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

DOE PAGES

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

2015-08-05

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

Initial dislocation structure and dynamic dislocation multiplication in Mo single crystals

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

Hsiung, L M; Lassila, D H

Initial dislocation structure in annealed high-purity Mo single crystals and deformation substructure in a crystal subjected to 1% compression have been examined and studied in order to investigate dislocation multiplication mechanisms in the early stages of plastic deformation. The initial dislocation density is in a range of 10{sup 6} {approx} 10{sup 7} cm{sup -2}, and the dislocation structure is found to contain many grown-in superjogs along dislocation lines. The dislocation density increases to a range of 10{sup 8} {approx} 10{sup 9} cm{sup -2}, and the average jog height is also found to increase after compressing for a total strain ofmore » 1%. It is proposed that the preexisting jogged screw dislocations can act as (multiple) dislocation multiplication sources when deformed under quasi-static conditions. Both the jog height and length of link segment (between jogs) can increase by stress-induced jog coalescence, which takes place via the lateral migration (drift) of superjogs driven by unbalanced line-tension partials acting on link segments of unequal lengths. Applied shear stress begins to push each link segment to precede dislocation multiplication when link length and jog height are greater than critical lengths. This dynamic dislocation multiplication source is subsequently verified by direct simulations of dislocation dynamics under stress to be crucial in the early stages of plastic deformation in Mo single crystals.« less

High purity, low dislocation GaAs single crystals

NASA Technical Reports Server (NTRS)

Chen, R. T.; Holmes, D. E.; Kirkpatrick, C. G.

1983-01-01

Liquid encapsulated Czochralski crystal growth techniques for producing undoped, high resistivity, low dislocation material suitable for device applications is described. Technique development resulted in reduction of dislocation densities in 3 inch GaAs crystals. Control over the melt stoichiometry was determined to be of critical importance for the reduction of twinning and polycrystallinity during growth.

High purith low defect FZ silicon

NASA Technical Reports Server (NTRS)

Kimura, H.; Robertson, G.

1985-01-01

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

A STUDY OF DISLOCATION STRUCTURE OF SUBBOUNDARIES IN MOLYBDENUM SINGLE CRYSTALS,

DTIC Science & Technology

MOLYBDENUM, *DISLOCATIONS), GRAIN STRUCTURES(METALLURGY), SINGLE CRYSTALS, ZONE MELTING, ELECTRON BEAM MELTING, GRAIN BOUNDARIES, MATHEMATICAL ANALYSIS, ETCHED CRYSTALS, ETCHING, ELECTROEROSIVE MACHINING, CHINA

Growth and dislocation studies of β-HMX.

PubMed

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

2014-01-01

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

Magnesium Vacancy Segregation and Fast Pipe Diffusion for the ½<110>{110} Edge Dislocation in MgO

NASA Astrophysics Data System (ADS)

Walker, A. M.; Zhang, F.; Wright, K.; Gale, J. D.

2009-12-01

The movement of point defects in minerals plays a key role in determining their rheological properties, both by permitting diffusional creep and by allowing recovery by dislocation climb. Point defect diffusion can also control the kinetics of phase transitions and grain growth, and can determine the rate of chemical equilibration between phases. Because of this, and the difficulties associated with experimental studies of diffusion, the simulation of point defect formation and migration has been a subject of considerable interest in computational mineral physics. So far, studies have concentrated on point defects moving through otherwise perfect crystals. In this work we examine the behavior of magnesium vacancies close to the core of an edge dislocation in MgO and find that the dislocation dramatically changes the behavior of the point defect. An atomic scale model of the ½<110>{110} edge dislocation in MgO was constructed by applying the anisotropic linear elastic displacement field to the crystal structure and subsequently minimizing the energy of the crystal close to the dislocation core using a parameterized potential model. This process yielded the structure of an isolated edge dislocation in an otherwise perfect crystal. The energy cost associated with introducing magnesium vacancies around the dislocation was then mapped and compared to the formation energy of an isolated magnesium vacancy in bulk MgO. We find that the formation energy of magnesium vacancies around the dislocation mirrors the elastic strain field. Above the dislocation line σxx and σyy are negative and the strain field is compressional. Atoms are squeezed together to make room for the extra half plane effectively increasing the pressure in this region. Below the dislocation line σxx and σyy are positive and the strain field is dilatational. Planes of atoms are pulled apart to avoid a discontinuity across the glide plane and the effective pressure is decreased. In the region with a compressional strain field the vacancies become less stable than those in perfect MgO. In contrast, the region with a dilatational strain field hosts vacancies which are stabilized compared to the perfect crystal. This is in agreement with the previously observed tendency for increasing pressure to decrease the stability of vacancies in MgO. The most stable position for a magnesium vacancy was found to be 1.7 eV more stable than the vacancy in the bulk crystal, suggesting that vacancies will strongly partition to dislocations in MgO. Finally, the energy profile traced out by a vacancy moving through the bulk crystal was compared with that experienced by a vacancy moving along the dislocation core. A low energy pathway for vacancy migration along the dislocation line was found with a migration energy of 1.6 eV compared with a migration energy in the perfect crystal of 1.9 eV. This shows that vacancies segregated to the dislocation line will be significantly more mobile than vacancies in the perfect crystal. Dislocations will act as pipes, allowing material to be rapidly transported through crystals of MgO.

Three-dimensional analysis of dislocation multiplication during thermal process of grown silicon with different orientations

NASA Astrophysics Data System (ADS)

Gao, B.; Nakano, S.; Harada, H.; Miyamura, Y.; Kakimoto, K.

2017-09-01

We used an advanced 3D model to study the effect of crystal orientation on the dislocation multiplication in single-crystal silicon under accurate control of the cooling history of temperature. The incorporation of the anisotropy effect of the crystal lattice into the model has been explained in detail, and an algorithm for accurate control of the temperature in the furnace has also been presented. This solver can dynamically track the history of dislocation generation for different orientations during thermal processing of single-crystal silicon. Four orientations, [001], [110], [111], and [112], have been examined, and the comparison of dislocation distributions has been provided.

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

NASA Astrophysics Data System (ADS)

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

2017-11-01

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

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

DOE PAGES

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

2017-08-02

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

Effect of dislocation pile-up on size-dependent yield strength in finite single-crystal micro-samples

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

Pan, Bo; Shibutani, Yoji, E-mail: sibutani@mech.eng.osaka-u.ac.jp; Zhang, Xu

2015-07-07

Recent research has explained that the steeply increasing yield strength in metals depends on decreasing sample size. In this work, we derive a statistical physical model of the yield strength of finite single-crystal micro-pillars that depends on single-ended dislocation pile-up inside the micro-pillars. We show that this size effect can be explained almost completely by considering the stochastic lengths of the dislocation source and the dislocation pile-up length in the single-crystal micro-pillars. The Hall–Petch-type relation holds even in a microscale single-crystal, which is characterized by its dislocation source lengths. Our quantitative conclusions suggest that the number of dislocation sources andmore » pile-ups are significant factors for the size effect. They also indicate that starvation of dislocation sources is another reason for the size effect. Moreover, we investigated the explicit relationship between the stacking fault energy and the dislocation “pile-up” effect inside the sample: materials with low stacking fault energy exhibit an obvious dislocation pile-up effect. Our proposed physical model predicts a sample strength that agrees well with experimental data, and our model can give a more precise prediction than the current single arm source model, especially for materials with low stacking fault energy.« less

Dislocation dynamics and crystal plasticity in the phase-field crystal model

NASA Astrophysics Data System (ADS)

Skaugen, Audun; Angheluta, Luiza; Viñals, Jorge

2018-02-01

A phase-field model of a crystalline material is introduced to develop the necessary theoretical framework to study plastic flow due to dislocation motion. We first obtain the elastic stress from the phase-field crystal free energy under weak distortion and show that it obeys the stress-strain relation of linear elasticity. We focus next on dislocations in a two-dimensional hexagonal lattice. They are composite topological defects in the weakly nonlinear amplitude equation expansion of the phase field, with topological charges given by the standard Burgers vector. This allows us to introduce a formal relation between the dislocation velocity and the evolution of the slowly varying amplitudes of the phase field. Standard dissipative dynamics of the phase-field crystal model is shown to determine the velocity of the dislocations. When the amplitude expansion is valid and under additional simplifications, we find that the dislocation velocity is determined by the Peach-Koehler force. As an application, we compute the defect velocity for a dislocation dipole in two setups, pure glide and pure climb, and compare it with the analytical predictions.

How to identify dislocations in molecular dynamics simulations?

NASA Astrophysics Data System (ADS)

Li, Duo; Wang, FengChao; Yang, ZhenYu; Zhao, YaPu

2014-12-01

Dislocations are of great importance in revealing the underlying mechanisms of deformed solid crystals. With the development of computational facilities and technologies, the observations of dislocations at atomic level through numerical simulations are permitted. Molecular dynamics (MD) simulation suggests itself as a powerful tool for understanding and visualizing the creation of dislocations as well as the evolution of crystal defects. However, the numerical results from the large-scale MD simulations are not very illuminating by themselves and there exist various techniques for analyzing dislocations and the deformed crystal structures. Thus, it is a big challenge for the beginners in this community to choose a proper method to start their investigations. In this review, we summarized and discussed up to twelve existing structure characterization methods in MD simulations of deformed crystal solids. A comprehensive comparison was made between the advantages and disadvantages of these typical techniques. We also examined some of the recent advances in the dynamics of dislocations related to the hydraulic fracturing. It was found that the dislocation emission has a significant effect on the propagation and bifurcation of the crack tip in the hydraulic fracturing.

The low thermal gradient CZ technique as a way of growing of dislocation-free germanium crystals

NASA Astrophysics Data System (ADS)

Moskovskih, V. A.; Kasimkin, P. V.; Shlegel, V. N.; Vasiliev, Y. V.; Gridchin, V. A.; Podkopaev, O. I.

2014-09-01

This paper considers the possibility of growth of dislocation-free germanium single crystals. This is achieved by reducing the temperature gradients at the level of 1 K/cm and lower. Single germanium crystals 45-48 mm in diameter with a dislocation density of 102 cm-2 were grown by a Low Thermal Gradient Czochralski technique (LTG CZ).

Modeling dislocation generation in high pressure Czochralski growth of indium phosphide single crystals

NASA Astrophysics Data System (ADS)

Pendurti, Srinivas

InP is an important material for opto-electronic and high speed electronics applications. Its main use today is as the substrate material for epitaxy to produce GaInAsP lasers. The present technology for growing bulk InP is the high pressure Czochralski process. Bulk InP grown through this technique suffers from presence of a high density of line defects or dislocations, which are produced by thermal stresses the material goes through during its growth in the high temperature furnace. Modeling of these thermal stresses and the resulting plastic deformation, giving rise to dislocation densities, entails simulation of the entire thermal history of the crystal during its growth in the furnace, and studying the deformation of the crystal through suitable visco-plastic constitutive equations. Accordingly, a suitable visco-plastic model for deformation of InP was constructed, integrated with the ABAQUS finite element code, and verified through experimental data for uniaxial constant strain rate deformation tests available in literature. This was then coupled with a computation fluid dynamics model, predicting the entire temperature history in the furnace during crystal growth, to study the plastic deformation and dislocation density evolution in the crystal during growth. Growth in a variety of conditions was simulated and those conditions that generate minimum dislocation density identified. Macroscopic controllable parameters that affect the dislocation densities the most, have also been delineated. It was found that the strength of gas convection in the Czochralski furnace has the strongest effect on the dislocation densities in the fully grown crystal. Comparison of the simulated dislocation densities on wafers, with experimentally recorded etch pit profiles on as-grown crystals was reasonable. Finally some limitations in the work are discussed and avenues for future work identified.

Transmission electron microscopy of a refractory inclusion from the Allende meteorite - Anatomy of a pyroxene

NASA Astrophysics Data System (ADS)

Doukhan, N.; Doukhan, J. C.; Poirier, J. P.

1991-06-01

A crystal of clinopyroxene from the coarse-grained refractory inclusion Egg 6 of the Allende meteorite has been studied in detail by transmission electron microscopy. The pyroxene crystal contains euhedral, dislocation-free inclusions of pure spinel MgAl2O4, without any topotactic relation to the host. Extensive dislocation walls at equilibrium, characteristic of high-temperature anneal, are present in the crystal. Alteration products are occasionaly observed at the spinel-pyroxene interface close to regions where dislocation walls decorated with bubbles (or voids) are present. The bubbles, often in the shape of tubes along the dislocation lines, are thought to be due to the precipitation of a fluid migrating along the dislocations. The observations are compatible with crystallization of the refractory inclusions from the melt and with the existence of a later stage of metasomatism.

Strain-relief by single dislocation loops in calcite crystals grown on self-assembled monolayers

DOE PAGES

Ihli, Johannes; Clark, Jesse N.; Côté, Alexander S.; ...

2016-06-15

Most of our knowledge of dislocation-mediated stress relaxation during epitaxial crystal growth comes from the study of inorganic heterostructures. In this study, we use Bragg coherent diffraction imaging to investigate a contrasting system, the epitaxial growth of calcite (CaCO 3) crystals on organic self-assembled monolayers, where these are widely used as a model for biomineralization processes. The calcite crystals are imaged to simultaneously visualize the crystal morphology and internal strain fields. Our data reveal that each crystal possesses a single dislocation loop that occupies a common position in every crystal. The loops exhibit entirely different geometries to misfit dislocations generatedmore » in conventional epitaxial thin films and are suggested to form in response to the stress field, arising from interfacial defects and the nanoscale roughness of the substrate. In conclusion, this work provides unique insight into how self-assembled monolayers control the growth of inorganic crystals and demonstrates important differences as compared with inorganic substrates.« less

Strain-relief by single dislocation loops in calcite crystals grown on self-assembled monolayers

PubMed Central

Ihli, Johannes; Clark, Jesse N.; Côté, Alexander S.; Kim, Yi-Yeoun; Schenk, Anna S.; Kulak, Alexander N.; Comyn, Timothy P.; Chammas, Oliver; Harder, Ross J.; Duffy, Dorothy M.; Robinson, Ian K.; Meldrum, Fiona C.

2016-01-01

Most of our knowledge of dislocation-mediated stress relaxation during epitaxial crystal growth comes from the study of inorganic heterostructures. Here we use Bragg coherent diffraction imaging to investigate a contrasting system, the epitaxial growth of calcite (CaCO3) crystals on organic self-assembled monolayers, where these are widely used as a model for biomineralization processes. The calcite crystals are imaged to simultaneously visualize the crystal morphology and internal strain fields. Our data reveal that each crystal possesses a single dislocation loop that occupies a common position in every crystal. The loops exhibit entirely different geometries to misfit dislocations generated in conventional epitaxial thin films and are suggested to form in response to the stress field, arising from interfacial defects and the nanoscale roughness of the substrate. This work provides unique insight into how self-assembled monolayers control the growth of inorganic crystals and demonstrates important differences as compared with inorganic substrates. PMID:27302863

Uncovering the inertia of dislocation motion and negative mechanical response in crystals.

PubMed

Tang, Yizhe

2018-01-09

Dislocations are linear defects in crystals and their motion controls crystals' mechanical behavior. The dissipative nature of dislocation propagation is generally accepted although the specific mechanisms are still not fully understood. The inertia, which is undoubtedly the nature of motion for particles with mass, seems much less convincing for configuration propagation. We utilize atomistic simulations in conditions that minimize dissipative effects to enable uncovering of the hidden nature of dislocation motion, in three typical model metals Mg, Cu and Ta. We find that, with less/no dissipation, dislocation motion is under-damped and explicitly inertial at both low and high velocities. The inertia of dislocation motion is intrinsic, and more fundamental than the dissipative nature. The inertia originates from the kinetic energy imparted from strain energy and stored in the moving core. Peculiar negative mechanical response associated with the inertia is also discovered. These findings shed light on the fundamental nature of dislocation motion, reveal the underlying physics, and provide a new physical explanation for phenomena relevant to high-velocity dislocations.

Renormalization group study of the melting of a two-dimensional system of collapsing hard disks

NASA Astrophysics Data System (ADS)

Ryzhov, V. N.; Tareyeva, E. E.; Fomin, Yu. D.; Tsiok, E. N.; Chumakov, E. S.

2017-06-01

We consider the melting of a two-dimensional system of collapsing hard disks (a system with a hard-disk potential to which a repulsive step is added) for different values of the repulsive-step width. We calculate the system phase diagram by the method of the density functional in crystallization theory using equations of the Berezinskii-Kosterlitz-Thouless-Halperin-Nelson-Young theory to determine the lines of stability with respect to the dissociation of dislocation pairs, which corresponds to the continuous transition from the solid to the hexatic phase. We show that the crystal phase can melt via a continuous transition at low densities (the transition to the hexatic phase) with a subsequent transition from the hexatic phase to the isotropic liquid and via a first-order transition. Using the solution of renormalization group equations with the presence of singular defects (dislocations) in the system taken into account, we consider the influence of the renormalization of the elastic moduli on the form of the phase diagram.

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

NASA Astrophysics Data System (ADS)

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

2007-10-01

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

Evaluation of the increase in threading dislocation during the initial stage of physical vapor transport growth of 4H-SiC

NASA Astrophysics Data System (ADS)

Suo, Hiromasa; Tsukimoto, Susumu; Eto, Kazuma; Osawa, Hiroshi; Kato, Tomohisa; Okumura, Hajime

2018-06-01

The increase in threading dislocation during the initial stage of physical vapor transport growth of n-type 4H-SiC crystals was evaluated by cross-sectional X-ray topography. Crystals were grown under two different temperature conditions. A significant increase in threading dislocation was observed in crystals grown at a high, not low, temperature. The local strain distribution in the vicinity of the grown/seed crystal interface was evaluated using the electron backscatter diffraction technique. The local nitrogen concentration distribution was also evaluated by time-of-flight secondary ion mass spectrometry. We discuss the relationship between the increase in threading dislocation and the local strain due to thermal stress and nitrogen concentration.

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

DOE PAGES

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

2015-05-18

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

Lateral Movement of Screw Dislocations During Homoepitaxial Growth and Devices Yielded Therefrom Free of the Detrimental Effects of Screw Dislocations

NASA Technical Reports Server (NTRS)

Neudeck, Philip G. (Inventor); Powell, J. Anthony (Inventor)

2004-01-01

The present invention is related to a method that enables and improves wide bandgap homoepitaxial layers to be grown on axis single crystal substrates, particularly SiC. The lateral positions of the screw dislocations in epitaxial layers are predetermined instead of random, which allows devices to be reproducibly patterned to avoid performance degrading crystal defects normally created by screw dislocations.

Elasticity and dislocation inelasticity of crystals

NASA Astrophysics Data System (ADS)

Nikanorov, S. P.; Kardashev, B. K.

The use of methods of physical acoustics for studying the elasticity and dislocation inelasticity of crystals is discussed, as is the application of the results of such studies to the analysis of interatomic and lattice defect interactions. The analysis of the potential functions determining the energy of interatomic interactions is based on an analysis of the elastic properties of crystals over a wide temperature range. The data on the dislocation structure and the interaction between dislocations and point defects are obtained from a study of inelastic effects. Particular attention is given to the relationship between microplastic effects under conditions of elastic oscillations and the initial stage of plastic deformation.

Effect of oxygen on dislocation multiplication in silicon crystals

NASA Astrophysics Data System (ADS)

Fukushima, Wataru; Harada, Hirofumi; Miyamura, Yoshiji; Imai, Masato; Nakano, Satoshi; Kakimoto, Koichi

2018-03-01

This paper aims to clarify the effect of oxygen on dislocation multiplication in silicon single crystals grown by the Czochralski and floating zone methods using numerical analysis. The analysis is based on the Alexander-Haasen-Sumino model and involves oxygen diffusion from the bulk to the dislocation cores during the annealing process in a furnace. The results show that after the annealing process, the dislocation density in silicon single crystals decreases as a function of oxygen concentration. This decrease can be explained by considering the unlocking stress caused by interstitial oxygen atoms. When the oxygen concentration is 7.5 × 1017 cm-3, the total stress is about 2 MPa and the unlocking stress is less than 1 MPa. As the oxygen concentration increases, the unlocking stress also increases; however, the dislocation velocity decreases.

Single-crystal sapphire microstructure for high-resolution synchrotron X-ray monochromators

DOE PAGES

Asadchikov, Victor E.; Butashin, Andrey V.; Buzmakov, Alexey V.; ...

2016-03-22

We report on the growth and characterization of several sapphire single crystals for the purpose of x-ray optics applications. Structural defects were studied by means of laboratory double-crystal X-ray diffractometry and white beam synchrotron-radiation topography. The investigations confirmed that the main defect types are dislocations. The best quality crystal was grown using the Kyropoulos technique with a dislocation density of 10 2-10 3 cm -2 and a small area with approximately 2*2 mm 2 did not show dislocation contrast in many reflections and has suitable quality for application as a backscattering monochromator. As a result, a clear correlation between growthmore » rate and dislocation density is observed, though growth rate is not the only parameter impacting the quality.« less

Gradient Plasticity Model and its Implementation into MARMOT

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

Barker, Erin I.; Li, Dongsheng; Zbib, Hussein M.

2013-08-01

The influence of strain gradient on deformation behavior of nuclear structural materials, such as boby centered cubic (bcc) iron alloys has been investigated. We have developed and implemented a dislocation based strain gradient crystal plasticity material model. A mesoscale crystal plasticity model for inelastic deformation of metallic material, bcc steel, has been developed and implemented numerically. Continuum Dislocation Dynamics (CDD) with a novel constitutive law based on dislocation density evolution mechanisms was developed to investigate the deformation behaviors of single crystals, as well as polycrystalline materials by coupling CDD and crystal plasticity (CP). The dislocation density evolution law in thismore » model is mechanism-based, with parameters measured from experiments or simulated with lower-length scale models, not an empirical law with parameters back-fitted from the flow curves.« less

Dislocations and other topological oddities

NASA Astrophysics Data System (ADS)

Pieranski, Pawel

2016-03-01

We will show that the book Dislocations by Jacques Friedel, published half a century ago, can still be recommended, in agreement with the author's intention, as a textbook ;for research students at University and for students at engineering schools as well as for research engineers;. Indeed, today dislocations are known to occur not only in solid crystals but also in many other systems discovered more recently such as colloidal crystals or liquid crystals having periodic structures. Moreover, the concept of dislocations is an excellent starting point for lectures on topological defects occurring in systems equipped with order parameters resulting from broken symmetries: disclinations in nematic or hexatic liquid crystals, dispirations in chiral smectics or disorientations in lyotropic liquid crystals. The discussion of dislocations in Blue Phases will give us an opportunity to call on mind Sir Charles Frank, friend of Jacques Friedel since his Bristol years, who called these ephemeral mesophases ;topological oddities;. Being made of networks of disclinations, Blue Phases are similar to Twist Grain Boundary (TGB) smectic phases, which are made of networks of screw dislocations and whose existence was predicted by de Gennes in 1972 on the basis of the analogy between smectics and superconductors. We will stress that the book by Jacques Friedel contains seeds of this analogy.

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

NASA Astrophysics Data System (ADS)

Luscher, Darby

2017-06-01

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

Dislocation Multiplication in the Early Stage of Deformation in Mo Single Crystals

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

Hsiung, L.; Lassila, D.H.

Initial dislocation structure in annealed high-purity Mo single crystals and deformation substructure in a crystal subjected to 1% compression have been examined and studied using transmission electron microscopy (TEM) techniques in order to investigate dislocation multiplication mechanisms in the early stage of plastic deformation. The initial dislocation density is in a range of 10{sup 6} {approx} 10{sup 7} cm{sup -2}, and the dislocation structure is found to contain many grown-in superjogs along dislocation lines. The dislocation density increases to a range of 10{sup 8} {approx} 10{sup 9} cm{sup -2}, and the average jog height is also found to increase aftermore » compressing for a total strain of 1%. It is proposed that the preexisting jogged screw dislocations can act as (multiple) dislocation multiplication sources when deformed under quasi-static conditions. The jog height can increase by stress-induced jog coalescence, which takes place via the lateral migration (drift) of superjogs driven by unbalanced line-tension partials acting on link segments of unequal lengths. The coalescence of superjogs results in an increase of both link length and jog height. Applied shear stress begins to push each link segment to precede dislocation multiplication when link length and jog height are greater than critical lengths. This ''dynamic'' dislocation multiplication source is suggested to be crucial for the dislocation multiplication in the early stage of plastic deformation in Mo.« less

Predicting the structure of screw dislocations in nanoporous materials

NASA Astrophysics Data System (ADS)

Walker, Andrew M.; Slater, Ben; Gale, Julian D.; Wright, Kate

2004-10-01

Extended microscale crystal defects, including dislocations and stacking faults, can radically alter the properties of technologically important materials. Determining the atomic structure and the influence of defects on properties remains a major experimental and computational challenge. Using a newly developed simulation technique, the structure of the 1/2a <100> screw dislocation in nanoporous zeolite A has been modelled. The predicted channel structure has a spiral form that resembles a nanoscale corkscrew. Our findings suggest that the dislocation will enhance the transport of molecules from the surface to the interior of the crystal while retarding transport parallel to the surface. Crucially, the dislocation creates an activated, locally chiral environment that may have enantioselective applications. These predictions highlight the influence that microscale defects have on the properties of structurally complex materials, in addition to their pivotal role in crystal growth.

Study of the possibility of growing germanium single crystals under low temperature gradients

NASA Astrophysics Data System (ADS)

Moskovskih, V. A.; Kasimkin, P. V.; Shlegel, V. N.; Vasiliev, Y. V.; Gridchin, V. A.; Podkopaev, O. I.; Zhdankov, V. N.

2014-03-01

The possibility of growing germanium single crystals under low temperature gradients in order to produce a dislocation-free material has been studied. Germanium crystals with a dislocation density of about 100-200 cm-2 have been grown in a system with a weight control of crystal growth at maximum axial gradients of about 1.5 K/cm.

A combined dislocation fan-finite element (DF-FE) method for stress field simulation of dislocations emerging at the free surfaces of 3D elastically anisotropic crystals

NASA Astrophysics Data System (ADS)

Balusu, K.; Huang, H.

2017-04-01

A combined dislocation fan-finite element (DF-FE) method is presented for efficient and accurate simulation of dislocation nodal forces in 3D elastically anisotropic crystals with dislocations intersecting the free surfaces. The finite domain problem is decomposed into half-spaces with singular traction stresses, an infinite domain, and a finite domain with non-singular traction stresses. As such, the singular and non-singular parts of the traction stresses are addressed separately; the dislocation fan (DF) method is introduced to balance the singular traction stresses in the half-spaces while the finite element method (FEM) is employed to enforce the non-singular boundary conditions. The accuracy and efficiency of the DF method is demonstrated using a simple isotropic test case, by comparing it with the analytical solution as well as the FEM solution. The DF-FE method is subsequently used for calculating the dislocation nodal forces in a finite elastically anisotropic crystal, which produces dislocation nodal forces that converge rapidly with increasing mesh resolutions. In comparison, the FEM solution fails to converge, especially for nodes closer to the surfaces.

Molecular dynamics study on the evolution of interfacial dislocation network and mechanical properties of Ni-based single crystal superalloys

NASA Astrophysics Data System (ADS)

Li, Nan-Lin; Wu, Wen-Ping; Nie, Kai

2018-05-01

The evolution of misfit dislocation network at γ /γ‧ phase interface and tensile mechanical properties of Ni-based single crystal superalloys at various temperatures and strain rates are studied by using molecular dynamics (MD) simulations. From the simulations, it is found that with the increase of loading, the dislocation network effectively inhibits dislocations emitted in the γ matrix cutting into the γ‧ phase and absorbs the matrix dislocations to strengthen itself which increases the stability of structure. Under the influence of the temperature, the initial mosaic structure of dislocation network gradually becomes irregular, and the initial misfit stress and the elastic modulus slowly decline as temperature increasing. On the other hand, with the increase of the strain rate, it almost has no effect on the elastic modulus and the way of evolution of dislocation network, but contributes to the increases of the yield stress and tensile strength. Moreover, tension-compression asymmetry of Ni-based single crystal superalloys is also presented based on MD simulations.

Dislocation Mobility and Anomalous Shear Modulus Effect in ^4He Crystals

NASA Astrophysics Data System (ADS)

Malmi-Kakkada, Abdul N.; Valls, Oriol T.; Dasgupta, Chandan

2017-02-01

We calculate the dislocation glide mobility in solid ^4He within a model that assumes the existence of a superfluid field associated with dislocation lines. Prompted by the results of this mobility calculation, we study within this model the role that such a superfluid field may play in the motion of the dislocation line when a stress is applied to the crystal. To do this, we relate the damping of dislocation motion, calculated in the presence of the assumed superfluid field, to the shear modulus of the crystal. As the temperature increases, we find that a sharp drop in the shear modulus will occur at the temperature where the superfluid field disappears. We compare the drop in shear modulus of the crystal arising from the temperature dependence of the damping contribution due to the superfluid field, to the experimental observation of the same phenomena in solid ^4He and find quantitative agreement. Our results indicate that such a superfluid field plays an important role in dislocation pinning in a clean solid ^4He at low temperatures and in this regime may provide an alternative source for the unusual elastic phenomena observed in solid ^4He.

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

NASA Astrophysics Data System (ADS)

Gao, Yuan; Zhuang, Zhuo; You, XiaoChuan

2011-04-01

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

Constitutive modeling of intrinsic and oxygen-contaminated silicon monocrystals in easy glide

NASA Astrophysics Data System (ADS)

Cochard, J.; Yonenaga, I.; Gouttebroze, S.; M'Hamdi, M.; Zhang, Z. L.

2010-11-01

We generalize in this work the constitutive model for silicon crystals of Alexander and Haasen. Strain-rate and temperature dependency of the mechanical behavior of intrinsic crystals are correctly accounted for into stage I of hardening. We show that the steady-state of deformation in stage I is very well reproduced in a wide range of temperature and strain rate. The case of extrinsic crystals containing high levels of dissolved oxygen is examined. The introduction of an effective density of mobile dislocations dependent on the unlocking stress created by oxygen atoms gathered at the dislocation cores is combined to an alteration of the dislocation multiplication rate, due to pinning of the dislocation line by oxygen atoms. This increases the upper yield stress with the bulk oxygen concentration in agreement with experimental observations. The fraction of effectively mobile dislocations is found to decay exponentially with the unlocking stress. Finally, the influence of oxygen migration back onto the dislocations from the bulk on the stress distribution in silicon bars is investigated.

Interfacial dislocations in (111) oriented (Ba 0.7Sr 0.3)TiO 3 films on SrTiO 3 single crystal

DOE PAGES

Shen, Xuan; Yamada, Tomoaki; Lin, Ruoqian; ...

2015-10-08

In this study, we have investigated the interfacial structure of epitaxial (Ba,Sr)TiO 3 films grown on (111)-oriented SrTiO 3 single-crystal substrates using transmission electron microscopy (TEM) techniques. Compared with the (100) epitaxial perovskite films, we observe dominant dislocation half-loop with Burgers vectors of a<110> comprised of a misfit dislocation along <112>, and threading dislocations along <110> or <100>. The misfit dislocation with Burgers vector of a <110> can dissociate into two ½ a <110> partial dislocations and one stacking fault. We found the dislocation reactions occur not only between misfit dislocations, but also between threading dislocations. Via three-dimensional electron tomography,more » we retrieved the configurations of the threading dislocation reactions. The reactions between threading dislocations lead to a more efficient strain relaxation than do the misfit dislocations alone in the near-interface region of the (111)-oriented (Ba 0.7Sr 0.3)TiO 3 films.« less

Dislocation mechanisms in stressed crystals with surface effects

NASA Astrophysics Data System (ADS)

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

2014-03-01

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

The relationship between strain geometry and geometrically necessary dislocations

NASA Astrophysics Data System (ADS)

Hansen, Lars; Wallis, David

2016-04-01

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

Computational analysis of heat transfer, thermal stress and dislocation density during resistively Czochralski growth of germanium single crystal

NASA Astrophysics Data System (ADS)

Tavakoli, Mohammad Hossein; Renani, Elahe Kabiri; Honarmandnia, Mohtaram; Ezheiyan, Mahdi

2018-02-01

In this paper, a set of numerical simulations of fluid flow, temperature gradient, thermal stress and dislocation density for a Czochralski setup used to grow IR optical-grade Ge single crystal have been done for different stages of the growth process. A two-dimensional steady state finite element method has been applied for all calculations. The obtained numerical results reveal that the thermal field, thermal stress and dislocation structure are mainly dependent on the crystal height, heat radiation and gas flow in the growth system.

Elasticity and dislocation anelasticity of crystals

NASA Astrophysics Data System (ADS)

Nikanorov, S. P.; Kardashev, B. K.

The book is concerned with the application of the results of physical acoustic studies of elasticity and dislocation anelasticity to the investigation of interatomic interactions and interactions between lattice defects. The analysis of the potential functions determining the energy of interatomic interactions is based on a study of the elastic properties of crystals over a wide temperature range; data on the dislocation structure and on the interaction between dislocations and point defects are based mainly on a study of inelastic effects. Particular attention is given to the relationship between microplastic effects and the initial stage of plastic deformation under conditions of elastic oscillations, when the multiplication of dislocations is negligible.

