthquake reveal that large and rapid afterslip developed at discrete areas of the megathrust that had previously hosted slow slip events. Regardless of whether they were locked or not before the earthquake, these areas appear to persistently release stress by aseismic slip throughout the earthquake cycle and outline the seismic rupture, an observation potentially leading to a better anticipation of future large earthquakes. PMID:29404404

Indentation Schmid factor and orientation dependence of nanoindentation pop-in behavior of NiAl single crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Tianlei; Gao, Yanfei; Bei, Hongbin

2011-01-01

Instrumented nanoindentation techniques have been widely used to characterize the small-scale mechanical behavior of materials. The elastic-plastic transition during nanoindentation is often indicated by a sudden displacement burst (pop-in) in the measured load-displacement curve. In defect-free single crystals, the pop-in is believed to be the result of homogeneous dislocation nucleation because the maximum shear stress corresponding to the pop-in load approaches the theoretical strength of the materials and because the statistical distribution of pop-in stresses is consistent with what is expected for a thermally activated process of homogeneous dislocation nucleation. This paper investigates whether this process is affected by crystallographymore » and stress components other than the resolved shear stress. A Stroh formalism coupled with the two-dimensional Fourier transformation is used to derive the analytical stress fields in elastically anisotropic solids under Hertzian contact, which allows the determination of an indentation Schmid factor, namely, the ratio of maximum resolved shear stress to the maximum contact pressure. Nanoindentation tests were conducted on B2-structured NiAl single crystals with different surface normal directions. This material was chosen because it deforms at room temperature by {110}<001> slip and thus avoids the complexity of partial dislocation nucleation. Good agreement is obtained between the experimental data and the theoretically predicted orientation dependence of pop-in loads based on the indentation Schmid factor. Pop-in load is lowest for indentation directions close to <111> and highest for those close to <001>. In nanoindentation, since the stress component normal to the slip plane is typically comparable in magnitude to the resolved shear stress, we find that the pressure sensitivity of homogeneous dislocation nucleation cannot be determined from pop-in tests. Our statistical measurements generally confirm the thermal activation model of homogeneous dislocation nucleation. That is, the extracted dependence of activation energy on resolved shear stress is almost the same for all the indentation directions considered in this study, except for those close to <001>. Because very high pop-in loads are measured for orientations close to <001>, which implies a large contact area at pop-in, there is a higher probability of activating pre-existing dislocations in these orientations, which may explain the discrepancy near <001>.« less

Characterization of deformation mechanisms in zirconium alloys: effect of temperature and irradiation

NASA Astrophysics Data System (ADS)

Long, Fei

Zirconium alloys have been widely used in the CANDU (CANada Deuterium Uranium) reactor as core structural materials. Alloy such as Zircaloy-2 has been used for calandria tubes; fuel cladding; the pressure tube is manufactured from alloy Zr-2.5Nb. During in-reactor service, these alloys are exposed to a high flux of fast neutron at elevated temperatures. It is important to understand the effect of temperature and irradiation on the deformation mechanism of zirconium alloys. Aiming to provide experimental guidance for future modeling predictions on the properties of zirconium alloys this thesis describes the result of an investigation of the change of slip and twinning modes in Zircaloy-2 and Zr-2.5Nb as a function of temperature and irradiation. The aim is to provide scientific fundamentals and experimental evidences for future industry modeling in processing technique design, and in-reactor property change prediction of zirconium components. In situ neutron diffraction mechanical tests carried out on alloy Zircaloy-2 at three temperatures: 100¢ªC, 300¢ªC, and 500¢ªC, and described in Chapter 3. The evolution of the lattice strain of individual grain families in the loading and Poisson's directions during deformation, which probes the operation of slip and twinning modes at different stress levels, are described. By using the same type of in situ neutron diffraction technique, tests on Zr-2.5Nb pressure tube material samples, in either the fast-neutron irradiated or un-irradiated condition, are reported in Chapter 4. In Chapter 5, the measurement of dislocation density by means of line profile analysis of neutron diffraction patterns, as well as TEM observations of the dislocation microstructural evolution, is described. In Chapter 6 a hot-rolled Zr-2.5Nb with a larger grain size compared with the pressure tubing was used to study the development of dislocation microstructures with increasing plastic strain. In Chapter 7, in situ loading of heavy ion irradiated hot-rolled Zr-2.5Nb alloy is described, providing evidence for the interaction between moving dislocations and irradiation induced loops. Chapter 8 gives the effect on the dislocation structure of different levels of compressive strains along two directions in the hot-rolled Zr-2.5Nb alloy. By using high resolution neutron diffraction and TEM observations, the evolution of type and dislocation densities, as well as changes of dislocation microstructure with plastic strain were characterized.

The deformation mechanisms and size effects of single-crystal magnesium

NASA Astrophysics Data System (ADS)

Byer, Cynthia M.

In this work, we seek to understand the deformation mechanisms and size effects of single-crystal magnesium at the micrometer scale through both microcompression experiments and finite element simulations. Microcompression experiments are conducted to investigate the impact of initial dislocation density and orientation on size effects. Micropillars are fabricated using a focused ion beam and tested in a Nanoindenter using a diamond fiat tip as a compression platen. Two different initial dislocation densities are examined for [0001] oriented micropillars. Our results demonstrate that decreasing the initial dislocation density results in an increased size effect in terms of increased strength and stochasticity. Microcompression along the [23¯14] axis results in much lower strengths than for [0001] oriented samples. Post-mortem analysis reveals basal slip in both [0001] and [23¯14] micropillars. The application of a stochastic probability model shows good agreement between theoretical predictions and experimental results for size effects with our values of initial dislocation density and micropillar dimensions. Size effects are then incorporated into a single-crystal plasticity model (modified from Zhang and Joshi [1]) implemented in ABAQUS/STANDARD as a user-material subroutine. The model successfully captures the phenomena typically associated with size effects of increasing stochasticity and strength with decreasing specimen size and also accounts for the changing trends resulting from variations in initial dislocation density that we observe in the experiments. Finally, finite element simulations are performed with the original (traditional, without size effects) crystal plasticity model [1] to investigate the relative activities of the deformation modes of single-crystal magnesium for varying degrees of misalignment in microcompression. The simulations reveal basal activity in all micropillars, even for perfectly aligned compression along the [0001] axis. Pyramidal < c + a > activity dominates until the misalignment increases to 2°, when basal slip takes over as the dominant mode. The stress-strain curves for the case of 0° misalignment agrees well with experimental curves, indicating that good alignment was achieved during the experiments. Through this investigation, we gain a better understanding of how to control the size effects, as well as the deformation mechanisms operating at the small scale in magnesium.

Size dependence of yield strength simulated by a dislocation-density function dynamics approach

NASA Astrophysics Data System (ADS)

Leung, P. S. S.; Leung, H. S.; Cheng, B.; Ngan, A. H. W.

2015-04-01

The size dependence of the strength of nano- and micron-sized crystals is studied using a new simulation approach in which the dynamics of the density functions of dislocations are modeled. Since any quantity of dislocations can be represented by a density, this approach can handle large systems containing large quantities of dislocations, which may handicap discrete dislocation dynamics schemes due to the excessive computation time involved. For this reason, pillar sizes spanning a large range, from the sub-micron to micron regimes, can be simulated. The simulation results reveal the power-law relationship between strength and specimen size up to a certain size, beyond which the strength varies much more slowly with size. For specimens smaller than ∼4000b, their strength is found to be controlled by the dislocation depletion condition, in which the total dislocation density remains almost constant throughout the loading process. In specimens larger than ∼4000b, the initial dislocation distribution is of critical importance since the presence of dislocation entanglements is found to obstruct deformation in the neighboring regions within a distance of ∼2000b. This length scale suggests that the effects of dense dislocation clusters are greater in intermediate-sized specimens (e.g. 4000b and 8000b) than in larger specimens (e.g. 16 000b), according to the weakest-link concept.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, D.F.; Zhang, X.M.; Gautier, E.

The morphology and crystallography of deformation-induced martensites formed during isothermal tensile tests in Fe-30Ni-0.34C and Fe-25Ni-0.66C alloys were investigated by means of optical, transmission electron and scanning electron microscopy. Transitions in morphology from thin plate to coupled plate, to lenticular coupled-plate martensite and from thin plate to lenticular to compact martensite have been observed with increasing deformation. Stress favors the growth of martensite when concurrent plastic strain allows accommodation of macroscopic transformation strains and the change of the Bain strain accommodation mechanism. Mobile dislocations and emission dislocations are directly related to the change of the Bain strain accommodation mechanism frommore » twinning to slip.« less

Solution softening in magnesium alloys: the effect of solid solutions on the dislocation core structure and nonbasal slip.

PubMed

Tsuru, T; Udagawa, Y; Yamaguchi, M; Itakura, M; Kaburaki, H; Kaji, Y

2013-01-16

There is a pressing need to improve the ductility of magnesium alloys so that they can be applied as lightweight structural materials. In this study, a mechanism for enhancing the ductility of magnesium alloys has been pursued using the atomistic method. The generalized stacking fault (GSF) energies for basal and prismatic planes in magnesium were calculated by using density functional theory, and the effect of the GSF energy on the dislocation core structures was examined using a semidiscrete variational Peierls-Nabarro model. Yttrium was found to have an anomalous influence on the solution softening owing to a reduction in the GSF energy gradient.

Time-dependent stress concentration and microcrack nucleation in TiAl

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yoo, M.H.

1995-07-01

Localized stress evolution associated with the interaction of slip or twinning with an interface is treated by means of a superposition of the {open_quotes}internal loading{close_quotes} of a crystalline subsystem by dynamic dislocation pile-up and the stress relaxation by climb of interfacial dislocations. The peak value of a stress concentration factor depends on both the angular function that includes the effect of mode mixity and the ratio of characteristic times for stress relaxation and internal loading. The available experimental data on orientation and strain-rate dependences of interfacial fracture mode in polysynthetically twinned TiAl crystals are discussed in view of the theoreticalmore » concepts presented in this paper.« less

Re-examination of the Present Stress State of the Atera Fault, Central Japan, Based on the Calibrated Crustal Stress Data of Hydraulic Fracturing Test by Measuring the Tensile Strength of Rocks

NASA Astrophysics Data System (ADS)

Yamashita, F.; Mizoguchi, K.; Fukuyama, E.; Omura, K.

2008-12-01

To infer the activity and physical state of intraplate faults in Japan, we re-examined the crustal stress with the hydraulic fracturing test by measuring the tensile strength of rocks. The tensile strength was measured by fracturing hollow cylindrical rock samples (inner and outer radius are 25.0-25.2 mm and 55.1-101.5 mm, respectively, length is 137.0-140.1 mm) which were obtained close to the in situ stress measurement locations by pressurizing the inner hole of the sample. Confining pressure is not applied to the samples in this test. To check the reliability and accuracy of this test, we conducted similar experiments with the standard rock sample (Inada granite) whose physical property is well known. Then, we measured the tensile strength of all available core samples including the Atera fault (at Ueno, Fukuoka, and Hatajiri), the Atotsugawa fault, and the Nojima fault (at Hirabayashi, Iwaya and Kabutoyama), in central Japan, which had been obtained by the National Research Institute for Earth Science and Disaster Prevention (NIED) by the stress measurement with the hydraulic fracturing method. The measured tensile strength data reveals that the in situ re- opening pressure, which is one of the parameters needed for the determination of the maximum in situ horizontal stress, was obviously biased. We re-estimated the re-opening pressure using the measured tensile strength and the in situ breakdown pressure, and re-calculated the in situ stress around the Atera fault. Although the past dislocation of the Atera fault has been considered to be left lateral from the geographical features around the fault, the re-estimated stress suggests that the present dislocation of the Atera fault is right lateral. And the shear stress decreases from the fault. The right lateral dislocation is also supported by the present-day horizontal crustal deformation observed by the triangular and GPS surveys by Geographical Survey Institute in Japan. Therefore, the dislocation direction of the Atera fault seems to change from left lateral to right lateral some time ago. The amount of accumulated right lateral dislocation estimated from the stress data with the dislocation model by Okada (1992) is 2.2-2.6 m. Because the current slip rate from the GPS survey is 2.1-2.3 mm/yr, the accumulation period of the dislocation becomes 960-1240 years if the slip rate is stable. This estimation suggests that during the last 1586 Tensho earthquake the Atera fault dislocated right laterally.

Fault geometry inversion and slip distribution of the 2010 Mw 7.2 El Mayor-Cucapah earthquake from geodetic data

NASA Astrophysics Data System (ADS)

Huang, Mong-Han; Fielding, Eric J.; Dickinson, Haylee; Sun, Jianbao; Gonzalez-Ortega, J. Alejandro; Freed, Andrew M.; Bürgmann, Roland

2017-01-01

The 4 April 2010 Mw 7.2 El Mayor-Cucapah (EMC) earthquake in Baja, California, and Sonora, Mexico, had primarily right-lateral strike-slip motion and a minor normal-slip component. The surface rupture extended about 120 km in a NW-SE direction, west of the Cerro Prieto fault. Here we use geodetic measurements including near- to far-field GPS, interferometric synthetic aperture radar (InSAR), and subpixel offset measurements of radar and optical images to characterize the fault slip during the EMC event. We use dislocation inversion methods and determine an optimal nine-segment fault geometry, as well as a subfault slip distribution from the geodetic measurements. With systematic perturbation of the fault dip angles, randomly removing one geodetic data constraint, or different data combinations, we are able to explore the robustness of the inferred slip distribution along fault strike and depth. The model fitting residuals imply contributions of early postseismic deformation to the InSAR measurements as well as lateral heterogeneity in the crustal elastic structure between the Peninsular Ranges and the Salton Trough. We also find that with incorporation of near-field geodetic data and finer fault patch size, the shallow slip deficit is reduced in the EMC event by reductions in the level of smoothing. These results show that the outcomes of coseismic inversions can vary greatly depending on model parameterization and methodology.

Dynamic mortar finite element method for modeling of shear rupture on frictional rough surfaces

NASA Astrophysics Data System (ADS)

Tal, Yuval; Hager, Bradford H.

2017-09-01

This paper presents a mortar-based finite element formulation for modeling the dynamics of shear rupture on rough interfaces governed by slip-weakening and rate and state (RS) friction laws, focusing on the dynamics of earthquakes. The method utilizes the dual Lagrange multipliers and the primal-dual active set strategy concepts, together with a consistent discretization and linearization of the contact forces and constraints, and the friction laws to obtain a semi-smooth Newton method. The discretization of the RS friction law involves a procedure to condense out the state variables, thus eliminating the addition of another set of unknowns into the system. Several numerical examples of shear rupture on frictional rough interfaces demonstrate the efficiency of the method and examine the effects of the different time discretization schemes on the convergence, energy conservation, and the time evolution of shear traction and slip rate.

On simulation of no-slip condition in the method of discrete vortices

NASA Astrophysics Data System (ADS)

Shmagunov, O. A.

2017-10-01

When modeling flows of an incompressible fluid, it is convenient sometimes to use the method of discrete vortices (MDV), where the continuous vorticity field is approximated by a set of discrete vortex elements moving in the velocity field. The vortex elements have a clear physical interpretation, they do not require the construction of grids and are automatically adaptive, since they concentrate in the regions of greatest interest and successfully describe the flows of a non-viscous fluid. The possibility of using MDV in simulating flows of a viscous fluid was considered in the previous papers using the examples of flows past bodies with sharp edges with the no-penetration condition at solid boundaries. However, the appearance of vorticity on smooth boundaries requires the no-slip condition to be met when MDV is realized, which substantially complicates the initially simple method. In this connection, an approach is considered that allows solving the problem by simple means.

A Modification of the Dunn Osteotomy With Preservation of the Ligamentum Teres.

PubMed

Bali, Navi; Harrison, James; Laugharne, Edward; Bache, C Edward

2017-06-01

We aimed to determine if a modified Dunn osteotomy could be safely performed without surgical dislocation and consequent preservation of the ligamentum teres. All patients undergoing a modified Dunn osteotomy for a slipped capital femoral epiphysis over an 8-year period were included in this study, and all had a severe slip with an open physis. The modified Dunn procedure was performed on 34 hips in 34 patients. The mean age was 13.1 years (range, 11 to 16 y) with a mean follow-up time of 54 months (range, 15 to 102 mo). All slips were severe (grade 3) with a mean slip angle of 73.2 degrees (range, 60 to 90 degrees). Nineteen slips were stable and 15 were unstable. Of the unstable slips, the average time from initial presentation to the emergency department until surgery was 9.4 days (range, 2 to 42 d). Excluding 1 patient who developed complete collapse of the femoral head (NAHS 56), the average Nonarthritic Hip score was 98 (range, 93.7 to 100). Four (11.8%) patients developed avascular necrosis of the femoral head, of which 3 were unstable slips. A modified Dunn osteotomy with preservation of the ligamentum teres allows an excellent restoration of the anatomic alignment of the femoral head and neck. Rates of AVN are not increased compared with other techniques of subcapital osteotomy but this complication cannot be eliminated particularly in patients with unstable slips. Level III.

Characterization of crack-tip microstructures via synchrotron fractography in Mo and Mo-Nb alloy crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hmelo, A.B.

1987-01-01

The nature of the plastic relaxation associated with the semi-brittle cleavage fracture of a series of pre-cracked molybdenum-niobium alloy single crystals was investigated as a function of composition and temperature from 77/sup 0/ to 298/sup 0/K. Conventional optical microscopy and white-beam Synchrotron X-Ray Fractography (SXRF) were used to examined the structure of a thin layer a few microns thick at the remnant of the precursor crack plastic zone. The plastic work of fracture was evaluated by measuring the lattice curvature associated with networks of dislocations beneath the cleavage surface. Using SXRF, lattice curvature is detected as asterism on photographic plates,more » and is associated with an excess density of edge dislocations of one sign. The results are in qualitative agreement with a previous determination of the fracture toughness of these specimens. Excess edge-dislocation density of one sign has been shown to vary as a function of temperature and composition, in a way consistent with previous studies of total dislocation content in these materials. Unlike the etch-pit analysis that can reveal the total dislocation content only, the tensor bases analysis described here allows the activity on individual slip systems to be distinguished.« less

Fragmentation of wall rock garnets during deep crustal earthquakes

PubMed Central

Austrheim, Håkon; Dunkel, Kristina G.; Plümper, Oliver; Ildefonse, Benoit; Liu, Yang; Jamtveit, Bjørn

2017-01-01

Fractures and faults riddle the Earth’s crust on all scales, and the deformation associated with them is presumed to have had significant effects on its petrological and structural evolution. However, despite the abundance of directly observable earthquake activity, unequivocal evidence for seismic slip rates along ancient faults is rare and usually related to frictional melting and the formation of pseudotachylites. We report novel microstructures from garnet crystals in the immediate vicinity of seismic slip planes that transected lower crustal granulites during intermediate-depth earthquakes in the Bergen Arcs area, western Norway, some 420 million years ago. Seismic loading caused massive dislocation formations and fragmentation of wall rock garnets. Microfracturing and the injection of sulfide melts occurred during an early stage of loading. Subsequent dilation caused pervasive transport of fluids into the garnets along a network of microfractures, dislocations, and subgrain and grain boundaries, leading to the growth of abundant mineral inclusions inside the fragmented garnets. Recrystallization by grain boundary migration closed most of the pores and fractures generated by the seismic event. This wall rock alteration represents the initial stages of an earthquake-triggered metamorphic transformation process that ultimately led to reworking of the lower crust on a regional scale. PMID:28261660

3D DDD modelling of dislocation-precipitate interaction in a nickel-based single crystal superalloy under cyclic deformation

NASA Astrophysics Data System (ADS)

Lin, Bing; Huang, Minsheng; Zhao, Liguo; Roy, Anish; Silberschmidt, Vadim; Barnard, Nick; Whittaker, Mark; McColvin, Gordon

2018-06-01

Strain-controlled cyclic deformation of a nickel-based single crystal superalloy has been modelled using three-dimensional (3D) discrete dislocation dynamics (DDD) for both [0 0 1] and [1 1 1] orientations. The work focused on the interaction between dislocations and precipitates during cyclic plastic deformation at elevated temperature, which has not been well studied yet. A representative volume element with cubic γ‧-precipitates was chosen to represent the material, with enforced periodical boundary conditions. In particular, cutting of superdislocations into precipitates was simulated by a back-force method. The global cyclic stress-strain responses were captured well by the DDD model when compared to experimental data, particularly the effects of crystallographic orientation. Dislocation evolution showed that considerably high density of dislocations was produced for [1 1 1] orientation when compared to [0 0 1] orientation. Cutting of dislocations into the precipitates had a significant effect on the plastic deformation, leading to material softening. Contour plots of in-plane shear strain proved the development of heterogeneous strain field, resulting in the formation of shear-band embryos.

Heterogeneous Earth Structure, Deformation, and Slip During the 2010 Mw 7.2 El Mayor-Cucapah Earthquake from Geodetic Data

NASA Astrophysics Data System (ADS)

Huang, M. H.; Dickinson, H.; Fielding, E. J.; Sun, J.; Freed, A. M.; Burgmann, R.

2015-12-01

The 4th of April 2010 Mw 7.2 El Mayor-Cucapah (EMC) earthquake in Baja California and Sonora, Mexico has primarily right-lateral strike-slip motion and a minor normal slip component. The surface rupture extends about 120 km west of the boundary between the Pacific and the North American plates. The EMC event initiated near the center and ruptured bilaterally into an east-dipping strike-slip fault zone to the north and a west-dipping strike-slip zone to the south. Here we use geodetic measurements including GPS, InSAR (SAR interferometry), and sub-pixel offset measurements to characterize the fault slip during the EMC event. We use dislocation inversion methods to determine fault geometry as well as sub-fault slip distribution based on geodetic measurements. We find that assuming layered earth elastic structure increased the inferred deep slip (10-15 km depth) by up to 1.6 m (60%) compared to assuming a homogeneous elastic structure. Inferred slip was also strongly (up to 2 m) influenced by the choice of observational constraints used in the inversion. The choice of constraints also influenced the inverted seismic moment from Mw 7.20 to 7.26, and the difference is equivalent to a Mw 6.5 event. Our results show that the outcomes of coseismic inversions can vary greatly depending on the methodology, something that needs to be considered both for characterizing an earthquake and when using such results in subsequent studies of postseismic deformation.

The 1999 Hector Mine Earthquake, Southern California: Vector Near-Field Displacements from ERS InSAR

NASA Technical Reports Server (NTRS)

Sandwell, David T.; Sichoix, Lydie; Smith, Bridget

2002-01-01

Two components of fault slip are uniquely determined from two line-of-sight (LOS) radar interferograms by assuming that the fault-normal component of displacement is zero. We use this approach with ascending and descending interferograms from the ERS satellites to estimate surface slip along the Hector Mine earthquake rupture. The LOS displacement is determined by visually counting fringes to within 1 km of the outboard ruptures. These LOS estimates and uncertainties are then transformed into strike- and dip-slip estimates and uncertainties; the transformation is singular for a N-S oriented fault and optimal for an E-W oriented fault. In contrast to our previous strike-slip estimates, which were based only on a descending interferogram, we now find good agreement with the geological measurements, except at the ends of the rupture. The ascending interferogram reveals significant west-sidedown dip-slip (approximately 1.0 m) which reduces the strike-slip estimates by 1 to 2 m, especially along the northern half of the rupture. A spike in the strike-slip displacement of 6 m is observed in central part of the rupture. This large offset is confirmed by subpixel cross correlation of features in the before and after amplitude images. In addition to strike slip and dip slip, we identify uplift and subsidence along the fault, related to the restraining and releasing bends in the fault trace, respectively. Our main conclusion is that at least two look directions are required for accurate estimates of surface slip even along a pure strike-slip fault. Models and results based only on a single look direction could have major errors. Our new estimates of strike slip and dip slip along the rupture provide a boundary condition for dislocation modeling. A simple model, which has uniform slip to a depth of 12 km, shows good agreement with the observed ascending and descending interferograms.

Free energy change of a dislocation due to a Cottrell atmosphere

DOE PAGES

Sills, R. B.; Cai, W.

2018-03-07

The free energy reduction of a dislocation due to a Cottrell atmosphere of solutes is computed using a continuum model. In this work, we show that the free energy change is composed of near-core and far-field components. The far-field component can be computed analytically using the linearized theory of solid solutions. Near the core the linearized theory is inaccurate, and the near-core component must be computed numerically. The influence of interactions between solutes in neighbouring lattice sites is also examined using the continuum model. We show that this model is able to reproduce atomistic calculations of the nickel–hydrogen system, predictingmore » hydride formation on dislocations. The formation of these hydrides leads to dramatic reductions in the free energy. Lastly, the influence of the free energy change on a dislocation’s line tension is examined by computing the equilibrium shape of a dislocation shear loop and the activation stress for a Frank–Read source using discrete dislocation dynamics.« less

Free energy change of a dislocation due to a Cottrell atmosphere

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sills, R. B.; Cai, W.

The free energy reduction of a dislocation due to a Cottrell atmosphere of solutes is computed using a continuum model. In this work, we show that the free energy change is composed of near-core and far-field components. The far-field component can be computed analytically using the linearized theory of solid solutions. Near the core the linearized theory is inaccurate, and the near-core component must be computed numerically. The influence of interactions between solutes in neighbouring lattice sites is also examined using the continuum model. We show that this model is able to reproduce atomistic calculations of the nickel–hydrogen system, predictingmore » hydride formation on dislocations. The formation of these hydrides leads to dramatic reductions in the free energy. Lastly, the influence of the free energy change on a dislocation’s line tension is examined by computing the equilibrium shape of a dislocation shear loop and the activation stress for a Frank–Read source using discrete dislocation dynamics.« less

Effect of He+ fluence on surface morphology and ion-irradiation induced defect evolution in 7075 aluminum alloys

NASA Astrophysics Data System (ADS)

Ni, Kai; Ma, Qian; Wan, Hao; Yang, Bin; Ge, Junjie; Zhang, Lingyu; Si, Naichao

2018-02-01

The evolution of microstructure for 7075 aluminum alloys with 50 Kev helium ions irradiation were studied by using optical microscopy (OM), scanning electron microscopy (SEM), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The fluences of 1 × 1015, 1 × 1016 and 1 × 1017 ions cm-2 were selected, and irradiation experiments were conducted at room temperatures. The transmission process of He+ ions was simulated by using SRIM software, including distribution of ion ranges, energy losses and atomic displacements. Experimental results show that irradiated pits and micro-cracks were observed on irradiation sample surface, and the size of constituent particles (not including Mg2Si) decreased with the increasing dose. The x-ray diffraction results of the pair of peaks is better resolved in irradiated samples might indicate that the stressed structure consequence due to crystal defects (vacancies and interstitials) after He+ implantation. TEM observation indicated that the density of MgZn2 phase was significantly reduced after helium ion irradiation which is harmful to strength. Besides, the development of compressive stress produced a large amount of dislocation defects in the 1015 ions cm-2 sample. Moreover, higher fluence irradiation produced more dislocations in sample. At fluence of 1016 ions cm-2, dislocation wall formed by dislocation slip and aggregation in the interior of grains, leading to the refinement of these grains. As fluence increased to 1017 ions cm-2, dislocation loops were observed in pinned dislocation. Moreover, dislocation as effective defect sink, irradiation-induced vacancy defects aggregated to these sinks, and resulted in the formation of helium bubbles in dislocation.

The strength and dislocation microstructure evolution in superalloy microcrystals

NASA Astrophysics Data System (ADS)

Hussein, Ahmed M.; Rao, Satish I.; Uchic, Michael D.; Parthasarathay, Triplicane A.; El-Awady, Jaafar A.

2017-02-01

In this work, the evolution of the dislocations microstructure in single crystal two-phase superalloy microcrystals under monotonic loading has been studied using the three-dimensional discrete dislocation dynamics (DDD) method. The DDD framework has been extended to properly handle the collective behavior of dislocations and their interactions with large collections of arbitrary shaped precipitates. Few constraints are imposed on the initial distribution of the dislocations or the precipitates, and the extended DDD framework can support experimentally-obtained precipitate geometries. Full tracking of the creation and destruction of anti-phase boundaries (APB) is accounted for. The effects of the precipitate volume fraction, APB energy, precipitate size, and crystal size on the deformation of superalloy microcrystals have been quantified. Correlations between the precipitate microstructure and the dominant deformation features, such as dislocation looping versus precipitate shearing, are also discussed. It is shown that the mechanical strength is independent of the crystal size, increases linearly with increasing the volume fraction, follows a near square-root relationship with the APB energy and an inverse square-root relationship with the precipitate size. Finally, the flow strength in simulations having initial dislocation pair sources show a flow strength that is about one half of that predicted from simulations starting with single dislocation sources. The method developed can be used, with minimal extensions, to simulate dislocation microstructure evolution in general multiphase materials.

The barrier to misfit dislocation glide in continuous, strained, epitaxial layers on patterned substrates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Watson, G.P.; Ast, D.G.; Anderson, T.J.

