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Sample records for deformation induced dislocation

  1. Intermittent dislocation flow in viscoplastic deformation.

    PubMed

    Miguel, M C; Vespignani, A; Zapperi, S; Weiss, J; Grasso, J R

    2001-04-01

    The viscoplastic deformation (creep) of crystalline materials under constant stress involves the motion of a large number of interacting dislocations. Analytical methods and sophisticated 'dislocation dynamics' simulations have proved very effective in the study of dislocation patterning, and have led to macroscopic constitutive laws of plastic deformation. Yet, a statistical analysis of the dynamics of an assembly of interacting dislocations has not hitherto been performed. Here we report acoustic emission measurements on stressed ice single crystals, the results of which indicate that dislocations move in a scale-free intermittent fashion. This result is confirmed by numerical simulations of a model of interacting dislocations that successfully reproduces the main features of the experiment. We find that dislocations generate a slowly evolving configuration landscape which coexists with rapid collective rearrangements. These rearrangements involve a comparatively small fraction of the dislocations and lead to an intermittent behaviour of the net plastic response. This basic dynamical picture appears to be a generic feature in the deformation of many other materials. Moreover, it should provide a framework for discussing fundamental aspects of plasticity that goes beyond standard mean-field approaches that see plastic deformation as a smooth laminar flow. PMID:11287948

  2. Sessile dislocations by reactions in NiAl severely deformed at room temperature

    SciTech Connect

    Geist, D.; Gammer, C.; Rentenberger, C.; Karnthaler, H. P.

    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 dislocation 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.

  3. Dislocations: 75 years of Deformation Mechanisms

    NASA Technical Reports Server (NTRS)

    Schneider, Judy

    2009-01-01

    The selection of papers presented in this section reflect on themes to be explored at the "Dislocations: 75 years of Deformation Mechanisms" Symposium to be held at the Annual 2009 TMS meeting. The symposium was sponsored by the Mechanical Behavior of Materials Committee to give tribute to the evolution of a concept that has formed the basis of our mechanistic understanding of how crystalline solids plastically deform and how they fail.

  4. Dislocation Mechanics of High-Rate Deformations

    NASA Astrophysics Data System (ADS)

    Armstrong, Ronald W.; Li, Qizhen

    2015-10-01

    Four topics associated with constitutive equation descriptions of rate-dependent metal plastic deformation behavior are reviewed in honor of previous research accomplished on the same issues by Professor Marc Meyers along with colleagues and students, as follow: (1) increasing strength levels attributed to thermally activated dislocation migration at higher loading rates; (2) inhomogeneous adiabatic shear banding; (3) controlling mechanisms of deformation in shock as compared with shock-less isentropic compression experiments and (4) Hall-Petch-based grain size-dependent strain rate sensitivities exhibited by nanopolycrystalline materials. Experimental results are reviewed on the topics for a wide range of metals.

  5. Surface dislocation nucleation controlled deformation of Au nanowires

    SciTech Connect

    Roos, B.; Kapelle, B.; Volkert, C. A.; Richter, G.

    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 the 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.

  6. Effect of dislocation and grain boundary on deformation mechanism in ultrafine-grained interstitial-free steel

    NASA Astrophysics Data System (ADS)

    Nakazawa, K.; Itoh, S.; Matsunaga, T.; Matsukawa, Y.; Satoh, Y.; Murase, Y.; Abe, H.

    2014-08-01

    Ultrafine-grained interstitial-free steel fabricated by the accumulative roll-bonding method was subjected to tensile tests and analyses of AFM, TEM and XRD to identify the effects of interaction between dislocations and grain boundaries (GB) on the deformation mechanism. The AFM analyses indicated that the main deformation mechanism of this material changed from dislocation motion to grain boundary sliding (GBS) with decreasing strain rate. TEM observations and XRD analysis revealed showed that dislocations piled up at GB and the dislocation density decreased with increasing strain. Those suggest the dislocations are absorbed into GB during deformation, activating slip-induced GBS.

  7. Sessile dislocations by reactions in NiAl severely deformed at room temperature

    PubMed Central

    Geist, D.; Gammer, C.; Rentenberger, C.; Karnthaler, H.P.

    2015-01-01

    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 dislocation 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. PMID:25663749

  8. Sessile dislocations by reactions in NiAl severely deformed at room temperature

    DOE PAGESBeta

    Geist, D.; Gammer, C.; Rentenberger, C.; Karnthaler, H. P.

    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

  9. Second yield via dislocation-induced premelting in copper

    NASA Astrophysics Data System (ADS)

    Wang, L.; Cai, Y.; He, A. M.; Luo, S. N.

    2016-05-01

    Premelting or virtual melting was proposed previously as an important deformation mechanism for high strain-rate loading. However, two questions remain outstanding: how premelting occurs exactly, and whether it plays a role in plastic deformation independent of, parasitic on, or synergetic with, dislocation motion. By virtue of double-shock compression, our large-scale molecular dynamics simulations reveal two yields in single-crystal copper, with the first yield achieved via dislocation motion, and the second, via dislocation-induced premelting as well. The clean capture of melting during dislocation motion suggests that premelting occurs on slip planes and at their intersections, facilitating gliding and leading to yield together with dislocation motion.

  10. Deformations of the spin currents by topological screw dislocation and cosmic dispiration

    NASA Astrophysics Data System (ADS)

    Wang, Jianhua; Ma, Kai; Li, Kang; Fan, Huawei

    2015-11-01

    We study the spin currents induced by topological screw dislocation and cosmic dispiration. By using the extended Drude model, we find that the spin dependent forces are modified by the nontrivial geometry. For the topological screw dislocation, only the direction of spin current is bent by deforming the spin polarization vector. In contrast, the force induced by cosmic dispiration could affect both the direction and magnitude of the spin current. As a consequence, the spin-Hall conductivity does not receive corrections from screw dislocation.

  11. Material yielding and irreversible deformation mediated by dislocation motion

    NASA Astrophysics Data System (ADS)

    Miguel, M.-Carmen; Laurson, L.; Alava, M. J.

    2008-08-01

    We study the collective behavior of dislocation assemblies in simplified models of plastic deformation. We first review several numerical results on long range dislocation interactions with simplified dislocation motion constraints. These typically give rise to a yielding transition separating stationary and moving dislocation phases. Furthermore, we discuss the intermittent relaxation of the plastic strain-rate observed around this transition at mesoscopic scales, and how this intermittent behavior gives rise to an average slow power law relaxation in time known in the literature as Andrade’s creep. We analyze the coherent dynamics and the average stress-strain relationship in the steady regime of plastic deformation. In this steady regime, plastic deformation proceeds in the form of plastic avalanches whose size and duration are broadly distributed and statistically characterized. One signature of the time correlations of this heterogeneous collective dislocation dynamics is a power spectrum scaling with frequency as f - a with an exponent α close to 1.5. This feature appears to be peculiar of dislocation and grain boundary motion as has been observed in other physical situations in the vicinity of a yielding transition.

  12. Inelastic deformation and dislocation structure of a nickel alloy - Effects of deformation and thermal histories

    NASA Technical Reports Server (NTRS)

    Chan, K. S.; Page, R. A.

    1988-01-01

    Inelastic deformation behavior of the cast Ni-base alloy, B1900 + Hf, was investigated using data from step-temperature tensile tests and thermomechanical cyclic tests in the temperature ranges 538-760 C and 760-982 C. The deformation results were correlated with the dislocation structures of deformed specimens, identified by TEM. It was found that, in the 760-982 C temperature range, there are no thermal history effects in the inelastic deformation behavior of B1900 + Hf. In the 538-760 range, anomalous cyclic hardening and, possibly, thermal history effects were observed in thermomechanically deformed alloy, caused by sessile (010) dislocations in the gamma-prime phase.

  13. Atomistic processes of dislocation generation and plastic deformation during nanoindentation

    SciTech Connect

    Begau, C.; Hartmaier, A.; George, Easo P; Pharr, George M

    2011-01-01

    To enable plastic deformation during nanoindentation of an initially defect-free crystal, it is necessary first to produce dislocations. While it is now widely accepted that the nucleation of the first dislocations occurs at the start of the pop-in event frequently observed in experiments, it is unclear how these initial dislocations multiply during the early stages of plastic deformation and produce pop-in displacements that are typically much larger than the magnitude of the Burgers vector. This uncertainty about the complex interplay between dislocation multiplication and strain hardening during nanoindentation makes a direct correlation between force-displacement curves and macroscopic material properties difficult. In this paper, we study the early phase of plastic deformation during nanoindentation with the help of large-scale molecular dynamics simulations. A skeletonization method to simplify defect structures in atomistic simulations enables the direct observation and quantitative analysis of dislocation nucleation and multiplication processes occurring in the bulk as well as at the surface.

  14. Stochastically forced dislocation density distribution in plastic deformation.

    PubMed

    Chattopadhyay, Amit K; Aifantis, Elias C

    2016-08-01

    The dynamical evolution of dislocations in plastically deformed metals is controlled by both deterministic factors arising out of applied loads and stochastic effects appearing due to fluctuations of internal stress. Such types of stochastic dislocation processes and the associated spatially inhomogeneous modes lead to randomness in the observed deformation structure. Previous studies have analyzed the role of randomness in such textural evolution, but none of these models have considered the impact of a finite decay time (all previous models assumed instantaneous relaxation which is "unphysical") of the stochastic perturbations in the overall dynamics of the system. The present article bridges this knowledge gap by introducing a colored noise in the form of an Ornstein-Uhlenbeck noise in the analysis of a class of linear and nonlinear Wiener and Ornstein-Uhlenbeck processes that these structural dislocation dynamics could be mapped on to. Based on an analysis of the relevant Fokker-Planck model, our results show that linear Wiener processes remain unaffected by the second time scale in the problem, but all nonlinear processes, both the Wiener type and Ornstein-Uhlenbeck type, scale as a function of the noise decay time τ. The results are expected to ramify existing experimental observations and inspire new numerical and laboratory tests to gain further insight into the competition between deterministic and random effects in modeling plastically deformed samples. PMID:27627278

  15. Local decomposition induced by dislocation motions inside precipitates in an Al-alloy

    PubMed Central

    Yang, B.; Zhou, Y. T.; Chen, D.; Ma, X. L.

    2013-01-01

    Dislocations in crystals are linear crystallographic defects, which move in lattice when crystals are plastically deformed. Motion of a partial dislocation may remove or create stacking fault characterized with a partial of a lattice translation vector. Here we report that motion of partial dislocations inside an intermetallic compound result in a local composition deviation from its stoichiometric ratio, which cannot be depicted with any vectors of the primary crystal. Along dislocation slip bands inside the deformed Al2Cu particles, redistribution of Cu and Al atoms leads to a local decomposition and collapse of the original crystal structure. This finding demonstrates that dislocation slip may induce destabilization in complex compounds, which is fundamentally different from that in monometallic crystals. This phenomenon of chemical unmixing of initially homogeneous multicomponent solids induced by dislocation motion might also have important implications for understanding the geologic evolvement of deep-focus peridotites in the Earth. PMID:23301160

  16. Local decomposition induced by dislocation motions inside precipitates in an Al-alloy.

    PubMed

    Yang, B; Zhou, Y T; Chen, D; Ma, X L

    2013-01-01

    Dislocations in crystals are linear crystallographic defects, which move in lattice when crystals are plastically deformed. Motion of a partial dislocation may remove or create stacking fault characterized with a partial of a lattice translation vector. Here we report that motion of partial dislocations inside an intermetallic compound result in a local composition deviation from its stoichiometric ratio, which cannot be depicted with any vectors of the primary crystal. Along dislocation slip bands inside the deformed Al(2)Cu particles, redistribution of Cu and Al atoms leads to a local decomposition and collapse of the original crystal structure. This finding demonstrates that dislocation slip may induce destabilization in complex compounds, which is fundamentally different from that in monometallic crystals. This phenomenon of chemical unmixing of initially homogeneous multicomponent solids induced by dislocation motion might also have important implications for understanding the geologic evolvement of deep-focus peridotites in the Earth. PMID:23301160

  17. Dislocation substructure in NiAl single crystals deformed at ambient temperature

    SciTech Connect

    Shi, X.; Pollock, T.M.; Mahajan, S.; Arunachalam, V.S.

    1997-12-31

    Dislocation substructure in NiAl single crystals oriented for single slip and deformed at ambient temperature has been studied using weak-beam transmission electron microscopy. Deformation is localized in bands that consists mostly of near-edge dislocations, with an interspersion of a high density of elongated prismatic loops. Pure screw dislocations are not observed, but dislocations having zigzag configurations that are near-screw in orientation are present. A high density of jogs is observed on both near-edge and zigzag dislocation segments. The mechanisms for the development of this substructure are discussed, emphasizing the role of double cross slip and resulting glissile and sessile jogs of varying heights.

  18. Dislocation models of interseismic deformation in the western United States

    USGS Publications Warehouse

    Pollitz, F.F.; McCrory, P.; Svarc, J.; Murray, J.

    2008-01-01

    The GPS-derived crustal velocity field of the western United States is used to construct dislocation models in a viscoelastic medium of interseismic crustal deformation. The interseismic velocity field is constrained by 1052 GPS velocity vectors spanning the ???2500-km-long plate boundary zone adjacent to the San Andreas fault and Cascadia subduction zone and extending ???1000 km into the plate interior. The GPS data set is compiled from U.S. Geological Survey campaign data, Plate Boundary Observatory data, and the Western U.S. Cordillera velocity field of Bennett et al. (1999). In the context of viscoelastic cycle models of postearthquake deformation, the interseismic velocity field is modeled with a combination of earthquake sources on ???100 known faults plus broadly distributed sources. Models that best explain the observed interseismic velocity field include the contributions of viscoelastic relaxation from faulting near the major plate margins, viscoelastic relaxation from distributed faulting in the plate interior, as well as lateral variations in depth-averaged rigidity in the elastic lithosphere. Resulting rigidity variations are consistent with reduced effective elastic plate thickness in a zone a few tens of kilometers wide surrounding the San Andreas fault (SAF) system. Primary deformation characteristics are captured along the entire SAF system, Eastern California Shear Zone, Walker Lane, the Mendocino triple junction, the Cascadia margin, and the plate interior up to ???1000 km from the major plate boundaries.

  19. Dislocation models of interseismic deformation in the western United States

    NASA Astrophysics Data System (ADS)

    Pollitz, Fred F.; McCrory, Patricia; Svarc, Jerry; Murray, Jessica

    2008-04-01

    The GPS-derived crustal velocity field of the western United States is used to construct dislocation models in a viscoelastic medium of interseismic crustal deformation. The interseismic velocity field is constrained by 1052 GPS velocity vectors spanning the ˜2500-km-long plate boundary zone adjacent to the San Andreas fault and Cascadia subduction zone and extending ˜1000 km into the plate interior. The GPS data set is compiled from U.S. Geological Survey campaign data, Plate Boundary Observatory data, and the Western U.S. Cordillera velocity field of Bennett et al. (1999). In the context of viscoelastic cycle models of postearthquake deformation, the interseismic velocity field is modeled with a combination of earthquake sources on ˜100 known faults plus broadly distributed sources. Models that best explain the observed interseismic velocity field include the contributions of viscoelastic relaxation from faulting near the major plate margins, viscoelastic relaxation from distributed faulting in the plate interior, as well as lateral variations in depth-averaged rigidity in the elastic lithosphere. Resulting rigidity variations are consistent with reduced effective elastic plate thickness in a zone a few tens of kilometers wide surrounding the San Andreas fault (SAF) system. Primary deformation characteristics are captured along the entire SAF system, Eastern California Shear Zone, Walker Lane, the Mendocino triple junction, the Cascadia margin, and the plate interior up to ˜1000 km from the major plate boundaries.

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

    SciTech Connect

    Fan, Haidong; Aubry, Sylvie; Arsenlis, Athanasios; El-Awady, Jaafar A.

    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 Mg 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.

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

    DOE PAGESBeta

    Fan, Haidong; Aubry, Sylvie; Arsenlis, Athanasios; El-Awady, Jaafar A.

    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

  2. Modeling and 2-D discrete simulation of dislocation dynamics for plastic deformation of metal

    NASA Astrophysics Data System (ADS)

    Liu, Juan; Cui, Zhenshan; Ou, Hengan; Ruan, Liqun

    2013-05-01

    Two methods are employed in this paper to investigate the dislocation evolution during plastic deformation of metal. One method is dislocation dynamic simulation of two-dimensional discrete dislocation dynamics (2D-DDD), and the other is dislocation dynamics modeling by means of nonlinear analysis. As screw dislocation is prone to disappear by cross-slip, only edge dislocation is taken into account in simulation. First, an approach of 2D-DDD is used to graphically simulate and exhibit the collective motion of a large number of discrete dislocations. In the beginning, initial grains are generated in the simulation cells according to the mechanism of grain growth and the initial dislocation is randomly distributed in grains and relaxed under the internal stress. During the simulation process, the externally imposed stress, the long range stress contribution of all dislocations and the short range stress caused by the grain boundaries are calculated. Under the action of these forces, dislocations begin to glide, climb, multiply, annihilate and react with each other. Besides, thermal activation process is included. Through the simulation, the distribution of dislocation and the stress-strain curves can be obtained. On the other hand, based on the classic dislocation theory, the variation of the dislocation density with time is described by nonlinear differential equations. Finite difference method (FDM) is used to solve the built differential equations. The dislocation evolution at a constant strain rate is taken as an example to verify the rationality of the model.

  3. Dislocation dynamics during plastic deformations of complex plasma crystals.

    PubMed

    Durniak, C; Samsonov, D; Ralph, J F; Zhdanov, S; Morfill, G

    2013-11-01

    The internal structures of most periodic crystalline solids contain defects. This affects various important mechanical and thermal properties of crystals. Since it is very difficult and expensive to track the motion of individual atoms in real solids, macroscopic model systems, such as complex plasmas, are often used. Complex plasmas consist of micrometer-sized grains immersed into an ion-electron plasma. They exist in solidlike, liquidlike, and gaseouslike states and exhibit a range of nonlinear and dynamic effects, most of which have direct analogies in solids and liquids. Slabs of a monolayer hexagonal complex plasma were subjected to a cycle of uniaxial compression and decompression of large amplitudes to achieve plastic deformations, both in experiments and simulations. During the cycle, the internal structure of the lattice exhibited significant rearrangements. Dislocations (point defects) were generated and displaced in the stressed lattice. They tended to glide parallel to their Burgers vectors under load. It was found that the deformation cycle was macroscopically reversible but irreversible at the particle scale. PMID:24329366

  4. Dislocation dynamics during plastic deformations of complex plasma crystals

    NASA Astrophysics Data System (ADS)

    Durniak, C.; Samsonov, D.; Ralph, J. F.; Zhdanov, S.; Morfill, G.

    2013-11-01

    The internal structures of most periodic crystalline solids contain defects. This affects various important mechanical and thermal properties of crystals. Since it is very difficult and expensive to track the motion of individual atoms in real solids, macroscopic model systems, such as complex plasmas, are often used. Complex plasmas consist of micrometer-sized grains immersed into an ion-electron plasma. They exist in solidlike, liquidlike, and gaseouslike states and exhibit a range of nonlinear and dynamic effects, most of which have direct analogies in solids and liquids. Slabs of a monolayer hexagonal complex plasma were subjected to a cycle of uniaxial compression and decompression of large amplitudes to achieve plastic deformations, both in experiments and simulations. During the cycle, the internal structure of the lattice exhibited significant rearrangements. Dislocations (point defects) were generated and displaced in the stressed lattice. They tended to glide parallel to their Burgers vectors under load. It was found that the deformation cycle was macroscopically reversible but irreversible at the particle scale.

  5. Transitions of Dislocation Glide to Twinning and Shear Transformation in Shock-Deformed Tantalum

    SciTech Connect

    Hsiung, L L; Campbell, G H; McNaney, J M

    2009-10-19

    tantalum. The observation of {alpha} {yields} {omega} transition in shock-compressed tantalum and tantalum-tungsten alloys at {approx}45 GPa in fact reveals the occurrence of a non-equilibrium phase transformation at such a low pressure. We therefore postulated that the equation of state (EOS) based on static thermodynamics, which asserts that the system free energy (G) is a function of volume (V), pressure (P), and temperature (T), i.e., G = F(V, P, T) is insufficient to rationalize the system free energy under dynamic-pressure conditions. Since shear deformation was found to play a crucial role in shock-induced deformation twins and {omega} phase, the density and arrangement of dislocations, which can alter and increase the system free energy, should also be taken into account to rationalize the non-equilibrium phase transformation in shocked tantalum. Typical arrangements of high-density dislocations formed in pure tantalum shocked at {approx}45 GPa are shown in Figs. 1a and 1b. Figure 1a reveals a cellular dislocation structure but no twins or {omega} phase-domains were observed in this region. The formation of low-energy type cellular dislocation structures indicates the occurrence of dynamic-recovery reactions to reduce dislocation density in this region. Figure 1b shows an evenly distributed dislocation structure with a local dislocation density ({rho}) as high as {approx}5 x 10{sup 12} cm{sup -2} according to {rho} {approx} 1/l{sup 2}, where l ({approx}4.5 nm) is the spacing between two dislocations. Here shock-induced twin plates and {omega} phase-domains can be readily seen. These observations provide us a clue that dislocation arrangement and density population, which can alter system free energy through the changes of dislocation self-energy (E{sub s}) and dislocation interaction energy (E{sub ij}), are relevant to the occurrence of shock-induced twinning and phase transformation in tantalum. The objective of this paper is to report new results obtained from

  6. High dislocation density of tin induced by electric current

    SciTech Connect

    Liao, Yi-Han; Liang, Chien-Lung; Lin, Kwang-Lung; Wu, Albert T.

    2015-12-15

    A dislocation density of as high as 10{sup 17} /m{sup 2} in a tin strip, as revealed by high resolution transmission electron microscope, was induced by current stressing at 6.5 x 10{sup 3} A/ cm{sup 2}. The dislocations exist in terms of dislocation line, dislocation loop, and dislocation aggregates. Electron Backscattered Diffraction images reflect that the high dislocation density induced the formation of low deflection angle subgrains, high deflection angle Widmanstätten grains, and recrystallization. The recrystallization gave rise to grain refining.

  7. Evidence of dislocation cross-slip in MAX phase deformed at high temperature

    PubMed Central

    Guitton, Antoine; Joulain, Anne; Thilly, Ludovic; Tromas, Christophe

    2014-01-01

    Ti2AlN nanolayered ternary alloy has been plastically deformed under confining pressure at 900°C. The dislocation configurations of the deformed material have been analyzed by transmission electron microscopy. The results show a drastic evolution compared to the dislocation configurations observed in the Ti2AlN samples deformed at room temperature. In particular, they evidence out-of-basal-plane dislocations and interactions. Moreover numerous cross-slip events from basal plane to prismatic or pyramidal planes are observed. These original results are discussed in the context of the Brittle-to-Ductile Transition of the nanolayered ternary alloys. PMID:25220949

  8. Modeling of ultrasonic nonlinearities for dislocation evolution in plastically deformed materials: Simulation and experimental validation.

    PubMed

    Zhu, Wujun; Deng, Mingxi; Xiang, Yanxun; Xuan, Fu-Zhen; Liu, Changjun; Wang, Yi-Ning

    2016-05-01

    A nonlinear constitutive relationship was established to investigate nonlinear behaviors of ultrasonic wave propagation in plastically damaged media based on analyses of mixed dislocation evolution. Finite element simulations of longitudinal wave propagation in plastically deformed martensite stainless steel were performed based on the proposed nonlinear constitutive relationship, in which the contribution of mixed dislocation to acoustic nonlinearity was considered. The simulated results were validated by experimental measurements of plastically deformed 30Cr2Ni4MoV martensite stainless steels. Simulated and experimental results both reveal a monotonically increasing tendency of the normalized acoustic nonlinearity parameter as a function of plastic strain. Microscopic studies revealed that the changes of the acoustic nonlinearity are mainly attributed to dislocation evolutions, such as dislocation density, dislocation length, and the type and fraction of dislocations during plastic loading. PMID:26950888

  9. Dislocation

    MedlinePlus

    Joint dislocation ... It may be hard to tell a dislocated joint from a broken bone . Both are emergencies. You ... to repair a ligament that tears when the joint is dislocated is needed. Injuries to nerves and ...

  10. Orientation dependence of shock induced dislocations in Tantalum single crystals

    NASA Astrophysics Data System (ADS)

    Pang, Bo; Jones, I.; Chiu, Yulung; Millett, J.; Whiteman, Glenn; Bourne, N.

    2013-06-01

    Shock wave deformation of monocrystalline tantalum to a pressure of 6.2 GPa and duration of 1.7 μs generates profuse dislocations. Three orientations (100),(110),(111) were tested to examine the orientation dependence of the dislocation generation. The dislocations were characterised by transmission electron microscopy. The difference in the Burgers vectors of the primary dislocations in the specimens with different orientations showed a distinct anisotropy and will be discussed in light of the models of slip behaviour in one-dimensional strain (Smith 1958) and (Meyers 1978). The front and rear surfaces of the specimens were both investigated to examine the effects of wave duration.

  11. Contribution of mixed dislocations to the acoustic nonlinearity in plastically deformed materials

    NASA Astrophysics Data System (ADS)

    Xiang, Yanxun; Deng, Mingxi; Liu, Chang-Jun; Xuan, Fu-Zhen

    2015-06-01

    An analytical model is proposed based on dislocation line energy and variable line tension to describe the influence of mixed dislocations on acoustic nonlinearity in plastically deformed materials. Numerical results indicate that the acoustic nonlinearity parameter β is strongly dependent on the Poisson's ratio, and the fraction and type of the dislocation. For edge dislocations, β is found to increase with increasing Poisson's ratio, which is different from the behavior predicted by existing models. Moreover, this result indicates that β should be more sensitive to edge dislocations in materials with large Poisson's ratios. The proposed model is validated by experimental measurements of cold-rolled 304 stainless steel plates and plastically deformed 30Cr2Ni4MoV martensite stainless steels.

  12. X-Ray Diffraction Study on the Strain Anisotropy and Dislocation Structure of Deformed Lath Martensite

    NASA Astrophysics Data System (ADS)

    Hossein Nedjad, S.; Hosseini Nasab, F.; Movaghar Garabagh, M. R.; Damadi, S. R.; Nili Ahmadabadi, M.

    2011-08-01

    18Ni (300) maraging steel possessing lath martensite structure was deformed by four passes of equal-channel angular pressing (ECAP) at ambient temperature. Line profile analysis (LPA) of X-ray diffraction (XRD) patterns identified strong strain anisotropy and remarkable increases in the relative fraction of screw dislocations after ECAP. The strain anisotropy was reasonably accounted for by the anisotropy of elastic constants. Domination of screw dislocations in the deformed structure was attributed to the preferred annihilation of edge dislocations in the early stages of deformation along with the difficulties for annihilation of screw dislocations by cross slipping. Cobalt addition was mainly assumed to make cross slipping difficult by reducing stacking-fault energy and favoring short-range ordering.

  13. Dislocations

    MedlinePlus

    Dislocations are joint injuries that force the ends of your bones out of position. The cause is often a fall or a blow, sometimes from playing a contact sport. You can dislocate your ankles, knees, shoulders, hips, elbows and jaw. You can also dislocate your finger and toe joints. Dislocated joints often are ...

  14. Dislocation dynamics simulations of interactions between gliding dislocations and radiation induced prismatic loops in zirconium

    NASA Astrophysics Data System (ADS)

    Drouet, Julie; Dupuy, Laurent; Onimus, Fabien; Mompiou, Frédéric; Perusin, Simon; Ambard, Antoine

    2014-06-01

    The mechanical behavior of Pressurized Water Reactor fuel cladding tubes made of zirconium alloys is strongly affected by neutron irradiation due to the high density of radiation induced dislocation loops. In order to investigate the interaction mechanisms between gliding dislocations and loops in zirconium, a new nodal dislocation dynamics code, adapted to Hexagonal Close Packed metals, has been used. Various configurations have been systematically computed considering different glide planes, basal or prismatic, and different characters, edge or screw, for gliding dislocations with -type Burgers vectors. Simulations show various interaction mechanisms such as (i) absorption of a loop on an edge dislocation leading to the formation of a double super-jog, (ii) creation of a helical turn, on a screw dislocation, that acts as a strong pinning point or (iii) sweeping of a loop by a gliding dislocation. It is shown that the clearing of loops is more favorable when the dislocation glides in the basal plane than in the prismatic plane explaining the easy dislocation channeling in the basal plane observed after neutron irradiation by transmission electron microscopy.

  15. Plastic Deformation of Transition Zone Minerals: Effect of Temperature on Dislocation Mobility

    NASA Astrophysics Data System (ADS)

    Ritterbex, S.; Carrez, P.; Gouriet, K.; Cordier, P.