Avalanches and plastic flow in crystal plasticity: an overview

NASA Astrophysics Data System (ADS)

Papanikolaou, Stefanos; Cui, Yinan; Ghoniem, Nasr

2018-01-01

Crystal plasticity is mediated through dislocations, which form knotted configurations in a complex energy landscape. Once they disentangle and move, they may also be impeded by permanent obstacles with finite energy barriers or frustrating long-range interactions. The outcome of such complexity is the emergence of dislocation avalanches as the basic mechanism of plastic flow in solids at the nanoscale. While the deformation behavior of bulk materials appears smooth, a predictive model should clearly be based upon the character of these dislocation avalanches and their associated strain bursts. We provide here a comprehensive overview of experimental observations, theoretical models and computational approaches that have been developed to unravel the multiple aspects of dislocation avalanche physics and the phenomena leading to strain bursts in crystal plasticity.

Plastic deformation of tubular crystals by dislocation glide.

PubMed

Beller, Daniel A; Nelson, David R

2016-09-01

Tubular crystals, two-dimensional lattices wrapped into cylindrical topologies, arise in many contexts, including botany and biofilaments, and in physical systems such as carbon nanotubes. The geometrical principles of botanical phyllotaxis, describing the spiral packings on cylinders commonly found in nature, have found application in all these systems. Several recent studies have examined defects in tubular crystals associated with crystalline packings that must accommodate a fixed tube radius. Here we study the mechanics of tubular crystals with variable tube radius, with dislocations interposed between regions of different phyllotactic packings. Unbinding and separation of dislocation pairs with equal and opposite Burgers vectors allow the growth of one phyllotactic domain at the expense of another. In particular, glide separation of dislocations offers a low-energy mode for plastic deformations of solid tubes in response to external stresses, reconfiguring the lattice step by step. Through theory and simulation, we examine how the tube's radius and helicity affects, and is in turn altered by, the mechanics of dislocation glide. We also discuss how a sufficiently strong bending rigidity can alter or arrest the deformations of tubes with small radii.

Plastic deformation of tubular crystals by dislocation glide

NASA Astrophysics Data System (ADS)

Beller, Daniel A.; Nelson, David R.

2016-09-01

Tubular crystals, two-dimensional lattices wrapped into cylindrical topologies, arise in many contexts, including botany and biofilaments, and in physical systems such as carbon nanotubes. The geometrical principles of botanical phyllotaxis, describing the spiral packings on cylinders commonly found in nature, have found application in all these systems. Several recent studies have examined defects in tubular crystals associated with crystalline packings that must accommodate a fixed tube radius. Here we study the mechanics of tubular crystals with variable tube radius, with dislocations interposed between regions of different phyllotactic packings. Unbinding and separation of dislocation pairs with equal and opposite Burgers vectors allow the growth of one phyllotactic domain at the expense of another. In particular, glide separation of dislocations offers a low-energy mode for plastic deformations of solid tubes in response to external stresses, reconfiguring the lattice step by step. Through theory and simulation, we examine how the tube's radius and helicity affects, and is in turn altered by, the mechanics of dislocation glide. We also discuss how a sufficiently strong bending rigidity can alter or arrest the deformations of tubes with small radii.

Ubiquity of quantum zero-point fluctuations in dislocation glide

NASA Astrophysics Data System (ADS)

Landeiro Dos Reis, Marie; Choudhury, Anshuman; Proville, Laurent

2017-03-01

Modeling the dislocation glide through atomic scale simulations in Al, Cu, and Ni and in solid solution alloys Al(Mg) and Cu(Ag), we show that in the course of the plastic deformation the variation of the crystal zero-point energy (ZPE) and the dislocation potential energy barriers are of opposite sign. The multiplicity of situations where we have observed the same trend allows us to conclude that quantum fluctuations, giving rise to the crystal ZPE, make easier the dislocation glide in most materials, even those constituted of atoms heavier than H and He.

The evolution of machining-induced surface of single-crystal FCC copper via nanoindentation

NASA Astrophysics Data System (ADS)

Zhang, Lin; Huang, Hu; Zhao, Hongwei; Ma, Zhichao; Yang, Yihan; Hu, Xiaoli

2013-05-01

The physical properties of the machining-induced new surface depend on the performance of the initial defect surface and deformed layer in the subsurface of the bulk material. In this paper, three-dimensional molecular dynamics simulations of nanoindentation are preformed on the single-point diamond turning surface of single-crystal copper comparing with that of pristine single-crystal face-centered cubic copper. The simulation results indicate that the nucleation of dislocations in the nanoindentation test on the machining-induced surface and pristine single-crystal copper is different. The dislocation embryos are gradually developed from the sites of homogeneous random nucleation around the indenter in the pristine single-crystal specimen, while the dislocation embryos derived from the vacancy-related defects are distributed in the damage layer of the subsurface beneath the machining-induced surface. The results show that the hardness of the machining-induced surface is softer than that of pristine single-crystal copper. Then, the nanocutting simulations are performed along different crystal orientations on the same crystal surface. It is shown that the crystal orientation directly influences the dislocation formation and distribution of the machining-induced surface. The crystal orientation of nanocutting is further verified to affect both residual defect generations and their propagation directions which are important in assessing the change of mechanical properties, such as hardness and Young's modulus, after nanocutting process.

A phase field crystal model simulation of morphology evolution and misfit dislocation generation in nanoheteroepitaxy

NASA Astrophysics Data System (ADS)

Zhang, J.; Chen, Z.; Cheng, C.; Wang, Y. X.

2017-10-01

A phase field crystal (PFC) model is employed to study morphology evolution of nanoheteroepitaxy and misfit dislocation generation when applied with enhanced supercooling, lattice mismatch and substrate vicinal angle conditions. Misfit strain that rises due to lattice mismatch causes rough surfaces or misfit dislocations, deteriorates film properties, hence, efforts taken to reveal their microscopic mechanism are significant for film quality improvement. Uniform islands, instead of misfit dislocations, are developed in subcritical thickness film, serving as a way of strain relief by surface mechanism. Misfit dislocations generate when strain relief by surface mechanism is deficient in higher supercooling, multilayers of misfit dislocations dominate, but the number of layers reduces gradually when the supercooling is further enhanced. Rough surfaces like islands or cuspate pits are developed which is ascribed to lattice mismatch, multilayers of misfit dislocations generate to further enhance lattice mismatch. Layers of misfit dislocations generate at a thickening position at enhanced substrate vicinal angle, this further enhancing the angle leading to sporadic generation of misfit dislocations.

Unravelling the physics of size-dependent dislocation-mediated plasticity

NASA Astrophysics Data System (ADS)

El-Awady, Jaafar A.

2015-01-01

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

Dislocations in bilayer graphene

NASA Astrophysics Data System (ADS)

Butz, Benjamin; Dolle, Christian; Niekiel, Florian; Weber, Konstantin; Waldmann, Daniel; Weber, Heiko B.; Meyer, Bernd; Spiecker, Erdmann

2014-01-01

Dislocations represent one of the most fascinating and fundamental concepts in materials science. Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals. In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy. However, in layered crystals such as graphite, dislocation movement is mainly restricted to the basal plane. Thus, the dislocations cannot escape, enabling their confinement in crystals as thin as only two monolayers. To explore the nature of dislocations under such extreme boundary conditions, the material of choice is bilayer graphene, the thinnest possible quasi-two-dimensional crystal in which such linear defects can be confined. Homogeneous and robust graphene membranes derived from high-quality epitaxial graphene on silicon carbide provide an ideal platform for their investigation. Here we report the direct observation of basal-plane dislocations in freestanding bilayer graphene using transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. Our investigation reveals two striking size effects. First, the absence of stacking-fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern that corresponds to an alternating ABAC change of the stacking order. Second, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane that results directly from accommodation of strain. In fact, the buckling changes the strain state of the bilayer graphene and is of key importance for its electronic properties. Our findings will contribute to the understanding of dislocations and of their role in the structural, mechanical and electronic properties of bilayer and few-layer graphene.

Dislocations in bilayer graphene.

PubMed

Butz, Benjamin; Dolle, Christian; Niekiel, Florian; Weber, Konstantin; Waldmann, Daniel; Weber, Heiko B; Meyer, Bernd; Spiecker, Erdmann

2014-01-23

Dislocations represent one of the most fascinating and fundamental concepts in materials science. Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals. In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy. However, in layered crystals such as graphite, dislocation movement is mainly restricted to the basal plane. Thus, the dislocations cannot escape, enabling their confinement in crystals as thin as only two monolayers. To explore the nature of dislocations under such extreme boundary conditions, the material of choice is bilayer graphene, the thinnest possible quasi-two-dimensional crystal in which such linear defects can be confined. Homogeneous and robust graphene membranes derived from high-quality epitaxial graphene on silicon carbide provide an ideal platform for their investigation. Here we report the direct observation of basal-plane dislocations in freestanding bilayer graphene using transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. Our investigation reveals two striking size effects. First, the absence of stacking-fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern that corresponds to an alternating AB B[Symbol: see text]AC change of the stacking order. Second, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane that results directly from accommodation of strain. In fact, the buckling changes the strain state of the bilayer graphene and is of key importance for its electronic properties. Our findings will contribute to the understanding of dislocations and of their role in the structural, mechanical and electronic properties of bilayer and few-layer graphene.

The strength and dislocation microstructure evolution in superalloy microcrystals

NASA Astrophysics Data System (ADS)

Hussein, Ahmed M.; Rao, Satish I.; Uchic, Michael D.; Parthasarathay, Triplicane A.; El-Awady, Jaafar A.

2017-02-01

In this work, the evolution of the dislocations microstructure in single crystal two-phase superalloy microcrystals under monotonic loading has been studied using the three-dimensional discrete dislocation dynamics (DDD) method. The DDD framework has been extended to properly handle the collective behavior of dislocations and their interactions with large collections of arbitrary shaped precipitates. Few constraints are imposed on the initial distribution of the dislocations or the precipitates, and the extended DDD framework can support experimentally-obtained precipitate geometries. Full tracking of the creation and destruction of anti-phase boundaries (APB) is accounted for. The effects of the precipitate volume fraction, APB energy, precipitate size, and crystal size on the deformation of superalloy microcrystals have been quantified. Correlations between the precipitate microstructure and the dominant deformation features, such as dislocation looping versus precipitate shearing, are also discussed. It is shown that the mechanical strength is independent of the crystal size, increases linearly with increasing the volume fraction, follows a near square-root relationship with the APB energy and an inverse square-root relationship with the precipitate size. Finally, the flow strength in simulations having initial dislocation pair sources show a flow strength that is about one half of that predicted from simulations starting with single dislocation sources. The method developed can be used, with minimal extensions, to simulate dislocation microstructure evolution in general multiphase materials.

Surface dislocation nucleation controlled deformation of Au nanowires

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

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

2014-11-17

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

Phonon-enhanced crystal growth and lattice healing

DOEpatents

Buonassisi, Anthony; Bertoni, Mariana; Newman, Bonna

2013-05-28

A system for modifying dislocation distributions in semiconductor materials is provided. The system includes one or more vibrational sources for producing at least one excitation of vibrational mode having phonon frequencies so as to enhance dislocation motion through a crystal lattice.

Transformations of the dislocation structure of nickel single crystals

NASA Astrophysics Data System (ADS)

Alfyorova, E. A.; Lychagin, D. V.; Lychagina, L. L.; Tsvetkov, N. A.

2017-12-01

A relationship between different-scale deformations of crystals has not been established yet. In order to solve this task, we investigate the development of a deformation relief and dislocation structure in nickel single crystals after deformation. The stress tensor, crystallography, and geometry of specimens affect the organization of some shear along corresponding systems of sliding. The organization of shear shows some features of self-organization. It is associated with the self-organization in the dislocation subsystem analyzed previously. The effectiveness of reducing external and internal stresses determines patterns of deformation processes at different scale levels.

Cyclic deformation leads to defect healing and strengthening of small-volume metal crystals

DOE PAGES

Wang, Zhang-Jie; Li, Qing-Jie; Cui, Yi-Nan; ...

2015-10-19

When microscopic and macroscopic specimens of metals are subjected to cyclic loading, the creation, interaction, and accumulation of defects lead to damage, cracking, and failure. We demonstrate that when aluminum single crystals of submicrometer dimensions are subjected to low-amplitude cyclic deformation at room temperature, the density of preexisting dislocation lines and loops can be dramatically reduced with virtually no change of the overall sample geometry and essentially no permanent plastic strain. Furthermore, this “cyclic healing” of the metal crystal leads to significant strengthening through dramatic reductions in dislocation density, in distinct contrast to conventional cyclic strain hardening mechanisms arising frommore » increases in dislocation density and interactions among defects in microcrystalline and macrocrystalline metals and alloys. Our real-time, in situ transmission electron microscopy observations of tensile tests reveal that pinned dislocation lines undergo shakedown during cyclic straining, with the extent of dislocation unpinning dependent on the amplitude, sequence, and number of strain cycles. Those unpinned mobile dislocations moving close enough to the free surface of the thin specimens as a result of such repeated straining are then further attracted to the surface by image forces that facilitate their egress from the crystal. Our results point to a versatile pathway for controlled mechanical annealing and defect engineering in submicrometer-sized metal crystals, thereby obviating the need for thermal annealing or significant plastic deformation that could cause change in shape and/or dimensions of the specimen.« less

Cyclic deformation leads to defect healing and strengthening of small-volume metal crystals

PubMed Central

Wang, Zhang-Jie; Li, Qing-Jie; Cui, Yi-Nan; Liu, Zhan-Li; Ma, Evan; Li, Ju; Sun, Jun; Zhuang, Zhuo; Dao, Ming; Shan, Zhi-Wei; Suresh, Subra

2015-01-01

When microscopic and macroscopic specimens of metals are subjected to cyclic loading, the creation, interaction, and accumulation of defects lead to damage, cracking, and failure. Here we demonstrate that when aluminum single crystals of submicrometer dimensions are subjected to low-amplitude cyclic deformation at room temperature, the density of preexisting dislocation lines and loops can be dramatically reduced with virtually no change of the overall sample geometry and essentially no permanent plastic strain. This “cyclic healing” of the metal crystal leads to significant strengthening through dramatic reductions in dislocation density, in distinct contrast to conventional cyclic strain hardening mechanisms arising from increases in dislocation density and interactions among defects in microcrystalline and macrocrystalline metals and alloys. Our real-time, in situ transmission electron microscopy observations of tensile tests reveal that pinned dislocation lines undergo shakedown during cyclic straining, with the extent of dislocation unpinning dependent on the amplitude, sequence, and number of strain cycles. Those unpinned mobile dislocations moving close enough to the free surface of the thin specimens as a result of such repeated straining are then further attracted to the surface by image forces that facilitate their egress from the crystal. These results point to a versatile pathway for controlled mechanical annealing and defect engineering in submicrometer-sized metal crystals, thereby obviating the need for thermal annealing or significant plastic deformation that could cause change in shape and/or dimensions of the specimen. PMID:26483463

Generalized continuum modeling of scale-dependent crystalline plasticity

NASA Astrophysics Data System (ADS)

Mayeur, Jason R.

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

Mesoscale modeling of strain induced solid state amorphization in crystalline materials

NASA Astrophysics Data System (ADS)

Lei, Lei

Solid state amorphization, and in particular crystalline to amorphous transformation, can be observed in metallic alloys, semiconductors, intermetallics, minerals, and also molecular crystals when they undergo irradiation, hydrogen gas dissolution, thermal interdiffusion, mechanical alloying, or mechanical milling. Although the amorphization mechanisms may be different, the transformation occurs due to the high level of disorder introduced into the material. Milling induced solid state amorphization is proposed to be the result of accumulation of crystal defects, specifically dislocations, as the material is subjected to large deformations during the high energy process. Thus, understanding the deformation mechanisms of crystalline materials will be the first step in studying solid state amorphization in crystalline materials, which not only has scientific contributions, but also technical consequences. A phase field dislocation dynamics (PFDD) approach is employed in this work to simulate plastic deformation of molecular crystals. This PFDD model has the advantage of tracking all of the dislocations in a material simultaneously. The model takes into account the elastic interaction between dislocations, the lattice resistance to dislocation motion, and the elastic interaction of dislocations with an external stress field. The PFDD model is employed to describe the deformation of molecular crystals with pharmaceutical applications, namely, single crystal sucrose, acetaminophen, gamma-indomethacin, and aspirin. Stress-strain curves are produced that result in expected anisotropic material response due to the activation of different slip systems and yield stresses that agree well with those from experiments. The PFDD model is coupled to a phase transformation model to study the relation between plastic deformation and the solid state amorphization of crystals that undergo milling. This model predicts the amorphous volume fraction in excellent agreement with experimental observation. Finally, we incorporate the effect of stress free surfaces to model the behavior of dislocations close to these surfaces and in the presence of voids.

Electron energy can oscillate near a crystal dislocation

DOE PAGES

Li, Mingda; Cui, Wenping; Dresselhaus, Mildred S.; ...

2017-01-25

Crystal dislocations govern the plastic mechanical properties of materials but also affect the electrical and optical properties. However, a fundamental and quantitative quantum field theory of a dislocation has remained undiscovered for decades. Here in this article we present an exactly-solvable one-dimensional quantum field theory of a dislocation, for both edge and screw dislocations in an isotropic medium, by introducing a new quasiparticle which we have called the ‘dislon’. The electron-dislocation relaxation time can then be studied directly from the electron self-energy calculation, which is reducible to classical results. In addition, we predict that the electron energy will experience anmore » oscillation pattern near a dislocation. Compared with the electron density’s Friedel oscillation, such an oscillation is intrinsically different since it exists even with only single electron is present. With our approach, the effect of dislocations on materials’ non-mechanical properties can be studied at a full quantum field theoretical level.« less

Oscillations of kinks on dislocation lines in crystals and low-temperature transport anomalies as a ``passport'' of newly-induced defects

NASA Astrophysics Data System (ADS)

Mezhov-Deglin, L. P.; Mukhin, S. I.

2011-10-01

The possible interpretation of experimental data on low-temperature anomalies in weakly deformed metallic crystals prepared form ultra-pure lead, copper, and silver, as well as in crystals of 4He is discussed within the previously proposed theoretical picture of dislocations with dynamical kinks. In the case of pure metals the theoretical predictions give a general picture of interaction of conduction electrons in a sample with newly-introduced dislocations, containing dynamic kinks in the Peierls potential relief. In the field of random stresses appearing due to plastic deformation of a sample, kinks on the dislocation line form a set of one-dimensional oscillators in potential wells of different shapes. In the low temperature region at low enough density of defects pinning kinks the inelastic scattering of electrons on kinks should lead to deviations from the Wiedemann-Franz law. In particular, the inelastic scattering on kinks should result in a quadratic temperature dependence of the thermal conductivity in a metallic sample along preferential directions of dislocation axes. In the plane normal to the dislocation axis the elastic large-angle scattering of electrons is prevalent. The kink pinning by a point defect or by additional dislocations as well as the sample annealing leading to the disappearance of kinks should induce suppression of transport anomalies. Thus, the energy interval for the spectrum of kink oscillations restricted by characteristic amplitude of the Peierls relief is a "passport of deformation history" for each specific sample. For instance, in copper the temperature/energy region of the order of 1 K corresponds to it. It is also planned to discuss in the other publication applicability of mechanism of phonon scattering on mobile dislocation kinks and pinning of kinks by impurities in order to explain anomalies of phonon thermal conductivity of 4He crystals and deformed crystals of pure lead in a superconducting state.

Polychromatic Microdiffraction Analysis of Defect Self-Organization in Shock Deformed Single Crystals

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

Barabash, Rozaliya; Ice, Gene E; Liu, Wenjun

A spatially resolved X-ray diffraction method - with a submicron 3D resolution together with SEM and OIM analysis are applied to understand the arrangements of voids, geometrically necessary dislocations and strain gradient distributions in samples of Al (1 2 3) and Cu (0 0 1) single crystals shocked to incipient spallation fracture. We describe how geometrically necessary dislocations and the effective strain gradient alter white beam Laue patterns of the shocked materials. Several distinct structural zones are observed at different depths under the impact surface. The density of geometrically necessary dislocations (GNDs) is extremely high near the impact and backmore » surface of the shock recovered crystals. The spall region is characterized by a large density of mesoscale voids and GNDs. The spall region is separated from the impact and back surfaces by compressed regions with high total dislocation density but lower GNDs density. Self-organization of shear bands is observed in the shock recovered Cu single crystal.« less

Dislocation formation in seed crystals induced by feedstock indentation during growth of quasimono crystalline silicon ingots

NASA Astrophysics Data System (ADS)

Trempa, M.; Beier, M.; Reimann, C.; Roßhirth, K.; Friedrich, J.; Löbel, C.; Sylla, L.; Richter, T.

2016-11-01

In this work the dislocation formation in the seed crystal induced by feedstock indentation during the growth of quasimono (QM) silicon ingots for photovoltaic application was investigated. It could be shown by special laboratory indentation experiments that the formed dislocations propagate up to several millimeters deep into the volume of the seed crystal in dependence on the applied pressure of the feedstock particles on the surface of the seed crystal. Further, it was demonstrated that these dislocations if they were not back-melted during the seeding process grow further into the silicon ingot and drastically reduce its material quality. An estimation of the apparent pressure values in a G5 industrial crucible/feedstock setup reveals that the indentation phenomenon is a critical issue for the industrial production of QM silicon ingots. Therefore, some approaches to avoid/reduce the indentation events were tested with the result, that the most promising solution should be the usage of suitable feedstock particles as coverage of the seed.

Analysis of synthetic diamond single crystals by X-ray topography and double-crystal diffractometry

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

Prokhorov, I. A., E-mail: igor.prokhorov@mail.ru; Ralchenko, V. G.; Bolshakov, A. P.

2013-12-15

Structural features of diamond single crystals synthesized under high pressure and homoepitaxial films grown by chemical vapor deposition (CVD) have been analyzed by double-crystal X-ray diffractometry and topography. The conditions of a diffraction analysis of diamond crystals using Ge monochromators have been optimized. The main structural defects (dislocations, stacking faults, growth striations, second-phase inclusions, etc.) formed during crystal growth have been revealed. The nitrogen concentration in high-pressure/high-temperature (HPHT) diamond substrates is estimated based on X-ray diffraction data. The formation of dislocation bundles at the film-substrate interface in the epitaxial structures has been revealed by plane-wave topography; these dislocations are likelymore » due to the relaxation of elastic macroscopic stresses caused by the lattice mismatch between the substrate and film. The critical thicknesses of plastic relaxation onset in CVD diamond films are calculated. The experimental techniques for studying the real diamond structure in optimizing crystal-growth technology are proven to be highly efficient.« less

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

NASA Astrophysics Data System (ADS)

Silbermann, C. B.; Ihlemann, J.

2016-03-01

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

Origin of dislocation luminescence centers and their reorganization in p-type silicon crystal subjected to plastic deformation and high temperature annealing.

PubMed

Pavlyk, Bohdan; Kushlyk, Markiyan; Slobodzyan, Dmytro

2017-12-01

Changes of the defect structure of silicon p-type crystal surface layer under the influence of plastic deformation and high temperature annealing in oxygen atmosphere were investigated by deep-level capacitance-modulation spectroscopy (DLCMS) and IR spectroscopy of molecules and atom vibrational levels. Special role of dislocations in the surface layer of silicon during the formation of its energy spectrum and rebuilding the defective structure was established. It is shown that the concentration of linear defects (N ≥ 10 4  cm -2 ) enriches surface layer with electrically active complexes (dislocation-oxygen, dislocation-vacancy, and dislocation-interstitial atoms of silicon) which are an effective radiative recombination centers.

Observation of Threading Dislocations in Ammonothermal Gallium Nitride Single Crystal Using Synchrotron X-ray Topography

NASA Astrophysics Data System (ADS)

Yao, Y.; Ishikawa, Y.; Sugawara, Y.; Takahashi, Y.; Hirano, K.

2018-04-01

Synchrotron monochromatic-beam x-ray topography observation has been performed on high-quality ammonothermal gallium nitride single crystal to evaluate threading dislocations (TD) in a nondestructive manner. Asymmetric diffractions with six equivalent g-vectors of 11-26, in addition to a symmetric diffraction with g = 0008, were applied to determine the Burgers vectors (b) of dislocations. It was found that pure edge-type TDs with \\varvec b = < {11 - 20} > /3 did not exist in the sample. A dominant proportion of TDs were of mixed type with \\varvec b = < {11 - 20} > /3 + < {0001} > , i.e., so-called c + a dislocations. Pure 1c screw dislocations with \\varvec b = < {0001} > and TDs with c-component larger than 1c were also observed.

A discrete dislocation dynamics model of creeping single crystals

NASA Astrophysics Data System (ADS)

Rajaguru, M.; Keralavarma, S. M.

2018-04-01

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

The growth of dislocation-free crystals of benzil

NASA Astrophysics Data System (ADS)

Katoh, K.; Kato, N.

1985-11-01

Dislocation-free crystals of benzil have been obtained by repeated Czochralski growth and have been characterized using X-ray diffraction topography. At each stage of growth, the parts containing the defects were etched off and the rest was used for the seed in the next growth. The growth behaviour could be interpreted in connection with the shape of the solid-liquid interface. The double image of the screw dislocation could be explained elementarily and it was concluded that the Burgers vector was c/3.

The Fundamentals of Dislocation Transport of Hydrogen in BCC Iron.

DTIC Science & Technology

1984-10-01

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

Hollow-core screw dislocations in 6H-SiC single crystals: A test of Frank`s theory

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

Si, W.; Dudley, M.; Glass, R.

1997-03-01

Hollow-core screw dislocations, also known as `micropipes`, along the [0001] axis in 6H-SiC single crystals, have been studied by synchrotron white beam x-ray topography (SWBXT), scanning electron microscopy (SEM), and Nomarski optical microscopy (NOM). Using SWBXT, the magnitude of the burgers vector of screw dislocations has been determined by measuring the following four parameters: (1) the diameter of dislocation images in back-reflection topographs; (2) the width of bimodal dislocation images in transmission topographs; (3) the magnitude of the tilt of lattice planes on both sides of dislocation core in projection topographs; and (4) the magnitude of the tilt of latticemore » planes in section topographs. The four methods show good agreement. The burgers vector magnitude of screw dislocations, b, and the diameter of associated micropipes, D, were fitted to Frank`s prediction for hollow-core screw dislocations: D = {mu}b{sup 2}/4{pi}{sup 2}{gamma}, where {mu} is shear modulus, and {gamma} is specific surface energy. 15 refs., 17 figs.« less

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

NASA Astrophysics Data System (ADS)

Huang, Minsheng; Li, Zhenhuan

2013-12-01

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

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.

Mechanical annealing under low-amplitude cyclic loading in micropillars

NASA Astrophysics Data System (ADS)

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

2016-04-01

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

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

NASA Astrophysics Data System (ADS)

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

2017-05-01

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

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

DOE PAGES

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

2016-03-28

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

Study of Te Inclusion and Related Point Defects in THM-Growth CdMnTe Crystal

NASA Astrophysics Data System (ADS)

Mao, Yifei; Zhang, Jijun; Min, Jiahua; Liang, Xiaoyan; Huang, Jian; Tang, Ke; Ling, Liwen; Li, Ming; Zhang, Ying; Wang, Linjun

2018-02-01

This study establishes a model for describing the interaction between Te inclusions, dislocations and point defects in CdMnTe crystals. The role of the complex environment surrounding the formation of Te inclusions was analyzed. Images of Te inclusions captured by scanning electron microscope and infrared microscope were used to observe the morphology of Te inclusions. The morphology of Te inclusions is discussed in light of crystallography, from the crystal growth temperature at 900°C to the melting temperature of Te inclusions using the traveling heater method. The dislocation nets around Te inclusions were calculated by counting lattice mismatches between the Te inclusions and the bulk CdMnTe at 470°C. The point defects of Te antisites were found to be gathered around Te inclusions, with dislocation climb during the cooling phase of crystal growth from 470°C to room temperature. The Te inclusions, dislocation nets and surrounding point defects are considered to be an entirety for evaluating the effect of Te inclusions on CdMnTe detector performance, and an effective mobility-lifetime product (μτ) was obtained.

Phyllotactic transformations as plastic deformations of tubular crystals with defects

NASA Astrophysics Data System (ADS)

Beller, Daniel; Nelson, David

Tubular crystals are 2D lattices in cylindrical topologies, which could be realized as assemblies of colloidal particles, and occur naturally in biological microtubules and in single-walled carbon nanotubes. Their geometry can be understood in the language of phyllotaxis borrowed from botany. We study the mechanics of plastic deformations in tubular crystals in response to tensile stress, as mediated by the formation and separation of dislocation pairs in a triangular lattice. Dislocation motion allows the growth of one phyllotactic arrangement at the expense of another, offering a low-energy, stepwise mode of plastic deformation in response to external stresses. Through theory and simulation, we examine how the tube's radius and helicity affects, and is in turn altered by, dislocation glide. The crystal's bending modulus is found to produce simple but important corrections to the tube's deformation mechanics.

The measurement of the stacking fault energy in copper, nickel and copper-nickel alloys

NASA Technical Reports Server (NTRS)

Leighly, H. P., Jr.

1982-01-01

The relationship of hydrogen solubility and the hydrogen embrittlement of high strength, high performance face centered cubic alloys to the stacking fault energy of the alloys was investigated. The stacking fault energy is inversely related to the distance between the two partial dislocations which are formed by the dissociation of a perfect dislocation. The two partial dislocations define a stacking fault in the crystal which offers a region for hydrogen segregation. The distance between the partial dislocations is measured by weak beam, dark field transmission electron microscopy. The stacking fault energy is calculated. Pure copper, pure nickel and copper-nickel single crystals are used to determine the stacking fault energy.

Three-dimensional evaluation of gettering ability for oxygen atoms at small-angle tilt boundaries in Czochralski-grown silicon crystals

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

Ohno, Yutaka, E-mail: yutakaohno@imr.tohoku.ac.jp; Inoue, Kaihei; Fujiwara, Kozo

2015-06-22

Three-dimensional distribution of oxygen atoms at small-angle tilt boundaries (SATBs) in Czochralski-grown p-type silicon ingots was investigated by atom probe tomography combined with transmission electron microscopy. Oxygen gettering along edge dislocations composing SATBs, post crystal growth, was observed. The gettering ability of SATBs would depend both on the dislocation strain and on the dislocation density. Oxygen atoms would agglomerate in the atomic sites under the tensile hydrostatic stress larger than about 2.0 GPa induced by the dislocations. It was suggested that the density of the atomic sites, depending on the tilt angle of SATBs, determined the gettering ability of SATBs.

Probing the limits of metal plasticity with molecular dynamics simulations

NASA Astrophysics Data System (ADS)

Zepeda-Ruiz, Luis A.; Stukowski, Alexander; Oppelstrup, Tomas; Bulatov, Vasily V.

2017-10-01

Ordinarily, the strength and plasticity properties of a metal are defined by dislocations--line defects in the crystal lattice whose motion results in material slippage along lattice planes. Dislocation dynamics models are usually used as mesoscale proxies for true atomistic dynamics, which are computationally expensive to perform routinely. However, atomistic simulations accurately capture every possible mechanism of material response, resolving every ``jiggle and wiggle'' of atomic motion, whereas dislocation dynamics models do not. Here we present fully dynamic atomistic simulations of bulk single-crystal plasticity in the body-centred-cubic metal tantalum. Our goal is to quantify the conditions under which the limits of dislocation-mediated plasticity are reached and to understand what happens to the metal beyond any such limit. In our simulations, the metal is compressed at ultrahigh strain rates along its [001] crystal axis under conditions of constant pressure, temperature and strain rate. To address the complexity of crystal plasticity processes on the length scales (85-340 nm) and timescales (1 ns-1μs) that we examine, we use recently developed methods of in situ computational microscopy to recast the enormous amount of transient trajectory data generated in our simulations into a form that can be analysed by a human. Our simulations predict that, on reaching certain limiting conditions of strain, dislocations alone can no longer relieve mechanical loads; instead, another mechanism, known as deformation twinning (the sudden re-orientation of the crystal lattice), takes over as the dominant mode of dynamic response. Below this limit, the metal assumes a strain-path-independent steady state of plastic flow in which the flow stress and the dislocation density remain constant as long as the conditions of straining thereafter remain unchanged. In this distinct state, tantalum flows like a viscous fluid while retaining its crystal lattice and remaining a strong and stiff metal.

Molecular dynamic simulations of the high-speed copper nanoparticles collision with the aluminum surface

NASA Astrophysics Data System (ADS)

Pogorelko, V. V.; Mayer, A. E.