1993-09-01

In a previous report [G. P. Watson, D. G. Ast, T. J. Anderson, and Y. Hayakawa, Appl. Phys. Lett. [bold 58], 2517 (1991)] we demonstrated that the motion of misfit dislocations in InGaAs, grown by organometallic vapor phase epitaxy on patterned GaAs substrates, can be impeded even if the strained epitaxial layer is continuous. Trenches etched into GaAs before growth are known to act as a barrier to misfit dislocation propagation [E. A. Fitzgerald, G. P. Watson, R. E. Proano, D. G. Ast, P. D. Kirchner, G. D. Pettit, and J. M. Woodall, J. Appl. Phys. [bold 65], 2220 (1989)]more » when those trenches create discontinuities in the epitaxial layers; but even shallow trenches, with continuous strained layers following the surface features, can act as barriers. By considering the strain energy required to change the length of the dislocation glide segments that stretch from the interface to the free surface, a simple model is developed that explains the major features of the unique blocking action observed at the trench edges. The trench wall angle is found to be an important parameter in determining whether or not a trench will block dislocation glide. The predicted blocking angles are consistent with observations made on continuous 300 and 600 nm thick In[sub 0.04]Ga[sub 0.96]As films on patterned GaAs. Based on the model, a structure is proposed that may be used as a filter to yield misfit dislocations with identical Burgers vectors or dislocations which slip in only one glide plane.« less

Atomistic simulations of deformation mechanisms in ultralight weight Mg-Li alloys

NASA Astrophysics Data System (ADS)

Karewar, Shivraj

Mg alloys have spurred a renewed academic and industrial interest because of their ultra-light-weight and high specific strength properties. Hexagonal close packed Mg has low deformability and a high plastic anisotropy between basal and non-basal slip systems at room temperature. Alloying with Li and other elements is believed to counter this deficiency by activating non-basal slip by reducing their nucleation stress. In this work I study how Li addition affects deformation mechanisms in Mg using atomistic simulations. In the first part, I create a reliable and transferable concentration dependent embedded atom method (CD-EAM) potential for my molecular dynamics study of deformation. This potential describes the Mg-Li phase diagram, which accurately describes the phase stability as a function of Li concentration and temperature. Also, it reproduces the heat of mixing, lattice parameters, and bulk moduli of the alloy as a function of Li concentration. Most importantly, our CD-EAM potential reproduces the variation of stacking fault energy for basal, prismatic, and pyramidal slip systems that in uences the deformation mechanisms as a function of Li concentration. This success of CD-EAM Mg-Li potential in reproducing different properties, as compared to literature data, shows its reliability and transferability. Next, I use this newly created potential to study the effect of Li addition on deformation mechanisms in Mg-Li nanocrystalline (NC) alloys. Mg-Li NC alloys show basal slip, pyramidal type-I slip, tension twinning, and two-compression twinning deformation modes. Li addition reduces the plastic anisotropy between basal and non-basal slip systems by modifying the energetics of Mg-Li alloys. This causes the solid solution softening. The inverse relationship between strength and ductility therefore suggests a concomitant increase in alloy ductility. A comparison of the NC results with single crystal deformation results helps to understand the qualitative and quantitative effect of Li addition in Mg on nucleation stress and fault energies of each deformation mode. The nucleation stress and fault energies of basal dislocations and compression twins in single crystal Mg-Li alloy increase while those for pyramidal dislocations and tension twinning decrease. This variation in respective values explains the reduction in plastic anisotropy and increase in ductility for Mg-Li alloys.

Measuring twinning and slip in shock-compressed Ta from in-situ x-ray diffraction

NASA Astrophysics Data System (ADS)

Wehrenberg, Christopher; McGonegle, David; Sliwa, Marcin; Suggit, Matt; Wark, Justin; Lee, Hae Ja; Nagler, Bob; Tavella, Franz; Remington, Bruce; Rudd, Rob; Lazicki, Amy; Park, Hye-Sook; Swift, Damian; Zepeda-Ruiz, Louis; Higginbotham, Andrew; Bolme, Cindy

2017-06-01

A fundamental understanding of high-pressure and high-strain-rate deformation rests on grasping the underlying microstructural processes, such as twinning and dislocation generation and transport (slip), yet simulations and ex-post-facto recovery experiments provide conflicting answers to these basic issues. Here, we report direct, in-situ observation of twinning and slip in shock compressed Ta using in-situ x-ray diffraction. A series of shock experiments were performed on the Matter in Extreme Conditions end station at LCLS. Direct laser ablation was used to drive a shock, ranging in pressure from 10-300 GPa, into a Ta sample with an initial (110) fiber texture. The subsequent changes in texture were observed in-situ by examining the azimuthal distribution of the diffraction intensity and found to match twinning and lattice rotation. Measurements of the twin fraction and lattice rotation were used to calculate the equivalent plastic strain from twinning and slip. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.

Analysis of Slip Activity and Deformation Modes in Tension and Tension-Creep Tests of Cast Mg-10Gd-3Y-0.5Zr (Wt Pct) at Elevated Temperatures Using In Situ SEM Experiments

NASA Astrophysics Data System (ADS)

Wang, Huan; Boehlert, Carl J.; Wang, Qudong; Yin, Dongdi; Ding, Wenjiang

2016-05-01

The tension and tension-creep deformation behavior at elevated temperatures of a cast Mg-10Gd-3Y-0.5Zr (wt pct, GW103) alloy was investigated using in situ scanning electron microscopy. The tests were performed at temperatures ranging from 473 K to 598 K (200 °C to 325 °C). The active slip systems were identified using an EBSD-based slip trace analysis methodology. The results showed that for all of the tests, basal slip was the most likely system to be activated, and non-basal slip was activated to some extent depending on the temperature. No twinning was observed. For the tension tests, non-basal slip consisted of ~35 pct of the deformation modes at low temperatures (473 K and 523 K (200 °C and 250 °C)), while non-basal slip accounted for 12 and 7 pct of the deformation modes at high temperatures (573 K and 598 K (300 °C and 325 °C)), respectively. For the tension-creep tests, non-basal slip accounted for 31 pct of the total slip systems at low temperatures, while this value decreased to 10 to 16 pct at high temperatures. For a given temperature, the relative activity for prismatic slip in the tension-creep tests was slightly greater than that for the tension tests, while the activity for pyramidal slip was lower. Slip-transfer in neighboring grains was observed for the low-temperature tests. Intergranular cracking was the main cracking mode, while some intragranular cracks were observed for the tension-creep tests at high temperature and low stress. Grain boundary ledges were prevalently observed for both the tension and tension-creep tests at high temperatures, which suggests that besides dislocation slip, grain boundary sliding also contributed to the deformation.

Scale-free avalanche dynamics in crystal plasticity

NASA Astrophysics Data System (ADS)

Ispanovity, Pater Dusan; Laurson, Lasse; Zaiser, Michael; Zapperi, Stefano; Groma, Istvan; Alava, Mikko

2015-03-01

We investigate the properties of strain bursts (dislocation avalanches) occurring during plastic deformation of crystalline matter using two dimensional discrete dislocation dynamics (DDD). We perform quasistatic stress-controlled simulations with three DDD models differing in the spatiotemporal discretization and the mobility law assumed for individual dislocations. We find that each model exhibits identical avalanche dynamics with the following properties: (i) strain burst sizes follow a power law distribution characterized by an exponent τ ~ 1 . 0 and (ii) the distribution in truncated at a cutoff that diverges with increasing system size at any applied stress level. It has been proposed earlier that plastic yielding can be described in terms of a continuous phase transition of depinning type and its critical point is at the yield stress. We will demonstrate, however, that our results are inconsistent with cutoff scaling in depinning systems (like magnetic domain walls or earthquakes) and that the system behaves as critical at every stress level. We, therefore, conclude that in the models studied plastic yielding cannot be associated with a continuous phase transition. Financial supports of the Hungarian Scientific Research Fund (OTKA) under Contract Numbers PD-105256 and K-105335 and of the European Commission under Grant Agreement No. CIG-321842 are acknowledged.

Dislocation dynamics simulations of plasticity at small scales

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhou, Caizhi

2010-01-01

As metallic structures and devices are being created on a dimension comparable to the length scales of the underlying dislocation microstructures, the mechanical properties of them change drastically. Since such small structures are increasingly common in modern technologies, there is an emergent need to understand the critical roles of elasticity, plasticity, and fracture in small structures. Dislocation dynamics (DD) simulations, in which the dislocations are the simulated entities, offer a way to extend length scales beyond those of atomistic simulations and the results from DD simulations can be directly compared with the micromechanical tests. The primary objective of this researchmore » is to use 3-D DD simulations to study the plastic deformation of nano- and micro-scale materials and understand the correlation between dislocation motion, interactions and the mechanical response. Specifically, to identify what critical events (i.e., dislocation multiplication, cross-slip, storage, nucleation, junction and dipole formation, pinning etc.) determine the deformation response and how these change from bulk behavior as the system decreases in size and correlate and improve our current knowledge of bulk plasticity with the knowledge gained from the direct observations of small-scale plasticity. Our simulation results on single crystal micropillars and polycrystalline thin films can march the experiment results well and capture the essential features in small-scale plasticity. Furthermore, several simple and accurate models have been developed following our simulation results and can reasonably predict the plastic behavior of small scale materials.« less

On the micromechanics of slip events in sheared, fluid-saturated fault gouge

NASA Astrophysics Data System (ADS)

Dorostkar, Omid; Guyer, Robert A.; Johnson, Paul A.; Marone, Chris; Carmeliet, Jan

2017-06-01

We used a three-dimensional discrete element method coupled with computational fluid dynamics to study the poromechanical properties of dry and fluid-saturated granular fault gouge. The granular layer was sheared under dry conditions to establish a steady state condition of stick-slip dynamic failure, and then fluid was introduced to study its effect on subsequent failure events. The fluid-saturated case showed increased stick-slip recurrence time and larger slip events compared to the dry case. Particle motion induces fluid flow with local pressure variation, which in turn leads to high particle kinetic energy during slip due to increased drag forces from fluid on particles. The presence of fluid during the stick phase of loading promotes a more stable configuration evidenced by higher particle coordination number. Our coupled fluid-particle simulations provide grain-scale information that improves understanding of slip instabilities and illuminates details of phenomenological, macroscale observations.

Olivine Slip-system Activity at High Pressure: Implications for Upper-Mantle Rheology and Seismic Anisotropy (Invited)

NASA Astrophysics Data System (ADS)

Raterron, P.; Castelnau, O.; Geenen, T.; Merkel, S.

2013-12-01

The past decade abounded in technical developments allowing the investigation of materials rheology at high pressure (P > 3 GPa) [1]. This had a significant impact on our understanding of olivine rheology in the Earth asthenosphere, where P is in the range 3 - 13 GPa. A dislocation slip-system transition induced by pressure has been documented in dry Fe-bearing olivine [2]; it induces changes in olivine aggregate lattice preferred orientation (LPO) [3,4], which may explain the seismic velocity anisotropy attenuation observed at depths > 200 km in the upper mantle [5]. Deformation experiments carried out on olivine single crystals at high pressure allowed quantifying the effect of P on individual slip system activities [6]. Integration of these data, together with data on lattice friction arising from computational models (e.g., [7]), into analytical or mean-field numerical models for aggregate plasticity gave insight on the viscosity and LPO of olivine aggregates deformed at geological conditions in the dislocation creep regime [8,9]. We will review these recent findings and their implications for upper mantle rheology and seismic anisotropy. [1] Raterron & Merkel, 2009, J. Sync. Rad., 16, 748 ; [2] Raterron et al., 2009, PEPI, 172, 74 ; [3] Jung et al., 2009, Nature Geoscience, 2, 73 ; [4] Ohuchi et al., 2011, EPSL, 304, 55 ; [5] Mainprice et al., 2005, Nature, 433, 731 ; [6] Raterron et al., 2012, PEPI, 200-201, 105 ; [7] Durinck et al., 2007, EJM, 19, 631 ; [8] Castelnau et al., 2010, C.R. Physique, 11, 304 ; [9] Raterron et al., 2011, PEPI, 188, 26

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Zijiao; Sheng, Hongwei; Wang, Zhangjie

Combinations of high strength and ductility are hard to attain in metals. Exceptions include materials exhibiting twinning-induced plasticity. To understand how the strength-ductility trade-off can be defeated, we apply in situ, and aberration-corrected scanning, transmission electron microscopy to examine deformation mechanisms in the medium-entropy alloy CrCoNi that exhibits one of the highest combinations of strength, ductility and toughness on record. Ab initio modelling suggests that it has negative stacking-fault energy at 0K and high propensity for twinning. With deformation we find that a three-dimensional (3D) hierarchical twin network forms from the activation of three twinning systems. This serves a dualmore » function: conventional twin-boundary (TB) strengthening from blockage of dislocations impinging on TBs, coupled with the 3D twin network which offers pathways for dislocation glide along, and cross-slip between, intersecting TB-matrix interfaces. The stable twin architecture is not disrupted by interfacial dislocation glide, serving as a continuous source of strength, ductility and toughness.« less

Shock compression of [001] single crystal silicon

DOE PAGES

Zhao, S.; Remington, B.; Hahn, E. N.; ...

2016-03-14

Silicon is ubiquitous in our advanced technological society, yet our current understanding of change to its mechanical response at extreme pressures and strain-rates is far from complete. This is due to its brittleness, making recovery experiments difficult. High-power, short-duration, laser-driven, shock compression and recovery experiments on [001] silicon (using impedance-matched momentum traps) unveiled remarkable structural changes observed by transmission electron microscopy. As laser energy increases, corresponding to an increase in peak shock pressure, the following plastic responses are are observed: surface cleavage along {111} planes, dislocations and stacking faults; bands of amorphized material initially forming on crystallographic orientations consistent withmore » dislocation slip; and coarse regions of amorphized material. Molecular dynamics simulations approach equivalent length and time scales to laser experiments and reveal the evolution of shock-induced partial dislocations and their crucial role in the preliminary stages of amorphization. Furthermore, application of coupled hydrostatic and shear stresses produce amorphization below the hydrostatically determined critical melting pressure under dynamic shock compression.« less

Shock compression of [001] single crystal silicon

NASA Astrophysics Data System (ADS)

Zhao, S.; Hahn, E. N.; Kad, B.; Remington, B. A.; Bringa, E. M.; Meyers, M. A.

2016-05-01

Silicon is ubiquitous in our advanced technological society, yet our current understanding of change to its mechanical response at extreme pressures and strain-rates is far from complete. This is due to its brittleness, making recovery experiments difficult. High-power, short-duration, laser-driven, shock compression and recovery experiments on [001] silicon (using impedance-matched momentum traps) unveiled remarkable structural changes observed by transmission electron microscopy. As laser energy increases, corresponding to an increase in peak shock pressure, the following plastic responses are are observed: surface cleavage along {111} planes, dislocations and stacking faults; bands of amorphized material initially forming on crystallographic orientations consistent with dislocation slip; and coarse regions of amorphized material. Molecular dynamics simulations approach equivalent length and time scales to laser experiments and reveal the evolution of shock-induced partial dislocations and their crucial role in the preliminary stages of amorphization. Application of coupled hydrostatic and shear stresses produce amorphization below the hydrostatically determined critical melting pressure under dynamic shock compression.

Creep substructure formation in sodium chloride single crystals in the power law and exponential creep regimes

NASA Technical Reports Server (NTRS)

Raj, S. V.; Pharr, G. M.

1989-01-01

Creep tests conducted on NaCl single crystals in the temperature range from 373 to 1023 K show that true steady state creep is obtained only above 873 K when the ratio of the applied stress to the shear modulus is less than or equal to 0.0001. Under other stress and temperature conditions, corresponding to both power law and exponential creep, the creep rate decreases monotonically with increasing strain. The transition from power law to exponential creep is shown to be associated with increases in the dislocation density, the cell boundary width, and the aspect ratio of the subgrains along the primary slip planes. The relation between dislocation structure and creep behavior is also assessed.

Fragmentation and growth processes in ultramafic pseudotachylites and associated wall rocks from Alpine Corsica

NASA Astrophysics Data System (ADS)

Dunkel, K.; Austrheim, H.; Plümper, O.; Ildefonse, B.; Ohl, M.; Jamtveit, B.

2017-12-01

Pseudotachylites, often taken as indicators of seismic slip, can form by comminution and/or melting. How the seismic energy released during a pseudotachylite-generating earthquake affects the wall rock surrounding the fault plane is not well known, although damage of the wall rock can have a substantial influence on the rheological properties of the rock and therefore on its further deformation and metamorphism. Micro- and nanostructures of pseudotachylites and their wall rocks in peridotites from Alpine Corsica have been characterized using optical, scanning and transmission electron microscopy, electron backscatter diffraction, and electron microprobe analysis. Pseudotachylites in dunites from the Cima di Gratera area in Cape Corse show a combination of comminution and melting. Olivine in the pseudotachylites is dislocation-rich whereas pyroxene is almost free of dislocations, indicating a fragmentation of the olivine during seismic slip and incomplete melting. The temperature pulse that induced melting was likely short-lived, since evidence for recovery in olivine is limited. Olivines in the wall rocks show a similarly high dislocation density and are strongly fragmented without being sheared. A fragmentation without shear strain, comparable to the "pulverization" described from shallower earthquakes, can probably only be explained by a high stress pulse released during a seismic event. Previous observations from the same area describe a transition from cataclastic wall rocks over mylonites and ultramylonites to pseudotachylite. This was taken as an indication that cataclasis weakened the rock and finally, in connection with thermal runaway, allowed for seismic slip. This difference between previous observations and those described here may be explained by a spatial heterogeneity in the deformation conditions. While earthquakes may have nucleated by self-localizing thermal runaway in cataclastic zones, they propagated also through previously undeformed rocks, causing fragmentation and partial melting at high stress. The fragmentation of the wall rock, whether it occurred by brittle or crystal-plastic mechanisms, allowed for the infiltration of fluids. Thus, earthquakes cause not only a physical, but also a chemical alteration of the wall rocks surrounding the fault plane

Synchrotron Radial X-ray Diffraction Studies of Deformation of Polycrystalline MgO

NASA Astrophysics Data System (ADS)

Girard, J.; Tsujino, N.; Mohiuddin, A.; Karato, S. I.

2016-12-01

X-ray diffraction analyses have been used for decades to study mechanical properties of polycrystalline samples during in-situ high-pressure deformation. When polycrystalline materials are deformed, stresses develop in grains and lead to lattice distortion. Using X-ray diffraction we can estimate the lattice strain for each (hkl) diffraction plans and calculate the applied stress for each (hkl), using [Singh, 1993] relation. However, this method doesn't take into account plastic anisotropy. As a results of plastic anisotropy present in the material, stress estimated from this method can be largely differ depending on (hkl) diffraction planes [Karato, 2009]. Studying the stress estimate for each (hkl) plane, might help us distinguish dominant deformation mechanisms activated during deformation such as diffusion (we will observe small stress variation as a function of (hkl) diffraction planes) or dislocation creep (we will observe a stress variation as a function of (hkl) diffraction planes that could also give us clues on potential slip system activity). In this study we observed stress evolution in MgO polycrystalline samples deformed under mantle pressure and temperature for (200) and (220) diffraction planes. Using a range MgO grain sizes we were able to control the active deformation mechanism (for e.g. diffusion creep or dislocation creep). For coarse-grained specimens, we observed strong (hkl) dependence of radial strain indicating the operation of dislocation creep. The observed (hkl) dependence changes with pressure suggesting a change in the slip system: at pressures higher than 27 GPa, (200) shows larger stress estimate than (220). In contrast, at lower pressures, (220) shows larger stress estimate than (200). This might indicate a slip system transition in MgO occurring under lower mantle conditions. From {110} plane to {100} plane. This is in good agreement with theoretical predictions and numerical calculation [Amodeo et al., 2012] and has an important implication for the interpretation of seismic anisotropy in the D" layer [Karato, 1998]. Amodeo, J., Carrey P., and P. Cordier (2012), Philosophical Magazine, 92(12). Karato, S-I. (1998), Earth and planets Space, 50, 1019-1028 Karato, S.-I. (2009), Physical Review. B, 79(21). Singh, A. K., (1993), Journal of Applied Physic, 73, 4278.

Forecasting Low-Cycle Fatigue Performance of Twinning-Induced Plasticity Steels: Difficulty and Attempt

NASA Astrophysics Data System (ADS)

Shao, C. W.; Zhang, P.; Zhang, Z. J.; Liu, R.; Zhang, Z. F.

2017-12-01

We find the existing empirical relations based on monotonic tensile properties and/or hardness cannot satisfactorily predict the low-cycle fatigue (LCF) performance of materials, especially for twinning-induced plasticity (TWIP) steels. Given this, we first identified the different deformation mechanisms under monotonic and cyclic deformation after a comprehensive study of stress-strain behaviors and microstructure evolutions for Fe-Mn-C alloys during tension and LCF, respectively. It is found that the good tensile properties of TWIP steel mainly originate from the large activation of multiple twinning systems, which may be attributed to the grain rotation during tensile deformation; while its LCF performance depends more on the dislocation slip mode, in addition to its strength and plasticity. Based on this, we further investigate the essential relations between microscopic damage mechanism (dislocation-dislocation interaction) and cyclic stress response, and propose a hysteresis loop model based on dislocation annihilation theory, trying to quickly assess the LCF resistance of Fe-Mn-C steels as well as other engineering materials. It is suggested that the hysteresis loop and its evolution can provide significant information on cyclic deformation behavior, e.g., (point) defect multiplication and vacancy aggregation, which may help estimate the LCF properties.

The inverse hall-petch relation in nanocrystalline metals: A discrete dislocation dynamics analysis

NASA Astrophysics Data System (ADS)

Quek, Siu Sin; Chooi, Zheng Hoe; Wu, Zhaoxuan; Zhang, Yong Wei; Srolovitz, David J.

2016-03-01

When the grain size in polycrystalline materials is reduced to the nanometer length scale (nanocrystallinity), observations from experiments and atomistic simulations suggest that the yield strength decreases (softening) as the grain size is decreased. This is in contrast to the Hall-Petch relation observed in larger sized grains. We incorporated grain boundary (GB) sliding and dislocation emission from GB junctions into the classical DDD framework, and recovered the smaller is weaker relationship observed in nanocrystalline materials. This current model shows that the inverse Hall-Petch behavior can be obtained through a relief of stress buildup at GB junctions from GB sliding by emitting dislocations from the junctions. The yield stress is shown to vary with grain size, d, by a d 1 / 2 relationship when grain sizes are very small. However, pure GB sliding alone without further plastic accomodation by dislocation emission is grain size independent.

Heterogeneous rupture in the great Cascadia earthquake of 1700 inferred from coastal subsidence estimates

USGS Publications Warehouse

Wang, Pei-Ling; Engelhart, Simon E.; Wang, Kelin; Hawkes, Andrea D.; Horton, Benjamin P.; Nelson, Alan R.; Witter, Robert C.

2013-01-01

Past earthquake rupture models used to explain paleoseismic estimates of coastal subsidence during the great A.D. 1700 Cascadia earthquake have assumed a uniform slip distribution along the megathrust. Here we infer heterogeneous slip for the Cascadia margin in A.D. 1700 that is analogous to slip distributions during instrumentally recorded great subduction earthquakes worldwide. The assumption of uniform distribution in previous rupture models was due partly to the large uncertainties of then available paleoseismic data used to constrain the models. In this work, we use more precise estimates of subsidence in 1700 from detailed tidal microfossil studies. We develop a 3-D elastic dislocation model that allows the slip to vary both along strike and in the dip direction. Despite uncertainties in the updip and downdip slip extensions, the more precise subsidence estimates are best explained by a model with along-strike slip heterogeneity, with multiple patches of high-moment release separated by areas of low-moment release. For example, in A.D. 1700, there was very little slip near Alsea Bay, Oregon (~44.4°N), an area that coincides with a segment boundary previously suggested on the basis of gravity anomalies. A probable subducting seamount in this area may be responsible for impeding rupture during great earthquakes. Our results highlight the need for more precise, high-quality estimates of subsidence or uplift during prehistoric earthquakes from the coasts of southern British Columbia, northern Washington (north of 47°N), southernmost Oregon, and northern California (south of 43°N), where slip distributions of prehistoric earthquakes are poorly constrained.

Deformation-related microstructures in magmatic zircon and implications for diffusion

NASA Astrophysics Data System (ADS)

Reddy, Steven Michael; Timms, Nicholas E.; Hamilton, Patrick Joseph; Smyth, Helen R.

2009-02-01

An undeformed glomeroporphyritic andesite from the Sunda Arc of Java, Indonesia, contains zoned plagioclase and amphibole glomerocrysts in a fine-grained groundmass and records a complex history of adcumulate formation and subsequent magmatic disaggregation. A suite of xenocrystic zircon records Proterozoic and Archaean dates whilst a discrete population of zoned, euhedral, igneous zircon yields a SHRIMP U-Pb crystallisation age of 9.3 ± 0.2 Ma. Quantitative microstructural analysis of zircon by electron backscatter diffraction (EBSD) shows no deformation in the inherited xenocrysts, but intragrain orientation variations of up to 30° in 80% of the young zircon population. These variations are typically accommodated by both progressive crystallographic bending and discrete low angle boundaries that overprint compositional growth zoning. Dispersion of crystallographic orientations are dominantly by rotation about an axis parallel to the zircon c-axis [001], which is coincident with the dominant orientation of misorientation axes of adjacent analysis points in EBSD maps. Less common <100> misorientation axes account for minor components of crystallographic dispersion. These observations are consistent with zircon deformation by dislocation creep and the formation of tilt and twist boundaries associated with the operation of <001>{100} and <100>{010} slip systems. The restriction of deformation microstructures to large glomerocrysts and the young magmatic zircon population, and the absence of deformation within the host igneous rock and inherited zircon grains, indicate that zircon deformation took place within a low-melt fraction (<5% melt), mid-lower crustal cumulate prior to fragmentation during magmatic disaggregation and entrainment of xenocrystic zircons during magmatic decompression. Tectonic stresses within the compressional Sunda Arc at the time of magmatism are considered to be the probable driver for low-strain deformation of the cumulate in the late stages of initial crystallisation. These results provide the first evidence of crystal plastic dislocation creep in zircon associated with magmatic crystallisation and indicate that the development of crystal-plastic microstructures in zircon is not restricted to high-strain rocks. Such microstructures have previously been shown to enhance bulk diffusion of trace elements (U, Th and REE) in zircon. The development of deformation microstructures, and therefore multiple diffusion pathways in zircon in the magmatic environment, has significant implications for the interpretation of geochemical data from igneous zircon and the trace element budgets of melts due to the potential enhancement of bulk diffusion and dissolution rates.

Role of weakest links and system-size scaling in multiscale modeling of stochastic plasticity

NASA Astrophysics Data System (ADS)

Ispánovity, Péter Dusán; Tüzes, Dániel; Szabó, Péter; Zaiser, Michael; Groma, István

2017-02-01

Plastic deformation of crystalline and amorphous matter often involves intermittent local strain burst events. To understand the physical background of the phenomenon a minimal stochastic mesoscopic model was introduced, where details of the microstructure evolution are statistically represented in terms of a fluctuating local yield threshold. In the present paper we propose a method for determining the corresponding yield stress distribution for the case of crystal plasticity from lower scale discrete dislocation dynamics simulations which we combine with weakest link arguments. The success of scale linking is demonstrated by comparing stress-strain curves obtained from the resulting mesoscopic and the underlying discrete dislocation models in the microplastic regime. As shown by various scaling relations they are statistically equivalent and behave identically in the thermodynamic limit. The proposed technique is expected to be applicable to different microstructures and also to amorphous materials.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hu, Shenyang; Setyawan, Wahyu; Joshi, Vineet V.

Xe gas bubble superlattice formation is observed in irradiated uranium–10 wt% molybdenum (U10Mo) fuels. However, the thermodynamic properties of the bubbles (the relationship among bubble size, equilibrium Xe concentration, and bubble pressure) and the mechanisms of bubble growth and superlattice formation are not well known. In this work, molecular dynamics is used to study these properties and mechanisms. The results provide important inputs for quantitative mesoscale models of gas bubble evolution and fuel performance. In the molecular dynamics simulations, the embedded-atom method (EAM) potential of U10Mo-Xe (Smirnova et al. 2013) is employed. Initial gas bubbles with low Xe concentration aremore » generated in a U10Mo single crystal. Then Xe atom atoms are continuously added into the bubbles, and the evolution of pressure and dislocation emission around the bubbles is analyzed. The relationship between pressure, equilibrium Xe concentration, and radius of the bubbles is established. It was found that the gas bubble growth is accompanied by partial dislocation emission, which results in a star-shaped dislocation structure and an anisotropic stress field. The emitted partial dislocations have a Burgers vector along the <111> direction and a slip plane of (11-2). Dislocation loop punch-out was not observed. A tensile stress was found along <110> directions around the bubble, favoring the nucleation and formation of a face-centered cubic bubble superlattice in body-centered cubic U10Mo fuels.« less

Cross-scale MD simulations of dynamic strength of tantalum

NASA Astrophysics Data System (ADS)

Bulatov, Vasily

2017-06-01

Dislocations are ubiquitous in metals where their motion presents the dominant and often the only mode of plastic response to straining. Over the last 25 years computational prediction of plastic response in metals has relied on Discrete Dislocation Dynamics (DDD) as the most fundamental method to account for collective dynamics of moving dislocations. Here we present first direct atomistic MD simulations of dislocation-mediated plasticity that are sufficiently large and long to compute plasticity response of single crystal tantalum while tracing the underlying dynamics of dislocations in all atomistic details. Where feasible, direct MD simulations sidestep DDD altogether thus reducing uncertainties of strength predictions to those of the interatomic potential. In the specific context of shock-induced material dynamics, the same MD models predict when, under what conditions and how dislocations interact and compete with other fundamental mechanisms of dynamic response, e.g. twinning, phase-transformations, fracture. In collaboration with: Luis Zepeda-Ruiz, Lawrence Livermore National Laboratory; Alexander Stukowski, Technische Universitat Darmstadt; Tomas Oppelstrup, Lawrence Livermore National Laboratory. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Mixed-mode singularity and temperature effects on dislocation nucleation in strained interconnects

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Jinhaeng; Gao, Yanfei

2011-01-01

Dislocations can be nucleated from sharp geometric features in strained interconnects due to thermal expansion coefficient mismatch, lattice mismatch, or stresses that arise during material processing. The asymptotic stress fields near the edge root can be described by mixed-mode singularities, which depend on the dihedral angle and material properties, and a transverse T-stress, which depends on how residual stress is realized in the interconnects. The critical condition for stress nucleation can be determined when an appropriate measure of the stress intensity factors (SIFs) reaches a critical value. Such a method, however, does not offer an explicit picture of the dislocationmore » nucleation process so that it has difficulties in studying complicated structures, mode mixity effects, and more importantly the temperature effects. Based on the Peierls concept, a dislocation can be described by a continuous slip field, and the dislocation nucleation condition corresponds when the total potential energy reaches a stationary state. Through implementing this ad hoc interface model into a finite element framework, it is found that dislocation nucleation becomes more difficult with the increase of mode mixity and T-stress, or the decrease of the width-to-height ratio of the surface pad, while the shape of the surface pad, being a square or a long line, plays a less important role. The Peierls dislocation model also allows us to determine the activation energy, which is the energy needed for the thermal activation of a dislocation when the applied load is lower than the athermal critical value. The calculated saddle point configuration compares favorably the molecular simulations in literature. Suggestions on making immortal strained interconnects are provided.« less

Structure-Property Relations in Aluminum-Lithium Alloys

DTIC Science & Technology

1989-01-01

adsorbed ( Mughrabi et al , 1983). Such dislocations could not re-enter the grain during the reverse cycle of stress and the associated slip would be...ASTM STP 601, ASTM, 33 Mughrabi , H., Wang, R., Differt, D., Essmann, U. (1983) ASTM STP 811, ASTM, 5 Muller, W. et al (1986) Aluminium-Lithium Alloys...behaviour of Al -Li-Cu-Mg-Zr alloys ...... 34 2.4 Mechanical behaviour ......................................... 35 2.4.1 Elastic modulus

Effects of Loading Frequency on Fatigue Behavior, Residual Stress, and Microstructure of Deep-Rolled Stainless Steel AISI 304 at Elevated Temperatures

NASA Astrophysics Data System (ADS)

Nikitin, I.; Juijerm, P.