    2014-12-01

    Mantle convection is the fundamental process by which the Earth expels its internal heat. It is controlled at the microscopic scale by the motion of crystal defects responsable for plastic deformation at high temperature and pressure conditions of the deep Earth. In this study we focus on dislocations which are usually considered as the most efficient defects contributing to intracrystalline deformation. The influence of temperature is a key parameter in determining the behaviour of dislocations. We propose a model to describe the temperature-dependent mobility of dislocations based on a computational materials science approach, connecting the atomic to the grain scale. This provides elementary knowledge to both interpret seismic anisotropy and to improve geodynamic modelling. Here we focus on plastic deformation of the transition zone minerals wadsleyite and ringwoodite, dominating the boundary separating the upper from the lower mantle, a region over which the viscosity is thought to increase rapidly. Using the Peierls-Nabarro-Galerkin model enabled us to select potential glide planes, to predict the dislocation core structures and fundamental properties of both Mg2SiO4 high-pressure polymorphs integrating the non-elastic nature of dislocations from atomic scale based calculations. Macroscopic deformation results from the mobility of these distinct dislocations. High finite mantle temperatures activates unstable double-kink configurations on the dislocation line which allow the dislocation to move under stress. The original contribution of the present work is the formulation of a mobility law for dissociated dislocations as they occur in wadsleyite and ringwoodite. This permits us to predict the critical activation enthalpy required to overcome lattice friction associated to the onset of glide. From this, the effective glide velocities can be derived as a function of stress and temperature leading to the first lower bound estimates of transition zone viscosities

  16. Dislocation Density Tensor Characterization of Deformation Using 3D X-Ray Microscopy

    SciTech Connect

    Larson, Ben C; Tischler, Jonathan Zachary; El-Azab, Anter; Liu, Wenjun

    2008-01-01

    Three-dimensional (3D) X-ray microscopy with submicron resolution has been used to make spatially resolved measurements of lattice curvature and elastic strain over two-dimensional slices in thin deformed Si plates. The techniques and capabilities associated with white-beam 3D X-ray microscopy are discussed, and both theoretical and experimental considerations associated with the measurement of Nye dislocation density tensors in deformed materials are presented. The ability to determine the local geometrically necessary dislocation (GND) density in the form of a dislocation density tensor, with micron spatial resolution over mesoscopic length scales, is demonstrated. Results are shown for the special case of an elastically bent (dislocation free) thin Si plate and for a similar thin Si plate that was bent plastically, above the brittle-to-ductile transition temperature, to introduce dislocations. Within the uncertainties of the measurements, the known result that GND density is zero for elastic bending is obtained, and well-defined GND distributions are observed in the plastically deformed Si plate. The direct and absolute connection between experimental measurements of GND density and multiscale modeling and computer simulations of deformation microstructures is discussed to highlight the importance of submicron-resolution 3D X-ray microscopy for mesoscale characterization of material defects and to achieve a fundamental understanding of deformation in ductile materials.

  17. Dislocation density tensor characterization of deformation using 3D x-ray microscopy.

    SciTech Connect

    Larson, B. C.; Tischler, J. Z.; El-Azab, A.; Liu, W.; ORNL; Florida State Univ.

    2008-04-01

    Three-dimensional (3D) X-ray microscopy with submicron resolution has been used to make spatially resolved measurements of lattice curvature and elastic strain over two-dimensional slices in thin deformed Si plates. The techniques and capabilities associated with white-beam 3D X-ray microscopy are discussed, and both theoretical and experimental considerations associated with the measurement of Nye dislocation density tensors in deformed materials are presented. The ability to determine the local geometrically necessary dislocation (GND) density in the form of a dislocation density tensor, with micron spatial resolution over mesoscopic length scales, is demonstrated. Results are shown for the special case of an elastically bent (dislocation free) thin Si plate and for a similar thin Si plate that was bent plastically, above the brittle-to-ductile transition temperature, to introduce dislocations. Within the uncertainties of the measurements, the known result that GND density is zero for elastic bending is obtained, and well-defined GND distributions are observed in the plastically deformed Si plate. The direct and absolute connection between experimental measurements of GND density and multiscale modeling and computer simulations of deformation microstructures is discussed to highlight the importance of submicron-resolution 3D X-ray microscopy for mesoscale characterization of material defects and to achieve a fundamental understanding of deformation in ductile materials.

  18. Three-dimensional deformation analysis of two-phase dislocation substructures

    NASA Technical Reports Server (NTRS)

    Freed, Alan D.; Raj, S. V.; Walker, Kevin P.

    1992-01-01

    Three-dimensional deformation analysis of two-phase dislocation substructures was carried out, extending the Qian and Fan (1991) approach to 3D stress-strain fields by using the Budiansky and Wu (1962) criterion for strain compatibility between the 'hard' and 'soft' regions. The result is a rate-dependent viscoplastic theory, named the dislocation substructure viscoplasticity (DSV), which incorporates a self-consistent effect of dislocation substructure on material response. An algorithm developed for numerical implementation of the DSV theory is presented.

  19. Impact of dislocation cell elastic strain variations on line profiles from deformed copper.

    SciTech Connect

    Levine, L. E.; Larson, B. C.; Tischler, J. Z.; Geantil, P.; Kassner, M. E.; Liu, W.; Stoudt, M. R.; NIST; ORNL; Univ. of Southern California

    2008-01-01

    Energy scanned, sub-micrometer X-ray beams were used to obtain diffraction line profiles from individual dislocation cells in copper single crystals deformed in compression. Sub-micrometer depth resolution was provided by translating a wire through the diffracted beams and using triangulation to determine the depths of the diffracting volumes. Connection to classic volume-averaged results was made by adding the line profiles from 52 spatially resolved dislocation cell measurements. The resulting sub profile is smooth and symmetric, in agreement with early assumptions; the mean strain and full width half maximum are consistent with the average of the parameters extracted from the more exact individual dislocation cell measurements.

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

    SciTech Connect

    Kwon, J.; Bowers, M. L.; Brandes, M. C.; McCreary, V.; Robertson, Ian M.; Phani, P. Sudaharshan; Bei, H.; Gao, Y. F.; Pharr, George M.; George, Easo P.; Mills, M. J.

    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 in 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

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

    DOE PAGESBeta

    Kwon, J.; Bowers, M. L.; Brandes, M. C.; McCreary, V.; Robertson, Ian M.; Phani, P. Sudaharshan; Bei, H.; Gao, Y. F.; Pharr, George M.; George, Easo P.; et al

    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

  2. Single-dislocation-based deformation mechanisms in nanolayered composites

    NASA Astrophysics Data System (ADS)

    Misra, Amit

    2004-03-01

    Nanolayered metallic composites, composed of alternating layers of soft metals, are shown to possess several GPa level strengths, often within a factor of two to three of the theoretical strength limit, when the bilayer periods are on the order of a few nanometers. Experiments on model systems such as Cu-Ni and Cu-Nb indicate that, in the micron to sub-micron range, the strengths of these materials increase with microstructural refinement according to the Hall-Petch model. However, as the layer thickness is reduced to the nm-scale, the number of dislocations in a pile-up approaches unity and the pile-up based Hall-Petch model ceases to apply. In the few to a few tens of nanometers range, the increase in yield strength of nanolaminates with decreasing layer thickness is interpreted in terms of the confined layer slip of single dislocations. As layer thickness is decreased to a couple nanometers, strength reaches a peak with weak or no dependence on layer thickness. We show that the peak strength is determined by the stress needed to transmit single dislocations across the interface. Atomistic modeling is used to estimate the single dislocation interface crossing stress for coherent (Cu-Ni), incoherent (Cu-Nb) and nanoscale twinned (austenitic stainless steel 330) interfaces, and compared with experimental data. This work is supported by the U. S. Dept. of Energy, Office of Science, Office of Basic Energy Sciences.

  3. The Interaction of Dislocations and Radiation-Induced Obstacles at High-Strain Rate

    SciTech Connect

    Young, J A; Wirth, B D; Robach, J; Robertson, I

    2003-07-14

    Improved understanding of the plastic deformation of metals during high strain rate shock loading is key to predicting their resulting material properties. This paper presents the results of molecular dynamics simulations that identify the deformation modes of aluminum over a range of applied shear stresses and examines the interaction between dislocations and irradiation induced obstacles. These simulations show that while super-sonic dislocation motion can occur during impact loading, the finite dimensions of the materials render this motion transient. Larger applied loads do not stabilize supersonic dislocations, but instead lead an alternate deformation mode, namely twinning. Finally, the atomistic mechanisms that underlie the observed changes in the mechanical properties of metals as a function of irradiation are examined. Specifically, simulations of the interactions between moving edge dislocations and nanometer-sized helium bubbles provide insight into increases of the critical shear stresses but also reveal the effect of internal gas pressure on the deformation mode. The information gained in these studies provides fundamental insight into materials behavior, as well as important inputs for multi-scale models of materials deformation.

  4. Dislocation processes in the deformation of nanocrystalline aluminum by molecular-dynamics simulation.

    SciTech Connect

    Yamakov, V.; Wolf, D.; Phillpot, S. R.; Mukherjee, A. K.; Gleiter, H.; Materials Science Division; Univ. of California; Forschungszentrum Karlsruhe

    2002-09-01

    The mechanical behaviour of nanocrystalline materials (that is, polycrystals with a grain size of less than 100 nm) remains controversial. Although it is commonly accepted that the intrinsic deformation behaviour of these materials arises from the interplay between dislocation and grain-boundary processes, little is known about the specific deformation mechanisms. Here we use large-scale molecular-dynamics simulations to elucidate this intricate interplay during room-temperature plastic deformation of model nanocrystalline Al microstructures. We demonstrate that, in contrast to coarse-grained Al, mechanical twinning may play an important role in the deformation behaviour of nanocrystalline Al. Our results illustrate that this type of simulation has now advanced to a level where it provides a powerful new tool for elucidating and quantifying-in a degree of detail not possible experimentally-the atomic-level mechanisms controlling the complex dislocation and grain-boundary processes in heavily deformed materials with a submicrometre grain size.

  5. Dislocation processes in the deformation of nanocrystalline aluminium by molecular-dynamics simulation.

    PubMed

    Yamakov, Vesselin; Wolf, Dieter; Phillpot, Simon R; Mukherjee, Amiya K; Gleiter, Herbert

    2002-09-01

    The mechanical behaviour of nanocrystalline materials (that is, polycrystals with a grain size of less than 100 nm) remains controversial. Although it is commonly accepted that the intrinsic deformation behaviour of these materials arises from the interplay between dislocation and grain-boundary processes, little is known about the specific deformation mechanisms. Here we use large-scale molecular-dynamics simulations to elucidate this intricate interplay during room-temperature plastic deformation of model nanocrystalline Al microstructures. We demonstrate that, in contrast to coarse-grained Al, mechanical twinning may play an important role in the deformation behaviour of nanocrystalline Al. Our results illustrate that this type of simulation has now advanced to a level where it provides a powerful new tool for elucidating and quantifying--in a degree of detail not possible experimentally--the atomic-level mechanisms controlling the complex dislocation and grain-boundary processes in heavily deformed materials with a submicrometre grain size. PMID:12618848

  6. On the hierarchy of deformation processes in nanocrystalline alloys: Grain boundary mediated plasticity vs. dislocation slip

    NASA Astrophysics Data System (ADS)

    Schäfer, Jonathan; Stukowski, Alexander; Albe, Karsten

    2013-10-01

    Hybrid molecular dynamics and Monte-Carlo simulations on the deformation behavior of nanocrystalline Pd-Au are presented. A semi-grandcanonical Monte-Carlo scheme is employed during straining to allow for local relaxation by chemical equilibration and to effectively shortcut diffusional processes. Altering the balance between an imposed straining and local relaxation reveals a strong correlation of the irreversible plastic deformation and the frequency of local relaxation. Using a novel method to quantify the amount of crystal slip strain from atomistic data, it is demonstrated how plastic deformation carried by dislocations changes as a function of the local relaxation. The results indicate that conventional molecular dynamics simulations overestimate the contributions of dislocation slip to the overall plastic deformation of nanocrystalline samples.

  7. 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.

  8. Dislocation distributions in an Al-4. 5% Mg alloy during superplastic deformation

    SciTech Connect

    Li, F.; Roberts, W.T.; Bate, P.S. . IRC in Materials for High Performance Applications)

    1993-10-01

    Superplasticity has been studied actively for several decades. Many models of the phenomenon have been proposed, in most of which normal dislocation glide has either not been considered or been considered only as an accommodation process to maintain continuity during grain boundary sliding. Although evidence of dislocation activity during superplastic deformation has been observed several times by TEM studies following deformation, the published results appear to be inconsistent and occasionally contradict themselves, and so are unable to provide systematic and convincing evidence for the development of any theoretical model. A major experimental problem associated with those results is the delay between unloading and quenching the specimens from the high testing temperatures. Dislocations can be lost by relaxation and annealing during the unloading and also during the preparation of TEM thin foils. A facility was developed to preserve the high temperature deformation microstructure for consequent TEM examination by quenching and aging under constant stress. This technique and some preliminary results are given elsewhere. The work presented here is from a more comprehensive TEM study of the dislocation distributions in an Al-4.5%Mg alloy developed during deformation at an elevated temperature and a range of strain rates.

  9. PREFACE: DISLOCATIONS 2008 An International Conference on the Fundamentals of Plastic Deformation

    NASA Astrophysics Data System (ADS)

    Cai, Wei; Edagawa, Keiichi; Ngan, Alfonso H. W.

    2009-07-01

    Crystal dislocations are fundamental carriers of plastic deformation in crystalline materials and a good understanding of their behavior forms the scientific basis for improving our ability to manufacture new materials of high mechanical strength. After their initial proposal in 1934, crystal dislocations have become the focus of intense research following their first direct observation in a transmission electron microscope in the 1950's. The last decade has witnessed a new burst of interest and activity in the field, driven in part by the desire to understand the fundamental mechanisms of plasticity in materials at smaller scales, and enabled by significant breakthroughs in both experimental and computational tools. To facilitate information exchange and academic discussions, the ''Dislocations 200X'' international conference series was established in 2000 and has since become the major world forum on crystal dislocations taking place every four years on average. The relatively long period between the conferences leaves sufficient time for new ideas to develop and become worthy of a wide discussion. An important goal of the ''Dislocations 200X'' series is to encourage young researchers to engage in direct communications with the scientists of older generations. The scope of the ''Dislocations 200X'' conference includes fundamental research on dislocations in all types of materials and their role in plasticity. The ''Dislocations 2008'', the third in the series, was held in Hong Kong, 13-17 October 2008. The choice of this conference site reflects the desire to strengthen the connection between scientists on the continents of North America, Europe and Asia. Over 60 talks and 60 posters were presented at the Dislocations 2008 conference. Some presentations have led to papers that are published in this volume after review. Editors: Wei Cai (Stanford University, USA) Keiichi Edagawa (Tokyo University, Japan) Alfonso H W Ngan (Hong Kong University, China)

  10. 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.

  11. Discrete dislocation plasticity analysis of contact between deformable bodies of simple geometry

    NASA Astrophysics Data System (ADS)

    Siang, Kelvin Ng Wei; Nicola, Lucia

    2016-05-01

    A contact mechanical model is presented where both metal bodies can deform by discrete dislocation plasticity. The model intends to improve on previous dislocation dynamics models of contact, where only a plastically deformable body was considered, flattened by a rigid platen. The effect of the rigid platen was mimicked through boundary conditions acting on the deformable body. While the formulation is general, the simulations presented here are only performed for contact between a plastically deforming body with sinusoidal surface and a flat body that is either elastic or rigid. Results show that the contact conditions, i.e. frictionless and full stick, affect the morphology of the contact as well as the contact pressure distribution. This is because dislocations can glide through the frictionless contact and fragment it, but do not penetrate a sticking contact. Average quantities like mean apparent contact pressure and total plastic slip are, instead, independent of contact conditions and of the details of the contact area. A size dependence is observed in relation to the onset of plastic deformation, where surfaces with smaller wavelength and amplitude require a larger contact pressure to yield than self similar surfaces with larger wavelength. The size dependence is very pronounced when the flat body is rigid, but fades when the compliance of the flat body is large.

  12. Dislocation

    MedlinePlus

    ... likely to happen again. Follow-up with an orthopedic surgeon is recommended after a dislocation. ... SE, Mehta A, Maddow C, Luber SD. Critical orthopedic skills and procedures. Emergency Medicine Clinics of North ...

  13. Dislocation-induced superfluidity in a model supersolid

    NASA Astrophysics Data System (ADS)

    Dasbiswas, Kinjal; Goswami, Debajit; Yoo, Chi-Deuk; Dorsey, Alan

    2010-03-01

    The effect of an edge dislocation in inducing superfluidity is explored by coupling the elastic strain field of the dislocation to the superfluid density, and solving the corresponding Ginzburg-Landau theory. It is shown that superfluid density is induced along a single dislocation below a critical temperature determined by the ground state solution of a 2D Schr"odinger equation with a dipolar potential. This superfluid behavior can be described by a 1D Ginzburg-Landau equation obtained through a weakly nonlinear analysis. We then extend our analysis to a network of dislocation lines considered before by Shevchenko and Toner, which could serve as a model for superflow through solid ^4He. The effect of fluctuations and dynamics are included through a full time dependent Ginzburg-Landau theory.

  14. Transmission electron microscopy of dislocations in cementite deformed at high pressure and high temperature

    NASA Astrophysics Data System (ADS)

    Mussi, A.; Cordier, P.; Ghosh, S.; Garvik, N.; Nzogang, B. C.; Carrez, Ph.; Garruchet, S.

    2016-06-01

    Polycrystalline aggregates of cementite (Fe3C) and (Fe,Ni)3C have been synthesised at 10 GPa and 1250 °C in the multianvil apparatus. Further, deformation of the carbides by stress relaxation has been carried out at temperature of 1250 °C and for 8 h at the same pressure. Dislocations have been characterised by transmission electron microscopy. They are of the [1 0 0] and [0 0 1] type, [1 0 0] being the most frequent. [1 0 0] dislocations are dissociated and glide in the (0 1 0) plane. [0 0 1] dislocations glide in (1 0 0) and (0 1 0). Given the plastic anisotropy of cementite, the morphology of the lamellae in pearlitic steels appears to have a major role in the strengthening role played by this phase, since activation of easy slip systems is geometrically inhibited in most cases.

  15. Dynamic instability of dislocation motion at high-strain-rate deformation of crystals with high concentration of point defects

    NASA Astrophysics Data System (ADS)

    Malashenko, V. V.

    2015-12-01

    The motion of an ensemble of edge dislocations has been studied under conditions of high-strainrate deformation of a crystal with a high concentration of point defects. The conditions of existence of the region of dynamic instability of dislocation motion have been found. It has been shown that the existence of the region and its boundaries is determined by the proportion of the point defect concentration and the dislocation density.

  16. Dislocation model for aseismic crustal deformation at Hollister, California

    NASA Technical Reports Server (NTRS)

    Matsuura, Mitsuhiro; Jackson, David D.; Cheng, Abe

    1986-01-01

    A model of crustal deformation during the interseismic phase is developed and applied (using the improved Bayesian inversion algorithm described by Jackson and Matsu'ura, 1985) to trilateration data for the USGS Hollister (CA) network. In the model, rigid blocks in motion relative to each other experience friction only in a brittle upper zone, while their ductile lower zones slide freely; the Hollister model comprises five blocks and nine rectangular fault patches. The data and results are presented in tables, graphs, and maps and characterized in detail. The model predicts steady block motion on time scales between 10 yr and 1 Myr, with net motion across the San Andreas/Calaveras fault system 38 + or - 3 mm/yr and brittle/ductile transition depths ranging from 0.4 to 11 km. Two San Andreas segments with higher probabilities of moderate-to-large earthquakes are identified.

  17. Dislocations

    MedlinePlus

    ... Attempting to move or jam a dislocated bone back in can damage blood vessels, muscles, ligaments, and nerves. Apply an ice pack. Ice can ease swelling and pain in and around the joint. Use ibuprofen or acetaminophen for pain. Think Prevention! Make sure kids wear the appropriate safety gear ...

  18. Transmission electron microscopy characterization of the dislocations of phase A deformed at 11 GPa, 400°C in the multianvil apparatus

    NASA Astrophysics Data System (ADS)

    Cordier, P.; Mussi, A.; Frost, D. J.

    2011-12-01

    Several dense hydrous magnesium silicate (DHMS) phases have been identified as possible carriers for water in subducting slabs. The style and distribution of deformation in subduction zones depends on the nature and rheological properties of subducted materials. If we have reasonable estimates of the rheological properties of anhydrous high-pressure phases, the properties of DHMS are largely unconstrained. In this study, we investigate the deformation mechanisms of phase A (Mg7Si2H6O14) which is hexagonal ( a = 7.86 Å and c = 9.57 Å). Phase A has been synthesized in quasi-hydrostatic conditions from high-purity oxides at 11 GPa, 900°C in a multi-anvil apparatus. After synthesis, the high-pressure phase was recovered and placed in another high-pressure cell designed to induce deviatoric stresses during the compression at 11 GPa, 400°C, i.e. at a temperature lower to the one investigated previously (700°C). Electron transparent thin foils suitable for TEM have been prepared by ion milling at liquid nitrogen temperature. To prevent electron beam damage, TEM characterizations were carried out under low illumination conditions, with a 300 kV accelerating voltage microscope (a Philips° CM30), and a Gatan° cold stage (liquid nitrogen temperature). The grain size of the microstructure is approximately 8 ± 2 μm, with very few sub-grains and the dislocation density is in the order of 2.1013 m-2. At this temperature, we observe many dissociated dislocations with extended stacking faults (dissociation width of the order of 1 μm). Supposing that dislocations are in glide conditions, and using the Ishida's method, we could identify: Partial 1/3 <1 -1 0 0> dislocations and perfect 1/3 <1 1 -2 0> dislocations (in equivalent proportion), in the (0 0 0 1) plane; Partial 1/3 <1 -1 0 0> and 1/3 <-1 1 0 3> dislocations, resulting from the dissociation of 1/3 <2 -1 -1 3> dislocations, and to a lesser extend perfect 1/3 <2 -1 -1 3> dislocations on the {1 -2 1 -1} plane

  19. Direct measurement of hydrogen dislocation pipe diffusion in deformed polycrystalline Pd using quasielastic neutron scattering.

    PubMed

    Heuser, Brent J; Trinkle, Dallas R; Jalarvo, Niina; Serio, Joseph; Schiavone, Emily J; Mamontov, Eugene; Tyagi, Madhusudan

    2014-07-11

    The temperature-dependent diffusivity D(T) of hydrogen solute atoms trapped at dislocations-dislocation pipe diffusion of hydrogen-in deformed polycrystalline PdH(x) (x∼10(-3)  [H]/[Pd]) has been quantified with quasielastic neutron scattering between 150 and 400 K. We observe diffusion coefficients for trapped hydrogen elevated by one to two orders of magnitude above bulk diffusion. Arrhenius diffusion behavior has been observed for dislocation pipe diffusion and regular bulk diffusion, the latter in well-annealed polycrystalline Pd. For regular bulk diffusion of hydrogen in Pd we find D(T)=D(0)exp(-E(a)/kT)=0.005exp(-0.23  eV/kT)  cm(2)/s, in agreement with the known diffusivity of hydrogen in Pd. For hydrogen dislocation pipe diffusion we find D(T)≃10(-5)exp(-E(a)/kT)  cm(2)/s, where E(a)=0.042 and 0.083 eV for concentrations of 0.52×10(-3) and 1.13×10(-3)[H]/[Pd], respectively. Ab initio computations provide a physical basis for the pipe diffusion pathway and confirm the reduced barrier height. PMID:25062206

  20. Deformation localization and dislocation channel dynamics in neutron-irradiated austenitic stainless steels

    DOE PAGESBeta

    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

  1. Deformation localization and dislocation channel dynamics in neutron-irradiated austenitic stainless steels

    NASA Astrophysics Data System (ADS)

    Gussev, Maxim N.; Field, Kevin G.; Busby, Jeremy T.

    2015-05-01

    The dynamics of deformation localization and dislocation channel formation were investigated 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 (TEM). Channel formation was observed at ∼70% of the polycrystalline yield stress of the irradiated materials (σ0.2). It was shown that triple junction points do not always serve as a source of dislocation channels; at stress levels below the σ0.2, channels often formed near the middle of the grain boundary. For a single grain, the role of elastic stiffness value (Young's modulus) in 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. 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. In the AISI 304 steel, channels in grains oriented close to <0 0 1>||TA (tensile axis) and <1 0 1>||TA were twin free and grain with <1 1 1>||TA and grains oriented close to a Schmid factor maximum contained deformation twins.

  2. Deformation localization and dislocation channel dynamics in neutron-irradiated austenitic stainless steels

    SciTech Connect

    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 a 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.

  3. Probing deformation substructure by synchrotron X-ray diffraction and dislocation dynamics modelling.

    PubMed

    Korsunsky, Alexander M; Hofmann, Felix; Song, Xu; Eve, Sophie; Collins, Steve P

    2010-09-01

    Materials characterization at the nano-scale is motivated by the desire to resolve the structural aspects and deformation behavior at length scales relevant to those mechanisms that define the novel and unusual properties of nano-structured materials. A range of novel techniques has recently become accessible with the help of synchrotron X-ray beams that can be focused down to spot sizes of less than a few microns on the sample. The unique combination of tunability (energy selection), parallelism and brightness of synchrotron X-ray beams allows their use for high resolution diffraction (determination of crystal structure and transformations, analysis of dislocation sub-structures, orientation and texture analysis, strain mapping); small angle X-ray scattering (analysis of nano-scale voids and defects; orientation analysis) and imaging (radiography and tomography). After a brief review of the state-of-the-art capabilities for monochromatic and white beam synchrotron diffraction, we consider the usefulness of these techniques for the task of bridging the gap between experiment and modeling. Namely, we discuss how the experiments can be configured to provide information relevant to the validation and improvement of modeling approaches, and also how the results of various simulations can be post-processed to improve the possibility of (more or less) direct comparison with experiments. Using the example of some recent experiments carried out on beamline 116 at Diamond Light Source near Oxford, we discuss how such experimental results can be interpreted in view and in conjunction with numerical deformation models, particularly those incorporating dislocation effects, e.g., finite-element based pseudo-continuum strain gradient formulations, and discrete dislocation simulations. Post-processing of FE and discrete dislocation simulations is described, illustrating the kind of information that can be extracted from comparisons between modeling and experimental data. PMID

  4. Dislocation decorrelation and relationship to deformation microtwins during creep of a y' precipitate strengthened Ni-based superalloy

    SciTech Connect

    Unocic, R. R.; Zhou, N.; Kovarik, Libor; Shen, C.; Wang, Y.; Mills, M. J.

    2011-11-01

    The evolution of microtwins during high temperature creep deformation in a strengthened Ni-base superalloy has been investigated through a combination of creep testing, TEM characterization, theoretical modeling and computer simulation. Experimentally, microtwin nucleation sources were identified and their evolution was tracked by characterizing the deformation substructure at different stages of the creep deformation. Initially, deformation is highly localized around stress concentrators such as carbides, borides and serrated grain boundaries, which act as sources of a/2<110> matrix type dislocations. Due to microstructural effects such as fine channels between particles and low matrix stacking fault energies, the a/2<110> matrix dislocations dissociate into a/6<112> Shockley partials, which were commonly observed to be decorrelated from one another, creating extended intrinsic stacking faults in the matrix. As deformation progress further, microtwins form via partial dislocations cooperatively shearing both and phases on adjacent {111} glide planes. The TEM observations lead directly to an analysis of dislocation-precipitate interactions. Through phase field simulations and theoretical analysis based on Orowan looping, the important processes of dislocation dissociation and decorrelation are modeled in detail, providing comprehensive insight into the microstructural features and applied stress conditions that favor the microtwinning deformation mode in strengthened Ni-based superalloys.

  5. Electronic-structure study of an edge dislocation in Aluminum and the role of macroscopic deformations on its energetics

    NASA Astrophysics Data System (ADS)

    Iyer, Mrinal; Radhakrishnan, Balachandran; Gavini, Vikram

    2015-03-01

    We employed a real-space formulation of orbital-free density functional theory using finite-element basis to study the defect-core and energetics of an edge dislocation in Aluminum. Our study shows that the core-size of a perfect edge dislocation is around ten times the magnitude of the Burgers vector. This finding is contrary to the widely accepted notion that continuum descriptions of dislocation energetics are accurate beyond ∼1-3 Burgers vector from the dislocation line. Consistent with prior electronic-structure studies, we find that the perfect edge dislocation dissociates into two Shockley partials with a partial separation distance of 12.8 Å. Interestingly, our study revealed a significant influence of macroscopic deformations on the core-energy of Shockley partials. We show that this dependence of the core-energy on macroscopic deformations results in an additional force on dislocations, beyond the Peach-Koehler force, that is proportional to strain gradients. Further, we demonstrate that this force from core-effects can be significant and can play an important role in governing the dislocation behavior in regions of inhomogeneous deformations.

  6. Multifunctional Alloys Obtained via a Dislocation-Free Plastic Deformation Mechanism.

    PubMed

    Saito, Takashi; Furuta, Tadahiko; Hwang, Jung-Hwan; Kuramoto, Shigeru; Nishino, Kazuaki; Suzuki, Nobuaki; Chen, Rong; Yamada, Akira; Ito, Kazuhiko; Seno, Yoshiki; Nonaka, Takamasa; Ikehata, Hideaki; Nagasako, Naoyuki; Iwamoto, Chihiro; Ikuhara, Yuuichi; Sakuma, Taketo

    2003-04-18

    We describe a group of alloys that exhibit "super" properties, such as ultralow elastic modulus, ultrahigh strength, super elasticity, and super plasticity, at room temperature and that show Elinvar and Invar behavior. These "super" properties are attributable to a dislocation-free plastic deformation mechanism. In cold-worked alloys, this mechanism forms elastic strain fields of hierarchical structure that range in size from the nanometer scale to several tens of micrometers. The resultant elastic strain energy leads to a number of enhanced material properties. PMID:12702870

  7. Dislocation decorrelation and relationship to deformation microtwins during creep of a precipitate strengthened Ni-based superalloy

    SciTech Connect

    Unocic, Raymond R; Zhou, Ning; Kovarik, Libor; Shen, Chen; Wang, Yunzhi; Mills, Michael J.