2016-11-01

With the use of the molecular dynamic simulations, we investigated the effect of the high-speed (500 m/s, 1000 m/s) copper nanoparticle impact on the mechanical properties of an aluminum surface. Dislocation analysis shows that a large number of dislocations are formed in the impact area; the total length of dislocations is determined not only by the speed and size of the incoming copper nanoparticle (kinetic energy of the nanoparticle), but by a temperature of the system as well. The dislocations occupy the whole area of the aluminum single crystal at high kinetic energy of the nanoparticle. With the decrease of the nanoparticle kinetic energy, the dislocation structures are formed in the near-surface layer; formation of the dislocation loops takes place. Temperature rise of the system (aluminum substrate + nanoparticle) reduces the total dislocation length in the single crystal of aluminum; there is deeper penetration of the copper atoms in the aluminum at high temperatures. Average energy of the nanoparticles and room temperature of the system are optimal for production of high-quality layers of copper on the aluminum surface.

Three-dimensional imaging of dislocation propagation during crystal growth and dissolution

PubMed Central

Schenk, Anna S.; Kim, Yi-Yeoun; Kulak, Alexander N.; Campbell, James M.; Nisbet, Gareth; Meldrum, Fiona C.; Robinson, Ian K.

2015-01-01

Atomic level defects such as dislocations play key roles in determining the macroscopic properties of crystalline materials 1,2. Their effects range from increased chemical reactivity 3,4 to enhanced mechanical properties 5,6. Dislocations have been widely studied using traditional techniques such as X-ray diffraction and optical imaging. Recent advances have enabled atomic force microscopy to study single dislocations 7 in two-dimensions (2D), while transmission electron microscopy (TEM) can now visualise strain fields in three-dimensions (3D) with near atomic resolution 8–10. However, these techniques cannot offer 3D imaging of the formation or movement of dislocations during dynamic processes. Here, we describe how Bragg Coherent Diffraction Imaging (BCDI) 11,12 can be used to visualize in 3D, the entire network of dislocations present within an individual calcite crystal during repeated growth and dissolution cycles. These investigations demonstrate the potential of BCDI for studying the mechanisms underlying the response of crystalline materials to external stimuli. PMID:26030304

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

NASA Astrophysics Data System (ADS)

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

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

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

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

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

Chirality-controlled crystallization via screw dislocations.

PubMed

Sung, Baeckkyoung; de la Cotte, Alexis; Grelet, Eric

2018-04-11

Chirality plays an important role in science from enantiomeric separation in chemistry to chiral plasmonics in nanotechnology. However, the understanding of chirality amplification from chiral building blocks to ordered helical superstructures remains a challenge. Here, we demonstrate that topological defects, such as screw dislocations, can drive the chirality transfer from particle to supramolecular structure level during the crystallization process. By using a model system of chiral particles, which enables direct imaging of single particle incorporation into growing crystals, we show that the crystallization kinetic pathway is the key parameter for monitoring, via the defects, the chirality amplification of the crystalline structures from racemic to predominantly homohelical. We provide an explanation based on the interplay between geometrical frustration, racemization induced by thermal fluctuations, and particle chirality. Our results demonstrate that screw dislocations not only promote the growth, but also control the chiral morphology and therefore the functionality of crystalline states.

Defect sensitive etching of hexagonal boron nitride single crystals

NASA Astrophysics Data System (ADS)

Edgar, J. H.; Liu, S.; Hoffman, T.; Zhang, Yichao; Twigg, M. E.; Bassim, Nabil D.; Liang, Shenglong; Khan, Neelam

2017-12-01

Defect sensitive etching (DSE) was developed to estimate the density of non-basal plane dislocations in hexagonal boron nitride (hBN) single crystals. The crystals employed in this study were precipitated by slowly cooling (2-4 °C/h) a nickel-chromium flux saturated with hBN from 1500 °C under 1 bar of flowing nitrogen. On the (0001) planes, hexagonal-shaped etch pits were formed by etching the crystals in a eutectic mixture of NaOH and KOH between 450 °C and 525 °C for 1-2 min. There were three types of pits: pointed bottom, flat bottom, and mixed shape pits. Cross-sectional transmission electron microscopy revealed that the pointed bottom etch pits examined were associated with threading dislocations. All of these dislocations had an a-type burgers vector (i.e., they were edge dislocations, since the line direction is perpendicular to the [ 2 11 ¯ 0 ]-type direction). The pit widths were much wider than the pit depths as measured by atomic force microscopy, indicating the lateral etch rate was much faster than the vertical etch rate. From an Arrhenius plot of the log of the etch rate versus the inverse temperature, the activation energy was approximately 60 kJ/mol. This work demonstrates that DSE is an effective method for locating threading dislocations in hBN and estimating their densities.

Monte Carlo simulations of backscattering process in dislocation-containing SrTiO3 single crystal

NASA Astrophysics Data System (ADS)

Jozwik, P.; Sathish, N.; Nowicki, L.; Jagielski, J.; Turos, A.; Kovarik, L.; Arey, B.

2014-05-01

Studies of defects formation in crystals are of obvious importance in electronics, nuclear engineering and other disciplines where materials are exposed to different forms of irradiation. Rutherford Backscattering/Channeling (RBS/C) and Monte Carlo (MC) simulations are the most convenient tool for this purpose, as they allow one to determine several features of lattice defects: their type, concentration and damage accumulation kinetic. On the other hand various irradiation conditions can be efficiently modeled by ion irradiation method without leading to the radioactivity of the sample. Combination of ion irradiation with channeling experiment and MC simulations appears thus as a most versatile method in studies of radiation damage in materials. The paper presents the results on such a study performed on SrTiO3 (STO) single crystals irradiated with 320 keV Ar ions. The samples were analyzed also by using HRTEM as a complementary method which enables the measurement of geometrical parameters of crystal lattice deformation in the vicinity of dislocations. Once the parameters and their variations within the distance of several lattice constants from the dislocation core are known, they may be used in MC simulations for the quantitative determination of dislocation depth distribution profiles. The final outcome of the deconvolution procedure are cross-sections values calculated for two types of defects observed (RDA and dislocations).

Scale-free avalanche dynamics in crystal plasticity

NASA Astrophysics Data System (ADS)

Ispanovity, Pater Dusan; Laurson, Lasse; Zaiser, Michael; Zapperi, Stefano; Groma, Istvan; Alava, Mikko

2015-03-01

We investigate the properties of strain bursts (dislocation avalanches) occurring during plastic deformation of crystalline matter using two dimensional discrete dislocation dynamics (DDD). We perform quasistatic stress-controlled simulations with three DDD models differing in the spatiotemporal discretization and the mobility law assumed for individual dislocations. We find that each model exhibits identical avalanche dynamics with the following properties: (i) strain burst sizes follow a power law distribution characterized by an exponent τ ~ 1 . 0 and (ii) the distribution in truncated at a cutoff that diverges with increasing system size at any applied stress level. It has been proposed earlier that plastic yielding can be described in terms of a continuous phase transition of depinning type and its critical point is at the yield stress. We will demonstrate, however, that our results are inconsistent with cutoff scaling in depinning systems (like magnetic domain walls or earthquakes) and that the system behaves as critical at every stress level. We, therefore, conclude that in the models studied plastic yielding cannot be associated with a continuous phase transition. Financial supports of the Hungarian Scientific Research Fund (OTKA) under Contract Numbers PD-105256 and K-105335 and of the European Commission under Grant Agreement No. CIG-321842 are acknowledged.

Investigation of dislocation cluster evolution during directional solidification of multicrystalline silicon

NASA Astrophysics Data System (ADS)

Oriwol, Daniel; Trempa, Matthias; Sylla, Lamine; Leipner, Hartmut S.

2017-04-01

Dislocation clusters are the main crystal defects in multicrystalline silicon and are detrimental for solar cell efficiency. They were formed during the silicon ingot casting due to the relaxation of strain energy. The evolution of the dislocation clusters was studied by means of automated analysing tools of the standard wafer and cell production giving information about the cluster development as a function of the ingot height. Due to the observation of the whole wafer surface the point of view is of macroscopic nature. It was found that the dislocations tend to build clusters of high density which usually expand in diameter as a function of ingot height. According to their structure the dislocation clusters can be divided into light and dense clusters. The appearance of both types shows a clear dependence on the orientation of the grain growth direction. Additionally, a process of annihilation of dislocation clusters during the crystallization has been observed. To complement the macroscopic description, the dislocation clusters were also investigates by TEM. It is shown that the dislocations within the subgrain boundaries are closely arranged. Distances of 40-30 nm were found. These results lead to the conclusion that the dislocation density within the cluster structure is impossible to quantify by means of etch pit counting.

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

NASA Astrophysics Data System (ADS)

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

2017-01-01

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

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

DOE PAGES

Patra, Anirban; McDowell, David L.

2016-03-25

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

High purity low dislocation GaAs single crystals

NASA Technical Reports Server (NTRS)

Chen, R. T.; Holmes, D. E.; Kirkpatrick, C. G.

1982-01-01

Recent advances in GaAs bulk crystal growth using the LEC (liquid encapsulated Czochralski) technique are described. The dependence of the background impurity concentration and the dislocation density distribution on the materials synthesis and growth conditions were investigated. Background impurity concentrations as low as 4 x 10 to the 15th power were observed in undoped LEC GaAs. The dislocation density in selected regions of individual ingots was very low, below the 3000 cm .3000/sq cm threshold. The average dislocation density over a large annular ring on the wafers fell below the 10000/sq cm level for 3 inch diameter ingots. The diameter control during the program advanced to a diameter variation along a 3 inch ingot less than 2 mm.

Understanding dislocation mechanics at the mesoscale using phase field dislocation dynamics

PubMed Central

Hunter, A.

2016-01-01

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

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

NASA Astrophysics Data System (ADS)

Romero, Ignacio; Segurado, Javier; LLorca, Javier

2008-04-01

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

Dislocation, crystallite size distribution and lattice strain of magnesium oxide nanoparticles

NASA Astrophysics Data System (ADS)

Sutapa, I. W.; Wahid Wahab, Abdul; Taba, P.; Nafie, N. L.

2018-03-01

The oxide of magnesium nanoparticles synthesized using sol-gel method and analysis of the structural properties was conducted. The functional groups of nanoparticles has been analysed by Fourier Transform Infrared Spectroscopy (FT-IR). Dislocations, average size of crystal, strain, stress, the energy density of crystal, crystallite size distribution and morphologies of the crystals were determined based on X-ray diffraction profile analysis. The morphological of the crystal was analysed based on the image resulted from SEM analysis. The crystallite size distribution was calculated with the contention that the particle size has a normal logarithmic form. The most orientations of crystal were determined based on the textural crystal from diffraction data of X-ray diffraction profile analysis. FT-IR results showed the stretching vibration mode of the Mg-O-Mg in the range of 400.11-525 cm-1 as a broad band. The average size crystal of nanoparticles resulted is 9.21 mm with dislocation value of crystal is 0.012 nm-2. The strains, stress, the energy density of crystal are 1.5 x 10-4 37.31 MPa; 0.72 MPa respectively. The highest texture coefficient value of the crystal is 0.98. This result is supported by morphological analysis using SEM which shows most of the regular cubic-shaped crystals. The synthesis method is suitable for simple and cost-effective synthesis model of MgO nanoparticles.

On damping of screw dislocation bending vibrations in dissipative crystal: limiting cases

NASA Astrophysics Data System (ADS)

Dezhin, V. V.

2018-03-01

The expression for the generalized susceptibility of the dislocation obtained earlier was used. The electronic drag mechanism of dislocations is considered. The study of small dislocation oscillations was limited. The contribution of the attenuation of low-frequency bending screw dislocation vibrations to the overall coefficient of dynamic dislocation drag in the long-wave and short-wave limits is calculated. The damping of short-wave bending screw dislocation vibrations caused by an external action of an arbitrary frequency has been investigated. The contribution of long-wave bending screw dislocation vibrations damping in the total drag coefficient at an arbitrary frequency is found.

FAST TRACK COMMUNICATION High rate straining of tantalum and copper

NASA Astrophysics Data System (ADS)

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

2010-12-01

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

Singular orientations and faceted motion of dislocations in body-centered cubic crystals.

PubMed

Kang, Keonwook; Bulatov, Vasily V; Cai, Wei

2012-09-18

Dislocation mobility is a fundamental material property that controls strength and ductility of crystals. An important measure of dislocation mobility is its Peierls stress, i.e., the minimal stress required to move a dislocation at zero temperature. Here we report that, in the body-centered cubic metal tantalum, the Peierls stress as a function of dislocation orientation exhibits fine structure with several singular orientations of high Peierls stress-stress spikes-surrounded by vicinal plateau regions. While the classical Peierls-Nabarro model captures the high Peierls stress of singular orientations, an extension that allows dislocations to bend is necessary to account for the plateau regions. Our results clarify the notion of dislocation kinks as meaningful only for orientations within the plateau regions vicinal to the Peierls stress spikes. These observations lead us to propose a Read-Shockley type classification of dislocation orientations into three distinct classes-special, vicinal, and general-with respect to their Peierls stress and motion mechanisms. We predict that dislocation loops expanding under stress at sufficiently low temperatures, should develop well defined facets corresponding to two special orientations of highest Peierls stress, the screw and the M111 orientations, both moving by kink mechanism. We propose that both the screw and the M111 dislocations are jointly responsible for the yield behavior of BCC metals at low temperatures.

The size effects upon shock plastic compression of nanocrystals

NASA Astrophysics Data System (ADS)

Malygin, G. A.; Klyavin, O. V.

2017-10-01

For the first time a theoretical analysis of scale effects upon the shock plastic compression of nanocrystals is implemented in the context of a dislocation kinetic approach based on the equations and relationships of dislocation kinetics. The yield point of crystals τy is established as a quantitative function of their cross-section size D and the rate of shock deformation as τy ɛ2/3 D. This dependence is valid in the case of elastic stress relaxation on account of emission of dislocations from single-pole Frank-Read sources near the crystal surface.

Edge facet dynamics during the growth of heavily doped n-type silicon by the Czochralski-method

NASA Astrophysics Data System (ADS)

Stockmeier, L.; Kranert, C.; Raming, G.; Miller, A.; Reimann, C.; Rudolph, P.; Friedrich, J.

2018-06-01

During the growth of [0 0 1]-oriented, heavily n-type doped silicon crystals by the Czochralski (CZ) method dislocation formation occurs frequently which leads to a reduction of the crystal yield. In this publication the evolution of the solid-liquid interface and the formation of the {1 1 1} edge facets are analyzed on a microscopic scale as possible reason for dislocation formation in heavily n-type doped [0 0 1]-oriented CZ crystals. A correlation between the length of the {1 1 1} edge facets and the curvature of the interface is found. They ultimately promote supercooled areas and interrupted growth kinetics, which increase the probability for dislocation formation at the boundary between the {1 1 1} edge facets and the atomically rough interface.

Physics-Based Crystal Plasticity Modeling of Single Crystal Niobium

NASA Astrophysics Data System (ADS)

Maiti, Tias

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

Appearance of singularities of optical fields under torsion of crystals containing threefold symmetry axes.

PubMed

Skab, Ihor; Vasylkiv, Yurij; Zapeka, Bohdan; Savaryn, Viktoriya; Vlokh, Rostyslav

2011-07-01

We present an analysis of the effect of torsion stresses on the spatial distribution of optical birefringence in crystals of different point symmetry groups. The symmetry requirements needed so that the optical beam carries dislocations of the phase front are evaluated for the case when the crystals are twisted and the beam closely corresponds to a plane wave. It is shown that the torsion stresses can produce screw-edge, pure screw, or pure edge dislocations of the phase front in the crystals belonging to cubic and trigonal systems. The conditions for appearance of canonical and noncanonical vortices in the conditions of crystal torsion are analyzed. © 2011 Optical Society of America

Continuous modeling of a grain boundary in MgO and its disclination induced grain-boundary migration mechanism

NASA Astrophysics Data System (ADS)

Cordier, P.; Sun, X.; Taupin, V.; Fressengeas, C.

2016-12-01

Grain boundaries (GBs) are thin material layers where the lattice rotates from one orientation to the next one within a few nanometers. Because they treat these layers as infinitely thin interfaces, large-scale polycrystalline representations fail to describe their structure. Conversely, atomistic representations provide a detailed description of the GBs, but their character remains discrete and not prone to coarse-graining procedures. Continuum descriptions based on kinematic and crystal defect fields defined at interatomic scale are appealing because they can provide smooth and thorough descriptions of GBs, recovering in some sense the atomistic description and potentially serving as a basis for coarse-grained polycrystalline representations. In this work, a crossover between atomistic description and continuous representation of a MgO tilt boundary in polycrystals is set-up to model the periodic arrays of structural units by using dislocation and disclination dipole arrays along GBs. The strain, rotation, curvature, disclination and dislocation density fields are determined in the boundary area by using the discrete atomic positions generated by molecular dynamics simulations. Then, this continuous disclination/dislocation model is used as part of the initial conditions in elasto-plastic continuum mechanics simulations to investigate the shear-coupled boundary migration of tilt boundaries. The present study leads to better understanding of the structure and mechanical architecture of grain boundaries.

Binary dislocation junction formation and strength in hexagonal close-packed crystals

DOE PAGES

Wu, Chi -Chin; Aubry, Sylvie; Arsenlis, Athanasios; ...

2015-12-17

This work examines binary dislocation interactions, junction formation and junction strengths in hexagonal close-packed ( hcp ) crystals. Through a line-tension model and dislocation dynamics (DD) simulations, the interaction and dissociation of different sets of binary junctions are investigated involving one dislocation on the (011¯0) prismatic plane and a second dislocation on one of the following planes: (0001) basal, (11¯00) prismatic, (11¯01) primary pyramidal, or (2¯112) secondary pyramidal. Varying pairs of Burgers vectors are chosen from among the common types the basal type < a > 1/3 < 112¯0 >, prismatic type < c > <0001>, and pyramidal type 1/3 < 112¯3¯ >. For binary interaction due to dislocation intersection, both the analytical results and DD-simulations indicate a relationship between symmetry of interaction maps and the relative magnitude of the Burgers vectors that constitute the junction. Using analytical formulae, a simple regressive model is also developed to represent the junction yield surface. The equation is treated as a degenerated super elliptical equation to quantify the aspect ratio and tilting angle. Lastly, the results provide analytical insights on binary dislocation interactions that may occur in general hcp metals.« less

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

NASA Astrophysics Data System (ADS)

Wentlent, Luke Arthur

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

Capping spheres with scarry crystals: Organizing principles of multi-dislocation, ground-state patterns

NASA Astrophysics Data System (ADS)

Azadi, Amir; Grason, Gregory M.

2014-03-01

Predicting the ground state ordering of curved crystals remains an unsolved, century-old challenge, beginning with the classic Thomson problem to more recent studies of particle-coated droplets. We study the structural features and underlying principles of multi-dislocation ground states of a crystalline cap adhered to a spherical substrate. In the continuum limit, vanishing lattice spacing, a --> 0 , dislocations proliferate and we show that ground states approach a characteristic sequence of patterns of n-fold radial grain boundary ``scars,'' extending from the boundary and terminating in the bulk. A combination of numerical and asymptotic analysis reveals that energetic hierarchy gives rise to a structural hierarchy, whereby the number of dislocation and scars diverge as a --> 0 while the scar length and number of dislocations per scar become remarkably independent of lattice spacing. We show the that structural hierarchy remains intact when n-fold symmetry becomes unstable to polydispersed forked-scar morphologies. We expect this analysis to resolve previously open questions about the optimal symmetries of dislocation patterns in Thomson-like problems, both with and without excess 5-fold defects.

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

NASA Astrophysics Data System (ADS)

Lin, Bing; Huang, Minsheng; Zhao, Liguo; Roy, Anish; Silberschmidt, Vadim; Barnard, Nick; Whittaker, Mark; McColvin, Gordon

2018-06-01

Strain-controlled cyclic deformation of a nickel-based single crystal superalloy has been modelled using three-dimensional (3D) discrete dislocation dynamics (DDD) for both [0 0 1] and [1 1 1] orientations. The work focused on the interaction between dislocations and precipitates during cyclic plastic deformation at elevated temperature, which has not been well studied yet. A representative volume element with cubic γ‧-precipitates was chosen to represent the material, with enforced periodical boundary conditions. In particular, cutting of superdislocations into precipitates was simulated by a back-force method. The global cyclic stress-strain responses were captured well by the DDD model when compared to experimental data, particularly the effects of crystallographic orientation. Dislocation evolution showed that considerably high density of dislocations was produced for [1 1 1] orientation when compared to [0 0 1] orientation. Cutting of dislocations into the precipitates had a significant effect on the plastic deformation, leading to material softening. Contour plots of in-plane shear strain proved the development of heterogeneous strain field, resulting in the formation of shear-band embryos.

The Effect of the Wall Contact and Post-Growth C001-Down on Defects in CdTe Crystals Grown by Contactless PVT

NASA Technical Reports Server (NTRS)

Palosz, W.; Grasza, K.; Dudley, M.; Raghothamachar, B.; Cai, L.; Durose, K.; Halliday, D.; Boyall, N. M.; Rose, M. Franklin (Technical Monitor)

2001-01-01

In crystal growth, the quality of the final material may depend, among other factors, on its interaction with the walls of the ampoule during and after the growth, and on the rate of the crystal cool-down at the end of ate the process. To investigate the above phenomena, a series of CdTe crystal growth processes was carried out, The crystals were grown by physical vapor transport without contact with the side walls of the silica glass ampoules, applying the Low Supersaturation Nucleation technique. The source temperature was 930 C, the undercooling was a few degrees. The crystals, having the diameter of 25 mm, grew at the rate of a few mm per day. The post-growth cool-down to the room temperature was conducted at different rates, and lasted from a few minutes to four days. The crystals were characterized using chemical etching low temperature luminescence, and Synchrotron White Beam X-ray Topography techniques. The dislocation (etch pit) density was measured and its distribution was analyzed by comparison with Poisson curves and with the Normalized Radial Distribution Correlation Function. It was found that the contact of the crystal with silica leads to a strain field and high (in the 105 sq cm range) dislocation (etch pit) density. Similar defect concentrations were found in crystals subjected to fast post-growth cool-down. Typical EPD values for lower cool-down rates and in regions not affected by wall interactions are in the lower 10(exp 4) sq cm range. In some areas the actual dislocation density was about 10(exp 3) sq cm or even less. No apparent effect of the cool-down rate on polygonization was observed. A fine structure could be discerned in low-temperature PL spectra of crystals with low dislocation density.

TEM study of 〈110〉-type 35.26° dislocations specially induced by polishing of SrTiO₃ single crystals.

PubMed

Jin, L; Guo, X; Jia, C L

2013-11-01

The dislocations created by mechanical polishing of SrTiO₃ (100) single crystals were investigated by means of transmission electron microscopy (TEM) techniques combined with scanning TEM (STEM) techniques. A high density of dislocations was observed in the surface layer with a thickness of about 5 μm. These dislocations were found to be straight and highly aligned along the 〈111〉 directions. In most cases they appear in pairs or as a bundle. The nature of the dislocations was determined as mixed 〈110〉-type with the line vector t=〈111〉. They are 〈110〉-type 35.26° dislocations. The isolated 〈110〉-type 35.26° dislocations possess a compact core structure with a core spreading of ~0.5 nm. Dissociation of the dislocation occurs on the {1−10} glide plane, leading to the formation of two b=a/2〈110〉 partials separated by a stacking fault. The separation of the two partials was estimated to be 2.53 ± 0.32 nm based on a cross-correlation analysis of atomic-resolution images. Our results provide a solid experimental evidence for this special type of dislocation in SrTiO₃. The high density of straight and highly 〈111〉-orientated dislocations is expected to have an important influence on the anisotropy in electrical and mass transport properties. © 2013 Elsevier B.V. All rights reserved.

Ultrahigh strength single crystalline nanowhiskers grown by physical vapor deposition.

PubMed

Richter, Gunther; Hillerich, Karla; Gianola, Daniel S; Mönig, Reiner; Kraft, Oliver; Volkert, Cynthia A

2009-08-01

The strength of metal crystals is reduced below the theoretical value by the presence of dislocations or by flaws that allow easy nucleation of dislocations. A straightforward method to minimize the number of defects and flaws and to presumably increase its strength is to increase the crystal quality or to reduce the crystal size. Here, we describe the successful fabrication of high aspect ratio nanowhiskers from a variety of face-centered cubic metals using a high temperature molecular beam epitaxy method. The presence of atomically smooth, faceted surfaces and absence of dislocations is confirmed using transmission electron microscopy investigations. Tensile tests performed in situ in a focused-ion beam scanning electron microscope on Cu nanowhiskers reveal strengths close to the theoretical upper limit and confirm that the properties of nanomaterials can be engineered by controlling defect and flaw densities.

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

NASA Technical Reports Server (NTRS)

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

2012-01-01

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

Microstructural changes in Beta-silicon nitride grains upon crystallizing the grain-boundary glass

NASA Technical Reports Server (NTRS)

Lee, William E.; Hilmas, Gregory E.; Lange, F. F. (Editor)

1991-01-01

Crystallizing the grain boundary glass of a liquid phase sintered Si3N4 ceramic for 2 h or less at 1500 C led to formation of gamma Y2Si2O7. After 5 h at 1500 C, the gamma Y2Si2O7 had transformed to beta Y2Si2O7 with a concurrent dramatic increase in dislocation density within beta Si3N4 grains. Reasons for the increased dislocation density is discussed. Annealing for 20 h at 1500 C reduced dislocation densities to the levels found in as-sintered materials.

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

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

Guo, Xiaobin; Deng, Yunlai, E-mail: luckdeng@csu.edu.cn; State Key Laboratory of High Performance and Complex Manufacturing, Central South University, Changsha

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 precipitationmore » 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.« less

Thermodynamic dislocation theory: Bauschinger effect

NASA Astrophysics Data System (ADS)

Le, K. C.; Tran, T. M.

2018-04-01

The thermodynamic dislocation theory developed for nonuniform plastic deformations is used here to simulate the stress-strain curves for crystals subjected to antiplane shear-controlled load reversal. We show that the presence of the positive back stress during the load reversal reduces the magnitude of shear stress required to pull excess dislocations back to the center of the specimen. There, the excess dislocations of opposite signs meet and annihilate each other leading to the Bauschinger effect.

Penetration Depth and Defect Image Contrast Formation in Grazing-Incidence X-ray Topography of 4H-SiC Wafers

NASA Astrophysics Data System (ADS)

Yang, Yu; Guo, Jianqiu; Goue, Ouloide Yannick; Kim, Jun Gyu; Raghothamachar, Balaji; Dudley, Michael; Chung, Gill; Sanchez, Edward; Manning, Ian

2018-02-01

Synchrotron x-ray topography in grazing-incidence geometry is useful for discerning defects at different depths below the crystal surface, particularly for 4H-SiC epitaxial wafers. However, the penetration depths measured from x-ray topographs are much larger than theoretical values. To interpret this discrepancy, we have simulated the topographic contrast of dislocations based on two of the most basic contrast formation mechanisms, viz. orientation and kinematical contrast. Orientation contrast considers merely displacement fields associated with dislocations, while kinematical contrast considers also diffraction volume, defined as the effective misorientation around dislocations and the rocking curve width for given diffraction vector. Ray-tracing simulation was carried out to visualize dislocation contrast for both models, taking into account photoelectric absorption of the x-ray beam inside the crystal. The results show that orientation contrast plays the key role in determining both the contrast and x-ray penetration depth for different types of dislocation.

The Effect of the Wall Contact and Post-Growth, Cool-Down on Defects in CdTe Crystals Grown By 'Contactless' PVT

NASA Technical Reports Server (NTRS)

Palosz, W.; Grasza, K.; Dudley, M.; Raghothamachar, B.; Cai, L.; Dunrose, K.; Halliday, D.; Boyall, N. M.; Rose, M. Franklin (Technical Monitor)

2001-01-01

To take a maximum advantage of materials processing in microgravity for understanding the effects of gravity, gravity-independent effects should be minimized. In crystal growth, the quality of the grown crystals may depend, among other factors, on their interaction with the walls of the processing container during and after growth, and on the rate of the crystal cool-down at the end of the process. To investigate the above phenomena, a series of CdTe crystal growth processes was carried out. The crystals were grown by physical vapor transport without contact with the side walls of the silica glass ampoules. To eliminate the effect of the seed quality, and to reduce the number of nuclei and related crystal grains, the Low Supersaturation Nucleation technique was applied. The source temperature was 930 C, the undercooling was a few degrees. The crystals, having the diameter of 25 mm, grew at the rate of a few mm per day. The post-growth cool-down to the room temperature was conducted at different rates, and lasted from a few minutes to four days. The crystals were characterized using chemical etching, low temperature luminescence, and Synchrotron White Beam X-ray Topography techniques. The dislocation (etch pit) density was measured and its distribution was analyzed by comparison with Poisson curves and with the Normalized Radial Distribution Correlation Function. In the regions where the crystal is in contact with silica, the materials show a considerable strain field which extends for a few mm or more from the silica-crystal interface. In the reference crystal grown with contact with the ampoule walls, and when the crystals are cooled at the highest rates, the etch pit/dislocation density is in the high 10(exp 5) per square centimeter region. Typical EPD values for lower cool-down rates are in the lower 10(exp 4) per square centimeter region. In some areas the actual dislocation density was about 10(exp 3) per square centimeter or even less. No apparent effect of the cool-down rate on polygonization was observed. Low temperature PL spectra show, that the dominant peak is (D(sup 0), h) and (A(sup 0), e) for samples with low and high dislocation densities, respectively. For low EPD crystals a peak at 1.45 eV with 21 meV phonon replicas was observed and attributed to donor-acceptor pair to neutral copper-acceptor transition. In high EPD crystals this PL structure was not observed, apparently due to the masking effect of the strong contribution from the dislocation band.

Effect of copper on the recombination activity of extended defects in silicon

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

Feklisova, O. V., E-mail: feklisov@iptm.ru; Yakimov, E. B.

2015-06-15

The effect of copper atoms introduced by high-temperature diffusion on the recombination properties of dislocations and dislocation trails in p-type single-crystal silicon is studied by the electron-beam-induced current technique. It is shown that, in contrast to dislocations, dislocation trails exhibit an increase in recombination activity after the introduction of copper. Bright contrast appearance in the vicinity of dislocation trails is detected after the diffusion of copper and quenching of the samples. The contrast depends on the defect density in these trails.

Unzipping and movement of Lomer-type edge dislocations in Ge/GeSi/Si(0 0 1) heterostructures

NASA Astrophysics Data System (ADS)

Bolkhovityanov, Yu. B.; Deryabin, A. S.; Gutakovskii, A. K.; Sokolov, L. V.

2018-02-01

Edge dislocations in face-centered crystals are formed from two mixed dislocations gliding along intersecting {1 -1 1} planes, forming the so-called Lomer locks. This process, which is called zipping, is energetically beneficial. It is experimentally demonstrated in this paper that a reverse process may occur in Ge/GeSi strained buffer/Si(0 0 1) heterostructures under certain conditions, namely, decoupling of two 60° dislocations that formed the Lomer-type dislocation, i.e., unzipping. It is assumed that the driving force responsible for separation of Lomer dislocations into two 60° dislocations is the strain remaining in the GeSi buffer layer.

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

Hu, Shenyang; Setyawan, Wahyu; Joshi, Vineet V.

Xe gas bubble superlattice formation is observed in irradiated uranium–10 wt% molybdenum (U10Mo) fuels. However, the thermodynamic properties of the bubbles (the relationship among bubble size, equilibrium Xe concentration, and bubble pressure) and the mechanisms of bubble growth and superlattice formation are not well known. In this work, molecular dynamics is used to study these properties and mechanisms. The results provide important inputs for quantitative mesoscale models of gas bubble evolution and fuel performance. In the molecular dynamics simulations, the embedded-atom method (EAM) potential of U10Mo-Xe (Smirnova et al. 2013) is employed. Initial gas bubbles with low Xe concentration aremore » generated in a U10Mo single crystal. Then Xe atom atoms are continuously added into the bubbles, and the evolution of pressure and dislocation emission around the bubbles is analyzed. The relationship between pressure, equilibrium Xe concentration, and radius of the bubbles is established. It was found that the gas bubble growth is accompanied by partial dislocation emission, which results in a star-shaped dislocation structure and an anisotropic stress field. The emitted partial dislocations have a Burgers vector along the <111> direction and a slip plane of (11-2). Dislocation loop punch-out was not observed. A tensile stress was found along <110> directions around the bubble, favoring the nucleation and formation of a face-centered cubic bubble superlattice in body-centered cubic U10Mo fuels.« less

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

DOE PAGES

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

2017-01-01

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

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

NASA Astrophysics Data System (ADS)

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

2017-02-01

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

Clusters of Point Defects Near Dislocations as a Tool to Control CdZnTe Electrical Parameters by Ultrasound

NASA Astrophysics Data System (ADS)

Olikh, Ya. M.; Tymochko, M. D.; Olikh, O. Ya.; Shenderovsky, V. A.