2018-02-01

The effects of loading frequency on the fatigue behavior of non-deep-rolled (NDR) and deep-rolled (DR) austenitic stainless steel AISI 304 were systematically clarified at elevated temperatures, especially at temperatures exhibiting the dynamic strain aging (DSA) phenomena. Tension-compression fatigue tests were performed isothermally at temperatures of 573 K and 773 K (300 °C and 500 °C) with different loading frequencies of 5, 0.5, 0.05, and 0.005 Hz. For the DR condition, the residual stresses and work-hardening states will be presented. It was found that DSA would be detected at appropriate temperatures and deformation rates. The cyclic deformation curves and the fatigue lives of the investigated austenitic stainless steel AISI 304 are considerably affected by the DSA, especially on the DR condition having high dislocation densities at the surface and in near-surface regions. In the temperature range of the DSA, residual stresses and work-hardening states of the DR condition seem to be stabilized. The microstructural alterations were investigated by transmission electron microscopy (TEM). At an appropriate temperature with low loading frequency, the plastic deformation mechanism shifted from a wavy slip manner to a planar slip manner in the DSA regimes, whereas the dislocation movements were obstructed.

Low viscosity and high attenuation in MgSiO3 post-perovskite inferred from atomic-scale calculations

PubMed Central

Goryaeva, Alexandra M.; Carrez, Philippe; Cordier, Patrick

2016-01-01

This work represents a numerical study of the thermal activation for dislocation glide of the [100](010) slip system in MgSiO3 post-perovskite (Mg-ppv) at 120 GPa. We propose an approach based on a one-dimensional line tension model in conjunction with atomic-scale calculations. In this model, the key parameters, namely, the line tension and the Peierls barrier, are obtained from density functional theory calculations. We find a Peierls stress σp = 2.1 GPa and a line tension Γ = 9.2 eV/Å, which lead to a kink-pair enthalpy (under zero stress) of 2.69 eV. These values confirm that this slip system bears a very low lattice friction because it vanishes for temperatures above approximately 500 K under mantle conditions. In the Earth’s mantle, high-pressure Mg-ppv silicate is thus expected to become as ductile as ferropericlase. These results confirm the hypothesis of a weak layer in the D″ layer where Mg-ppv is present. Easy glide along [100](010) suggests strong preferred orientations with (010) planes aligned. Highly mobile [100] dislocations are also likely to respond to stresses related to seismic waves, leading to energy dissipation and strong attenuation. PMID:27708386

Probability Assessment of Mega-thrust Earthquakes in Global Subduction Zones　-from the View of Slip Deficit-

NASA Astrophysics Data System (ADS)

Ikuta, R.; Mitsui, Y.; Ando, M.

2014-12-01

We studied inter-plate slip history for about 100 years using earthquake catalogs. On assumption that each earthquake has stick-slip patch centered in its centroid, we regard cumulative seismic slips around the centroid as representing the inter-plate dislocation. We evaluated the slips on the stick-slip patches of over-M5-class earthquakes prior to three recent mega-thrust earthquakes, the 2004 Sumatra (Mw9.2), the 2010 Chile (Mw8.8), and the 2011 Tohoku (Mw9.0) around them. Comparing the cumulative seismic slips with the plate convergence, the slips before the mega-thrust events are significantly short in large area corresponding to the size of the mega-thrust events. We also researched cumulative seismic slips after other three mega-thrust earthquakes occurred in this 100 years, the 1952 Kamchatka (Mw9.0), the 1960 Chile (Mw9.5), the 1964 Alaska (Mw9.2). The cumulative slips have been significantly short in and around the focal area after their occurrence. The result should reflect persistency of the strong or/and large inter-plate coupled area capable of mega-thrust earthquakes. We applied the same procedure to global subduction zones to find that 21 regions including the focal area of above mega-thrust earthquakes show slip deficit over large area corresponding to the size of M9-class earthquakes. Considering that at least six M9-class earthquakes occurred in this 100 years and each recurrence interval should be 500-1000 years, it would not be surprised that from five to ten times of the already known regions (30 to 60 regions) are capable of M9 class earthquakes. The 21 regions as expected M9 class focal areas in our study is less than 5 to 10 times of the known 6, some of these regions may be divided into a few M9 class focal area because they extend to much larger area than typical M9 class focal area.

Elevated Temperature Effects on the Plastic Anisotropy of an Extruded Mg-4 Wt Pct Li Alloy: Experiments and Polycrystal Modeling

NASA Astrophysics Data System (ADS)

Risse, Marcel; Lentz, Martin; Fahrenson, Christoph; Reimers, Walter; Knezevic, Marko; Beyerlein, Irene J.

2017-01-01

In this work, we study the deformation behavior of Mg-4 wt pct Li in uniaxial tension as a function of temperature and loading direction. Standard tensile tests were performed at temperatures in the range of 293 K (20 °C) ≤ T ≤ 473 K (200 °C) and in two in-plane directions: the extrusion and the transverse. We find that while the in-plane plastic anisotropy (PA) decreases with temperature, the anisotropy in failure strain and texture development increases. To uncover the temperature dependence in the critical stresses for slip and in the amounts of slip and twinning systems mediating deformation, we employ the elastic-plastic self-consistent polycrystal plasticity model with a thermally activated dislocation density based hardening law for activating slip with individual crystals. We demonstrate that the model, with a single set of intrinsic material parameters, achieves good agreement with the stress-strain curves, deformation textures, and intragranular misorientation axis analysis for all test directions and temperatures. With the model, we show that at all temperatures the in-plane tensile behavior is driven primarily by < a rangle slip and both < {c + a} rangle slip and twinning play a minor role. The analysis explains that the in-plane PA decreases and failure strains increase with temperature as a result of a significant reduction in the activation stress for pyramidal < {c + a} rangle slip, which effectively promotes strain accommodation from multiple types of < a rangle and < {c + a} rangle slip. The results also show that because of the strong initial texture, in-plane texture development is anisotropic since prismatic slip dominates the deformation in one test, although it is not the easiest slip mode, and basal slip in the other. These findings reveal the relationship between the temperature-sensitive thresholds needed to activate crystallographic slip and the development of texture and macroscopic PA.

Reexaming Owens Valley: Partitioning of Discrete and Distributed Transtension, Structural Controls on Magmatism, and Seismic Potential within an Active Rift Zone, Eastern California.

NASA Astrophysics Data System (ADS)

Levy, D. A.; Haproff, P. J.; Yin, A.

2016-12-01

Crustal-scale transtensional deformation is common in intracontinental extensional settings. However, along-strike variations in the geometry, kinematics, and linkages between rift-related faults, along with controls on local magmatic plumbing, remain inadequately examined. In this study, we conducted geologic mapping of active structures within central and northern Owens Valley of eastern California. C. Owens Valley features right-slip oblique deformation accommodated by three discrete north-south-trending faults: (1) the right-slip Owens Valley fault (OVF) and rift-bounding (2) Sierra Nevada Frontal fault (SNFF) and (3) the White-Inyo Mountains fault (WIMF). The OVF also serves as a lithospheric-scale, vertical conduit for asthenospheric-derived magma to migrate upwards and erupt at Big Pine Volcanic Field. Right-slip shear within C. Owens Valley is transferred to the SNFF of N. Owens Valley via the Poverty Hills restraining bend. In contrast to C. Owens Valley, the northern segment is dominated by distributed E-W to NE-SW-oriented extension, evidenced by normal fault scarps throughout Volcanic Tablelands and basin floor. Furthermore, the White Mountain fault which bounds N. Owens Valley to the east consists of a master west-dipping detachment fault that thinned the lithosphere, allowing for asthenospheric upwelling into the crust beneath the western rift shoulder. Subvertical, right-slip faults of the SNFF provide a conduit for magma to erupt on the surface throughout the Long Valley Caldera, Mono-Inyo Craters, and Mono Basin region. Our mapping demonstrates complex strain partitioning of discrete and distributed deformation within an alternating pure and simple shear, transtensional rift zone. Lastly, we present previously unknown relationships in Owens Valley between lithospheric-scale fault systems, seismic potential, and rift magmatism.

Modeling plastic deformation of post-irradiated copper micro-pillars

NASA Astrophysics Data System (ADS)

Crosby, Tamer; Po, Giacomo; Ghoniem, Nasr M.

2014-12-01

We present here an application of a fundamentally new theoretical framework for description of the simultaneous evolution of radiation damage and plasticity that can describe both in situ and ex situ deformation of structural materials [1]. The theory is based on the variational principle of maximum entropy production rate; with constraints on dislocation climb motion that are imposed by point defect fluxes as a result of irradiation. The developed theory is implemented in a new computational code that facilitates the simulation of irradiated and unirradiated materials alike in a consistent fashion [2]. Discrete Dislocation Dynamics (DDD) computer simulations are presented here for irradiated fcc metals that address the phenomenon of dislocation channel formation in post-irradiated copper. The focus of the simulations is on the role of micro-pillar boundaries and the statistics of dislocation pinning by stacking-fault tetrahedra (SFTs) on the onset of dislocation channel and incipient surface crack formation. The simulations show that the spatial heterogeneity in the distribution of SFTs naturally leads to localized plastic deformation and incipient surface fracture of micro-pillars.

Some methodological aspects of tectonic stress reconstruction based on geological indicators

NASA Astrophysics Data System (ADS)

Sim, Lidiya A.

2012-03-01

The impact of the initial heterogeneity in rocks on the distribution of slickensides (slip planes), which are used to reconstruct local stress-state, is discussed. The stress-state was reconstructed by a graphic variant of the cinematic method. A new type of stress-state - Variation of Stress-State Type (VSST) - is introduced. The VSST is characterized by dislocation vectors originated both by uniaxial compression and uniaxial tension in the same rock volume, i.e. coefficient μσ = 1-2R varies from +1 to -1. The reasons for differences between slip planes from those predicted by the model are discussed. The distribution of slip planes depends on the stress-state type (μσ coefficient) and on the initial heterogeneity of the rocks. A criterion for the identification of tectonic stress rank is suggested. It is based on the models of tectonic stress distribution in the fracture vicinities. Examples of results of tectonic stress studies are given. Studies of tectonic stresses based on geological indicators are of methodological and practical importance.

Effects of gap junction inhibition on contraction waves in the murine small intestine in relation to coupled oscillator theory.

PubMed

Parsons, Sean P; Huizinga, Jan D

2015-02-15

Waves of contraction in the small intestine correlate with slow waves generated by the myenteric network of interstitial cells of Cajal. Coupled oscillator theory has been used to explain steplike gradients in the frequency (frequency plateaux) of contraction waves along the length of the small intestine. Inhibition of gap junction coupling between oscillators should lead to predictable effects on these plateaux and the wave dislocation (wave drop) phenomena associated with their boundaries. It is these predictions that we wished to test. We used a novel multicamera diameter-mapping system to measure contraction along 25- to 30-cm lengths of murine small intestine. There were typically two to three plateaux per length of intestine. Dislocations could be limited to the wavefronts immediately about the terminated wave, giving the appearance of a three-pronged fork, i.e., a fork dislocation; additionally, localized decreases in velocity developed across a number of wavefronts, ending with the terminated wave, which could appear as a fork, i.e., slip dislocations. The gap junction inhibitor carbenoxolone increased the number of plateaux and dislocations and decreased contraction wave velocity. In some cases, the usual frequency gradient was reversed, with a plateau at a higher frequency than its proximal neighbor; thus fork dislocations were inverted, and the direction of propagation was reversed. Heptanol had no effect on the frequency or velocity of contractions but did reduce their amplitude. To understand intestinal motor patterns, the pacemaker network of the interstitial cells of Cajal is best evaluated as a system of coupled oscillators. Copyright © 2015 the American Physiological Society.

Role of interfaces in deformation and fracture of ordered intermetallics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yoo, M.H.; Fu, C.L.

1996-12-31

While sub- and grain-boundaries are the primary dislocation sources in Ll{sub 2} alloys, yield and flow stresses are strongly influenced by the multiplication and exhaustion of mobile dislocations from the secondary sources. The concept of enhanced microplasticity at grain boundaries due to chemical disordering is well supported by theoretical modeling, but no conclusive direct evidence exist for Ni{sub 3}Al bicrystals. The strong plastic anisotropy reported in TiAl PST (polysynthetically twinned) crystals is attributed in part to localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. Calculations of work of adhesion suggest that, intrinsically, interfacial cracking ismore » more likely to initiate on {gamma}/{gamma}-type interfaces than on the {alpha}{sub 2}/{gamma} boundary. 70 refs, 5 tabs, 5 figs.« less

Post-1906 stress recovery of the San Andreas fault system calculated from three-dimensional finite element analysis

USGS Publications Warehouse

Parsons, T.

2002-01-01

The M = 7.8 1906 San Francisco earthquake cast a stress shadow across the San Andreas fault system, inhibiting other large earthquakes for at least 75 years. The duration of the stress shadow is a key question in San Francisco Bay area seismic hazard assessment. This study presents a three-dimensional (3-D) finite element simulation of post-1906 stress recovery. The model reproduces observed geologic slip rates on major strike-slip faults and produces surface velocity vectors comparable to geodetic measurements. Fault stressing rates calculated with the finite element model are evaluated against numbers calculated using deep dislocation slip. In the finite element model, tectonic stressing is distributed throughout the crust and upper mantle, whereas tectonic stressing calculated with dislocations is focused mostly on faults. In addition, the finite element model incorporates postseismic effects such as deep afterslip and viscoelastic relaxation in the upper mantle. More distributed stressing and postseismic effects in the finite element model lead to lower calculated tectonic stressing rates and longer stress shadow durations (17-74 years compared with 7-54 years). All models considered indicate that the 1906 stress shadow was completely erased by tectonic loading no later than 1980. However, the stress shadow still affects present-day earthquake probability. Use of stressing rate parameters calculated with the finite element model yields a 7-12% reduction in 30-year probability caused by the 1906 stress shadow as compared with calculations not incorporating interactions. The aggregate interaction-based probability on selected segments (not including the ruptured San Andreas fault) is 53-70% versus the noninteraction range of 65-77%.

Glacier seismology: eavesdropping on the ice-bed interface

NASA Astrophysics Data System (ADS)

Walter, F.; Röösli, C.

2015-12-01

Glacier sliding plays a central role in ice dynamics. A number of remote sensing and deep drilling initiatives have therefore focused on the ice-bed interface. Although these techniques have provided valuable insights into bed properties, they do not supply theorists with data of sufficient temporal and spatial resolution to rigorously test mathematical sliding laws. As an alternative, passive seismic techniques have gained popularity in glacier monitoring. Analysis of glacier-related seismic sources ('icequakes') has become a useful technique to study inaccessible regions of the cryosphere, including the ice-bed interface. Seismic monitoring networks on the polar ice sheets have shown that ice sliding is not only a smooth process involving viscous deformation and regelation of basal ice layers. Instead, ice streams exhibit sudden slip episodes over their beds and intermittent phases of partial or complete stagnation. Here we discuss new and recently published discoveries of basal seismic sources beneath various glacial bodies. We revisit basal seismicity of hard-bedded Alpine glaciers, which is not the result of pure stick-slip motion. Sudden changes in seismicity suggest that the local configuration of the subglacial drainage system undergoes changes on sub daily time scales. Accordingly, such observations place constraints on basal resistance and sliding of hard-bedded glaciers. In contrast, certain clusters of stick-slip dislocations associated with micro seismicity beneath the Greenland ice sheet undergo diurnal variations in magnitudes and inter event times. This is best explained with a soft till bed, which hosts the shear dislocations and whose strength varies in response to changes in subglacial water pressure. These results suggest that analysis of basal icequakes is well suited for characterizing glacier and ice sheet beds. Future studies should address the relative importance between "smooth" and seismogenic sliding in different glacial environments.

Hydrogeologic Controls on Water Dynamics in a Discontinuous Permafrost, Lake-Rich Landscape

NASA Astrophysics Data System (ADS)

Walvoord, M. A.; Briggs, M. A.; Day-Lewis, F. D.; Jepsen, S. M.; Lane, J. W., Jr.; McKenzie, J. M.; Minsley, B. J.; Striegl, R. G.; Voss, C. I.; Wellman, T. P.

2014-12-01

Glacier sliding plays a central role in ice dynamics. A number of remote sensing and deep drilling initiatives have therefore focused on the ice-bed interface. Although these techniques have provided valuable insights into bed properties, they do not supply theorists with data of sufficient temporal and spatial resolution to rigorously test mathematical sliding laws. As an alternative, passive seismic techniques have gained popularity in glacier monitoring. Analysis of glacier-related seismic sources ('icequakes') has become a useful technique to study inaccessible regions of the cryosphere, including the ice-bed interface. Seismic monitoring networks on the polar ice sheets have shown that ice sliding is not only a smooth process involving viscous deformation and regelation of basal ice layers. Instead, ice streams exhibit sudden slip episodes over their beds and intermittent phases of partial or complete stagnation. Here we discuss new and recently published discoveries of basal seismic sources beneath various glacial bodies. We revisit basal seismicity of hard-bedded Alpine glaciers, which is not the result of pure stick-slip motion. Sudden changes in seismicity suggest that the local configuration of the subglacial drainage system undergoes changes on sub daily time scales. Accordingly, such observations place constraints on basal resistance and sliding of hard-bedded glaciers. In contrast, certain clusters of stick-slip dislocations associated with micro seismicity beneath the Greenland ice sheet undergo diurnal variations in magnitudes and inter event times. This is best explained with a soft till bed, which hosts the shear dislocations and whose strength varies in response to changes in subglacial water pressure. These results suggest that analysis of basal icequakes is well suited for characterizing glacier and ice sheet beds. Future studies should address the relative importance between "smooth" and seismogenic sliding in different glacial environments.

High-temperature in-situ TEM straining of the interaction with dislocations and particles for Cu-added ferritic stainless steel.

PubMed

Kobayashi, Shuhei; Kaneko, Kenji; Yamada, Kazuhiro; Kikuchi, Masao; Kanno, Norihiro; Hamada, Junichi

2014-11-01

IntroductionCu is always present in the matrix when ferritic steels were prepared from ferrous scrap. When the ferritic steels are aged thermally, Cu precipitates start appear and disperse finely and homogeneously [1], which may make the steels strengthened by precipitation hardening. In this study, the interaction between Cu precipitates and dislocations was exmined via high-temperature in-situ TEM straining. ExperimentalCu-added ferritic stainless steel (Fe-18.4%Cr-1.5%Cu) was used in the present study. Specimen was aged at1073 K for 360 ks. Samples for TEM observation were prepared by focused ion beam (FIB; Quanta 3D 200i) method. Microstructure of specimen was analyzed by JEM-3200FSK and high-temperature in-situ TEM straining was conducted using JEM-1300NEF. Results and discussionInteraction between Cu precipitates and dislocation is seen from consecutive TEM images acquired by in-situ TEM straining at 1073 K, as shown in Fig.1. The size of Cu precipitates was about 70 nm and several dislocations were present within the field of view. In particular, progressing dislocations contacted with the Cu precipitate at right angle, as indicated by arrows in Fig.1 (b) to (d). This result implies that there is an attractive interaction between dislocations and the Cu precipitate. This is attributed to the fact that Stress field of dislocations was easily relaxed in interface between the Cu precipitate and matrix because of lattice and interface diffusion as well as slip in the interface [2,3]. Furthermore, dislocations pass through the particle after contacting it, so that the interaction with dislocations and particles should be explained by Srolovitz mechanism [4].jmicro;63/suppl_1/i28/DFU083F1F1DFU083F1Fig. 1.TEM images foucused on interaction with dislocations and partticles. © The Author 2014. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

Wrinkle-like slip pulse on a fault between different materials

USGS Publications Warehouse

Andrews, D.J.; Ben-Zion, Y.

1997-01-01

Pulses of slip velocity can propagate on a planar interface governed by a constant coefficient of friction, where the interface separates different elastic materials. Such pulses have been found in two-dimensional plane strain finite difference calculations of slip on a fault between elastic media with wave speeds differing by 20%. The self-sustaining propagation of the slip pulse arises from interaction between normal and tangential deformation that exists only with a material contrast. These calculations confirm the prediction of Weertman [1980] that a dislocation propagating steadily along a material interface has a tensile change of normal traction with the same pulse shape as slip velocity. The self-sustaining pulse is associated with a rapid transition from a head wave traveling along the interface with the S wave speed of the faster material, to an opposite polarity body wave traveling with the slower S speed. Slip occurs during the reversal of normal particle velocity. The pulse can propagate in a region with constant coefficient of friction and an initial stress state below the frictional criterion. Propagation occurs in only one direction, the direction of slip in the more compliant medium, with rupture velocity near the slower S wave speed. Displacement is larger in the softer medium, which is displaced away from the fault during the passage of the slip pulse. Motion is analogous to a propagating wrinkle in a carpet. The amplitude of slip remains approximately constant during propagation, but the pulse width decreases and the amplitudes of slip velocity and stress change increase. The tensile change of normal traction increases until absolute normal traction reaches zero. The pulse can be generated as a secondary effect of a drop of shear stress in an asperity. The pulse shape is unstable, and the initial slip pulse can change during propagation into a collection of sharper pulses. Such a pulse enables slip to occur with little loss of energy to friction, while at the same time increasing irregularity of stress and slip at the source. Copyright 1997 by the American Geophysical Union.

A continuum theory of edge dislocations

NASA Astrophysics Data System (ADS)

Berdichevsky, V. L.

2017-09-01

Continuum theory of dislocation aims to describe the behavior of large ensembles of dislocations. This task is far from completion, and, most likely, does not have a "universal solution", which is applicable to any dislocation ensemble. In this regards it is important to have guiding lines set by benchmark cases, where the transition from a discrete set of dislocations to a continuum description is made rigorously. Two such cases have been considered recently: equilibrium of dislocation walls and screw dislocations in beams. In this paper one more case is studied, equilibrium of a large set of 2D edge dislocations placed randomly in a 2D bounded region. The major characteristic of interest is energy of dislocation ensemble, because it determines the structure of continuum equations. The homogenized energy functional is obtained for the periodic dislocation ensembles with a random contents of the periodic cell. Parameters of the periodic structure can change slowly on distances of order of the size of periodic cells. The energy functional is obtained by the variational-asymptotic method. Equilibrium positions are local minima of energy. It is confirmed the earlier assertion that energy density of the system is the sum of elastic energy of averaged elastic strains and microstructure energy, which is elastic energy of the neutralized dislocation system, i.e. the dislocation system placed in a constant dislocation density field making the averaged dislocation density zero. The computation of energy is reduced to solution of a variational cell problem. This problem is solved analytically. The solution is used to investigate stability of simple dislocation arrays, i.e. arrays with one dislocation in the periodic cell. The relations obtained yield two outcomes: First, there is a state parameter of the system, dislocation polarization; averaged stresses affect only dislocation polarization and cannot change other characteristics of the system. Second, the structure of dislocation phase space is strikingly simple. Dislocation phase space is split in a family of subspaces corresponding to constant values of dislocation polarizations; in each equipolarization subspace there are many local minima of energy; for zero external stresses the system is stuck in a local minimum of energy; for non-zero slowly changing external stress, dislocation polarization evolves, while the system moves over local energy minima of equipolarization subspaces. Such a simple picture of dislocation dynamics is due to the presence of two time scales, slow evolution of dislocation polarization and fast motion of the system over local minima of energy. The existence of two time scales is justified for a neutral system of edge dislocations.

Coseismic microstructures of experimental fault zones in Carrara marble

NASA Astrophysics Data System (ADS)

Ree, Jin-Han; Ando, Jun-ichi; Han, Raehee; Shimamoto, Toshihiko

2014-09-01

Experimental fault zones developed in Carrara marble that were deformed at seismic slip rates (1.18-1.30 m s-1) using a high-velocity-rotary-shear apparatus exhibit very low friction (friction coefficient as low as 0.06) at steady state due to nanoparticle lubrication of the decomposition product (lime). The fault zones show a layered structure; a central slip-localization layer (5-60 μm thick) of lime nanograins mantled by gouge layers (5-150 μm thick) and a plastically deformed layer (45-500 μm thick) between the wall rock and gouge layer in the marginal portion of cylindrical specimens. Calcite grains of the wall rock adjacent to the slip zone deform by dislocation glide when subjected to frictional heating and a lower strain rate than that of the principal slip zone. The very fine (2-5 μm) calcite grains in the gouge layer show a foam structure with relatively straight grain boundaries and 120° triple junctions. This foam structure is presumed to develop by welding at high temperature and low strain once slip is localized along the central layer. We suggest that a seismic event can be inferred from deformed marbles, given: (i) the presence of welded gouge with foam structure in a fault zone where wall rocks show no evidence of thermal metamorphism and (ii) a thin plastically deformed layer immediately adjacent to the principal slip zone of a cataclastic fault zone.

The effect of twinning and slip on the bauschinger effect of hadfield steel single crystals

NASA Astrophysics Data System (ADS)

Karaman, Ibrahim; Sehitoglu, Huseyin; Chumlyakov, Y. I.; Maier, Hans J.; Kireeva, I. V.

2001-03-01

The Bauschinger effect (BE) in single crystals of Hadfield manganese steel (Fe, 12.3 Mn, 1.0C in wt pct) was studied for three crystallographic orientations, [overline 1 11],{text{ [}}overline {text{1}} 23], and [001]. Both forward tensionreverse compression (FT/RC) and forward compression-reverse tension (FC/RT) loading schemes were used to investigate the role of deformation history on the BE. The evolution of stress-strain response and a dimensionless Bauschinger parameter were used to study the BE. The BE stems from long-range back stress generated by the dislocation pileups, at the twin and localized slip boundaries. Twinning boundaries present a strong obstacle and lead to a strong BE If localized slip followed twinning, permanent softening was evident, such as in the case of the [overline 1 11] FT/RC scheme. Localized slip and multiple slip in the forward loading provided a transient effect in the stress-strain response without a significant permanent softening. Hadfield steel single crystals have demonstrated a high BE for orientations conducive to combined twinning/slip deformation. The BE increased with increasing prestrain, then saturated and started to decrease, in contrast with precipitation-hardened alloys. A unique strain-hardening approach along with the back stress calculation was introduced into a viscoplastic self-consistent (VPSC) formulation. The strain-hardening formulation incorporates length scales associated with spacing between twin lamellae. The simulations correctly predicted the BE and the stress-strain response for both forward and reverse loading.

Multiscale Modeling of Inclusions and Precipitation Hardening in Metal Matrix Composites: Application to Advanced High-Strength Steels

DOE Office of Scientific and Technical Information (OSTI.GOV)

Askari, Hesam; Zbib, Hussein M.; Sun, Xin

In this study, the strengthening effect of inclusions and precipitates in metals is investigated within a multiscale approach that utilizes models at various length scales, namely, Molecular Mechanics (MM), discrete Dislocation Dynamics (DD), and an Eigenstrain Inclusion Method (EIM). Particularly, precipitates are modeled as hardsoft particles whose stress fields interact with dislocations. The stress field resulting from the elastic mismatch between the particles and the matrix is accounted for through the EIM. While the MM method is employed for the purpose of developing rules for DD for short range interaction between a single dislocation and an inclusion, the DD methodmore » is used to predict the strength of the composite resulting from the interaction between ensembles of dislocations and particles. As an application to this method, the mechanical behavior of Advanced High Strength Steel (AHSS) is investigated and the results are then compared to the experimental data. The results show that the finely dispersive precipitates can strengthen the material by pinning the dislocations up to a certain shear stress and retarding the recovery, as well as annihilation of dislocations. The DD results show that strengthening due to nano sized particles is a function of the density and size of the precipitates. This size effect is then explained using a mechanistic model developed based on dislocation-particle interaction.« less

Triangular dislocation: an analytical, artefact-free solution

NASA Astrophysics Data System (ADS)

Nikkhoo, Mehdi; Walter, Thomas R.

2015-05-01

Displacements and stress-field changes associated with earthquakes, volcanoes, landslides and human activity are often simulated using numerical models in an attempt to understand the underlying processes and their governing physics. The application of elastic dislocation theory to these problems, however, may be biased because of numerical instabilities in the calculations. Here, we present a new method that is free of artefact singularities and numerical instabilities in analytical solutions for triangular dislocations (TDs) in both full-space and half-space. We apply the method to both the displacement and the stress fields. The entire 3-D Euclidean space {R}3 is divided into two complementary subspaces, in the sense that in each one, a particular analytical formulation fulfils the requirements for the ideal, artefact-free solution for a TD. The primary advantage of the presented method is that the development of our solutions involves neither numerical approximations nor series expansion methods. As a result, the final outputs are independent of the scale of the input parameters, including the size and position of the dislocation as well as its corresponding slip vector components. Our solutions are therefore well suited for application at various scales in geoscience, physics and engineering. We validate the solutions through comparison to other well-known analytical methods and provide the MATLAB codes.

Length-dependent mechanical properties of gold nanowires

NASA Astrophysics Data System (ADS)

Han, Jing; Fang, Liang; Sun, Jiapeng; Han, Ying; Sun, Kun

2012-12-01

The well-known "size effect" is not only related to the diameter but also to the length of the small volume materials. It is unfortunate that the length effect on the mechanical behavior of nanowires is rarely explored in contrast to the intensive studies of the diameter effect. The present paper pays attention to the length-dependent mechanical properties of <111>-oriented single crystal gold nanowires employing the large-scale molecular dynamics simulation. It is discovered that the ultrashort Au nanowires exhibit a new deformation and failure regime-high elongation and high strength. The constrained dislocation nucleation and transient dislocation slipping are observed as the dominant mechanism for such unique combination of high strength and high elongation. A mechanical model based on image force theory is developed to provide an insight to dislocation nucleation and capture the yield strength and nucleation site of first partial dislocation indicated by simulation results. Increasing the length of the nanowires, the ductile-to-brittle transition is confirmed. And the new explanation is suggested in the predict model of this transition. Inspired by the superior properties, a new approach to strengthen and toughen nanowires-hard/soft/hard sandwich structured nanowires is suggested. A preliminary evidence from the molecular dynamics simulation corroborates the present opinion.