    2011-01-01

    The evolution of microtwins during high temperature creep deformation in a strengthened Ni-base superalloy has been investigated through a combination of creep testing, transmission electron microscopy (TEM), theoretical modeling, and computer simulation. Experimentally, microtwin nucleation sources were identified and their evolution was tracked by characterizing the deformation substructure at different stages of creep deformation. Deformation is highly localized around stress concentrators such as carbides, borides and serrated grain boundaries, which act as sources of a/2<110> matrix-type dislocations. Due to fine channels between particles, coupled with the low matrix stacking fault energy, the a/2<110> matrix dislocations dissociate into a/6<112> Shockley partials, which were commonly observed to be decorrelated from one another, creating extended intrinsic stacking faults in the matrix. Microtwins are common and form via Shockley partial dislocations cooperatively shearing both and phases on adjacent {111} glide planes. The TEM observations lead directly to an analysis of dislocation-precipitate interactions. Through phase field simulations and theoretical analyses based on Orowan looping, the important processes of dislocation dissociation and decorrelation are modeled in detail, providing comprehensive insight into the microstructural features and applied stress conditions that favor the microtwinning deformation mode in strengthened Ni-based superalloys.

  8. Role of the static and dynamic aging of dislocations in the kinetics of deformation of doped crystals

    NASA Astrophysics Data System (ADS)

    Petukhov, B. V.

    2014-06-01

    A method has been developed for calculating yield stresses in doped crystals and solid solutions with inclusion of the static and dynamic aging of dislocations. An analytical description of the kinetics of deformation of these materials in the vicinity of the upper yield stress has been proposed. The stage character of stress-strain curves due to the dislocation immobilization by impurity atmospheres formed around dislocations has been demonstrated. The theory is applicable to the calculation of the impurity and solid solution hardening of semiconductors, metals with body-centered cubic (bcc) structure, intermetallic compounds, and other materials.

  9. Effect of plastic deformation on the magnetic properties and dislocation luminescence of isotopically enriched silicon {sup 29}Si:B

    SciTech Connect

    Koplak, O. V.; Shteynman, E. A.; Tereschenko, A. N.; Morgunov, R. B.

    2015-09-15

    A correlation between the temperature dependences of the D1-line intensity of dislocation luminescence and the magnetic moment of plastically deformed isotopically enriched crystals {sup 29}Si:B is found. It is established that the magnetic susceptibility of the deformed crystals obtained by integration of the spectra of electron spin resonance and the D1-line intensity undergo similar nonmonotonic variations with temperature varying in the range of 20–32 K.

  10. Dislocation creep accommodated Grain Boundary Sliding: A high strain rate/low temperature deformation mechanism in calcite ultramylonites

    NASA Astrophysics Data System (ADS)

    Rogowitz, Anna; Grasemann, Bernhard

    2014-05-01

    Grain boundary sliding (GBS) is an important grain size sensitive deformation mechanism that is often associated with extreme strain localization and superplasticity. Another mechanism has to operate simultaneously to GBS in order to prevent overlaps and voids between sliding grains. One of the most common accommodating mechanisms is diffusional creep but, recently, dislocation creep has been reported to operate simultaneous to GBS. Due to the formation of a flanking structure in nearly pure calcite marble on Syros (Cyclades, Greece) at lower greenschist facies conditions, an extremely fine grained ultramylonite developed. The microstructure of the layer is characterized by (1) calcite grains with an average grain size of 3.6 µm (developed by low temperature/high strain rate grain boundary migration recrystallization, BLG), (2) grain boundary triple junctions with nearly 120° angles and (3) small cavities preferentially located at triple junctions and at grain boundaries in extension. These features suggest that the dominant deformation mechanism was GBS. In order to get more information on the accommodation mechanism detailed microstructural and textural analyses have been performed on a FEI Quanta 3D FEG instrument equipped with an EDAX Digiview IV EBSD camera. The misorientation distribution curves for correlated and uncorrelated grains follow almost perfect the calculated theoretical curve for a random distribution, which is typical for polycrystalline material deformed by GBS. However, the crystallographic preferred orientation indicates that dislocation creep might have operated simultaneously. We also report Zener-Stroh cracks resulting from dislocation pile up, indicating that dislocation movement was active. We, therefore, conclude that the dominant deformation mechanism was dislocation creep accommodated grain boundary sliding. This is consistent with the observed grain size range that plots at the field boundary between grain size insensitive and grain

  11. Stochastic dislocation kinetics and fractal structures in deforming metals probed by acoustic emission and surface topography measurements

    SciTech Connect

    Vinogradov, A.; Yasnikov, I. S.; Estrin, Y.

    2014-06-21

    We demonstrate that the fractal dimension (FD) of the dislocation population in a deforming material is an important quantitative characteristic of the evolution of the dislocation structure. Thus, we show that peaking of FD signifies a nearing loss of uniformity of plastic flow and the onset of strain localization. Two techniques were employed to determine FD: (i) inspection of surface morphology of the deforming crystal by white light interferometry and (ii) monitoring of acoustic emission (AE) during uniaxial tensile deformation. A connection between the AE characteristics and the fractal dimension determined from surface topography measurements was established. As a common platform for the two methods, the dislocation density evolution in the bulk was used. The relations found made it possible to identify the occurrence of a peak in the median frequency of AE as a harbinger of plastic instability leading to necking. It is suggested that access to the fractal dimension provided by AE measurements and by surface topography analysis makes these techniques important tools for monitoring the evolution of the dislocation structure during plastic deformation—both as stand-alone methods and especially when used in tandem.

  12. Dislocation structure in AlN films induced by in situ transmission electron microscope nanoindentation

    NASA Astrophysics Data System (ADS)

    Tokumoto, Yuki; Kutsukake, Kentaro; Ohno, Yutaka; Yonenaga, Ichiro

    2012-11-01

    To elucidate dislocation generation and propagation processes in AlN films containing a high density of grown-in threading dislocations (TDs), in situ nanoindentation (NI) was performed in a transmission electron microscope at room temperature. Dislocations with the Burgers vector b = 1/3<12¯10> were introduced not only on the primary slip plane, i.e., the (0001) basal planes, but also on the {101¯1} and {101¯2} pyramidal planes. The results are explained by considering the distribution of the resolved shear stress. It was found that the dislocations induced by NI interact with grown-in TDs: (1) for the NI-induced dislocations on pyramidal planes, edge grown-in TDs induce cross slip to basal planes, and (2) for the NI-induced dislocations on basal planes, screw grown-in TDs prevent their propagation, while edge grown-in TDs do not.

  13. Hydrogen induced plastic deformation of stainless steel

    SciTech Connect

    Gadgil, V.J.; Keim, E.G.; Geijselaers, H.J.M.

    1998-12-31

    Hydrogen can influence the behavior of materials significantly. The effects of hydrogen are specially pronounced in high fugacities of hydrogen which can occur at the surface of steels in contact with certain aqueous environments. In this investigation the effect of high fugacity hydrogen on the surface of stainless steel was investigated using electrochemical cathodic charging. Microhardness was measured on the cross section. Transmission electron microscopy was used to investigate the dislocation substructure just below the surface. Computer simulation using finite element method was carried out to estimate the extent and severity of the deformation. The significance of the results are discussed in relation to the loss of ductility due to hydrogen.

  14. Flux line lattice in deformed superconductor Nb-Mo: Collective pinning and dielastic interaction with screw dislocations

    SciTech Connect

    Grigoryeva, I.V.; Vinnikov, L.Y.

    1989-01-01

    A detailed investigation of the flux line lattice (FLL) interaction with nonuniformly distributed, mainly screw dislocations in deformed Nb-Mo single crystals has been made using the high resolution technique of decorating a sample by small Fe particles. The interaction has been considered from the point of view both of elementary pinning force summation and of the particular pinning mechanism. The FLL has been observed to split into short-range order regions at a low dislocation density and to be amorphous at densities exceeding 10/sup 10/ cm/sup /minus/2/. The dimensions of these regions determine the critical current value in agreement with the collective pinning theory. The observed distribution of the flux lines has been shown to result from the dielastic /Delta/E-interaction of the dislocations with the FLL.

  15. Spatial organization of plastic deformation in single crystals with different structure of slip dislocation

    SciTech Connect

    Kunitsyna, T. S.; Teplyakova, L. A. Koneva, N. A.; Poltaranin, M. A.

    2015-10-27

    It is established that different structure of slip dislocation at the end of the linear hardening stage results in different distribution of dislocation charges in the volume of a single crystal. In the alloy with a near atomic order the slip of single dislocations leads to formation of planar structures—layers with the excess density of dislocations. In the alloy with long-range atomic order the slip of superdislocations brings the formation of the system of parallel rod-like charged dislocation linking.

  16. Deformation-Induced Anisotropy of Polymers

    NASA Technical Reports Server (NTRS)

    Peng, S. T. J.; Landel, R. F.

    1982-01-01

    New theory calculates anisotropies induced by large deformations in polymers. Theory was developed primarily for calculating anistropy of thermal expansivity, but is also applicable to thermal conductivity, elastic moduli and other properties. Theory assumes that in isotropic state, long polymer chains are randomly coiled and not oriented in particular direction. They acquire an orientation when material is deformed. As average molecular orientation increases with deformation, properties of bulk material exhibit averaging of the microscopic anistropies of the oriented molecular segments.

  17. Ion irradiation induced disappearance of dislocations in a nickel-based alloy

    NASA Astrophysics Data System (ADS)

    Chen, H. C.; Li, D. H.; Lui, R. D.; Huang, H. F.; Li, J. J.; Lei, G. H.; Huang, Q.; Bao, L. M.; Yan, L.; Zhou, X. T.; Zhu, Z. Y.

    2016-06-01

    Under Xe ion irradiation, the microstructural evolution of a nickel based alloy, Hastelloy N (US N10003), was studied. The intrinsic dislocations are decorated with irradiation induced interstitial loops and/or clusters. Moreover, the intrinsic dislocations density reduces as the irradiation damage increases. The disappearance of the intrinsic dislocations is ascribed to the dislocations climb to the free surface by the absorption of interstitials under the ion irradiation. Moreover, the in situ annealing experiment reveals that the small interstitial loops and/or clusters induced by the ion irradiation are stable below 600 °C.

  18. Post-irradiation plastic deformation in bcc Fe grains investigated by means of 3D dislocation dynamics simulations

    NASA Astrophysics Data System (ADS)

    Gururaj, K.; Robertson, C.; Fivel, M.

    2015-04-01

    Post-irradiation tensile straining is investigated by means of three-dimensional dislocation dynamics simulations adapted to body centred cubic Fe. Namely, 1 μm Fe grains are strained at various temperatures in the 100-300 K range, in absence and in presence of radiation-induced defect dispersions. The defect-induced hardening is consistent with the disperse barrier effect up to 5 ×1021m-3 loops and is weakly dependent on the straining temperature. The dislocation-loops interaction rate augments with the accumulated plastic strain, loop density and strength; while it is mainly independent of the number of active slip systems and thermally activated screw dislocation mobility. An additional, radiation-induced hardening mechanism known as dislocation "decoration" is also implemented and tested for comparison. Those results show that the plastic flow localisation transition depends on the total yield point rise rather than on the lone, dispersed loop density. The simulation results are then rationalized through an original micro-mechanical model relating the grain-scale stress-strain behaviour to dislocation sub-structure formation and spreading. That model combines strain dependent and strain independent hardening mechanisms, which both contribute to the associated stress-strain response and plastic flow spreading.

  19. Heavy ion irradiation induced dislocation loops in AREVA's M5® alloy

    NASA Astrophysics Data System (ADS)

    Hengstler-Eger, R. M.; Baldo, P.; Beck, L.; Dorner, J.; Ertl, K.; Hoffmann, P. B.; Hugenschmidt, C.; Kirk, M. A.; Petry, W.; Pikart, P.; Rempel, A.

    2012-04-01

    Pressurized water reactor (PWR) Zr-based alloy structural materials show creep and growth under neutron irradiation as a consequence of the irradiation induced microstructural changes in the alloy. A better scientific understanding of these microstructural processes can improve simulation programs for structural component deformation and simplify the development of advanced deformation resistant alloys. As in-pile irradiation leads to high material activation and requires long irradiation times, the objective of this work was to study whether ion irradiation is an applicable method to simulate typical PWR neutron damage in Zr-based alloys, with AREVA's M5® alloy as reference material. The irradiated specimens were studied by electron backscatter diffraction (EBSD), positron Doppler broadening spectroscopy (DBS) and in situ transmission electron microscopy (TEM) at different dose levels and temperatures. The irradiation induced microstructure consisted of - and -type dislocation loops with their characteristics corresponding to typical neutron damage in Zr-based alloys; it can thus be concluded that heavy ion irradiation under the chosen conditions is an excellent method to simulate PWR neutron damage.

  20. 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.

  1. A novel unified dislocation density-based model for hot deformation behavior of a nickel-based superalloy under dynamic recrystallization conditions

    NASA Astrophysics Data System (ADS)

    Lin, Y. C.; Wen, Dong-Xu; Chen, Ming-Song; Chen, Xiao-Min

    2016-09-01

    In this study, a novel unified dislocation density-based model is presented for characterizing hot deformation behaviors in a nickel-based superalloy under dynamic recrystallization (DRX) conditions. In the Kocks-Mecking model, a new softening item is proposed to represent the impacts of DRX behavior on dislocation density evolution. The grain size evolution and DRX kinetics are incorporated into the developed model. Material parameters of the developed model are calibrated by a derivative-free method of MATLAB software. Comparisons between experimental and predicted results confirm that the developed unified dislocation density-based model can nicely reproduce hot deformation behavior, DRX kinetics, and grain size evolution in wide scope of initial grain size, strain rate, and deformation temperature. Moreover, the developed unified dislocation density-based model is well employed to analyze the time-variant forming processes of the studied superalloy.

  2. A second-order phase-transformation of the dislocation structure during plastic deformation determined by in situ synchrotron X-ray diffraction

    SciTech Connect

    Schafler, E.; Simon, K.; Bernstorff, S.; Tichy, G.; Ungar, T. . E-mail: ungar@ludens.elte.hu; Zehetbauer, M.J.

    2005-01-10

    In situ X-ray diffraction peak profile analysis during plastic deformation in [0 0 1] oriented copper single crystals was carried out using synchrotron radiation. Characteristic changes of the hardening coefficient indicate that a transition occurs from stage III to stage IV which has been observed for the first time in a single crystal under low temperature deformation conditions. The long-range internal stresses, the dislocation arrangement parameters and the fluctuations of the dislocation density show non-monotonous changes at this transition suggesting that the dislocation structure, especially within the cell-wall regions, reveals a second-order phase transition. A microscopic dislocation model is introduced which not only illustrates the break of symmetry, but also describes well the development of new grains ('fragmentation') during plastic deformation.

  3. Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth’s upper mantle

    PubMed Central

    Ohuchi, Tomohiro; Kawazoe, Takaaki; Higo, Yuji; Funakoshi, Ken-ichi; Suzuki, Akio; Kikegawa, Takumi; Irifune, Tetsuo

    2015-01-01

    Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth’s upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 1019.6 to 1020.7 Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size–sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle. PMID:26601281

  4. Spiderweb deformation induced by electrostatically charged insects

    PubMed Central

    Ortega-Jimenez, Victor Manuel; Dudley, Robert

    2013-01-01

    Capture success of spider webs has been associated with their microstructure, ornamentation, and wind-induced vibrations. Indirect evidence suggests that statically charged objects can attract silk thread, but web deformations induced by charged insects have not yet been described. Here, we show under laboratory conditions that electrostatically charged honeybees, green bottle flies, fruit flies, aphids, and also water drops falling near webs of cross-spiders (Araneus diadematus) induce rapid thread deformation that enhances the likelihood of physical contact, and thus of prey capture. PMID:23828093

  5. Spiderweb deformation induced by electrostatically charged insects

    NASA Astrophysics Data System (ADS)

    Ortega-Jimenez, Victor Manuel; Dudley, Robert

    2013-07-01

    Capture success of spider webs has been associated with their microstructure, ornamentation, and wind-induced vibrations. Indirect evidence suggests that statically charged objects can attract silk thread, but web deformations induced by charged insects have not yet been described. Here, we show under laboratory conditions that electrostatically charged honeybees, green bottle flies, fruit flies, aphids, and also water drops falling near webs of cross-spiders (Araneus diadematus) induce rapid thread deformation that enhances the likelihood of physical contact, and thus of prey capture.

  6. Fields induced by three-dimensional dislocation loops in anisotropic magneto-electro-elastic bimaterials

    NASA Astrophysics Data System (ADS)

    Han, Xueli; Pan, Ernie; Sangghaleh, Ali

    2013-08-01

    The coupled elastic, electric and magnetic fields produced by an arbitrarily shaped three-dimensional dislocation loop in general anisotropic magneto-electro-elastic (MEE) bimaterials are derived. First, we develop line-integral expressions for the fields induced by a general dislocation loop. Then, we obtain analytical solutions for the fields, including the extended Peach-Koehler force, due to some useful dislocation segments such as straight line and elliptic arc. The present solutions contain the piezoelectric, piezomagnetic and purely elastic solutions as special cases. As numerical examples, the fields induced by a square and an elliptic dislocation loop in MEE bimaterials are studied. Our numerical results show the coupling effects among different fields, along with various interesting features associated with the dislocation and interface.

  7. The breakdown of superlubricity by driving-induced commensurate dislocations

    PubMed Central

    Benassi, A.; Ma, Ming; Urbakh, M.; Vanossi, A.

    2015-01-01

    In the framework of a Frenkel-Kontorova-like model, we address the robustness of the superlubricity phenomenon in an edge-driven system at large scales, highlighting the dynamical mechanisms leading to its failure due to the slider elasticity. The results of the numerical simulations perfectly match the length critical size derived from a parameter-free analytical model. By considering different driving and commensurability interface configurations, we explore the distinctive nature of the transition from superlubric to high-friction sliding states which occurs above the critical size, discovering the occurrence of previously undetected multiple dissipative jumps in the friction force as a function of the slider length. These driving-induced commensurate dislocations in the slider are then characterized in relation to their spatial localization and width, depending on the system parameters. Setting the ground to scale superlubricity up, this investigation provides a novel perspective on friction and nanomanipulation experiments and can serve as a theoretical basis for designing high-tech devices with specific superlow frictional features. PMID:26553308

  8. A dislocation-based, strain–gradient–plasticity strengthening model for deformation processed metal-metal composites

    SciTech Connect

    Tian, Liang; Russell, Alan; Anderson, Iver

    2014-01-03

    Deformation processed metal–metal composites (DMMCs) are high-strength, high-electrical conductivity composites developed by severe plastic deformation of two ductile metal phases. The extraordinarily high strength of DMMCs is underestimated using the rule of mixture (or volumetric weighted average) of conventionally work-hardened metals. In this article, a dislocation-density-based, strain–gradient–plasticity model is proposed to relate the strain-gradient effect with the geometrically necessary dislocations emanating from the interface to better predict the strength of DMMCs. The model prediction was compared with the experimental findings of Cu–Nb, Cu–Ta, and Al–Ti DMMC systems to verify the applicability of the new model. The results show that this model predicts the strength of DMMCs better than the rule-of-mixture model. The strain-gradient effect, responsible for the exceptionally high strength of heavily cold worked DMMCs, is dominant at large deformation strain since its characteristic microstructure length is comparable with the intrinsic material length.

  9. A dislocation-based, strain–gradient–plasticity strengthening model for deformation processed metal–metal composites

    DOE PAGESBeta

    Tian, Liang; Russell, Alan; Anderson, Iver

    2014-01-03

    Deformation processed metal–metal composites (DMMCs) are high-strength, high-electrical conductivity composites developed by severe plastic deformation of two ductile metal phases. The extraordinarily high strength of DMMCs is underestimated using the rule of mixture (or volumetric weighted average) of conventionally work-hardened metals. A dislocation-density-based, strain–gradient–plasticity model is proposed to relate the strain-gradient effect with the geometrically necessary dislocations emanating from the interface to better predict the strength of DMMCs. The model prediction was compared with our experimental findings of Cu–Nb, Cu–Ta, and Al–Ti DMMC systems to verify the applicability of the new model. The results show that this model predicts themore » strength of DMMCs better than the rule-of-mixture model. The strain-gradient effect, responsible for the exceptionally high strength of heavily cold worked DMMCs, is dominant at large deformation strain since its characteristic microstructure length is comparable with the intrinsic material length.« less

  10. A dislocation-based, strain–gradient–plasticity strengthening model for deformation processed metal–metal composites

    SciTech Connect

    Tian, Liang; Russell, Alan; Anderson, Iver

    2014-01-03

    Deformation processed metal–metal composites (DMMCs) are high-strength, high-electrical conductivity composites developed by severe plastic deformation of two ductile metal phases. The extraordinarily high strength of DMMCs is underestimated using the rule of mixture (or volumetric weighted average) of conventionally work-hardened metals. A dislocation-density-based, strain–gradient–plasticity model is proposed to relate the strain-gradient effect with the geometrically necessary dislocations emanating from the interface to better predict the strength of DMMCs. The model prediction was compared with our experimental findings of Cu–Nb, Cu–Ta, and Al–Ti DMMC systems to verify the applicability of the new model. The results show that this model predicts the strength of DMMCs better than the rule-of-mixture model. The strain-gradient effect, responsible for the exceptionally high strength of heavily cold worked DMMCs, is dominant at large deformation strain since its characteristic microstructure length is comparable with the intrinsic material length.

  11. Doping-induced suppression of dislocation formation in semiconductors

    SciTech Connect

    Walukiewicz, W.

    1989-04-15

    A mechanism explaining suppression of dislocation formation in doped semiconductors is proposed. The mechanism is based on the recently introduced concept of amphoteric native defects. It is shown that supersaturation of vacancylike defects depends on the Fermi energy and thus also on the doping level. The calculated dependence of supersaturation on the doping level quantitatively accounts for experimentally observed trends in dislocation suppression in GaAs and InP.

  12. Chloroquine-induced bilateral anterior shoulder dislocation: a unique aetiology for a rare clinical problem.

    PubMed

    Martin, Alexander Nicholas; Tsekes, Dimitris; White, William James; Rossouw, Dan

    2016-01-01

    Bilateral anterior shoulder dislocation is a rare clinical entity with few case reports and limited series published in the literature. Bilateral shoulder dislocations are rare and of them, most are posterior. We present a highly unusual case of bilateral, atraumatic, anterior shoulder dislocation with concomitant comminuted greater tuberosity fracture on the right side, secondary to seizure, in a patient without known epilepsy, induced by oral chloroquine medication. We demonstrate the treatment approach that led to a satisfactory clinical outcome, evidenced by radiological union, clinical assessment and Patient Reported Outcome Measure data, following non-operative management of both shoulders. The unusual mechanism for anterior shoulder dislocation, the asymmetric dislocation pattern and peculiar precipitant for the causative seizure all provide interesting learning points from this case. PMID:27005796

  13. Cyclic deformation behavior and dislocation substructures of hexagonal Zircaloy-4 under out-of-phase loading

    SciTech Connect

    Lin, X.

    2000-01-01

    Macroscopic response and microscopic dislocation structures of Zr-4 subjected to biaxial fatigue under different phase angles of 30{degree}, 60{degree}, 90{degree}, and different equivalent strain ranges of 0.8%, 0.6%, 0.4% were studied. The testing results show that the delay angle between the stress deviators and strain increment tensors is strongly dependent on phase angle and the equivalent strain range. When phase angle equals 60{degree}, the delay angle has the minimum variation range for all specimens. The mean value of the delay angle decreases with increasing phase angle or the equivalent strain range. The variation range and average value of the Mises equivalent stress have the maximum in S3 with the phase angle of 90{degree}. They decrease as the equivalent strain range decreases. Zr-4 displays a pronounced initial hardening followed by a continuous softening for all specimens during out-of-phase cycling. The stabilized saturation stresses of Zr-4 under out-of-phase cycling are much higher than that under uniaxial cycling. It indicates that Zr-4 displays an obvious additional hardening. As the phase angle increases, the typical dislocation structure changes from dislocation cells to tangles. The dislocation-dislocation interactions increase resulting in an additional hardening. In essence, the degree of additional hardening depends, among other factors, on the maximum shear stress ratio of resolved shear stresses and critical resolved shear stresses (RSS/CRSS).

  14. Determination of the activation enthalpy for migration of dislocations in plastically deformed 8006 Al-alloy by positron annihilation lifetime technique

    NASA Astrophysics Data System (ADS)

    Salah, Mohammed; Abdel-Rahman, M.; Badawi, Emad A.; Abdel-Rahman, M. A.

    2016-06-01

    The activation enthalpy for migration of dislocations of plastically deformed 8006 Al-alloy was investigated by positron annihilation lifetime technique. Plastic deformation using a hydraulic press produces mainly dislocations and may produce point defects. The type of defect was studied by isochronal annealing which determines the temperature range of recovery of each type. Only one type of defect (dislocations) was observed for the investigated sample and was found to be recovered within the range 455-700 K. Isothermal annealing by slow cooling was performed through this range and used in determination of the activation enthalpy of migration of dislocations which was found to be 0.26 ± 0.01 eV.

  15. Dislocation Mechanics Under Extreme Pressures

    NASA Astrophysics Data System (ADS)

    Armstrong, Ronald; Arnold, Werner; Zerilli, Frank

    2007-06-01

    The shock-induced plasticity of copper, Armco iron, and tantalum materials is attributed to strain rate control by a substantial dislocation density being generated at the shock front. A thermal activation type constitutive equation is employed for the dislocation generation based on achievement of a limiting small activation volume for the process. A linear dependence of the equivalent compressive stress on logarithm of the plastic strain rate is predicted. The prediction compares favorably with Swegle-Grady and Meyers measurements previously fitted to a power law relationship. For Armco iron and tantalum, control is matched with a dislocation description of deformation twinning at the shock front. By comparison, the uniform shock-less loading in an isentropic compression experiment (ICE) provides for plastic strain rate control by the drag-resisted movement of mobile dislocations within the resident dislocation density.

  16. Shock-induced deformation of nanocrystalline Al: Characterization with orientation mapping and selected area electron diffraction

    NASA Astrophysics Data System (ADS)

    Wang, L.; E, J. C.; Cai, Y.; Zhao, F.; Fan, D.; Luo, S. N.

    2015-02-01

    We investigate shock-induced deformation of columnar nanocrystalline Al with large-scale molecular dynamics simulations and implement orientation mapping (OM) and selected area electron diffraction (SAED) for microstructural analysis. Deformation mechanisms include stacking fault formation, pronounced twinning, dislocation slip, grain boundary (GB) sliding and migration, and lattice or partial grain rotation. GBs and GB triple junctions serve as the nucleation sites for crystal plasticity including twinning and dislocations, due to GB weakening, and stress concentrations. Grains with different orientations exhibit different densities of twins or stacking faults nucleated from GBs. GB migration occurs as a result of differential deformation between two grains across the GB. High strain rates, appropriate grain orientation and GBs contribute to deformation twinning. Upon shock compression, intra-grain dislocation and twinning nucleated from GBs lead to partial grain rotation and the formation of subgrains, while whole grain rotation is not observed. During tension, stress gradients associated with the tensile pulse give rise to intra-grain plasticity and then partial grain rotation. The simulated OM and SAED are useful to describe lattice/grain rotation, the formation of subgrains, GB migration and other microstructures.

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

    PubMed

    Feuerbacher, Michael

    2016-01-01

    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. PMID:27430993

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

    PubMed Central

    Feuerbacher, Michael

    2016-01-01

    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. PMID:27430993

  19. 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.

  20. Vortex dislocations in wake-type flow induced by spanwise disturbances

    NASA Astrophysics Data System (ADS)

    Ling, Guo Can; Zhao, Hong Liang

    2009-07-01

    Vortex dislocations in wake-type flow induced by three types of spanwise disturbances superimposed on an upstream velocity profile are investigated by direct numerical simulations. Three distinct modes of vortex dislocations and flow transitions have been found. A local spanwise exponential decay disturbance leads to the appearance of a twisted chainlike mode of vortex dislocation. A stepped spanwise disturbance causes a streamwise periodic spotlike mode of vortex dislocation. A spanwise sinusoidal wavy disturbance with a moderate waviness causes a strong unsteadiness of wake behavior. This unsteadiness starts with a systematic periodic mode of vortex dislocation in the spanwise direction followed by the spanwise vortex shedding suppressed completely with increased time and the near wake becoming a steady shear flow. Characteristics of these modes of vortex dislocation and complex vortex linkages over the dislocation, as well as the corresponding dynamic processes related to the appearance of dislocations, are described by examining the variations of vortex lines and vorticity distribution. The nature of the vortex dislocation is demonstrated by the substantial vorticity modification of the spanwise vortex from the original spanwise direction to streamwise and vertical directions, accompanied by the appearance of noticeable vortex branching and complex vortex linking, all of which are produced at the locations with the biggest phase difference or with a frequency discontinuity between shedding cells. The effect of vortex dislocation on flow transition, either to an unsteady irregular vortex flow or suppression of the Kármán vortex shedding making the wake flow steady state, is analyzed. Distinct similarities are found in the mechanism and main flow phenomena between the present numerical results obtained in wake-type flows and the experimental-numerical results of cylinder wakes reported in previous studies.

  1. Deformation-induced {alpha}{sub 2} {yields} {gamma} phase transformation in TiAl alloys

    SciTech Connect

    Chen, C.L.; Lu, W.; Sun Dai; He, L.L.; Ye, H.Q.

    2010-11-15

    Deformation-induced {alpha}{sub 2} {yields} {gamma} phase transformation in high Nb containing TiAl alloys was investigated using high-resolution transmission electron microscopy (HREM) and energy dispersive X-ray spectroscopy (EDS). The dislocations appearing at the tip of deformation-induced {gamma} plate (DI-{gamma}) and the stacking sequence change of the {alpha}{sub 2} matrix were two key evidences for determining the occurrence of the deformation-induced {alpha}{sub 2} {yields} {gamma} phase transformation. Compositional analysis revealed that the product phase of the room-temperature transformation was not standard {gamma} phase; on the contrary, the product phase of the high-temperature transformation was standard {gamma} phase.