2018-05-01

We studied the temperature dependence (77-300 K) of the electron concentration and mobility using the Hall method under ultrasound (the acoustic Hall method) to determine the mechanisms by which ultrasound influences the electrical activity of near-dislocation clusters in n-type low-ohmic Cd1-x Zn x Te single crystals (N Cl ≈ 1024 m-3; x = 0; 0.04) with different dislocation density (0.4-5.1) × 1010 m-2. Changes in electrophysical parameters were found to occur as a function of temperature and ultrasound intensity. To evaluate the relative contribution of different charge carrier scattering mechanisms (lattice scattering, ionized impurity scattering, neutral impurity scattering, and dislocation scattering) and their change under ultrasound, a differential evolution method was used. This method made it possible to analyze experimental mobility μ H(T) by its nonlinear approximation with characteristic temperature dependence for each mechanism. An increase in neutral impurity scattering and a decrease in ionized impurity and dislocation scattering components were observed under ultrasound. The character and the amount of these acoustically induced changes correlate with particular sample dislocation characteristics. It was concluded that the observed effects are related to the acoustically induced transformation of the point-defect structure, mainly in the near dislocation crystal regions.

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

DTIC Science & Technology

2010-01-01

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

The Strength of Binary Junctions in Hexagonal Close-Packed Crystals

DTIC Science & Technology

2014-03-01

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

Accumulated distribution of material gain at dislocation crystal growth

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

Rakin, V. I., E-mail: rakin@geo.komisc.ru

2016-05-15

A model for slowing down the tangential growth rate of an elementary step at dislocation crystal growth is proposed based on the exponential law of impurity particle distribution over adsorption energy. It is established that the statistical distribution of material gain on structurally equivalent faces obeys the Erlang law. The Erlang distribution is proposed to be used to calculate the occurrence rates of morphological combinatorial types of polyhedra, presenting real simple crystallographic forms.

Implementation and application of a gradient enhanced crystal plasticity model

NASA Astrophysics Data System (ADS)

Soyarslan, C.; Perdahcıoǧlu, E. S.; Aşık, E. E.; van den Boogaard, A. H.; Bargmann, S.

2017-10-01

A rate-independent crystal plasticity model is implemented in which description of the hardening of the material is given as a function of the total dislocation density. The evolution of statistically stored dislocations (SSDs) is described using a saturating type evolution law. The evolution of geometrically necessary dislocations (GNDs) on the other hand is described using the gradient of the plastic strain tensor in a non-local manner. The gradient of the incremental plastic strain tensor is computed explicitly during an implicit FE simulation after each converged step. Using the plastic strain tensor stored as state variables at each integration point and an efficient numerical algorithm to find the gradients, the GND density is obtained. This results in a weak coupling of the equilibrium solution and the gradient enhancement. The algorithm is applied to an academic test problem which considers growth of a cylindrical void in a single crystal matrix.

A tale of two mechanisms. Strain-softening versus strain-hardening in single crystals under small stressed volumes

DOE PAGES

Bei, Hongbin; Xia, Yuzhi; Barabash, Rozaliya; ...

2015-08-10

Pre-straining defect-free single crystals will introduce heterogeneous dislocation nucleation sources that reduce the measured strength from the theoretical value, while pre-straining bulk samples will lead to strain hardening. Their competition is investigated by nanoindentation pop-in tests on variously pre-strained Mo single crystals with several indenter radii (~micrometer). Pre-straining primarily shifts deformation mechanism from homogeneous dislocation nucleation to a stochastic behavior, while strain hardening plays a secondary role, as summarized in a master plot of pop-in strength versus normalized indenter radius.

Mechanisms of protein and virus crystal growth: An atomic force microscopy study of canavalin and STMV crystallization

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

Land, T.A.; De Yoreo, J.J.; Malkin, A.J.

1996-05-01

The evolution of surface morphology and step dynamics during growth of rhombohedral crystals of the protein canavalin and crystals of the cubic satellite tobacco mosaic virus (STMV) have been investigated for the first time by in situ atomic force microscopy. These two crystals were observed to grow by very different mechanisms. Growth of canavalin occurs on complex vicinal hillocks formed by multiple, independently acting screw dislocations. Small clusters were observed on the terraces. STMV on the other hand, was observed to grow by 2D nucleation of islands. No dislocations were found on the crystal. The results are used to determinemore » the growth mechanisms and estimate the fundamental materials parameters. The images also illustrate the important mechanism of defect incorporation and provide insight to the processes that limit the growth rate and uniformity of these crystals.« less

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

NASA Astrophysics Data System (ADS)

Saha, Biswadeep

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

Anisotropic Dislocation Line Energy and Crack Tip Dislocation Nucleation in (alpha)RDX

DTIC Science & Technology

2013-11-01

Mitchell, R.L.; Knap, J.; Chung, P.W., Role of molecule flexibility on the nucleation of dislocations in molecular crystals. App. Phys. Lett. 2013...uniform stres , under-estimate of stressses (Get from S) Br=0; for i=1:3 Br=Br+sum(S(i,1:3)); end Br=1/Br; % Shear Modulus Gr=4/15*(S(1,1

Non-basal dislocations should be accounted for in simulating ice mass flow

NASA Astrophysics Data System (ADS)

Chauve, T.; Montagnat, M.; Piazolo, S.; Journaux, B.; Wheeler, J.; Barou, F.; Mainprice, D.; Tommasi, A.

2017-09-01

Prediction of ice mass flow and associated dynamics is pivotal at a time of climate change. Ice flow is dominantly accommodated by the motion of crystal defects - the dislocations. In the specific case of ice, their observation is not always accessible by means of the classical tools such as X-ray diffraction or transmission electron microscopy (TEM). Part of the dislocation population, the geometrically necessary dislocations (GNDs) can nevertheless be constrained using crystal orientation measurements via electron backscattering diffraction (EBSD) associated with appropriate analyses based on the Nye (1950) approach. The present study uses the Weighted Burgers Vectors, a reduced formulation of the Nye theory that enables the characterization of GNDs. Applied to ice, this method documents, for the first time, the presence of dislocations with non-basal [ c ] or < c + a > Burgers vectors. These [ c ] or < c + a > dislocations represent up to 35% of the GNDs observed in laboratory-deformed ice samples. Our findings offer a more complex and comprehensive picture of the key plasticity processes responsible for polycrystalline ice creep and provide better constraints on the constitutive mechanical laws implemented in ice sheet flow models used to predict the response of Earth ice masses to climate change.

Synchrotron X-ray topographic study on nature of threading mixed dislocations in 4H–SiC crystals grown by PVT method

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

Guo, Jianqiu; Yang, Yu; Wu, Fangzhen

Synchrotron X-ray Topography is a powerful technique to study defects structures particularly dislocation configurations in single crystals. Complementing this technique with geometrical and contrast analysis can enhance the efficiency of quantitatively characterizing defects. In this study, the use of Synchrotron White Beam X-ray Topography (SWBXT) to determine the line directions of threading dislocations in 4H–SiC axial slices (sample cut parallel to the growth axis from the boule) is demonstrated. This technique is based on the fact that the projected line directions of dislocations on different reflections are different. Another technique also discussed is the determination of the absolute Burgers vectorsmore » of threading mixed dislocations (TMDs) using Synchrotron Monochromatic Beam X-ray Topography (SMBXT). This technique utilizes the fact that the contrast from TMDs varies on SMBXT images as their Burgers vectors change. By comparing observed contrast with the contrast from threading dislocations provided by Ray Tracing Simulations, the Burgers vectors can be determined. Thereafter the distribution of TMDs with different Burgers vectors across the wafer is mapped and investigated.« less

Disclinations, dislocations, and continuous defects: A reappraisal

NASA Astrophysics Data System (ADS)

Kleman, M.; Friedel, J.

2008-01-01

Disclinations were first observed in mesomorphic phases. They were later found relevant to a number of ill-ordered condensed-matter media involving continuous symmetries or frustrated order. Disclinations also appear in polycrystals at the edges of grain boundaries; but they are of limited interest in solid single crystals, where they can move only by diffusion climb and, owing to their large elastic stresses, mostly appear in close pairs of opposite signs. The relaxation mechanisms associated with a disclination in its creation, motion, and change of shape involve an interplay with continuous or quantized dislocations and/or continuous disclinations. These are attached to the disclinations or are akin to Nye’s dislocation densities, which are particularly well suited for consideration here. The notion of an extended Volterra process is introduced, which takes these relaxation processes into account and covers different situations where this interplay takes place. These concepts are illustrated by a variety of applications in amorphous solids, mesomorphic phases, and frustrated media in their curved habit space. These often involve disclination networks with specific node conditions. The powerful topological theory of line defects considers only defects stable against any change of boundary conditions or relaxation processes compatible with the structure considered. It can be seen as a simplified case of the approach considered here, particularly suited for media of high plasticity or/and complex structures. It cannot analyze the dynamical properties of defects nor the elastic constants involved in their static properties; topological stability cannot guarantee energetic stability, and sometimes cannot distinguish finer details of the structure of defects.

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

NASA Astrophysics Data System (ADS)

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

2016-04-01

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

Water weakening in experimentally deformed milky quartz single crystals

NASA Astrophysics Data System (ADS)

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

2015-12-01

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

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

NASA Astrophysics Data System (ADS)

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

2015-03-01

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

The possibility of identifying the spatial location of single dislocations by topo-tomography on laboratory setups

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

Zolotov, D. A., E-mail: zolotovden@crys.ras.ru; Buzmakov, A. V.; Elfimov, D. A.

2017-01-15

The spatial arrangement of single linear defects in a Si single crystal (input surface (111)) has been investigated by X-ray topo-tomography using laboratory X-ray sources. The experimental technique and the procedure of reconstructing a 3D image of dislocation half-loops near the Si crystal surface are described. The sizes of observed linear defects with a spatial resolution of about 10 μm are estimated.

Signature of dislocations and stacking faults of face-centred cubic nanocrystals in coherent X-ray diffraction patterns: a numerical study.

PubMed

Dupraz, Maxime; Beutier, Guillaume; Rodney, David; Mordehai, Dan; Verdier, Marc

2015-06-01

Crystal defects induce strong distortions in diffraction patterns. A single defect alone can yield strong and fine features that are observed in high-resolution diffraction experiments such as coherent X-ray diffraction. The case of face-centred cubic nanocrystals is studied numerically and the signatures of typical defects close to Bragg positions are identified. Crystals of a few tens of nanometres are modelled with realistic atomic potentials and 'relaxed' after introduction of well defined defects such as pure screw or edge dislocations, or Frank or prismatic loops. Diffraction patterns calculated in the kinematic approximation reveal various signatures of the defects depending on the Miller indices. They are strongly modified by the dissociation of the dislocations. Selection rules on the Miller indices are provided, to observe the maximum effect of given crystal defects in the initial and relaxed configurations. The effect of several physical and geometrical parameters such as stacking fault energy, crystal shape and defect position are discussed. The method is illustrated on a complex structure resulting from the simulated nanoindentation of a gold nanocrystal.

Signature of dislocations and stacking faults of face-centred cubic nanocrystals in coherent X-ray diffraction patterns: a numerical study1

PubMed Central

Dupraz, Maxime; Beutier, Guillaume; Rodney, David; Mordehai, Dan; Verdier, Marc

2015-01-01

Crystal defects induce strong distortions in diffraction patterns. A single defect alone can yield strong and fine features that are observed in high-resolution diffraction experiments such as coherent X-ray diffraction. The case of face-centred cubic nanocrystals is studied numerically and the signatures of typical defects close to Bragg positions are identified. Crystals of a few tens of nanometres are modelled with realistic atomic potentials and ‘relaxed’ after introduction of well defined defects such as pure screw or edge dislocations, or Frank or prismatic loops. Diffraction patterns calculated in the kinematic approximation reveal various signatures of the defects depending on the Miller indices. They are strongly modified by the dissociation of the dislocations. Selection rules on the Miller indices are provided, to observe the maximum effect of given crystal defects in the initial and relaxed configurations. The effect of several physical and geometrical parameters such as stacking fault energy, crystal shape and defect position are discussed. The method is illustrated on a complex structure resulting from the simulated nanoindentation of a gold nanocrystal. PMID:26089755

From Modeling of Plasticity in Single-Crystal Superalloys to High-Resolution X-rays Three-Crystal Diffractometer Peaks Simulation

NASA Astrophysics Data System (ADS)

Jacques, Alain

2016-12-01

The dislocation-based modeling of the high-temperature creep of two-phased single-crystal superalloys requires input data beyond strain vs time curves. This may be obtained by use of in situ experiments combining high-temperature creep tests with high-resolution synchrotron three-crystal diffractometry. Such tests give access to changes in phase volume fractions and to the average components of the stress tensor in each phase as well as the plastic strain of each phase. Further progress may be obtained by a new method making intensive use of the Fast Fourier Transform, and first modeling the behavior of a representative volume of material (stress fields, plastic strain, dislocation densities…), then simulating directly the corresponding diffraction peaks, taking into account the displacement field within the material, chemical variations, and beam coherence. Initial tests indicate that the simulated peak shapes are close to the experimental ones and are quite sensitive to the details of the microstructure and to dislocation densities at interfaces and within the soft γ phase.

Three-dimensional imaging of threading dislocations in GaN crystals using two-photon excitation photoluminescence

NASA Astrophysics Data System (ADS)

Tanikawa, Tomoyuki; Ohnishi, Kazuki; Kanoh, Masaya; Mukai, Takashi; Matsuoka, Takashi

2018-03-01

The three-dimensional imaging of threading dislocations in GaN films was demonstrated using two-photon excitation photoluminescence. The threading dislocations were shown as dark lines. The spatial resolutions near the surface were about 0.32 and 3.2 µm for the in-plane and depth directions, respectively. The threading dislocations with a density less than 108 cm-2 were resolved, although the aberration induced by the refractive index mismatch was observed. The decrease in threading dislocation density was clearly observed by increasing the GaN film thickness. This can be considered a novel method for characterizing threading dislocations in GaN films without any destructive preparations.

High-resolution transmission electron microscopy of hexagonal and rhombohedral molybdenum disulfide crystals.

PubMed

Isshiki, T; Nishio, K; Saijo, H; Shiojiri, M; Yabuuchi, Y; Takahashi, N

1993-07-01

Natural (molybdenite) and synthesized molybdenum disulfide crystals have been studied by high-resolution transmission electron microscopy. The image simulation demonstrates that the [0001] and [0110] HRTEM images of hexagonal and rhombohedral MoS2 crystals hardly disclose their stacking sequences, and that the [2110] images can distinguish the Mo and S columns along the incident electron beam and enable one to determine not only the crystal structure but also the fault structure. Observed [0001] images of cleaved molybdenite and synthesized MoS2 crystals, however, reveal the strain field around partial dislocations limiting an extended dislocation. A cross-sectional image of a single molecular (S-Mo-S) layer cleaved from molybdenite has been observed. Synthesized MoS2 flakes which were prepared by grinding have been found to be rhombohedral crystals containing many stacking faults caused by glides between S/S layers.

Structural properties and defects of GaN crystals grown at ultra-high pressures: A molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Gao, Tinghong; Li, Yidan; Xie, Quan; Tian, Zean; Chen, Qian; Liang, Yongchao; Ren, Lei; Hu, Xuechen

2018-01-01

The growth of GaN crystals at different pressures was studied by molecular dynamics simulation employing the Stillinger-Weber potential, and their structural properties and defects were characterized using the radial distribution function, the Voronoi polyhedron index method, and a suitable visualization technology. Crystal structures formed at 0, 1, 5, 10, and 20 GPa featured an overwhelming number of <4 0 0 0> Voronoi polyhedra, whereas amorphous structures comprising numerous disordered polyhedra were produced at 50 GPa. During quenching, coherent twin boundaries were easily formed between zinc-blende and wurtzite crystal structures in GaN. Notably, point defects usually appeared at low pressure, whereas dislocations were observed at high pressure, since the simultaneous growth of two crystal grains with different crystal orientations and their boundary expansion was hindered in the latter case, resulting in the formation of a dislocation between these grains.

Hydrolytic weakening in olivine single crystals

NASA Astrophysics Data System (ADS)

Tielke, Jacob A.; Zimmerman, Mark E.; Kohlstedt, David L.

2017-05-01

Deformation experiments on single crystals of San Carlos olivine under hydrous conditions were performed to investigate the microphysical processes responsible for hydrolytic weakening during dislocation creep. Hydrogen was supplied to the crystals using either talc or brucite sealed in nickel capsules with the crystal. Deformation experiments were carried out using a gas medium apparatus at temperatures of 1050° to 1250°C, a confining pressure of 300 MPa, differential stresses of 45 to 294 MPa, and resultant strain rates of 1.5 × 10-6 to 4.4 × 10-4 s-1. For talc-buffered (i.e., water and orthopyroxene-buffered) samples at high temperatures, the dependence of strain rate on stress follows a power law relationship with a stress exponent (n) of ˜2.5 and an activation energy of ˜490 kJ/mol. Brucite-buffered samples deformed faster than talc-buffered samples but contained similar hydrogen concentrations, demonstrating that strain rate is influenced by orthopyroxene activity under hydrous conditions. The values of n and dependence of strain rate on orthopyroxene activity are consistent with hydrolytic weakening occurring in the climb-controlled dislocation creep regime that is associated with deformation controlled by lattice diffusion under hydrous conditions and by pipe diffusion under anhydrous conditions. Analyses of postdeformation electron-backscatter diffraction data demonstrate that dislocations with [100] Burgers vectors are dominant in the climb-controlled regime and dislocations with [001] are dominant in the glide-controlled regime. Comparison of the experimentally determined constitutive equations demonstrates that under hydrous conditions crystals deform 1 to 2 orders of magnitude faster than under anhydrous conditions.

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

NASA Astrophysics Data System (ADS)

Canadinc, Demircan

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

Modelling irradiation-induced softening in BCC iron by crystal plasticity approach

NASA Astrophysics Data System (ADS)

Xiao, Xiazi; Terentyev, Dmitry; Yu, Long; Song, Dingkun; Bakaev, A.; Duan, Huiling

2015-11-01

Crystal plasticity model (CPM) for BCC iron to account for radiation-induced strain softening is proposed. CPM is based on the plastically-driven and thermally-activated removal of dislocation loops. Atomistic simulations are applied to parameterize dislocation-defect interactions. Combining experimental microstructures, defect-hardening/absorption rules from atomistic simulations, and CPM fitted to properties of non-irradiated iron, the model achieves a good agreement with experimental data regarding radiation-induced strain softening and flow stress increase under neutron irradiation.

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

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

Koplak, O. V.; Shteynman, E. A.; Tereschenko, A. N.

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.

Creep and Fatigue Interaction in the PWA 1484 Single Crystal Nickel-Base Alloy (Preprint)

DTIC Science & Technology

2011-07-01

work by Zhang et al . has shown that during the early part of fatigue cycling dislocations are formed primarily in the γ matrix with the number of...dislocations increasing with the number of applied cycles [11]. Ott and Mughrabi showed that during fatigue of single crystal nickel base superalloys... al . and their research determined that the fatigue behavior of PWA1484 could be well represented by a Walker type fatigue model that also included

From Atomistic Model to the Peierls-Nabarro Model with {γ} -surface for Dislocations

NASA Astrophysics Data System (ADS)

Luo, Tao; Ming, Pingbing; Xiang, Yang

2018-05-01

The Peierls-Nabarro (PN) model for dislocations is a hybrid model that incorporates the atomistic information of the dislocation core structure into the continuum theory. In this paper, we study the convergence from a full atomistic model to the PN model with {γ} -surface for the dislocation in a bilayer system. We prove that the displacement field and the total energy of the dislocation solution of the PN model are asymptotically close to those of the full atomistic model. Our work can be considered as a generalization of the analysis of the convergence from atomistic model to Cauchy-Born rule for crystals without defects.

Spatiotemporal periodicity of dislocation dynamics in a two-dimensional microfluidic crystal flowing in a tapered channel

DOE PAGES

Gai, Ya; Min Leong, Chia; Cai, Wei; ...

2016-10-10

When a many-body system is driven away from equilibrium, order can spontaneously emerge in places where disorder might be expected. Here we report an unexpected order in the flow of a concentrated emulsion in a tapered microfluidic channel. The velocity profiles of individual drops in the emulsion show periodic patterns in both space and time. Such periodic patterns appear surprising from both a fluid and a solid mechanics point of view. In particular, when the emulsion is considered as a soft crystal under extrusion, a disordered scenario might be expected based on the stochastic nature of dislocation dynamics in microscopicmore » crystals. However, an orchestrated sequence of dislocation nucleation and migration is observed to give rise to a highly ordered deformation mode. This discovery suggests that nanocrystals can be made to deform more controllably than previously thought. It can also lead to novel flow control and mixing strategies in droplet microfluidics.« less

Micromechanical properties of single crystals and polycrystals of pure α-titanium: anisotropy of microhardness, size effect, effect of the temperature (77-300 K)

NASA Astrophysics Data System (ADS)

Lubenets, S. V.; Rusakova, A. V.; Fomenko, L. S.; Moskalenko, V. A.

2018-01-01

The anisotropy of microhardness of pure α-Ti single crystals, indentation size effect in single-crystal, course grained (CG) pure and nanocrystalline (NC) VT1-0 titanium, as well as the temperature dependences of the microhardness of single-crystal and CG Ti in the temperature range 77-300 K were studied. The minimum value of hardness was obtained when indenting into the basal plane (0001). The indentation size effect (ISE) was clearly observed in the indentation of soft high-purity single-crystal iodide titanium while it was the least pronounced in a sample of nanocrystalline VT1-0 titanium. It has been demonstrated that the ISE can be described within the model of geometrically necessary dislocations (GND), which follows from the theory of strain gradient plasticity. The true hardness and others parameters of the GND model were determined for all materials. The temperature dependence of the microhardness is in agreement with the idea of the governing role of Peierls relief in the dislocation thermally-activated plastic deformation of pure titanium as has been earlier established and justified in macroscopic tensile investigations at low temperatures. The activation energy and activation volume of dislocation motion in the strained region under the indenter were estimated.

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

Li, B.; The Peac Institute of Multiscale Sciences, Chengdu, Sichuan 610207; Wang, L.

With large-scale molecular dynamics simulations, we investigate shock response of He nanobubbles in single crystal Cu. For sufficient bubble size or internal pressure, a prismatic dislocation loop may form around a bubble in unshocked Cu. The internal He pressure helps to stabilize the bubble against plastic deformation. However, the prismatic dislocation loops may partially heal but facilitate nucleation of new shear and prismatic dislocation loops. For strong shocks, the internal pressure also impedes internal jetting, while a bubble assists local melting; a high speed jet breaks a He bubble into pieces dispersed among Cu. Near-surface He bubbles may burst andmore » form high velocity ejecta containing atoms and small fragments, while the ejecta velocities do not follow the three-dimensional Maxwell-Boltzmann distributions expected for thermal equilibrium. The biggest fragment size deceases with increasing shock strength. With a decrease in ligament thickness or an increase in He bubble size, the critical shock strength required for bubble bursting decreases, while the velocity range, space extension and average velocity component along the shock direction, increase. Small bubbles are more efficient in mass ejecting. Compared to voids and perfect single crystal Cu, He bubbles have pronounced effects on shock response including bubble/void collapse, Hugoniot elastic limit (HEL), deformation mechanisms, and surface jetting. HEL is the highest for perfect single crystal Cu with the same orientations, followed by He bubbles without pre-existing prismatic dislocation loops, and then voids. Complete void collapse and shear dislocations occur for embedded voids, as opposed to partial collapse, and shear and possibly prismatic dislocations for He bubbles. He bubbles lower the threshhold shock strength for ejecta formation, and increase ejecta velocity and ejected mass.« less

Crystal growth and dislocation etch pits observation of chalcopyrite CdSiP2

NASA Astrophysics Data System (ADS)

He, Zhiyu; Zhao, Beijun; Zhu, Shifu; Chen, Baojun; Huang, Wei; Lin, Li; Feng, Bo

2018-01-01

CdSiP2 is the only crystal that can offer Non-critical Phase Matching (NCPM) for a 1064 nm pumped optical parametric oscillation (OPO) with idler output in the 6 μm range. In this paper, a large, crack-free CdSiP2 single crystal measuring 18 mm in diameter and 65 mm in length was successfully grown by the Vertical Bridgman method (MVB) with an explosion-proof quartz ampoule. The results of lattice parameters, element composition and IR transmittance of the as-grown crystal characterized by X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDS) and Fourier transformation infrared spectrometer (FTIR) showed the as grown crystal crystallized well and the absorption coefficients at 4878 cm-1 and 2500 cm-1 were 0.14 cm-1 and 0.06 cm-1. Moreover, a new etchant composed of Br2, HCl, HNO3, CH3OH and H2O (1:800:800:400:400 in volume ratio) was prepared and the dislocation etch pits on oriented faces of as-grown CdSiP2 crystal were observed for the first time. It is found the etch pits are in rectangular structure on the (1 0 1) face, but in trigonal pyramid structure on (3 1 2) face. According to the quantities of the etch pits, the average densities of dislocation were evaluated to be 2.28 × 105/cm2 and 1.4 × 105/cm2, respectively.

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

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

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

Theory of interacting dislocations on cylinders.

PubMed

Amir, Ariel; Paulose, Jayson; Nelson, David R

2013-04-01

We study the mechanics and statistical physics of dislocations interacting on cylinders, motivated by the elongation of rod-shaped bacterial cell walls and cylindrical assemblies of colloidal particles subject to external stresses. The interaction energy and forces between dislocations are solved analytically, and analyzed asymptotically. The results of continuum elastic theory agree well with numerical simulations on finite lattices even for relatively small systems. Isolated dislocations on a cylinder act like grain boundaries. With colloidal crystals in mind, we show that saddle points are created by a Peach-Koehler force on the dislocations in the circumferential direction, causing dislocation pairs to unbind. The thermal nucleation rate of dislocation unbinding is calculated, for an arbitrary mobility tensor and external stress, including the case of a twist-induced Peach-Koehler force along the cylinder axis. Surprisingly rich phenomena arise for dislocations on cylinders, despite their vanishing Gaussian curvature.

Defects at grain boundaries: A coarse-grained, three-dimensional description by the amplitude expansion of the phase-field crystal model

NASA Astrophysics Data System (ADS)

Salvalaglio, Marco; Backofen, Rainer; Elder, K. R.; Voigt, Axel

2018-05-01

We address a three-dimensional, coarse-grained description of dislocation networks at grain boundaries between rotated crystals. The so-called amplitude expansion of the phase-field crystal model is exploited with the aid of finite element method calculations. This approach allows for the description of microscopic features, such as dislocations, while simultaneously being able to describe length scales that are orders of magnitude larger than the lattice spacing. Moreover, it allows for the direct description of extended defects by means of a scalar order parameter. The versatility of this framework is shown by considering both fcc and bcc lattice symmetries and different rotation axes. First, the specific case of planar, twist grain boundaries is illustrated. The details of the method are reported and the consistency of the results with literature is discussed. Then, the dislocation networks forming at the interface between a spherical, rotated crystal embedded in an unrotated crystalline structure, are shown. Although explicitly accounting for dislocations which lead to an anisotropic shrinkage of the rotated grain, the extension of the spherical grain boundary is found to decrease linearly over time in agreement with the classical theory of grain growth and recent atomistic investigations. It is shown that the results obtained for a system with bcc symmetry agree very well with existing results, validating the methodology. Furthermore, fully original results are shown for fcc lattice symmetry, revealing the generality of the reported observations.

Strain amplitude-dependent anelasticity in Cu-Ni solid solution due to thermally activated and athermal dislocation-point obstacle interactions

NASA Astrophysics Data System (ADS)

Kustov, S.; Gremaud, G.; Benoit, W.; Golyandin, S.; Sapozhnikov, K.; Nishino, Y.; Asano, S.

1999-02-01

Experimental investigations of the internal friction and the Young's modulus defect in single crystals of Cu-(1.3-7.6) at. % Ni have been performed for 7-300 K over a wide range of oscillatory strain amplitudes. Extensive data have been obtained at a frequency of vibrations around 100 kHz and compared with the results obtained for the same crystals at a frequency of ˜1 kHz. The strain amplitude dependence of the anelastic strain amplitude and the average friction stress acting on a dislocation due to solute atoms are also analyzed. Several stages in the strain amplitude dependence of the internal friction and the Young's modulus defect are revealed for all of the alloy compositions, at different temperatures and in different frequency ranges. For the 100 kHz frequency, low temperatures and low strain amplitudes (˜10-7-10-5), the amplitude-dependent internal friction and the Young's modulus defect are essentially temperature independent, and are ascribed to a purely hysteretic internal friction component. At higher strain amplitudes, a transition stage and a steep strain amplitude dependence of the internal friction and the Young's modulus defect are observed, followed by saturation at the highest strain amplitudes employed. These stages are temperature and frequency dependent and are assumed to be due to thermally activated motion of dislocations. We suggest that the observed regularities in the entire strain amplitude, temperature and frequency ranges correspond to a motion of dislocations in a two-component system of obstacles: weak but long-range ones, due to the elastic interaction of dislocations with solute atoms distributed in the bulk of the crystal; and strong short-range ones, due to the interaction of dislocations with solute atoms distributed close to dislocation glide planes. Based on these assumptions, a qualitative explanation is given for the variety of experimental observations.

Basal-plane dislocations in bilayer graphene - Peculiarities in a quasi-2D material

NASA Astrophysics Data System (ADS)

Butz, Benjamin

2015-03-01

Dislocations represent one of the most fascinating and fundamental concepts in materials science. First and foremost, they are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly alter the local electronic or optical properties of semiconductors and ionic crystals. In layered crystals like graphite dislocation movement is restricted to the basal plane. Thus, those basal-plane dislocations cannot escape enabling their confinement in between only two atomic layers of the material. So-called bilayer graphene is the thinnest imaginable quasi-2D crystal to explore the nature and behavior of dislocations under such extreme boundary conditions. Robust graphene membranes derived from epitaxial graphene on SiC provide an ideal platform for their investigation. The presentation will give an insight in the direct observation of basal-plane partial dislocations by transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. The investigation reveals striking size effects. First, the absence of stacking fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern, which corresponds to an alternating AB <--> BA change of the stacking order. Most importantly, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane, which directly results from accommodation of strain. In fact, the buckling completely changes the strain state of the bilayer graphene and is of key importance for its electronic/spin transport properties. Due to the high degree of disorder in our quasi-2D material it is one of the very few examples for a perfect linear magnetoresistance, i.e. the linear dependency of the in-plane electrical resistance on a magnetic field applied perpendicular to the graphene sheet up to field strengths of more than 60 T. This research is financed by the German Research Foundation through the SFB 953 ``Synthetic Carbon Allotropes.''

Applications of Real Space Crystallography in Characterization of Dislocations in Geological Materials in a Scanning Electron Microscope (SEM)

NASA Astrophysics Data System (ADS)

Kaboli, S.; Burnley, P. C.

2017-12-01

Imaging and characterization of defects in crystalline materials is of significant importance in various disciplines including geoscience, materials science, and applied physics. Linear defects such as dislocations and planar defects such as twins and stacking faults, strongly influence many of the properties of crystalline materials and also reflect the conditions and degree of deformation. Dislocations have been conventionally imaged in thin foils in a transmission electron microscope (TEM). Since the development of field emission scanning electron microscopes (FE-SEM) with high gun brightness and small spot size, extensive efforts have been dedicated to the imaging and characterization of dislocations in semi-conductors using electron channeling contrast imaging (ECCI) in the SEM. The obvious advantages of using SEM over TEM include easier and non-destructive sample preparation and a large field of view enabling statistical examination of the density and distribution of dislocations and other defects. In this contribution, we extend this technique to geological materials and introduce the Real Space Crystallography methodology for imaging and complete characterization of dislocations based on bend contour contrast obtained by ECCI in FE-SEM. Bend contours map out the distortion in the crystal lattice across a deformed grain. The contrast of dislocations is maximum in the vicinity of bend contours where crystal planes diffract at small and positive deviations from the Bragg positions (as defined by Bragg's law of electron diffraction). Imaging is performed in a commercial FE-SEM equipped with a standard silicon photodiode backscattered (BSE) detector and an electron backscatter diffraction (EBSD) system for crystal orientation measurements. We demonstrate the practice of this technique in characterization of a number of geological materials in particular quartz, forsterite olivine and corundum, experimentally deformed at high pressure-temperature conditions. This new approach in microstructure characterization of deformed geologic materials in FE-SEM, without the use of etching or decoration techniques, has valuable applications to both experimentally deformed and naturally deformed specimens.

Geometrical Characteristics of Cd-Rich Inclusion Defects in CdZnTe Materials

NASA Astrophysics Data System (ADS)

Xu, Chao; Sheng, Fengfeng; Yang, Jianrong

2017-08-01

The geometrical characteristics of Cd-rich inclusion defects in CdZnTe crystals have been investigated by infrared transmission (IRT) microscopy and chemical etching methods, revealing that they are composed of a Cd-rich inclusion core zone with high dislocation density and defect extension belts. Based on the experimental results, the orientation and shape of these belts were determined, showing that their extension directions in three-dimensional (3-D) space are along <211> crystal orientation. To explain the observed IRT images of Cd-rich inclusion defects, a 3-D model with plate-shaped structure for dislocation extension belts is proposed. Greyscale IRT images of dislocation extension belts thus depend on their absorption layer thickness. Assuming that defects can be discerned by IRT microscopy only when their absorption layer thickness is greater than twice that of the plate-shaped dislocation extension belts, this 3-D defect model can rationalize the IRT images of Cd-rich inclusion defects.