The Recycling and Reclamation of Used Tank Track Pins

DTIC Science & Technology

1985-08-01

dislocation models that show crack formation as the accumu- lation of defects and subsequent loss of coherency across a slip plane; and, models based on...specifications, thus removing any damage that has occurred. The success of this project is based on the ability of the reheat treatment to eliminate the...develop into cracks 8 . The general models of fatigue crack nucleation have been grouped into five main categories which are: models which consider the

Analysis of plastic deformation in silicon web crystals

NASA Technical Reports Server (NTRS)

Spitznagel, J. A.; Seidensticker, R. G.; Lien, S. Y.; Mchugh, J. P.; Hopkins, R. H.

1987-01-01

Numerical calculation of 111-plane 110-line slip activity in silicon web crystals generated by thermal stresses is in good agreement with etch pit patterns and X-ray topographic data. The data suggest that stress redistribution effects are small and that a model, similar to that proposed by Penning (1958) and Jordan (1981) but modified to account for dislocation annihilation and egress, can be used to describe plastic flow effects during silicon web growth.

In Situ Deformation of Olivine in the Transmission Electron Microscope: from Dislocation Velocity Measurements to Stress-Strain Curves

NASA Astrophysics Data System (ADS)

Bollinger, C.; Idrissi, H.; Boioli, F.; Cordier, P.

2015-12-01

There is a growing consensus to recognize that rheological law established for olivine at high-temperature (ca. >1000°C) fail when extrapolated to low temperatures relevant for the lithospheric mantle. Hence it appears necessary to fit rheological laws against data at low temperatures where olivine tends to become more and more brittle. The usual approach consists in applying confining pressure to inhibit brittleness. Here we propose an innovative approach based on the use of very small samples and numerical modelling. New commercial in situ TEM nanotensile testing equipment recently developed by Hysitron.Inc is combined with weak-beam dark-field TEM diffraction contrast imaging in order to obtain information on the elementary mechanisms controlling the plasticity of olivine: namely glide of [001] screw dislocations. The olivine tensile beams dedicated for in situ TEM nanomechanical testing were produced using microfabrication techniques based on MEMS-type procedures. The testing geometry was designed as to induce maximum resolved shear stresses on the [001](110) slip system. Under tensile loads between 2 and 3 GPa, ductile behaviour was reached with the development and propagation of dislocation loops across the sample allowing to measure the velocity of screw and non-screw dislocations as a function of stress. This information is introduced into a numerical model involving Dislocation Dynamics in order to obtain the stress-strain curves describing the mechanical response of olivine single crystals deformed in tension at room temperature.

Measurements of stress fields near a grain boundary: Exploring blocked arrays of dislocations in 3D

DOE PAGES

Guo, Y.; Collins, D. M.; Tarleton, E.; ...

2015-06-24

The interaction between dislocation pile-ups and grain boundaries gives rise to heterogeneous stress distributions when a structural metal is subjected to mechanical loading. Such stress heterogeneity leads to preferential sites for damage nucleation and therefore is intrinsically linked to the strength and ductility of polycrystalline metals. To date the majority of conclusions have been drawn from 2D experimental investigations at the sample surface, allowing only incomplete observations. Our purpose here is to significantly advance the understanding of such problems by providing quantitative measurements of the effects of dislocation pile up and grain boundary interactions in 3D. This is accomplished throughmore » the application of differential aperture X-ray Laue micro-diffraction (DAXM) and high angular resolution electron backscatter diffraction (HR-EBSD) techniques. Our analysis demonstrates a similar strain characterization capability between DAXM and HR-EBSD and the variation of stress intensity in 3D reveals that different parts of the same grain boundary may have different strengths in resisting slip transfer, likely due to the local grain boundary curvature.« less

A Continuum-Atomistic Analysis of Transgranular Crack Propagation in Aluminum

NASA Technical Reports Server (NTRS)

Yamakov, V.; Saether, E.; Glaessgen, E.

2009-01-01

A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain is used to study plastic processes at a crack tip in a single crystal of aluminum. The case of mode I loading is studied. A transition from deformation twinning to full dislocation emission from the crack tip is found when the crack plane is rotated around the [111] crystallographic axis. When the crack plane normal coincides with the [112] twinning direction, the crack propagates through a twinning mechanism. When the crack plane normal coincides with the [011] slip direction, the crack propagates through the emission of full dislocations. In intermediate orientations, a transition from full dislocation emission to twinning is found to occur with an increase in the stress intensity at the crack tip. This finding confirms the suggestion that the very high strain rates, inherently present in MD simulations, which produce higher stress intensities at the crack tip, over-predict the tendency for deformation twinning compared to experiments. The present study, therefore, aims to develop a more realistic and accurate predictive modeling of fracture processes.

Orientation and temperature dependence of some mechanical properties of the single-crystal nickel-base superalloy Rene N4. I - Tensile behavior

NASA Technical Reports Server (NTRS)

Miner, R. V.; Gayda, J.; Gabb, T. P.; Voigt, R. C.

1986-01-01

Single crystal specimens of a nickel-base superalloy with axes near 001, 011, and -112 were tested in tension at room temperature, 760, and 980 C. The alloy Rene N-4, was developed for gas turbine engine blades and has the nominal composition 3.7 Al, 4.2 Ti, 4 Ta, 0.5 Nb, 6 W, 1.5 Mo, 9 Cr, 7.5 Co, balance Ni, (all in weight percent). Analysis of slip band traces, specimen axis rotation, and dislocation Burgers vectors showed that at 760 and 980 C primary cube slip supplanted normal octahedral slip for the -112 line-oriented specimens. The other two orientations, which have lower resolved shear stresses on the cube system, exhibited octahedral slip at all three temperatures. The critical resolved shear stress is considerably greater on the cube system than on the octahedral system at room temperature. However, at 760 and 980 C the critical resolved shear stresses on the two systems are about the same. While the room temperature and 980 C yield strengths for the two orientations exhibiting octahedral slip could be rationalized on the basis of resolved shear stress, those at 760 C could not. Such violations of Schmid's law have previously been observed in other superalloys and single phase gamma-prime.

Assessing the capability of continuum and discrete particle methods to simulate gas-solids flow using DNS predictions as a benchmark

DOE PAGES

Lu, Liqiang; Liu, Xiaowen; Li, Tingwen; ...

2017-08-12

For this study, gas–solids flow in a three-dimension periodic domain was numerically investigated by direct numerical simulation (DNS), computational fluid dynamic-discrete element method (CFD-DEM) and two-fluid model (TFM). DNS data obtained by finely resolving the flow around every particle are used as a benchmark to assess the validity of coarser DEM and TFM approaches. The CFD-DEM predicts the correct cluster size distribution and under-predicts the macro-scale slip velocity even with a grid size as small as twice the particle diameter. The TFM approach predicts larger cluster size and lower slip velocity with a homogeneous drag correlation. Although the slip velocitymore » can be matched by a simple modification to the drag model, the predicted voidage distribution is still different from DNS: Both CFD-DEM and TFM over-predict the fraction of particles in dense regions and under-predict the fraction of particles in regions of intermediate void fractions. Also, the cluster aspect ratio of DNS is smaller than CFD-DEM and TFM. Since a simple correction to the drag model can predict a correct slip velocity, it is hopeful that drag corrections based on more elaborate theories that consider voidage gradient and particle fluctuations may be able to improve the current predictions of cluster distribution.« less

Assessing the capability of continuum and discrete particle methods to simulate gas-solids flow using DNS predictions as a benchmark

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lu, Liqiang; Liu, Xiaowen; Li, Tingwen

For this study, gas–solids flow in a three-dimension periodic domain was numerically investigated by direct numerical simulation (DNS), computational fluid dynamic-discrete element method (CFD-DEM) and two-fluid model (TFM). DNS data obtained by finely resolving the flow around every particle are used as a benchmark to assess the validity of coarser DEM and TFM approaches. The CFD-DEM predicts the correct cluster size distribution and under-predicts the macro-scale slip velocity even with a grid size as small as twice the particle diameter. The TFM approach predicts larger cluster size and lower slip velocity with a homogeneous drag correlation. Although the slip velocitymore » can be matched by a simple modification to the drag model, the predicted voidage distribution is still different from DNS: Both CFD-DEM and TFM over-predict the fraction of particles in dense regions and under-predict the fraction of particles in regions of intermediate void fractions. Also, the cluster aspect ratio of DNS is smaller than CFD-DEM and TFM. Since a simple correction to the drag model can predict a correct slip velocity, it is hopeful that drag corrections based on more elaborate theories that consider voidage gradient and particle fluctuations may be able to improve the current predictions of cluster distribution.« less

The Dependence of Portevin-Le Châtelier Effect on the γ' Precipitates in a Wrought Ni-Base Superalloy

NASA Astrophysics Data System (ADS)

Wang, Xinguang; Han, Guoming; Cui, Chuanyong; Guan, Shuai; Jin, Tao; Sun, Xiaofeng; Hu, Zhuangqi

2016-12-01

The dependence of Portevin-Le Châtelier (PLC) effect on the γ' precipitates of the Nimonic 263 alloy in different microstructural conditions has been studied by analyzing the parameters of the tensile curves and the deformation mechanisms. It is shown that the γ' precipitates with different sizes, edge-to-edge interprecipitate distance, and areal number density are obtained by altering the aging time. It is demonstrated that when the mean size of the γ' precipitates is less than 28 nm (aging less than 25 hours), the deformation mechanisms are dominated by APB-coupled a/2<101> dislocations shearing the small γ' precipitates and the slip bands continuously cutting the γ and γ' phases. When the γ' size is between 28 and 45 nm (aging time between 25 and 50 hours), the deformation mechanism is controlled by the APB-coupled a/2<101> dislocations shearing the small γ' precipitates, the a/6<112> Shockley partial dislocation continuously shearing the γ and γ' phases combined with matrix dislocations by-passing the γ' precipitates; If the γ' size over 45 nm (aging time more than 50 hours), Orowan by-passing becomes the main deformation mechanism. Moreover, with increasing the aging time, the critical plastic strain for the onset of the PLC effect increases and reaches a maximum after aging for 50 hours, and then gradually decreases. At last, the dependence of critical plastic strain on the deformation mechanisms is well explained by the elementary incremental strain (γ). The precipitation process of the γ' phase can directly influence the PLC effect by changing the interactions among solutes atoms, mobile dislocations, and forest dislocations.

Stress and temperature dependence of screw dislocation mobility in {alpha}-Fe by molecular dynamics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gilbert, M. R.; Queyreau, S.; Marian, J.

2011-11-01

The low-temperature plastic yield of {alpha}-Fe single crystals is known to display a strong temperature dependence and to be controlled by the thermally activated motion of screw dislocations. In this paper, we present molecular dynamics simulations of (1/2)<111>{l_brace}112{r_brace} screw dislocation motion as a function of temperature and stress in order to extract mobility relations that describe the general dynamic behavior of screw dislocations in pure {alpha}-Fe. We find two dynamic regimes in the stress-velocity space governed by different mechanisms of motion. Consistent with experimental evidence, at low stresses and temperatures, the dislocations move by thermally activated nucleation and propagation ofmore » kink pairs. Then, at a critical stress, a temperature-dependent transition to a viscous linear regime is observed. Critical output from the simulations, such as threshold stresses and the stress dependence of the kink activation energy, are compared to experimental data and other atomistic works with generally very good agreement. Contrary to some experimental interpretations, we find that glide on {l_brace}112{r_brace} planes is only apparent, as slip always occurs by elementary kink-pair nucleation/propagation events on {l_brace}110{r_brace} planes. Additionally, a dislocation core transformation from compact to dissociated has been identified above room temperature, although its impact on the general mobility is seen to be limited. This and other observations expose the limitations of inferring or presuming dynamic behavior on the basis of only static calculations. We discuss the relevance and applicability of our results and provide a closed-form functional mobility law suitable for mesoscale computational techniques.« less

A fast immersed boundary method for external incompressible viscous flows using lattice Green's functions

NASA Astrophysics Data System (ADS)

Liska, Sebastian; Colonius, Tim

2017-02-01

A new parallel, computationally efficient immersed boundary method for solving three-dimensional, viscous, incompressible flows on unbounded domains is presented. Immersed surfaces with prescribed motions are generated using the interpolation and regularization operators obtained from the discrete delta function approach of the original (Peskin's) immersed boundary method. Unlike Peskin's method, boundary forces are regarded as Lagrange multipliers that are used to satisfy the no-slip condition. The incompressible Navier-Stokes equations are discretized on an unbounded staggered Cartesian grid and are solved in a finite number of operations using lattice Green's function techniques. These techniques are used to automatically enforce the natural free-space boundary conditions and to implement a novel block-wise adaptive grid that significantly reduces the run-time cost of solutions by limiting operations to grid cells in the immediate vicinity and near-wake region of the immersed surface. These techniques also enable the construction of practical discrete viscous integrating factors that are used in combination with specialized half-explicit Runge-Kutta schemes to accurately and efficiently solve the differential algebraic equations describing the discrete momentum equation, incompressibility constraint, and no-slip constraint. Linear systems of equations resulting from the time integration scheme are efficiently solved using an approximation-free nested projection technique. The algebraic properties of the discrete operators are used to reduce projection steps to simple discrete elliptic problems, e.g. discrete Poisson problems, that are compatible with recent parallel fast multipole methods for difference equations. Numerical experiments on low-aspect-ratio flat plates and spheres at Reynolds numbers up to 3700 are used to verify the accuracy and physical fidelity of the formulation.

Experimental Deformation of Enstatite Single Crystals at Mantle Pressure and Temperature

NASA Astrophysics Data System (ADS)

Raterron, P. C.; Holyoke, C. W.; Girard, J.

2012-12-01

Orthopyroxenes (OPx) is the second most abundant constituent of the upper mantle, thus may significantly influence mantle plasticity. However, little is known on OPx rheology at high pressure; this is because apparatuses allowing controlled deformation experiments at asthenospheric pressures (P > 3 GPa) are available since only a decade (see Raterron and Merkel, 2009, J. Synch. Rad., 16, 748-756). Mackwell (1991, GRL, 18, 2027-2030) reports a preliminary study of the high-temperature rheology of enstatite single crystals, but these experiments were carried out at room pressure, i.e. in the protoenstatite stability field, and the results cannot directly apply to mantle phases. In order to quantify the effect of pressure on OPx rheology, deformation experiments were carried out in compression on natural enstatite (En90) single crystals in the Deformation-DIA apparatus (D-DIA), at P > 5 GPa and high temperature (T > 1200°C) within the orthoenstatite stability field. The applied stress and specimen strain rates were measured in situ using X-ray diffraction and imaging techniques at the X17B2 beamline of the National Synchrotron Light Source (NSLS, NY, USA). The cylindrical specimens were oriented with their axis - the compression direction - along [101]c crystallographic direction which forms a 45° angle with both [100] and [001] directions. This geometry imposes during compression a maximum shear stress in (001) plane along [001] direction, which results in the activation of [001](001) dislocation slip system known as the weakest slip in OPx. Specimens were deformed together with other crystals or aggregates - two by two, one atop the other in the compression column - in order to compare [101]c crystal plasticity with that of either [011]c enstatite crystals - within which [001](010) slip system is activated - or enstatite aggregates, or again orientated San Carlos olivine crystals of known rheology. Run products microstructures were investigated by electron microscopy. We will present these results which bring new light on the activity of orthoenstatite dislocation slips at mantle P and T conditions.

Aseismic slip and surface creep on the Hazar-Palu Section of the East Anatolian Fault, Turkey

NASA Astrophysics Data System (ADS)

Ergintav, S.; Cakir, Z.; Dogan, U.; Cetin, S.; Senturk, S.; Karabulut, H.; Saroglu, F.; Dikmen, U.; Bilham, R. G.; Ozdemir, A.; Julaiti, W.; Ozener, H.

2017-12-01

Forming the boundary between the Anatolian and Arabian plates in Turkey, the East Anatolian fault (EAF) is one of the most important tectonic structures in the Eastern Mediterranean region. Together with its conjugate, the North Anatolian Fault (NAF), it accommodates the westward motion of the Anatolian plate at a rate of 10 mm/yr. We study the interseismic deformation along the eastern section of the EAF using SAR data (2012-2017). Interferograms are calculated, using GMT5SAR software (Sandwell et al., 2011). The interferograms are then used to map the velocity field with the Stanford Method for Persistent Scatterers technique (STAMPS; Hooper et al., 2012). In 2015 a new GPS network was established with 6 fault perpendicular profiles crossing segments identified to be creeping from SAR analysis. The closest GPS sites are within 2 km from the surface EAF. Far-field continuous GPS sites permit us to determine the long-term slip rate and hence the depth of creep on the fault using dislocation models. Additionally, two creepmeters were installed to monitor fault creep in a railway tunnel crossing the fault at ≈50 m depth below the ruined medieval village of Palu, at a location where the walls of the tunnel have been offset by 10-20 cm since construction in the middle of the last century. To confirm these results, earthquake catalogs were, also, re-analyzed. The preliminary results, which are obtained from multidisciplinary data, confirm the average slip rate of the EAF is about 10 mm/yr. The results also reveal that the 100-km-long Palu segment in the Elazıg-Bingöl seismic gap is exhibiting aseismic creep at the surface. The surface creep rate varies along the fault locally attaining the far field plate velocity (i.e. 10 mm/yr), implying that significant portion of the elastic strain has been released aseismically. Preliminary modelling with elastic dislocations suggests that some sections of the fault may be creeping from the surface down to the entire seismogenic crust. Our data are the first to confirm aseismic slip on the EAF (supported by TUBITAK 1001 project no:114Y250)

The 2011 Mw 7.1 Van (Eastern Turkey) earthquake

USGS Publications Warehouse

Elliot, John R.; Copley, Alex C.; Holley, R.; Scharer, Katherine M.; Parsons, Barry

2013-01-01

We use interferometric synthetic aperture radar (InSAR), body wave seismology, satellite imagery, and field observations to constrain the fault parameters of the Mw 7.1 2011 Van (Eastern Turkey) reverse-slip earthquake, in the Turkish-Iranian plateau. Distributed slip models from elastic dislocation modeling of the InSAR surface displacements from ENVISAT and COSMO-SkyMed interferograms indicate up to 9 m of reverse and oblique slip on a pair of en echelon NW 40 °–54 ° dipping fault planes which have surface extensions projecting to just 10 km north of the city of Van. The slip remained buried and is relatively deep, with a centroid depth of 14 km, and the rupture reaching only within 8–9 km of the surface, consistent with the lack of significant ground rupture. The up-dip extension of this modeled WSW striking fault plane coincides with field observations of weak ground deformation seen on the western of the two fault segments and has a dip consistent with that seen at the surface in fault gouge exposed in Quaternary sediments. No significant coseismic slip is found in the upper 8 km of the crust above the main slip patches, except for a small region on the eastern segment potentially resulting from the Mw 5.9 aftershock on the same day. We perform extensive resolution tests on the data to confirm the robustness of the observed slip deficit in the shallow crust. We resolve a steep gradient in displacement at the point where the planes of the two fault segments ends are inferred to abut at depth, possibly exerting some structural control on rupture extent.

Micromechanics of an Extrusion in High-Cycle Fatigue

DTIC Science & Technology

1988-08-22

length of R would be longer than the slot by an amount referred to as the "static extrusion" by Mughrabi et al . (1983). This e,,. causes an initial... Mughrabi (1980) in their studies of persistent slip bands. Extrusions and intru- sions in fatigue specimens were also observed by a number of other...interstitial dislocation dipoles, and a negative e("i by vacancy dipoles. Recently, Antoncpoulus, Brown and Winter (1976) and Mughrabi , Wang, Differt and

Phase stability tuning in the NbxZr1-xN thin-film system for large stacking fault density and enhanced mechanical strength

NASA Astrophysics Data System (ADS)

Joelsson, T.; Hultman, L.; Hugosson, H. W.; Molina-Aldareguia, J. M.

2005-03-01

The phase stability of hexagonal WC-structure and cubic NaCl-structure 4d transition metal nitrides was calculated using first-principles density functional theory. It is predicted that there is a multiphase or polytypic region for the 4d transition metal nitrides with a valence electron concentration around 9.5 to 9.7 per formula unit. For verification, epitaxial NbxZr1-xN (0⩽x⩽1) was grown by reactive magnetron sputter deposition on MgO(001) substrates and analyzed with transmission electron microscopy (TEM) and x-ray diffraction. The defects observed in the films were threading dislocations due to nucleation and growth on the lattice-mismatched substrate and planar defects (stacking faults) parallel to the substrate surface. The highest defect density was found at the x =0.5 composition. The nanoindentation hardness of the films varied between 21GPa for the binary nitrides, and 26GPa for Nb0.5Zr0.5N. Unlike the cubic binary nitrides, no slip on the preferred ⟨11¯0⟩{110} slip system was observed. The increase in hardness is attributed to the increase in defect density at x =0.5, as the defects act as obstacles for dislocation glide during deformation. The findings present routes for the design of wear-resistant nitride coatings by phase stability tuning.

Implications of basal micro-earthquakes and tremor for ice stream mechanics: Stick-slip basal sliding and till erosion

NASA Astrophysics Data System (ADS)

Barcheck, C. Grace; Tulaczyk, Slawek; Schwartz, Susan Y.; Walter, Jacob I.; Winberry, J. Paul

2018-03-01

The Whillans Ice Plain (WIP) is unique among Antarctic ice streams because it moves by stick-slip. The conditions allowing stick-slip and its importance in controlling ice dynamics remain uncertain. Local basal seismicity previously observed during unstable slip is a clue to the mechanism of ice stream stick-slip and a window into current basal conditions, but the spatial extent and importance of this basal seismicity are unknown. We analyze data from a 2010-2011 ice-plain-wide seismic and GPS network to show that basal micro-seismicity correlates with large-scale patterns in ice stream slip behavior: Basal seismicity is common where the ice moves the least between unstable slip events, with small discrete basal micro-earthquakes happening within 10s of km of the central stick-slip nucleation area and emergent basal tremor occurring downstream of this area. Basal seismicity is largely absent in surrounding areas, where inter-slip creep rates are high. The large seismically active area suggests that a frictional sliding law that can accommodate stick-slip may be appropriate for ice stream beds on regional scales. Variability in seismic behavior over inter-station distances of 1-10 km indicates heterogeneity in local bed conditions and frictional complexity. WIP unstable slips may nucleate when stick-slip basal earthquake patches fail over a large area. We present a conceptual model in which basal seismicity results from slip-weakening frictional failure of over-consolidated till as it is eroded and mobilized into deforming till.

Subatomic movements of a domain wall in the Peierls potential.

PubMed

Novoselov, K S; Geim, A K; Dubonos, S V; Hill, E W; Grigorieva, I V

2003-12-18

The discrete nature of crystal lattices plays a role in virtually every material property. But it is only when the size of entities hosted by a crystal becomes comparable to the lattice period--as occurs for dislocations, vortices in superconductors and domain walls--that this discreteness is manifest explicitly. The associated phenomena are usually described in terms of a background Peierls 'atomic washboard' energy potential, which was first introduced for the case of dislocation motion in the 1940s. This concept has subsequently been invoked in many situations to describe certain features in the bulk behaviour of materials, but has to date eluded direct detection and experimental scrutiny at a microscopic level. Here we report observations of the motion of a single magnetic domain wall at the scale of the individual peaks and troughs of the atomic energy landscape. Our experiments reveal that domain walls can become trapped between crystalline planes, and that they propagate by distinct jumps that match the lattice periodicity. The jumps between valleys are found to involve unusual dynamics that shed light on the microscopic processes underlying domain-wall propagation. Such observations offer a means for probing experimentally the physics of topological defects in discrete lattices--a field rich in phenomena that have been subject to extensive theoretical study.

Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops

PubMed Central

Peterman, Emily M.; Reddy, Steven M.; Saxey, David W.; Snoeyenbos, David R.; Rickard, William D. A.; Fougerouse, Denis; Kylander-Clark, Andrew R. C.

2016-01-01

Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 μm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe microscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The 207Pb/206Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages. PMID:27617295

Multiscale Analysis of Structurally-Graded Microstructures Using Molecular Dynamics, Discrete Dislocation Dynamics and Continuum Crystal Plasticity

NASA Technical Reports Server (NTRS)

Saether, Erik; Hochhalter, Jacob D.; Glaessgen, Edward H.; Mishin, Yuri

2014-01-01

A multiscale modeling methodology is developed for structurally-graded material microstructures. Molecular dynamic (MD) simulations are performed at the nanoscale to determine fundamental failure mechanisms and quantify material constitutive parameters. These parameters are used to calibrate material processes at the mesoscale using discrete dislocation dynamics (DD). Different grain boundary interactions with dislocations are analyzed using DD to predict grain-size dependent stress-strain behavior. These relationships are mapped into crystal plasticity (CP) parameters to develop a computationally efficient finite element-based DD/CP model for continuum-level simulations and complete the multiscale analysis by predicting the behavior of macroscopic physical specimens. The present analysis is focused on simulating the behavior of a graded microstructure in which grain sizes are on the order of nanometers in the exterior region and transition to larger, multi-micron size in the interior domain. This microstructural configuration has been shown to offer improved mechanical properties over homogeneous coarse-grained materials by increasing yield stress while maintaining ductility. Various mesoscopic polycrystal models of structurally-graded microstructures are generated, analyzed and used as a benchmark for comparison between multiscale DD/CP model and DD predictions. A final series of simulations utilize the DD/CP analysis method exclusively to study macroscopic models that cannot be analyzed by MD or DD methods alone due to the model size.

Continuous modeling of a grain boundary in MgO and its disclination induced grain-boundary migration mechanism

NASA Astrophysics Data System (ADS)

Cordier, P.; Sun, X.; Taupin, V.; Fressengeas, C.

2016-12-01

Grain boundaries (GBs) are thin material layers where the lattice rotates from one orientation to the next one within a few nanometers. Because they treat these layers as infinitely thin interfaces, large-scale polycrystalline representations fail to describe their structure. Conversely, atomistic representations provide a detailed description of the GBs, but their character remains discrete and not prone to coarse-graining procedures. Continuum descriptions based on kinematic and crystal defect fields defined at interatomic scale are appealing because they can provide smooth and thorough descriptions of GBs, recovering in some sense the atomistic description and potentially serving as a basis for coarse-grained polycrystalline representations. In this work, a crossover between atomistic description and continuous representation of a MgO tilt boundary in polycrystals is set-up to model the periodic arrays of structural units by using dislocation and disclination dipole arrays along GBs. The strain, rotation, curvature, disclination and dislocation density fields are determined in the boundary area by using the discrete atomic positions generated by molecular dynamics simulations. Then, this continuous disclination/dislocation model is used as part of the initial conditions in elasto-plastic continuum mechanics simulations to investigate the shear-coupled boundary migration of tilt boundaries. The present study leads to better understanding of the structure and mechanical architecture of grain boundaries.

Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere

USGS Publications Warehouse

Geist, E.L.; Andrews, D.J.

2000-01-01

Long-term slip rates on major faults in the San Francisco Bay area are predicted by modeling the anelastic deformation of the continental lithosphere in response to regional relative plate motion. The model developed by Bird and Kong [1994] is used to simulate lithospheric deformation according to a Coulomb frictional rheology of the upper crust and a dislocation creep rheology at depth. The focus of this study is the long-term motion of faults in a region extending from the creeping section of the San Andreas fault to the south up to the latitude of Cape Mendocino to the north. Boundary conditions are specified by the relative motion between the Pacific plate and the Sierra Nevada - Great Valley microplate [Argus and Gordon, 2000]. Rheologic-frictional parameters are specified as independent variables, and prediction errors are calculated with respect to geologic estimates of slip rates and maximum compressive stress directions. The model that best explains the region-wide observations is one in which the coefficient of friction on all of the major faults is less than 0.15, with the coefficient of friction for the San Andreas fault being approximately 0.09, consistent with previous inferences of San Andreas fault friction. Prediction error increases with lower fault friction on the San Andreas, indicating a lower bound of ??SAF > 0.08. Discrepancies with respect to previous slip rate estimates include a higher than expected slip rate along the peninsula segment of the San Andreas fault and a slightly lower than expected slip rate along the San Gregorio fault.

New insights into fault activation and stress transfer between en echelon thrusts: The 2012 Emilia, Northern Italy, earthquake sequence

NASA Astrophysics Data System (ADS)

Cheloni, D.; Giuliani, R.; D'Agostino, N.; Mattone, M.; Bonano, M.; Fornaro, G.; Lanari, R.; Reale, D.; Atzori, S.

2016-06-01

Here we present the results of the inversion of a new geodetic data set covering the 2012 Emilia seismic sequence and the following 1 year of postseismic deformation. Modeling of the geodetic data together with the use of a catalog of 3-D relocated aftershocks allows us to constrain the rupture geometries and the coseismic and postseismic slip distributions for the two main events (Mw 6.1 and 6.0) of the sequence and to explore how these thrust events have interacted with each other. Dislocation modeling reveals that the first event ruptured a slip patch located in the center of the Middle Ferrara thrust with up to 1 m of reverse slip. The modeling of the second event, located about 15 km to the southwest, indicates a main patch with up to 60 cm of slip initiated in the deeper and flatter portion of the Mirandola thrust and progressively propagated postseismically toward the top section of the rupture plane, where most of the aftershocks and afterslip occurred. Our results also indicate that between the two main events, a third thrust segment was activated releasing a pulse of aseismic slip equivalent to a Mw 5.8 event. Coulomb stress changes suggest that the aseismic event was likely triggered by the preceding main shock and that the aseismic slip event probably brought the second fault closer to failure. Our findings show significant correlations between static stress changes and seismicity and suggest that stress interaction between earthquakes plays a significant role among continental en echelon thrusts.

Coseismic source model of the 2003 Mw 6.8 Chengkung earthquake, Taiwan, determined from GPS measurements

USGS Publications Warehouse

Ching, K.-E.; Rau, R.-J.; Zeng, Y.