  2. Dislocation-nucleation-controlled deformation of Ni3Al nanocubes in molecular dynamics simulations

    NASA Astrophysics Data System (ADS)

    Shreiber, Koren; Mordehai, Dan

    2015-12-01

    The strength of Ni3Al nanocubes under compression, as well as the underlying dislocation mechanisms, are analysed in molecular dynamics simulations. For this purpose, the bond-order parameters analysis is extended to multi-atomic systems in order to identify the intrinsic planar defects in Ni3Al. We benchmarked different interatomic potentials and compared the results with experimental ones. The different potentials resulted in different elastic responses under compression but all yielded abruptly at a compressive stress of about 7-8 GPa, followed by a large strain burst. The nanocubes yielded by nucleating Shockley partial dislocations at the vertices on ≤ft\\{1 1 1\\right\\} planes, leaving a structure of faulted planes. The mechanical response was found to be size independent, which we attribute to the cubical shape of the nanoparticle and the lack of stress gradients at its vertices.

  3. Modeling of friction-induced deformation and microstructures.

    SciTech Connect

    Michael, Joseph Richard; Prasad, Somuri V.; Jungk, John Michael; Cordill, Megan J.; Bammann, Douglas J.; Battaile, Corbett Chandler; Moody, Neville Reid; Majumdar, Bhaskar Sinha (New Mexico Institure of Mining and Technology)

    2006-12-01

    Frictional contact results in surface and subsurface damage that could influence the performance, aging, and reliability of moving mechanical assemblies. Changes in surface roughness, hardness, grain size and texture often occur during the initial run-in period, resulting in the evolution of subsurface layers with characteristic microstructural features that are different from those of the bulk. The objective of this LDRD funded research was to model friction-induced microstructures. In order to accomplish this objective, novel experimental techniques were developed to make friction measurements on single crystal surfaces along specific crystallographic surfaces. Focused ion beam techniques were used to prepare cross-sections of wear scars, and electron backscattered diffraction (EBSD) and TEM to understand the deformation, orientation changes, and recrystallization that are associated with sliding wear. The extent of subsurface deformation and the coefficient of friction were strongly dependent on the crystal orientation. These experimental observations and insights were used to develop and validate phenomenological models. A phenomenological model was developed to elucidate the relationships between deformation, microstructure formation, and friction during wear. The contact mechanics problem was described by well-known mathematical solutions for the stresses during sliding friction. Crystal plasticity theory was used to describe the evolution of dislocation content in the worn material, which in turn provided an estimate of the characteristic microstructural feature size as a function of the imposed strain. An analysis of grain boundary sliding in ultra-fine-grained material provided a mechanism for lubrication, and model predictions of the contribution of grain boundary sliding (relative to plastic deformation) to lubrication were in good qualitative agreement with experimental evidence. A nanomechanics-based approach has been developed for characterizing the

  4. Gravitational effects of process-induced dislocations in silicon. [during thermal cycling

    NASA Technical Reports Server (NTRS)

    Porter, W. A.; Parker, D. L.

    1974-01-01

    Matters pertaining to semiconductor device fabrication were studied in terms of the influence of gravity on the production of dislocations in silicon wafers during thermal cycling in a controlled ambient where no impurities are present and oxidation is minimal. Both n-type and p-type silicon wafers having a diameter of 1.25 in to 1.5 in, with fixed orientation and resistivity values, were used. The surface dislocation densities were measured quantitatively by the Sirtl etch technique. The results show two significant features of the plastic flow phenomenon as it is related to gravitational stress: (1) the density of dislocations generated during a given thermal cycle is directly related to the duration of the cycle; and (2) the duration of the thermal cycle required to produce a given dislocation density is inversely related to the equilibrium temperature. Analysis of the results indicates that gravitational stress is instrumental in process-induced defect generation.

  5. Experimental Deformation of Olivine Single Crystal at Mantle P and T: Pressure Effect on Olivine Dislocation Slip-System Activities

    NASA Astrophysics Data System (ADS)

    Paul, R.; Girard, J.; Chen, J.; Amiguet, E.

    2008-12-01

    Seismic velocity anisotropies observed in the upper mantle are interpreted from lattice preferred orientations (LPO) produced experimentally in olivine, which depends on the dominant dislocation slip systems. At low pressure P<3 GPa, mantle temperature (T) and in dry conditions, olivine [100] dislocation slip dominates the less active [001] slip. This tends to align crystal fast velocity [100] axis with the principal shear direction. Yet recent high-pressure deformation experiments (Couvy et al., 2004, EJM, 16, 877; Raterron et al., 2007, Am. Min., 92, 1436; Raterron et al., 2008, Phys. Earth Planet. Int., doi:10.1016/j.pepi.2008.07.026) show that [001](010) slip system dominates [100](010) system in the (P,T) range of the deep upper mantle. This may promote a shear-parallel slow-velocity [001] axis and may explain the seismic-velocity attenuation observed at depth >200 km (Mainprice et al., 2005, Nature, 433, 731). In order to further constrain the effect of P on olivine slip system activities, which is classically quantified by the activation volume V* in power creep laws, 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 Deformation-DIA apparatus at the X17B2 beamline of the NSLS (Upton, NY). Ten 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 σ <300 MPa and specimen strain rates were monitored in situ using time-resolved x-ray diffraction and radiography, respectively, for a total of 27 investigated steady state conditions. The obtained rheological data were compared with data previously obtained in comparable T and σ conditions, but at room P, by Darot and Gueguen (1981, JGR, 86, 6219) for Fo100 and by Bai et al. (1991, JGR, 96, 2441) for San Carlos olivine. This new set of data confirms previous deformation data

  6. Fracture toughness from atomistic simulations: Brittleness induced by emission of sessile dislocations

    SciTech Connect

    Farkas, D.

    1998-08-04

    Using atomistic simulations of crack response for intermetallic materials the author shows that when the emitted dislocations are sessile and stay in the immediate vicinity of the crack tip the emitted dislocations can actually lead to brittle failure. She present the results of an atomistic simulation study of the simultaneous dislocation emission and crack propagation process in this class of materials. She used a molecular statics technique with embedded atom (EAM) potentials developed for NiAl. The crystal structure of NiAl is the CsCl type (B2) with a lattice parameter of 0.287 nm, which is reproduced by the potential together with the cohesive energy and elastic constants. The compound stays ordered up to the melting point, indicating a strong tendency towards chemical ordering with a relatively high energy of the antiphase boundary (APB). As a result of this relatively large energy the dislocations of 1/2<111> type Burgers vectors imply a high energy and the deformation process occurs via the larger <100> type dislocations.

  7. Stress induced roughening of superclimbing dislocation in solid 4He

    NASA Astrophysics Data System (ADS)

    Aleinikava, Darya; Kuklov, Anatoly

    2011-03-01

    We investigate numerically superclimb of dislocation in solid 4 He biased by externally imposed chemical potential μ . The effective action takes into account quantum phase slips in the core superfluid as well as the core displacement in Peierls potential within the Granato-Lücke string model. The bias produces stress on the core and this can result in dislocation roughening. Such roughening is characterized by hysteretic behavior at temperatures (T) below some threshold Thyst . At T >Thyst strongresonantpeaksdevelopinthedislocationdifferentialresponse . Thesepeaksexhibitperiodicbehaviorvs μ, with the period determined by Peierls potential and dislocation length. We explain these effects by thermally assisted tunneling of jog-antijog pairs across the barrier created by Peierls potential and the bias. Since superclimbing is controlled by core superflow, speed of sound along the superfluid core exhibits dip-like features at the peak positions. We propose that this effect is seen in the mass transport experiment. We acknowledge support by NSF, grants PHY1005527 and PHY0653135,and by CUNY, grant 63071-00 41.

  8. Plastic deformation and impaction of the retroacetabular surface associated with posterior fracture-dislocation of the hip: description of two cases.

    PubMed

    Meier, Sandra; Isler, Balz; Gautier, Emanuel

    2007-10-01

    In two cases of posterior fracture-dislocation of the hip, we found an impaction and plastic deformation of the retroacetabular surface, in addition to other typically associated lesions. This deformation creates a mismatch between the stable and the free fragments of the posterior wall, and if unrecognized makes an anatomic reconstruction of the posterior wall impossible. On the computed tomography scan, the most characteristic sign is a concave deformation of the retroacetabular region as compared to the contralateral side. To our knowledge, this specific fracture-dislocation pattern has not been previously described in the literature. If orthopedic surgeons know about the existence of this deformation, then they will easily recognize the impaction on preoperative computed tomography scan and will be able to achieve an anatomic reduction of the acetabulum. Diagnostic aspects and the operative technique to approach this problem are described and illustrated in two cases. PMID:17921844

  9. Sensitivity of ion induced charge pulses to the electrical and crystallographic properties of 60° dislocations

    NASA Astrophysics Data System (ADS)

    Breese, M. B. H.; King, P. J. C.; Grime, G. W.

    1994-12-01

    This letter reports evidence that the size of MeV ion induced charge pulses measured from epitaxial Si0.875Ge0.125/Si depends on both the crystallographic and electrical properties of the 60° misfit dislocations present. The results are correlated with both backscattered and transmission ion channeling analysis. With the sample in nonchanneled alignment the measured ion induced charge pulses depend on the number of charge carriers which recombine at the dislocations. With the sample in channeled alignment the rotated (110) and (11¯0) planes around the 60° dislocations affect the local rate of carrier generation and so alter the size of the measured ion induced charge pulses.

  10. Intraplate rotational deformation induced by faults

    NASA Astrophysics Data System (ADS)

    Dembo, Neta; Hamiel, Yariv; Granot, Roi

    2015-11-01

    Vertical axis rotations provide important constraints on the tectonic history of plate boundaries. Geodetic measurements can be used to calculate interseismic rotations, whereas paleomagnetic remanence directions provide constraints on the long-term rotations accumulated over geological timescales. Here we present a new mechanical modeling approach that links between intraplate deformational patterns of these timescales. We construct mechanical models of active faults at their locked state to simulate the presumed to be elastic interseismic deformation rate observed by GPS measurements. We then apply a slip to the faults above the locking depth to simulate the long-term deformation of the crust from which we derive the accumulated rotations. We test this approach in northern Israel along the Dead Sea Fault and Carmel-Gilboa fault system. We use 12 years of interseismic GPS measurements to constrain a slip model of the major faults found in this region. Next, we compare the modeled rotations against long-term rotations determined based on new primary magnetic remanence directions from 29 sites with known age. The distributional pattern of site mean declinations is in general agreement with the vertical axis rotations predicted by the mechanical model, both showing anomalously high rotations near fault tips and bending points. Overall, the results from northern Israel validate the effectiveness of our approach and indicate that rotations induced by motion along faults may act in parallel (or alone) to rigid block rotations. Finally, the new suggested method unravels important insights on the evolution (timing, magnitude, and style) of deformation along major faults.

  11. A model for nucleation of tin whisker through dislocation behavior

    NASA Astrophysics Data System (ADS)

    Nakai, K.; Sakamoto, T.; Kobayashi, S.; Takamizawa, M.; Murakami, K.; Hino, M.

    2009-05-01

    A model for the nucleation and growth processes of Sn whisker is offered. High density of localized screw dislocations by deformation form the dense spiral steps of atomic scale on Sn surface. The spiral steps would induce the nucleation of Sn whisker. Edge dislocations localized at the same region where dense screw dislocations exist supply Sn atoms to the Sn whisker through pipe diffusion. Both screw and edge dislocations would bend along almost one direction, namely, to relax the external shear stress. The image force also helps to bend the dislocations perpendicular to the whisker side-surface. The bending of dislocations at root of whisker leads the bend of whisker. The pipe diffusion of Sn atoms through edge dislocations from bulk Sn toward whisker is suppressed at the bent part of edge dislocation, resulting in release of Sn atoms inside whisker and leading to the growth of whisker near its root.

  12. Deformation-induced dissolution of the intermetallics Ni3Ti and Ni3Al in austenitic steels at cryogenic temperatures

    NASA Astrophysics Data System (ADS)

    Sagaradze, V. V.; Shabashov, V. A.; Kataeva, N. V.; Zavalishin, V. A.; Kozlov, K. A.; Kuznetsov, A. R.; Litvinov, A. V.; Pilyugin, V. P.

    2016-06-01

    An anomalous deformation-induced dissolution of the intermetallics Ni3Al and Ni3Ti in the matrix of austenitic Fe-Ni-Al(Ti) alloys has been revealed in experiment at cryogenic temperatures (down to 77 K) under rolling and high pressure torsion. The observed phenomenon is explained as the result of migration of deformation-stipulated interstitial atoms from a particle into the matrix in the stress field of moving dislocations. With increasing the temperature of deformation, the dissolution is replaced by the deformation-induced precipitation of the intermetallics, which is accelerated due to a sufficient amount of point defects in the matrix, gained as well in the course of deformation at lower temperatures.

  13. 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

    SciTech Connect

    Long, Fei; Daymond, Mark R. Yao, Zhongwen

    2015-03-07

    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 the sample that had been deformed with 1.5% plastic strain, implying that these dislocations require larger stresses to be activated.

  14. 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

    NASA Astrophysics Data System (ADS)

    Long, Fei; Daymond, Mark R.; Yao, Zhongwen

    2015-03-01

    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 the sample that had been deformed with 1.5% plastic strain, implying that these dislocations require larger stresses to be activated.

  15. Morphology transition of deformation-induced lenticular martensite in Fe-Ni-C alloys

    SciTech Connect

    Zhang, X.M.; Li, D.F.; Xing, Z.S. . Inst. of Metal Research); Gautier, E.; Zhang, J.S.; Simon, A. . Lab. de Science et Genie des Materiaux Metalliques)

    1993-06-01

    The morphology and habit planes of deformation-induced lenticular martensite were investigated by optical and transmission electron microscopy in Fe-30Ni and Fe-30Ni-0.11C alloys. Transitions in morphology were observed with progressive deformation levels going from lenticular to butterfly martensite for the Fe-30Ni-0.11C alloy. The habit planes changed from (225)[sub f] or (259)[sub f] for the thermal lenticular martensite to (111)[sub f] for the strain-induced martensite. The morphology and crystallography of the small butterfly martensites was also investigated. A change in the orientation relationships from K-S to N-W relations was also observed. These changes were attributed to the contribution of mobile dislocations which modified the shear mode form twinning to slip, and to a plastic accommodation of transformation strains.

  16. Investigation of dislocations in Nb-doped SrTiO{sub 3} by electron-beam-induced current and transmission electron microscopy

    SciTech Connect

    Chen, Jun Sekiguchi, Takashi; Li, Jianyong; Ito, Shun; Yi, Wei; Ogura, Atsushi

    2015-03-09

    This paper aims to clarify the electrical activities of dislocations in Nb-doped SrTiO{sub 3} substrates and the role of dislocations in the resistance switching phenomenon in Pt/SrTiO{sub 3} Schottky contacts. The electrical activities of dislocations have been studied by electron-beam-induced current (EBIC) technique. EBIC has found that dislocations can exhibit dark or bright contrast depending on their character and band bending condition. The character of dislocations has been analysed based on chemical etching and transmission electron microscopy. These data suggested that not all the dislocations contribute to the switching phenomenon. The active dislocations for resistance switching were discussed.

  17. Role of superposition of dislocation avalanches in the statistics of acoustic emission during plastic deformation.

    PubMed

    Lebyodkin, M A; Shashkov, I V; Lebedkina, T A; Mathis, K; Dobron, P; Chmelik, F

    2013-10-01

    Various dynamical systems with many degrees of freedom display avalanche dynamics, which is characterized by scale invariance reflected in power-law statistics. The superposition of avalanche processes in real systems driven at a finite velocity may influence the experimental determination of the underlying power law. The present paper reports results of an investigation of this effect using the example of acoustic emission (AE) accompanying plastic deformation of crystals. Indeed, recent studies of AE did not only prove that the dynamics of crystal defects obeys power-law statistics, but also led to a hypothesis of universality of the scaling law. We examine the sensitivity of the apparent statistics of AE to the parameters applied to individualize AE events. Two different alloys, MgZr and AlMg, both displaying strong AE but characterized by different plasticity mechanisms, are investigated. It is shown that the power-law indices display a good robustness in wide ranges of parameters even in the conditions leading to very strong superposition of AE events, although some deviations from the persistent values are also detected. The totality of the results confirms the scale-invariant character of deformation processes on the scale relevant to AE, but uncovers essential differences between the power-law exponents found for two kinds of alloys. PMID:24229184

  18. The evolution of internal stress and dislocation during tensile deformation in a 9Cr ferritic/martensitic (F/M) ODS steel investigated by high-energy X-rays

    SciTech Connect

    Zhang, Guangming; Zhou, Zhangjian; Mo, Kun; Miao, Yinbin; Liu, Xiang; Almer, Jonathan; Stubbins, James F.

    2015-12-01

    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 high 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.

  19. The evolution of internal stress and dislocation during tensile deformation in a 9Cr ferritic/martensitic (F/M) ODS steel investigated by high-energy X-rays

    NASA Astrophysics Data System (ADS)

    Zhang, Guangming; Zhou, Zhangjian; Mo, Kun; Miao, Yinbin; Liu, Xiang; Almer, Jonathan; Stubbins, James F.

    2015-12-01

    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 X-ray 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 high 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.

  20. Screw dislocation-induced influence of transverse modes on Hall conductivity

    NASA Astrophysics Data System (ADS)

    de Lima, André G.; Poux, Armelle; Assafrão, Denise; Filgueiras, Cleverson

    2013-11-01

    The Hall conductivity of an electron gas on an interface showing a topological defect called screw dislocation is investigated. This kind of defect induces a singular torsion on the medium which in turn induces transverse modes in the quantum Hall effect. It is shown that this topology decreases the plateaus' widths and shifts the steps in the Hall conductivity to lower magnetic fields. The Hall conductivity is neither enhanced nor diminished by the presence of this kind of defect alone. We also consider the presence of two defects on a sample, a screw dislocation together with a disclination. For a specific value of deficit angle, there is a reduction in the Hall conductivity. For an excess of angle, the steps shift to higher magnetic fields and the Hall conductivity is enhanced. Our work could be tested only in common semiconductors but we think it opens a road to the investigation on how topological defects can influence other classes of Hall effect.

  1. Silicon based light emitters utilizing radiation from dislocations; electric field induced shift of the dislocation-related luminescence

    NASA Astrophysics Data System (ADS)

    Arguirov, T.; Mchedlidze, T.; Kittler, M.; Reiche, M.; Wilhelm, T.; Hoang, T.; Holleman, J.; Schmitz, J.

    2009-05-01

    Dislocation rich regions can be controllably formed at a certain location inside a silicon wafer. We studied the light emission properties of such regions located in an electric field of a p-n junction under different excitation conditions. It was found that the luminescence spectra of the dislocations are significantly influenced by the presence of the junction. The dislocation-related luminescence peak position appears red-shifted due to the built-in electric field. A suppression of that field by photo-generation of carriers or by applying a forward bias voltage at the junction leads to a gradual decrease in the energy position of the peaks. The dependence of the peak position on the electric field was found to be a quadratic function, similar to that observed for semiconductor nanostructures. We show that the shift of the peak position is due to the Stark effect on dislocation-related excitonic states. The characteristic constant of the shift, obtained by fitting the data with the quadratic Stark effect equation, was 0.0186 meV/(kV/cm) 2. The observed effect opens new possibilities for integration of a silicon based light emitter, combining the radiation from dislocations with a Stark effect based modulator.

  2. Plasticity of Cu nanoparticles: Dislocation-dendrite-induced strain hardening and a limit for displacive plasticity

    PubMed Central

    Albe, Karsten

    2013-01-01

    Summary The plastic behaviour of individual Cu crystallites under nanoextrusion is studied by molecular dynamics simulations. Single-crystal Cu fcc nanoparticles are embedded in a spherical force field mimicking the effect of a contracting carbon shell, inducing pressure on the system in the range of gigapascals. The material is extruded from a hole of 1.1–1.6 nm radius under athermal conditions. Simultaneous nucleation of partial dislocations at the extrusion orifice leads to the formation of dislocation dendrites in the particle causing strain hardening and high flow stress of the material. As the extrusion orifice radius is reduced below 1.3 Å we observe a transition from displacive plasticity to solid-state amorphisation. PMID:23616936

  3. 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.

  4. Dislocation density evolution during high pressure torsion of a nanocrystalline Ni-Fe alloy

    SciTech Connect

    Li, Hongqi; Wang, Y B; Ho, J C; Cao, Y; Liao, X Z; Ringer, S P; Zhu, Y T; Zhao, Y H; Lavernia, E J

    2009-01-01

    High-pressure torsion (HPT) induced dislocation density evolution in a nanocrystalline Ni-20wt.%Fe alloy was investigated using X-ray diffraction and transmission electron microscopy. Results suggest that the dislocation density evolution is different from that in coarse-grained materials. An HPT process first reduces the dislocation density within nanocrystalline grains and produces a large number of dislocations located at small-angle sub grain boundaries that are formed via grain rotation and coalescence. Continuing the deformation process eliminates the sub grain boundaries but significantly increases the dislocation density in grains. This phenomenon provides an explanation of the mechanical behavior of some nanostructured materials.

  5. Dislocation motion and instability

    NASA Astrophysics Data System (ADS)

    Zhu, Yichao; Chapman, Stephen Jonathan; Acharya, Amit

    2013-08-01

    The Peach-Koehler expression for the stress generated by a single (non-planar) curvilinear dislocation is evaluated to calculate the dislocation self stress. This is combined with a law of motion to give the self-induced motion of a general dislocation curve. A stability analysis of a rectilinear, uniformly translating dislocation is then performed. The dislocation is found to be susceptible to a helical instability, with the maximum growth rate occurring when the dislocation is almost, but not exactly, pure screw. The non-linear evolution of the instability is determined numerically, and implications for slip band formation and non-Schmid behavior in yielding are discussed.

  6. Microstructure and Deformation Behavior of Phase-Reversion-Induced Nanograined/Ultrafine-Grained Austenitic Stainless Steel

    NASA Astrophysics Data System (ADS)

    Misra, R. D. K.; Nayak, S.; Mali, S. A.; Shah, J. S.; Somani, M. C.; Karjalainen, L. P.

    2009-10-01

    Materials with submicron to nanometer-sized grains by virtue of their high grain boundary area to grain size ratio provide valuable tools for studying deformation behavior in ultrafine-grained structures. In this regard, the well-known strain-induced martensite transformation and its reversal to the parent austenite phase were used to produce nanograins/ultrafine grains via controlled annealing of heavily cold-worked metastable austenite. The results of the electron microscopy study of phase-reversion-induced microstructure and deformation behavior of nanograined/ultrafine-grained (NG/UFG) austenitic stainless steel during tensile straining are described here. The phase-reversion-induced structure was observed to depend on the cold rolling reduction and temperature-time annealing cycle. The optimized structure consisted of nanocrystalline ( d < 100 nm), ultrafine ( d ≈ 100 to 500 nm), and submicron ( d ≈ 500 to 1000 nm) grains and was characterized by a high yield strength (800 to 1000 MPa)-high ductility (30 to 40 pct) combination. Austenite nucleation during phase-reversion annealing occurred in the form of thin plates or as equiaxed grains along the martensite laths. Twinning and dislocation glide were identified as the primary deformation mechanisms, where twinning had a varied character. However, the high elongation seems to be associated with the gradual transformation of metastable austenite, with twinning having only a minor contribution.

  7. PSGRN/PSCMP—a new code for calculating co- and post-seismic deformation, geoid and gravity changes based on the viscoelastic-gravitational dislocation theory

    NASA Astrophysics Data System (ADS)

    Wang, Rongjiang; Lorenzo-Martín, Francisco; Roth, Frank

    2006-05-01

    We present a new numerical code for modeling co- and post-seismic response of the Earth's crust to earthquakes. The code consists of two FORTRAN programs: the first program, PSGRN, calculates the time-dependent Green functions of a given layered viscoelastic-gravitational half-space for four fundamental dislocation sources [the strike-slip double-couple, the dip-slip double-couple, the compensated linear vertical dipole (CLVD) and the point inflation] at different depths. The results provide a data base for the second program, PSCMP, which automatically discretizes the earthquake's extended rupture area into a number of discrete point dislocations and calculates the co- and post-seismic deformation by linear superposition. According to the correspondence principle, the same propagator algorithm used in our previously published elastic modeling software, EDGRN/EDCMP, is adopted to compute the spectral Green functions. The temporal Green functions are then obtained by the fast Fourier transform extended with an anti-aliasing technique, that ensures numerical stability when calculating the post-seismic transients. Moreover, the new software considers the coupling between the deformation and the Earth's gravity field, so that its output includes not only the complete deformation field consisting of 3 displacement components, 6 stress (strain) components and 2 tilt components, but also the geoid and gravity changes. In particular, the gravity effect is treated using a new consistent approach that remedies an incorrect formulation used in many earlier publications. The performance of the software is shown by an example.

  8. Parallel Dislocation Simulator

    Energy Science and Technology Software Center (ESTSC)

    2006-10-30

    ParaDiS is software capable of simulating the motion, evolution, and interaction of dislocation networks in single crystals using massively parallel computer architectures. The software is capable of outputting the stress-strain response of a single crystal whose plastic deformation is controlled by the dislocation processes.

  9. Patterns of postural deformity in non-ambulant people with cerebral palsy: what is the relationship between the direction of scoliosis, direction of pelvic obliquity, direction of windswept hip deformity and side of hip dislocation?

    PubMed Central

    Michael, Shona; Kirkwood, Craig

    2008-01-01

    Objective: To investigate: (a) associations between the direction of scoliosis, direction of pelvic obliquity, direction of windswept deformity and side of hip subluxation/dislocation in non-ambulant people with cerebral palsy; and (b) the lateral distribution of these postural asymmetries. Design: Cross-sectional observational study. Setting: Posture management services in three centres in the UK. Subjects: Non-ambulant people at level five on the gross motor function classification system for cerebral palsy. Main measures: Direction of pelvic obliquity and lateral spinal curvature determined from physical examination, direction of windswept hip deformity derived from range of hip abduction/adduction, and presence/side of unilateral hip subluxation defined by hip migration percentage. Results: A total of 747 participants were included in the study, aged 6–80 years (median 18 years 10 months). Associations between the direction of scoliosis and direction of pelvic obliquity, and between the direction of windswept hip deformity and side hip subluxation/dislocation were confirmed. A significant association was also seen between the direction of scoliosis and the direction of the windswept hip deformity (P < 0.001) such that the convexity of the lateral spinal curve was more likely to be opposite to the direction of windsweeping. Furthermore, significantly more windswept deformities to the right (P = 0.007), hips subluxed on the left (P = 0.002) and lateral lumbar/lower thoracic spinal curves convex to the left (P = 0.03) were observed. Conclusions: The individual asymmetrical postural deformities are not unrelated in terms of direction and not equally distributed to the left/right. A pattern of postural deformity was observed. PMID:18042604

  10. Shock metamorphism of deformed quartz

    NASA Technical Reports Server (NTRS)

    Gratz, Andrew J.; Christie, John; Tyburczy, James; Ahrens, Thomas; Pongratz, Peter

    1988-01-01

    The effect produced by shock loading (to peak pressures of 12 and 24) on deformed synthetic quartz containing a dislocation and abundant bubbles and small inclusions was investigated, and the relationships between preexisting dislocation density shock lamellae in the target material were examined. The resultant material was found to be inhomogeneously deformed and extremely fractured. Results of TEM examinations indicate that no change in dislocation density was caused by shock loading except in regions containing shock lamellae, where the dislocation density was lowered. The shock-induced defects tend to nucleate on and be controlled by preexisting stress concentrators; shock lamellae, glassy veins, and most curviplanar defects form in tension, presumably during release. An extremely mobile silica fluid is formed and injected into fractures during release, which forcibly removes crystalline fragments from vein walls. It is concluded that shock deformation in quartz is dominated by fracture and melting.

  11. EBIC (electron beam induced current) contrast of clean, decorated and deuterium passivated Si(Ge) epitaxial misfit dislocations

    SciTech Connect

    Zhou, T.Q.; Buczkowski, A.; Radzimski, Z.J.; Rozgonyi, G.A. . Dept. of Materials Science and Engineering); Seager, C.H.; Panitz, J. )

    1991-01-01

    The electrical activity of as-grown and intentionally decorated misfit dislocations in an epitaxial Si/Si(Ge) heterostructure was examined using the electron beam induced current (EBIC) technique in a scanning electron microscope. Misfit dislocations, which were not visible initially, were subsequently activated either by an unknown processing contaminant or a backside metallic impurity. Passivation of these contaminated dislocations was then studied using low energy deuterium ion implantation in a Kaufman ion source. EBIC results show that the recombination activity of the decorated misfit dislocations was dramatically reduced by the deuterium treatment. Although a front side passivation treatment was more effective than a backside treatment, a surface ion bombardment damage problem is still evident. 5 refs., 3 figs.

  12. Hydrologically induced slope deformations detected by GPS and clinometric surveys in the Cansiglio Plateau, southern Alps

    NASA Astrophysics Data System (ADS)

    Devoti, R.; Zuliani, D.; Braitenberg, C.; Fabris, P.; Grillo, B.