Fracture of single crystals of the nickel-base superalloy PWA 1480E in helium at 22 C

NASA Technical Reports Server (NTRS)

Chen, P. S.; Wilcox, R. C.

1991-01-01

The fracture behavior and deformation of He-charged (at 22 C) single crystals of PWA 1480E Ni-base superalloy were investigated using SEM and TEM techniques to observe the behavior of tensile fractures in notched single crystals with seven different crystal growth orientations: 100-line, 110-line, 111-line, 013-line, 112-line, 123-line, and 223-line. To identify the cleavage plane orientation, a stereoscopic technique, combined with the use of planar gamma-prime morphologies, was applied. It was found that gamma-prime particles were orderly and closely aligned with edges along the 100-line, 010-line, and 001-line-oriented directions of the gamma matrix. Different crystal growth orientations were found not to affect the morphology of gamma-prime particles. The accumulation of dislocations around gamma/gamma-prime interfaces formed strong barriers to subsequent dislocation movement and was the primary strengthening mechanism at room temperature.

Drag of a Cottrell atmosphere by an edge dislocation in a smectic-A liquid crystal.

PubMed

Oswald, P; Lejček, L

2017-10-01

In a recent letter (P. Oswald et al., EPL 103, 46004 (2013)), we have shown that a smectic-A phase hardens in compression normal to the layers when the liquid crystal is doped with gold nanoparticles. This is due to the formation of Cottrell clouds nearby the core of the edge dislocations and the appearance of an additional drag force that reduces their mobility. We theoretically calculate the shape of the Cottrell cloud and the associated drag force as a function of the climb velocity of the dislocations. The main result is that the drag force depends on velocity and vanishes when the temperature tends to the smectic-A-to-nematic transition temperature. The role of the diffusion anisotropy is also evaluated.

Disentangling nonradiative recombination processes in Ge micro-crystals on Si substrates

NASA Astrophysics Data System (ADS)

Pezzoli, Fabio; Giorgioni, Anna; Gallacher, Kevin; Isa, Fabio; Biagioni, Paolo; Millar, Ross W.; Gatti, Eleonora; Grilli, Emanuele; Bonera, Emiliano; Isella, Giovanni; Paul, Douglas J.; Miglio, Leo

2016-06-01

We address nonradiative recombination pathways by leveraging surface passivation and dislocation management in μm-scale arrays of Ge crystals grown on deeply patterned Si substrates. The time decay photoluminescence (PL) at cryogenic temperatures discloses carrier lifetimes approaching 45 ns in band-gap engineered Ge micro-crystals. This investigation provides compelling information about the competitive interplay between the radiative band-edge transitions and the trapping of carriers by dislocations and free surfaces. Furthermore, an in-depth analysis of the temperature dependence of the PL, combined with capacitance data and finite difference time domain modeling, demonstrates the effectiveness of GeO2 in passivating the surface of Ge and thus in enhancing the room temperature PL emission.

Size-Dependent Grain-Boundary Structure with Improved Conductive and Mechanical Stabilities in Sub-10-nm Gold Crystals

NASA Astrophysics Data System (ADS)

Wang, Chunyang; Du, Kui; Song, Kepeng; Ye, Xinglong; Qi, Lu; He, Suyun; Tang, Daiming; Lu, Ning; Jin, Haijun; Li, Feng; Ye, Hengqiang

2018-05-01

Low-angle grain boundaries generally exist in the form of dislocation arrays, while high-angle grain boundaries (misorientation angle >15 ° ) exist in the form of structural units in bulk metals. Here, through in situ atomic resolution aberration corrected electron microscopy observations, we report size-dependent grain-boundary structures improving both stabilities of electrical conductivity and mechanical properties in sub-10-nm-sized gold crystals. With the diameter of a nanocrystal decreasing below 10 nm, the high-angle grain boundary in the crystal exists as an array of dislocations. This size effect may be of importance to a new generation of interconnects applications.

Extreme Response in Tension and Compression of Tantalum

NASA Astrophysics Data System (ADS)

Remington, Tane Perry

This research on a model bcc metal, tantalum, has three components: the study of tensile failure; defects generated under a nanoindenter; and dislocation velocities in an extreme regime generated by pulsed lasers. The processes of dynamic failure by spalling were established in nano, poly, and mono crystalline tantalum in recovery experiments following laser compression and release. The process of spall was characterized by different techniques: optical microscopy, scanning electron microscopy, microcomputerized tomography and electron backscatter diffraction. Additionally, the pull back signal was measured by VISAR and the pressure decay was compared with HYADES simulations. There are clear differences in the microscopic fracture mechanisms, dictated by the grain sizes. In the nano and poly crystals, spalling occurred by ductile fracture favoring grain boundaries. In the monocrystals, grain boundaries are absent, and the process was of ductile failure by void initiation, growth and coalescence. The spall strength of single crystalline tantalum was higher than the poly and nano crystals. It was experimentally confirmed that spall strength in tantalum increases with strain rate. In order to generate dislocations close to the surface, single crystalline tantalum with orientations (100), (110) and (111) was nanoindented with a Berkovich tip. Atomic force microscopy showed pile-ups of dislocations around the perimeter of the nanoindentations. Sections of nanoindentations were focused ion beam cut into transmission electron microscope foils. The mechanisms of deformation under a nanoindentation in tantalum were identified and quantified. Molecular dynamics simulations were conducted and the simulated plastic deformation proceeds by the formation of nanotwins, which rapidly evolve into shear dislocation loops. Dislocation densities under the indenter were estimated experimentally (~1.2 x 1015 m-2), by MD (~7 x1015 m-2) and through an analytical calculation (2.6--19 x10 15 m-2). Considering the assumptions and simplifications, this agreement is considered satisfactory. These indented crystals were subjected to shock compression and the results are being analyzed with the objective of establishing the velocities of dislocations. A novel technique to establish dislocation velocities is being tested. It consists of subjecting tantalum containing a matrix of nanoindentations to shock compression for post shock characterization enabling the determination of mean dislocation displacements.

The Relevant Role of Dislocations in the Martensitic Transformations in Cu-Al-Ni Single Crystals

NASA Astrophysics Data System (ADS)

Gastien, R.; Sade, M.; Lovey, F. C.

2018-03-01

The interaction between dislocations and martensitic transformations in Cu-Al-Ni alloys is shortly reviewed. Results from many researchers are critically analyzed towards a clear interpretation of the relevant role played by dislocations on the properties of shape memory alloys in Cu-based alloys. Both thermally and stress-induced transformations are considered and focus is paid on two types of transitions, the β→β' and the formation of a mixture of martensites: β→β' + γ'. After cycling in the range where both martensites are formed, the twinned γ' phase is inhibited and cycling evolves into the formation of only β'. A model which considers the difference in energy of each γ' twin variant due to the introduced dislocations quantitatively explains the inhibition of γ' in both thermally and stress-induced cycling. The type of dislocations which are mainly introduced, mixed with Burgers vector belonging to the basal plane of the β' martensite, enables also to explain the unmodified mechanical behavior during β→β' cycling. The reported behavior shows interesting advantages of Cu-Al-Ni single crystals if mechanical properties are comparatively considered with those in other Cu-based alloys.

Structural Characterization of Lateral-grown 6H-SiC am-plane Seed Crystals by Hot Wall CVD Epitaxy

NASA Technical Reports Server (NTRS)

Goue, Ouloide Yannick; Raghothamachar, Balaji; Dudley, Michael; Trunek, Andrew J.; Neudeck, Philip G.; Woodworth, Andrew A.; Spry, David J.

2014-01-01

The performance of commercially available silicon carbide (SiC) power devices is limited due to inherently high density of screw dislocations (SD), which are necessary for maintaining polytype during boule growth and commercially viable growth rates. The NASA Glenn Research Center (GRC) has recently proposed a new bulk growth process based on axial fiber growth (parallel to the c-axis) followed by lateral expansion (perpendicular to the c-axis) for producing multi-faceted m-plane SiC boules that can potentially produce wafers with as few as one SD per wafer. In order to implement this novel growth technique, the lateral homoepitaxial growth expansion of a SiC fiber without introducing a significant number of additional defects is critical. Lateral expansion is being investigated by hot wall chemical vapor deposition (HWCVD) growth of 6H-SiC am-plane seed crystals (0.8mm x 0.5mm x 15mm) designed to replicate axially grown SiC single crystal fibers. The post-growth crystals exhibit hexagonal morphology with approximately 1500 m (1.5 mm) of total lateral expansion. Preliminary analysis by synchrotron white beam x-ray topography (SWBXT) confirms that the growth was homoepitaxial, matching the polytype of the respective underlying region of the seed crystal. Axial and transverse sections from the as grown crystal samples were characterized in detail by a combination of SWBXT, transmission electron microscopy (TEM) and Raman spectroscopy to map defect types and distribution. X-ray diffraction analysis indicates the seed crystal contained stacking disorders and this appears to have been reproduced in the lateral growth sections. Analysis of the relative intensity for folded transverse acoustic (FTA) and optical (FTO) modes on the Raman spectra indicate the existence of stacking faults. Further, the density of stacking faults is higher in the seed than in the grown crystal. Bundles of dislocations are observed propagating from the seed in m-axis lateral directions. Contrast extinction analysis of these dislocation lines reveals they are edge type basal plane dislocations that track the growth direction. Polytype phase transition and stacking faults were observed by high-resolution TEM (HRTEM), in agreement with SWBXT and Raman scattering.

Constitutive Model for Anisotropic Creep Behaviors of Single-Crystal Ni-Base Superalloys in the Low-Temperature, High-Stress Regime (Postprint)

DTIC Science & Technology

2012-01-19

specific dislocation reactions. Rae et al .[4,5,7] proposed micromechanisms for primary creep caused by SF shearing of c0 precipitates by ah112i...near the [0 0 1] was done by Matan et al .[3] They proposed a phenomenological creep model, which was adopted from Gilman’s dislocation density model...the original loading orientation). MacLachlan et al .[18 21] proposed a series of creep models for anisotropic creep of single-crystal superalloys. Their

Modeling of surface effects in crystalline materials within the framework of gradient crystal plasticity

NASA Astrophysics Data System (ADS)

Peng, Xiang-Long; Husser, Edgar; Huang, Gan-Yun; Bargmann, Swantje

2018-03-01

A finite-deformation gradient crystal plasticity theory is developed, which takes into account the interaction between dislocations and surfaces. The model captures both energetic and dissipative effects for surfaces penetrable by dislocations. By taking advantage of the principle of virtual power, the surface microscopic boundary equations are obtained naturally. Surface equations govern surface yielding and hardening. A thin film under shear deformation serves as a benchmark problem for validation of the proposed model. It is found that both energetic and dissipative surface effects significantly affect the plastic behavior.

Anisotropic Defect-Mediated Melting of Two-Dimensional Colloidal Crystals

NASA Astrophysics Data System (ADS)

Eisenmann, C.; Gasser, U.; Keim, P.; Maret, G.

2004-09-01

The melting transition of anisotropic two-dimensional (2D) crystals is studied in a model system of superparamagnetic colloids. The anisotropy of the induced dipole-dipole interaction is varied by tilting the external magnetic field off the normal to the particle plane. By analyzing the time-dependent Lindemann parameter as well as translational and orientational order we observe a 2D smecticlike phase. The Kosterlitz-Thouless-Halperin-Nelson-Young scenario of isotropic melting is modified: dislocation pairs and dislocations appear with different probabilities depending on their orientation with respect to the in-plane field.

Deformation twinning of a silver nanocrystal under high pressure. Supplementary materials

DOE PAGES

Huang, X. J.; Yang, W. G.; Harder, R.; ...

2015-10-20

Within a high-pressure environment, crystal deformation is controlled by complex processes such as dislocation motion, twinning, and phase transitions, which change materials’ microscopic morphology and alter their properties. Likewise, understanding a crystal’s response to external stress provides a unique opportunity for rational tailoring of its functionalities. It is very challenging to track the strain evolution and physical deformation from a single nanoscale crystal under high-pressure stress. Here, we report an in situ three-dimensional mapping of morphology and strain evolutions in a single-crystal silver nanocube within a high-pressure environment using the Bragg Coherent Diffractive Imaging (CDI) method. We also observed amore » continuous lattice distortion, followed by a deformation twining process at a constant pressure. The ability to visualize stress-introduced deformation of nanocrystals with high spatial resolution and prominent strain sensitivity provides an important route for interpreting and engineering novel properties of nanomaterials.« less

Homogeneity and variation of donor doping in Verneuil-grown SrTiO3:Nb single crystals

PubMed Central

Rodenbücher, C.; Luysberg, M.; Schwedt, A.; Havel, V.; Gunkel, F.; Mayer, J.; Waser, R.

2016-01-01

The homogeneity of Verneuil-grown SrTiO3:Nb crystals was investigated. Due to the fast crystal growth process, inhomogeneities in the donor dopant distribution and variation in the dislocation density are expected to occur. In fact, for some crystals optical studies show variations in the density of Ti3+ states on the microscale and a cluster-like surface conductivity was reported in tip-induced resistive switching studies. However, our investigations by TEM, EDX mapping, and 3D atom probe reveal that the Nb donors are distributed in a statistically random manner, indicating that there is clearly no inhomogeneity on the macro-, micro-, and nanoscale in high quality Verneuil-grown crystals. In consequence, the electronic transport in the bulk of donor-doped crystals is homogeneous and it is not significantly channelled by extended defects such as dislocations which justifies using this material, for example, as electronically conducting substrate for epitaxial oxide film growth. PMID:27577508

Crystal front shape control by use of an additional heater in a Czochralski sapphire single crystal growth system

NASA Astrophysics Data System (ADS)

Hur, Min-Jae; Han, Xue-Feng; Choi, Ho-Gil; Yi, Kyung-Woo

2017-09-01

The quality of sapphire single crystals used as substrates for LED production is largely influenced by two defects: dislocation density and bubbles trapped in the crystal. In particular, the dislocation density has a higher value in sapphire grown by the Czochralski (CZ) method than by other methods. In the present study, we predict a decreased value for the convexity and thermal gradient at the crystal front (CF) through the use of an additional heater in an induction-heated CZ system. In addition, we develop a solute concentration model by which the location of bubble formation in CZ growth is calculated, and the results are compared with experimental results. We further calculate the location of bubble entrapment corresponding with the use of an additional heater. We find that sapphire crystal growth with an additional heater yields a decreased thermal gradient at the CF, together with decreased CF convexity, improved energy efficiency, and improvements in terms of bubble formation location.

Displacive transformation of virus protein crystal

NASA Astrophysics Data System (ADS)

Celotto, S.; Pond, R. C.

2003-10-01

A crystalline protein undergoes a displacive transformation in the T-even bacteriophage. In the present work, the transformation mechanism is modelled in terms of interfacial dislocations whose motion gives rise to the observed deformation. The topological properties (Burgers vector, {b}, and `overlap' step height, h) of the dislocations involved are defined rigorously and a recent theory is used that quantifies the diffusional flux arising due to their movement. The circumstance under which passage of transformation dislocations is diffusionless is identified. Thus, dislocation modelling is used successfully to describe a diffusionless displacive transformation in a process where the phenomenological theory of martensite crystallography cannot be applied.

Dislocation Multiplication by Single Cross Slip for FCC at Submicron Scales

NASA Astrophysics Data System (ADS)

Cui, Yi-Nan; Liu, Zhan-Li; Zhuang, Zhuo

2013-04-01

The operation mechanism of single cross slip multiplication (SCSM) is investigated by studying the response of one dislocation loop expanding in face-centered-cubic (FCC) single crystal using three-dimensional discrete dislocation dynamic (3D-DDD) simulation. The results show that SCSM can trigger highly correlated dislocation generation in a short time, which may shed some light on understanding the large strain burst observed experimentally. Furthermore, we find that there is a critical stress and material size for the operation of SCSM, which agrees with that required to trigger large strain burst in the compression tests of FCC micropillars.

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

Li, Mingda; Cui, Wenping; Dresselhaus, Mildred S.

Crystal dislocations govern the plastic mechanical properties of materials but also affect the electrical and optical properties. However, a fundamental and quantitative quantum field theory of a dislocation has remained undiscovered for decades. Here in this article we present an exactly-solvable one-dimensional quantum field theory of a dislocation, for both edge and screw dislocations in an isotropic medium, by introducing a new quasiparticle which we have called the ‘dislon’. The electron-dislocation relaxation time can then be studied directly from the electron self-energy calculation, which is reducible to classical results. In addition, we predict that the electron energy will experience anmore » oscillation pattern near a dislocation. Compared with the electron density’s Friedel oscillation, such an oscillation is intrinsically different since it exists even with only single electron is present. With our approach, the effect of dislocations on materials’ non-mechanical properties can be studied at a full quantum field theoretical level.« less

Effect of Te inclusions in CdZnTe crystals at different temperatures

NASA Astrophysics Data System (ADS)

Hossain, A.; Bolotnikov, A. E.; Camarda, G. S.; Gul, R.; Kim, K.-H.; Cui, Y.; Yang, G.; Xu, L.; James, R. B.

2011-02-01

CdZnTe crystals often exhibit nonuniformities due to the presence of Te inclusions and dislocations. High concentrations of such defects in these crystals generally entail severe charge-trapping, a major problem in ensuring the device's satisfactory performance. In this study, we employed a high-intensity, high-spatial-resolution synchrotron x-ray beam as the ideal tool to generate charges by focusing it over the large Te inclusions, and then observing the carrier's response at room- and at low-temperatures. A high spatial 5-μm resolution raster scan revealed the fine details of the presence of extended defects, like Te inclusions and dislocations in the CdZnTe crystals. A noticeable change was observed in the efficiency of electron charge collection at low temperature (1 °C), but it was hardly altered at room-temperature.

Deformation induced microtwins and stacking faults in aluminum single crystal.

PubMed

Han, W Z; Cheng, G M; Li, S X; Wu, S D; Zhang, Z F

2008-09-12

Microtwins and stacking faults in plastically deformed aluminum single crystal were successfully observed by high-resolution transmission electron microscope. The occurrence of these microtwins and stacking faults is directly related to the specially designed crystallographic orientation, because they were not observed in pure aluminum single crystal or polycrystal before. Based on the new finding above, we propose a universal dislocation-based model to judge the preference or not for the nucleation of deformation twins and stacking faults in various face-centered-cubic metals in terms of the critical stress for dislocation glide or twinning by considering the intrinsic factors, such as stacking fault energy, crystallographic orientation, and grain size. The new finding of deformation induced microtwins and stacking faults in aluminum single crystal and the proposed model should be of interest to a broad community.

Atomistic simulation of shocks in single crystal and polycrystalline Ta

NASA Astrophysics Data System (ADS)

Bringa, E. M.; Higginbotham, A.; Park, N.; Tang, Y.; Suggit, M.; Mogni, G.; Ruestes, C. J.; Hawreliak, J.; Erhart, P.; Meyers, M. A.; Wark, J. S.

2011-06-01

Non-equilibrium molecular dynamics (MD) simulations of shocks in Ta single crystals and polycrystals were carried out using up to 360 million atoms. Several EAM and FS type potentials were tested up to 150 GPa, with varying success reproducing the Hugoniot and the behavior of elastic constants under pressure. Phonon modes were studied to exclude possible plasticity nucleation by soft-phonon modes, as observed in MD simulations of Cu crystals. The effect of loading rise time in the resulting microstructure was studied for ramps up to 0.2 ns long. Dislocation activity was not observed in single crystals, unless there were defects acting as dislocation sources above a certain pressure. E.M.B. was funded by CONICET, Agencia Nacional de Ciencia y Tecnología (PICT2008-1325), and a Royal Society International Joint Project award.

Fracture toughness of materials

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

Burns, S.J.

Crack tip dislocation emission in bulk specimens have been measured in single crystal specimens and the measurements are well below the accepted theoretical values for dislocation emission. The image forces on a dislocation due to the presence of a semi-infinite crack are used to calculate the potential energy of the dislocation around the crack. Expressions for the radial and tangential forces and for slip and climb forces have been found. Crack tip deformation in Mode I and Mode II fractures on both {l brace}100{r brace} and {l brace}110{r brace} planes have been observed in crystals of LiF. The deformation ismore » shown to nearly completely shield {l brace}110{r brace} plane cracks and prevent their propagation while deformation is less effective in shielding {l brace}100{r brace} plane cracks. The fracture toughness of MgO-partially-stabilized ZrO{sub 2} exhibiting transformation toughening been measured. The equations of linear elastic fracture mechanics have been self-consistantly formulated to include the residual displacement from the transformation wake. MgO single crystals were fatigued in plastic strain control at elevated temperatures. At high temperatures, dense bundles of dislocations were observed in transmission electron microscopy aligned perpendicular to the Burgers' vector directions. The thermodynamics of a superconducting second order phase transformation has been related to jumps in physical properties. A simple energy balance, without assuming an equation of state, is used to relate the rate of change of state variables to measurable physical properties. There are no preconceived assumptions about the superconducting mechanism.« less

The Role of Geometrically Necessary Dislocations in Cantilever Beam Bending Experiments of Single Crystals

PubMed Central

Husser, Edgar; Bargmann, Swantje

2017-01-01

The mechanical behavior of single crystalline, micro-sized copper is investigated in the context of cantilever beam bending experiments. Particular focus is on the role of geometrically necessary dislocations (GNDs) during bending-dominated load conditions and their impact on the characteristic bending size effect. Three different sample sizes are considered in this work with main variation in thickness. A gradient extended crystal plasticity model is presented and applied in a three-dimensional finite-element (FE) framework considering slip system-based edge and screw components of the dislocation density vector. The underlying mathematical model contains non-standard evolution equations for GNDs, crystal-specific interaction relations, and higher-order boundary conditions. Moreover, two element formulations are examined and compared with respect to size-independent as well as size-dependent bending behavior. The first formulation is based on a linear interpolation of the displacement and the GND density field together with a full integration scheme whereas the second is based on a mixed interpolation scheme. While the GND density fields are treated equivalently, the displacement field is interpolated quadratically in combination with a reduced integration scheme. Computational results indicate that GND storage in small cantilever beams strongly influences the evolution of statistically stored dislocations (SSDs) and, hence, the distribution of the total dislocation density. As a particular example, the mechanical bending behavior in the case of a physically motivated limitation of GND storage is studied. The resulting impact on the mechanical bending response as well as on the predicted size effect is analyzed. Obtained results are discussed and related to experimental findings from the literature. PMID:28772657

The deformation mechanisms and size effects of single-crystal magnesium

NASA Astrophysics Data System (ADS)

Byer, Cynthia M.

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

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

NASA Astrophysics Data System (ADS)

Chen, Yali

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

Ultrasonic Study of Dislocation Dynamics in Lithium -

NASA Astrophysics Data System (ADS)

Han, Myeong-Deok

1987-09-01

Experimental studies of dislocation dynamics in LiF single crystals, using ultrasonic techniques combined with dynamic loading, were performed to investigate the time evolution of the plastic deformation process under a short stress pulse at room temperature, and the temperature dependence of the dislocation damping mechanism in the temperature range 25 - 300(DEGREES)K. From the former, the time dependence of the ultrasonic attenuation was understood as resulting from dislocation multiplication followed by the evolution of mobile dislocations to immobile ones under large stress. From the latter, the temperature dependence of the ultrasonic attenuation was interpreted as due to the motion of the dislocation loops overcoming the periodic Peierls potential barrier in a manner analogous to the motion of a thermalized sine-Gordon chain under a small stress. The Peierls stress obtained from the experimental results by application of Seeger's relaxation model with exponential dislocation length distribution was 4.26MPa, which is consistent with the lowest stress for the linear relation between the dislocation velocity and stress observed by Flinn and Tinder.

On the tungsten single crystal coatings achieved by chemical vapor transportation deposition

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

Shi, J.Q.; Shen, Y.B.; Yao, S.Y.

2016-12-15

The tungsten single crystal has many excellent properties, namely a high melting point, high anti-creeping strength. Chemical vapor transportation deposition (CVTD) is a possible approach to achieve large-sized W single crystals for high-temperature application such as the cathode of a thermionic energy converter. In this work, CVTD W coatings were deposited on the monocrystalline molybdenum substrate (a tube with < 111 > axial crystalline orientation) using WCl{sub 6} as a transport medium. The microstructures of the coatings were investigated by a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The as-deposited coatings are hexagonal prisms—rough surfaces perpendicular to with alternating hill-like bulges and pits at the side edges of the prisms, and flat surfaces perpendicular to < 112 > with arc-shaped terraces at the side faces. This can be explained by two-dimensional nucleation -mediated lateral growth model. Some parts of the coatings contain hillocks of an exotic morphology (noted as “abnormal growth”). The authors hypothesize that the abnormal growth is likely caused by the defects of the Mo substrate, which facilitate W nucleation sites, cause orientation difference, and may even form boundaries in the coatings. A dislocation density of 10{sup 6} to 10{sup 7} (counts/cm{sup 2}) was revealed by an etch-pit method and synchrotron X-ray diffraction. As the depositing temperature rises, the dislocation density decreases, and no sub-boundaries are found on samples deposited over 1300 °C, as a result of atom diffusion and dislocation climbing. - Highlights: •The varied growth rate causes the different morphologies of different planes. •The W coating is a single crystal when only single hillocks appear. •The (110) plane tends to have the lowest dislocation density. •The dislocation density tends to decrease as the temperature increases.« less

Density of bunched threading dislocations in epitaxial GaN layers as determined using X-ray diffraction

NASA Astrophysics Data System (ADS)

Barchuk, M.; Holý, V.; Rafaja, D.

2018-04-01

X-ray diffraction is one of the most popular experimental methods employed for determination of dislocation densities, as it can recognize both the strain fields and the local lattice rotations produced by dislocations. The main challenge of the quantitative analysis of the dislocation density is the formulation of a suitable microstructure model, which describes the dislocation arrangement and the effect of the interactions between the strain fields from neighboring dislocations reliably in order to be able to determine the dislocation densities precisely. The aim of this study is to prove the capability of X-ray diffraction and two computational methods, which are frequently used for quantification of the threading dislocation densities from X-ray diffraction measurements, in the special case of partially bunched threading dislocations. The first method is based on the analysis of the dislocation-controlled crystal mosaicity, and the other one on the analysis of diffuse X-ray scattering from threading dislocations. The complementarity of both methods is discussed. Furthermore, it is shown how the complementarity of these methods can be used to improve the results of the quantitative analysis of bunched and thus inhomogeneously distributed threading dislocations and to get a better insight into the dislocation arrangement.

Modeling Plasticity of Ni3Al-Based L12 Intermetallic Single Crystals-I. Anomalous Temperature Dependence of the Flow Behavior (Preprint)

DTIC Science & Technology

2006-07-01

dislocation-loop expansion . The new model was used to simulate the thermally reversible flow behaviour for C-S type two-step deformation, and the results are...implemented into the finite element software ABAQUS through a User MATerial subroutine (UMAT). A tangent modulus method [48] was used for the time...locking under a dislocation loop- expansion configuration. This approach was motivated by modern understanding of dislocation mechanisms for Ni3Al

Stationary Shock Waves with Oscillating Front in Dislocation Systems of Semiconductors

NASA Astrophysics Data System (ADS)

Gestrin, S. G.; Shchukina, E. V.

2018-05-01

The paper presents a study of weakly nonlinear wave processes in the cylindrical region of a hole gas surrounding a negatively charged dislocation in an n-type semiconductor crystal. It is shown that shock waves propagating along the dislocation are the solutions of the Korteweg-de Vries-Burgers equation when the dispersion and dissipation of medium are taken into account. Estimates are obtained for the basic physical parameters characterizing the shock wave and the region inside the Reed cylinder.

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

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

Qian, Dan; Xue, Jiawei; Zhang, Anfeng

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

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

DOE PAGES

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

2017-06-06

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

Chain Ends and the Ultimate Tensile Strength of Polyethylene Fibers

NASA Astrophysics Data System (ADS)

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

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

Influence of dislocation strain fields on the diffusion of interstitial iron impurities in silicon

NASA Astrophysics Data System (ADS)

Ziebarth, Benedikt; Mrovec, Matous; Elsässer, Christian; Gumbsch, Peter

2015-09-01

The efficiency of silicon (Si)-based solar cells is strongly affected by crystal defects and impurities. Metallic impurities, in particular interstitial iron (Fe) atoms, cause large electric losses because they act as recombination centers for photogenerated charge carriers. Here, we present a systematic first-principles density functional theory (DFT) study focusing on the influence of hydrostatic, uniaxial, and shear strains on the thermodynamic stability and the diffusivity of Fe impurities in crystalline Si. Our calculations show that the formation energy of neutral Fe interstitials in tetrahedral interstitial sites is almost unaffected by uniform deformations of the Si crystal up to strains of 5%. In contrast, the migration barrier varies significantly with strain, especially for hydrostatic deformation. In order to determine effective diffusion coefficients for different strain states, a kinetic Monte Carlo (kMC) model was set up based on the activation energy barriers and frequency factors obtained from the DFT simulations. By using the strain dependence of the migration barrier, we examined the migration of Fe interstitials in the vicinity of perfect 1 /2 <110 > screw and 60∘ mixed dislocations, and 1 /6 <112 > 90∘ and 30∘ partial dislocations. While the strain field of the perfect screw dislocation always enhances the local Fe diffusion, the existence of tensile and compressive regions around the 60∘ mixed dislocation results in a strong anisotropic diffusion profile with significantly faster and slower diffusivities on its tensile and compressive sides. The influences of the partial dislocations are qualitatively similar to that of the 60∘ mixed dislocation.

The Cyclic Stress-Strain Behavior of a Single Crystal Nickel-Base Superalloy. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Gabb, Timothy P.

1988-01-01

The cyclic stress-strain response and similar deformation structures of the single crystal nickel based superalloy was described under a specific set of conditions. The isothermal low cycle fatigue response and deformation structures were described at a typical intermediate temperature and at high temperature. Specimens oriented near the (001) and (111) crystallographic orientations were tested at 1050 C, where more moderate orientation effects were expected. This enabled the description of the deformation structures at each of the 2 temperatures and their relationship to the observed cyclic stress-strain behavior. The initial yield strength of all specimens tested at 650 C was controlled by the shearing of the gamma prime precipitates by dislocation pairs. Low cycle fatigue tests at 650 C had cyclic hardening, which was associated with dislocation interactions in the gamma matrix. The initial yield strength of specimens tested at 1050 C was associated with dislocation bypassing of the gamma prime precipitates. Low cycle fatigue tests at 1050 C had cyclic softening, associated with extensive dislocation recovery at the gamma-gamma prime interfaces along with some gamma prime precipitate coarsening.

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.

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

NASA Astrophysics Data System (ADS)

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

2017-03-01

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

Recent Progress in Discrete Dislocation Dynamics and Its Applications to Micro Plasticity

NASA Astrophysics Data System (ADS)

Po, Giacomo; Mohamed, Mamdouh S.; Crosby, Tamer; Erel, Can; El-Azab, Anter; Ghoniem, Nasr

2014-10-01

We present a self-contained review of the discrete dislocation dynamics (DDD) method for the numerical investigation of plasticity in crystals, focusing on recent development and implementation progress. The review covers the theoretical foundations of DDD within the framework of incompatible elasticity, its numerical implementation via the nodal method, the extension of the method to finite domains and several implementation details. Applications of the method to current topics in micro-plasticity are presented, including the size effects in nano-indentation, the evolution of the dislocation microstructure in persistent slip bands, and the phenomenon of dislocation avalanches in micro-pillar compression.

Electric-field-induced domain intersection in BaTiO3 single crystal

NASA Astrophysics Data System (ADS)

He, Ming; Wang, Mengxia; Zhang, Zhihua

2017-03-01

Large-angle convergent beam electron diffraction was used to determine the directions of polarization vectors in a BaTiO3 single crystal. Domain intersections driven by an electric field were investigated by in situ transmission electron microscopy. The dark triangles observed in the domain intersection region can be accounted for by dislocations and the strain field. Domains nucleate at the domain tip depending on the dislocations and strain field to relieve the accumulated stress. Schematic representations of the intersecting domains and the microscopic structure are given, clarifying the special electric-field-induced domain structure.

Mechanisms for Ductile Rupture - FY16 ESC Progress Report

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

Boyce, Brad L.; Carroll, Jay D.; Noell, Phillip

2017-01-01

Ductile rupture in metals is generally a multi-step process of void nucleation, growth, and coalescence. Particle decohesion and particle fracture are generally invoked as the primary microstructural mechanisms for room-temperature void nucleation. However, because high-purity materials also fail by void nucleation and coalescence, other microstructural features must also act as sites for void nucleation. Early studies of void initiation in high-purity materials, which included post-mortem fracture surface characterization using scanning electron microscopy (SEM) and high-voltage electron microscopy (HVEM) and in-situ HVEM observations of fracture, established the presence of dislocation cell walls as void initiation sites in high-purity materials. Direct experimentalmore » evidence for this contention was obtained during in-situ HVEM tensile tests of Be single crystals. Voids between 0.2 and 1 μm long appeared suddenly along dislocation cell walls during tensile straining. However, subsequent attempts to replicate these results in other materials, particularly α -Fe single crystals, were unsuccessful because of the small size of the dislocation cells, and these remain the only published in-situ HVEM observations of void nucleation at dislocation cell walls in the absence of a growing macrocrack. Despite this challenge, other approaches to studying void nucleation in high-purity metals also indicate that dislocation cell walls are nucleation sites for voids.« less

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

NASA Astrophysics Data System (ADS)

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

2018-02-01

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

Crystal defect studies using x-ray diffuse scattering

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

Larson, B.C.