2007-01-01

A coseismic source model of the 2003 Mw 6.8 Chengkung, Taiwan, earthquake was well determined with 213 GPS stations, providing a unique opportunity to study the characteristics of coseismic displacements of a high-angle buried reverse fault. Horizontal coseismic displacements show fault-normal shortening across the fault trace. Displacements on the hanging wall reveal fault-parallel and fault-normal lengthening. The largest horizontal and vertical GPS displacements reached 153 and 302 mm, respectively, in the middle part of the network. Fault geometry and slip distribution were determined by inverting GPS data using a three-dimensional (3-D) layered-elastic dislocation model. The slip is mainly concentrated within a 44 ?? 14 km slip patch centered at 15 km depth with peak amplitude of 126.6 cm. Results from 3-D forward-elastic model tests indicate that the dome-shaped folding on the hanging wall is reproduced with fault dips greater than 40??. Compared with the rupture area and average slip from slow slip earthquakes and a compilation of finite source models of 18 earthquakes, the Chengkung earthquake generated a larger rupture area and a lower stress drop, suggesting lower than average friction. Hence the Chengkung earthquake seems to be a transitional example between regular and slow slip earthquakes. The coseismic source model of this event indicates that the Chihshang fault is divided into a creeping segment in the north and the locked segment in the south. An average recurrence interval of 50 years for a magnitude 6.8 earthquake was estimated for the southern fault segment. Copyright 2007 by the American Geophysical Union.

Mesoscale modeling of strain induced solid state amorphization in crystalline materials

NASA Astrophysics Data System (ADS)

Lei, Lei

Solid state amorphization, and in particular crystalline to amorphous transformation, can be observed in metallic alloys, semiconductors, intermetallics, minerals, and also molecular crystals when they undergo irradiation, hydrogen gas dissolution, thermal interdiffusion, mechanical alloying, or mechanical milling. Although the amorphization mechanisms may be different, the transformation occurs due to the high level of disorder introduced into the material. Milling induced solid state amorphization is proposed to be the result of accumulation of crystal defects, specifically dislocations, as the material is subjected to large deformations during the high energy process. Thus, understanding the deformation mechanisms of crystalline materials will be the first step in studying solid state amorphization in crystalline materials, which not only has scientific contributions, but also technical consequences. A phase field dislocation dynamics (PFDD) approach is employed in this work to simulate plastic deformation of molecular crystals. This PFDD model has the advantage of tracking all of the dislocations in a material simultaneously. The model takes into account the elastic interaction between dislocations, the lattice resistance to dislocation motion, and the elastic interaction of dislocations with an external stress field. The PFDD model is employed to describe the deformation of molecular crystals with pharmaceutical applications, namely, single crystal sucrose, acetaminophen, gamma-indomethacin, and aspirin. Stress-strain curves are produced that result in expected anisotropic material response due to the activation of different slip systems and yield stresses that agree well with those from experiments. The PFDD model is coupled to a phase transformation model to study the relation between plastic deformation and the solid state amorphization of crystals that undergo milling. This model predicts the amorphous volume fraction in excellent agreement with experimental observation. Finally, we incorporate the effect of stress free surfaces to model the behavior of dislocations close to these surfaces and in the presence of voids.

Fault Geometry and Slip Distribution at Depth of the 1997 Mw 7.2 Zirkuh Earthquake: Contribution of Near-Field Displacement Data

NASA Astrophysics Data System (ADS)

Marchandon, Mathilde; Vergnolle, Mathilde; Sudhaus, Henriette; Cavalié, Olivier

2018-02-01

In this study, we reestimate the source model of the 1997 Mw 7.2 Zirkuh earthquake (northeastern Iran) by jointly optimizing intermediate-field Interferometry Synthetic Aperture Radar data and near-field optical correlation data using a two-step fault modeling procedure. First, we estimate the geometry of the multisegmented Abiz fault using a genetic algorithm. Then, we discretize the fault segments into subfaults and invert the data to image the slip distribution on the fault. Our joint-data model, although similar to the Interferometry Synthetic Aperture Radar-based model to the first order, highlights differences in the fault dip and slip distribution. Our preferred model is ˜80° west dipping in the northern part of the fault, ˜75° east dipping in the southern part and shows three disconnected high slip zones separated by low slip zones. The low slip zones are located where the Abiz fault shows geometric complexities and where the aftershocks are located. We interpret this rough slip distribution as three asperities separated by geometrical barriers that impede the rupture propagation. Finally, no shallow slip deficit is found for the overall rupture except on the central segment where it could be due to off-fault deformation in quaternary deposits.

Deformation Twins in Nanocrystalline Body-Centered Cubic Mo as Predicted by Molecular Dynamics Simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Michael Tonks; Bulent biner; Yongfeng Zhang

2012-10-01

This work studies deformation twins in nanocrystalline body-centered cubic Mo, including the nucleation and growth mechanisms as well as their effects on ductility, through molecular dynamics simulations. The deformation processes of nanocrystalline Mo are simulated using a columnar grain model with three different orientations. The deformation mechanisms identified, including dislocation slip, grain-boundary-mediated plasticity, deformation twins and martensitic transformation, are in agreement with previous studies. In (1 1 0) columnar grains, the deformation is dominated by twinning, which nucleates primarily from the grain boundaries by successive emission of twinning partials and thickens by jog nucleation in the grain interiors. Upon arrestmore » by a grain boundary, the twin may either produce continuous plastic strain across the grain boundary by activating compatible twinning/slip systems or result in intergranular failure in the absence of compatible twinning/slip systems in the neighboring grain. Multiple twinning systems can be activated in the same grain, and the competition between them favors those capable of producing continuous deformation across the grain boundary.« less

Surface Rupture Effects on Earthquake Moment-Area Scaling Relations

NASA Astrophysics Data System (ADS)

Luo, Yingdi; Ampuero, Jean-Paul; Miyakoshi, Ken; Irikura, Kojiro

2017-09-01

Empirical earthquake scaling relations play a central role in fundamental studies of earthquake physics and in current practice of earthquake hazard assessment, and are being refined by advances in earthquake source analysis. A scaling relation between seismic moment ( M 0) and rupture area ( A) currently in use for ground motion prediction in Japan features a transition regime of the form M 0- A 2, between the well-recognized small (self-similar) and very large (W-model) earthquake regimes, which has counter-intuitive attributes and uncertain theoretical underpinnings. Here, we investigate the mechanical origin of this transition regime via earthquake cycle simulations, analytical dislocation models and numerical crack models on strike-slip faults. We find that, even if stress drop is assumed constant, the properties of the transition regime are controlled by surface rupture effects, comprising an effective rupture elongation along-dip due to a mirror effect and systematic changes of the shape factor relating slip to stress drop. Based on this physical insight, we propose a simplified formula to account for these effects in M 0- A scaling relations for strike-slip earthquakes.

A study of spectrum fatigue crack propagation in two aluminum alloys. 2: Influence of microstructures

NASA Technical Reports Server (NTRS)

Telesman, J.; Antolovich, S. D.

1985-01-01

The important metallurgical factors that influence both constant amplitude and spectrum crack growth behavior in aluminum alloys were investigated. The effect of microstructural features such as grain size, inclusions, and dispersoids was evaluated. It was shown that a lower stress intensities, the I/M 7050 alloy showed better fatigue crack propagation (FCP) resistance than P/M 7091 alloy for both constant amplitude and spectrum testing. It was suggested that the most important microstructural variable accounting for superior FCP resistance of 7050 alloy is its large grain size. It was further postulated that the inhomogenous planar slip and large grain size of 7050 limit dislocation interactions and thus increase slip reversibility which improves FCP performance. The hypothesis was supported by establishing that the cyclic strain hardening exponent for the 7091 alloy is higher than that of 7050.

Tensile Deformation Temperature Impact on Microstructure and Mechanical Properties of AISI 316LN Austenitic Stainless Steel

NASA Astrophysics Data System (ADS)

Xiong, Yi; He, Tiantian; Lu, Yan; Ren, Fengzhang; Volinsky, Alex A.; Cao, Wei

2018-03-01

Uniaxial tensile tests were conducted on AISI 316LN austenitic stainless steel from - 40 to 300 °C at a rate of 0.5 mm/min. Microstructure and mechanical properties of the deformed steel were investigated by optical, scanning and transmission electron microscopies, x-ray diffraction, and microhardness testing. The yield strength, ultimate tensile strength, elongation, and microhardness increase with the decrease in the test temperature. The tensile fracture morphology has the dimple rupture feature after low-temperature deformations and turns to a mixture of transgranular fracture and dimple fracture after high-temperature ones. The dominating deformation microstructure evolves from dislocation tangle/slip bands to large deformation twins/slip bands with temperature decrease. The deformation-induced martensite transformation can only be realized at low temperature, and its quantity increases with the decrease in the temperature.

Parallel algorithm for multiscale atomistic/continuum simulations using LAMMPS

NASA Astrophysics Data System (ADS)

Pavia, F.; Curtin, W. A.

2015-07-01

Deformation and fracture processes in engineering materials often require simultaneous descriptions over a range of length and time scales, with each scale using a different computational technique. Here we present a high-performance parallel 3D computing framework for executing large multiscale studies that couple an atomic domain, modeled using molecular dynamics and a continuum domain, modeled using explicit finite elements. We use the robust Coupled Atomistic/Discrete-Dislocation (CADD) displacement-coupling method, but without the transfer of dislocations between atoms and continuum. The main purpose of the work is to provide a multiscale implementation within an existing large-scale parallel molecular dynamics code (LAMMPS) that enables use of all the tools associated with this popular open-source code, while extending CADD-type coupling to 3D. Validation of the implementation includes the demonstration of (i) stability in finite-temperature dynamics using Langevin dynamics, (ii) elimination of wave reflections due to large dynamic events occurring in the MD region and (iii) the absence of spurious forces acting on dislocations due to the MD/FE coupling, for dislocations further than 10 Å from the coupling boundary. A first non-trivial example application of dislocation glide and bowing around obstacles is shown, for dislocation lengths of ∼50 nm using fewer than 1 000 000 atoms but reproducing results of extremely large atomistic simulations at much lower computational cost.

Fracture toughness of materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Burns, S.J.

Crack tip dislocation emission in bulk specimens have been measured in single crystal specimens and the measurements are well below the accepted theoretical values for dislocation emission. The image forces on a dislocation due to the presence of a semi-infinite crack are used to calculate the potential energy of the dislocation around the crack. Expressions for the radial and tangential forces and for slip and climb forces have been found. Crack tip deformation in Mode I and Mode II fractures on both {l brace}100{r brace} and {l brace}110{r brace} planes have been observed in crystals of LiF. The deformation ismore » shown to nearly completely shield {l brace}110{r brace} plane cracks and prevent their propagation while deformation is less effective in shielding {l brace}100{r brace} plane cracks. The fracture toughness of MgO-partially-stabilized ZrO{sub 2} exhibiting transformation toughening been measured. The equations of linear elastic fracture mechanics have been self-consistantly formulated to include the residual displacement from the transformation wake. MgO single crystals were fatigued in plastic strain control at elevated temperatures. At high temperatures, dense bundles of dislocations were observed in transmission electron microscopy aligned perpendicular to the Burgers' vector directions. The thermodynamics of a superconducting second order phase transformation has been related to jumps in physical properties. A simple energy balance, without assuming an equation of state, is used to relate the rate of change of state variables to measurable physical properties. There are no preconceived assumptions about the superconducting mechanism.« less

The Role of Geometrically Necessary Dislocations in Cantilever Beam Bending Experiments of Single Crystals

PubMed Central

Husser, Edgar; Bargmann, Swantje

2017-01-01

The mechanical behavior of single crystalline, micro-sized copper is investigated in the context of cantilever beam bending experiments. Particular focus is on the role of geometrically necessary dislocations (GNDs) during bending-dominated load conditions and their impact on the characteristic bending size effect. Three different sample sizes are considered in this work with main variation in thickness. A gradient extended crystal plasticity model is presented and applied in a three-dimensional finite-element (FE) framework considering slip system-based edge and screw components of the dislocation density vector. The underlying mathematical model contains non-standard evolution equations for GNDs, crystal-specific interaction relations, and higher-order boundary conditions. Moreover, two element formulations are examined and compared with respect to size-independent as well as size-dependent bending behavior. The first formulation is based on a linear interpolation of the displacement and the GND density field together with a full integration scheme whereas the second is based on a mixed interpolation scheme. While the GND density fields are treated equivalently, the displacement field is interpolated quadratically in combination with a reduced integration scheme. Computational results indicate that GND storage in small cantilever beams strongly influences the evolution of statistically stored dislocations (SSDs) and, hence, the distribution of the total dislocation density. As a particular example, the mechanical bending behavior in the case of a physically motivated limitation of GND storage is studied. The resulting impact on the mechanical bending response as well as on the predicted size effect is analyzed. Obtained results are discussed and related to experimental findings from the literature. PMID:28772657

Seafloor Deformation and Localized Source Mechanisms of the 2011 M9 Tohoku Earthquake and Tsunami.

NASA Astrophysics Data System (ADS)

Masterlark, T.; Grilli, S. T.; Tappin, D. R.; Kirby, J. T.

2012-12-01

The 2011 M9 Tohoku Earthquake (TE) ruptured the interface separating the Pacific and Okhotsk Plates. This rupture was about hundred kilometers in the along-strike direction and 200 kilometers in the down-dip direction. The TE was primarily thrust having substantial slip along the up-dip portion of the rupture, near the Japan Trench. The regional-scale seafloor deformation from the TE triggered a tsunami with run-ups of a few tens of meters that caused extensive damage along the east coast of Tohoku, Japan. We construct finite element models (FEMs) to simulate the deformation caused by a distribution of coseismic slip along the curved rupture surface of the TE. The FEMs include a distribution of material properties that accounts for the subduction zone structure -a weak forearc, volcanic arc, and backarc basin of the overriding Okhotsk Plate overriding the relatively strong subducting slab that is capped by basaltic oceanic crust. The coseismic rupture is simulated as a distribution of elastic dislocations along the interface separating the forearc of the overriding plate and the oceanic crust of the subducting slab. The slip distribution is calibrated to both onshore and offshore geodetic data, using linear least-squares inverse methods with FEM-generated Greens Functions and second order regularization. The regularization is imposed with a conductance matrix, constructed using Galerkin's Method to account for the curvilinear relationships among the dislocating node pairs. The estimated slip distribution is generally characterized as a few tens of meters of slip over the entire rupture, with greater slip magnitudes (>50 meters) concentrated up-dip and near the Japan Trench. The offshore geodetic data provide critical constraints for the location of the polarity reversal of predicted seafloor vertical deformation. Wave models excited by the predicted regional-scale seafloor deformation generally well predict observed tsunami run-ups and the vertical displacement magnitudes of low frequency waves of coastal GPS buoys. However, coastal areas near Sanriku, Japan experienced anomalously high run-ups of 40 meters and local offshore GPS buoys indicate high frequency waveforms that are incompatible with the coseismic seafloor deformation of the TE. These observations require a localized deformation source near the Japan Trench and just to the north of the TE rupture zone, which models solely based on tsunami waveform inversion predict. Others suggest that a submarine mass failure at this location, presumably triggered by the TE, can excite such waveforms. In this study, we investigate an alternative hypothesis that localized splay faulting, also presumably triggered by the TE, can excite the anomalous waveforms. To do so, we will estimate plausible suites of splay fault and slip parameters that can account for the anomalously high magnitude and high frequency tsunami waves sourced from a localized area near the Japan Trench and north of the TE rupture.

In situ straining investigation of slip transfer across α2 lamellae at room temperature in a lamellar TiAl alloy

NASA Astrophysics Data System (ADS)

Singh, J. B.; Molénat, G.; Sundararaman, M.; Banerjee, S.; Saada, G.; Veyssière, P.; Couret, A.

2006-01-01

Processes by which deformation spreads throughout a lamellar TiAl alloy have been investigated by in situ tensile experiments performed at room temperature in a transmission electron microscope. Several situations are found and analysed in which dislocations cross the ?/a2 interfaces and the a2 lamellae - the hard phase of the structure. Conditions by which strain transfer can be elastically mediated across sufficiently thin a2 lamellae are discussed.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sun Dongsheng; Wang Hongcai; Ma Yinsheng

In-situ stress change near the fault before and after a great earthquake is a key issue in the geosciences field. In this work, based on the 2008 Great Wenchuan earthquake fault slip dislocation model, the co-seismic stress tensor change due to the Wenchuan earthquake and the distribution functions around the Longmen Shan fault are given. Our calculated results are almost consistent with the before and after great Wenchuan earthquake in-situ measuring results. The quantitative assessment results provide a reference for the study of the mechanism of earthquakes.

Strength and Dislocation Structure Evolution of Small Metals under Vibrations

NASA Astrophysics Data System (ADS)

Ngan, Alfonso

2015-03-01

It is well-known that ultrasonic vibration can soften metals, and this phenomenon has been widely exploited in industrial applications concerning metal forming and bonding. In this work, we explore the effects of a superimposed small oscillatory load on metal plasticity, from the nano- to macro-size range, and from audible to ultrasonic frequency ranges. Macroscopic and nano-indentation were performed on aluminum, copper and molybdenum, and the results show that the simultaneous application of oscillatory stresses can lower the hardness of these samples. More interestingly, EBSD and TEM observations show that subgrain formation and reduction in dislocation density generally occurred when stress oscillations were applied. These findings point to an important knowledge gap in metal plasticity - the existing understanding of ultrasound softening in terms of the vibrations either imposing additional stress waves to augment the quasi-static applied load, or heating up the metal, whereas the metal's intrinsic deformation resistance or dislocation interactive processes are assumed unaltered by the ultrasound, is proven wrong by the present results. Furthermore, in the case of nanoindentation, the Continuous Stiffness Measurement technique for contact stiffness measurement assumes that the imposed signal-carrier oscillations do not intrinsically alter the material properties of the specimen, and again, the present results prove that this can be wrong. To understand the enhanced subgrain formation and dislocation annihilation, Discrete Dislocation Dynamics (DDD) simulations were carried out and these show that when an oscillatory stress is superimposed on a quasi-static applied stress, reversals of motion of dislocations may occur, and these allow the dislocations to revisit repeatedly suitable configurations for annihilation. DDD, however, was unable to predict the observed subgrain formation presumably because the number of dislocations that can be handled is not large enough. Subgrain formation was directly predicted by a new simulation method of dislocation plasticity based on the dynamics of dislocation density functions.

Coseismic temporal changes of slip direction: the effect of absolute stress on dynamic rupture

USGS Publications Warehouse

Guatteri, Mariagiovanna; Spudich, P.

1998-01-01

We investigate the dynamics of rupture at low-stress level. We show that one main difference between the dynamics of high- and low-stress events is the amount of coseismic temporal rake rotation occurring at given points on the fault. Curved striations on exposed fault surfaces and earthquake dislocation models derived from ground-motion inversion indicate that the slip direction may change with time at a point on the fault during dynamic rupture. We use a 3D boundary integral method to model temporal rake variations during dynamic rupture propagation assuming a slip-weakening friction law and isotropic friction. The points at which the slip rotates most are characterized by an initial shear stress direction substantially different from the average stress direction over the fault plane. We show that for a given value of stress drop, the level of initial shear stress (i.e., the fractional stress drop) determines the amount of rotation in slip direction. We infer that seismic events that show evidence of temporal rake rotations are characterized by a low initial shear-stress level with spatially variable direction on the fault (possibly due to changes in fault surface geometry) and an almost complete stress drop.Our models motivate a new interpretation of curved and cross-cutting striations and put new constraints on their analysis. The initial rake is in general collinear with the initial stress at the hypocentral zone, supporting the assumptions made in stress-tensor inversion from first-motion analysis. At other points on the fault, especially away from the hypocenter, the initial slip rake may not be collinear with the initial shear stress, contradicting a common assumption of structural geology. On the other hand, the later part of slip in our models is systematically more aligned with the average stress direction than the early slip. Our modeling suggests that the length of the straight part of curved striations is usually an upper bound of the slip-weakening distance if this parameter is uniform over the fault plane, and the direction of the late part of slip of curved striations should have more weight in the estimate of initial stress direction.

Water pumping in mantle shear zones

PubMed Central

Précigout, Jacques; Prigent, Cécile; Palasse, Laurie; Pochon, Anthony

2017-01-01

Water plays an important role in geological processes. Providing constraints on what may influence the distribution of aqueous fluids is thus crucial to understanding how water impacts Earth's geodynamics. Here we demonstrate that ductile flow exerts a dynamic control on water-rich fluid circulation in mantle shear zones. Based on amphibole distribution and using dislocation slip-systems as a proxy for syn-tectonic water content in olivine, we highlight fluid accumulation around fine-grained layers dominated by grain-size-sensitive creep. This fluid aggregation correlates with dislocation creep-accommodated strain that localizes in water-rich layers. We also give evidence of cracking induced by fluid pressure where the highest amount of water is expected. These results emphasize long-term fluid pumping attributed to creep cavitation and associated phase nucleation during grain size reduction. Considering the ubiquitous process of grain size reduction during strain localization, our findings shed light on multiple fluid reservoirs in the crust and mantle. PMID:28593947

The microstructure and tensile properties of extruded melt-spun ribbons of iron-rich B2 FeAl

NASA Technical Reports Server (NTRS)

Baker, I.; Gaydosh, D. J.

1987-01-01

The microstructure of extruded rods of iron-rich FeAl(B2-structure), as characterized by TEM, SEM, optical microscopy and x-ray diffractometry, consisted of elongated grains with a 111-line fibre texture containing a high dislocation density. Numerous oxide particles were found, mostly in lines which reflected the matrix flow during extrusion. In addition, some large inclusions were present. Tensile testing of annealed, relatively dislocation-free specimens as a function of increasing temperature found increasing ductility up to 900K, above which a ductility drop occurred accompanied by a change in fracture mode, from transgranular cleavage to intergranular fracture. The yield strength, which was independent of temperature up to 800K (at about 500MPa), also decreased rapidly as diffusion became more important. The predominant slip vector changed from 111-line to 100-line around 700K.

Molecular Dynamics Modeling and Simulation of Diamond Cutting of Cerium.

PubMed

Zhang, Junjie; Zheng, Haibing; Shuai, Maobing; Li, Yao; Yang, Yang; Sun, Tao

2017-12-01

The coupling between structural phase transformations and dislocations induces challenges in understanding the deformation behavior of metallic cerium at the nanoscale. In the present work, we elucidate the underlying mechanism of cerium under ultra-precision diamond cutting by means of molecular dynamics modeling and simulations. The molecular dynamics model of diamond cutting of cerium is established by assigning empirical potentials to describe atomic interactions and evaluating properties of two face-centered cubic cerium phases. Subsequent molecular dynamics simulations reveal that dislocation slip dominates the plastic deformation of cerium under the cutting process. In addition, the analysis based on atomic radial distribution functions demonstrates that there are trivial phase transformations from the γ-Ce to the δ-Ce occurred in both machined surface and formed chip. Following investigations on machining parameter dependence reveal the optimal machining conditions for achieving high quality of machined surface of cerium.

Molecular Dynamics Modeling and Simulation of Diamond Cutting of Cerium

NASA Astrophysics Data System (ADS)

Zhang, Junjie; Zheng, Haibing; Shuai, Maobing; Li, Yao; Yang, Yang; Sun, Tao

2017-07-01

The coupling between structural phase transformations and dislocations induces challenges in understanding the deformation behavior of metallic cerium at the nanoscale. In the present work, we elucidate the underlying mechanism of cerium under ultra-precision diamond cutting by means of molecular dynamics modeling and simulations. The molecular dynamics model of diamond cutting of cerium is established by assigning empirical potentials to describe atomic interactions and evaluating properties of two face-centered cubic cerium phases. Subsequent molecular dynamics simulations reveal that dislocation slip dominates the plastic deformation of cerium under the cutting process. In addition, the analysis based on atomic radial distribution functions demonstrates that there are trivial phase transformations from the γ-Ce to the δ-Ce occurred in both machined surface and formed chip. Following investigations on machining parameter dependence reveal the optimal machining conditions for achieving high quality of machined surface of cerium.

Transmission electron microscopy investigation of interfaces in a two-phase TiAl alloy

NASA Astrophysics Data System (ADS)

Mahon, G. J.; Howe, J. M.

1990-06-01

The atomic structures of the γ/α2 and γ/γT interfaces in a TiAl alloy were investigated using conventional and high-resolution transmission electron microscopy (TEM) in order to understand the growth mechanisms and deformation behavior of the two-phase alloy. The results show that the α2 plates grow from the γ phase by the migration of a/6<112> partial dislocation ledges across the faces and that the γ/α2 interface usually contains closely spaced arrays of interfacial dislocations. Deformation twins cut through both γ twin boundaries and α2 plates during deformation, although slip of twinning c slocations through α2 appears to be a difficult process. Both the γ/α2 and γ/γT interfaces can be imaged and modeled at the atomic level, although slight crystal and/or beam tilt can complicate image interpretation.

Application of microdynamics and lattice mechanics to problems in plastic flow and fracture. Final report, 1 April 1973--31 March 1978

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bilello, J C; Liu, J M

Progress in an investigation of the application of microdynamics and lattice mechanics to the problems in plastic flow and fracture is described. The research program consisted of both theoretical formulations and experimental measurements of a number of intrinsic material parameters in bcc metals and alloys including surface energy, phonon-dispersion curves for dislocated solids, dislocation-point defect interaction energy, slip initiation and microplastic flow behavior. The study has resulted in an improved understanding in the relationship among the experimentally determined fracture surface energy, the intrinsic cohesive energy between atomic planes, and the plastic deformation associated with the initial stages of crack propagation.more » The values of intrinsic surface energy of tungsten, molybdenum, niobium and niobium-molybdenum alloys, deduced from the measurements, serve as a starting point from which fracture toughness of these materials in engineering service may be intelligently discussed.« less

A review of the rupture characteristics of the 2011 Tohoku-oki Mw 9.1 earthquake

NASA Astrophysics Data System (ADS)

Lay, Thorne

2018-05-01

The 2011 March 11 Tohoku-oki great (Mw 9.1) earthquake ruptured the plate boundary megathrust fault offshore of northern Honshu with estimates of shallow slip of 50 m and more near the trench. Non-uniform slip extended 220 km across the width and 400 km along strike of the subduction zone. Extensive data provided by regional networks of seismic and geodetic stations in Japan and global networks of broadband seismic stations, regional and global ocean bottom pressure sensors and sea level measurement stations, seafloor GPS/Acoustic displacement sites, repeated multi-channel reflection images, extensive coastal runup and inundation observations, and in situ sampling of the shallow fault zone materials and temperature perturbation, make the event the best-recorded and most extensively studied great earthquake to date. An effort is made here to identify the more robust attributes of the rupture as well as less well constrained, but likely features. Other issues involve the degree to which the rupture corresponded to geodetically-defined preceding slip-deficit regions, the influence of re-rupture of slip regions for large events in the past few centuries, and relationships of coseismic slip to precursory slow slip, foreshocks, aftershocks, afterslip, and relocking of the megathrust. Frictional properties associated with the slip heterogeneity and in situ measurements of frictional heating of the shallow fault zone support low stress during shallow sliding and near-total shear stress drop of 10-30 MPa in large-slip regions in the shallow megathrust. The roles of fault morphology, sediments, fluids, and dynamical processes in the rupture behavior continue to be examined; consensus has not yet been achieved. The possibility of secondary sources of tsunami excitation such as inelastic deformation of the sedimentary wedge or submarine slumping remains undemonstrated; dislocation models in an elastic continuum appear to sufficiently account for most mainshock observations, although afterslip and viscoelastic processes remain contested.

Can grain size sensitive flow lubricate faults during the initial stages of earthquake propagation?

NASA Astrophysics Data System (ADS)

De Paola, Nicola; Holdsworth, Robert E.; Viti, Cecilia; Collettini, Cristiano; Bullock, Rachael

2015-12-01

Recent friction experiments carried out under upper crustal P-T conditions have shown that microstructures typical of high temperature creep develop in the slip zone of experimental faults. These mechanisms are more commonly thought to control aseismic viscous flow and shear zone strength in the lower crust/upper mantle. In this study, displacement-controlled experiments have been performed on carbonate gouges at seismic slip rates (1 m s-1), to investigate whether they may also control the frictional strength of seismic faults at the higher strain rates attained in the brittle crust. At relatively low displacements (<1 cm) and temperatures (≤100 °C), brittle fracturing and cataclasis produce shear localisation and grain size reduction in a thin slip zone (150 μm). With increasing displacement (up to 15 cm) and temperatures (T up to 600 °C), due to frictional heating, intracrystalline plasticity mechanisms start to accommodate intragranular strain in the slip zone, and play a key role in producing nanoscale subgrains (≤100 nm). With further displacement and temperature rise, the onset of weakening coincides with the formation in the slip zone of equiaxial, nanograin aggregates exhibiting polygonal grain boundaries, no shape or crystal preferred orientation and low dislocation densities, possibly due to high temperature (>900 °C) grain boundary sliding (GBS) deformation mechanisms. The observed micro-textures are strikingly similar to those predicted by theoretical studies, and those observed during experiments on metals and fine-grained carbonates, where superplastic behaviour has been inferred. To a first approximation, the measured drop in strength is in agreement with our flow stress calculations, suggesting that strain could be accommodated more efficiently by these mechanisms within the weaker bulk slip zone, rather than by frictional sliding along the main slip surfaces in the slip zone. Frictionally induced, grainsize-sensitive GBS deformation mechanisms can thus account for the self-lubrication and dynamic weakening of carbonate faults during earthquake propagation in nature.

HR-EBSD as a new tool for quantifying geometrically necessary dislocations in quartz: Application to chessboard subgrain boundaries

NASA Astrophysics Data System (ADS)

Wallis, D.; Parsons, A. J.; Hansen, L. N.