    2015-06-01

    Changes in groundwater or surface water level may cause observable deformation of the drainage basins in different ways. We describe an active slope deformation monitored with GPS and tiltmeter stations in a karstic limestone plateau in southeastern Alps (Cansiglio Plateau). The observed transient GPS deformation clearly correlates with the rainfall. Both GPS and tiltmeter equipments react instantly to heavy rains displaying abrupt offsets, but with different time constants, demonstrating the response to different catchment volumes. The GPS movement is mostly confined in the horizontal plane (SSW direction) showing a systematic tendency to rebound in the weeks following the rain. Four GPS stations concur to define a coherent deformation pattern of a wide area (12 × 5km2), concerning the whole southeastern slope of the plateau. The plateau expands and rebounds radially after rain by an amount up to a few centimeters and causing only small vertical deformation. The effect is largest where karstic features are mostly developed, at the margin of the plateau where a thick succession of Cretaceous peritidal carbonates faces the Venetian lowland. A couple of tiltmeters installed in a cave at the top of the plateau, detect a much faster deformation, that has the tendency to rebound in less than 6 h. The correlation to rainfall is less straightforward, and shows a more complex behavior during rainy weather. The different responses demonstrate a fast hydrologic flow in the more permeable epikarst for the tiltmeters, drained by open fractures and fissures in the neighborhood of the cave, and a rapid tensile dislocation of the bedrock measured at the GPS stations that affect the whole slope of the mountain. In the days following the rain, both tiltmeter and GPS data show a tendency to retrieve the displacement which is consistent with the phreatic discharge curve. We propose that hydrologically active fractures recharged by rainfall are the most likely features capable to

  13. Strain-induced phase transformation at the surface of an AISI-304 stainless steel irradiated to 4.4 dpa and deformed to 0.8% strain

    NASA Astrophysics Data System (ADS)

    Gussev, M. N.; Field, K. G.; Busby, J. T.

    2014-03-01

    Surface relief due to localized deformation in a 4.4-dpa neutron-irradiated AISI 304 stainless steel was investigated using scanning electron microscopy coupled with electron backscattering diffraction and scanning transmission electron microscopy. It was found a body-centered-cubic (BCC) phase (deformation-induced martensite) had formed at the surface of the deformed specimen along the steps generated from dislocation channels. Martensitic hill-like formations with widths of ˜1 μm and depths of several microns were observed at channels with heights greater than ˜150 nm above the original surface. Martensite at dislocation channels was observed in grains along the [0 0 1]-[1 1 1] orientation but not in those along the [1 0 1] orientation.

  14. In-situ TEM observation of dynamic interaction between dislocation and cavity in BCC metals in tensile deformation

    NASA Astrophysics Data System (ADS)

    Tougou, Kouichi; Shikata, Akihito; Kawase, Uchu; Onitsuka, Takashi; Fukumoto, Ken-ichi

    2015-10-01

    To investigate the effect of irradiation hardening of structural materials due to cavity formation in BCC metals for nuclear applications, an in-situ transmission electron microscopy (TEM) observation in tensile test was performed for the helium ion-irradiated specimens of pure molybdenum and pure iron. The obstacle barrier strength, α was calculated from the bow-out dislocation based on line tension model, and the obstacle barrier strengths of cavity in pure molybdenum and pure iron were about 0.5-0.7. The fractions of cross-slip generation of dislocation of screw type due to interaction with the cavities were about 16-18 % for pure molybdenum.

  15. 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.

  16. Prediction of dislocation generation during Bridgman growth of GaAs crystals

    NASA Technical Reports Server (NTRS)

    Tsai, C. T.; Yao, M. W.; Chait, Arnon

    1992-01-01

    Dislocation densities are generated in GaAs single crystals due to the excessive thermal stresses induced by temperature variations during growth. A viscoplastic material model for GaAs, which takes into account the movement and multiplication of dislocations in the plastic deformation, is developed according to Haasen's theory. The dislocation density is expressed as an internal state variable in this dynamic viscoplastic model. The deformation process is a nonlinear function of stress, strain rate, dislocation density and temperature. The dislocation density in the GaAs crystal during vertical Bridgman growth is calculated using a nonlinear finite element model. The dislocation multiplication in GaAs crystals for several temperature fields obtained from thermal modeling of both the GTE GaAs experimental data and artificially designed data are investigated.

  17. Statistical characterization of dislocation ensembles

    SciTech Connect

    El-Azab, A; Deng, J; Tang, M

    2006-05-17

    We outline a method to study the spatial and orientation statistics of dynamical dislocation systems by modeling the dislocations as a stochastic fiber process. Statistical measures have been introduced for the density, velocity, and flux of dislocations, and the connection between these measures and the dislocation state and plastic distortion rate in the crystal is explained. A dislocation dynamics simulation model has been used to extract numerical data to study the evolution of these statistical measures numerically in a body-centered cubic crystal under deformation. The orientation distribution of the dislocation density, velocity and dislocation flux, as well as the dislocation correlations have been computed. The importance of the statistical measures introduced here in building continuum models of dislocation systems is highlighted.

  18. Analysis of Obstacle Hardening Models Using Dislocation Dynamics: Application to Irradiation-Induced Defects

    NASA Astrophysics Data System (ADS)

    Sobie, Cameron; Bertin, Nicolas; Capolungo, Laurent

    2015-08-01

    Irradiation hardening in -iron represents a critical factor in nuclear reactor design and lifetime prediction. The dispersed barrier hardening, Friedel Kroupa Hirsch (FKH), and Bacon Kocks Scattergood (BKS) models have been proposed to predict hardening caused by dislocation obstacles in metals, but the limits of their applicability have never been investigated for varying defect types, sizes, and densities. In this work, dislocation dynamics calculations of irradiation-induced obstacle hardening in the athermal case were compared to these models for voids, self-interstitial atom (SIA) loops, and a combination of the two types. The BKS model was found to accurately predict hardening due to voids, whereas the FKH model was superior for SIA loops. For both loops and voids, the hardening from a normal distribution of defects was compared to that from the mean size, and was shown to have no statistically significant dependence on the distribution. A mean size approach was also shown to be valid for an asymmetric distribution of voids. A non-linear superposition principle was shown to predict the hardening from the simultaneous presence of voids and SIA loops.

  19. Coseismic deformation induced by the Sumatra earthquake

    NASA Astrophysics Data System (ADS)

    Boschi, E.; Casarotti, E.; Devoti, R.; Melini, D.; Piersanti, A.; Pietrantonio, G.; Riguzzi, F.

    2006-08-01

    The giant Sumatra-Andaman earthquake of December 26, 2004 caused permanent deformations effects in a region of previously never observed extension. The GPS data from the worldwide network of permanent IGS sites show significant coseismic displacements in an area exceeding 10 7 km 2, reaching most of South-East Asia, besides Indonesia and India. We have analyzed long GPS time series histories in order to characterize the noise type of each site and, consequently, to precisely assess the formal errors of the coseismic offset estimates. The synthetic simulations of the coseismic displacement field obtained by means of a spherical model using different rupture histories indicate that a major part of the energy release took place in a fault plane similar to that obtained by Ammon et al. (2005) and Vigny et al. (2005) but with a larger amount of compressional slip on the northern segment of the fault area.

  20. Role of plastic deformation in shock-induced phase transitions

    NASA Astrophysics Data System (ADS)

    Ghimire, Punam; Germann, T. C.; Ravelo, R.

    2015-06-01

    Non-equilibrium molecular dynamics (NEMD) simulations of shock-wave propagation in fcc single crystals exhibit high elastic limits and large anisotropies in the yield strength. They can be used to explore the role of plastic deformation in the morphology and kinetics of solid-solid phase transformations. We report on large-scale atomistic simulations of defect-mediated phase transformations under shock and quasi-isentropic compression (QIC). An analytical embedded atom method (EAM) description is used to model a fcc-bcc phase transition (PT) boundary fitted to occur below or above the elastic-plastic threshold in order to model systems undergoing a PT with and without plasticity. For cases where plastic deformation precedes the phase transformation, the defect-mediated PT proceeds at faster rates than the defect-free ones. The bcc fraction growth rate can be correlated with a sharp decrease in the dislocation densities originally present in the parent phase. This work was supported by the Air Force Office of Scientific Research under AFOSR Award FA9550-12-1-0476. Work at Los Alamos was performed under the auspices of the U.S. Department of Energy (DOE) under Contract No. DE-AC52-06NA25396.

  1. Deformation twinning induced decomposition of lamellar LPSO structure and its re-precipitation in an Mg-Zn-Y alloy

    PubMed Central

    Shao, X. H.; Zheng, S. J.; Chen, D.; Jin, Q. Q.; Peng, Z. Z.; Ma, X. L.

    2016-01-01

    The high hardness or yield strength of an alloy is known to benefit from the presence of small-scale precipitation, whose hardening effect is extensively applied in various engineering materials. Stability of the precipitates is of critical importance in maintaining the high performance of a material under mechanical loading. The long period stacking ordered (LPSO) structures play an important role in tuning the mechanical properties of an Mg-alloy. Here, we report deformation twinning induces decomposition of lamellar LPSO structures and their re-precipitation in an Mg-Zn-Y alloy. Using atomic resolution scanning transmission electron microscopy (STEM), we directly illustrate that the misfit dislocations at the interface between the lamellar LPSO structure and the deformation twin is corresponding to the decomposition and re-precipitation of LPSO structure, owing to dislocation effects on redistribution of Zn/Y atoms. This finding demonstrates that deformation twinning could destabilize complex precipitates. An occurrence of decomposition and re-precipitation, leading to a variant spatial distribution of the precipitates under plastic loading, may significantly affect the precipitation strengthening. PMID:27435638

  2. Deformation twinning induced decomposition of lamellar LPSO structure and its re-precipitation in an Mg-Zn-Y alloy

    NASA Astrophysics Data System (ADS)

    Shao, X. H.; Zheng, S. J.; Chen, D.; Jin, Q. Q.; Peng, Z. Z.; Ma, X. L.

    2016-07-01

    The high hardness or yield strength of an alloy is known to benefit from the presence of small-scale precipitation, whose hardening effect is extensively applied in various engineering materials. Stability of the precipitates is of critical importance in maintaining the high performance of a material under mechanical loading. The long period stacking ordered (LPSO) structures play an important role in tuning the mechanical properties of an Mg-alloy. Here, we report deformation twinning induces decomposition of lamellar LPSO structures and their re-precipitation in an Mg-Zn-Y alloy. Using atomic resolution scanning transmission electron microscopy (STEM), we directly illustrate that the misfit dislocations at the interface between the lamellar LPSO structure and the deformation twin is corresponding to the decomposition and re-precipitation of LPSO structure, owing to dislocation effects on redistribution of Zn/Y atoms. This finding demonstrates that deformation twinning could destabilize complex precipitates. An occurrence of decomposition and re-precipitation, leading to a variant spatial distribution of the precipitates under plastic loading, may significantly affect the precipitation strengthening.

  3. Deformation twinning induced decomposition of lamellar LPSO structure and its re-precipitation in an Mg-Zn-Y alloy.

    PubMed

    Shao, X H; Zheng, S J; Chen, D; Jin, Q Q; Peng, Z Z; Ma, X L

    2016-01-01

    The high hardness or yield strength of an alloy is known to benefit from the presence of small-scale precipitation, whose hardening effect is extensively applied in various engineering materials. Stability of the precipitates is of critical importance in maintaining the high performance of a material under mechanical loading. The long period stacking ordered (LPSO) structures play an important role in tuning the mechanical properties of an Mg-alloy. Here, we report deformation twinning induces decomposition of lamellar LPSO structures and their re-precipitation in an Mg-Zn-Y alloy. Using atomic resolution scanning transmission electron microscopy (STEM), we directly illustrate that the misfit dislocations at the interface between the lamellar LPSO structure and the deformation twin is corresponding to the decomposition and re-precipitation of LPSO structure, owing to dislocation effects on redistribution of Zn/Y atoms. This finding demonstrates that deformation twinning could destabilize complex precipitates. An occurrence of decomposition and re-precipitation, leading to a variant spatial distribution of the precipitates under plastic loading, may significantly affect the precipitation strengthening. PMID:27435638

  4. Temperature dependence of optically induced cell deformations

    NASA Astrophysics Data System (ADS)

    Fritsch, Anatol; Kiessling, Tobias R.; Stange, Roland; Kaes, Josef A.

    2012-02-01

    The mechanical properties of any material change with temperature, hence this must be true for cellular material. In biology many functions are known to undergo modulations with temperature, like myosin motor activity, mechanical properties of actin filament solutions, CO2 uptake of cultured cells or sex determination of several species. As mechanical properties of living cells are considered to play an important role in many cell functions it is surprising that only little is known on how the rheology of single cells is affected by temperature. We report the systematic temperature dependence of single cell deformations in Optical Stretcher (OS) measurements. The temperature is changed on a scale of about 20 minutes up to hours and compared to defined temperature shocks in the range of milliseconds. Thereby, a strong temperature dependence of the mechanics of single suspended cells is revealed. We conclude that the observable differences arise rather from viscosity changes of the cytosol than from structural changes of the cytoskeleton. These findings have implications for the interpretation of many rheological measurements, especially for laser based approaches in biological studies.

  5. Superplastic deformation induced by cyclic hydrogen charging

    SciTech Connect

    Choe, Heeman; Schuh, Christopher A.; Dunand, David C.

    2008-05-15

    Deformation under the combined action of external stress and cyclic hydrogen charging/discharging is studied in a model material, titanium. Cyclic charging with hydrogen is carried out at 860 deg. C, which repeatedly triggers the transformation between hydrogen-lean {alpha}-Ti and hydrogen-rich {beta}-Ti. Due to bias from the externally applied tensile stress, the internal mismatch strains produced by this isothermal {alpha}-{beta} transformation accumulate preferentially along the loading axis. These strain increments are linearly proportional to the applied stress, i.e., flow is ideally Newtonian, at small stress levels (below {approx}2 MPa). Therefore, after multiple chemical cycles, a tensile engineering strain of 100% is achieved without fracture, with an average strain rate of 10{sup -5} s{sup -1}, which demonstrates for the first time that superplastic elongations can be achieved by chemical cycling. The effect of hydrogen partial pressure, cycle time, and external stress on the value of the superplastic strain increments is experimentally measured and discussed in light of a diffusional phase transformation model. Special attention is paid to understanding the two contributions to the internal mismatch strains from the phase transformation and lattice swelling.

  6. Elastic image registration via rigid object motion induced deformation

    NASA Astrophysics Data System (ADS)

    Zheng, Xiaofen; Udupa, Jayaram K.; Hirsch, Bruce E.

    2011-03-01

    In this paper, we estimate the deformations induced on soft tissues by the rigid independent movements of hard objects and create an admixture of rigid and elastic adaptive image registration transformations. By automatically segmenting and independently estimating the movement of rigid objects in 3D images, we can maintain rigidity in bones and hard tissues while appropriately deforming soft tissues. We tested our algorithms on 20 pairs of 3D MRI datasets pertaining to a kinematic study of the flexibility of the ankle complex of normal feet as well as ankles affected by abnormalities in foot architecture and ligament injuries. The results show that elastic image registration via rigid object-induced deformation outperforms purely rigid and purely nonrigid approaches.

  7. Freezing-induced deformation of biomaterials in cryomedicine

    NASA Astrophysics Data System (ADS)

    Ozcelikkale, Altug

    Cryomedicine utilizes low temperature treatments of biological proteins, cells and tissues for cryopreservation, materials processing and cryotherapy. Lack of proper understanding of cryodamage that occurs during these applications remains to be the primary bottleneck for development of successful tissue cryopreservation and cryosurgery procedures. An engineering approach based on a view of biological systems as functional biomaterials can help identify, predict and control the primary cryodamage mechanisms by developing an understanding of underlying freezing-induced biophysical processes. In particular, freezing constitutes the main structural/mechanical origin of cryodamage and results in significant deformation of biomaterials at multiple length scales. Understanding of these freezing-induced deformation processes and their effects on post-thaw biomaterial functionality is currently lacking but will be critical to engineer improved cryomedicine procedures. This dissertation addresses this problem by presenting three separate but related studies of freezing-induced deformation at multiple length scales including nanometer-scale protein fibrils, single cells and whole tissues. A combination of rigorous experimentation and computational modeling is used to characterize post-thaw biomaterial structure and properties, predict biomaterial behavior and assess its post-thaw biological functionality. Firstly, freezing-induced damage on hierarchical extracellular matrix structure of collagen is investigated at molecular, fibril and matrix levels. Results indicate to a specific kind of fibril damage due to freezing-induced expansion of intrafibrillar fluid. This is followed by a study of freezing-induced cell and tissue deformation coupled to osmotically driven cellular water transport. Computational and semi empirical modeling of these processes indicate that intracellular deformation of the cell during freezing is heterogeneous and can interfere with cellular water

  8. Relative Stability of Deformed Cube in Warm and Hot Deformed AA6022: Possible Role of Strain-Induced Boundary Migration

    NASA Astrophysics Data System (ADS)

    Raveendra, S.; Paranjape, H.; Mishra, S.; Weiland, H.; Doherty, R. D.; Samajdar, I.

    2009-09-01

    AA6022 samples were deformed at different temperatures and strain rates and to respective true strains of 1.0 and 2.0. Though the deformation was immediately followed by quenching, the as-deformed samples contained recrystallized grains. A convention was adopted, based on grain size and in-grain misorientation developments, to distinguish between the deformed and recrystallized grains/orientations. At the highest deformation temperature(s), a clear pattern of increase (with increase in strain) in deformed cube left\\{ {001} right\\}left< {100} rightrangle was established. Such a pattern could not be explained from Taylor type deformation texture simulations, even after allowing nonoctahedral as well as octahedral slip. Microstructural observations could link, statistically, the increased cube presence with the presence of thicker deformed cube bands. The process of thickening was hypothesized as strain-induced boundary migration (SIBM) of the deformed cube bands into the adjacent noncube.

  9. Interaction of a screw dislocation with a nano-sized, arbitrarily shaped inhomogeneity with interface stresses under anti-plane deformations.

    PubMed

    Wang, Xu; Schiavone, Peter

    2014-10-01

    We propose an elegant and concise general method for the solution of a problem involving the interaction of a screw dislocation and a nano-sized, arbitrarily shaped, elastic inhomogeneity in which the contribution of interface/surface elasticity is incorporated using a version of the Gurtin-Murdoch model. The analytic function inside the arbitrarily shaped inhomogeneity is represented in the form of a Faber series. The real periodic function arising from the contribution of the surface mechanics is then expanded as a Fourier series. The resulting system of linear algebraic equations is solved through the use of simple matrix algebra. When the elastic inhomogeneity represents a hole, our solution method simplifies considerably. Furthermore, we undertake an analytical investigation of the challenging problem of a screw dislocation interacting with two closely spaced nano-sized holes of arbitrary shape in the presence of surface stresses. Our solutions quite clearly demonstrate that the induced elastic fields and image force acting on the dislocation are indeed size-dependent. PMID:25294965

  10. Hydrogen-Induced Plastic Deformation in ZnO

    NASA Astrophysics Data System (ADS)

    Lukáč, F.; Čížek, J.; Vlček, M.; Procházka, I.; Anwand, W.; Brauer, G.; Traeger, F.; Rogalla, D.; Becker, H.-W.

    In the present work hydrothermally grown ZnO single crystals covered with Pd over-layer were electrochemically loaded with hydrogen and the influence of hydrogen on ZnO micro structure was investigated by positron annihilation spectroscopy (PAS). Nuclear reaction analysis (NRA) was employed for determination of depth profile of hydrogen concentration in the sample. NRA measurements confirmed that a substantial amount of hydrogen was introduced into ZnO by electrochemical charging. The bulk hydrogen concentration in ZnO determined by NRA agrees well with the concentration estimated from the transported charge using the Faraday's law. Moreover, a subsurface region with enhanced hydrogen concentration was found in the loaded crystals. Slow positron implantation spectroscopy (SPIS) investigations of hydrogen-loaded crystal revealed enhanced concentration of defects in the subsurface region. This testifies hydrogen-induced plastic deformation of the loaded crystal. Absorbed hydrogen causes a significant lattice expansion. At low hydrogen concentrations this expansion is accommodated by elastic straining, but at higher concentrations hydrogen-induced stress exceeds the yield stress in ZnO and plastic deformation of the loaded crystal takes place. Enhanced hydrogen concentration detected in the subsurface region by NRA is, therefore, due to excess hydrogen trapped at open volume defects introduced by plastic deformation. Moreover, it was found that hydrogen-induced plastic deformation in the subsurface layer leads to typical surface modification: formation of hexagonal shape pyramids on the surface due to hydrogen-induced slip in the [0001] direction.

  11. Imaging Dyke-Induced Deformation in the Lab

    NASA Astrophysics Data System (ADS)

    Ellwood, H.; Rivalta, E.; Wright, T. J.; O'Shea, K.; Hogg, D. C.; Boyle, R. D.

    2008-12-01

    The presence of a dike in Earth's crust can be inferred from a combination of seismological and geodetic techniques. Often, geodetic measurements (e.g. GPS and InSAR) of surface deformation near a dike are inverted for dike parameters using the equations for rectangular tensile dislocations in an elastic, homogeneous half-space, as presented in Okada (1985). Here, we create artificial dikes in the lab and simulate surface geodetic observations. Because the dike parameters are known, or can be directly observed, we are able to test the validity of standard geodetic models for dike injections. A fluid injection into gelatin is analogous to dike ascent driven by magma buoyancy in the crust; gelatin is an elastic medium (Menand and Tait, 2002) and away from the tip of a magma dike, strains are small, suggesting that deformation of the host rock may also be largely elastic (Delaney and Pollard, 1981). Gelatin has been used in several investigations into the propagation of fluid-filled cracks in the Earth's crust, including the shape and velocity of fluid-filled fractures (Takada, 1990; Dahm, 2000), propagation in layered media (Rivalta et al., 2005) and laccolith growth (Johnson and Pollard, 1973), among others. This study focuses on the deformation at the surface caused by this propagation. We seeded the upper surface of a homogeneous gelatin mass with markers and injected some fluid at the bottom of the gelatin container. We recorded the ascent of the resulting buoyancy-driven fluid-filled fracture from above with two camcorders and from the side with an additional camcorder. Surfaces markers were observed by the camcorders and tracked from one frame to the next. 3D positions were determined using photogrammetry after matching the markers. The resultant time series of surface deformation at each marker are analogous to continuous GPS observations from real dikes. The horizontal accuracy obtainable with HD camcorders was about 0.1 mm. We inverted the surface

  12. Supersonic Dislocation Bursts in Silicon

    PubMed Central

    Hahn, E. N.; Zhao, S.; Bringa, E. M.; Meyers, M. A.

    2016-01-01

    Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolution we successfully predict a dislocation density of 1.5 × 1012 cm−2 within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon. PMID:27264746

  13. Supersonic Dislocation Bursts in Silicon

    DOE PAGESBeta

    Hahn, E. N.; Zhao, S.; Bringa, E. M.; Meyers, M. A.

    2016-06-06

    Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolutionmore » we successfully predict a dislocation density of 1.5 x 10(12) cm(-2) within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon.« less

  14. Supersonic Dislocation Bursts in Silicon.

    PubMed

    Hahn, E N; Zhao, S; Bringa, E M; Meyers, M A

    2016-01-01

    Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolution we successfully predict a dislocation density of 1.5 × 10(12) cm(-2) within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon. PMID:27264746

  15. Supersonic Dislocation Bursts in Silicon

    NASA Astrophysics Data System (ADS)

    Hahn, E. N.; Zhao, S.; Bringa, E. M.; Meyers, M. A.

    2016-06-01

    Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolution we successfully predict a dislocation density of 1.5 × 1012 cm‑2 within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon.

  16. Dislocation dynamics in nanocrystalline nickel.

    PubMed

    Shan, Z W; Wiezorek, J M K; Stach, E A; Follstaedt, D M; Knapp, J A; Mao, S X

    2007-03-01

    It is believed that the dynamics of dislocation processes during the deformation of nanocrystalline materials can only be visualized by computational simulations. Here we demonstrate that observations of dislocation processes during the deformation of nanocrystalline Ni with grain sizes as small as 10 nm can be achieved by using a combination of in situ tensile straining and high-resolution transmission electron microscopy. Trapped unit lattice dislocations are observed in strained grains as small as 5 nm, but subsequent relaxation leads to dislocation recombination. PMID:17359167

  17. Spatiotemporal measurement of freezing-induced deformation of engineered tissues

    PubMed Central

    Teo, Ka Yaw; Dutton, J. Craig; Han, Bumsoo

    2010-01-01

    In order to cryopreserve functional engineered tissues (ETs), the microstructure of the extracellular matrix (ECM) should be maintained as well as the cellular viability since the functionality is closely related to the ECM microstructure. Since the post-thaw ECM microstructure is determined by the deformation of ETs during cryopreservation, freezing-induced deformation of ETs was measured with a newly developed quantum dot (QD)-mediated cell image deformetry system using dermal equivalents as a model tissue. The dermal equivalents were constructed by seeding QD-labeled fibroblasts in type I collagen matrices. After 24 hour incubation, the ETs were directionally frozen by exposing them to a spatial temperature gradient (from 4 °C to −20 °C over a distance of 6 mm). While being frozen, the ETs were consecutively imaged, and consecutive pairs of these images were two-dimensionally cross-correlated to determine the local deformation during freezing. The results showed that freezing induced the deformation of ET, and its magnitude varied with both time and location. The maximum local dilatation was 0.006 s−1 and was always observed at the phase change interface. Due to this local expansion, the unfrozen region in front of the freezing interface experienced compression. This expansion-compression pattern was observed throughout the freezing process. In the unfrozen region, the deformation rate gradually decreased away from the freezing interface. After freezing/thawing, the ET experienced an approximately 28% decrease in thickness and 8% loss in weight. These results indicate that freezing-induced deformation caused the transport of interstitial fluid and the interstitial fluid was extruded. In summary, the results suggest that complex cell-fluid-matrix interactions occur within ETs during freezing, and these interactions determine the post-thaw ECM microstructure and eventual post-thaw tissue functionality. PMID:20459191

  18. Compensation of Gravity-Induced Structural Deformations on a Beam- Waveguide Antenna Using a Deformable Mirror

    NASA Technical Reports Server (NTRS)

    Imbriale, W. A.; Moore, M.; Rochblatt, D. J.; Veruttipong, W.

    1995-01-01

    At the NASA Deep Space Network (DSN) Goldstone Complex, a 34-meter- diameter beam-waveguide antenna, DSS-13, was constructed in 1988-1990 and has become an integral part of an advanced systems program and a test bed for technologies being developed to introduce Ka-band (32 GHz) frequencies into the DSN. A method for compensating the gravity- induced structural deformations in this large antenna is presented.

  19. Understanding Gas-Induced Structural Deformation of ZIF-8.

    PubMed

    Ania, Conchi O; García-Pérez, E; Haro, M; Gutiérrez-Sevillano, J J; Valdés-Solís, T; Parra, J B; Calero, S

    2012-05-01

    ZIF-8 is a zeolitic imidazolate framework with very good thermal and chemical stability that opens up many applications that are not feasible by other metal-organic frameowrks (MOFs) and zeolites. Several works report the adsorption properties of ZIF-8 for strategic gases. However, despite the vast experimental corpus of data reported, there seems yet to be a dearth in the understanding of the gas adsorption properties. In this work we provide insights at a molecular level on the mechanisms governing the ZIF-8 structural deformation during molecular adsorption. We demonstrate that the ZIF-8 structural deformation during the adsorption of different molecules at cryogenic temperature goes beyond the gas-induced rotation of the imidazolate linkers. We combine experimental and simulation studies to demonstrate that this deformation is governed by the polarizability and molecular size and shape of the gases, and that the stepped adsorption behavior is defined by the packing arrangement of the guest inside the host. PMID:26288051

  20. Texture developed during deformation of Transformation Induced Plasticity (TRIP) steels

    NASA Astrophysics Data System (ADS)

    Bhargava, M.; Shanta, C.; Asim, T.; Sushil, M.

    2015-04-01

    Automotive industry is currently focusing on using advanced high strength steels (AHSS) due to its high strength and formability for closure applications. Transformation Induced Plasticity (TRIP) steel is promising material for this application among other AHSS. The present work is focused on the microstructure development during deformation of TRIP steel sheets. To mimic complex strain path condition during forming of automotive body, Limit Dome Height (LDH) tests were conducted and samples were deformed in servo hydraulic press to find the different strain path. FEM Simulations were done to predict different strain path diagrams and compared with experimental results. There is a significant difference between experimental and simulation results as the existing material models are not applicable for TRIP steels. Micro texture studies were performed on the samples using EBSD and X-RD techniques. It was observed that austenite is transformed to martensite and texture developed during deformation had strong impact on limit strain and strain path.

  1. A variational formulation of constrained dislocation dynamics coupled with heat and vacancy diffusion

    NASA Astrophysics Data System (ADS)

    Po, Giacomo; Ghoniem, Nasr

    2014-05-01

    We present a formulation of the discrete Dislocation Dynamics (DD) method based on Onsager's variational principle. The motion of discrete dislocations is treated as a generalized irreversible flux associated with conjugate thermodynamic forces causing internal production of entropy. Intrinsic in the variational principle is the role of physical constraints that limit the choice of generalized fluxes. We leverage the concept of constrained maximization to introduce the requirement that dislocation climb must be sustained by the flux of vacancies into the dislocation core. The constrained variational approach results naturally in the coupling between plastic deformation induced by discrete dislocations, vacancy diffusion, and heat propagation in solid crystals. In particular, this coupling requires that dislocation velocity and chemical potential of vacancies at the dislocation core be found simultaneously. A new numerical formulation of DD that accounts for generalized constraints imposed on dislocations is presented, based on a network discretization of the dislocation configuration. Applications illustrate the significance of constrained motion of dislocations confined in channels and pillars, and the attainment of heterogeneous dislocation structures.

  2. Collective dynamics of dislocations interacting with mobile solute atoms

    NASA Astrophysics Data System (ADS)

    Ovaska, Markus; Paananen, Topi; Laurson, Lasse; Alava, Mikko J.

    2016-04-01

    We study the effect of diffusing solute atoms on the collective dynamics of dislocations in plastically deforming crystals, by simulating a two-dimensional discrete dislocation dynamics model with solute atoms included. We employ various protocols to apply the external stress, including constant, oscillatory and quasistatically increasing stress, and study the resulting dynamics for various values of the solute mobility, temperature, and interaction strength with the dislocations. The values of these parameters dictate if Cottrell clouds are formed around the dislocations, and whether the dislocations are able to drag them along as they move. The relevant solute-induced processes include a temporally increasing average Cottrell cloud size due to cloud merging during the evolution of the dislocation structures subject to constant stresses, and a crossover between a solute-free ‘phase’ and a regime where solute drag is important for cyclic stresses, controlled by the solute mobility and temperature. Statistics of deformation bursts under quasistatic loading exhibit atypical scaling where the average burst size is directly proportional to its duration, and are also affected by solute-induced strain hardening in the high-stress regime.