1980-01-01

Microscopic lattice defects such as point (single atom) defects, dislocation loops, and solute precipitates are characterized by local electronic density changes at the defect sites and by distortions of the lattice structure surrounding the defects. The effect of these interruptions of the crystal lattice on the scattering of x-rays is considered in this paper, and examples are presented of the use of the diffuse scattering to study the defects. X-ray studies of self-interstitials in electron irradiated aluminum and copper are discussed in terms of the identification of the interstitial configuration. Methods for detecting the onset of point defect aggregation intomore » dislocation loops are considered and new techniques for the determination of separate size distributions for vacancy loops and interstitial loops are presented. Direct comparisons of dislocation loop measurements by x-rays with existing electron microscopy studies of dislocation loops indicate agreement for larger size loops, but x-ray measurements report higher concentrations in the smaller loop range. Methods for distinguishing between loops and three-dimensional precipitates are discussed and possibilities for detailed studies considered. A comparison of dislocation loop size distributions obtained from integral diffuse scattering measurements with those from TEM show a discrepancy in the smaller sizes similar to that described above.« less

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

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

Hmelo, A.B.

1987-01-01

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

The role of subgrain boundaries in partial melting

NASA Astrophysics Data System (ADS)

Levine, Jamie S. F.; Mosher, Sharon; Rahl, Jeffrey M.

2016-08-01

Evidence for partial melting along subgrain boundaries in quartz and plagioclase is documented for rocks from the Lost Creek Gneiss of the Llano Uplift, central Texas, the Wet Mountains of central Colorado, and the Albany-Fraser Orogen, southwestern Australia. Domains of quartz or plagioclase crystals along subgrain boundaries are preferentially involved in partial melting over unstrained domains of these minerals. Material along subgrain boundaries in quartz and plagioclase has the same morphology as melt pseudomorphs present along grain boundaries and is commonly laterally continuous with this former grain boundary melt, indicating the material along subgrain boundaries can also be categorized as a melt pseudomorph. Subgrain boundaries consist of arrays of dislocations within a crystal lattice, and unlike fractures would not act as conduits for melt migration. Instead, the presence of former melt along subgrain boundaries requires that partial melting occurred in these locations because it is kinetically more favorable for melting reactions to occur there. Preferential melting in high strain locations may be attributed to strain energy, which provides a minor energetic contribution to the reaction and leads to preferential melting in locations with weakened bonds, and/or the presence of small quantities of water associated with dislocations, which may enhance diffusion rates or locally lower the temperature needed for partial melting.

Dual gauge field theory of quantum liquid crystals in two dimensions

NASA Astrophysics Data System (ADS)

Beekman, Aron J.; Nissinen, Jaakko; Wu, Kai; Liu, Ke; Slager, Robert-Jan; Nussinov, Zohar; Cvetkovic, Vladimir; Zaanen, Jan

2017-04-01

We present a self-contained review of the theory of dislocation-mediated quantum melting at zero temperature in two spatial dimensions. The theory describes the liquid-crystalline phases with spatial symmetries in between a quantum crystalline solid and an isotropic superfluid: quantum nematics and smectics. It is based on an Abelian-Higgs-type duality mapping of phonons onto gauge bosons (;stress photons;), which encode for the capacity of the crystal to propagate stresses. Dislocations and disclinations, the topological defects of the crystal, are sources for the gauge fields and the melting of the crystal can be understood as the proliferation (condensation) of these defects, giving rise to the Anderson-Higgs mechanism on the dual side. For the liquid crystal phases, the shear sector of the gauge bosons becomes massive signaling that shear rigidity is lost. After providing the necessary background knowledge, including the order parameter theory of two-dimensional quantum liquid crystals and the dual theory of stress gauge bosons in bosonic crystals, the theory of melting is developed step-by-step via the disorder theory of dislocation-mediated melting. Resting on symmetry principles, we derive the phenomenological imaginary time actions of quantum nematics and smectics and analyze the full spectrum of collective modes. The quantum nematic is a superfluid having a true rotational Goldstone mode due to rotational symmetry breaking, and the origin of this 'deconfined' mode is traced back to the crystalline phase. The two-dimensional quantum smectic turns out to be a dizzyingly anisotropic phase with the collective modes interpolating between the solid and nematic in a non-trivial way. We also consider electrically charged bosonic crystals and liquid crystals, and carefully analyze the electromagnetic response of the quantum liquid crystal phases. In particular, the quantum nematic is a real superconductor and shows the Meissner effect. Their special properties inherited from spatial symmetry breaking show up mostly at finite momentum, and should be accessible by momentum-sensitive spectroscopy.

Dual gauge field theory of quantum liquid crystals in two dimensions

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

Beekman, Aron J.; Nissinen, Jaakko; Wu, Kai

We present a self-contained review of the theory of dislocation-mediated quantum melting at zero temperature in two spatial dimensions. The theory describes the liquid-crystalline phases with spatial symmetries in between a quantum crystalline solid and an isotropic superfluid: quantum nematics and smectics. It is based on an Abelian-Higgs-type duality mapping of phonons onto gauge bosons (“stress photons”), which encode for the capacity of the crystal to propagate stresses. Dislocations and disclinations, the topological defects of the crystal, are sources for the gauge fields and the melting of the crystal can be understood as the proliferation (condensation) of these defects, givingmore » rise to the Anderson–Higgs mechanism on the dual side. For the liquid crystal phases, the shear sector of the gauge bosons becomes massive signaling that shear rigidity is lost. After providing the necessary background knowledge, including the order parameter theory of two-dimensional quantum liquid crystals and the dual theory of stress gauge bosons in bosonic crystals, the theory of melting is developed step-by-step via the disorder theory of dislocation-mediated melting. Resting on symmetry principles, we derive the phenomenological imaginary time actions of quantum nematics and smectics and analyze the full spectrum of collective modes. The quantum nematic is a superfluid having a true rotational Goldstone mode due to rotational symmetry breaking, and the origin of this ‘deconfined’ mode is traced back to the crystalline phase. The two-dimensional quantum smectic turns out to be a dizzyingly anisotropic phase with the collective modes interpolating between the solid and nematic in a non-trivial way. We also consider electrically charged bosonic crystals and liquid crystals, and carefully analyze the electromagnetic response of the quantum liquid crystal phases. In particular, the quantum nematic is a real superconductor and shows the Meissner effect. Furthermore, their special properties inherited from spatial symmetry breaking show up mostly at finite momentum, and should be accessible by momentum-sensitive spectroscopy.« less

Dual gauge field theory of quantum liquid crystals in two dimensions

DOE PAGES

Beekman, Aron J.; Nissinen, Jaakko; Wu, Kai; ...

2017-04-18

We present a self-contained review of the theory of dislocation-mediated quantum melting at zero temperature in two spatial dimensions. The theory describes the liquid-crystalline phases with spatial symmetries in between a quantum crystalline solid and an isotropic superfluid: quantum nematics and smectics. It is based on an Abelian-Higgs-type duality mapping of phonons onto gauge bosons (“stress photons”), which encode for the capacity of the crystal to propagate stresses. Dislocations and disclinations, the topological defects of the crystal, are sources for the gauge fields and the melting of the crystal can be understood as the proliferation (condensation) of these defects, givingmore » rise to the Anderson–Higgs mechanism on the dual side. For the liquid crystal phases, the shear sector of the gauge bosons becomes massive signaling that shear rigidity is lost. After providing the necessary background knowledge, including the order parameter theory of two-dimensional quantum liquid crystals and the dual theory of stress gauge bosons in bosonic crystals, the theory of melting is developed step-by-step via the disorder theory of dislocation-mediated melting. Resting on symmetry principles, we derive the phenomenological imaginary time actions of quantum nematics and smectics and analyze the full spectrum of collective modes. The quantum nematic is a superfluid having a true rotational Goldstone mode due to rotational symmetry breaking, and the origin of this ‘deconfined’ mode is traced back to the crystalline phase. The two-dimensional quantum smectic turns out to be a dizzyingly anisotropic phase with the collective modes interpolating between the solid and nematic in a non-trivial way. We also consider electrically charged bosonic crystals and liquid crystals, and carefully analyze the electromagnetic response of the quantum liquid crystal phases. In particular, the quantum nematic is a real superconductor and shows the Meissner effect. Furthermore, their special properties inherited from spatial symmetry breaking show up mostly at finite momentum, and should be accessible by momentum-sensitive spectroscopy.« less

Growth of Ca 4YO(BO 3) 3 crystals by vertical Bridgman method

NASA Astrophysics Data System (ADS)

Luo, Jun; Fan, Shiji; Wang, Jinchang; Zhong, Zhenwu; Qian, Guoxing; Sun, Renying

2001-07-01

Growth of single crystals of Ca 4YO(BO 3) 3 (YCOB) by the vertical Bridgman method is reported in this paper. By using near-sealed Pt crucibles to prevent volatilization of B 2O 3, the high-optical-quality YCOB crystals of 25 mm diameter and more than 40 mm in length have been grown at the furnace temperature of 50-80°C above the melting point of YCOB and the crucible lowering rates of 0.2-0.6 mm/h. Owing to the low vertical and radial temperature gradient, crack-free YCOB crystals have been obtained in the <0 1 0> and <0 0 1> directions. At the top of a YCOB boule, the dislocation density was found to decrease from the center to the outer area, and the average dislocation density is about 600/cm 2.

Extraordinary plasticity of an inorganic semiconductor in darkness.

PubMed

Oshima, Yu; Nakamura, Atsutomo; Matsunaga, Katsuyuki

2018-05-18

Inorganic semiconductors generally tend to fail in a brittle manner. Here, we report that extraordinary "plasticity" can take place in an inorganic semiconductor if the deformation is carried out "in complete darkness." Room-temperature deformation tests of zinc sulfide (ZnS) were performed under varying light conditions. ZnS crystals immediately fractured when they deformed under light irradiation. In contrast, it was found that ZnS crystals can be plastically deformed up to a deformation strain of ε t = 45% in complete darkness. In addition, the optical bandgap of the deformed ZnS crystals was distinctly decreased after deformation. These results suggest that dislocations in ZnS become mobile in complete darkness and that multiplied dislocations can affect the optical bandgap over the whole crystal. Inorganic semiconductors are not necessarily intrinsically brittle. Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

Dual gauge field theory of quantum liquid crystals in three dimensions

NASA Astrophysics Data System (ADS)

Beekman, Aron J.; Nissinen, Jaakko; Wu, Kai; Zaanen, Jan

2017-10-01

The dislocation-mediated quantum melting of solids into quantum liquid crystals is extended from two to three spatial dimensions, using a generalization of boson-vortex or Abelian-Higgs duality. Dislocations are now Burgers-vector-valued strings that trace out worldsheets in space-time while the phonons of the solid dualize into two-form (Kalb-Ramond) gauge fields. We propose an effective dual Higgs potential that allows for restoring translational symmetry in either one, two, or three directions, leading to the quantum analogues of columnar, smectic, or nematic liquid crystals. In these phases, transverse phonons turn into gapped, propagating modes, while compressional stress remains massless. Rotational Goldstone modes emerge whenever translational symmetry is restored. We also consider the effective electromagnetic response of electrically charged quantum liquid crystals, and find among other things that as a hard principle only two out of the possible three rotational Goldstone modes are observable using propagating electromagnetic fields.

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

NASA Astrophysics Data System (ADS)

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

2018-05-01

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

Microstructure and Sn crystal orientation evolution in Sn-3.5Ag lead-free solders in high temperature packaging applications

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

Zhou, Bite; Muralidharan, Govindarajan; Kurumaddali, Nalini Kanth

2014-01-01

Understanding the reliability of eutectic Sn-3.5Ag lead-free solders in high temperature packaging applications is of significant interest in power electronics for the next generation electric grid. Large area (2.5mm 2.5mm) Sn-3.5Ag solder joints between silicon dies and direct bonded copper substrates were thermally cycled between 5 C and 200 C. Sn crystal orientation and microstructure evolution during thermal cycling were characterized by electron backscatter diffraction (EBSD) in scanning electron microscope (SEM). Comparisons are made between observed initial texture and microstructure and its evolution during thermal cycling. Gradual lattice rotation and grain boundary misorientation evolution suggested the continuous recrystallization mechanism. Recrystallizationmore » behavior was correlated with dislocation slip activities.« less

Containerless processing of single crystals in low-G environment

NASA Technical Reports Server (NTRS)

Walter, H. U.

1974-01-01

Experiments on containerless crystal growth from the melt were conducted during Skylab missions SL3 and SL4 (Skylab Experiment M-560). Six samples of InSb were processed, one of them heavily doped with selenium. The concept of the experiment is discussed and related to general crystal growth methods and their merits as techniques for containerless processing in space. The morphology of the crystals obtained is explained in terms of volume changes associated with solidification and wetting conditions during solidification. All samples exhibit extremely well developed growth facets. Analysis by X-ray topographical methods and chemical etching shows that the crystals are of high structural perfection. Average dislocation density as revealed by etching is of the order of 100 per sq cm; no dislocation clusters could be observed in the space-grown samples. A sequence of striations that is observed in the first half of the selenium-doped sample is explained as being caused by periodic surface breakdown.

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

DOE PAGES

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

2015-02-26

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

Very High Quality Crystals of Wide-Gap II-VI Semiconductors: What for?

DTIC Science & Technology

2001-01-01

the reciprocal space mapping , by the etch pit density (EPD) measurements (to determine the density of dislocations) and by the measurement of the width...crystals. The EPD was in the range 5 x 1 + 104 cmn2 for Cdl.,ZnxTe crystals and about 104 cmz for ZnTe. The reciprocal space mapping of the crystals

Three-dimensional modelling of thermal stress in floating zone silicon crystal growth

NASA Astrophysics Data System (ADS)

Plate, Matiss; Krauze, Armands; Virbulis, Jānis

2018-05-01

During the growth of large diameter silicon single crystals with the industrial floating zone method, undesirable level of thermal stress in the crystal is easily reached due to the inhomogeneous expansion as the crystal cools down. Shapes of the phase boundaries, temperature field and elastic material properties determine the thermal stress distribution in the solid mono crystalline silicon during cylindrical growth. Excessive stress can lead to fracture, generation of dislocations and altered distribution of intrinsic point defects. Although appearance of ridges on the crystal surface is the decisive factor of a dislocation-free growth, the influence of these ridges on the stress field is not completely clear. Here we present the results of thermal stress analysis for 4” and 5” diameter crystals using a quasi-stationary three dimensional mathematical model including the material anisotropy and the presence of experimentally observed ridges which cannot be addressed with axis-symmetric models. The ridge has a local but relatively strong influence on thermal stress therefore its relation to the origin of fracture is hypothesized. In addition, thermal stresses at the crystal rim are found to increase for a particular position of the crystal radiation reflector.

Elastic-plastic deformation of molybdenum single crystals shocked along [100

DOE PAGES

Mandal, A.; Gupta, Y. M.

2017-01-24

To understand the elastic-plastic deformation response of shock-compressed molybdenum (Mo) – a body-centered cubic (BCC) metal, single crystal samples were shocked along the [100] crystallographic orientation to an elastic impact stress of 12.5 GPa. Elastic-plastic wave profiles, measured at different propagation distances ranging between ~0.23 to 2.31 mm using laser interferometry, showed a time-dependent material response. Within experimental scatter, the measured elastic wave amplitudes were nearly constant over the propagation distances examined. These data point to a large and rapid elastic wave attenuation near the impact surface, before reaching a threshold value (elastic limit) of ~3.6 GPa. Numerical simulations ofmore » the measured wave profiles, performed using a dislocation-based continuum model, suggested that {110}<111> and/or {112}<111> slip systems are operative under shock loading. In contrast to shocked metal single crystals with close-packed structures, the measured wave profiles in Mo single crystals could not be explained in terms of dislocation multiplication alone. A dislocation generation mechanism, operative for shear stresses larger than that at the elastic limit, was required to model the rapid elastic wave attenuation and to provide a good overall match to the measured wave profiles. However, the physical basis for this mechanism was not established for the high-purity single crystal samples used in this study. As a result, the numerical simulations also suggested that Mo single crystals do not work harden significantly under shock loading in contrast to the behavior observed under quasi-static loading.« less

Crystal Plasticity Model of Reactor Pressure Vessel Embrittlement in GRIZZLY

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

Chakraborty, Pritam; Biner, Suleyman Bulent; Zhang, Yongfeng

2015-07-01

The integrity of reactor pressure vessels (RPVs) is of utmost importance to ensure safe operation of nuclear reactors under extended lifetime. Microstructure-scale models at various length and time scales, coupled concurrently or through homogenization methods, can play a crucial role in understanding and quantifying irradiation-induced defect production, growth and their influence on mechanical behavior of RPV steels. A multi-scale approach, involving atomistic, meso- and engineering-scale models, is currently being pursued within the GRIZZLY project to understand and quantify irradiation-induced embrittlement of RPV steels. Within this framework, a dislocation-density based crystal plasticity model has been developed in GRIZZLY that captures themore » effect of irradiation-induced defects on the flow stress behavior and is presented in this report. The present formulation accounts for the interaction between self-interstitial loops and matrix dislocations. The model predictions have been validated with experiments and dislocation dynamics simulation.« less

Macrodefect-free, large, and thick GaN bulk crystals for high-quality 2–6 in. GaN substrates by hydride vapor phase epitaxy with hardness control

NASA Astrophysics Data System (ADS)

Fujikura, Hajime; Konno, Taichiro; Suzuki, Takayuki; Kitamura, Toshio; Fujimoto, Tetsuji; Yoshida, Takehiro

2018-06-01

On the basis of a novel crystal hardness control, we successfully realized macrodefect-free, large (2–6 in.) and thick +c-oriented GaN bulk crystals by hydride vapor phase epitaxy. Without the hardness control, the introduction of macrodefects including inversion domains and/or basal-plane dislocations seemed to be indispensable to avoid crystal fracture in GaN growth with millimeter thickness. However, the presence of these macrodefects tended to limit the applicability of the GaN substrate to practical devices. The present technology markedly increased the GaN crystal hardness from below 20 to 22 GPa, thus increasing the available growth thickness from below 1 mm to over 6 mm even without macrodefect introduction. The 2 and 4 in. GaN wafers fabricated from these crystals had extremely low dislocation densities in the low- to mid-105 cm‑2 range and low off-angle variations (2 in.: <0.1° 4 in.: ∼0.2°). The realization of such high-quality 6 in. wafers is also expected.

Intrinsic luminescence and core structure of freshly introduced a-screw dislocations in n-GaN

NASA Astrophysics Data System (ADS)

Medvedev, O.; Vyvenko, O.; Ubyivovk, E.; Shapenkov, S.; Bondarenko, A.; Saring, P.; Seibt, M.

2018-04-01

Dislocations introduced by the scratching or by the indentation of the basal and prismatic surfaces of low-ohmic unintentionally n-type doped GaN crystals were investigated by means of cathodoluminescence and transmission electron microscopy (TEM). A strong luminescence of straight segments of a-screw dislocations was observed in the temperature range of 70-420 K. The spectrum of dislocation related luminescence (DRL) consisted of a doublet of narrow lines red shifted by about 0.3 eV with respect to the band gap. TEM revealed dissociated character of the screw dislocations and the formation of extended nodes at their intersection. From the analysis of the DRL spectral doublet temperature, power and strain dependences DRL was ascribed to direct and indirect excitons bound by 1D quantum wells formed by partials and stacking fault (SF) ribbon of dissociated screw dislocation.

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

NASA Astrophysics Data System (ADS)

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

2018-02-01

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

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.

High breakdown single-crystal GaN p-n diodes by molecular beam epitaxy

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

Qi, Meng; Zhao, Yuning; Yan, Xiaodong

2015-12-07

Molecular beam epitaxy grown GaN p-n vertical diodes are demonstrated on single-crystal GaN substrates. A low leakage current <3 nA/cm{sup 2} is obtained with reverse bias voltage up to −20 V. With a 400 nm thick n-drift region, an on-resistance of 0.23 mΩ cm{sup 2} is achieved, with a breakdown voltage corresponding to a peak electric field of ∼3.1 MV/cm in GaN. Single-crystal GaN substrates with very low dislocation densities enable the low leakage current and the high breakdown field in the diodes, showing significant potential for MBE growth to attain near-intrinsic performance when the density of dislocations is low.

Sessile dislocations by reactions in NiAl severely deformed at room temperature

DOE PAGES

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

2015-02-05

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

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

Mandal, A.; Gupta, Y. M.

To understand the elastic-plastic deformation response of shock-compressed molybdenum (Mo) – a body-centered cubic (BCC) metal, single crystal samples were shocked along the [100] crystallographic orientation to an elastic impact stress of 12.5 GPa. Elastic-plastic wave profiles, measured at different propagation distances ranging between ~0.23 to 2.31 mm using laser interferometry, showed a time-dependent material response. Within experimental scatter, the measured elastic wave amplitudes were nearly constant over the propagation distances examined. These data point to a large and rapid elastic wave attenuation near the impact surface, before reaching a threshold value (elastic limit) of ~3.6 GPa. Numerical simulations ofmore » the measured wave profiles, performed using a dislocation-based continuum model, suggested that {110}<111> and/or {112}<111> slip systems are operative under shock loading. In contrast to shocked metal single crystals with close-packed structures, the measured wave profiles in Mo single crystals could not be explained in terms of dislocation multiplication alone. A dislocation generation mechanism, operative for shear stresses larger than that at the elastic limit, was required to model the rapid elastic wave attenuation and to provide a good overall match to the measured wave profiles. However, the physical basis for this mechanism was not established for the high-purity single crystal samples used in this study. As a result, the numerical simulations also suggested that Mo single crystals do not work harden significantly under shock loading in contrast to the behavior observed under quasi-static loading.« less

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

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

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

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

Hydrogen diffusion in the elastic fields of dislocations in iron

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

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

2016-12-15

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

Dynamics and Removal Pathway of Edge Dislocations in Imperfectly Attached PbTe Nanocrystal Pairs: Toward Design Rules for Oriented Attachment.

PubMed

Ondry, Justin C; Hauwiller, Matthew R; Alivisatos, A Paul

2018-04-24

Using in situ high-resolution TEM, we study the structure and dynamics of well-defined edge dislocations in imperfectly attached PbTe nanocrystals. We identify that attachment of PbTe nanocrystals on both {100} and {110} facets gives rise to b = a/2⟨110⟩ edge dislocations. Based on the Burgers vector of individual dislocations, we can identify the glide plane of the dislocations. We observe that defects in particles attached on {100} facets have glide planes that quickly intersect the surface, and HRTEM movies show that the defects follow the glide plane to the surface. For {110} attached particles, the glide plane is collinear with the attachment direction, which does not provide an easy path for the dislocation to reach the surface. Indeed, HRTEM movies of dislocations for {110} attached particles show that defect removal is much slower. Further, we observe conversion from pure edge dislocations in imperfectly attached particles to dislocations with mixed edge and screw character, which has important implications for crystal growth. Finally, we observe that dislocations initially closer to the surface have a higher speed of removal, consistent with the strong dislocation free surface attractive force. Our results provide important design rules for defect-free attachment of preformed nanocrystals into epitaxial assemblies.

The microstructure of laterally seeded silicon-on-oxide

NASA Astrophysics Data System (ADS)

Pinizzotto, R. F.; Lam, H. W.; Vaandrager, B. L.

1982-03-01

The production of large scale integrated circuits in thin silicon films on insulating substrates is currently of much interest in the electronics industry. One of the most promising techniques of forming this composite structure is by lateral seeding. We have used optical microscopy and transmission electron microscopy to characterize the microstructure of silicon-on-oxide formed by scanning CW laser induced lateral epitaxy. The primary defects are dislocations. Dislocation rearrangement leads to the formation of both small angle boundaries (stable, regular dislocation arrays) and grain boundaries. The grains were found to be misoriented to the <100> direction perpendicular to the film plane by ≤ 4° and to the <100> directions in the plane of the film by ≤ 2°. Internal reflection twins are a common defect. Microtwinning was found to occur at the vertical step caused by the substrate-oxide interface if the substrate to oxide step height was > 120 nm. The microstructure is continuous across successive scan lines. Microstructural defects are found to initiate at the same topographical location in different oxide pads. We propose that this is due to the meeting of two crystallization growth fronts. The liquid silicon between the fronts causes large stresses in this area because of the 9% volume increase during solidification. The defects observed in the bulk may form by a similar mechanism or by dislocation generation at substrate-oxide interface irregularities. The models predict that slower growth leads to improved material quality. This has been observed experimentally.

Processing of energy materials in electromagnetic field

NASA Astrophysics Data System (ADS)

Rodzevich, A. P.; Kuzmina, L. V.; Gazenaur, E. G.; Krasheninin, V. I.

2015-09-01

This paper presents the research results of complex impact of mechanical stress and electromagnetic field on the defect structure of energy materials. As the object of research quite a typical energy material - silver azide was chosen, being a model in chemistry of solids. According to the experiments co-effect of magnetic field and mechanical stress in silver azide crystals furthers multiplication, stopper breakaway, shift of dislocations, and generation of superlattice dislocations - micro-cracks. A method of mechanical and electric strengthening has been developed and involves changing the density of dislocations in whiskers.

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

NASA Astrophysics Data System (ADS)

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

2017-12-01

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

Recombination-related properties of a-screw dislocations in GaN: A combined CL, EBIC, TEM study

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

Medvedev, O. S., E-mail: o.s.medvedev@spbu.ru; Mikhailovskii, V. Yu.; IRC for Nanotechnology, Research Park, St.-Petersburg State University

2016-06-17

Cathodoluminescence (CL), electron beam current (EBIC) and transmission electron microscopy (TEM) techniques have been applied to investigate recombination properties and structure of freshly introduced dislocations in low-ohmic GaN crystals. It was confirmed that the only a-screw dislocations exhibited an intense characteristic dislocation-related luminescence (DRL) which persisted up to room temperature and was red-shifted by about 0.3 eV with respect to the band gap energy not only in HVPE but also in MOCVD grown samples. EBIC contrast of the dislocations was found to be temperature independent indicating that the dislocation-related recombination level is situated below 200 meV with respect of conductionmore » band minimum. With the increasing of the magnification of the dislocation TEM cross-sectional images they were found to disappear, probably, due to the recombination enhanced dislocation glide (REDG) under electron beam exposure which was immediately observed in CL investigations on a large scale. The stacking fault ribbon in the core of dissociated a-screw dislocation which form a quantum well for electrons was proposed to play an important role both in DRL spectrum formation and in REDG.« less

Static and dynamic properties of incommensurate smectic-A(IC) liquid crystals

NASA Technical Reports Server (NTRS)

Lubensky, T. C.; Ramaswamy, Sriram; Toner, John

1988-01-01

The elasticity, topological defects, and hydrodynamics of the incommensurate smectic A(IC) phase liquid crystals are studied. The phase is characterized by two colinear mass density waves of incommensurate spatial frequency. The elastic free energy is formulated in terms of a displacement field and a phason field. It is found that the topological defects of the system are dislocations with a nonzero phason field and phason field components. A two-dimensional Burgers lattice for these dislocations is introduced. It is shown that the hydrodynamic modes of the phase include first- and second-sound modes whose direction-dependent velocities are identical to those in ordinary smectics.

Correlations between critical current density, j{sub c}, critical temperature, T{sub c}, and structural quality of Y{sub 1}B{sub 2}Cu{sub 3}O{sub 7-x} thin superconducting films

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

Chrzanowski, J.; Xing, W.B.; Atlan, D.

1994-12-31

Correlations between critical current density (j{sub c}) critical temperature (T{sub c}) and the density of edge dislocations and nonuniform strain have been observed in YBCO thin films deposited by pulsed laser ablation on (001) LaAlO{sub 3} single crystals. Distinct maxima in j{sub c} as a function of the linewidths of the (00{ell}) Bragg reflections and as a function of the mosaic spread have been found in the epitaxial films. These maxima in j{sub c} indicate that the magnetic flux lines, in films of structural quality approaching that of single crystals, are insufficiently pinned which results in a decreased critical currentmore » density. T{sub c} increased monotonically with improving crystalline quality and approached a value characteristic of a pure single crystal. A strong correlation between j{sub c} and the density of edge dislocations N{sub D} was found. At the maximum of the critical current density the density of edge dislocations was estimated to be N{sub D}{approximately}1-2 x 10{sup 9}/cm{sup 2}.« less

Electrical Impact of SiC Structural Crystal Defects on High Electric Field Devices

NASA Technical Reports Server (NTRS)

Neudeck, Philip G.

1999-01-01

Commercial epilayers are known to contain a variety of crystallographic imperfections. including micropipes, closed core screw dislocations. low-angle boundaries, basal plane dislocations, heteropolytypic inclusions, and non-ideal surface features like step bunching and pits. This paper reviews the limited present understanding of the operational impact of various crystal defects on SiC electrical devices. Aside from micropipes and triangular inclusions whose densities have been shrinking towards manageably small values in recent years, many of these defects appear to have little adverse operational and/or yield impact on SiC-based sensors, high-frequency RF, and signal conditioning electronics. However high-power switching devices used in power management and distribution circuits have historically (in silicon experience) demanded the highest material quality for prolonged safe operation, and are thus more susceptible to operational reliability problems that arise from electrical property nonuniformities likely to occur at extended crystal defects. A particular emphasis is placed on the impact of closed-core screw dislocations on high-power switching devices, because these difficult to observe defects are present in densities of thousands per cm,in commercial SiC epilayers. and their reduction to acceptable levels seems the most problematic at the present time.

Correlations between critical current density, j(sub c), critical temperature, T(sub c),and structural quality of Y1B2Cu3O(7-x) thin superconducting films

NASA Technical Reports Server (NTRS)

Chrzanowski, J.; Xing, W. B.; Atlan, D.; Irwin, J. C.; Heinrich, B.; Cragg, R. A.; Zhou, H.; Angus, V.; Habib, F.; Fife, A. A.

1995-01-01

Correlations between critical current density (j(sub c)) critical temperature (T(sub c)) and the density of edge dislocations and nonuniform strain have been observed in YBCO thin films deposited by pulsed laser ablation on (001) LaAlO3 single crystals. Distinct maxima in j(sub c) as a function of the linewidths of the (00 l) Bragg reflections and as a function of the mosaic spread have been found in the epitaxial films. These maxima in j(sub c) indicate that the magnetic flux lines, in films of structural quality approachingthat of single crystals, are insufficiently pinned which results in a decreased critical current density. T(sub c) increased monotonically with improving crystalline quality and approached a value characteristic of a pure single crystal. A strong correlation between j(sub c) and the density of edge dislocations ND was found. At the maximum of the critical current density the density of edge dislocations was estimated to be N(sub D) approximately 1-2 x 10(exp 9)/sq cm.

20 CFR 663.115 - What are the eligibility criteria for core services for dislocated workers in the adult and...

Code of Federal Regulations, 2012 CFR

2012-04-01

... services for dislocated workers in the adult and dislocated worker programs? 663.115 Section 663.115 Employees' Benefits EMPLOYMENT AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated...

20 CFR 663.115 - What are the eligibility criteria for core services for dislocated workers in the adult and...

Code of Federal Regulations, 2014 CFR

2014-04-01

... services for dislocated workers in the adult and dislocated worker programs? 663.115 Section 663.115 Employees' Benefits EMPLOYMENT AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated...

20 CFR 663.115 - What are the eligibility criteria for core services for dislocated workers in the adult and...

Code of Federal Regulations, 2013 CFR

2013-04-01

... services for dislocated workers in the adult and dislocated worker programs? 663.115 Section 663.115 Employees' Benefits EMPLOYMENT AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated...

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

NASA Astrophysics Data System (ADS)

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

2018-06-01

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

Trapping of edge dislocations by a moving smectic-A smectic-B interface

NASA Astrophysics Data System (ADS)

Oswald, P.; Lejcek, L.

1991-09-01

We analyze how the motion of the edge dislocations of the smectic-A liquid crystal allows the system to relax plastically the stresses that are generated during the growth of the smectic-B plastic crystal. These stresses are both due, to the density difference between the two phases, and to the layer thickness variation at the phase transition. In particular, we calculate under which conditions a dislocation can be trapped by the smectic-B phase. Finally, we suggest that this dynamical trapping is responsible for the very large amount of stacking faults observed by X-ray diffraction. Nous analysons comment le mouvement des dislocations coin du cristal liquide smectique A permet de relaxer plastiquement les contraintes induites par la croissance du cristal plastique smectique B. Ces contraintes sont engendrées à la fois par la différence de densité qui existe entre les deux phases et par la variation d'épaisseur des couches à la transition. Nous calculons en particulier dans quelles conditions une dislocation coin peut être piégée par le smectique B. Enfin, nous suggérons que ce piégeage dynamique est à l'origine de la très forte densité de fautes d'empilement qui est couramment observée aux rayons X dans la phase B.