2017-12-01

Chessboard subgrains in quartz, with boundaries composed of {m}[c] edge dislocations, are widely used as evidence for high-temperature deformation and have been suggested to form only in β-quartz. However, the origins and dislocation structure of chessboard subgrains remain poorly constrained and, without precise constraints on axes of misorientations across subgrain boundaries, other subgrain types formed at lower temperatures can be misidentified as chessboard subgrains. The technique most commonly employed to investigate subgrain structures, electron backscatter diffraction, can only resolve misorientation angles and axes for a portion of the substructure. This limitation hinders detailed interpretation of the dislocation types, densities, and processes that generate characteristic subgrain structures. We overcome these limitations by employing high-angular resolution electron backscatter diffraction (HR-EBSD), which employs cross-correlation of diffraction patterns to achieve angular resolution on the order of 0.01° with well-constrained misorientation axes. We analyse chessboard subgrains in samples from the Greater Himalayan Sequence, Nepal, which were deformed along well constrained pressure-temperature paths confined to the stability field of α-quartz. HR-EBSD analysis demonstrates that the subgrain boundaries consist of two sets. One set consists primarily of {m}[c] edge dislocations and the other consists of dislocations primarily with Burgers vectors. Apparent densities of geometrically necessary dislocations vary from > 1013 m-2 within some subgrain boundaries to < 1012 m-2 within subgrain interiors. This analysis provides new insight into the structure of chessboard subgrain boundaries, and a new tool to distinguish them from superficially similar deformation microstructures formed by other dislocation types at lower temperatures. Application of HR-EBSD to quartz from the Greater Himalayan Sequence confirms the activity of {m}[c] slip in the α-quartz stability field and demonstrates that formation of chessboard subgrains is not restricted to the stability field of β-quartz. Most importantly, this study demonstrates the potential of HR-EBSD as an improved method for analysis of quartz microstructures used as indicators of deformation conditions.

Microstructure of calcite deformed by high-pressure torsion: An X-ray line profile study

NASA Astrophysics Data System (ADS)

Schuster, Roman; Schafler, Erhard; Schell, Norbert; Kunz, Martin; Abart, Rainer

2017-11-01

Calcite aggregates were deformed to high strain using high-pressure torsion and applying confining pressures of 1-6 GPa and temperatures between room temperature and 450 °C. The run products were characterized by X-ray diffraction, and key microstructural parameters were extracted employing X-ray line profile analysis. The dominant slip system was determined as r { 10 1 bar 4 } ⟨ 2 bar 021 ⟩ with edge dislocation character. The resulting dislocation density and the size of the coherently scattering domains (CSD) exhibit a systematic dependence on the P-T conditions of deformation. While high pressure generally impedes recovery through reducing point defect mobility, the picture is complicated by pressure-induced phase transformations in the CaCO3 system. Transition from the calcite stability field to those of the high-pressure polymorphs CaCO3-II, CaCO3-III and CaCO3-IIIb leads to a change of the microstructural evolution with deformation. At 450 °C and pressures within the calcite stability field, dislocation densities and CSD sizes saturate at shear strains exceeding 10 in agreement with earlier studies at lower pressures. In the stability field of CaCO3-II, the dislocation density exhibits a more complex behavior. Furthermore, at a given strain and strain rate, the dislocation density increases and the CSD size decreases with increasing pressure within the stability fields of either calcite or of the high-pressure polymorphs. There is, however, a jump from high dislocation densities and small CSDs in the upper pressure region of the calcite stability field to lower dislocation densities and larger CSDs in the low-pressure region of the CaCO3-II stability field. This jump is more pronounced at higher temperatures and less so at room temperature. The pressure influence on the deformation-induced evolution of dislocation densities implies that pressure variations may change the rheology of carbonate rocks. In particular, a weakening is expected to occur at the transition from the calcite to the CaCO3-II stability field, if aragonite does not form.

Flow and fracture behavior of NiAl in relation to the brittle-to-ductile transition temperature

NASA Technical Reports Server (NTRS)

Noebe, R. D.; Bowman, R. R.; Cullers, C. L.; Raj, S. V.

1991-01-01

NiAl has only three independent slip systems operating at low and intermediate temperatures whereas five independent deformation mechanisms are required to satisfy the von Mises criterion for general plasticity in polycrystalline materials. Yet, it is generally recognized that polycrystalline NiAl can be deformed extensively in compression at room temperature and that limited tensile ductility can be obtained in extruded materials. In order to determine whether these results are in conflict with the von Mises criterion, tension and compression tests were conducted on powder-extruded, binary NiAl between 300 and 1300 K. The results indicate that below the brittle-to-ductile transition temperature (BDTT) the failure mechanism in NiAl involves the initiation and propagation of cracks at the grain boundaries which is consistent with the von Mises analysis. Furthermore, evaluation of the flow behavior of NiAl indicates that the transition from brittle to ductile behavior with increasing temperature coincides with the onset of recovery mechanisms such as dislocation climb. The increase in ductility above the BDTT is therefore attributed to the climb of the 001 line type dislocations which in combination with dislocation glide enable grain boundary compatibility to be maintained at the higher temperatures.

Deformation mechanism study of a hot rolled Zr-2.5Nb alloy by transmission electron microscopy. II. In situ transmission electron microscopy study of deformation mechanism change of a Zr-2.5Nb alloy upon heavy ion irradiation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Long, Fei; Daymond, Mark R., E-mail: mark.daymond@queensu.ca; Yao, Zhongwen

2015-03-14

The effect of heavy-ion irradiation on deformation mechanisms of a Zr-2.5Nb alloy was investigated by using the in situ transmission electron microscopy deformation technique. The gliding behavior of prismatic 〈a〉 dislocations has been dynamically observed before and after irradiation at room temperature and 300 °C. Irradiation induced loops were shown to strongly pin the gliding dislocations. Unpinning occurred while loops were incorporated into or eliminated by 〈a〉 dislocations. In the irradiated sample, loop depleted areas with a boundary parallel to the basal plane trace were found by post-mortem observation after room temperature deformation, supporting the possibility of basal channel formation inmore » bulk neutron irradiated samples. Strong activity of pyramidal slip was also observed at both temperatures, which might be another important mechanism to induce plastic instability in irradiated zirconium alloys. Finally, (011{sup ¯}1)〈01{sup ¯}12〉 twinning was identified in the irradiated sample deformed at 300 °C.« less

Deformation mechanism study of a hot rolled Zr-2.5Nb alloy by transmission electron microscopy. II. In situ transmission electron microscopy study of deformation mechanism change of a Zr-2.5Nb alloy upon heavy ion irradiation.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Long, Fei; Daymond, Mark R.; Yao, Zhongwen

2015-03-14

The effect of heavy-ion irradiation on deformation mechanisms of a Zr-2.5Nb alloy was investigated by using the in situ transmission electron microscopy deformation technique. The gliding behavior of prismatic < a > dislocations has been dynamically observed before and after irradiation at room temperature and 300 degrees C. Irradiation induced loops were shown to strongly pin the gliding dislocations. Unpinning occurred while loops were incorporated into or eliminated by < a > dislocations. In the irradiated sample, loop depleted areas with a boundary parallel to the basal plane trace were found by post-mortem observation after room temperature deformation, supporting themore » possibility of basal channel formation in bulk neutron irradiated samples. Strong activity of pyramidal slip was also observed at both temperatures, which might be another important mechanism to induce plastic instability in irradiated zirconium alloys. Finally, {01 (1) over bar1}< 0 (1) over bar 12 > twinning was identified in the irradiated sample deformed at 300 degrees C.« less

MaRIE first experiments summaries version: May 9, 2010

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sarrao, John L

2010-01-01

A predictive understanding of microstructure-based heterogeneity and its consequences for materials damage & failure and phase transformation initiation is presently lacking. Most metallic materials used in applications are polycrystalline aggregates - individual single crystals separated by grain boundaries. Most of these materials are either metallic alloys or contain impurities. In either case, there is spatial variability in their chemical composition. These materials also contain dislocations which will be distributed in some way throughout the individual grains and increase in density with deformation and typically form dislocation sub-cell arrangements - producing spatial distribution in dislocation density. Many materials also produce twinmore » or slip band structures with deformation which produce further heterogeneity within individual crystals. The objective of this first experiment is to probe the physics of dynamic solid-solid phase transformation and damage at length scales approaching those at which they nucleate in order to gain a detailed understanding of this process and the influence real material microstructure has on these events. These experiments would simultaneously be simulated by the appropriate modeling tools to further develop these predictive tools and to assist in our interpretation of experimental results.« less

Effects of Gas Rarefaction on Dynamic Characteristics of Micro Spiral-Grooved Thrust Bearing.

PubMed

Liu, Ren; Wang, Xiao-Li; Zhang, Xiao-Qing

2012-04-01

The effects of gas-rarefaction on dynamic characteristics of micro spiral-grooved-thrust-bearing are studied. The Reynolds equation is modified by the first order slip model, and the corresponding perturbation equations are then obtained on the basis of the linear small perturbation method. In the converted spiral-curve-coordinates system, the finite-volume-method (FVM) is employed to discrete the surface domain of micro bearing. The results show, compared with the continuum-flow model, that under the slip-flow regime, the decrease in the pressure and stiffness become obvious with the increasing of the compressibility number. Moreover, with the decrease of the relative gas-film-thickness, the deviations of dynamic coefficients between slip-flow-model and continuum-flow-model are increasing.

A silent slip event on the deeper Cascadia subduction interface.

PubMed

Dragert, G; Wang, K; James, T S

2001-05-25

Continuous Global Positioning System sites in southwestern British Columbia, Canada, and northwestern Washington state, USA, have been moving landward as a result of the locked state of the Cascadia subduction fault offshore. In the summer of 1999, a cluster of seven sites briefly reversed their direction of motion. No seismicity was associated with this event. The sudden displacements are best explained by approximately 2 centimeters of aseismic slip over a 50-kilometer-by-300-kilometer area on the subduction interface downdip from the seismogenic zone, a rupture equivalent to an earthquake of moment magnitude 6.7. This provides evidence that slip of the hotter, plastic part of the subduction interface, and hence stress loading of the megathrust earthquake zone, can occur in discrete pulses.

Distributional and regularized radiation fields of non-uniformly moving straight dislocations, and elastodynamic Tamm problem

NASA Astrophysics Data System (ADS)

Lazar, Markus; Pellegrini, Yves-Patrick

2016-11-01

This work introduces original explicit solutions for the elastic fields radiated by non-uniformly moving, straight, screw or edge dislocations in an isotropic medium, in the form of time-integral representations in which acceleration-dependent contributions are explicitly separated out. These solutions are obtained by applying an isotropic regularization procedure to distributional expressions of the elastodynamic fields built on the Green tensor of the Navier equation. The obtained regularized field expressions are singularity-free, and depend on the dislocation density rather than on the plastic eigenstrain. They cover non-uniform motion at arbitrary speeds, including faster-than-wave ones. A numerical method of computation is discussed, that rests on discretizing motion along an arbitrary path in the plane transverse to the dislocation, into a succession of time intervals of constant velocity vector over which time-integrated contributions can be obtained in closed form. As a simple illustration, it is applied to the elastodynamic equivalent of the Tamm problem, where fields induced by a dislocation accelerated from rest beyond the longitudinal wave speed, and thereafter put to rest again, are computed. As expected, the proposed expressions produce Mach cones, the dynamic build-up and decay of which is illustrated by means of full-field calculations.

Discrete effect on the halfway bounce-back boundary condition of multiple-relaxation-time lattice Boltzmann model for convection-diffusion equations.

PubMed

Cui, Shuqi; Hong, Ning; Shi, Baochang; Chai, Zhenhua

2016-04-01

In this paper, we will focus on the multiple-relaxation-time (MRT) lattice Boltzmann model for two-dimensional convection-diffusion equations (CDEs), and analyze the discrete effect on the halfway bounce-back (HBB) boundary condition (or sometimes called bounce-back boundary condition) of the MRT model where three different discrete velocity models are considered. We first present a theoretical analysis on the discrete effect of the HBB boundary condition for the simple problems with a parabolic distribution in the x or y direction, and a numerical slip proportional to the second-order of lattice spacing is observed at the boundary, which means that the MRT model has a second-order convergence rate in space. The theoretical analysis also shows that the numerical slip can be eliminated in the MRT model through tuning the free relaxation parameter corresponding to the second-order moment, while it cannot be removed in the single-relaxation-time model or the Bhatnagar-Gross-Krook model unless the relaxation parameter related to the diffusion coefficient is set to be a special value. We then perform some simulations to confirm our theoretical results, and find that the numerical results are consistent with our theoretical analysis. Finally, we would also like to point out the present analysis can be extended to other boundary conditions of lattice Boltzmann models for CDEs.

Preslip and cascade processes initiating laboratory stick slip

USGS Publications Warehouse

McLaskey, Gregory C.; Lockner, David A.

2014-01-01

Recent modeling studies have explored whether earthquakes begin with a large aseismic nucleation process or initiate dynamically from the rapid growth of a smaller instability in a “cascade-up” process. To explore such a case in the laboratory, we study the initiation of dynamic rupture (stick slip) of a smooth saw-cut fault in a 76mm diameter cylindrical granite laboratory sample at 40–120MPa confining pressure. We use a high dynamic range recording system to directly compare the seismic waves radiated during the stick-slip event to those radiated from tiny (M _6) discrete seismic events, commonly known as acoustic emissions (AEs), that occur in the seconds prior to each large stick slip. The seismic moments, focal mechanisms, locations, and timing of the AEs all contribute to our understanding of their mechanics and provide us with information about the stick-slip nucleation process. In a sequence of 10 stick slips, the first few microseconds of the signals recorded from stick-slip instabilities are nearly indistinguishable from those of premonitory AEs. In this sense, it appears that each stick slip begins as an AE event that rapidly (~20 μs) grows about 2 orders of magnitude in linear dimension and ruptures the entire 150mm length of the simulated fault. We also measure accelerating fault slip in the final seconds before stick slip. We estimate that this slip is at least 98% aseismic and that it both weakens the fault and produces AEs that will eventually cascade-up to initiate the larger dynamic rupture.

Numerical study of the flow in a three-dimensional thermally driven cavity

NASA Astrophysics Data System (ADS)

Rauwoens, Pieter; Vierendeels, Jan; Merci, Bart

2008-06-01

Solutions for the fully compressible Navier-Stokes equations are presented for the flow and temperature fields in a cubic cavity with large horizontal temperature differences. The ideal-gas approximation for air is assumed and viscosity is computed using Sutherland's law. The three-dimensional case forms an extension of previous studies performed on a two-dimensional square cavity. The influence of imposed boundary conditions in the third dimension is investigated as a numerical experiment. Comparison is made between convergence rates in case of periodic and free-slip boundary conditions. Results with no-slip boundary conditions are presented as well. The effect of the Rayleigh number is studied. Results are computed using a finite volume method on a structured, collocated grid. An explicit third-order discretization for the convective part and an implicit central discretization for the acoustic part and for the diffusive part are used. To stabilize the scheme an artificial dissipation term for the pressure and the temperature is introduced. The discrete equations are solved using a time-marching method with restrictions on the timestep corresponding to the explicit parts of the solver. Multigrid is used as acceleration technique.

Thermo-Hydro-Micro-Mechanical 3D Modeling of a Fault Gouge During Co-seismic Slip

NASA Astrophysics Data System (ADS)

Papachristos, E.; Stefanou, I.; Sulem, J.; Donze, F. V.

2017-12-01

A coupled Thermo-Hydro-Micro-Mechanical (THMM) model based on the Discrete Elements method (DEM) is presented for studying the evolving fault gouge properties during pre- and co-seismic slip. Modeling the behavior of the fault gouge at the microscale is expected to improve our understanding on the various mechanisms that lead to slip weakening and finally control the transition from aseismic to seismic slip.The gouge is considered as a granular material of spherical particles [1]. Upon loading, the interactions between particles follow a frictional behavior and explicit dynamics. Using regular triangulation, a pore network is defined by the physical pore space between the particles. The network is saturated by a compressible fluid, and flow takes place following Stoke's equations. Particles' movement leads to pore deformation and thus to local pore pressure increase. Forces exerted from the fluid onto the particles are calculated using mid-step velocities. The fluid forces are then added to the contact forces resulting from the mechanical interactions before the next step.The same semi-implicit, two way iterative coupling is used for the heat-exchange through conduction.Simple tests have been performed to verify the model against analytical solutions and experimental results. Furthermore, the model was used to study the effect of temperature on the evolution of effective stress in the system and to highlight the role of thermal pressurization during seismic slip [2, 3].The analyses are expected to give grounds for enhancing the current state-of-the-art constitutive models regarding fault friction and shed light on the evolution of fault zone propertiesduring seismic slip.[1] Omid Dorostkar, Robert A Guyer, Paul A Johnson, Chris Marone, and Jan Carmeliet. On the role of fluids in stick-slip dynamics of saturated granular fault gouge using a coupled computational fluid dynamics-discrete element approach. Journal of Geophysical Research: Solid Earth, 122(5):3689-3700, 2017.[2] James R Rice. Heating and weakening of faults during earthquake slip. Journal of Geophysical Research: Solid Earth, 111(B5), 2006.[3] Jean Sulem, Ioannis Stefanou, and Emmanuil Veveakis. Stability analysis of undrained adiabatic shearing of a rock layer with cosserat microstructure. Granular Matter, 13(3):261-268,2011.

Fault slip and seismic moment of the 1700 Cascadia earthquake inferred from Japanese tsunami descriptions

USGS Publications Warehouse

Satake, K.; Wang, K.; Atwater, B.F.

2003-01-01

The 1700 Cascadia earthquake attained moment magnitude 9 according to new estimates based on effects of its tsunami in Japan, computed coseismic seafloor deformation for hypothetical ruptures in Cascadia, and tsunami modeling in the Pacific Ocean. Reports of damage and flooding show that the 1700 Casscadia tsunami reached 1-5 m heights at seven shoreline sites in Japan. Three sets of estimated heights express uncertainty about location and depth of reported flooding, landward decline in tsunami heights from shorelines, and post-1700 land-level changes. We compare each set with tsunami heights computed from six Cascadia sources. Each source is vertical seafloor displacement calculated with a three-dimensional elastic dislocation model, for three sources the rupture extends the 1100 km length of the subduction zone and differs in width and shallow dip; for the other sources, ruptures of ordinary width extend 360-670 km. To compute tsunami waveforms, we use a linear long-wave approximation with a finite difference method, and we employ modern bathymetry with nearshore grid spacing as small as 0.4 km. The various combinations of Japanese tsunami heights and Cascadia sources give seismic moment of 1-9 ?? 1022 N m, equivalent to moment magnitude 8.7-9.2. This range excludes several unquantified uncertainties. The most likely earthquake, of moment magnitude 9.0, has 19 m of coseismic slip on an offshore, full-slip zone 1100 km long with linearly decreasing slip on a downdip partial-slip zone. The shorter rupture models require up to 40 m offshore slip and predict land-level changes inconsistent with coastal paleoseismological evidence. Copyright 2003 by the American Geophysical Union.

Dislocation imaging for orthopyroxene using an atom-resolved scanning transmission electron microscopy.

PubMed

Kumamoto, Akihito; Kogure, Toshihiro; Raimbourg, Hugues; Ikuhara, Yuichi

2014-11-01

Dislocations, one-dimensional lattice defects, appear as a microscopic phenomenon while they are formed in silicate minerals by macroscopic dynamics of the earth crust such as shear stress. To understand ductile deformation mechanisms of silicates, atomic structures of the dislocations have been examined using transmission electron microscopy (TEM). Among them, it has been proposed that {100}<001> primary slip system of orthopyroxene (Opx) is dissociated into partial dislocations, and a stacking fault with the clinopyroxene (Cpx) structure is formed between the dislocations. This model, however, has not been determined completely due to the complex structures of silicates. Scanning transmission electron microscopy (STEM) has a potential to determine the structure of dislocations with single-atomic column sensitivity, particularly by using high-angle annular dark field (HAADF) and annular bright field (ABF) imaging with a probing aberration corrector.[1] Furthermore, successive analyses from light microscopy to atom-resolved STEM have been achieved by focused ion beam (FIB) sampling techniques.[2] In this study, we examined dislocation arrays at a low-angle grain boundary of ∼1° rotation about the b-axis in natural deformed Opx using a simultaneous acquisition of HAADF/ABF (JEM-ARM200F, JEOL) equipped with 100 mm2 silicon drift detector (SDD) for energy dispersive X-ray spectroscopy (EDS). Figure 1 shows averaged STEM images viewed along the b- axis of Opx extracted from repeating units. HAADF provides the cation-site arrangement, and ABF distinguishes the difference of slightly rotated SiO4 tetrahedron around the a- axis. This is useful to distinguish the change of stacking sequence between the partial dislocations. Two types of stacking faults with Cpx and protopyroxene (Ppx) structures were identified between three partial dislocations. Furthermore, Ca accumulation in M2 (Fe) site around the stacking faults was detected by STEM-EDS. Interestingly, Ca is distributed not only in these stacking faults but also Opx matrix around the faults. jmicro;63/suppl_1/i17/DFU063F1F1DFU063F1Fig. 1. (a) HAADF and (b) ABF of Opx view of [010] direction with inset simulation images and models of its unit cell (a = 0.52, c = 1.83 nm). © The Author 2014. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

Mechanical Failure Mode of Metal Nanowires: Global Deformation versus Local Deformation

PubMed Central

Ho, Duc Tam; Im, Youngtae; Kwon, Soon-Yong; Earmme, Youn Young; Kim, Sung Youb

2015-01-01

It is believed that the failure mode of metal nanowires under tensile loading is the result of the nucleation and propagation of dislocations. Such failure modes can be slip, partial slip or twinning and therefore they are regarded as local deformation. Here we provide numerical and theoretical evidences to show that global deformation is another predominant failure mode of nanowires under tensile loading. At the global deformation mode, nanowires fail with a large contraction along a lateral direction and a large expansion along the other lateral direction. In addition, there is a competition between global and local deformations. Nanowires loaded at low temperature exhibit global failure mode first and then local deformation follows later. We show that the global deformation originates from the intrinsic instability of the nanowires and that temperature is a main parameter that decides the global or local deformation as the failure mode of nanowires. PMID:26087445

Load relaxation of olivine single crystals

NASA Astrophysics Data System (ADS)

Cooper, Reid F.; Stone, Donald S.; Plookphol, Thawatchai

2016-10-01

Single crystals of ferromagnesian olivine (San Carlos, AZ, peridot; Fo88-90) have been deformed in both uniaxial creep and load relaxation under conditions of ambient pressure, T = 1500°C and pO2 = 10-10 atm; creep stresses were in the range 40 ≤ σ1 (MPa) ≤ 220. The crystals were oriented such that the applied stress was parallel to [011]c, which promotes single slip on the slowest slip system in olivine, (010)[001]. The creep rates at steady state match well the results of earlier investigators, as does the stress sensitivity (a power law exponent of n = 3.6). Dislocation microstructures, including spatial distribution of low-angle (subgrain) boundaries, additionally confirm previous investigations. Inverted primary creep (an accelerating strain rate with an increase in stress) was observed. Load relaxation, however, produced a singular response—a single hardness curve—regardless of the magnitude of creep stress or total accumulated strain preceding relaxation. The log stress versus log strain rate data from load-relaxation and creep experiments overlap to within experimental error. The load-relaxation behavior is distinctly different than that described for other crystalline solids, where the flow stress is affected strongly by work hardening such that a family of distinct hardness curves is generated, which are related by a scaling function. The response of olivine for the conditions studied, we argue, indicates flow that is rate limited by dislocation glide, reflecting specifically a high intrinsic lattice resistance (Peierls stress).

Load Relaxation of Olivine Single Crystals

NASA Astrophysics Data System (ADS)

Cooper, R. F.; Stone, D. S.; Plookphol, T.

2016-12-01

Single crystals of ferromagnesian olivine (San Carlos, AZ, peridot; Fo90-92) have been deformed in both uniaxial creep and load relaxation under conditions of ambient pressure, T = 1500ºC and pO2 = 10-10 atm; creep stresses were in the range 40 ≤ σ1 (MPa) ≤ 220. The crystals were oriented such that the applied stress was parallel to [011]c, which promotes single slip on the slowest slip system in olivine, (010)[001]. The creep rates at steady state match well the results of earlier investigators, as does the stress sensitivity (a power-law exponent of n = 3.6). Dislocation microstructures, including spatial distribution of low-angle (subgrain) boundaries, additionally confirm previous investigations. Inverted primary creep (an accelerating strain rate with an increase in stress) was observed. Load-relaxation, however, produced a singular response—a single hardness curve—regardless of the magnitude of creep stress or total accumulated strain preceding relaxation. The log-stress v. log-strain rate data from load-relaxation and creep experiments overlap to within experimental error. The load-relaxation behavior is distinctly different that that described for other crystalline solids, where the flow stress is affected strongly by work hardening such that a family of distinct hardness curves is generated, which are related by a scaling function. The response of olivine for the conditions studied, thus, indicates flow that is rate-limited by dislocation glide, reflecting specifically a high intrinsic lattice resistance (Peierls stress).

Heterogeneous Rupture in the Great Cascadia Earthquake of 1700 Inferred from Coastal Subsidence Estimates

NASA Astrophysics Data System (ADS)

Wang, P.; Wang, K.; Hawkes, A.; Horton, B. P.; Engelhart, S. E.; Nelson, A. R.; Witter, R. C.

2011-12-01

Abrupt coastal subsidence induced by the great AD 1700 Cascadia earthquake has been estimated from paleoseismic evidence of buried soils and overlying mud and associated tsunamis deposits. These records have been modeled using a rather uniform rupture model, a mirror image of the uniform interseismic fault locking based on modern GPS observations. However, as seen in other megathrust earthquakes such as at Sumatra, Chile, and Alaska, the rupture must have had multiple patches of concentrated slip. Variable moment release is also seen in the 2011 Tohoku-Oki earthquake in Japan, although there is only one patch. The use of a uniform rupture scenario for Cascadia is due mainly to the poor resolving power of the previous paleoseismic data. In this work, we invoke recently obtained more precise data from detailed microfossil studies to better constrain the slip distribution. Our 3-D elastic dislocation model allows the fault slip to vary along strike. Along any profile in the dip direction, we assume a bell-shaped slip distribution with the peak value scaling with local rupture width, consistent with rupture mechanics. We found that the coseismic slip is large in central Cascadia, and areas of high moment release are separated by areas of low moment release. The amount of slip in northern and southern Cascadia is poorly constrained. Although data uncertainties are large, the coastal variable subsidence can be explained with multiple slip patches. For example, there is an area near Alsea Bay, Oregon (about 44.5°N) that, in accordance with the minimum coseismic subsidence estimated by the microfossil data, had very little slip in the 1700 event. This area approximately coincides with a segment boundary previously defined on the basis of gravity anomalies. There is also reported evidence for the presence of a subducting seamount in this area, and the seamount might be responsible for impeding rupture during large earthquakes. The nature of this rupture barrier and whether it is a persistent feature are important topics of future research. Our results indicate that there is not always a one-to-one correlation between areas of more complete interseismic locking and larger coseismic slip.

Finite element model predictions of static deformation from dislocation sources in a subduction zone: Sensitivities to homogeneous, isotropic, Poisson-solid, and half-space assumptions

USGS Publications Warehouse

Masterlark, Timothy

2003-01-01

Dislocation models can simulate static deformation caused by slip along a fault. These models usually take the form of a dislocation embedded in a homogeneous, isotropic, Poisson-solid half-space (HIPSHS). However, the widely accepted HIPSHS assumptions poorly approximate subduction zone systems of converging oceanic and continental crust. This study uses three-dimensional finite element models (FEMs) that allow for any combination (including none) of the HIPSHS assumptions to compute synthetic Green's functions for displacement. Using the 1995 Mw = 8.0 Jalisco-Colima, Mexico, subduction zone earthquake and associated measurements from a nearby GPS array as an example, FEM-generated synthetic Green's functions are combined with standard linear inverse methods to estimate dislocation distributions along the subduction interface. Loading a forward HIPSHS model with dislocation distributions, estimated from FEMs that sequentially relax the HIPSHS assumptions, yields the sensitivity of predicted displacements to each of the HIPSHS assumptions. For the subduction zone models tested and the specific field situation considered, sensitivities to the individual Poisson-solid, isotropy, and homogeneity assumptions can be substantially greater than GPS. measurement uncertainties. Forward modeling quantifies stress coupling between the Mw = 8.0 earthquake and a nearby Mw = 6.3 earthquake that occurred 63 days later. Coulomb stress changes predicted from static HIPSHS models cannot account for the 63-day lag time between events. Alternatively, an FEM that includes a poroelastic oceanic crust, which allows for postseismic pore fluid pressure recovery, can account for the lag time. The pore fluid pressure recovery rate puts an upper limit of 10-17 m2 on the bulk permeability of the oceanic crust. Copyright 2003 by the American Geophysical Union.

Mechanical properties and crack growth behavior of polycrystalline copper using molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Qiu, Ren-Zheng; Li, Chi-Chen; Fang, Te-Hua

2017-08-01

This study investigated the mechanical properties and crack propagation behavior of polycrystalline copper using a molecular dynamics simulation. The effects of temperature, grain size, and crack length were evaluated in terms of atomic trajectories, slip vectors, common neighbor analysis, the material’s stress-strain diagram and Young’s modulus. The simulation results show that the grain boundary of the material is more easily damaged at high temperatures and that grain boundaries will combine at the crack tip. From the stress-strain diagram, it was observed that the maximum stress increased as the temperature decreased. In contrast, the maximum stress was reduced by increasing the temperature. With regard to the effect of the grain size, when the grain size was too small, the structure of the sample deformed due to the effect of atomic interactions, which caused the grain boundary structure to be disordered in general. However, when the grain size was larger, dislocations appeared and began to move from the tip of the crack, which led to a new dislocation phenomenon. With regards to the effect of the crack length, the tip of the crack did not affect the sample’s material when the crack length was less than 5 nm. However, when the crack length was above 7.5 nm, the grain boundary was damaged, and twinning structures and dislocations appeared on both sides of the crack tip. This is because the tip of the crack was blunt at first before sharpening due to the dislocation effect.

Deformation localization and dislocation channel dynamics in neutron-irradiated austenitic stainless steels

DOE PAGES

Gussev, Maxim N.; Field, Kevin G.; Busby, Jeremy T.