  3. Nonlinear resonance-assisted tunneling induced by microcavity deformation

    PubMed Central

    Kwak, Hojeong; Shin, Younghoon; Moon, Songky; Lee, Sang-Bum; Yang, Juhee; An, Kyungwon

    2015-01-01

    Noncircular two-dimensional microcavities support directional output and strong confinement of light, making them suitable for various photonics applications. It is now of primary interest to control the interactions among the cavity modes since novel functionality and enhanced light-matter coupling can be realized through intermode interactions. However, the interaction Hamiltonian induced by cavity deformation is basically unknown, limiting practical utilization of intermode interactions. Here we present the first experimental observation of resonance-assisted tunneling in a deformed two-dimensional microcavity. It is this tunneling mechanism that induces strong inter-mode interactions in mixed phase space as their strength can be directly obtained from a separatrix area in the phase space of intracavity ray dynamics. A selection rule for strong interactions is also found in terms of angular quantum numbers. Our findings, applicable to other physical systems in mixed phase space, make the interaction control more accessible. PMID:25759322

  4. Defect-induced incompatability of elastic strains: dislocations within the Landau theory of martensitic phase transformations

    SciTech Connect

    Groger, Roman1; Lockman, Turab; Saxena, Avadh

    2008-01-01

    In dislocation-free martensites the components of the elastic strain tensor are constrained by the Saint-Venant compatibility condition which guarantees continuity of the body during external loading. However, in dislocated materials the plastic part of the distortion tensor introduces a displacement mismatch that is removed by elastic relaxation. The elastic strains are then no longer compatible in the sense of the Saint-Venant law and the ensuing incompatibility tensor is shown to be proportional to the gradients of the Nye dislocation density tensor. We demonstrate that the presence of this incompatibility gives rise to an additional long-range contribution in the inhomogeneous part of the Landau energy functional and to the corresponding stress fields. Competition among the local and long-range interactions results in frustration in the evolving order parameter (elastic) texture. We show how the Peach-Koehler forces and stress fields for any distribution of dislocations in arbitrarily anisotropic media can be calculated and employed in a Fokker-Planck dynamics for the dislocation density. This approach represents a self-consistent scheme that yields the evolutions of both the order parameter field and the continuous dislocation density. We illustrate our method by studying the effects of dislocations on microstructure, particularly twinned domain walls, in an Fe-Pd alloy undergoing a martensitic transformation.

  5. Texture of Nanocrystalline Nickel: Probing the Lower Size Limit of Dislocation Activity

    NASA Astrophysics Data System (ADS)

    Chen, Bin; Lutker, Katie; Raju, Selva Vennila; Yan, Jinyuan; Kanitpanyacharoen, Waruntorn; Lei, Jialin; Yang, Shizhong; Wenk, Hans-Rudolf; Mao, Ho-kwang; Williams, Quentin

    2012-12-01

    The size of nanocrystals provides a limitation on dislocation activity and associated stress-induced deformation. Dislocation-mediated plastic deformation is expected to become inactive below a critical particle size, which has been proposed to be between 10 and 30 nanometers according to computer simulations and transmission electron microscopy analysis. However, deformation experiments at high pressure on polycrystalline nickel suggest that dislocation activity is still operative in 3-nanometer crystals. Substantial texturing is observed at pressures above 3.0 gigapascals for 500-nanometer nickel and at greater than 11.0 gigapascals for 20-nanometer nickel. Surprisingly, texturing is also seen in 3-nanometer nickel when compressed above 18.5 gigapascals. The observations of pressure-promoted texturing indicate that under high external pressures, dislocation activity can be extended down to a few-nanometers-length scale.

  6. Vitamin A Deficiency Induces Congenital Spinal Deformities in Rats

    PubMed Central

    Li, Zheng; Shen, Jianxiong; Wu, William Ka Kei; Wang, Xiaojuan; Liang, Jinqian; Qiu, Guixing; Liu, Jiaming

    2012-01-01

    Most cases of congenital spinal deformities were sporadic and without strong evidence of heritability. The etiology of congenital spinal deformities is still elusive and assumed to be multi-factorial. The current study seeks to elucidate the effect of maternal vitamin A deficiency and the production of congenital spinal deformities in the offsping. Thirty two female rats were randomized into two groups: control group, which was fed a normal diet; vitamin A deficient group, which were given vitamin A-deficient diet from at least 2 weeks before mating till delivery. Three random neonatal rats from each group were killed the next day of parturition. Female rats were fed an AIN-93G diet sufficient in vitamin A to feed the rest of neonates for two weeks until euthanasia. Serum levels of vitamin A were assessed in the adult and filial rats. Anteroposterior (AP) spine radiographs were obtained at week 2 after delivery to evaluate the presence of the skeletal abnormalities especially of spinal deformities. Liver and vertebral body expression of retinaldehyde dehydrogenase (RALDHs) and RARs mRNA was assessed by reverse transcription-real time PCR. VAD neonates displayed many skeletal malformations in the cervical, thoracic, the pelvic and sacral and limbs regions. The incidence of congenital scoliosis was 13.79% (8/58) in the filial rats of vitamin A deficiency group and 0% in the control group. Furthermore, vitamin A deficiency negatively regulate the liver and verterbral body mRNA levels of RALDH1, RALDH2, RALDH3, RAR-α, RAR-β and RAR-γ. Vitamin A deficiency in pregnancy may induce congenital spinal deformities in the postnatal rats. The decreases of RALDHs and RARs mRNA expression induced by vitamin A deprivation suggest that vertebral birth defects may be caused by a defect in RA signaling pathway during somitogenesis. PMID:23071590

  7. Strong luminescence induced by elastic deformation of piezoelectric crystals

    NASA Astrophysics Data System (ADS)

    Chandra, V. K.; Chandra, B. P.; Jha, Piyush

    2013-06-01

    The luminescence induced by elastic deformation of solids, called the phenomenon of elastico-mechanoluminescence (EML), is observed in several materials. For applied pressure in the range of 17 MPa, certain crystals emit intense EML, which can be seen in day light with naked eye. In the present paper, we explore that, as the piezoelectric constant near the photo-generated electric dipoles formed by trapping of charge carriers in crystals is several times higher as compared to that at normal sites, the piezoelectrically induced detrapping of charge carriers and EML emission may take place for less value of the pressure applied onto the crystals.

  8. Radiation-induced strengthening and absorption of dislocation loops in ferritic Fe-Cr alloys: the role of Cr segregation.

    PubMed

    Terentyev, D; Bakaev, A

    2013-07-01

    The understanding of radiation-induced strengthening in ferritic FeCr-based steels remains an essential issue in the assessment of materials for fusion and fission reactors. Both early and recent experimental works on Fe-Cr alloys reveal Cr segregation on radiation-induced nanostructural features (mainly dislocation loops), whose impact on the modification of the mechanical response of the material might be key for explaining quantitatively the radiation-induced strengthening in these alloys. In this work, we use molecular dynamics to study systematically the interaction of dislocations with 1/2<111> and <100> loops in all possible orientations, both enriched by Cr atoms and undecorated, for different temperatures, loop sizes and dislocation velocities. The configurations of the enriched loops have been obtained using a non-rigid lattice Monte Carlo method. The study reveals that Cr segregation influences the interaction mechanisms with both 1/2<111> and <100> loops. The overall effect of Cr enrichment is to penalize the mobility of intrinsically glissile 1/2<111> loops, modifying the reaction mechanisms as a result. The following three most important effects associated with Cr enrichment have been revealed: (i) absence of dynamic drag; (ii) suppression of complete absorption; (iii) enhanced strength of small dislocation loops (2 nm and smaller). Overall the effect of the Cr enrichment is therefore to increase the unpinning stress, so experimentally 'invisible' nanostructural features may also contribute to radiation-induced strengthening. The reasons for the modification of the mechanisms are explained and the impact of the loading conditions is discussed. PMID:23756468

  9. Dislocation-induced Charges in Quantum Dots: Step Alignment and Radiative Emission

    NASA Technical Reports Server (NTRS)

    Leon, R.; Okuno, J.; Lawton, R.; Stevens-Kalceff, M.; Phillips, M.; Zou, J.; Cockayne, D.; Lobo, C.

    1999-01-01

    A transition between two types of step alignment was observed in a multilayered InGaAs/GaAs quantum-dot (QD) structure. A change to larger QD sizes in smaller concentrations occurred after formation of a dislocation array.

  10. The analysis of severely deformed pure Fe structure aided by X-ray diffraction profile

    NASA Astrophysics Data System (ADS)

    Forouzanmehr, Nazanin; Nili-Ahmadabadi, Mahmoud; Bönisch, Matthias

    2016-06-01

    Pure Fe was severely deformed by a combination of shaped cold rolling and cold drawing. X-ray diffraction profiles analysis was applied in accordance with the Williamson-Hall (WH) and modified Williamson-Hall (MWH) methods to identify crystallite sizes of the deformed specimens. It was found that some differences exist between the results of WH and MWH procedures using the hkl dependent Young's modulus or considering the average dislocation contrast factor. The latter method is more accurate and enables the determination of the character of dislocations in plastically deformed Fe. It was shown that by increasing deformation strain, the screw dislocations dominated. The enhancement of hardness occurs in the deformed Fe due to grain refinement, dislocation accumulation and deformation-induced vacancies.

  11. Tritium deformation interactions in FCC austenitic steels

    SciTech Connect

    Chene, J.

    2008-07-15

    Hydrogen deformation interactions are known to control the environmental degradation effects (H and He embrittlement, stress corrosion cracking,...) associated with the presence of H and its isotopes in structural materials. Different types of interaction have been investigated: trapping on stress field and strain-induced defects, enhanced diffusion along dislocation networks, transport by moving dislocation. For several reasons, the quantification of these interactions is a major challenge in nuclear systems involving the presence of tritium: prevention of tritium-induced damage, tritium inventory, management of tritiated waste... This paper reports recent results on the quantitative characterization of tritium deformation interactions in fee materials. (authors)

  12. Deformation field of the soft substrate induced by capillary force

    NASA Astrophysics Data System (ADS)

    Liu, J. L.; Nie, Z. X.; Jiang, W. G.

    2009-05-01

    Prediction on the deformation of a soft substrate induced by capillary force has been widely paid attention in the broad range of applications, such as metallurgy, material science, astronavigation, micro/nano-technology, etc., which is also a supplementary result to the classical Young's equation. We quantitatively analyzed the deformation of an elastic substrate under capillary force by means of the energy principle and the continuum mechanics method. The actual drop's morphology was investigated and was compared with that calculated based on the classical spherical shape assumption of the droplet. The displacement field of the substrate was obtained, especially, its singularity at the droplet edge was also discussed. The results are beneficial to engineering application and micro/nano-measurement.

  13. Deformation mechanisms of bent Si nanowires governed by the sign and magnitude of strain

    NASA Astrophysics Data System (ADS)

    Wang, Lihua; Kong, Deli; Xin, Tianjiao; Shu, Xinyu; Zheng, Kun; Xiao, Lirong; Sha, Xuechao; Lu, Yan; Zhang, Ze; Han, Xiaodong; Zou, Jin

    2016-04-01

    In this study, the deformation mechanisms of bent Si nanowires are investigated at the atomic scale with bending strain up to 12.8%. The sign and magnitude of the applied strain are found to govern their deformation mechanisms, in which the dislocation types (full or partial dislocations) can be affected by the sign (tensile or compressive) and magnitude of the applied strain. In the early stages of bending, plastic deformation is controlled by 60° full dislocations. As the bending increases, Lomer dislocations can be frequently observed. When the strain increases to a significant level, 90° partial dislocations induced from the tensile surfaces of the bent nanowires are observed. This study provides a deeper understanding of the effect of the sign and magnitude of the bending strain on the deformation mechanisms in bent Si nanowires.

  14. Atomic-scale configurations of synchroshear-induced deformation twins in the ionic MnS crystal

    PubMed Central

    Zhou, Y. T.; Xue, Y. B.; Chen, D.; Wang, Y. J.; Zhang, B.; Ma, X. L.

    2014-01-01

    Deformation twinning was thought as impossible in ionic compounds with rock-salt structure due to the charge effect on {111} planes. Here we report the presence and formation mechanism of deformation {111} twins in the rock-salt manganese sulphide (MnS) inclusions embedded in a hot-rolled stainless steel. Based on the atomic-scale mapping under aberration-corrected scanning transmission electron microscopy, a dislocation-based mechanism involved two synchronized shear on adjacent atomic layers is proposed to describe the dislocation glide and consequently twinning formation. First-principles calculations of the energy barriers for twinning formation in MnS and comparing with that of PbS and MgO indicate the distinct dislocation glide scheme and deformation behaviors for the rock-salt compounds with different ionicities. This study may improve our understanding of the deformation mechanisms of rock-salt crystals and other ionic compounds. PMID:24874022

  15. Atomic-scale configurations of synchroshear-induced deformation twins in the ionic MnS crystal

    NASA Astrophysics Data System (ADS)

    Zhou, Y. T.; Xue, Y. B.; Chen, D.; Wang, Y. J.; Zhang, B.; Ma, X. L.

    2014-05-01

    Deformation twinning was thought as impossible in ionic compounds with rock-salt structure due to the charge effect on {111} planes. Here we report the presence and formation mechanism of deformation {111} twins in the rock-salt manganese sulphide (MnS) inclusions embedded in a hot-rolled stainless steel. Based on the atomic-scale mapping under aberration-corrected scanning transmission electron microscopy, a dislocation-based mechanism involved two synchronized shear on adjacent atomic layers is proposed to describe the dislocation glide and consequently twinning formation. First-principles calculations of the energy barriers for twinning formation in MnS and comparing with that of PbS and MgO indicate the distinct dislocation glide scheme and deformation behaviors for the rock-salt compounds with different ionicities. This study may improve our understanding of the deformation mechanisms of rock-salt crystals and other ionic compounds.

  16. The computation of induced drag with nonplanar and deformed wakes

    NASA Technical Reports Server (NTRS)

    Kroo, Ilan; Smith, Stephen

    1991-01-01

    The classical calculation of inviscid drag, based on far field flow properties, is reexamined with particular attention to the nonlinear effects of wake roll-up. Based on a detailed look at nonlinear, inviscid flow theory, it is concluded that many of the classical, linear results are more general than might have been expected. Departures from the linear theory are identified and design implications are discussed. Results include the following: Wake deformation has little effect on the induced drag of a single element wing, but introduces first order corrections to the induced drag of a multi-element lifting system. Far field Trefftz-plane analysis may be used to estimate the induced drag of lifting systems, even when wake roll-up is considered, but numerical difficulties arise. The implications of several other approximations made in lifting line theory are evaluated by comparison with more refined analyses.

  17. RNGCHN: a program to calculate displacement components from dislocations in an elastic half-space with applications for modeling geodetic measurements of crustal deformation

    NASA Astrophysics Data System (ADS)

    Feigl, Kurt L.; Dupré, Emmeline

    1999-07-01

    The RNGCHN program calculates a single component of the displacement field due to a finite or point-source dislocation buried in an elastic half space. This formulation approximates the surface movements produced by earthquake faulting or volcanic intrusion. As such, it is appropriate for modeling crustal deformation measured by geodetic surveying techniques, such as spirit leveling, trilateration, Very Long Baseline Interferometry (VLBI), Global Positioning System (GPS), or especially interferometric analysis of synthetic aperture radar (SAR) images. Examples suggest that this model can fit simple coseismic earthquake signatures to within their measurement uncertainties. The program's input parameters include fault position, depth, length, width, strike, dip, and three components of slip. The output consists of displacement components in the form of an ASCII list or a rectangular array of binary integers. The same program also provides partial derivatives of the displacement component with respect to all 10 input parameters. The FORTRAN source code for the program is in the public domain and available as the compressed tar file rngchn.tar.Z in the directory/pub/GRGS via the Internet by anonymous ftp to spike.cst. cnes.fr. This distribution includes worked examples and a MATLAB interface.

  18. Dislocation boundaries and active slip systems

    SciTech Connect

    Wert, J.A.; Hansen, N.

    1995-11-01

    Part of the dislocations which have participated in the plastic deformation of a polycrystalline metal are stored in dislocation boundaries in a two- or three-dimensional arrangement. The dislocation in such boundaries can be analyzed by determining the misorientation between neighboring crystallites and the boundary orientation. Information about the dislocations in the boundaries can also be obtained by an analysis of active slip systems based on the crystallite orientation and the imposed stress or strain state in combination with appropriate constraint conditions. In the present paper an analysis of the boundary dislocation structure and of the slip systems has been conducted for pure aluminium cold-rolled to a von Mises strain of 0.41. The results show that a substantial majority of dislocations in different types of dislocation boundaries are from the primary and conjugate slip system in the adjoining crystallites. A basis is therefore provided for integrating deformation structure observations with plastic deformation behavior.

  19. Transition of dislocation nucleation induced by local stress concentration in nanotwinned copper

    PubMed Central

    Lu, N.; Du, K.; Lu, L.; Ye, H. Q.

    2015-01-01

    Metals with a high density of nanometre-scale twins have demonstrated simultaneous high strength and good ductility, attributed to the interaction between lattice dislocations and twin boundaries. Maximum strength was observed at a critical twin lamella spacing (∼15 nm) by mechanical testing; hence, an explanation of how twin lamella spacing influences dislocation behaviours is desired. Here, we report a transition of dislocation nucleation from steps on the twin boundaries to twin boundary/grain boundary junctions at a critical twin lamella spacing (12–37 nm), observed with in situ transmission electron microscopy. The local stress concentrations vary significantly with twin lamella spacing, thus resulting in a critical twin lamella spacing (∼18 nm) for the transition of dislocation nucleation. This agrees quantitatively with the mechanical test. These results demonstrate that by quantitatively analysing local stress concentrations, a direct relationship can be resolved between the microscopic dislocation activities and macroscopic mechanical properties of nanotwinned metals. PMID:26179409

  20. Transition of dislocation nucleation induced by local stress concentration in nanotwinned copper.

    PubMed

    Lu, N; Du, K; Lu, L; Ye, H Q

    2015-01-01

    Metals with a high density of nanometre-scale twins have demonstrated simultaneous high strength and good ductility, attributed to the interaction between lattice dislocations and twin boundaries. Maximum strength was observed at a critical twin lamella spacing (∼15 nm) by mechanical testing; hence, an explanation of how twin lamella spacing influences dislocation behaviours is desired. Here, we report a transition of dislocation nucleation from steps on the twin boundaries to twin boundary/grain boundary junctions at a critical twin lamella spacing (12-37 nm), observed with in situ transmission electron microscopy. The local stress concentrations vary significantly with twin lamella spacing, thus resulting in a critical twin lamella spacing (∼18 nm) for the transition of dislocation nucleation. This agrees quantitatively with the mechanical test. These results demonstrate that by quantitatively analysing local stress concentrations, a direct relationship can be resolved between the microscopic dislocation activities and macroscopic mechanical properties of nanotwinned metals. PMID:26179409

  1. A Multiscale Model Based On Intragranular Microstructure — Prediction Of Dislocation Patterns At The Microscopic Scale

    NASA Astrophysics Data System (ADS)

    Franz, Gérald; Abed-Meraim, Farid; Zineb, Tarak Ben; Lemoine, Xavier; Berveiller, Marcel

    2007-04-01

    A large strain elastic-plastic single crystal constitutive law, based on dislocation annihilation and storage, is implemented in a new self-consistent scheme, leading to a multiscale model which achieves, for each grain, the calculation of plastic slip activity, with help of regularized formulation drawn from visco-plasticity, and dislocation microstructure evolution. This paper focuses on the relationship between the deformation history of a BCC grain and induced microstructure during monotonic and two-stage strain paths.

  2. Dislocation Multi-junctions and Strain Hardening

    SciTech Connect

    Bulatov, V; Hsiung, L; Tang, M; Arsenlis, A; Bartelt, M; Cai, W; Florando, J; Hiratani, M; Rhee, M; Hommes, G; Pierce, T; Diaz de la Rubia, T

    2006-06-20

    At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects--dislocations. First theorized in 1934 to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed only two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening: a common phenomenon in which continued deformation increases a crystal's strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions tying dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed hereafter multi-junctions. The existence of multi-junctions is first predicted by Dislocation Dynamics (DD) and atomistic simulations and then confirmed by the transmission electron microscopy (TEM) experiments in single crystal molybdenum. In large-scale Dislocation Dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in BCC crystals.

  3. Thermally induced stresses and deformations in layered composite tubes

    NASA Technical Reports Server (NTRS)

    Cooper, D. E.; Cohen, D.; Rousseau, C. Q.; Hyer, M. W.; Tompkins, S. S.

    1985-01-01

    The thermally induced stresses and deformations in layered, orthotropic tubes are studied. The motivation for studying tubes is their likely application for use in space structures. Tubes are a strong candidate for this application because of their high structural efficiency, as measured by stiffness per unit weight, and their relative ease of fabrication. Also, tubes have no free edges to deteriorate or delaminate. An anticipated thermal condition for tubes in space is a circumferential temperature gradient. This type of gradient will introduce dimensional changes into the structure and may cause stresses large enough to cause damage to the material. There are potentially large differences in temperatures at different circumferential locations on the tube. Because of this, the effects of temperature dependent material properties on the stresses and deformations may be important. The study is composed of three parts: experiments to determine the functional form of the circumferential gradient and to measure tube deflections; an elasticity solution to compute the stresses and deformations; and an approximate approach to determine the effects of temperature dependent material properties.

  4. Electron Beam Induced Artifacts During in situ TEM Deformation of Nanostructured Metals.

    PubMed

    Sarkar, Rohit; Rentenberger, Christian; Rajagopalan, Jagannathan

    2015-01-01

    A critical assumption underlying in situ transmission electron microscopy studies is that the electron beam (e-beam) exposure does not fundamentally alter the intrinsic deformation behavior of the materials being probed. Here, we show that e-beam exposure causes increased dislocation activation and marked stress relaxation in aluminum and gold films spanning a range of thicknesses (80-400 nanometers) and grain sizes (50-220 nanometers). Furthermore, the e-beam induces anomalous sample necking, which unusually depends more on the e-beam diameter than intensity. Notably, the stress relaxation in both aluminum and gold occurs at beam energies well below their damage thresholds. More remarkably, the stress relaxation and/or sample necking is significantly more pronounced at lower accelerating voltages (120 kV versus 200 kV) in both the metals. These observations in aluminum and gold, two metals with highly dissimilar atomic weights and properties, indicate that e-beam exposure can cause anomalous behavior in a broad spectrum of nanostructured materials, and simultaneously suggest a strategy to minimize such artifacts. PMID:26552934

  5. Electron Beam Induced Artifacts During in situ TEM Deformation of Nanostructured Metals

    PubMed Central

    Sarkar, Rohit; Rentenberger, Christian; Rajagopalan, Jagannathan

    2015-01-01

    A critical assumption underlying in situ transmission electron microscopy studies is that the electron beam (e-beam) exposure does not fundamentally alter the intrinsic deformation behavior of the materials being probed. Here, we show that e-beam exposure causes increased dislocation activation and marked stress relaxation in aluminum and gold films spanning a range of thicknesses (80–400 nanometers) and grain sizes (50–220 nanometers). Furthermore, the e-beam induces anomalous sample necking, which unusually depends more on the e-beam diameter than intensity. Notably, the stress relaxation in both aluminum and gold occurs at beam energies well below their damage thresholds. More remarkably, the stress relaxation and/or sample necking is significantly more pronounced at lower accelerating voltages (120 kV versus 200 kV) in both the metals. These observations in aluminum and gold, two metals with highly dissimilar atomic weights and properties, indicate that e-beam exposure can cause anomalous behavior in a broad spectrum of nanostructured materials, and simultaneously suggest a strategy to minimize such artifacts. PMID:26552934

  6. Theory of magnetoresistance due to lattice dislocations in face-centred cubic metals

    NASA Astrophysics Data System (ADS)

    Bian, Q.; Niewczas, M.

    2016-06-01

    A theoretical model to describe the low temperature magneto-resistivity of high purity copper single and polycrystals containing different density and distribution of dislocations has been developed. In the model, magnetoresistivity tensor is evaluated numerically using the effective medium approximation. The anisotropy of dislocation-induced relaxation time is considered by incorporating two independent energy bands with different relaxation times and the spherical and cylindrical Fermi surfaces representing open, extended and closed electron orbits. The effect of dislocation microstructure is introduced by means of two adjustable parameters corresponding to the length and direction of electron orbits in the momentum space, which permits prediction of magnetoresistance of FCC metals containing different density and distribution of dislocations. The results reveal that dislocation microstructure influences the character of the field-dependent magnetoresistivity. In the orientation of the open orbits, the quadratic variation in magnetoresistivity changes to quasi-linear as the density of dislocations increases. In the closed orbit orientation, dislocations delay the onset of magnetoresistivity saturation. The results indicate that in the open orbit orientations of the crystals, the anisotropic relaxation time due to small-angle dislocation scattering induces the upward deviation from Kohler's rule. In the closed orbit orientations Kohler's rule holds, independent of the density of dislocations. The results obtained with the model show good agreement with the experimental measurements of transverse magnetoresistivity in deformed single and polycrystal samples of copper at 2 K.

  7. Development of regional liquefaction-induced deformation hazard maps

    USGS Publications Warehouse

    Rosinski, A.; Knudsen, K.-L.; Wu, J.; Seed, R.B.; Real, C.R.

    2004-01-01

    This paper describes part of a project to assess the feasibility of producing regional (1:24,000-scale) liquefaction hazard maps that are based-on potential liquefaction-induced deformation. The study area is the central Santa Clara Valley, at the south end of San Francisco Bay in Central California. The information collected and used includes: a) detailed Quaternary geological mapping, b) over 650 geotechnical borings, c) probabilistic earthquake shaking information, and d) ground-water levels. Predictions of strain can be made using either empirical formulations or numerical simulations. In this project lateral spread displacements are estimated and new empirical relations to estimate future volumetric and shear strain are used. Geotechnical boring data to are used to: (a) develop isopach maps showing the thickness of sediment thatis likely to liquefy and deform under earthquake shaking; and (b) assess the variability in engineering properties within and between geologic map units. Preliminary results reveal that late Holocene deposits are likely to experience the greatest liquefaction-induced strains, while Holocene and late Pleistocene deposits are likely to experience significantly less horizontal and vertical strain in future earthquakes. Development of maps based on these analyses is feasible.

  8. Atomistic Simulation of shock induced dislocation dynamics and evolution of different plasticity mechanisms in Single Crystal Copper

    NASA Astrophysics Data System (ADS)

    Neogi, Anupam; Mitra, Nilanjan

    Deformation and observation of different types of plasticity mechanisms of FCC metals (e.g. Copper) under shock loading of various intensities has been investigated by several groups of researchers around the globe through different types of experiments and/or atomistic simulations. However, there still exists lacuna in this well researched area. In this study the temporal details of dislocation dynamics are provided. Simulations also demonstrate different types of temporal evolution of different loops observed for single crystal Cu under different intensities of shock loading. Observance of formation of twins and their temporal evolution at higher intensities of shock loading are also demonstrated as part of this study. Comparisons of these NEMD simulations using EAM potential are discussed with regards to different experimental and simulation studies in literature.

  9. Deformation-induced trace element redistribution in zircon revealed using atom probe tomography

    PubMed Central

    Piazolo, Sandra; La Fontaine, Alexandre; Trimby, Patrick; Harley, Simon; Yang, Limei; Armstrong, Richard; Cairney, Julie M.

    2016-01-01

    Trace elements diffuse negligible distances through the pristine crystal lattice in minerals: this is a fundamental assumption when using them to decipher geological processes. For example, the reliable use of the mineral zircon (ZrSiO4) as a U-Th-Pb geochronometer and trace element monitor requires minimal radiogenic isotope and trace element mobility. Here, using atom probe tomography, we document the effects of crystal–plastic deformation on atomic-scale elemental distributions in zircon revealing sub-micrometre-scale mechanisms of trace element mobility. Dislocations that move through the lattice accumulate U and other trace elements. Pipe diffusion along dislocation arrays connected to a chemical or structural sink results in continuous removal of selected elements (for example, Pb), even after deformation has ceased. However, in disconnected dislocations, trace elements remain locked. Our findings have important implications for the use of zircon as a geochronometer, and highlight the importance of deformation on trace element redistribution in minerals and engineering materials. PMID:26868040

  10. Deformation-induced trace element redistribution in zircon revealed using atom probe tomography.

    PubMed

    Piazolo, Sandra; La Fontaine, Alexandre; Trimby, Patrick; Harley, Simon; Yang, Limei; Armstrong, Richard; Cairney, Julie M

    2016-01-01

    Trace elements diffuse negligible distances through the pristine crystal lattice in minerals: this is a fundamental assumption when using them to decipher geological processes. For example, the reliable use of the mineral zircon (ZrSiO4) as a U-Th-Pb geochronometer and trace element monitor requires minimal radiogenic isotope and trace element mobility. Here, using atom probe tomography, we document the effects of crystal-plastic deformation on atomic-scale elemental distributions in zircon revealing sub-micrometre-scale mechanisms of trace element mobility. Dislocations that move through the lattice accumulate U and other trace elements. Pipe diffusion along dislocation arrays connected to a chemical or structural sink results in continuous removal of selected elements (for example, Pb), even after deformation has ceased. However, in disconnected dislocations, trace elements remain locked. Our findings have important implications for the use of zircon as a geochronometer, and highlight the importance of deformation on trace element redistribution in minerals and engineering materials. PMID:26868040

  11. Deformation-induced trace element redistribution in zircon revealed using atom probe tomography

    NASA Astrophysics Data System (ADS)

    Piazolo, Sandra; La Fontaine, Alexandre; Trimby, Patrick; Harley, Simon; Yang, Limei; Armstrong, Richard; Cairney, Julie M.