Resolving individual Shockley partials of a dissociated dislocation by STEM

NASA Astrophysics Data System (ADS)

Iwata, Hiroyuki; Saka, Hiroyasu

2017-02-01

A practical method was developed to image detailed features of defects in a crystal using STEM. This method is essentially a STEM version of the conventional CTEM g/3g weak beam dark field (WBDF) method. The method was successfully applied to resolving individual Shockley partials of a dissociated dislocation in a Cu-6.44at.%Al alloy.

Enhancing elastic stress relaxation in SiGe/Si heterostructures by Si pillar necking

NASA Astrophysics Data System (ADS)

Isa, F.; Salvalaglio, M.; Arroyo Rojas Dasilva, Y.; Jung, A.; Isella, G.; Erni, R.; Timotijevic, B.; Niedermann, P.; Gröning, P.; Montalenti, F.; von Känel, H.

2016-10-01

We demonstrate that the elastic stress relaxation mechanism in micrometre-sized, highly mismatched heterostructures may be enhanced by employing patterned substrates in the form of necked pillars, resulting in a significant reduction of the dislocation density. Compositionally graded Si1-xGex crystals were grown by low energy plasma enhanced chemical vapour deposition, resulting in tens of micrometres tall, three-dimensional heterostructures. The patterned Si(001) substrates consist of micrometre-sized Si pillars either with the vertical {110} or isotropically under-etched sidewalls resulting in narrow necks. The structural properties of these heterostructures were investigated by defect etching and transmission electron microscopy. We show that the dislocation density, and hence the competition between elastic and plastic stress relaxation, is highly influenced by the shape of the substrate necks and their proximity to the mismatched epitaxial material. The SiGe dislocation density increases monotonically with the crystal width but is significantly reduced by the substrate under-etching. The drop in dislocation density is interpreted as a direct effect of the enhanced compliance of the under-etched Si pillars, as confirmed by the three-dimensional finite element method simulations of the elastic energy distribution.

Rotation-limited growth of three-dimensional body-centered-cubic crystals

NASA Astrophysics Data System (ADS)

Tarp, Jens M.; Mathiesen, Joachim

2015-07-01

According to classical grain growth laws, grain growth is driven by the minimization of surface energy and will continue until a single grain prevails. These laws do not take into account the lattice anisotropy and the details of the microscopic rearrangement of mass between grains. Here we consider coarsening of body-centered-cubic polycrystalline materials in three dimensions using the phase field crystal model. We observe, as a function of the quenching depth, a crossover between a state where grain rotation halts and the growth stagnates and a state where grains coarsen rapidly by coalescence through rotation and alignment of the lattices of neighboring grains. We show that the grain rotation per volume change of a grain follows a power law with an exponent of -1.25 . The scaling exponent is consistent with theoretical considerations based on the conservation of dislocations.

Phase-field crystal modeling of compositional domain formation in ultrathin films.

PubMed

Muralidharan, Srevatsan; Haataja, Mikko

2010-09-17

Bulk-immiscible binary systems often form stress-induced miscible alloy phases when deposited on a substrate. Both alloying and surface dislocation formation lead to the decrease of the elastic strain energy, and the competition between these two strain-relaxation mechanisms gives rise to the emergence of pseudomorphic compositional nanoscale domains, often coexisting with a partially coherent single phase. In this work, we develop a phase-field crystal model for compositional patterning in monolayer aggregates of binary metallic systems. We first demonstrate that the model naturally incorporates the competition between alloying and misfit dislocations, and quantify the effects of misfit and line tension on equilibrium domain size. Then, we quantitatively relate the parameters of the phase-field crystal model to a specific system, CoAg/Ru(0001), and demonstrate that the simulations capture experimentally observed morphologies.

Tailoring Superconductivity with Quantum Dislocations.

PubMed

Li, Mingda; Song, Qichen; Liu, Te-Huan; Meroueh, Laureen; Mahan, Gerald D; Dresselhaus, Mildred S; Chen, Gang

2017-08-09

Despite the established knowledge that crystal dislocations can affect a material's superconducting properties, the exact mechanism of the electron-dislocation interaction in a dislocated superconductor has long been missing. Being a type of defect, dislocations are expected to decrease a material's superconducting transition temperature (T c ) by breaking the coherence. Yet experimentally, even in isotropic type I superconductors, dislocations can either decrease, increase, or have little influence on T c . These experimental findings have yet to be understood. Although the anisotropic pairing in dirty superconductors has explained impurity-induced T c reduction, no quantitative agreement has been reached in the case a dislocation given its complexity. In this study, by generalizing the one-dimensional quantized dislocation field to three dimensions, we reveal that there are indeed two distinct types of electron-dislocation interactions. Besides the usual electron-dislocation potential scattering, there is another interaction driving an effective attraction between electrons that is caused by dislons, which are quantized modes of a dislocation. The role of dislocations to superconductivity is thus clarified as the competition between the classical and quantum effects, showing excellent agreement with existing experimental data. In particular, the existence of both classical and quantum effects provides a plausible explanation for the illusive origin of dislocation-induced superconductivity in semiconducting PbS/PbTe superlattice nanostructures. A quantitative criterion has been derived, in which a dislocated superconductor with low elastic moduli and small electron effective mass and in a confined environment is inclined to enhance T c . This provides a new pathway for engineering a material's superconducting properties by using dislocations as an additional degree of freedom.

Effet Bauschinger lors de la plasticité cyclique de l'aluminium pur monocristallin

NASA Astrophysics Data System (ADS)

Alhamany, A.; Chicois, J.; Fougères, R.; Hamel, A.

1992-08-01

This paper is concerned with the study of microscopic mechanisms which control the cyclic deformation of pure aluminium and especially with the analysis of the Bauschinger effect which appears in aluminium single crystals deformed by cyclic straining. Fatigue tests are performed on Al single crystals with the crystal axis parallel to [ overline{1}23] at room temperature, at plastic shear strain amplitudes in the range from 10^{-4} to 3× 10^{-3}. Mechanical saturation is not obtained at any strain level. Instead, a hardening-softening-secondary hardening sequence is found. The magnitude of the Bauschinger effect as the difference between yield stresses in traction and in compression, changes all along the fatigue loop and during the fatigue test. The Bauschinger effect disappears at two points of the fatigue loop, one in the traction part, the other in the compression one. At these points, the Bauschinger effect is inverted. Dislocation arrangement evolutions with fatigue conditions can explain the cyclic behaviour of Al single crystals. An heterogeneous dislocation distribution can be observed in the cyclically strained metal : dislocation tangles, long dislocation walls and dislocation cell walls, separated by dislocation poor channels appear in the material as a function of the cycle number. The long range internal stress necessary to ensure the compatibility of deformation between the hard and soft regions controls the observed Bauschinger effect. Ce travail s'inscrit dans le cadre de l'étude des mécanismes microsocopiques intervenant lors de la déformation cyclique de l'aluminium pur et concerne en particulier l'analyse de l'effet Bauschinger apparaissant au cours de la solliciation cyclique des monocristaux. L'étude a été menée à température ambiante sur des monocristaux d'aluminium pur orientés pour un glissement simple (axe [ overline{1}23] ), à des amplitudes de déformation plastique comprise entre 10^{-4} et quelques 10^{-3}. Nous n'avons pas obtenu de véritable saturation mécanique. Nous sommes en présence d'une séquence durcissement-adoucissement-durcissement secondaire. L'amplitude de l'effet Bauschinger considéré comme la différence entre les limites élastiques en traction et en compression mesurées selon une procédure appropriée, évolue le long d'une boucle de fatigue, s'annule pour deux points particuliers l'un en traction l'autre en compression. De part et d'autre de ces points, le signe de l'effet Bauschinger est inversé. Les microstructures des états fatigués sont caractérisés par une répartition hétérogène des dislocations constituée d'amas, de murs ou des parois, suivant le degré de déformation cyclique, séparés par des zones à faible densité de dislocations. Les contraintes internes liées aux incompatibilités de déformation résultant de cette répartition hétérogène des dislocations sont à l'origine de l'effet Bauschinger observé dans les monocristaux. Ces contraintes et l'évolution de la quantité de cellules de dislocations avec la fatigue expliquent le durcissement secondaire.

Dual gauge field theory of quantum liquid crystals in three dimensions

DOE PAGES

Beekman, Aron J.; Nissinen, Jaakko; Wu, Kai; ...

2017-10-09

The dislocation-mediated quantum melting of solids into quantum liquid crystals is extended from two to three spatial dimensions, using a generalization of boson-vortex or Abelian-Higgs duality. Dislocations are now Burgers-vector-valued strings that trace out worldsheets in space-time while the phonons of the solid dualize into two-form (Kalb-Ramond) gauge fields. We propose an effective dual Higgs potential that allows for restoring translational symmetry in either one, two, or three directions, leading to the quantum analogues of columnar, smectic, or nematic liquid crystals. In these phases, transverse phonons turn into gapped, propagating modes, while compressional stress remains massless. Rotational Goldstone modes emergemore » whenever translational symmetry is restored. Lastly, we also consider the effective electromagnetic response of electrically charged quantum liquid crystals, and find among other things that as a hard principle only two out of the possible three rotational Goldstone modes are observable using propagating electromagnetic fields.« less

Dual gauge field theory of quantum liquid crystals in three dimensions

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

Beekman, Aron J.; Nissinen, Jaakko; Wu, Kai

The dislocation-mediated quantum melting of solids into quantum liquid crystals is extended from two to three spatial dimensions, using a generalization of boson-vortex or Abelian-Higgs duality. Dislocations are now Burgers-vector-valued strings that trace out worldsheets in space-time while the phonons of the solid dualize into two-form (Kalb-Ramond) gauge fields. We propose an effective dual Higgs potential that allows for restoring translational symmetry in either one, two, or three directions, leading to the quantum analogues of columnar, smectic, or nematic liquid crystals. In these phases, transverse phonons turn into gapped, propagating modes, while compressional stress remains massless. Rotational Goldstone modes emergemore » whenever translational symmetry is restored. Lastly, we also consider the effective electromagnetic response of electrically charged quantum liquid crystals, and find among other things that as a hard principle only two out of the possible three rotational Goldstone modes are observable using propagating electromagnetic fields.« less

SMALL ANGLE SCATTERING OF X-RAYS BY PLASTICALLY DEFORMED SINGLE CRYSTALS

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

Robinson, W.H.; Smoluchowski, R.

1959-05-01

The small-angle scattering of x rays from single crystals of magnesium plastically deformed by simple shear was measured in the angular range of 4' to 5 deg . The crystals were subjected to both unidirectional and cyclic shear stresses applied along the STAl 1 2-bar 0! direction. Thin slices of the deformed single crystals were prepared using strainfree cutting and polishing techniques. The thin slices had orientations such that the slip direction was either parallel or perpendicular to the incident x-ray beam in order to observe any anisotropy in the scattering that might be due to dislocations. It was foundmore » that those samples which contained deformation twins within the irradiated volume produced rather large scattered intensity. This scattered intensity is interpreted as being due to double Bragg scattering. The scattered intensity from other specimens was attributed to surface scattering. No evidence for small angle scattering by dislocations was found. (auth)« less

Melting of 2D colloidal crystals

NASA Astrophysics Data System (ADS)

Maret, G.; Eisenmann, C.; Gasser, U.; Vongruenberg, H. H.; Keim, P.; Zahn, K.

2004-11-01

We study melting of 2D crystals of super-paramagnetic colloidal particles confined by gravity to a flat air-water interface. The effective system temperature is given by the strength of the dipolar inter-particle interaction controlled by an external magnetic field B. Particle positions are obtained by video-microscopy. In vertical B-field crystals are hexagonal and we find all features of the 2-step melting scenario predicted by KTHNY-theory. In particular, quantitative agreement is found for the translational and orientational order parameters related to bound and isolated dislocations and disclinations. From particle position fluctuations wave-vector (q) dependent normal-mode spring constants are obtained in agreement with phonon band structure calculations. The elastic constants (q=0 limit) soften near melting in quantitative agreement with KTHNY. By tilting B away from vertical anisotropic 2D crystals are generated; at small tilting angles they melt through a quasi-hexatic phase, while at higher tilts a centered rectangular phase is found which melts into a 2D smectic-like phase through orientation-dependent dislocations.

Continuous cooling transformations and microstructures in a low-carbon, high-strength low-alloy plate steel

NASA Astrophysics Data System (ADS)

Thompson, S. W.; Vin, D. J., Col; Krauss, G.

1990-06-01

A continuous-cooling-transformation (CCT) diagram was determined for a high-strength low-alloy plate steel containing (in weight percent) 0.06 C, 1.45 Mn, 1.25 Cu, 0.97 Ni, 0.72 Cr, and 0.42 Mo. Dilatometric measurements were supplemented by microhardness testing, light microscopy, and transmission electron microscopy. The CCT diagram showed significant suppression of polygonal ferrite formation and a prominent transformation region, normally attributed to bainite formation, at temperatures intermediate to those of polygonal ferrite and martensite formation. In the intermediate region, ferrite formation in groups of similarly oriented crystals about 1 μm in size and containing a high density of dislocations dominated the transformation of austenite during continuous cooling. The ferrite grains assumed two morphologies, elongated or acicular and equiaxed or granular, leading to the terms “acicular ferrite” and “granular ferrite,” respectively, to describe these structures. Austenite regions, some transformed to martensite, were enriched in carbon and retained at interfaces between ferrite grains. Coarse interfacial ledges and the nonacicular morphology of the granular ferrite grains provided evidence for a phase transformation mechanism involving reconstructive diffusion of substitutional atoms. At slow cooling rates, polygonal ferrite and Widmanstätten ferrite formed. These latter structures contained low dislocation densities and e-copper precipitates formed by an interphase transformation mechanism.

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

NASA Astrophysics Data System (ADS)

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

2016-03-01

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

Structure of screw dislocation core in Ta at high pressure

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

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

2014-03-07

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

Reduced Moment-Based Models for Oxygen Precipitates and Dislocation Loops in Silicon

NASA Astrophysics Data System (ADS)

Trzynadlowski, Bart

The demand for ever smaller, higher-performance integrated circuits and more efficient, cost-effective solar cells continues to push the frontiers of process technology. Fabrication of silicon devices requires extremely precise control of impurities and crystallographic defects. Failure to do so not only reduces performance, efficiency, and yield, it threatens the very survival of commercial enterprises in today's fiercely competitive and price-sensitive global market. The presence of oxygen in silicon is an unavoidable consequence of the Czochralski process, which remains the most popular method for large-scale production of single-crystal silicon. Oxygen precipitates that form during thermal processing cause distortion of the surrounding silicon lattice and can lead to the formation of dislocation loops. Localized deformation caused by both of these defects introduces potential wells that trap diffusing impurities such as metal atoms, which is highly desirable if done far away from sensitive device regions. Unfortunately, dislocations also reduce the mechanical strength of silicon, which can cause wafer warpage and breakage. Engineers must negotiate this and other complex tradeoffs when designing fabrication processes. Accomplishing this in a complex, modern process involving a large number of thermal steps is impossible without the aid of computational models. In this dissertation, new models for oxygen precipitation and dislocation loop evolution are described. An oxygen model using kinetic rate equations to evolve the complete precipitate size distribution was developed first. This was then used to create a reduced model tracking only the moments of the size distribution. The moment-based model was found to run significantly faster than its full counterpart while accurately capturing the evolution of oxygen precipitates. The reduced model was fitted to experimental data and a sensitivity analysis was performed to assess the robustness of the results. Source code for both models is included. A moment-based model for dislocation loop formation from {311} defects in ion-implanted silicon was also developed and validated against experimental data. Ab initio density functional theory calculations of stacking faults and edge dislocations were performed to extract energies and elastic properties. This allowed the effect of applied stress on the evolution of {311} defects and dislocation loops to be investigated.

EBIC/TEM investigations of process-induced defects in EFG silicon ribbon

NASA Technical Reports Server (NTRS)

Cunningham, B.; Ast, D. G.

1981-01-01

Electron bombardment induced conductivity and scanning transmission electron microscopy observations on unprocessed and processed edge-defined film-fed growth ribbon show that the phosphorus diffused junction depth is not uniform, and that a variety of chemical impurities precipitate out during processing. Two kinds of precipitates are found (1) 10 nm or less in size, located at the dislocation nodes in sub-boundary like dislocation arrangements formed during processing and (2) large precipitates, the chemical composition of which has been partially identified. These large precipitates emit dense dislocations tangles into the adjacent crystal volume.

Interacting effects of strengthening and twin boundary migration in nanotwinned materials

NASA Astrophysics Data System (ADS)

Joshi, Kartikey; Joshi, Shailendra P.

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

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

NASA Astrophysics Data System (ADS)

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

2017-11-01

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

Remarks on the Particular Behavior in Martensitic Phase Transition in Cu-Based and Ni-Ti Shape Memory Alloys

NASA Astrophysics Data System (ADS)

Torra, Vicenç; Martorell, Ferran; Lovey, Francisco C.; Sade, Marcos

2018-05-01

Many macroscopic behaviors of the martensitic transformations are difficult to explain in the frame of the classical first-order phase transformations, without including the role of point and crystallographic defects (dislocations, stacking faults, interfaces, precipitates). A few major examples are outlined in the present study. First, the elementary reason for thermoelasticity and pseudoelasticity in single crystals of Cu-Zn-Al (β-18R transformation) arises from the interaction of a growing martensite plate with the existing dislocations in the material. Secondly, in Cu-Al-Ni, the twinned hexagonal (γ') martensite produces dislocations inhibiting this transformation and favoring the appearance of 18R in subsequent transformation cycles. Thirdly, single crystals of Cu-Al-Be visualize, via enhanced stress, a transformation primarily to 18R, a structural distortion of the 18R structure, and an additional transformation to another martensitic phase (i.e., 6R) with an increased strain. A dynamic behavior in Ni-Ti is also analyzed, where defects alter the pseudoelastic behavior after cycling.

Electronic and Chemical Properties of a Surface-Terminated Screw Dislocation in MgO

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

Mckenna, Keith P.

2013-12-18

Dislocations represent an important and ubiquitous class of topological defect found at the surfaces of metal oxide materials. They are thought to influence processes as diverse as crystal growth, corrosion, charge trapping, luminescence, molecular adsorption and catalytic activity, however, their electronic and chemical properties remain poorly understood. Here, through a detailed first principles investigation into the properties of a surface terminated screw dislocation in MgO we provide atomistic insight into these issues. We show that surface dislocations can exhibit intriguing electron trapping properties which are important for understanding the chemical and electronic characteristics of oxide surfaces. The results presented inmore » this article taken together with recent experimental reports show that surface dislocations can be equally as important as more commonly considered surface defects, such as steps, kinks and vacanies, but are now just beginning to be understood.« less

Electronic and Chemical Properties of a Surface-Terminated Screw Dislocation in MgO

PubMed Central

2013-01-01

Dislocations represent an important and ubiquitous class of topological defect found at the surfaces of metal oxide materials. They are thought to influence processes as diverse as crystal growth, corrosion, charge trapping, luminescence, molecular adsorption, and catalytic activity; however, their electronic and chemical properties remain poorly understood. Here, through a detailed first-principles investigation into the properties of a surface-terminated screw dislocation in MgO we provide atomistic insight into these issues. We show that surface dislocations can exhibit intriguing electron trapping properties which are important for understanding the chemical and electronic characteristics of oxide surfaces. The results presented in this article taken together with recent experimental reports show that surface dislocations can be equally as important as more commonly considered surface defects, such as steps, kinks, and vacancies, but are now just beginning to be understood. PMID:24279391

Molecular dynamics simulations of dislocations in TlBr crystals under an electrical field

DOE PAGES

Zhou, X. W.; Foster, M. E.; Yang, P.; ...

2016-07-13

TlBr crystals have superior radiation detection properties; however, their properties degrade in the range of hours to weeks when an operating electrical field is applied. To account for this rapid degradation using the widely-accepted vacancy migration mechanism, the vacancy concentration must be orders of magnitude higher than any conventional estimates. The present work has incorporated a new analytical variable charge model in molecular dynamics (MD) simulations to examine the structural changes of materials under electrical fields. Our simulations indicate that dislocations in TlBr move under electrical fields. As a result, this discovery can lead to new understanding of TlBr agingmore » mechanisms under external fields.« less

Computing arbitrary defect structures on arbitrary lattices on arbitrary geometries from arbitrary energies

NASA Astrophysics Data System (ADS)

Allen, Brian; Travesset, Alex

2004-03-01

Dislocations and disclinations play a fundamental role in the properties of two dimensional crystals. In this talk, it will be shown that a general computational framework can be developed by combining previous work of Seung and Nelson* and modern advances in objected oriented design. This allows separating the problem into independent classes such as: geometry (sphere, plane, torus..), lattice (triangular, square, etc..), type of defect (dislocation, disclinations, etc..), boundary conditions, type of order (crystalline, hexatic) or energy functional. As applications, the ground state of crystals in several geometries will be discussed. Experimental examples with colloidal particles will be shown. *S. Seung and D. Nelson, Phys. Rev. A 38, 1005 (1988)

Dislocation Content Measured Via 3D HR-EBSD Near a Grain Boundary in an AlCu Oligocrystal

NASA Technical Reports Server (NTRS)

Ruggles, Timothy; Hochhalter, Jacob; Homer, Eric

2016-01-01

Interactions between dislocations and grain boundaries are poorly understood and crucial to mesoscale plasticity modeling. Much of our understanding of dislocation-grain boundary interaction comes from atomistic simulations and TEM studies, both of which are extremely limited in scale. High angular resolution EBSD-based continuum dislocation microscopy provides a way of measuring dislocation activity at length scales and accuracies relevant to crystal plasticity, but it is limited as a two-dimensional technique, meaning the character of the grain boundary and the complete dislocation activity is difficult to recover. However, the commercialization of plasma FIB dual-beam microscopes have made 3D EBSD studies all the more feasible. The objective of this work is to apply high angular resolution cross correlation EBSD to a 3D EBSD data set collected by serial sectioning in a FIB to characterize dislocation interaction with a grain boundary. Three dimensional high angular resolution cross correlation EBSD analysis was applied to an AlCu oligocrystal to measure dislocation densities around a grain boundary. Distortion derivatives associated with the plasma FIB serial sectioning were higher than expected, possibly due to geometric uncertainty between layers. Future work will focus on mitigating the geometric uncertainty and examining more regions of interest along the grain boundary to glean information on dislocation-grain boundary interaction.

Dislocation confinement in the growth of Na flux GaN on metalorganic chemical vapor deposition-GaN

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

Takeuchi, S., E-mail: takeuchi@ee.es.osaka-u.ac.jp; Asazu, H.; Nakamura, Y.

2015-12-28

We have demonstrated a GaN growth technique in the Na flux method to confine c-, (a+c)-, and a-type dislocations around the interface between a Na flux GaN crystal and a GaN layer grown by metalorganic chemical vapor deposition (MOCVD) on a (0001) sapphire substrate. Transmission electron microscopy (TEM) clearly revealed detailed interface structures and dislocation behaviors that reduced the density of vertically aligned dislocations threading to the Na flux GaN surface. Submicron-scale voids were formed at the interface above the dislocations with a c component in MOCVD-GaN, while no such voids were formed above the a-type dislocations. The penetration ofmore » the dislocations with a c component into Na flux GaN was, in most cases, effectively blocked by the presence of the voids. Although some dislocations with a c component in the MOCVD-GaN penetrated into the Na flux GaN, their propagation direction changed laterally through the voids. On the other hand, the a-type dislocations propagated laterally and collectively near the interface, when these dislocations in the MOCVD-GaN penetrated into the Na flux GaN. These results indicated that the dislocation propagation behavior was highly sensitive to the type of dislocation, but all types of dislocations were confined to within several micrometers region of the Na flux GaN from the interface. The cause of void formation, the role of voids in controlling the dislocation behavior, and the mechanism of lateral and collective dislocation propagation are discussed on the basis of TEM results.« less

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

NASA Astrophysics Data System (ADS)

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

2015-04-01

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

Ion channeling study of defects in compound crystals using Monte Carlo simulations

NASA Astrophysics Data System (ADS)

Turos, A.; Jozwik, P.; Nowicki, L.; Sathish, N.

2014-08-01

Ion channeling is a well-established technique for determination of structural properties of crystalline materials. Defect depth profiles have been usually determined basing on the two-beam model developed by Bøgh (1968) [1]. As long as the main research interest was focused on single element crystals it was considered as sufficiently accurate. New challenge emerged with growing technological importance of compound single crystals and epitaxial heterostructures. Overlap of partial spectra due to different sublattices and formation of complicated defect structures makes the two beam method hardly applicable. The solution is provided by Monte Carlo computer simulations. Our paper reviews principal aspects of this approach and the recent developments in the McChasy simulation code. The latter made it possible to distinguish between randomly displaced atoms (RDA) and extended defects (dislocations, loops, etc.). Hence, complex defect structures can be characterized by the relative content of these two components. The next refinement of the code consists of detailed parameterization of dislocations and dislocation loops. Defect profiles for variety of compound crystals (GaN, ZnO, SrTiO3) have been measured and evaluated using the McChasy code. Damage accumulation curves for RDA and extended defects revealed non monotonous defect buildup with some characteristic steps. Transition to each stage is governed by the different driving force. As shown by the complementary high resolution XRD measurements lattice strain plays here the crucial role and can be correlated with the concentration of extended defects.

Oscillating-Crucible Technique for Silicon Growth

NASA Technical Reports Server (NTRS)

Daud, T.; Dumas, K. A.; Kim, K. M.; Schwuttke, G. H.; Smetana, P.

1984-01-01

Technique yields better mixing of impurities and superior qualiity crystals. Accellerated motion stirs melt which reduces temperature gradients and decreases boundary layer for diffusion of impurities near growing surface. Results better mixing of impurities into melt, decrease in tendency for dendritic growth or cellular growth and crystals with low dislocation density. Applied with success to solution growth and Czochralski growth, resulting in large crystals of superior quality.

Discrete Dislocation Dynamics Simulations of Twin Size-Effects in Magnesium

DTIC Science & Technology

2015-01-01

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

Structural defects in bulk GaN

NASA Astrophysics Data System (ADS)

Liliental-Weber, Z.; dos Reis, R.; Mancuso, M.; Song, C. Y.; Grzegory, I.; Porowski, S.; Bockowski, M.

2014-10-01

Transmission Electron Microscopy (TEM) studies of undoped and Mg doped GaN layers grown on the HVPE substrates by High Nitrogen Pressure Solution (HNPS) with the multi-feed-seed (MFS) configuration are shown. The propagation of dislocations from the HVPE substrate to the layer is observed. Due to the interaction between these dislocations in the thick layers much lower density of these defects is observed in the upper part of the HNPS layers. Amorphous Ga precipitates with attached voids pointing toward the growth direction are observed in the undoped layer. This is similar to the presence of Ga precipitates in high-pressure platelets, however the shape of these precipitates is different. The Mg doped layers do not show Ga precipitates, but MgO rectangular precipitates are formed, decorating the dislocations. Results of TEM studies of HVPE layers grown on Ammonothermal substrates are also presented. These layers have superior crystal quality in comparison to the HNPS layers, as far as density of dislocation is concern. Occasionally some small inclusions can be found, but their chemical composition was not yet determined. It is expected that growth of the HNPS layers on these substrate will lead to large layer thickness obtained in a short time and with high crystal perfection needed in devices.

Domain configurations in dislocations embedded hexagonal manganite systems: From the view of graph theory

NASA Astrophysics Data System (ADS)

Cheng, Shaobo; Zhang, Dong; Deng, Shiqing; Li, Xing; Li, Jun; Tan, Guotai; Zhu, Yimei; Zhu, Jing

2018-04-01

Topological defects and their interactions often arouse multiple types of emerging phenomena from edge states in Skyrmions to disclination pairs in liquid crystals. In hexagonal manganites, partial edge dislocations, a prototype topological defect, are ubiquitous and they significantly alter the topologically protected domains and their behaviors. Herein, combining electron microscopy experiment and graph theory analysis, we report a systematic study of the connections and configurations of domains in this dislocation embedded system. Rules for domain arrangement are established. The dividing line between domains, which can be attributed by the strain field of dislocations, is accurately described by a genus model from a higher dimension in the graph theory. Our results open a door for the understanding of domain patterns in topologically protected multiferroic systems.

20 CFR 663.105 - When must adults and dislocated workers be registered?

Code of Federal Regulations, 2012 CFR

2012-04-01

... 20 Employees' Benefits 4 2012-04-01 2012-04-01 false When must adults and dislocated workers be... LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One-Stop Delivery System § 663.105 When must...

20 CFR 663.105 - When must adults and dislocated workers be registered?

Code of Federal Regulations, 2014 CFR

2014-04-01

... 20 Employees' Benefits 4 2014-04-01 2014-04-01 false When must adults and dislocated workers be... LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One-Stop Delivery System § 663.105 When must...

20 CFR 663.105 - When must adults and dislocated workers be registered?

Code of Federal Regulations, 2013 CFR

2013-04-01

... 20 Employees' Benefits 4 2013-04-01 2013-04-01 false When must adults and dislocated workers be... LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One-Stop Delivery System § 663.105 When must...

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

NASA Astrophysics Data System (ADS)

El-Awady, Jaafar Abbas

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

Role of fluttering dislocations in the thermal interface resistance between a silicon crystal and plastic solid 4He

NASA Astrophysics Data System (ADS)

Amrit, Jay; Ramiere, Aymeric; Volz, Sebastian

2018-01-01

A quantum solid (solid 4He) in contact with a classical solid defines a new class of interfaces. In addition to its quantum nature, solid 4He is indeed a very plastic medium. We examine the thermal interface resistance upon solidification of superfluid 4He in contact with a silicon crystal surface (111) and show that dislocations play a crucial role in the thermal interface transport. The growth of solid 4He and the measurements are conducted at the minimum of the melting curve of helium (0.778 K and ˜25 bar ). The results display a first-order transition in the Kapitza resistance from a value of RK ,L=(80 ±8 ) c m2K /W at a pressure of 24.5 bar to a value of RK ,S=(41.7 ±8 ) c m2K /W after the formation of solid helium at ˜25.2 bar . The drop in RK ,S is only of a factor of ˜2 , although transverse phonon modes in solid 4He now participate in heat transmission at the interface. We provide an explanation for the measured RK ,S by considering the interaction of thermal phonons with vibrating dislocations in solid 4He. We demonstrate that this mechanism, also called fluttering, induces a thermal resistance RF l∝NdT-6 , where T is the temperature and Nd is the density of dislocations. We estimate that for dislocation densities on the order of ˜107c m-2 , RF l predominates over the boundary resistance RK ,S. These fundamental findings shed light on the role of dislocations and provide a quantitative explanation for previous experiments which showed no measurable change in the Kapitza resistance between Cu and superfluid 4He upon solidification of the latter. This demonstrates the possibility of using dislocations as an additional means to tailor thermal resistances at interfaces, formed especially with a plastic material.

Subgrain boundary analyses in deformed orthopyroxene by TEM/STEM with EBSD-FIB sample preparation technique

NASA Astrophysics Data System (ADS)

Kogure, Toshihiro; Raimbourg, Hugues; Kumamoto, Akihito; Fujii, Eiko; Ikuhara, Yuichi

2014-12-01

High-resolution structure analyses using electron beam techniques have been performed for the investigation of subgrain boundaries (SGBs) in deformed orthopyroxene (Opx) in mylonite from Hidaka Metamorphic Belt, Hokkaido, Japan, to understand ductile deformation mechanism of silicate minerals in shear zones. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analysis of Opx porphyroclasts in the mylonitic rock indicated that the crystal orientation inside the Opx crystals gradually changes by rotation about the b-axis by SGBs and crystal folding. In order to observe the SGBs along the b-axis by transmission electron microscopy (TEM) or scanning TEM (STEM), the following sample preparation protocol was adopted. First, petrographic thin sections were slightly etched with hydrofluoric acid to identify SGBs in SEM. The Opx crystals whose b-axes were oriented close to the normal of the surface were identified by EBSD, and the areas containing SGBs were picked and thinned for (S) TEM analysis with a focused ion beam instrument with micro-sampling system. High-resolution TEM imaging of the SGBs in Opx revealed various boundary structures from a periodic array of dissociated (100) [001] edge dislocations to partially or completely incoherent crystals, depending on the misorientation angle. Atomic-resolution STEM imaging clearly confirmed the formation of clinopyroxene (Cpx) structure between the dissociated partial dislocations. Moreover, X-ray microanalysis in STEM revealed that the Cpx contains a considerable amount of calcium replacing iron. Such chemical inhomogeneity may limit glide motion of the dislocation and eventually the plastic deformation of the Opx porphyroclasts at a low temperature. Chemical profiles across the high-angle incoherent SGB also showed an enrichment of the latter in calcium at the boundary, suggesting that SGBs are an efficient diffusion pathway of calcium out of host Opx grain during cooling.