2015-02-24

We investigated dynamics of deformation localization and dislocation channel formation in situ in a neutron irradiated AISI 304 austenitic stainless steel and a model 304-based austenitic alloy by combining several analytical techniques including optic microscopy and laser confocal microscopy, scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. Channel formation was observed at 70% of the formal tensile yield stress for both alloys. It was shown that triple junction points do not always serve as a source of dislocation channels; at stress levels below the yield stress, channels often formed near the middle of the grain boundary. For amore » single grain, the role of elastic stiffness value (Young modulus) in the channel formation was analyzed; it was shown that in the irradiated 304 steels the initial channels appeared in soft grains with a high Schmid factor located near stiff grains with high elastic stiffness. Moreover, the spatial organization of channels in a single grain was analyzed; it was shown that secondary channels operating in the same slip plane as primary channels often appeared at the middle or at one third of the way between primary channels. The twinning nature of dislocation channels was analyzed for grains of different orientation using TEM. Finally, it was shown that in the AISI 304 steel, channels were twin-free in grains oriented close to [001] and [101] of standard unit triangle; [111]-grains and grains oriented close to Schmid factor maximum contained deformation twins.« less

Mechanisms of fine extinction band development in vein quartz: new insights from correlative light and electron microscopy

NASA Astrophysics Data System (ADS)

Derez, Tine; Van Der Donck, Tom; Plümper, Oliver; Muchez, Philippe; Pennock, Gill; Drury, Martyn R.; Sintubin, Manuel

2017-07-01

Fine extinction bands (FEBs) (also known as deformation lamellae) visible with polarized light microscopy in quartz consist of a range of nanostructures, inferring different formation processes. Previous transmission electron microscopy studies have shown that most FEB nanostructures in naturally deformed quartz are elongated subgrains formed by recovery of dislocation slip bands. Here we show that three types of FEB nanostructure occur in naturally deformed vein quartz from the low-grade metamorphic High-Ardenne slate belt (Belgium). Prismatic oriented FEBs are defined by bands of dislocation walls. Dauphiné twin boundaries present along the FEB boundaries probably formed after FEB formation. In an example of two sub-rhombohedral oriented FEBs, developed as two sets in one grain, the finer FEB set consists of elongated subgrains, similar to FEBs described in previous transmission electron microscopy studies. The second wider FEB set consists of bands with different dislocation density and fluid-inclusion content. The wider FEB set is interpreted as bands with different plastic strain associated with the primary growth banding of the vein quartz grain. The nanometre-scale fluid inclusions are interpreted to have formed from structurally bounded hydroxyl groups that moreover facilitated formation of the elongate subgrains. Larger fluid inclusions aligned along FEBs are explained by fluid-inclusion redistribution along dislocation cores. The prismatic FEB nanostructure and the relation between FEBs and growth bands have not been recognized before, although related structures have been reported in experimentally deformed quartz.

Dual-phase steel sheets under cyclic tension-compression to large strains: Experiments and crystal plasticity modeling

NASA Astrophysics Data System (ADS)

Zecevic, Milovan; Korkolis, Yannis P.; Kuwabara, Toshihiko; Knezevic, Marko

2016-11-01

In this work, we develop a physically-based crystal plasticity model for the prediction of cyclic tension-compression deformation of multi-phase materials, specifically dual-phase (DP) steels. The model is elasto-plastic in nature and integrates a hardening law based on statistically stored dislocation density, localized hardening due to geometrically necessary dislocations (GNDs), slip-system-level kinematic backstresses, and annihilation of dislocations. The model further features a two level homogenization scheme where the first level is the overall response of a two-phase polycrystalline aggregate and the second level is the homogenized response of the martensite polycrystalline regions. The model is applied to simulate a cyclic tension-compression-tension deformation behavior of DP590 steel sheets. From experiments, we observe that the material exhibits a typical decreasing hardening rate during forward loading, followed by a linear and then a non-linear unloading upon the load reversal, the Bauschinger effect, and changes in hardening rate during strain reversals. To predict these effects, we identify the model parameters using a portion of the measured data and validate and verify them using the remaining data. The developed model is capable of predicting all the particular features of the cyclic deformation of DP590 steel, with great accuracy. From the predictions, we infer and discuss the effects of GNDs, the backstresses, dislocation annihilation, and the two-level homogenization scheme on capturing the cyclic deformation behavior of the material.

Structure and lithology of the Japan Trench subduction plate boundary fault

NASA Astrophysics Data System (ADS)

Kirkpatrick, James D.; Rowe, Christie D.; Ujiie, Kohtaro; Moore, J. Casey; Regalla, Christine; Remitti, Francesca; Toy, Virginia; Wolfson-Schwehr, Monica; Kameda, Jun; Bose, Santanu; Chester, Frederick M.

2015-01-01

The 2011 Mw9.0 Tohoku-oki earthquake ruptured to the trench with maximum coseismic slip located on the shallow portion of the plate boundary fault. To investigate the conditions and physical processes that promoted slip to the trench, Integrated Ocean Drilling Program Expedition 343/343T sailed 1 year after the earthquake and drilled into the plate boundary ˜7 km landward of the trench, in the region of maximum slip. Core analyses show that the plate boundary décollement is localized onto an interval of smectite-rich, pelagic clay. Subsidiary structures are present in both the upper and lower plates, which define a fault zone ˜5-15m thick. Fault rocks recovered from within the clay-rich interval contain a pervasive scaly fabric defined by anastomosing, polished, and lineated surfaces with two predominant orientations. The scaly fabric is crosscut in several places by discrete contacts across which the scaly fabric is truncated and rotated, or different rocks are juxtaposed. These contacts are inferred to be faults. The plate boundary décollement therefore contains structures resulting from both distributed and localized deformation. We infer that the formation of both of these types of structures is controlled by the frictional properties of the clay: the distributed scaly fabric formed at low strain rates associated with velocity-strengthening frictional behavior, and the localized faults formed at high strain rates characterized by velocity-weakening behavior. The presence of multiple discrete faults resulting from seismic slip within the décollement suggests that rupture to the trench may be characteristic of this margin.

The role of bed-parallel slip in the development of complex normal fault zones

NASA Astrophysics Data System (ADS)

Delogkos, Efstratios; Childs, Conrad; Manzocchi, Tom; Walsh, John J.; Pavlides, Spyros

2017-04-01

Normal faults exposed in Kardia lignite mine, Ptolemais Basin, NW Greece formed at the same time as bed-parallel slip-surfaces, so that while the normal faults grew they were intermittently offset by bed-parallel slip. Following offset by a bed-parallel slip-surface, further fault growth is accommodated by reactivation on one or both of the offset fault segments. Where one fault is reactivated the site of bed-parallel slip is a bypassed asperity. Where both faults are reactivated, they propagate past each other to form a volume between overlapping fault segments that displays many of the characteristics of relay zones, including elevated strains and transfer of displacement between segments. Unlike conventional relay zones, however, these structures contain either a repeated or a missing section of stratigraphy which has a thickness equal to the throw of the fault at the time of the bed-parallel slip event, and the displacement profiles along the relay-bounding fault segments have discrete steps at their intersections with bed-parallel slip-surfaces. With further increase in displacement, the overlapping fault segments connect to form a fault-bound lens. Conventional relay zones form during initial fault propagation, but with coeval bed-parallel slip, relay-like structures can form later in the growth of a fault. Geometrical restoration of cross-sections through selected faults shows that repeated bed-parallel slip events during fault growth can lead to complex internal fault zone structure that masks its origin. Bed-parallel slip, in this case, is attributed to flexural-slip arising from hanging-wall rollover associated with a basin-bounding fault outside the study area.

Imbricated slip rate processes during slow slip transients imaged by low-frequency earthquakes

NASA Astrophysics Data System (ADS)

Lengliné, O.; Frank, W.; Marsan, D.; Ampuero, J. P.

2017-12-01

Low Frequency Earthquakes (LFEs) often occur in conjunction with transient strain episodes, or Slow Slip Events (SSEs), in subduction zones. Their focal mechanism and location consistent with shear failure on the plate interface argue for a model where LFEs are discrete dynamic ruptures in an otherwise slowly slipping interface. SSEs are mostly observed by surface geodetic instruments with limited resolution and it is likely that only the largest ones are detected. The time synchronization of LFEs and SSEs suggests that we could use the recorded LFEs to constrain the evolution of SSEs, and notably of the geodetically-undetected small ones. However, inferring slow slip rate from the temporal evolution of LFE activity is complicated by the strong temporal clustering of LFEs. Here we apply dedicated statistical tools to retrieve the temporal evolution of SSE slip rates from the time history of LFE occurrences in two subduction zones, Mexico and Cascadia, and in the deep portion of the San Andreas fault at Parkfield. We find temporal characteristics of LFEs that are similar across these three different regions. The longer term episodic slip transients present in these datasets show a slip rate decay with time after the passage of the SSE front possibly as t-1/4. They are composed of multiple short term transients with steeper slip rate decay as t-α with α between 1.4 and 2. We also find that the maximum slip rate of SSEs has a continuous distribution. Our results indicate that creeping faults host intermittent deformation at various scales resulting from the imbricated occurrence of numerous slow slip events of various amplitudes.

Imbricated slip rate processes during slow slip transients imaged by low-frequency earthquakes

NASA Astrophysics Data System (ADS)

Lengliné, O.; Frank, W. B.; Marsan, D.; Ampuero, J.-P.

2017-10-01

Low Frequency Earthquakes (LFEs) often occur in conjunction with transient strain episodes, or Slow Slip Events (SSEs), in subduction zones. Their focal mechanism and location consistent with shear failure on the plate interface argue for a model where LFEs are discrete dynamic ruptures in an otherwise slowly slipping interface. SSEs are mostly observed by surface geodetic instruments with limited resolution and it is likely that only the largest ones are detected. The time synchronization of LFEs and SSEs suggests that we could use the recorded LFEs to constrain the evolution of SSEs, and notably of the geodetically-undetected small ones. However, inferring slow slip rate from the temporal evolution of LFE activity is complicated by the strong temporal clustering of LFEs. Here we apply dedicated statistical tools to retrieve the temporal evolution of SSE slip rates from the time history of LFE occurrences in two subduction zones, Mexico and Cascadia, and in the deep portion of the San Andreas fault at Parkfield. We find temporal characteristics of LFEs that are similar across these three different regions. The longer term episodic slip transients present in these datasets show a slip rate decay with time after the passage of the SSE front possibly as t - 1 / 4. They are composed of multiple short term transients with steeper slip rate decay as t-α with α between 1.4 and 2. We also find that the maximum slip rate of SSEs has a continuous distribution. Our results indicate that creeping faults host intermittent deformation at various scales resulting from the imbricated occurrence of numerous slow slip events of various amplitudes.

DYNAMIC PLANE-STRAIN SHEAR RUPTURE WITH A SLIP-WEAKENING FRICTION LAW CALCULATED BY A BOUNDARY INTEGRAL METHOD.

USGS Publications Warehouse

Andrews, D.J.

1985-01-01

A numerical boundary integral method, relating slip and traction on a plane in an elastic medium by convolution with a discretized Green function, can be linked to a slip-dependent friction law on the fault plane. Such a method is developed here in two-dimensional plane-strain geometry. Spontaneous plane-strain shear ruptures can make a transition from sub-Rayleigh to near-P propagation velocity. Results from the boundary integral method agree with earlier results from a finite difference method on the location of this transition in parameter space. The methods differ in their prediction of rupture velocity following the transition. The trailing edge of the cohesive zone propagates at the P-wave velocity after the transition in the boundary integral calculations. Refs.

A Self-Consistent Fault Slip Model for the 2011 Tohoku Earthquake and Tsunami

NASA Astrophysics Data System (ADS)

Yamazaki, Yoshiki; Cheung, Kwok Fai; Lay, Thorne

2018-02-01

The unprecedented geophysical and hydrographic data sets from the 2011 Tohoku earthquake and tsunami have facilitated numerous modeling and inversion analyses for a wide range of dislocation models. Significant uncertainties remain in the slip distribution as well as the possible contribution of tsunami excitation from submarine slumping or anelastic wedge deformation. We seek a self-consistent model for the primary teleseismic and tsunami observations through an iterative approach that begins with downsampling of a finite fault model inverted from global seismic records. Direct adjustment of the fault displacement guided by high-resolution forward modeling of near-field tsunami waveform and runup measurements improves the features that are not satisfactorily accounted for by the seismic wave inversion. The results show acute sensitivity of the runup to impulsive tsunami waves generated by near-trench slip. The adjusted finite fault model is able to reproduce the DART records across the Pacific Ocean in forward modeling of the far-field tsunami as well as the global seismic records through a finer-scale subfault moment- and rake-constrained inversion, thereby validating its ability to account for the tsunami and teleseismic observations without requiring an exotic source. The upsampled final model gives reasonably good fits to onshore and offshore geodetic observations albeit early after-slip effects and wedge faulting that cannot be reliably accounted for. The large predicted slip of over 20 m at shallow depth extending northward to 39.7°N indicates extensive rerupture and reduced seismic hazard of the 1896 tsunami earthquake zone, as inferred to varying extents by several recent joint and tsunami-only inversions.

Sensitivities of Near-field Tsunami Forecasts to Megathrust Deformation Predictions

NASA Astrophysics Data System (ADS)

Tung, S.; Masterlark, T.

2018-02-01

This study reveals how modeling configurations of forward and inverse analyses of coseismic deformation data influence the estimations of seismic and tsunami sources. We illuminate how the predictions of near-field tsunami change when (1) a heterogeneous (HET) distribution of crustal material is introduced to the elastic dislocation model, and (2) the near-trench rupture is either encouraged or suppressed to invert spontaneous coseismic displacements. Hypothetical scenarios of megathrust earthquakes are studied with synthetic Global Positioning System displacements in Cascadia. Finite-element models are designed to mimic the subsurface heterogeneity across the curved subduction margin. The HET lithospheric domain modifies the seafloor displacement field and alters tsunami predictions from those of a homogeneous (HOM) crust. Uncertainties persist as the inverse analyses of geodetic data produce nonrealistic slip artifacts over the HOM domain, which propagates into the prediction errors of subsequent tsunami arrival and amplitudes. A stochastic analysis further shows that the uncertainties of seismic tomography models do not degrade the solution accuracy of HET over HOM. Whether the source ruptures near the trench also controls the details of the seafloor disturbance. Deeper subsurface slips induce more seafloor uplift near the coast and cause an earlier arrival of tsunami waves than surface-slipping events. We suggest using the solutions of zero-updip-slip and zero-updip-slip-gradient rupture boundary conditions as end-members to constrain the tsunami behavior for forecasting purposes. The findings are important for the near-field tsunami warning that primarily relies on the near-real-time geodetic or seismic data for source calibration before megawaves hit the nearest shore upon tsunamigenic events.

In situ measurements of a homogeneous to heterogeneous transition in the plastic response of ion-irradiated <111> Ni microspecimens

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhao, Xinyu; Strickland, Daniel J.; Derlet, Peter M.

We report on the use of quantitative in situ microcompression experiments in a scanning electron microscope to systematically investigate the effect of self-ion irradiation damage on the full plastic response of <111> Ni. In addition to the well-known irradiationinduced increases in the yield and flow strengths with increasing dose, we measure substantial changes in plastic flow intermittency behavior, manifested as stress drops accompanying energy releases as the driven material transits critical states. At low irradiation doses, the magnitude of stress drops reduces relative to the unirradiated material and plastic slip proceeds on multiple slip systems, leading to quasi-homogeneous plastic flow.more » In contrast, highly irradiated specimens exhibit pronounced shear localization on parallel slip planes, which we ascribe to the onset of defect free channels normally seen in bulk irradiated materials. Our in situ testing system and approach allows for a quantitative study of the energy release and dynamics associated with defect free channel formation and subsequent localization. As a result, this study provides fundamental insight to the nature of interactions between mobile dislocations and irradiation-mediated and damage-dependent defect structures.« less

Microstructure, Texture, and Mechanical Behavior of As-cast Ni-Fe-W Matrix Alloy

NASA Astrophysics Data System (ADS)

Rao, A. Sambasiva; Manda, Premkumar; Mohan, M. K.; Nandy, T. K.; Singh, A. K.

2018-04-01

This article describes the tensile properties, flow, and work-hardening behavior of an experimental alloy 53Ni-29Fe-18W in as-cast condition. The microstructure of the alloy 53Ni-29Fe-18W displays single phase (fcc) in as-cast condition along with typical dendritic features. The bulk texture of the as-cast alloy reveals the triclinic sample symmetry and characteristic nature of coarse-grained materials. The alloy exhibits maximum strength ( σ YS and σ UTS) values along the transverse direction. The elongation values are maximum and minimum along the transverse and longitudinal directions, respectively. Tensile fracture surfaces of both the longitudinal and transverse samples display complete ductile fracture features. Two types of slip lines, namely, planar and intersecting, are observed in deformed specimens and the density of slip lines increases with increasing the amount of deformation. The alloy displays moderate in-plane anisotropy ( A IP) and reasonably low anisotropic index ( δ) values, respectively. The instantaneous or work-hardening rate curves portray three typical stages (I through III) along both the longitudinal and transverse directions. The alloy exhibits dislocation-controlled strain hardening during tensile testing, and slip is the predominant deformation mechanism.

On rate-dependent polycrystal deformation: the temperature sensitivity of cold dwell fatigue

PubMed Central

Zhang, Zhen; Cuddihy, M. A.; Dunne, F. P. E.

2015-01-01

A temperature and rate-dependent crystal plasticity framework has been used to examine the temperature sensitivity of stress relaxation, creep and load shedding in model Ti-6Al polycrystal behaviour under dwell fatigue conditions. A temperature close to 120°C is found to lead to the strongest stress redistribution and load shedding, resulting from the coupling between crystallographic slip rate and slip system dislocation hardening. For temperatures in excess of about 230°C, grain-level load shedding from soft to hard grains diminishes because of the more rapid stress relaxation, leading ultimately to the diminution of the load shedding and hence, it is argued, the elimination of the dwell debit. Under conditions of cyclic stress dwell, at temperatures between 20°C and 230°C for which load shedding occurs, the rate-dependent accumulation of local slip by ratcheting is shown to lead to the progressive cycle-by-cycle redistribution of stress from soft to hard grains. This phenomenon is termed cyclic load shedding since it also depends on the material's creep response, but develops over and above the well-known dwell load shedding, thus providing an additional rationale for the incubation of facet nucleation. PMID:26528078

In situ measurements of a homogeneous to heterogeneous transition in the plastic response of ion-irradiated <111> Ni microspecimens

DOE PAGES

Zhao, Xinyu; Strickland, Daniel J.; Derlet, Peter M.; ...

2015-02-11

We report on the use of quantitative in situ microcompression experiments in a scanning electron microscope to systematically investigate the effect of self-ion irradiation damage on the full plastic response of <111> Ni. In addition to the well-known irradiationinduced increases in the yield and flow strengths with increasing dose, we measure substantial changes in plastic flow intermittency behavior, manifested as stress drops accompanying energy releases as the driven material transits critical states. At low irradiation doses, the magnitude of stress drops reduces relative to the unirradiated material and plastic slip proceeds on multiple slip systems, leading to quasi-homogeneous plastic flow.more » In contrast, highly irradiated specimens exhibit pronounced shear localization on parallel slip planes, which we ascribe to the onset of defect free channels normally seen in bulk irradiated materials. Our in situ testing system and approach allows for a quantitative study of the energy release and dynamics associated with defect free channel formation and subsequent localization. As a result, this study provides fundamental insight to the nature of interactions between mobile dislocations and irradiation-mediated and damage-dependent defect structures.« less

Predictions of the shear response of (Mg,Fe)SiO3 post-perovskite

NASA Astrophysics Data System (ADS)

Metsue, A.; Tsuchiya, T.

2011-12-01

Observation of seismic data put in forth evidence of a spatial anisotropy in the seismic wave velocities in the D'' layer, the lowermost part of the mantle. (Mg,Fe)SiO3 post-perovskite (PPv) is thought to be the most abundant phase in this part of the mantle, and this mineral exhibits a strong elastic anisotropy and may contribute significantly to the seismic anisotropy in the D'' layer. However, the seismic anisotropy cannot be expressed at the rock scale if the orientations of the grains are distributed randomly. Consequently, the formation of lattice preferred orientations with an anisotropic mechanism of plasticity, such as dislocation creep, can cause the seismic anisotropy in the D'' layer. Some experiments have been done on the plasticity of pure and Fe-bearing MgSiO3 post-perovskite and lead to textures of deformation dominated by the (100) and (110) slip planes (Merkel et al., 2007) and by the (001) slip plane (Miyagi et al., 2010). On the other hand, theoretical calculations on the dislocations mobility on pure MgSiO3 (Carrez et al., 2007; Metsue et al., 2009) suggested a texture dominated by the (010) slip plane. A first step to understanding the mechanisms of plasticity and, therefore, the shear wave splitting occurring in the deep Earth is to test the response of the PPv phase to a plastic shear in a geophysical relevant composition. In this study, we present new results from first-principles calculations on the shear response of pure and ferrous iron-bearing MgSiO3 PPv. The originality of this work is the use of internally consistent LSDA+U formalism to accurately describe the local interactions between the d-states of iron. About 8% of iron is incorporated in the high spin state as a Mg substitution defect, since several studies suggest that iron is in the high spin in the D'' layer pressure range (Stackhouse et al., 2006; Metsue and Tsuchiya, 2011). We also performed the calculations for incorporated iron in the low spin state if an eventual spin transition of Fe occurs. The response of the PPv to a plastic shear is investigated at 120 GPa through the calculations of the Generalized Stacking Faults (GSF) energy in pure and iron-bearing systems for ten potential {hkl} slip systems, since these latter are not well constrained for the PPv phase. The GSF energies are obtained by shearing homogeneously half of an infinite crystal over the other half for every slip plane and give the value of the ideal shear stress (ISS), which can be defined as the theoretical elastic limit of the crystal. The [100](001) slip system in pure and iron-bearing phases exhibits the lowest ISS and may play an important role in the plastic deformation of the PPv phase. The activation of this slip system is compatible with the observed shear wave splitting VSH>VSV. We show that incorporation of iron decreases the GSF energy and the ISS of all slip systems. We discuss the plastic anisotropy of pure and iron-bearing phases from the values of the ISS and the orientation of applied tensile stress. Our results suggest that the incorporation of ferrous iron in the PPv phase has a limited effect on its plastic anisotropy.

Deformation bands, the LEDS theory, and their importance in texture development: Part II. Theoretical conclusions

NASA Astrophysics Data System (ADS)

Kuhlmann-Wilsdorf, D.

1999-09-01

The facts regarding “regular” deformation bands (DBs) outlined in Part I of this series of articles are related to the low-energy dislocation structure (LEDS) theory of dislocation-based plasticity. They prompt an expansion of the theory by including the stresses due to strain gradients on account of changing selections of slip systems to the previously known dislocation driving forces. This last and until now neglected driving force is much smaller than the components considered hitherto, principally due to the applied stress and to mutual stress-screening among neighbor dislocations. As a result, it permits a near-proof of the LEDS hypothesis, to wit that among all structures which, in principle, are accessible to the dislocations, that one is realized which has the lowest free energy. Specifically, the temperature rises that would result from annihilating the largest DBs amount to only several millidegrees Centigrade, meaning that they, and by implication the entire dislocation structures, are close to thermodynamical equilibrium. This is in stark contrast to the assumption of the presently widespread self-organizing dislocation structures (SODS) modeling that plastic deformation occurs far from equilibrium and is subject to Prigogine’s thermodynamics of energy-flow-through systems. It also holds out promise for future rapid advances in the construction of constitutive equations, since the LEDS hypothesis is the principal basis of the LEDS theory of plastic deformation and follows directly from the second law of thermodynamics in conjunction with Newton’s third law. By contrast, all other known models of metal plasticity are in conflict with the LEDS hypothesis. In regard to texture modeling, the present analysis shows that Taylor’s criterion of minimum plastic work is incorrect and should be replaced by the criterion of minimum free energy in the stressed state. Last, the LEDS hypothesis is but a special case of the more general low-energy structure (LES) hypothesis, applying to plastic deformation independent of the deformation mechanism. It is thus seen that plastic deformation is one of nature’s means to generate order, as a byproduct of the entropy generation when mechanical work is largely converted into heat.

The effect of hydrogen on the deformation behavior of a single crystal nickel-base superalloy

NASA Technical Reports Server (NTRS)

Walston, W. S.; Thompson, A. W.; Bernstein, I. M.

1989-01-01

The effect of hydrogen on the tensile deformation behavior of PWA 1480 is presented. Tensile tests were interrupted at different plastic strain levels to observe the development of the dislocation structure. Transmission electron microscopy (TEM) foils were cut perpendicular to the tensile axis to allow the deformation of both phases to be simultaneously observed as well as parallel to zone axes (III) to show the superdislocations on their slip planes. Similar to other nickel-base superalloys, hydrogen was detrimental to the room temperature tensile properties of PWA 1480. There was little effect on strength, however the material was severely embrittled. Even without hydrogen, the elongation-to-failure was only approximately 3 percent. The tensile fracture surface was made up primarily of ductile voids with regions of cleavage fracture. These cleavage facets are the eutectic (gamma') in the microstructure. It was shown by quantitative fractography that hydrogen embrittles the eutectic (gamma') and causes the crack path to seek out and fracture through the eutectic (gamma'). There was two to three times the amount of cleavage on the fracture surface of the hydrogen-charged samples than on the surface of the uncharged samples. The effect of hydrogen can also be seen in the dislocation structure. There is a marked tendency for dislocation trapping in the gamma matrix with and without hydrogen at all plastic strain levels. Without hydrogen there is a high dislocation density in the gamma matrix leading to strain exhaustion in this region and failure through the matrix. The dislocation structure at failure with hydrogen is slightly different. The TEM foils cut parallel to zone axes (III) showed dislocations wrapping around gamma precipitates. Zone axes (001) foils show that there is a lower dislocation density in the gamma matrix which can be linked to the effects of hydrogen on the fracture behavior. The primary activity in the gamma precipitates is in the form of superlattice intrinsic stacking faults (SISFs). These faults have also been reported in other ordered alloys and superalloys.

Interaction of Two Slip Planes on Extrusion Growth in Fatigue Band

DTIC Science & Technology

1987-01-01

observed under microscope in fatigue specimens as indicated by Essmann et al . [23] and Mughrabi [24]. t I 1 It I I 3 @ O.OS^ = 0 1S^ lAlONG SUP... Mughrabi , et. al . [25] have suggested a model of dislocation dipoles in a single crystal as shown in Fig. 2. The initial inelastic strain e^„ due to the...interesting question which was raised by Mughrabi , and Essmann et. al . [23] was, after the extrusion has reached the amount of static extrusion , will

Limitation of Current Hardening Models in Predicting Anisotropy by Twinning in HCP Metals: Application to a Rod-Textured AM30 Magnesium Alloy

DTIC Science & Technology

2011-03-01

model and a phenomenological Voce hard- ening model. The HCP material is exemplified by an extruded AM30 magnesium alloy with a 〈101̄0〉-fiber...effect accounted for by a sort of slip-twin latent hardening in the Voce type hardening model was not able to inflect the simulated curves with loading... Voce model is unable to cap- ture this effect, but the dislocation model [2] is. A pragmatic factor distinctly increasing the stored dis- locations in

Viscous Flow Causes Weakening in Calcite Nanogouges Sheared at Seismic Velocity

NASA Astrophysics Data System (ADS)

Pozzi, G.; De Paola, N.; Nielsen, S. B.; Holdsworth, R.

2016-12-01

Recent experimental studies have suggested that the activation of diffusion creep at high temperatures (T ≥ 800 °C) and strain rates in nanograin aggregates can weaken faults and facilitate earthquake propagation. However, the frictional properties of nanoscale aggregates at high strain rates and T are still poorly investigated and, in particular, their flow laws at these extreme conditions are poorly constrained due to lack of knowledge about the evolution of grain size and strain localization during seismic slip. Experiments performed in a rotary shear apparatus on micro- and nano-metric calcite gouges (d=63-90 µm and d 200nm, respectively) at seismic (up to 1.4 m/s) and subseismic (<10 cm/s) velocities, arrested at different amounts of slip, show that: (i) onset of dynamic weakening in the nanogouge is faster, with a significantly reduced initial phase of slip hardening, (ii) dynamic weakening of the nanogouge is achieved at velocities and temperatures as low as 1.4 cm/s and <300°C, respectively, compared to >10 cm/s and >500°C in the microgouge, (iii) shear strength shows a rate-dependent weakening. Microstructural analysis of samples shows a three stage evolution: (i) cataclastic comminution and development of Riedel shear bands during the pre-weakening slip-hardening stage, (ii) interconnection of Riedel shears to form a continuous horizontal, localised shear band at the onset of weakening and (iii) evolution of the latter into a thin discrete shear zone with thickness <200 µm composed by a low-porosity aggregate of equigranular recrystallized crystals displaying triple junctions, at the attainment of steady-state weakening stage. Microstructures up to stage (i) are achieved in samples that did not undergo weakening. Despite both gouges show the same microstructural evolution, the initial grainsize of nanoparticles allows a more efficient localisation as the development of a discrete slip zone requires smaller amounts of slip. Our experimental results and microstructural observations shed some light on the critical role that extreme comminution and localisation play on the onset of weakening dominated by viscous flow at high strain rate in carbonate gouges.

A Comparison of Slow Slip Events at Etna and Kilauea Volcanoes

NASA Astrophysics Data System (ADS)

Mattia, M.; Montgomery-Brown, E. K.; Bruno, V.; Scandura, D.

2016-12-01

Mt. Etna and Kilauea Volcano are both large basaltic volcanoes with unstable flanks, on which slow slip events have been observed by continuous GPS networks. The slow slip events (SSEs) last about two days at both volcanoes, although there are some differences in the depths and frequencies. While recurrence intervals were initially somewhat irregular at Kilauea, the most recent 5 events have become more regular with an inter-event time of about 2.4 years. At Mt. Etna, these events seem to be more frequent (about 2 per year) and are often related to the main recharge phases of the volcano. Ground deformation data have been used on both volcanoes for determining the source of the anomalous displacements and, from this point of view, the two volcanoes seem very different. Although slow slip events at Mt. Etna and Kilauea are much shallower than many subduction zone slow slip events, slip at Kilauea occurs on a discrete decollement at about 8 km deep. At Mt. Etna, a variety of data suggest that the sliding could be much shallower and more diffuse. In this work, we show some preliminary results of a "block-like" model of Mt. Etna's slow slip events that is able to explain the source of the flank displacements with slip on the Giarre Wedge near the coast. This work will allow a possible classification of different types of slip events affecting the flanks of large basaltic volcanoes, often densely populated, with a significant impact on the evaluation of seismic and volcanic hazard.

Acoustically induced slip in sheared granular layers: Application to dynamic earthquake triggering: TRIGGERED SLIP IN SHEARED GRANULAR GOUGE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ferdowsi, Behrooz; Griffa, Michele; Guyer, Robert A.