    2016-02-01

    Trace elements diffuse negligible distances through the pristine crystal lattice in minerals: this is a fundamental assumption when using them to decipher geological processes. For example, the reliable use of the mineral zircon (ZrSiO4) as a U-Th-Pb geochronometer and trace element monitor requires minimal radiogenic isotope and trace element mobility. Here, using atom probe tomography, we document the effects of crystal-plastic deformation on atomic-scale elemental distributions in zircon revealing sub-micrometre-scale mechanisms of trace element mobility. Dislocations that move through the lattice accumulate U and other trace elements. Pipe diffusion along dislocation arrays connected to a chemical or structural sink results in continuous removal of selected elements (for example, Pb), even after deformation has ceased. However, in disconnected dislocations, trace elements remain locked. Our findings have important implications for the use of zircon as a geochronometer, and highlight the importance of deformation on trace element redistribution in minerals and engineering materials.

  12. Deformation-Induced Amorphization of Copper-Titanium Intermetallics

    NASA Astrophysics Data System (ADS)

    Askenazy, Philip Douglas

    Two methods of inducing amorphization in Cu-Ti intermetallic compounds by mechanical means have been investigated. Ingots of compositions Cu_{35}Ti _{65} and Cu_ {33.3}Ti_{66.7} were rapidly quenched into ribbons. The microstructure consisted largely of microcrystals in an amorphous matrix, which were either quenched in or grown by annealing. The ribbons were cold-rolled, which reduced their effective thickness by a factor of about 8. The status of the intermetallic compound CuTi_2 was monitored by x-ray diffraction and transmission electron microscopy (TEM). The crystals were found to amorphize as rolling progressed. This behavior was not reproduced in polycrystalline samples that had no amorphous matrix present initially. The presence of the amorphous phase is thus necessary for amorphization of the crystal: it eliminates the need to nucleate the new glass, and it prevents the ribbon from disintegrating at high deformation stages. It may also change the deformation mechanism that occurs in the crystals, retarding the onset of amorphization. Diffuse scattering in close-packed directions is similar to that seen in electron irradiation experiments. It is postulated that the chemical disorder present in antiphase boundaries caused by deformation raises the free energy of the crystal higher than that of the amorphous phase. Ingots of the same compound were worn against each other in a custom-built wear apparatus. The design eliminates iron contamination of the wear sample and requires relatively small quantities of material. Alteration of the surface structure was monitored by plane-view and cross -sectional TEM. Larger subsurface crystals exhibit diffuse scattering, similar to that found in the rolled samples. A wide range of grain sizes was observed, due to the inhomogeneous nature of the wear process. An unusual phase was observed at the surface, consisting of a nanometer-scale mixture of aligned nanocrystalline regions and disordered areas. Some amorphous phase is

  13. Subtalar dislocation

    SciTech Connect

    El-Khoury, G.Y.; Yousefzadeh, D.K.; Mulligan, G.M.; Moore, T.E.

    1982-05-01

    Over a period of three years we have seen nine patients with subtalar dislocation, all of whom sustained violent trauma to the region of the ankle and hind foot. All but one patient were males. Clinically a subtalar dislocation resembles a complicated fracture dislocation of the ankle but a definitive diagnosis can only be made radiographically. The mechanism of injury and radiographic features of this injury are discussed.

  14. Formation of Nanostructures in Severely Deformed High-Strength Steel Induced by High-Frequency Ultrasonic Impact Treatment

    NASA Astrophysics Data System (ADS)

    Dutta, R. K.; Malet, L.; Gao, H.; Hermans, M. J. M.; Godet, S.; Richardson, I. M.

    2015-02-01

    Surface modification by the generation of a nanostructured surface layer induced via ultrasonic impact treatment was performed at the weld toe of a welded high-strength quenched and tempered structural steel, S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)). Such high-frequency peening techniques are known to improve the fatigue life of welded components. The nanocrystallized structure as a function of depth from the top-treated surface was characterized via a recently developed automated crystal orientation mapping in transmission electron microscopy. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation during the ultrasonic impact treatment is proposed. It involves the formation of low-angle misoriented lamellae displaying a high density of dislocations followed by the subdivision of microbands into blocks and the resulting formation of polygonal submicronic grains. These submicronic grains further breakdown into nano grains. The results show the presence of retained austenite even after severe surface plastic deformation. The average grain size of the retained austenite and martensite is 17 and 35 nm, respectively. The in-grain deformation mechanisms are different in larger and smaller grains. Larger grains show long-range lattice rotations, while smaller grains show plastic deformation through grain rotation. Also the smaller nano grains exhibit the presence of short-range disorder. Surface nanocrystallization also leads to an increased fraction of low angle and low energy coincident site lattice boundaries especially in the smaller grains ( nm).

  15. Synergy of plastic deformation and gas retention in tungsten

    NASA Astrophysics Data System (ADS)

    Terentyev, D.; De Temmerman, G.; Minov, B.; Zayachuk, Y.; Lambrinou, K.; Morgan, T. W.; Dubinko, A.; Bystrov, K.; Van Oost, G.

    2015-01-01

    Taking the example of tungsten, we demonstrate that high-flux plasma exposure of recrystallized and plastically deformed samples leads to principal differences in the gas trapping and associated surface modification. Surface of the exposed pre-deformed samples exhibits ruptured µm-sized blisters, a signature of bubbles nucleated close to the surface on the plastically induced dislocation network. Contrary to the recrystallized samples, no stage attributable to gas bubbles appeared in the desorption spectrum of the deformed samples demonstrating the strong impact of dislocations on hydrogen retention.

  16. Evidence for a transition in deformation mechanism in nanocrystalline pure titanium processed by high-pressure torsion

    NASA Astrophysics Data System (ADS)

    Yang, Chao; Song, Min; Liu, Yong; Ni, Song; Sabbaghianrad, Shima; Langdon, Terence G.

    2016-06-01

    Nanocrystalline titanium with an average grain size of about 60-70 nm was prepared by high-pressure torsion. The results of hardness and structural evolutions indicate that a strain-induced hardening-softening-hardening-softening behaviour occurs. For coarse-grained titanium, -type dislocation multiplication, twinning and a high pressure-induced α-to-ω phase transformation play major roles to accommodate deformation, leading to a significant strain hardening. As deformation proceeds, dynamic recrystallisation leads to a decrease in dislocation density, especially for -type dislocations, leading to a slight strain softening. The -component dislocation multiplication dominates the deformation when the grain size decreases to 100 nm and -component dislocation multiplication, grain refinement and the α-to-ω phase transformation contribute to the second strain hardening. The following strain softening is attributed to dynamic recovery.

  17. Determination of the inhomogeneous dislocation density in a crystallographic texture

    SciTech Connect

    Satdarova, F. F.; Kozlov, D. A.

    2007-03-15

    Diffraction analysis of the mixed dislocation structure of a polycrystal with an orthorhombic texture is reported. A significant difference in the dislocation densities in the texture components of cold-deformed low-carbon steel has been revealed.

  18. Transient luminescence induced by electrical refilling of charge carrier traps of dislocation network at hydrophilically bonded Si wafers interface

    SciTech Connect

    Bondarenko, Anton; Vyvenko, Oleg

    2014-02-21

    Dislocation network (DN) at hydrophilically bonded Si wafers interface is placed in space charge region (SCR) of a Schottky diode at a depth of about 150 nm from Schottky electrode for simultaneous investigation of its electrical and luminescent properties. Our recently proposed pulsed traps refilling enhanced luminescence (Pulsed-TREL) technique based on the effect of transient luminescence induced by refilling of charge carrier traps with electrical pulses is further developed and used as a tool to establish DN energy levels responsible for D1 band of dislocation-related luminescence in Si (DRL). In present work we do theoretical analysis and simulation of traps refilling kinetics dependence on refilling pulse magnitude (Vp) in two levels model: shallow and deep. The influence of initial charge state of deep level on shallow level occupation-Vp dependence is discussed. Characteristic features predicted by simulations are used for Pulsed-TREL experimental results interpretation. We conclude that only shallow (∼0.1 eV from conduction and valence band) energetic levels in the band gap participate in D1 DRL.

  19. AC electric field induced droplet deformation in a microfluidic T-junction.

    PubMed

    Xi, Heng-Dong; Guo, Wei; Leniart, Michael; Chong, Zhuang Zhi; Tan, Say Hwa

    2016-08-01

    We present for the first time an experimental study on the droplet deformation induced by an AC electric field in droplet-based microfluidics. It is found that the deformation of the droplets becomes stronger with increasing electric field intensity and frequency. The measured electric field intensity dependence of the droplet deformation is consistent with an early theoretical prediction for stationary droplets. We also proposed a simple equivalent circuit model to account for the frequency dependence of the droplet deformation. The model well explains our experimental observations. In addition, we found that the droplets can be deformed repeatedly by applying an amplitude modulation (AM) signal. PMID:27173587

  20. Hydrogen-induced change in core structures of {110}[111] edge and {110}[111] screw dislocations in iron

    PubMed Central

    Wang, Shuai; Hashimoto, Naoyuki; Ohnuki, Somei

    2013-01-01

    Employing the empirical embedded-atom method potentials, the evolution of edge and screw dislocation core structure is calculated at different hydrogen concentrations. With hydrogen, the core energy and Peierls potential are reduced for all dislocations. A broaden-core and a quasi-split core structure are observed for edge and screw dislocation respectively. The screw dislocation and hydrogen interaction in body-centred cubic iron is found to be not mainly due to the change of elastic modulus, but the variation of dislocation core structure. PMID:24067268

  1. Acute traumatic patellar dislocation.

    PubMed

    Duthon, V B

    2015-02-01

    Inaugural traumatic patellar dislocation is most often due to trauma sustained during physical or sports activity. Two-thirds of acute patellar dislocations occur in young active patients (less than 20 years old). Non-contact knee sprain in flexion and valgus is the leading mechanism in patellar dislocation, accounting for as many as 93% of all cases. The strong displacement of the patella tears the medial stabilizing structures, and notably the medial patellofemoral ligament (MPFL), which is almost always injured in acute patellar dislocation, most frequently at its femoral attachment. Lateral patellar glide can be assessed with the knee in extension or 20° flexion. Displacement by more than 50% of the patellar width is considered abnormal and may induce apprehension. Plain X-ray and CT are mandatory to diagnose bony risk factors for patellar dislocation, such as trochlear dysplasia or increased tibial tubercle-trochlear groove distance (TT-TG), and plan correction. MRI gives information on cartilage and capsulo-ligamentous status for treatment planning: free bodies or osteochondral fracture have to be treated surgically. If patellar dislocation occurs in an anatomically normal knee and osteochondral fracture is ruled out on MRI, non-operative treatment is usually recommended. PMID:25592052

  2. Electric field-induced deformation of polyelectrolyte gels

    SciTech Connect

    Adolf, D.; Hance, B.G.

    1995-08-01

    Water-swollen polyelectrolyte gels deform in an electric field. We observed that the sign and magnitude of the deformation is dependent on the nature of the salt bath in which the gel is immersed and electrocuted. These results are compatible with a deformation mechanism based upon creation of ion density gradients by the field which, in turn, creates osmotic pressure gradients within the gel. A consistent interpretation results only if gel mobility is allowed as well as free ion diffusion and migration.

  3. Nonlinear modeling and estimation of slew induced structural deformations

    NASA Technical Reports Server (NTRS)

    Dwyer, T. A. W., III; Karray, F.

    1988-01-01

    A model of the nonlinear dynamics of a deformable maneuvering multibody system is described, whereby elastic deformation are modeled by restoring forces and dissipative forces at point mass appendages. This model is brought into bilinear form. Estimation of deformations occasioned by rapid slewing maneuvers is carried out by a filter based on a globally equivalent linear model of the bilinear dynamics, and is shown to be an improvement over the extended Kalman filter. To further alleviate the computational burden, the estimated deformation state is propagated between observations by a low dimensional operator spline interpolator of bilinear system Volterra series, which is easily implemented.

  4. Using transverse isotropy to model arbitrary deformation-induced anisotropy

    SciTech Connect

    Brannon, R.M.

    1996-07-01

    A unifying framework is developed for the analysis of brittle materials. Heretofore diverse classes of models result from different choices for unspecified coefficient and distribution functions in the unified theory. Material response is described in terms of expectation integrals of transverse symmetry tensors. First, a canonical body containing cracks of all the same orientation is argued to possess macroscopic transverse isotropy. An orthogonal basis for the linear subspace consisting of all double-symmetric transversely-isotropic fourth-order tensors associated with a given material vector is introduced and applied to deduce the explicit functional dependence of the compliance of such contrived materials on the shared crack orientation. A principle of superposition of strain rates is used to write the compliance for a more realistic material consisting of cracks of random size and orientation as an expectation integral of the transverse compliance for each orientation times the joint distribution function for the size and orientation. Utilizing an evolving (initially exponential) size- dependence in the joint distribution, the general theory gives unprecedented agreement with measurements of the dynamic response of alumina to impact loading, especially upon release where the calculations predict the development of considerable deformation- induced anisotropy, challenging the conventional notion of shocks as isotropic phenomena.

  5. Shock induced deformation substructures in a copper bicrystal

    SciTech Connect

    Cao, Fang; Beyerlein, Irene J; Cerreta, Ellen K; Trujillo, Carl P; Gray Ill, George T; Sencer, Bulent H

    2008-01-01

    Controlled shock recovery experiments have been conducted to assess the role of shock pressure and orientation dependence on the substructure evolution of a [100]/[01{ovr 1}] copper bicrystal. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were utilized to characterize orientation variation and substructure evolution of the post-shock specimens. Well defined dislocation cell structures were displayed in both grains and the average cell size was observed to decrease with increasing shock pressure. Twinning was occasionally observed in the 5 GPa shocked [100] grain and became the dominant substructure at higher shock pressure. The stress and directional dependence of twinning in the bicrystal was analyzed with consideration of the energetically favorable dissociation of dislocations into Shockley partials and the stress-orientation effect on the partial width. Moreover, a critical 'tear apart' stress is proposed and a good agreement is obtained between the calculated value and the experimental observations.

  6. Kinetics of a Fast Moving Partial Dislocation

    NASA Astrophysics Data System (ADS)

    Daphalapurkar, Nitin; Ramesh, K. T.

    2013-03-01

    Plastic deformation in materials under extreme stresses requires a kinetic description of moving dislocations. The velocities with which the partial dislocations can propagate under an applied stress has implications for plasticity at high strain rates, specifically, the rate of plastic deformation and the rate-sensitivity. In this work, we focus our attention on motion of a twinning partial dislocation in a face-centered cubic (FCC) material, Ni. We use molecular dynamics simulations to simulate the velocity of a propagating twinning partial dislocation and investigate the effect of applied shear stress. Results suggest a limiting value for the speeds of a propagating partial dislocation. The material speeds based on the nonlinear part (under high stresses) of the stress-strain curve are shown to have an influence on the velocity with which a partial dislocation can propagate. Predicted velocities from simulations will be related to observations from high rate impact experiments. Supported by Hopkins Extreme Materials Institute

  7. Cyclic-loading-induced accumulation of geometrically necessary dislocations near grain boundaries in a an ni-based superalloy.

    SciTech Connect

    Huang, E. W.; Barabash, R. I.; Ice, G. I.; Liu, W.; Liu, Y. L.; Kai, J. J.; Liaw, P. K.; Univ.of Tennessee; ORNL; Tsing-Hua Univ.

    2009-01-01

    In this study, the fatigue-induced microstructure produced in a nickel-based polycrystalline superalloy that was subjected to cyclic loading was characterized by polychromatic x-ray microdiffraction (PXM) together with in-situ neutron diffraction and transmission-electron microscopy (TEM). In-situ neutron-diffraction measurements reveal two distinct stages of the fatigue damage: cyclic hardening followed by cyclic softening. Three-dimensional spatially resolved PXM micro-Laue measurements find an increase in the density of geometrically necessary dislocations near the grain boundaries, which is accompanied by lattice rotations and grain subdivisions. The PXM results are in agreement with the in-situ neutron-diffraction and TEM results.

  8. Structural anisotropy in metallic glasses induced by mechanical deformation

    SciTech Connect

    Dmowski, W.; Egami, T.

    2009-03-06

    We observed structural anisotropy in metallic glasses samples deformed by homogenous mechanical creep and by inhomogeneous compression using high energy X-ray diffraction. Pair distribution function analysis indicates bond anisotropy in the first atomic shell. This suggests that mechanical deformation involves rearrangements in a cluster of atoms by a bond reformation.

  9. Deformation-induced accelerated dynamics in polymer glasses

    SciTech Connect

    Warren, Mya; Rottler, Joerg

    2010-10-28

    Molecular dynamics simulations are used to investigate the effects of deformation on the segmental dynamics in an aging polymer glass. Individual particle trajectories are decomposed into a series of discontinuous hops, from which we obtain the full distribution of relaxation times and displacements under three deformation protocols: step stress (creep), step strain, and constant strain rate deformation. As in experiments, the dynamics can be accelerated by several orders of magnitude during deformation, and the history dependence is entirely erased during yield (mechanical rejuvenation). Aging can be explained as a result of the long tails in the relaxation time distribution of the glass, and similarly, mechanical rejuvenation is understood through the observed narrowing of this distribution during yield. Although the relaxation time distributions under deformation are highly protocol specific, in each case they may be described by a universal acceleration factor that depends only on the strain.

  10. Internal friction peaks observed in explosively deformed polycrystalline Mo, Nb, and Cu

    NASA Technical Reports Server (NTRS)

    Rieu, G. E.; Grimes, H. H.; Romain, J. P.; Defouquet, J.

    1974-01-01

    Explosive deformation (50 kbar range) induced, in Cu, Mo and Nb, internal friction peaks identical to those observed after large normal deformation. The variation of the peaks with pressure for Mo and Nb lead to an explanation of these processes in terms of double kink generation in screw and edge dislocations.

  11. Particle-Based Simulation of Shock-Induced Deformation of Elastic Bodies

    NASA Astrophysics Data System (ADS)

    Sakamura, Y.; Sugimoto, T.; Nakayama, K.

    Shock-induced deformations of solid bodies are of practical interest to those who are concerned with explosive processing of materials, demolition of buildings, precautions against accidental explosions, etc. In order to simulate the shock-induced deformations of solid bodies, a large number of numerical codes based on continuum mechanics, which are called hydrocodes, have been developed so far [1, 2]. When the amount of deformation is relatively small, Lagrangian hydrocodes have been used to simulate the dynamic response of shock-loaded materials. When the deformation is large, Eulerian hydrocodes have been utilized instead. This is because the computational grids distorted along with the deformation of materials in the Lagrangian approach make the simulations either inaccurate or unstable, while the Eulerian approach where grids are fixed in space can handle such large deformations of materials. On the contrary, material interfaces that are precisely defined in the Lagrangian approach are not traced exactly in the Eulerian one.

  12. Dislocations and crowdions in two-dimensional crystals. Part III: Plastic deformation of the crystal as a result of defect movement and defect interaction with the field of elastic stresses

    NASA Astrophysics Data System (ADS)

    Natsik, V. D.; Smirnov, S. N.

    2016-03-01

    A continuation of the theoretical study of the intrinsic properties of dislocation and crowdion structural defects in 2D crystals [V. D. Natsik and S. N. Smirnov, Fiz. Nizk. Temp. 40, 1366 (2014) and V. D. Natsik and S. N. Smirnov, Fiz. Nizk. Temp. 41, 271 (2015)]. The atomic lattice model of conservative (glide) and non-conservative (climb) defect movement is discussed in detail. It is shown that given a continuum description of the 2D crystal, an individual defect can be examined as a point carrier of plastic deformation, its value being determined by the topological charge, which is compliant with the crystal geometry defect parameters. It is found that the strain rate depends on the rate at which the defect center moves, as well as its topological charge. The elastic forces acting on the dislocation and crowdion centers in the field of applied mechanical stresses, and the forces of elastic interaction between defects, are calculated in terms of the linear theory of elasticity of a 2D crystal. The non-linear effect pertaining to the interaction between defects and bending deformation of the crystalline membrane, which is specific to 2D crystals, is also discussed.

  13. Discrete dislocation dynamics simulations in a cylinder

    NASA Astrophysics Data System (ADS)

    Li, Maosheng; Gao, Chan; Xu, Jianing

    2015-02-01

    Mechanical properties of material are closely related to the motion of dislocations, and predicting the interactions and resulting collective motion of dislocations is a major task in understanding and modelling plastically deforming materials. A discrete dislocation dynamics model is used to describe the orientation substructure within the microstructure. Discrete dislocation dynamics simulations in three dimensions have been used to examine the role of dislocation multiplication and mobility on the plasticity in small samples under uniaxial compression. In this paper we describe the application of the dislocation dynamics simulations in a cylindrical geometry. The boundary conditions for the simulation were estimated from the distribution of the geometrically necessary dislocation density which was obtained from the orientation map. Numerical studies benchmark could validate the accuracy of the algorithms and the importance of handling the singularity correctly. The results of the simulation explain the formation of the experimentally observed substructure.

  14. Local decomposition induced by dislocation motions inside tetragonal Al2Cu compound: slip system-dependent dynamics

    PubMed Central

    Chen, D.; Ma, X. L.

    2013-01-01

    Dislocations in a crystal are usually classified into several independent slip systems. Motion of a partial dislocation in monometallic crystals may remove or create stacking fault characterized with a partial of a lattice translation vector. However, it is recently known that motion of partial dislocations in complex structure, such as that inside an intermetallic Al2Cu compound, lead to a local composition deviation from its stoichiometric ratio and the resultant structure collapse. Here we report such a local decomposition behaviors are strongly dependent on slip system of dislocations. Under applied external stress, we have studied dislocation motion behaviors in the three independent slip systems of [001](110), [100]() and [110]() within tetragonal Al2Cu crystal by using molecular dynamics method. We found dislocation motions in all these slip systems result in local decomposition but their physical details differ significantly. PMID:24196169

  15. Local decomposition induced by dislocation motions inside tetragonal Al2Cu compound: slip system-dependent dynamics

    NASA Astrophysics Data System (ADS)

    Chen, D.; Ma, X. L.

    2013-11-01

    Dislocations in a crystal are usually classified into several independent slip systems. Motion of a partial dislocation in monometallic crystals may remove or create stacking fault characterized with a partial of a lattice translation vector. However, it is recently known that motion of partial dislocations in complex structure, such as that inside an intermetallic Al2Cu compound, lead to a local composition deviation from its stoichiometric ratio and the resultant structure collapse. Here we report such a local decomposition behaviors are strongly dependent on slip system of dislocations. Under applied external stress, we have studied dislocation motion behaviors in the three independent slip systems of [001](110), [100]() and [110]() within tetragonal Al2Cu crystal by using molecular dynamics method. We found dislocation motions in all these slip systems result in local decomposition but their physical details differ significantly.

  16. Local decomposition induced by dislocation motions inside tetragonal Al(2)Cu compound: slip system-dependent dynamics.

    PubMed

    Chen, D; Ma, X L

    2013-01-01

    Dislocations in a crystal are usually classified into several independent slip systems. Motion of a partial dislocation in monometallic crystals may remove or create stacking fault characterized with a partial of a lattice translation vector. However, it is recently known that motion of partial dislocations in complex structure, such as that inside an intermetallic Al2Cu compound, lead to a local composition deviation from its stoichiometric ratio and the resultant structure collapse. Here we report such a local decomposition behaviors are strongly dependent on slip system of dislocations. Under applied external stress, we have studied dislocation motion behaviors in the three independent slip systems of [001](110), [100]() and [110]() within tetragonal Al2Cu crystal by using molecular dynamics method. We found dislocation motions in all these slip systems result in local decomposition but their physical details differ significantly. PMID:24196169

  17. The inhibiting effect of dislocation helices on the stress-induced orientation of S' precipitates in Al–Cu–Mg alloy

    SciTech Connect

    Guo, Xiaobin; Deng, Yunlai; Zhang, Jin; Zhang, Xinming

    2015-09-15

    The phenomenon of restrained stress-induced preferential orientation of S′ precipitates is investigated using a single-crystal of Al–1.23Cu–0.43 Mg alloy. Al–1.23Cu–0.43 Mg single-crystal specimens are subjected to stress aging, and the microstructure is analyzed by transmission electron microscopy (TEM). It is found that the stress-induced preferential orientation of S′ precipitates is restrained owing to the dislocations produced by a higher stress. The effect of dislocations on the oriented precipitates depends on the total length of the intersection lines for precipitate habit planes and dislocation glide planes. This investigation not only provides important insight into solving the anisotropy problem attributed to precipitation strengthening, but also offers a benchmark for choosing the appropriate stress range in manufacturing of Al–Cu–Mg alloys. - Highlights: • Single crystals of an Al–Cu–Mg alloy were prepared for the investigations. • A phenomenon of restrained stress-induced preferential orientation of S′ precipitates was found. • The influence of dislocation helices on precipitation during stress-aging was studied. • Difference of orientation degree of S′ precipitates and θ′ precipitates was explained. • A basis for choosing the appropriate stress range in manufacturing of Al–Cu–Mg alloys is provided.

  18. Smectic Edge Dislocations under Shear

    NASA Astrophysics Data System (ADS)

    Chen, Peilong; Lu, Chun-Yi David

    2011-09-01

    Layer structures around an edge dislocation in a smectic phase under shear are studied with both phase field and order parameter models. It is shown that, contrast to a crystal solid, the conventional picture of the Peach--Koehler force experienced by dislocations when the sample is under a shear stress cannot be readily applied to the smectic phases. Under a uniform shear flow, we obtain the phase field and order parameter solutions around an edge dislocation. The solutions elucidate properties such as the layer distortion range around the dislocation and scaling of inter-dislocation interaction on dislocation separation. Calculations on energy dissipation indicate the extreme shear-thinning behavior that an edge dislocation induces a shear stress independent of the shear rate. Finally in a bulk sample with dislocation forming loops and networks, we argue that the uniform flow component around the dislocation is important to the energy dissipation and we show that its scaling exponent with the shear rate is very close to results from many previous rheology measurements.

  19. Plastic deformation of a magnesium oxide 001-plane surface produced by cavitation

    NASA Technical Reports Server (NTRS)

    Hattori, S.; Miyoshi, K.; Buckley, D. H.; Okada, T.

    1986-01-01

    An investigation was conducted to examine plastic deformation of a cleaved single-crystal magnesium oxide 001-plane surface exposed to cavitation. Cavitation damage experiments were carried out in distilled water at 25 C by using a magnetostrictive oscillator in close proximity (2 mm) to the surface of the cleaved specimen. The dislocation-etch-pit patterns induced by cavitation were examined and compared with that of microhardness indentations. The results revealed that dislocation-etch-pit patterns around hardness indentations contain both screw and edge dislocations, while the etch-pit patterns on the surface exposed to cavitation contain only screw dislocations. During cavitation, deformation occurred in a thin surface layer, accompanied by work-hardening of the ceramic. The row of screw dislocations underwent a stable growth, which was analyzed crystallographically.

  20. Deformation-induced damage and recovery in model hydrogels - A molecular dynamics simulation

    NASA Astrophysics Data System (ADS)

    Zidek, Jan; Milchev, Andrey; Jancar, Josef; Vilgis, Thomas A.

    2016-09-01

    Using molecular dynamics simulation of a model hybrid cross-link hydrogel, we investigate the network damage evolution and the related structure transformations. We model the hydrogel structure as a network-connected assembly of crosslinked clusters whereby deformation-induced damage is considered along with network recovery. The two principal mechanisms involved in hydrogel recovery from deformation include segment hops of the building structure units (segments) between clusters and cluster shape modification. These mechanisms act either instantaneously, or with a certain time delay after the onset of deformation. By elucidating the conditions under which one of the mechanisms prevails, one may design hydrogel materials with a desired response to deformation.

  1. Deformation behaviour and 6H-LPSO structure formation at nanoindentation in lamellar high Nb containing TiAl alloy

    NASA Astrophysics Data System (ADS)

    Song, L.; Xu, X. J.; Peng, C.; Wang, Y. L.; Liang, Y. F.; Shang, S. L.; Liu, Z. K.; Lin, J. P.

    2015-02-01

    Microstructure and deformation mechanisms at a nanoindentation in the lamellar colony of high Nb containing TiAl alloy have been studied using the focused ion beam and the transmission electron microscopy. Considerable deformation twins are observed around the nanoindentation, and a strain gradient is generated. A continuous change in the bending angle of the lamellar structure can be derived, and a strain-induced grain refinement process is observed as various active deformations split the γ grains into subgrains. In addition to all possible deformation mechanisms (ordinary dislocation, super-dislocation and deformation twining) activated due to the heavy plastic deformation, a 6-layer hexagonal (6H) long-period stacking ordered structure is identified for the first time near the contact zone and is thought to be closely related to the glide of partial dislocations.

  2. Knee Dislocations

    PubMed Central

    Schenck, Robert C.; Richter, Dustin L.; Wascher, Daniel C.

    2014-01-01

    Background: Traumatic knee dislocation is becoming more prevalent because of improved recognition and increased exposure to high-energy trauma, but long-term results are lacking. Purpose: To present 2 cases with minimum 20-year follow-up and a review of the literature to illustrate some of the fundamental principles in the management of the dislocated knee. Study Design: Review and case reports. Methods: Two patients with knee dislocations who underwent multiligamentous knee reconstruction were reviewed, with a minimum 20-year follow-up. These patients were brought back for a clinical evaluation using both subjective and objective measures. Subjective measures include the following scales: Lysholm, Tegner activity, visual analog scale (VAS), Short Form–36 (SF-36), International Knee Documentation Committee (IKDC), and a psychosocial questionnaire. Objective measures included ligamentous examination, radiographic evaluation (including Telos stress radiographs), and physical therapy assessment of function and stability. Results: The mean follow-up was 22 years. One patient had a vascular injury requiring repair prior to ligament reconstruction. The average assessment scores were as follows: SF-36 physical health, 52; SF-36 mental health, 59; Lysholm, 92; IKDC, 86.5; VAS involved, 10.5 mm; and VAS uninvolved, 2.5 mm. Both patients had excellent stability and were functioning at high levels of activity for their age (eg, hiking, skydiving). Both patients had radiographic signs of arthritis, which lowered 1 subject’s IKDC score to “C.” Conclusion: Knee dislocations have rare long-term excellent results, and most intermediate-term studies show fair to good functional results. By following fundamental principles in the management of a dislocated knee, patients can be given the opportunity to function at high levels. Hopefully, continued advances in the evaluation and treatment of knee dislocations will improve the long-term outcomes for these patients in the

  3. Characterization of Plastic Deformation Induced by Shot-Peening in a Ni-Base Superalloy

    NASA Astrophysics Data System (ADS)

    Messé, O. M. D. M.; Stekovic, S.; Hardy, M. C.; Rae, C. M. F.

    2014-12-01

    The shot-peening process is currently employed in most industries to improve the longevity of components by inhibiting crack initiation as well as crack growth at the surface. The protective effect of shot peening has been mainly attributed to compressive stresses within the deformed layer. Intensive research has been carried out to quantify the near-surface residual stresses on entry into service and evolution throughout life. In nickel-base superalloys, the focus of research on the effects of shot-peening has performed using x-rays from either laboratory or synchrotron-based sources. However, this approach cannot evaluate in detail the deformation mechanisms nor the role of the γ' precipitates in a nickel-base superalloy; the latter is responsible for its unique properties. Our study uses a complementary range of techniques to investigate in detail the microstructure and deformation mechanisms associated with shot-peening in a coarse-grained nickel-based superalloy strengthened with coherent γ' precipitates. These include scanning electron microscopy and transmission electron microscopy, nanoindentation and micropillar compression. Accurate mapping of the dislocation structure produced throughout the deformed layers have been performed. Using an unconventional specimen preparation technique, it provides the basis for a more complete interpretation of how shot-peening inhibits fatigue cracking.

  4. Evaluating Deformation-Induced Grain Orientation Change in a Polycrystal During In Situ Tensile Deformation using EBSD.

    PubMed

    Buchheit, Thomas E; Carroll, Jay D; Clark, Blythe G; Boyce, Brad L

    2015-08-01

    Using an in situ load frame within a scanning electron microscope, a microstructural section on the surface of an annealed tantalum (Ta) polycrystalline specimen was mapped at successive tensile strain intervals, up to ~20% strain, using electron backscatter diffraction. A grain identification and correlation technique was developed for characterizing the evolving microstructure during loading. Presenting the correlated results builds on the reference orientation deviation (ROD) map concept where individual orientation measurements within a grain are compared with a reference orientation associated with that grain. In this case, individual orientation measurements in a deformed grain are measured relative to a reference orientation derived from the undeformed (initial) configuration rather than the current deformed configuration as has been done for previous ROD schemes. Using this technique helps reveal the evolution of crystallographic orientation gradients and development of deformation-induced substructure within grains. Although overall crystallographic texture evolved slowly during deformation, orientation spread within grains developed quickly. In some locations, misorientation relative to the original orientation of a grain exceeded 20° by 15% strain. The largest orientation changes often appeared near grain boundaries suggesting that these regions were preferred locations for the initial development of subgrains. PMID:26189352

  5. Solute drag on perfect and extended dislocations

    NASA Astrophysics Data System (ADS)

    Sills, R. B.; Cai, W.

    2016-04-01

    The drag force exerted on a moving dislocation by a field of mobile solutes is studied in the steady state. The drag force is numerically calculated as a function of the dislocation velocity for both perfect and extended dislocations. The sensitivity of the non-dimensionalized force-velocity curve to the various controlling parameters is assessed, and an approximate analytical force-velocity expression is given. A non-dimensional parameter S characterizing the strength of the solute-dislocation interaction, the background solute fraction ?, and the dislocation character angle ?, are found to have the strongest influence on the force-velocity curve. Within the model considered here, a perfect screw dislocation experiences no solute drag, but an extended screw dislocation experiences a non-zero drag force that is about 10 to 30% of the drag on an extended edge dislocation. The solutes can change the spacing between the Shockley partials in both stationary and moving extended dislocations, even when the stacking fault energy remains unaltered. Under certain conditions, the solutes destabilize an extended dislocation by either collapsing it into a perfect dislocation or causing the partials to separate unboundedly. It is proposed that the latter instability may lead to the formation of large faulted areas and deformation twins in low stacking fault energy materials containing solutes, consistent with experimental observations of copper and stainless steel containing hydrogen.

  6. Particle deformation induced by AFM tapping under different setpoint voltages

    NASA Astrophysics Data System (ADS)

    Wu, Chung-Lin; Farkas, Natalia; Dagata, John A.; He, Bo-Ching; Fu, Wei-En

    2014-09-01

    The measured height of polystyrene nanoparticles varies with setpoint voltage during atomic force microscopy (AFM) tapping-mode imaging. Nanoparticle height was strongly influenced by the magnitude of the deformation caused by the AFM tapping forces, which was determined by the setpoint voltage. This influence quantity was studied by controlling the operational AFM setpoint voltage. A test sample consisting of well-dispersed 60-nm polystyrene and gold nanoparticles co-adsorbed on poly-l-lysine-coated mica was studied in this research. Gold nanoparticles have not only better mechanical property than polystyrene nanoparticles, but also obvious facets in AFM phase image. By using this sample of mixed nanoparticles, it allows us to confirm that the deformation resulted from the effect of setpoint voltage, not noise. In tapping mode, the deformation of polystyrene nanoparticles increased with decreasing setpoint voltage. Similar behavior was observed with both open loop and closed loop AFM instruments.

  7. Monitoring of deformation induced microcracking in polycrystalline NiAl

    SciTech Connect

    Wanner, A.; Schietinger, B.; Bidlingmaier, T.; Zalkind, H.; Arzt, E.

    1995-08-01

    Microcracking in polycrystalline near-stoichiometric NiAl produced by room temperature plastic deformation under uniaxial compression was investigated by means of optical microscopy, velocity of sound measurements, and acoustic emission monitoring. Results show that strains greater than 2% are required to produce microcrack populations which can be evaluated by microscopical investigation or velocity of sound measurements. However, acoustic emission monitoring during compression testing indicates that microcracking starts at about 0.7% compressive plastic strain which is identical with the typical tensile fracture strain for NiAl. Thus it is concluded that there is little or no stable microcracking prior to failure in tension. Acoustic emission results show also that the process of microcracking does not primarily occur during the applied compressive deformation. A considerable fraction of the microcracking takes place during the quasi-elastic unloading following deformation.

  8. Distribution of dislocations in nanostructured bainite

    SciTech Connect

    Cornide, J; Miyamoto, G; Caballero, Francesca G.; Furuhara, T; Miller, Michael K; Garcia-Mateo, C.

    2011-01-01

    The dislocation density in ferrite and austenite of a bainitic microstructure obtained by transformation at very low temperature (300 C) has been determined using transmission electron microscopy. Observations revealed that bainitic ferrite plates consist of two distinctive regions with different substructures. A central region in the ferrite plate is observed with dislocations that may result from lattice-invariant deformation at the earlier stage of bainite growth. As plastic deformation occurs in the surrounding austenite to accommodate the transformation strain as growth progresses, the Ferrite/Austenite interface has also a very distinctive dislocation profile. In addition, atom-probe tomography suggested that dislocation tangles observed in the vicinity of the ferrite/austenite interface might trap higher amount of carbon than single dislocations inside the bainitic ferrite plate.

  9. Response properties of axion insulators and Weyl semimetals driven by screw dislocations and dynamical axion strings

    NASA Astrophysics Data System (ADS)

    You, Yizhi; Cho, Gil Young; Hughes, Taylor L.

    2016-08-01

    In this paper, we investigate the theory of dynamical axion strings emerging from chiral symmetry breaking in three-dimensional Weyl semimetals. The chiral symmetry is spontaneously broken by a charge density wave (CDW) order which opens an energy gap and converts the Weyl semimetal into an axion insulator. Indeed, the phase fluctuations of the CDW order parameter act as a dynamical axion field θ (x ⃗,t ) and couple to electromagnetic field via Lθ=θ/(x ⃗,t ) 32 π2 ɛσ τ ν μFσ τFν μ. Additionally, when the axion insulator is coupled to deformations of the background geometry/strain fields via torsional defects, e.g., screw dislocations, there is interesting interplay between the crystal dislocations and dynamical axion strings. For example, the screw dislocation traps axial charge, and there is a Berry phase accumulation when an axion string (which carries axial flux) is braided with a screw dislocation. In addition, a cubic coupling between the axial current and the geometry fields is nonvanishing and indicates a Berry phase accumulation during a particular three-loop braiding procedure where a dislocation loop is braided with another dislocation and they are both threaded by an axion string. We also observe a chiral magnetic effect induced by a screw dislocation density in the absence of a nodal energy imbalance between Weyl points and describe an additional chiral geometric effect and a geometric Witten effect.

  10. Hot deformation induced defects and performance enhancement in FeSb2 thermoelectric materials

    NASA Astrophysics Data System (ADS)

    Wang, Yongzheng; Fu, Chenguang; Zhu, Tiejun; Hu, Lipeng; Jiang, Guangyu; Zhao, Guanghui; Huo, Dexuan; Zhao, Xinbing

    2013-11-01

    The effect of hot deformation induced defects and texture on thermoelectric properties of FeSb2 bulk crystals has been investigated. The transport properties of the samples along both parallel and perpendicular direction of pressing were measured from 3 K to 300 K. The results showed that thermal conductivity of the deformed samples was significantly reduced. After twice deformation, the thermal conductivity of the sample along the perpendicular direction of pressing was decreased to 4 W/mK, which was only one third of that before deformation. Transmission electron microscopy observation revealed the presence of high density of lattice defects in the deformed samples. The lattice thermal conductivity was analyzed using the Debye-Callaway approximation, and the results showed that the deformation induced lattice imperfections play an important role in enhancing phonon scattering. In addition, both the electrical resistivity and Seebeck coefficient exhibited a weak anisotropy in the deformed samples. The figure of merit ZT of the bulk FeSb2 was significantly improved from 0.010 to 0.021 after deformation.

  11. Hot deformation induced defects and performance enhancement in FeSb{sub 2} thermoelectric materials

    SciTech Connect

    Wang, Yongzheng; Fu, Chenguang; Zhu, Tiejun E-mail: zhaoxb@zju.edu.cn; Hu, Lipeng; Jiang, Guangyu; Zhao, Xinbing E-mail: zhaoxb@zju.edu.cn; Zhao, Guanghui; Huo, Dexuan

    2013-11-14

    The effect of hot deformation induced defects and texture on thermoelectric properties of FeSb{sub 2} bulk crystals has been investigated. The transport properties of the samples along both parallel and perpendicular direction of pressing were measured from 3 K to 300 K. The results showed that thermal conductivity of the deformed samples was significantly reduced. After twice deformation, the thermal conductivity of the sample along the perpendicular direction of pressing was decreased to 4 W/mK, which was only one third of that before deformation. Transmission electron microscopy observation revealed the presence of high density of lattice defects in the deformed samples. The lattice thermal conductivity was analyzed using the Debye-Callaway approximation, and the results showed that the deformation induced lattice imperfections play an important role in enhancing phonon scattering. In addition, both the electrical resistivity and Seebeck coefficient exhibited a weak anisotropy in the deformed samples. The figure of merit ZT of the bulk FeSb{sub 2} was significantly improved from 0.010 to 0.021 after deformation.

  12. Structural Anisotropy in Metallic Glasses Induced by Mechanical Deformation

    SciTech Connect

    Dmowski, Wojtek; Egami, Takeshi

    2008-01-01

    Metallic glasses have been studied vigorously since the first report on amorphous gold-silicon alloy back in 1960.[1] Initially soft magnetic properties were the most promising features for industrial applications. The recent development of bulk metallic glasses (BMGs)[2 5] initiated interests in engineering applications such as structural or biomedical materials because of attractive properties such as high strength,[6] high elasticity,[7,8] and good corrosion resistance,[9,10] among others. In addition, high temperature processing of BMGs allows for near-net-shape formability,[11 13] which could simplify and possibly reduce the cost of the final product. The glasses retain the disordered atomic structure of a liquid, and ideally are isotropic solids. Frequently because of processing conditions, such as directional heat flow, some structural anisotropy is produced during quenching, and has been observed by structural investigations. Usually, annealing at high temperatures results in an isotropic structure. Also, formation of uniaxial magnetic anisotropy[14] had been observed in studies of creep deformed ferromagnetic metallic glasses. Samples with a near-zero magnetostriction coefficient had been studied to establish the origin of the magnetic anisotropy. It was concluded that anisotropy resulted from the atomic level anisotropy[15] and not the heterogeneous internal stress distribution. Indeed X-ray diffraction study of the creep deformed metallic glass showed bond anisotropy.[ 16,17] Such structural studies had been cumbersome and lengthy because they required measurement of many orientations with high statistics. Recently we have shown that use of an area detector and high energy X-rays at a synchrotron source can speed up data collection without compromising statistics.[18] In this contribution, we present data showing structural anisotropy in glassy samples after homogenous (creep) and inhomogeneous (compression) mechanical deformation. The observation of the

  13. 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

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

    NASA Astrophysics Data System (ADS)

    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.

  15. 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. PMID:26871192

  16. Dislocation patterning in a two-dimensional continuum theory of dislocations

    NASA Astrophysics Data System (ADS)

    Groma, István; Zaiser, Michael; Ispánovity, Péter Dusán

    2016-06-01

    Understanding the spontaneous emergence of dislocation patterns during plastic deformation is a long standing challenge in dislocation theory. During the past decades several phenomenological continuum models of dislocation patterning were proposed, but few of them (if any) are derived from microscopic considerations through systematic and controlled averaging procedures. In this paper we present a two-dimensional continuum theory that is obtained by systematic averaging of the equations of motion of discrete dislocations. It is shown that in the evolution equations of the dislocation densities diffusionlike terms neglected in earlier considerations play a crucial role in the length scale selection of the dislocation density fluctuations. It is also shown that the formulated continuum theory can be derived from an averaged energy functional using the framework of phase field theories. However, in order to account for the flow stress one has in that case to introduce a nontrivial dislocation mobility function, which proves to be crucial for the instability leading to patterning.

  17. Coupling between meniscus and smectic-A films: circular and catenoid profiles, induced stress, and dislocation dynamics

    PubMed

    Picano; Holyst; Oswald

    2000-09-01

    In this paper we discuss the formation and shape of the meniscus between a free-standing film of a smectic-A phase and a wall (in practice the frame that supports the film). The wall may be flat or circular, and the system with or without a reservoir of particles. The formation of the meniscus is always an irreversible thermodynamic process, since it involves the creation of dislocations in the bulk (therefore it involves friction). The four basic shapes of meniscus discussed are the following: exponential, algebraic (x(3/2)), circular, and catenoid. Three principal regions of the whole meniscus must be distinguished: close to the wall with a high density of dislocations, away from the wall with medium density of dislocations, and far from the wall (i.e., close to the film) with a low density of dislocations (vicinal regime). The region with medium density of dislocations is observable using a microscope, and is determined by the competition between surface tension, energy of dislocations, and pressure difference set by the mass of the meniscus or by the reservoir. Its profile is circular as observed in recent experiments [J.-C. Geminard, R. Holyst, and P. Oswald, Phys. Rev. Lett. 78, 1924 (1997)]. By contrast, the vicinal regime with low density of dislocations is never observable with an optical microscope. In the regime with a high density of dislocations, the reasons why the dislocations tend to gather by forming giant dislocations and rows of focal conics are discussed. Finally, we discuss the stability of a smectic film with respect to the formation of a dislocation loop. We show experimentally that the critical radius of the loop is proportional to the curvature radius of the meniscus in its circular part, in agreement with the theory. In addition, we show that the mobility of edge dislocations measured in thick films is in agreement with that found in bulk samples from a creep experiment. This result confirms again our model of the meniscus. PMID:11088891

  18. Shear-Induced Deformation of Surfactant Multilamellar Vesicles

    NASA Astrophysics Data System (ADS)

    Pommella, Angelo; Caserta, Sergio; Guida, Vincenzo; Guido, Stefano

    2012-03-01

    Surfactant multilamellar vesicles (SMLVs) play a key role in the formulation of many industrial products, such as detergents, foodstuff, and cosmetics. In this Letter, we present the first quantitative investigation of the flow behavior of single SMLVs in a shearing parallel plate apparatus. We found that SMLVs are deformed and oriented by the action of shear flow while keeping constant volume and exhibit complex dynamic modes (i.e., tumbling, breathing, and tank treading). This behavior can be explained in terms of an excess area (as compared to a sphere of the same volume) and of microstructural defects, which were observed by 3D shape reconstruction through confocal microscopy. Furthermore, the deformation and orientation of SMLVs scale with radius R in analogy with emulsion droplets and elastic capsules (instead of R3, such as in unilamellar vesicles). A possible application of the physical insight provided by this Letter is in the rationale design of processing methods of surfactant-based systems.

  19. A Unified Material Description for Light Induced Deformation in Azobenzene Polymers

    PubMed Central

    Bin, Jonghoon; Oates, William S.

    2015-01-01

    Complex light-matter interactions in azobenzene polymers have limited our understanding of how photoisomerization induces deformation as a function of the underlying polymer network and form of the light excitation. A unified modeling framework is formulated to advance the understanding of surface deformation and bulk deformation of polymer films that are controlled by linear or circularly polarized light or vortex beams. It is shown that dipole forces strongly respond to polarized light in contrast to higher order quadrupole forces that are often used to describe surface relief grating deformation through a field gradient constitutive law. The modeling results and comparisons with a broad range of photomechanical data in the literature suggest that the molecular structure of the azobenzene monomers dramatically influences the photostrictive behavior. The results provide important insight for designing azobenzene monomers within a polymer network to achieve enhanced photo-responsive deformation. PMID:26437598

  20. A Unified Material Description for Light Induced Deformation in Azobenzene Polymers

    NASA Astrophysics Data System (ADS)

    Bin, Jonghoon; Oates, William S.

    2015-10-01

    Complex light-matter interactions in azobenzene polymers have limited our understanding of how photoisomerization induces deformation as a function of the underlying polymer network and form of the light excitation. A unified modeling framework is formulated to advance the understanding of surface deformation and bulk deformation of polymer films that are controlled by linear or circularly polarized light or vortex beams. It is shown that dipole forces strongly respond to polarized light in contrast to higher order quadrupole forces that are often used to describe surface relief grating deformation through a field gradient constitutive law. The modeling results and comparisons with a broad range of photomechanical data in the literature suggest that the molecular structure of the azobenzene monomers dramatically influences the photostrictive behavior. The results provide important insight for designing azobenzene monomers within a polymer network to achieve enhanced photo-responsive deformation.

  1. Flow-induced compaction of a deformable porous medium.

    PubMed

    Hewitt, Duncan R; Nijjer, Japinder S; Worster, M Grae; Neufeld, Jerome A

    2016-02-01

    Fluid flowing through a deformable porous medium imparts viscous drag on the solid matrix, causing it to deform. This effect is investigated theoretically and experimentally in a one-dimensional configuration. The experiments consist of the downwards flow of water through a saturated pack of small, soft, hydrogel spheres, driven by a pressure head that can be increased or decreased. As the pressure head is increased, the effective permeability of the medium decreases and, in contrast to flow through a rigid medium, the flux of water is found to increase towards a finite upper bound such that it becomes insensitive to changes in the pressure head. Measurements of the internal deformation, extracted by particle tracking, show that the medium compacts differentially, with the porosity being lower at the base than at the upper free surface. A general theoretical model is derived, and the predictions of the model give good agreement with experimental measurements from a series of experiments in which the applied pressure head is sequentially increased. However, contrary to theory, all the experimental results display a distinct and repeatable hysteresis: the flux through the material for a particular applied pressure drop is appreciably lower when the pressure has been decreased to that value compared to when it has been increased to the same value. PMID:26986422

  2. Sound Emission of Rotor Induced Deformations of Generator Casings

    NASA Technical Reports Server (NTRS)

    Polifke, W.; Mueller, B.; Yee, H. C.; Mansour, Nagi (Technical Monitor)

    2001-01-01

    The casing of large electrical generators can be deformed slightly by the rotor's magnetic field. The sound emission produced by these periodic deformations, which could possibly exceed guaranteed noise emission limits, is analysed analytically and numerically. From the deformation of the casing, the normal velocity of the generator's surface is computed. Taking into account the corresponding symmetry, an analytical solution for the acoustic pressure outside the generator is round in terms of the Hankel function of second order. The normal velocity or the generator surface provides the required boundary condition for the acoustic pressure and determines the magnitude of pressure oscillations. For the numerical simulation, the nonlinear 2D Euler equations are formulated In a perturbation form for low Mach number Computational Aeroacoustics (CAA). The spatial derivatives are discretized by the classical sixth-order central interior scheme and a third-order boundary scheme. Spurious high frequency oscillations are damped by a characteristic-based artificial compression method (ACM) filter. The time derivatives are approximated by the classical 4th-order Runge-Kutta method. The numerical results are In excellent agreement with the analytical solution.

  3. Flow-induced compaction of a deformable porous medium

    NASA Astrophysics Data System (ADS)

    Hewitt, Duncan R.; Nijjer, Japinder S.; Worster, M. Grae; Neufeld, Jerome A.

    2016-02-01

    Fluid flowing through a deformable porous medium imparts viscous drag on the solid matrix, causing it to deform. This effect is investigated theoretically and experimentally in a one-dimensional configuration. The experiments consist of the downwards flow of water through a saturated pack of small, soft, hydrogel spheres, driven by a pressure head that can be increased or decreased. As the pressure head is increased, the effective permeability of the medium decreases and, in contrast to flow through a rigid medium, the flux of water is found to increase towards a finite upper bound such that it becomes insensitive to changes in the pressure head. Measurements of the internal deformation, extracted by particle tracking, show that the medium compacts differentially, with the porosity being lower at the base than at the upper free surface. A general theoretical model is derived, and the predictions of the model give good agreement with experimental measurements from a series of experiments in which the applied pressure head is sequentially increased. However, contrary to theory, all the experimental results display a distinct and repeatable hysteresis: the flux through the material for a particular applied pressure drop is appreciably lower when the pressure has been decreased to that value compared to when it has been increased to the same value.

  4. Bauschinger effect in thin metal films: Discrete dislocation dynamics study

    NASA Astrophysics Data System (ADS)

    Davoudi, Kamyar M.; Nicola, Lucia; Vlassak, Joost J.

    2014-01-01

    The effects of dislocation climb on plastic deformation during loading and unloading are studied using a two-dimensional discrete dislocation dynamics model. Simulations are performed for polycrystalline thin films passivated on both surfaces. Dislocation climb lowers the overall level of the stress inside thin films and reduces the work hardening rate. Climb decreases the density of dislocations in pile-ups and reduces back stresses. These factors result in a smaller Bauschinger effect on unloading compared to simulations without climb. As dislocations continue to climb at the onset of unloading and the dislocation density continues to increase, the initial unloading slope increases with decreasing unloading rate. Because climb disperses dislocations, fewer dislocations are annihilated during unloading, leading to a higher dislocation density at the end of the unloading step.

  5. Bipolar dislocation of the clavicle.

    PubMed

    Jiang, Wei; Gao, Shu-Guang; Li, Yu-Sheng; Lei, Guang-Hua

    2012-11-01

    Bipolar dislocation of the clavicle at acromioclavicular and sternoclavicular joint is an uncommon traumatic injury. The conservative treatments adopted in the past is associated with redislocation dysfunction and deformity. A 41 years old lady with bipolar dislocation of right shoulder is treated surgically by open reduction and internal fixation by oblique T-plate at sternoclavicular joint and Kirschner wire stabilization at acromioclavicular joint. The patient showed satisfactory recovery with full range of motion of the right shoulder and normal muscular strength. The case reported in view of rarity and at 2 years followup. PMID:23325981

  6. Predictive Model for Temperature-Induced Deformation of Robot Mechanical Systems

    NASA Astrophysics Data System (ADS)

    Poonyapak, Pranchalee

    The positioning accuracy and repeatability of a robot are critical for many industrial applications. Drift in repeatability can occur with changes in environmental and internal conditions, such as those seen with temperature-induced deformation. Thermal instability causes dimensional deformation, and a warm-up cycle is typically required to bring the robot to a thermally stable working condition. The elimination of warm-up cycles will ultimately enhance the positioning accuracy of the robots, their productivity, and reduce unnecessary energy consumption. The main objective of this research was to develop a robot controller algorithm that would provide, a priori, compensation for temperature-induced deformation associated with warm-up in robot mechanical systems. The research started at the fundamental stage of gaining insight into the thermal behaviour and corresponding temperature-induced deformation of simplified, i.e., one-dimensional, robot mechanical systems consisting of slender links and heat sources. The systems were studied using concomitant experimental, numerical and analytical models to provide cross-checking of the results. For the experimental model, the deformation was measured by tracking the drift of a laser diode spot across a charge-coupled device (CCD) camera chip. A non-contact measurement system consisting of an infrared camera, a CCD camera and a laser diode was developed to provide high accuracy measurement for the deformation. The numerical model was generated with a coupled thermal-mechanical finite element analysis incorporating thermal effects due to conduction and convection. The models were tested with the analytical model that was further extended using a finite difference technique. Once the three models showed excellent agreement, it was possible to develop a controller algorithm. Deformations predicted by the finite difference model were used as input for a validation experiment of the compensation algorithm. Results of the

  7. An x-ray diffraction study of microstructural deformation induced by cyclic loading of selected steels

    SciTech Connect

    Fourspring, P.M.; Pangborn, R.N.

    1997-12-31

    X-ray double crystal diffractometry (XRDCD) was used to assess cyclic microstructural deformation in a face centered cubic (fcc) steel (AISI304) and a body centered cubic (bcc) steel (SA508 class 2). The objectives of the investigation were to determine if XRDCD could be used effectively to monitor cyclic microstructural deformation in polycrystalline Fe alloys and to study the distribution of the microstructural deformation induced by cyclic loading in these alloys. The approach used in the investigation was to induce fatigue damage in a material and to characterize the resulting microstructural deformation at discrete fractions of the fatigue life of the material. Also, characterization of microstructural deformation was carried out to identify differences in the accumulation of damage from the surface to the bulk, focusing on the following three regions: near surface (0--10 {micro}m), subsurface (10--300 {micro}m), and bulk. Characterization of the subsurface region was performed only on the AISI304 material because of the limited availability of the SA508 material. The results from the XRDCD data indicate a measurable change induced by fatigue from the initial state to subsequent states of both the AISI304 and the SA508 materials. Therefore, the XRDCD technique was shown to be sensitive to the microstructural deformation caused by fatigue in steels; thus, the technique can be used to monitor fatigue damage in steels.

  8. Modeling of porous scaffold deformation induced by medium perfusion.

    PubMed

    Podichetty, Jagdeep T; Madihally, Sundararajan V

    2014-05-01

    In this study, we tested the possibility of calculating permeability of porous scaffolds utilized in soft tissue engineering using pore size and shape. We validated the results using experimental measured pressure drop and simulations with the inclusion of structural deformation. We prepared Polycaprolactone (PCL) and Chitosan-Gelatin (CG) scaffolds by salt leaching and freeze drying technique, respectively. Micrographs were assessed for pore characteristics and mechanical properties. Porosity for both scaffolds was nearly same but the permeability varied 10-fold. Elastic moduli were 600 and 9 kPa for PCL and CG scaffolds, respectively, while Poisson's ratio was 0.3 for PCL scaffolds and ∼1.0 for CG scaffolds. A flow-through bioreactor accommodating a 10 cm diameter and 0.2 cm thick scaffold was used to determine the pressure-drop at various flow rates. Additionally, computational fluid dynamic (CFD) simulations were performed by coupling fluid flow, described by Brinkman equation, with structural mechanics using a dynamic mesh. The experimentally obtained pressure drop matched the simulation results of PCL scaffolds. Simulations were extended to a broad range of permeabilities (10(-10) m(2) to 10(-14) m(2) ), elastic moduli (10-100,000 kPa) and Poisson's ratio (0.1-0.49). The results showed significant deviation in pressure drop due to scaffold deformation compared to rigid scaffold at permeabilities near healthy tissues. Also, considering the scaffold as a nonrigid structure altered the shear stress profile. In summary, scaffold permeability can be calculated using scaffold pore characteristics and deformation could be predicted using CFD simulation. These relationships could potentially be used in monitoring tissue regeneration noninvasively via pressure drop. PMID:24259467

  9. 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.

    SciTech Connect

    Long, Fei; Daymond, Mark R.; Yao, Zhongwen; Kirk, Marquis A.

    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 the 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.

  10. Vibration-induced elastic deformation of Fabry-Perot cavities

    SciTech Connect

    Chen Lisheng; Hall, John L.; Ye Jun; Yang Tao; Zang Erjun; Li Tianchu

    2006-11-15

    We perform a detailed numerical analysis of Fabry-Perot cavities used for state-of-the-art laser stabilization. Elastic deformation of Fabry-Perot cavities with various shapes and mounting methods is quantitatively analyzed using finite-element analysis. We show that with a suitable choice of mounting schemes it is feasible to minimize the susceptibility of the resonator length to vibrational perturbations. This investigation offers detailed information on stable optical cavities that may benefit the development of ultrastable optical local oscillators in optical atomic clocks and precision measurements probing the fundamental laws of physics.