A fundamental mystery in earthquake physics is “how can an earthquake be triggered by distant seismic sources?” We use discrete element method simulations of a granular layer, during stick slip, that is subject to transient vibrational excitation to gain further insight into the physics of dynamic earthquake triggering. We also observe delayed triggering of slip in the granular gouge, using Coulomb friction law for grains interaction. We find that at a critical vibrational amplitude (strain) there is an abrupt transition from negligible time-advanced slip (clock advance) to full clock advance; i.e., transient vibration and triggered slip are simultaneous. Moreover, themore » critical strain is of order 10 -6, similar to observations in the laboratory and in Earth. The transition is related to frictional weakening of the granular layer due to a dramatic decrease in coordination number and the weakening of the contact force network. Associated with this frictional weakening is a pronounced decrease in the elastic modulus of the layer. The study has important implications for mechanisms of triggered earthquakes and induced seismic events and points out the underlying processes in response of the fault gouge to dynamic transient stresses.« less

Acoustically induced slip in sheared granular layers: Application to dynamic earthquake triggering: TRIGGERED SLIP IN SHEARED GRANULAR GOUGE

DOE PAGES

Ferdowsi, Behrooz; Griffa, Michele; Guyer, Robert A.; ...

2015-11-19

A fundamental mystery in earthquake physics is “how can an earthquake be triggered by distant seismic sources?” We use discrete element method simulations of a granular layer, during stick slip, that is subject to transient vibrational excitation to gain further insight into the physics of dynamic earthquake triggering. We also observe delayed triggering of slip in the granular gouge, using Coulomb friction law for grains interaction. We find that at a critical vibrational amplitude (strain) there is an abrupt transition from negligible time-advanced slip (clock advance) to full clock advance; i.e., transient vibration and triggered slip are simultaneous. Moreover, themore » critical strain is of order 10 -6, similar to observations in the laboratory and in Earth. The transition is related to frictional weakening of the granular layer due to a dramatic decrease in coordination number and the weakening of the contact force network. Associated with this frictional weakening is a pronounced decrease in the elastic modulus of the layer. The study has important implications for mechanisms of triggered earthquakes and induced seismic events and points out the underlying processes in response of the fault gouge to dynamic transient stresses.« less

Deformation Mechanisms of Gum Metals Under Nanoindentation

NASA Astrophysics Data System (ADS)

Sankaran, Rohini Priya

Gum Metal is a set of multi-component beta-Ti alloys designed and developed by Toyota Central R&D Labs in 2003 to have a nearly zero shear modulus in the direction. After significant amounts of cold-work (>90%), these alloys were found to have yield strengths at a significant fraction of the predicted ideal strengths and exhibited very little work hardening. It has been speculated that this mechanical behavior may be realized through an ideal shear mechanism as opposed to conventional plastic deformation mechanisms, such as slip, and that such a mechanism may be realized through a defect structure termed "nanodisturbance". It is furthermore theorized that for near ideal strength to be attained, dislocations need to be pinned at sufficiently high stresses. It is the search for these defects and pinning points that motivates the present study. However, the mechanism of plastic deformation and the true origin of specific defect structures unique to gum metals is still controversial, mainly due to the complexity of the beta-Ti alloy system and the heavily distorted lattice exhibited in cold worked gum metals, rendering interpretation of images difficult. Accordingly, the first aim of this study is to clarify the starting as-received microstructures of gum metal alloys through conventional transmission electron microscopy (TEM) and aberration-corrected high resolution scanning transmission electron microscopy with high-angle annular dark field detector (HAADF-HRSTEM) imaging. To elucidate the effects of beta-stability and starting microstructure on the deformation behavior of gum metals and thus to provide adequate context for potentially novel deformation structures, we investigate three alloy conditions: gum metal that has undergone solution heat treatment (STGM), gum metal that has been heavily cold worked (CWGM), and a solution treated alloy of nominal gum metal composition, but leaner in beta-stabilizing content (ST Ref-1). In order to directly relate observed defect structures to applied loading, we perform ex-situ nanoindentation. Nanoindentation is a convenient method as the plastic deformation is localized and probes a nominally defect free volume of the material. We subsequently characterize the defect structures in these alloys with both conventional TEM and advanced techniques such as HAADF HRSTEM and nanoprobe diffraction. These advanced techniques allow for a more thorough understanding of the observed deformation features. The main findings from this investigation are as follows. As expected we observe that a non-equilibrium phase, o, is present in the leaner beta-stabilized alloy, ST Ref-1. We do not find any direct evidence of secondary phases in STGM, and we find the beta phase in CWGM, along with lath microstructure with subgrain structure consisting of dislocation cell networks. Upon nanoindentation, we find twinning accompanied by beta nucleation on the twin boundary in ST Ref-1 samples. This result is consistent with previous findings and is reasonable considering the alloy is unstable with respect to beta transformation. We find deformation nanotwinning in cold worked gum metals under nanoindentation, which is initially surprising. We argue that when viewed as a nanocrystalline material, such a deformation mechanism is consistent with previous work, and furthermore, a deformation nanotwinned structure does not preclude an ideal shear mechanism from operating in the alloy. Lastly, we observe continuous lattice rotations in STGM under nanoindentation via nanoprobe diffraction. With this technique, for the first time we can demonstrate that the lattice rotations are truly continuous at the nanoscale. We can quantify this lattice rotation, and find that even though the rotation is large, it may be mediated by a reasonable geometrically necessary dislocation density, and note that similar rotations are typically observed in other materials under nanoindentation. HRSTEM and conventional TEM data confirm the presence of dislocations in regions that have sustained large lattice rotations. Finally, we report on the nature of indirectly observed "pinning points" in STGM under nanoindentation that was reported in a previous study. We find through ADF/HAADF STEM that the "pinning points" which cause dislocation bowing in STGM under nanoindentation are actually other dislocations with the line direction normal to the TEM foil, and, in support of this finding, we also observe other in-plane dislocation-dislocation interactions that is responsible for resultant bowing. We observe no direct evidence of any secondary phases, twinning, or nanodisturbances in the STGM case, and the majority of deformation features can be explained by conventional slip mechanism. However, it remains a possibility that an ideal shear mechanism may be accompanying conventional slip in STGMs that may account for the truly continuous nature of the lattice rotations.

Dislocation Onset and Glide in Carbon Nanotubes under Torsion

NASA Astrophysics Data System (ADS)

Dumitrica, Traian; Zhang, Dong-Bo; James, Richard

2009-03-01

The torsional plastic response of carbon nanotubes is comprehensively described in the objective molecular dynamics framework [1-3]. It is shown that an (n,m) tube is prone to slip along a nearly-axial helical path, which introduces a distinct (+1,-1) change in the wrapping index. The low energy realization occurs without loss of mass, via nucleation of a 5-7-7-5 dislocation dipole, followed by a nearly-axial glide of the 5-7 dislocation. The onset of plasticity depends not only on chirality but also on handedness. For a given handedness of the applied twist, chiral tubes of opposed handedness are most susceptible to yield. A right-handed applied twist on an armchair (zig-zag) tube leads to a right- (left-) handed tube. [4pt] [1] T. Dumitrica and R.D. James, Objective Molecular Dynamics, Journal of the Mechanics and Physics of Solids 55, 2206 (2007). [0pt] [2] D.-B. Zhang, M. Hua, and T. Dumitrica, Stability of Polycrystalline and Wurtzite Si Nanowires via Symmetry-Adapted Tight-Binding Objective Molecular Dynamics, Journal of Chemical Physics 128, 084104 (2008). [0pt] [3] D.-B. Zhang and T. Dumitrica, Elasticity of Ideal Single-Walled Carbon Nanotubes via Symmetry-Adapted Tight-Binding Objective Modeling, Applied Physics Letters 93, 031919 (2008).

Faulting mechanism of the El Asnam (Algeria) 1954 and 1980 earthquakes from modelling of vertical movements

NASA Astrophysics Data System (ADS)

Bezzeghoud, M.; Dimitro, D.; Ruegg, J. C.; Lammali, K.

1995-09-01

Since 1980, most of the papers published on the El Asnam earthquake concern the geological and seismological aspects of the fault zone. Only one paper, published by Ruegg et al. (1982), constrains the faulting mechanism with geodetic measurements. The purpose of this paper is to reexamine the faulting mechanism of the 1954 and 1980 events by modelling the associated vertical movements. For this purpose we used all available data, and particularly those of the levelling profiles along the Algiers-Oran railway that has been remeasured after each event. The comparison between 1905 and 1976 levelling data shows observed vertical displacement that could have been induced by the 1954 earthquake. On the basis of the 1954 and 1980 levelling data, we propose a possible model for the 1954 and 1980 fault systems. Our 1954 fault model is parallel to the 1980 main thrust fault, with an offset of 6 km towards the west. The 1980 dislocation model proposed in this study is based on a variable slip dislocation model and explains the observed surface break displacements given by Yielding et al. (1981). The Dewey (1991) and Avouac et al. (1992) models are compared with our dislocation model and discussed in this paper.

Crustal deformation associated with east Mediterranean strike-slip earthquakes: The 8 June 2008 Movri (NW Peloponnese), Greece, earthquake (M w6.4)

NASA Astrophysics Data System (ADS)

Papadopoulos, Gerassimos A.; Karastathis, Vassilis; Kontoes, Charalambos; Charalampakis, Marinos; Fokaefs, Anna; Papoutsis, Ioannis

2010-09-01

The 2008 mainshock ( Mw = 6.4) was the first modern, strong strike-slip earthquake in the Greek mainland. The fault strikes NE-SW, dips ˜ 85°NW while the motion was right-lateral with small reverse component. Historical seismicity showed no evidence that the fault ruptured in the last 300 years. For rectangular planar fault we estimated fault dimensions from aftershock locations. Dimensions are consistent with that a buried fault was activated, lateral expansion occurred only along length and the rupture stopped at depth ˜ 20 km implying that more rupture along length was favoured. We concluded that no major asperities remained unbroken and that the aftershock activity was dominated rather by creeping mechanism than by the presence of locked patches. For Mo = 4.56 × 10 25 dyn cm we calculated average slip of 76 cm and stress drop Δσ ˜ 13 bars. This Δσ is high for Greek strike-slip earthquakes, due rather to increased rigidity because of the relatively long recurrence ( Τ > 300 years) of strong earthquakes in the fault, than to high slip. Values of Δσ and Τ indicated that the fault is neither a typical strong nor a typical weak fault. Dislocation modeling of a buried fault showed uplift of ˜ 8.0 cm in Kato Achaia ( Δ ˜ 20 km) at the hanging wall of the reverse fault component. DInSAR analysis detected co-seismic motion only in Kato Achaia where interferogram fringes pattern showed vertical displacement from 3.0 to 6.0 cm. From field-surveys we estimated maximum intensity of VIII in Kato Achaia. The most important liquefaction spots were also observed there. These observations are attributable neither to surface fault-breaks nor to site effects but possibly to high ground acceleration due to the co-seismic uplift. The causal association between displacement and earthquake damage in the hanging wall described for dip-slip faults in Taiwan, Greece and elsewhere, becomes possible also for strike-slip faults with dip-slip component, as the 2008 earthquake.

Dynamic strain aging and plastic instabilities

NASA Astrophysics Data System (ADS)

Mesarovic, Sinisa Dj.

1995-05-01

A constitutive model proposed by McCormick [(1988) Theory of flow localization due to dynamic strain ageing. Acta. Metall.36, 3061-3067] based on dislocation-solute interaction and describing dynamic strain aging behavior, is analyzed for the simple loading case of uniaxial tension. The model is rate dependent and includes a time-varying state variable, representing the local concentration of the impurity atoms at dislocations. Stability of the system and its post-instability behavior are considered. The methods used include analytical and numerical stability and bifurcation analysis with a numerical continuation technique. Yield point behavior and serrated yielding are found to result for well defined intervals of temperature and strain rate. Serrated yielding emerges as a branch of periodic solutions of the relaxation oscillation type, similar to frictional stick-slip. The distinction between the temporal and spatial (loss of homogeneity of strain) instability is emphasized. It is found that a critical machine stiffness exists above which a purely temporal instability cannot occur. The results are compared to the available experimental data.

Surface Nanocrystallization and Amorphization of Dual-Phase TC11 Titanium Alloys under Laser Induced Ultrahigh Strain-Rate Plastic Deformation

PubMed Central

Luo, Sihai; Zhou, Liucheng; Wang, Xuede; Cao, Xin; Nie, Xiangfan

2018-01-01

As an innovative surface technology for ultrahigh strain-rate plastic deformation, laser shock peening (LSP) was applied to the dual-phase TC11 titanium alloy to fabricate an amorphous and nanocrystalline surface layer at room temperature. X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy (HRTEM) were used to investigate the microstructural evolution, and the deformation mechanism was discussed. The results showed that a surface nanostructured surface layer was synthesized after LSP treatment with adequate laser parameters. Simultaneously, the behavior of dislocations was also studied for different laser parameters. The rapid slipping, accumulation, annihilation, and rearrangement of dislocations under the laser-induced shock waves contributed greatly to the surface nanocrystallization. In addition, a 10 nm-thick amorphous structure layer was found through HRTEM in the top surface and the formation mechanism was attributed to the local temperature rising to the melting point, followed by its subsequent fast cooling. PMID:29642379

Surface Nanocrystallization and Amorphization of Dual-Phase TC11 Titanium Alloys under Laser Induced Ultrahigh Strain-Rate Plastic Deformation.

PubMed

Luo, Sihai; Zhou, Liucheng; Wang, Xuede; Cao, Xin; Nie, Xiangfan; He, Weifeng

2018-04-06

As an innovative surface technology for ultrahigh strain-rate plastic deformation, laser shock peening (LSP) was applied to the dual-phase TC11 titanium alloy to fabricate an amorphous and nanocrystalline surface layer at room temperature. X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy (HRTEM) were used to investigate the microstructural evolution, and the deformation mechanism was discussed. The results showed that a surface nanostructured surface layer was synthesized after LSP treatment with adequate laser parameters. Simultaneously, the behavior of dislocations was also studied for different laser parameters. The rapid slipping, accumulation, annihilation, and rearrangement of dislocations under the laser-induced shock waves contributed greatly to the surface nanocrystallization. In addition, a 10 nm-thick amorphous structure layer was found through HRTEM in the top surface and the formation mechanism was attributed to the local temperature rising to the melting point, followed by its subsequent fast cooling.

SMALL ANGLE SCATTERING OF X-RAYS BY PLASTICALLY DEFORMED SINGLE CRYSTALS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Robinson, W.H.; Smoluchowski, R.

1959-05-01

The small-angle scattering of x rays from single crystals of magnesium plastically deformed by simple shear was measured in the angular range of 4' to 5 deg . The crystals were subjected to both unidirectional and cyclic shear stresses applied along the STAl 1 2-bar 0! direction. Thin slices of the deformed single crystals were prepared using strainfree cutting and polishing techniques. The thin slices had orientations such that the slip direction was either parallel or perpendicular to the incident x-ray beam in order to observe any anisotropy in the scattering that might be due to dislocations. It was foundmore » that those samples which contained deformation twins within the irradiated volume produced rather large scattered intensity. This scattered intensity is interpreted as being due to double Bragg scattering. The scattered intensity from other specimens was attributed to surface scattering. No evidence for small angle scattering by dislocations was found. (auth)« less

Effect of microstructure on the coupled electromagnetic-thermo-mechanical response of cyclotrimethylenetrinitramine-estane energetic aggregates to infrared laser radiation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brown, Judith A.; Zikry, M. A., E-mail: zikry@ncsu.edu

2015-09-28

The coupled electromagnetic (EM)-thermo-mechanical response of cyclotrimethylenetrinitramine-estane energetic aggregates under laser irradiation and high strain rate loads has been investigated for various aggregate sizes and binder volume fractions. The cyclotrimethylenetrinitramine (RDX) crystals are modeled with a dislocation density-based crystalline plasticity formulation and the estane binder is modeled with finite viscoelasticity through a nonlinear finite element approach that couples EM wave propagation with laser heat absorption, thermal conduction, and inelastic deformation. Material property and local behavior mismatch at the crystal-binder interfaces resulted in geometric scattering of the EM wave, electric field and laser heating localization, high stress gradients, dislocation density, andmore » crystalline shear slip accumulation. Viscous sliding in the binder was another energy dissipation mechanism that reduced stresses in aggregates with thicker binder ligaments and larger binder volume fractions. This investigation indicates the complex interactions between EM waves and mechanical behavior, for accurate predictions of laser irradiation of heterogeneous materials.« less

Ductile shear zone rheology: the viewpoint of experimentally crept lower crustal rocks and analogues

NASA Astrophysics Data System (ADS)

Dimanov, Alexandre; Raphanel, Jean; Bornert, Michel; Bourcier, Mathieu; Gaye, Ababacar; Ludwig, Wolfgang

2015-04-01

With respect to lithosphere rheology, we are especially interested in the mechanical behavior and evolution of ductile shear zones at depth, which present polyphase and heterogeneous character and multi-scale strain localization patterns. According to structural geology, most strain concentrates in ultramylonitic layers, which exhibit along with metamorphism overprinted or concomitant microstructural signatures from several deformation mechanisms. The latter are either active in volume (crystal slip plasticity and dislocation recovery processes), or in the vicinity and along interfaces (grain sliding, phase transformations and solution mass transfer). Because all of these contribute to the drastic evolution of microstructures with respect to the wall rock and the protomylonite, and because the chronology of their activation and their interactions are unclear, inference of the overall rheology from these microstructural records seems illusory. Therefore, since more than a decade we investigate experimentally and numerically the rheology of synthetic rocks representative of lower crustal mineralogy (namely plagioclases and clinopyroxenes). Samples are elaborated with different microstructures and with variable phases, fluid and melt contents for the purpose of being representative of diverse geodynamical contexts. Experiments were performed either at constant stress or strain rate, in co-axial compression or in torsion. For macroscopic non-Newtonian flow we clearly identified dislocation glide and creep mechanisms. That is to say that power law rheology relates to dominant crystal slip plasticity accommodated by recovery processes, including dislocation climb and pile-up, sub-grain rotation and marginal recrystallization. We further refer to this regime as RCSP (recovery crystal slip plasticity). Conversely, Newtonian (linear viscous) behavior mostly involves grain boundary sliding (GBS) accommodated by diffusional mass transfer and grain boundary dislocation mechanisms, but local RCSP can still be active as well. We realized finite element (FE) modelling of a representative elementary volume (REV) constituted of volumetrically dominant fine-grained matrix and strong inclusions subjected to simple shear. The results confirmed that the microstructural heterogeneity and the contrasting rheology of the constituent phases induce very strong local stress and strain partitioning. On the one hand, GBS mechanisms dominate the overall response, which indicates Newtonian rheology as most appropriate for modelling the steady state mechanical behavior of deep crustal shear zones. On the other hand, the local stress heterogeneities trigger combined RCSP and GBS, which intimate interactions remain unclear. To clarify the respective roles of CSP and GBS we realized a multi-scale full field measurement investigation during deformation of rock analog synthetic halite. Uniaxial compression tests were performed in-situ a scanning electron microscope (SEM) and under X-ray contrast tomography (MCT). Full in-plane and volume strain fields were computed thanks to Digital Image Correlation (DIC) analysis performed at consecutive loading steps. Cooperative CSP and GBS were identified within early localized shear bands. When CSP dominates, GBS contributes as an accommodation mechanism for the local incompatibilities of plastic strain across some grain boundaries. Conversely, when GBS is pronounced strongly localized CSP allows for sliding accommodation at triple junctions. In opposition to the common assumption, our results indicate that GBS and CSP cannot be really dissociated as competing and independent strain mechanisms. They both and cooperatively ensure macroscopically homogeneous flow.

Mechanical Evolution and Dynamics of Decollement Slip in Contractional Systems: Correlating Macro- and Micro-Scale Processes in Particle Dynamics Simulation

NASA Astrophysics Data System (ADS)

Morgan, J. K.

2014-12-01

Particle-based numerical simulations allow detailed investigations of small-scale processes and mechanisms associated with fault initiation and slip, which emerge naturally in such models. This study investigates the evolving mechanical conditions and associated micro-mechanisms during transient slip on a weak decollement propagating beneath a growing contractional wedge (e.g., accretionary prism, fold and thrust belt). The models serve as analogs of the seismic cycle, although lacking full earthquake dynamics. Nonetheless, the mechanical evolution of both decollement and upper plate can be monitored, and correlated with the particle-scale physical and contact properties, providing insights into changes that accompany such stick-slip behavior. In this study, particle assemblages consolidated under gravity and bonded to impart cohesion, are pushed at a constant velocity above a weak, unbonded decollement surface. Forward propagation of decollement slip occurs in discrete pulses, modulated by heterogeneous stress conditions (e.g., roughness, contact bridging) along the fault. Passage of decollement slip resets the stress along this horizon, producing distinct patterns: shear stress is enhanced in front of the slipped decollement due to local contact bridging and fault locking; shear stress minima occur immediately above the tip, denoting local stress release and contact reorganization following slip; more mature portions of the fault exhibit intermediate shear stress, reflecting more stable contact force distributions and magnitudes. This pattern of shear stress pre-conditions the decollement for future slip events, which must overcome the high stresses at the fault tip. Long-term slip along the basal decollement induces upper plate contraction. When upper plate stresses reach critical strength conditions, new thrust faults break through the upper plate, relieving stresses and accommodating horizontal shortening. Decollement activity retreats back to the newly formed thrust fault. The cessation of upper plate fault slip causes gradual increases in upper plate stresses, rebuilding shear stresses along the decollement and enabling renewed pulses of decollement slip. Thus, upper plate deformation occurs out of phase with decollement propagation.

Gravitational spreading, bookshelf faulting, and tectonic evolution of the South Polar Terrain of Saturn's moon Enceladus

NASA Astrophysics Data System (ADS)

Yin, An; Pappalardo, Robert T.

2015-11-01

Despite a decade of intense research the mechanical origin of the tiger-stripe fractures (TSF) and their geologic relationship to the hosting South Polar Terrain (SPT) of Enceladus remain poorly understood. Here we show via systematic photo-geological mapping that the semi-squared SPT is bounded by right-slip, left-slip, extensional, and contractional zones on its four edges. Discrete deformation along the edges in turn accommodates translation of the SPT as a single sheet with its transport direction parallel to the regional topographic gradient. This parallel relationship implies that the gradient of gravitational potential energy drove the SPT motion. In map view, internal deformation of the SPT is expressed by distributed right-slip shear parallel to the SPT transport direction. The broad right-slip shear across the whole SPT was facilitated by left-slip bookshelf faulting along the parallel TSF. We suggest that the flow-like tectonics, to the first approximation across the SPT on Enceladus, is best explained by the occurrence of a transient thermal event, which allowed the release of gravitational potential energy via lateral viscous flow within the thermally weakened ice shell.

Geodetic exploration of strain along the El Pilar Fault in northeastern Venezuela

NASA Astrophysics Data System (ADS)

Reinoza, C.; Jouanne, F.; Audemard, F. A.; Schmitz, M.; Beck, C.

2015-03-01

We use Global Navigation Satellite Systems observations in northeastern Venezuela to constrain the El Pilar Fault (EPF) kinematics and to explore the effects of the variable elastic properties of the surrounding medium and of the fault geometry on inferred slip rates and locking depth. The velocity field exhibits an asymmetric velocity gradient on either side of the EPF. We use five different approaches to explore possible models to explain this asymmetry. First, we infer a 1.6 km locking depth using a classic elastic half-space dislocation model. Second, we infer a 1.5 km locking depth and a 0.33 asymmetry coefficient using a heterogeneous asymmetric model, including contrasting material properties on either side of a vertical fault, suggesting that the igneous-metamorphic terranes on the northern side are ~2 times more rigid than the sedimentary southern side. Third, we use a three-dimensional elastostatic model to evaluate the presence of a compliant zone, suggesting a 30% reduction of rigidity in the upper 3 km at the depth of a 1 to 5 km wide fault zone. Fourth, we evaluate the distribution of fault slip, revealing a widespread partial creep pattern in the eastern upper segment, while the upper western segment exhibits a partially locked area, which coincides with the rupture surface of the 1797 and 1929 earthquakes. To supplement these models, we upgrade the previously published displacement simulation method using nonvertical dislocations with data acquired between 2003 and 2013. The localized aseismic displacement pattern associated with creeping or partially creeping fault segments could explain the low level of historic seismicity.

The respective roles of bulk friction and slip velocity during a granular mass flow

NASA Astrophysics Data System (ADS)

Staron, Lydie

2016-04-01

Catastrophic granular mass flows form an important natural hazard. Mitigation has motivated numerous studies on the properties of natural granular flows, and in particular, their ability to travel long distances away from the release point. The mobility of granular flows is commonly characterised through the definition of rheological properties and effective friction. Yet, it is widely accepted that the description in term of effective friction may include various lubrication effects, softening at the base of the flow and large slip velocities being a most likely one. In this case, flow bulk properties may obliterate the flow boundary conditions. In this contribution, we investigate how disentangling bulk properties from boundary conditions may improve our understanding of the flow. Using discrete simulations, we induce increasing slip velocities in different flow configurations. We show that increased mobility may be achieved without changing bulk properties. The results are interpreted in terms of a Robin-Navier slip condition and implemented in a continuum Navier-Stokes solver. We quantify the respective role of rheological bulk properties and boundary conditions in the general behaviour of a transient mass flow. We show that omitting the description of boundary conditions leads to misinterpretation of the flow properties. The outcome is discussed in terms of models reliability. References P.-Y. Lagrée et al, The granular column collapse as a continuum: validity of a two-dimensional Navier-Stokes model with the mu(I) rheology, J. Fluid Mech. 686, 378-408 (2011) L. Staron and E. Lajeunesse, Understanding how the volume affects the mobility of dry debris flows, Geophys. Res. Lett. 36, L12402 (2009) L. Staron, Mobility of long-runout rock flows: a discrete numerical investigation, Geophys. J. Int. 172, 455-463 (2008)

GPS measurements of deformation associated with the 1987 Superstition Hills earthquake: Evidence for conjugate faulting

NASA Technical Reports Server (NTRS)

Larsen, Shawn; Reilinger, Robert; Neugebauer, Helen; Strange, William

1991-01-01

Large station displacements observed from Imperial Valley Global Positioning System (GPS) campaigns are attributed to the November 24, 1987 Superstition Hills earthquake sequence. Thirty sites from a 42 station GPS network established in 1986 were reoccupied during 1988 and/or 1990. Displacements at three sites within 3 kilometers of the surface rupture approach 0.5 m. Eight additional stations within 20 km of the seismic zone are displaced at least 10 cm. This is the first occurrence of a large earthquake (M(sub S) 6.6) within a preexisting GPS network. Best-fitting uniform slip models of rectangular dislocations in an elastic half-space indicate 130 + or - 8 cm right-lateral displacement along the northwest-trending Superstition Hills fault and 30 + or - 10 cm left-lateral displacement along the conjugate northeast-trending Elmore Ranch fault. The geodetic moments are 9.4 x 10(exp 25) dyne-cm and 2.3 x 10(exp 25) dyne-cm for the Superstition Hills and Elmore Ranch faults, respectively, consistent with teleseismic source parameters. The data also suggest the post seismic slip along the Superstition Hills fault is concentrated at shallow depths. Distributed slip solutions using Singular Value Decomposition indicate near uniform displacement along the Elmore Ranch fault and concentrated slip to the northwest and southeast along the Superstition Hills fault. A significant component of non-seismic displacement is observed across the Imperial Valley, which is attributed in part to interseismic plate-boundary deformation.

Effects of layered crust on the coseismic slip inversion and related CFF variations: Hints from the 2012 Emilia Romagna earthquake

NASA Astrophysics Data System (ADS)

Nespoli, Massimo; Belardinelli, Maria E.; Anderlini, Letizia; Bonafede, Maurizio; Pezzo, Giuseppe; Todesco, Micol; Rinaldi, Antonio P.

2017-12-01

The 2012 Emilia Romagna (Italy) seismic sequence has been extensively studied given the occurrence of two mainshocks, both temporally and spatially close to each other. The recent literature accounts for several fault models, obtained with different inversion methods and different datasets. Several authors investigated the possibility that the second event was triggered by the first mainshock with elusive results. In this work, we consider all the available InSAR and GPS datasets and two planar fault geometries, which are based on both seismological and geological constraints. We account for a layered, elastic half-space hosting the dislocation and compare the slip distribution resulting from the inversion and the related changes in Coulomb Failure Function (CFF) obtained with both a homogeneous and layered half-space. Finally, we focus on the interaction between the two main events, discriminating the contributions of coseismic and early postseismic slip of the mainshock on the generation of the second event and discuss the spatio-temporal distribution of the seismic sequence. When accounting for both InSAR and GPS geodetic data we are able to reproduce a detailed coseismic slip distribution for the two mainshocks that is in accordance with the overall aftershock seismicity distribution. Furthermore, we see that an elastic medium with depth dependent rigidity better accounts for the lack of the shallow seismicity, amplifying, with respect to the homogeneous case, the mechanical interaction of the two mainshocks.

Experimental validation of plastic constitutive hardening relationship based upon the direction of the Net Burgers Density Vector

NASA Astrophysics Data System (ADS)

Sarac, Abdulhamit; Kysar, Jeffrey W.

2018-02-01

We present a new methodology for experimental validation of single crystal plasticity constitutive relationships based upon spatially resolved measurements of the direction of the Net Burgers Density Vector, which we refer to as the β-field. The β-variable contains information about the active slip systems as well as the ratios of the Geometrically Necessary Dislocation (GND) densities on the active slip systems. We demonstrate the methodology by comparing single crystal plasticity finite element simulations of plane strain wedge indentations into face-centered cubic nickel to detailed experimental measurements of the β-field. We employ the classical Peirce-Asaro-Needleman (PAN) hardening model in this study due to the straightforward physical interpretation of its constitutive parameters that include latent hardening ratio, initial hardening modulus and the saturation stress. The saturation stress and the initial hardening modulus have relatively large influence on the β-variable compared to the latent hardening ratio. A change in the initial hardening modulus leads to a shift in the boundaries of plastic slip sectors with the plastically deforming region. As the saturation strength varies, both the magnitude of the β-variable and the boundaries of the plastic slip sectors change. We thus demonstrate that the β-variable is sensitive to changes in the constitutive parameters making the variable suitable for validation purposes. We identify a set of constitutive parameters that are consistent with the β-field obtained from the experiment.