Homoepitaxial "Web Growth" of SiC to Terminate C-Axis Screw Dislocations and Enlarge Step-Free Surfaces

NASA Technical Reports Server (NTRS)

Neudeck, Philip G.; Powell, J. Anthony; Trunek, Andrew; Spry, David; Beheim, Glenn M.; Benavage, Emye; Abel, Phillip; Vetter, William M.; Dudley, Michael

2001-01-01

Homoepitaxial CVD growth of thin lateral cantilevers emanating from the edges of mesa patterns dry-etched into on-axis commercial 4H-SiC substrates prior to growth is reported. Cantilevers on the order of a micrometer thick extending tens of micrometers from the edge of a mesa have been grown. The termination of vertically propagating screw dislocations, including a micropipe, that are overgrown by the cantilevers has been demonstrated, in large part because the crystal structure of the cantilevers is established laterally from the mesa sidewalls. This technique could help reduce performance-degrading dislocations in SiC electrical devices.

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

NASA Astrophysics Data System (ADS)

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

2017-09-01

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

Controllable Growth and Formation Mechanisms of Dislocated WS2 Spirals.

PubMed

Fan, Xiaopeng; Zhao, Yuzhou; Zheng, Weihao; Li, Honglai; Wu, Xueping; Hu, Xuelu; Zhang, Xuehong; Zhu, Xiaoli; Zhang, Qinglin; Wang, Xiao; Yang, Bin; Chen, Jianghua; Jin, Song; Pan, Anlian

2018-06-13

Two-dimensional (2D) layered metal dichalcogenides can form spiral nanostructures by a screw-dislocation-driven mechanism, which leads to changes in crystal symmetry and layer stackings that introduce attractive physical properties different from their bulk and few-layer nanostructures. However, controllable growth of spirals is challenging and their growth mechanisms are poorly understood. Here, we report the controllable growth of WS 2 spiral nanoplates with different stackings by a vapor phase deposition route and investigate their formation mechanisms by combining atomic force microscopy with second harmonic generation imaging. Previously not observed "spiral arm" features could be explained as covered dislocation spiral steps, and the number of spiral arms correlates with the number of screw dislocations initiated at the bottom plane. The supersaturation-dependent growth can generate new screw dislocations from the existing layers, or even new layers templated by existing screw dislocations. Different number of dislocations and orientation of new layers result in distinct morphologies, different layer stackings, and more complex nanostructures, such as triangular spiral nanoplates with hexagonal spiral pattern on top. This work provides the understanding and control of dislocation-driven growth of 2D nanostructures. These spiral nanostructures offer diverse candidates for probing the physical properties of layered materials and exploring new applications in functional nanoelectronic and optoelectronic devices.

Molecular dynamics simulation on the elastoplastic properties of copper nanowire under torsion

NASA Astrophysics Data System (ADS)

Yang, Yong; Li, Ying; Yang, Zailin; Zhang, Guowei; Wang, Xizhi; Liu, Jin

2018-02-01

Influences of different factors on the torsion properties of single crystal copper nanowire are studied by molecular dynamics method. The length, torsional rate, and temperature of the nanowire are discussed at the elastic-plastic critical point. According to the average potential energy curve and shear stress curve, the elastic-plastic critical angle is determined. Also, the dislocation at elastoplastic critical points is analyzed. The simulation results show that the single crystal copper nanowire can be strengthened by lengthening the model, decreasing the torsional rate, and lowering the temperature. Moreover, atoms move violently and dislocation is more likely to occur with a higher temperature. This work mainly describes the mechanical behavior of the model under different states.

Creep substructure formation in sodium chloride single crystals in the power law and exponential creep regimes

NASA Technical Reports Server (NTRS)

Raj, S. V.; Pharr, G. M.

1989-01-01

Creep tests conducted on NaCl single crystals in the temperature range from 373 to 1023 K show that true steady state creep is obtained only above 873 K when the ratio of the applied stress to the shear modulus is less than or equal to 0.0001. Under other stress and temperature conditions, corresponding to both power law and exponential creep, the creep rate decreases monotonically with increasing strain. The transition from power law to exponential creep is shown to be associated with increases in the dislocation density, the cell boundary width, and the aspect ratio of the subgrains along the primary slip planes. The relation between dislocation structure and creep behavior is also assessed.

Crystal plasticity modeling of irradiation growth in Zircaloy-2

DOE PAGES

Patra, Anirban; Tome, Carlos; Golubov, Stanislav I.

2017-05-10

A reaction-diffusion based mean field rate theory model is implemented in the viscoplastic self-consistent (VPSC) crystal plasticity framework to simulate irradiation growth in hcp Zr and its alloys. A novel scheme is proposed to model the evolution (both number density and radius) of irradiation-induced dislocation loops that can be informed directly from experimental data of dislocation density evolution during irradiation. This framework is used to predict the irradiation growth behavior of cold-worked Zircaloy-2 and trends compared to available experimental data. The role of internal stresses in inducing irradiation creep is discussed. Effects of grain size, texture, and external stress onmore » the coupled irradiation growth and creep behavior are also studied.« less

Crystal plasticity modeling of irradiation growth in Zircaloy-2

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

Patra, Anirban; Tome, Carlos; Golubov, Stanislav I.

A reaction-diffusion based mean field rate theory model is implemented in the viscoplastic self-consistent (VPSC) crystal plasticity framework to simulate irradiation growth in hcp Zr and its alloys. A novel scheme is proposed to model the evolution (both number density and radius) of irradiation-induced dislocation loops that can be informed directly from experimental data of dislocation density evolution during irradiation. This framework is used to predict the irradiation growth behavior of cold-worked Zircaloy-2 and trends compared to available experimental data. The role of internal stresses in inducing irradiation creep is discussed. Effects of grain size, texture, and external stress onmore » the coupled irradiation growth and creep behavior are also studied.« less

20 CFR 663.110 - What are the eligibility criteria for core services for adults in the adult and dislocated worker...

Code of Federal Regulations, 2013 CFR

2013-04-01

... services for adults in the adult and dislocated worker programs? 663.110 Section 663.110 Employees' Benefits EMPLOYMENT AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker...

20 CFR 663.110 - What are the eligibility criteria for core services for adults in the adult and dislocated worker...

Code of Federal Regulations, 2012 CFR

2012-04-01

... services for adults in the adult and dislocated worker programs? 663.110 Section 663.110 Employees' Benefits EMPLOYMENT AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker...

20 CFR 663.110 - What are the eligibility criteria for core services for adults in the adult and dislocated worker...

Code of Federal Regulations, 2014 CFR

2014-04-01

... services for adults in the adult and dislocated worker programs? 663.110 Section 663.110 Employees' Benefits EMPLOYMENT AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker...

X-Ray Diffraction and Imaging Study of Imperfections of Crystallized Lysozyme with Coherent X-Rays

NASA Technical Reports Server (NTRS)

Hu, Zheng-Wei; Chu, Y. S.; Lai, B.; Cai, Z.; Thomas, B. R.; Chernov, A. A.

2003-01-01

Phase-sensitive x-ray diffraction imaging and high angular-resolution diffraction combined with phase contrast radiographic imaging are employed to characterize defects and perfection of a uniformly grown tetragonal lysozyme crystal in symmetric Laue case. The fill width at half-maximum (FWHM) of a 4 4 0 rocking curve measured from the original crystal is approximately 16.7 arcseconds, and defects, which include point defects, line defects, and microscopic domains, have been clearly observed in the diffraction images of the crystal. The observed line defects carry distinct dislocation features running approximately along the <110> growth front, and they have been found to originate mostly at a central growth area and occasionally at outer growth regions. Individual point defects trapped at a crystal nucleus are resolved in the images of high sensitivity to defects. Slow dehydration has led to the broadening of the 4 4 0 rocking curve by a factor of approximately 2.4. A significant change of the defect structure and configuration with drying has been revealed, which suggests the dehydration induced migration and evolution of dislocations and lattice rearrangements to reduce overall strain energy. The sufficient details of the observed defects shed light upon perfection, nucleation and growth, and properties of protein crystals.

Numerical simulation of thermal stress distributions in Czochralski-grown silicon crystals

NASA Astrophysics Data System (ADS)

Kumar, M. Avinash; Srinivasan, M.; Ramasamy, P.

2018-04-01

Numerical simulation is one of the important tools in the investigation and optimization of the single-crystal silicon grown by the Czochralski (Cz) method. A 2D steady global heat transfer model was used to investigate the temperature distribution and the thermal stress distributions at particular crystal position during the Cz growth process. The computation determines the thermal stress such as von Mises stress and maximum shear stress distribution along grown crystal and shows possible reason for dislocation formation in the Cz-grown single-crystal silicon.

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

NASA Astrophysics Data System (ADS)

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

2015-08-01

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

Advances in Discrete Dislocation Dynamics Modeling of Size-Affected Plasticity

NASA Astrophysics Data System (ADS)

El-Awady, Jaafar A.; Fan, Haidong; Hussein, Ahmed M.

In dislocation-mediated plasticity of crystalline materials, discrete dislocation dynamics (DDD) methods have been widely used to predict the plastic deformation in a number of technologically important problems. These simulations have led to significant improvement in the understanding of the different mechanism that controls the mechanical properties of crystalline materials, which can greatly accelerate the future development of materials with superior properties. This chapter provides an overview of different practical applications of both two-dimensional and three-dimensional DDD simulations in the field of size-affected dislocation-mediated plasticity. The chapter is divided into two major tracks. First, DDD simulations focusing on aspects of modeling size-dependent plasticity in single crystals in uniaxial micro-compression/tension, microtorsion, microbending, and nanoindentation are discussed. Special attention is directed towards the role of cross-slip and dislocation nucleation on the overall response. Second, DDD simulations focusing on the role of interfaces, including grain and twin boundaries, on dislocation-mediated plasticity are discussed. Finally, a number of challenges that are withholding DDD simulations from reaching their full potential are discussed.

20 CFR 663.120 - Are displaced homemakers eligible for dislocated worker activities under WIA?

Code of Federal Regulations, 2012 CFR

2012-04-01

... ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One-Stop Delivery... the adult program. ...

20 CFR 663.120 - Are displaced homemakers eligible for dislocated worker activities under WIA?

Code of Federal Regulations, 2013 CFR

2013-04-01

... ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One-Stop Delivery... the adult program. ...

20 CFR 663.120 - Are displaced homemakers eligible for dislocated worker activities under WIA?

Code of Federal Regulations, 2014 CFR

2014-04-01

... ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One-Stop Delivery... the adult program. ...

The growth and perfection of β-cyclotetramethylene-tetranitramine (HMX) studied by laboratory and synchrotron X-ray topography

NASA Astrophysics Data System (ADS)

Gallagher, H. G.; Sherwood, J. N.; Vrcelj, R. M.

2017-10-01

An examination has been made of the defect structure of crystals of the energetic material β-cyclotetramethylene-tetranitramine (HMX) using both Laboratory (Lang method) and Synchrotron (Bragg Reflection and Laue method) techniques. The results of the three methods are compared with particular attention to the influence of potential radiation damage caused to the samples by the latter, more energetic, technique. The comparison shows that both techniques can be confidently used to evaluate the defect structures yielding closely similar results. The results show that, even under the relatively casual preparative methods used (slow evaporation of unstirred solutions at constant temperature), HMX crystals of high perfection can be produced. The crystals show well defined bulk defect structures characteristic of organic materials in general: growth dislocations, twins, growth sector boundaries, growth banding and solvent inclusions. The distribution of the defects in specific samples is correlated with the morphological variation of the grown crystals. The results show promise for the further evaluation and characterisation of the structure and properties of dislocations and other defects and their involvement in mechanical and energetic processes in this material.

Understanding Solidification Based Grain Refinement in Steels

DTIC Science & Technology

2014-12-18

sulfide.^^"^^ Another approach would be to react a sample of misch metal or rare earth silicide at elevated temperatures to form the desired oxide or...dislocation can travel through a metal crystal before being blocked by a grain boundary. Since the dislocation is impeded sooner, the material cannot...in the melt; 3) be wetted by the liquid metal ; and 4) have a similar crystallographic structure to the host metal . Using reference data and

Athermal Mechanisms of Size-Dependent Crystal Flow Gleaned from Three-Dimensional Discrete Dislocation Simulations

DTIC Science & Technology

2008-01-01

Sevillano, et. al . directly developed a definition of the single-slip glide-resistance correlation length, 10//5.8 o  , from 2d simulations of single...dislocation percolation through point-obstacle fields [35, 36]. Subsequently, predictions for the size- dependence of * from Gil Sevillano, et. al ...Sevillano, et. al ., and microcrystal deformation experiments was perhaps fortuitous, significant merit remains within their treatment. The more general

III-V compound semiconductor material characterization of microstructures and nanostructures on various optoelectronic devices with analytical transmission electron microscopy and high resolution electron microscopy

NASA Astrophysics Data System (ADS)

Zhou, Wei

Analytical Transmission Electron Microscopy (TEM) and High Resolution Electron Microscopy have been carried out to characterize microstructures and nanostructures in various III-V compound semiconductor devices by metalorganic chemical vapor deposition (MOCVD). The low-defect GaN nonplanar templates by lateral epitaxial overgrowth (LEO) has a trapezoidal cross-section with smooth (0001) and {112¯2} facets. Penetration of threading dislocations (TDs) beyond mask windows is observed in ordinary LEO substrates. In two-step LEO substrates, where TDs are engineered to bend 90° in the TD bending layer after the first LEO step, only perfect a-type dislocations with Burgers vector b = 1/3 <112¯0> are generated in the upper Post-bending layer with a density of ˜8 x 107cm-2. The demonstrated 3-dimensional dislocation spatial distribution in the LEO nonplanar substrate substantiates the dislocation reaction mechanism. Al0.07GaN/GaN superlattice can further decrease dislocations. InGaN QW thickness enhancement on top of GaN nonplanar templates has been verified to influence the optoelectronic properties significantly. Dense arrays of hexagonally ordered MOCVD-grown (In)(Ga)As nano-QDs by block copolymer nanolithography & selective area growth (SAG), approximately 20nm in diameter and 40nm apart with a density of 1011/cm 2, are perfect crystals by TEM. V-shaped defects and worse InAs growth uniformity have been observed in multiple layers of vertically coupled self-assembled InAs nanostructure arrays on strain-modulated GaAs substrates. TEM shows a smooth coalesced GaN surface with a thickness as thin as ˜200nm after Nano-LEO and a defect reduction of 70%-75%. The (In)GaAs 20 nm twist bonded compliant substrates have almost no compliant effect and higher dislocation density, but the 10nm compliant substrates are on the contrary. A 60nm oxygen-infiltrated crystallized transition layer is observed between the amorphous oxidized layer and the crystallized unoxidized aperture in Al xGa1-xAs wet lateral oxidation, potentially influencing the current confinement characteristic of the sub-micron oxide aperture. Almost no dislocation is aroused by the wet lateral oxidation of In0.52Al 0.48As in the InP microresonator waveguides. XTEM was performed to compare InP SAG regions with 10˜50mum masks, which shows the performance deterioration of laser threshold current densities in the case of 50mum mask results from high density of dislocations induced from the highly strained QW structures caused by the high enhancements.

A scanning defect mapping system for semiconductor characterization

NASA Technical Reports Server (NTRS)

Sopori, Bushnan L.

1994-01-01

We have developed an optical scanning system that generates maps of the spatial distributions of defects in single and polycrystalline silicon wafers. This instrument, called Scanning Defect Mapping System, utilizes differences in the scattering characteristics of dislocation etch pits and grain boundaries from a defect-etched sample to identify and count them. This system simultaneously operates in the dislocation mode and the grain boundary (GB) mode. In the 'dislocation mode,' the optical scattering from the etch pits is used to statistically count dislocations, while ignoring the GB's. Likewise, in the 'grain boundary mode' the system only recognizes the local scattering from the GB's to generate grain boundary distributions. The information generated by this instrument is valuable for material quality control, identifying mechanisms of defect generation and the nature of thermal stresses during the crystal growth, and the solar cell process design.

Domain configurations in dislocations embedded hexagonal manganite systems: From the view of graph theory

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

Cheng, Shaobo; Zhang, Dong; Deng, Shiqing

Topological defects and their interactions often arouse multiple types of emerging phenomena from edge states in Skyrmions to disclination pairs in liquid crystals. In hexagonal manganites, partial edge dislocations, a prototype topological defect, are ubiquitous and they significantly alter the topologically protected domains and their behaviors. In this work, combining electron microscopy experiment and graph theory analysis, we report a systematic study of the connections and configurations of domains in this dislocation embedded system. Rules for domain arrangement are established. The dividing line between domains, which can be attributed by the strain field of dislocations, is accurately described by amore » genus model from a higher dimension in the graph theory. In conclusion, our results open a door for the understanding of domain patterns in topologically protected multiferroic systems.« less

Domain configurations in dislocations embedded hexagonal manganite systems: From the view of graph theory

DOE PAGES

Cheng, Shaobo; Zhang, Dong; Deng, Shiqing; ...

2018-04-19

Topological defects and their interactions often arouse multiple types of emerging phenomena from edge states in Skyrmions to disclination pairs in liquid crystals. In hexagonal manganites, partial edge dislocations, a prototype topological defect, are ubiquitous and they significantly alter the topologically protected domains and their behaviors. In this work, combining electron microscopy experiment and graph theory analysis, we report a systematic study of the connections and configurations of domains in this dislocation embedded system. Rules for domain arrangement are established. The dividing line between domains, which can be attributed by the strain field of dislocations, is accurately described by amore » genus model from a higher dimension in the graph theory. In conclusion, our results open a door for the understanding of domain patterns in topologically protected multiferroic systems.« less

Formation mechanism of fivefold deformation twins in a face-centered cubic alloy.

PubMed

Zhang, Zhenyu; Huang, Siling; Chen, Leilei; Zhu, Zhanwei; Guo, Dongming

2017-03-28

The formation mechanism considers fivefold deformation twins originating from the grain boundaries in a nanocrystalline material, resulting in that fivefold deformation twins derived from a single crystal have not been reported by molecular dynamics simulations. In this study, fivefold deformation twins are observed in a single crystal of face-centered cubic (fcc) alloy. A new formation mechanism is proposed for fivefold deformation twins in a single crystal. A partial dislocation is emitted from the incoherent twin boundaries (ITBs) with high energy, generating a stacking fault along {111} plane, and resulting in the nucleating and growing of a twin by the successive emission of partials. A node is fixed at the intersecting center of the four different slip {111} planes. With increasing stress under the indentation, ITBs come into being close to the node, leading to the emission of a partial from the node. This generates a stacking fault along a {111} plane, nucleating and growing a twin by the continuous emission of the partials. This process repeats until the formation of fivefold deformation twins.

Formation mechanism of fivefold deformation twins in a face-centered cubic alloy

NASA Astrophysics Data System (ADS)

Zhang, Zhenyu; Huang, Siling; Chen, Leilei; Zhu, Zhanwei; Guo, Dongming

2017-03-01

The formation mechanism considers fivefold deformation twins originating from the grain boundaries in a nanocrystalline material, resulting in that fivefold deformation twins derived from a single crystal have not been reported by molecular dynamics simulations. In this study, fivefold deformation twins are observed in a single crystal of face-centered cubic (fcc) alloy. A new formation mechanism is proposed for fivefold deformation twins in a single crystal. A partial dislocation is emitted from the incoherent twin boundaries (ITBs) with high energy, generating a stacking fault along {111} plane, and resulting in the nucleating and growing of a twin by the successive emission of partials. A node is fixed at the intersecting center of the four different slip {111} planes. With increasing stress under the indentation, ITBs come into being close to the node, leading to the emission of a partial from the node. This generates a stacking fault along a {111} plane, nucleating and growing a twin by the continuous emission of the partials. This process repeats until the formation of fivefold deformation twins.

Dislocation blocking by AlGaN hot electron injecting layer in the epitaxial growth of GaN terahertz Gunn diode

NASA Astrophysics Data System (ADS)

Li, Liang; Yang, Lin'an; Zhang, Jincheng; Hao, Yue

2013-09-01

This paper reports an efficient method to improve the crystal quality of GaN Gunn diode with AlGaN hot electron injecting layer (HEI). An evident reduction of screw dislocation and edge dislocation densities is achieved by the strain management and the enhanced lateral growth in high temperature grown AlGaN HEI layer. Compared with the top hot electron injecting layer (THEI) structure, the bottom hot electron injecting layer (BHEI) structure enhances the crystal quality of transit region due to the growth sequence modulation of HEI layer. A high Hall mobility of 2934 cm2/Vs at 77 K, a nearly flat downtrend of Hall mobility at the temperature ranging from 300 to 573 K, a low intensity of ratio of yellow luminescence band to band edge emission, a narrow band edge emission line-width, and a smooth surface morphology are observed for the BHEI structural epitaxy of Gunn diode, which indicates that AlGaN BHEI structure is a promising candidate for fabrication of GaN Gunn diodes in terahertz regime.

3D Imaging of a Dislocation Loop at the Onset of Plasticity in an Indented Nanocrystal.

PubMed

Dupraz, M; Beutier, G; Cornelius, T W; Parry, G; Ren, Z; Labat, S; Richard, M-I; Chahine, G A; Kovalenko, O; De Boissieu, M; Rabkin, E; Verdier, M; Thomas, O

2017-11-08

Structural quality and stability of nanocrystals are fundamental problems that bear important consequences for the performances of small-scale devices. Indeed, at the nanoscale, their functional properties are largely influenced by elastic strain and depend critically on the presence of crystal defects. It is thus of prime importance to be able to monitor, by noninvasive means, the stability of the microstructure of nano-objects against external stimuli such as mechanical load. Here we demonstrate the potential of Bragg coherent diffraction imaging for such measurements, by imaging in 3D the evolution of the microstructure of a nanocrystal exposed to in situ mechanical loading. Not only could we observe the evolution of the internal strain field after successive loadings, but we also evidenced a transient microstructure hosting a stable dislocation loop. The latter is fully characterized from its characteristic displacement field. The mechanical behavior of this small crystal is clearly at odds with what happens in bulk materials where many dislocations interact. Moreover, this original in situ experiment opens interesting possibilities for the investigation of plastic deformation at the nanoscale.

The core structure and recombination energy of a copper screw dislocation: a Peierls study

NASA Astrophysics Data System (ADS)

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

2017-09-01

The recombination process of dislocations is central to cross-slip, and transmission through ?3 grain boundaries among other fundamental plastic deformation processes. Despite its importance, a detailed mechanistic understanding remains lacking. We apply a continuous dislocation model, inspired by Peierls and Nabarro, complete with an ab-initio computed ?-surface and continuous units of infinitesimal dislocation slip, towards computing the stress-dependent recombination path of both an isotropic and anisotropic Cu screw dislocation. Under no applied stress, our model reproduces the stacking fault width between Shockley partial dislocations as predicted by discrete linear elasticity. Upon application of a compressive Escaig stress, the two partial dislocations coalesce to a separation of ??. Upon increased loading the edge components of each partial dislocation recede, leaving behind a spread Peierls screw dislocation, indicating the recombined state. We demonstrate that the critical stress required to achieve the recombined state is independent of the shear modulus. Rather the critical recombination stress depends on an energy difference between an unstable fault energy (?) and the intrinsic stacking fault energy (?-?). We report recombination energies of ?W = 0.168 eV/Å and ?W = 0.084 eV/Å, respectively, for the Cu screw dislocation within isotropic and anisotropic media. We develop an analytic model which provides insight into our simulation results which compare favourably with other (similar) models.

20 CFR 663.100 - What is the role of the adult and dislocated worker programs in the One-Stop delivery system?

Code of Federal Regulations, 2012 CFR

2012-04-01

... 20 Employees' Benefits 4 2012-04-01 2012-04-01 false What is the role of the adult and dislocated... AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through...

20 CFR 663.100 - What is the role of the adult and dislocated worker programs in the One-Stop delivery system?

Code of Federal Regulations, 2014 CFR

2014-04-01

... 20 Employees' Benefits 4 2014-04-01 2014-04-01 false What is the role of the adult and dislocated... AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through...

20 CFR 663.100 - What is the role of the adult and dislocated worker programs in the One-Stop delivery system?

Code of Federal Regulations, 2013 CFR

2013-04-01

... 20 Employees' Benefits 4 2013-04-01 2013-04-01 false What is the role of the adult and dislocated... AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through...

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

NASA Astrophysics Data System (ADS)

Yang, Hui; Li, Zhenhuan; Huang, Minsheng

2014-12-01

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

The barrier to misfit dislocation glide in continuous, strained, epitaxial layers on patterned substrates

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

Watson, G.P.; Ast, D.G.; Anderson, T.J.

1993-09-01

In a previous report [G. P. Watson, D. G. Ast, T. J. Anderson, and Y. Hayakawa, Appl. Phys. Lett. [bold 58], 2517 (1991)] we demonstrated that the motion of misfit dislocations in InGaAs, grown by organometallic vapor phase epitaxy on patterned GaAs substrates, can be impeded even if the strained epitaxial layer is continuous. Trenches etched into GaAs before growth are known to act as a barrier to misfit dislocation propagation [E. A. Fitzgerald, G. P. Watson, R. E. Proano, D. G. Ast, P. D. Kirchner, G. D. Pettit, and J. M. Woodall, J. Appl. Phys. [bold 65], 2220 (1989)]more » when those trenches create discontinuities in the epitaxial layers; but even shallow trenches, with continuous strained layers following the surface features, can act as barriers. By considering the strain energy required to change the length of the dislocation glide segments that stretch from the interface to the free surface, a simple model is developed that explains the major features of the unique blocking action observed at the trench edges. The trench wall angle is found to be an important parameter in determining whether or not a trench will block dislocation glide. The predicted blocking angles are consistent with observations made on continuous 300 and 600 nm thick In[sub 0.04]Ga[sub 0.96]As films on patterned GaAs. Based on the model, a structure is proposed that may be used as a filter to yield misfit dislocations with identical Burgers vectors or dislocations which slip in only one glide plane.« less

Brittle to Semibrittle Transition in Quartz Sandstone: Energetics

NASA Astrophysics Data System (ADS)

Kanaya, Taka; Hirth, Greg

2018-01-01

Triaxial compression experiments were conducted on a quartz sandstone at effective pressures up to 175 MPa and temperatures up to 900°C. Our experiments show a transition from brittle faulting to semibrittle faulting with an increase in both pressure and temperature. The yield behavior of samples deformed in the semibrittle regime follows a compactant elliptical cap at low strain, but evolves to a dilatant Mohr-Coulomb relationship with continued compaction. Optical microscopy indicates that semibrittle deformation involves cataclastic flow through shear-enhanced compaction and grain crushing; however, transmission electron microscopy shows evidence for dislocation glide in limited portions of samples. To constrain the relative contribution of brittle and crystal plastic mechanisms, we estimate the partitioning of the inelastic work into the dissipation energy for microcracking, intergranular frictional slip, and dislocation glide. We conclude that semibrittle deformation is accommodated primarily by cataclastic mechanisms, with only a limited contribution from crystal plasticity. Mechanical data, acoustic emission records, and analysis of surface energy all indicate the activation of subcritical cracking at elevated temperature. Hence, we infer that the enhancement of subcritical cracking is responsible for the transition to semibrittle flow through promoting distributed grain-scale fractures and millimeter-scale shear bands. Subcritical cracking promotes the nucleation of microfractures at lower stresses, and the resulting decrease in flow stress retards the propagation of transgranular microfractures. Our study illuminates the important role of temperature on the micromechanics of the transition from brittle faulting to cataclastic flow in the Earth.

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

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

Li, Tianlei; Gao, Yanfei; Bei, Hongbin

2011-01-01

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

Theory of electron–phonon–dislon interacting system—toward a quantized theory of dislocations

DOE PAGES

Li, Mingda; Tsurimaki, Yoichiro; Meng, Qingping; ...

2018-02-05

In this paper, we provide a comprehensive theoretical framework to study how crystal dislocations influence the functional properties of materials, based on the idea of a quantized dislocation, namely a 'dislon'. In contrast to previous work on dislons which focused on exotic phenomenology, here we focus on their theoretical structure and computational power. We first provide a pedagogical introduction that explains the necessity and benefits of taking the dislon approach and why the dislon Hamiltonian takes its current form. Then, we study the electron–dislocation and phonon–dislocation scattering problems using the dislon formalism. Both the effective electron and phonon theories aremore » derived, from which the role of dislocations on electronic and phononic transport properties is computed. Compared with traditional dislocation scattering studies, which are intrinsically single-particle, low-order perturbation and classical quenched defect in nature, the dislon theory not only allows easy incorporation of quantum many-body effects such as electron correlation, electron–phonon interaction, and higher-order scattering events, but also allows proper consideration of the dislocation's long-range strain field and dynamic aspects on equal footing for arbitrary types of straight-line dislocations. This means that instead of developing individual models for specific dislocation scattering problems, the dislon theory allows for the calculation of electronic structure and electrical transport, thermal transport, optical and superconducting properties, etc, under one unified theory. Furthermore, the dislon theory has another advantage over empirical models in that it requires no fitting parameters. The dislon theory could serve as a major computational tool to understand the role of dislocations on multiple materials' functional properties at an unprecedented level of clarity, and may have wide applications in dislocated energy materials.« less

Theory of electron–phonon–dislon interacting system—toward a quantized theory of dislocations

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

Li, Mingda; Tsurimaki, Yoichiro; Meng, Qingping

In this paper, we provide a comprehensive theoretical framework to study how crystal dislocations influence the functional properties of materials, based on the idea of a quantized dislocation, namely a 'dislon'. In contrast to previous work on dislons which focused on exotic phenomenology, here we focus on their theoretical structure and computational power. We first provide a pedagogical introduction that explains the necessity and benefits of taking the dislon approach and why the dislon Hamiltonian takes its current form. Then, we study the electron–dislocation and phonon–dislocation scattering problems using the dislon formalism. Both the effective electron and phonon theories aremore » derived, from which the role of dislocations on electronic and phononic transport properties is computed. Compared with traditional dislocation scattering studies, which are intrinsically single-particle, low-order perturbation and classical quenched defect in nature, the dislon theory not only allows easy incorporation of quantum many-body effects such as electron correlation, electron–phonon interaction, and higher-order scattering events, but also allows proper consideration of the dislocation's long-range strain field and dynamic aspects on equal footing for arbitrary types of straight-line dislocations. This means that instead of developing individual models for specific dislocation scattering problems, the dislon theory allows for the calculation of electronic structure and electrical transport, thermal transport, optical and superconducting properties, etc, under one unified theory. Furthermore, the dislon theory has another advantage over empirical models in that it requires no fitting parameters. The dislon theory could serve as a major computational tool to understand the role of dislocations on multiple materials' functional properties at an unprecedented level of clarity, and may have wide applications in dislocated energy materials.« less

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

NASA Astrophysics Data System (ADS)

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

2007-06-01

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

Recombination activity of nickel, copper, and oxygen atoms segregating at grain boundaries in mono-like silicon crystals

NASA Astrophysics Data System (ADS)

Ohno, Yutaka; Kutsukake, Kentaro; Deura, Momoko; Yonenaga, Ichiro; Shimizu, Yasuo; Ebisawa, Naoki; Inoue, Koji; Nagai, Yasuyoshi; Yoshida, Hideto; Takeda, Seiji

2016-10-01

Three-dimensional distribution of impurity atoms was determined at functional Σ5{013} and small-angle grain boundaries (GBs) in as-grown mono-like silicon crystals by atom probe tomography combined with transmission electron microscopy, and it was correlated with the recombination activity of those GBs, CGB, revealed by photoluminescence imaging. Nickel (Ni), copper (Cu), and oxygen atoms preferentially segregated at the GBs on which arrays of dislocations existed, while those atoms scarcely segregated at Σ5{013} GBs free from dislocations. Silicides containing Ni and Cu about 5 nm in size and oxides about 1 nm in size were formed along the dislocation arrays on those GBs. The number of segregating impurity atoms per unit GB area for Ni and that for Cu, NNi and NCu, were in a trade-off correlation with that for oxygen, NO, as a function of CGB, while the sum of those numbers was almost constant irrespective of the GB character, CGB, and the dislocation density on GBs. CGB would be explained as a linear combination of those numbers: CGB (in %) ˜400(0.38NO + NNi + NCu) (in atoms/nm2). The GB segregation of oxygen atoms would be better for solar cells, rather than that of metal impurities, from a viewpoint of the conversion efficiency of solar cells.

Investigation of mechanical properties and deformation behavior of single-crystal Al-Cu core-shell nanowire generated using non-equilibrium molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Sarkar, Jit

2018-06-01

Molecular dynamics (MD) simulation studies were carried out to generate a cylindrical single-crystal Al-Cu core-shell nanowire and its mechanical properties like yield strength and Young's modulus were evaluated in comparison to a solid aluminum nanowire and hollow copper nanowire which combines to constitute the core-shell structure respectively. The deformation behavior due to changes in the number of Wigner-Seitz defects and dislocations during the entire tensile deformation process was thoroughly studied for the Al-Cu core-shell nanowire. The single-crystal Al-Cu core-shell nanowire shows much higher yield strength and Young's modulus in comparison to the solid aluminum core and hollow copper shell nanowire due to tangling of dislocations caused by lattice mismatch between aluminum and copper. Thus, the Al-Cu core-shell nanowire can be reinforced in different bulk matrix to develop new type of light-weight nanocomposite materials with greatly enhanced material properties.

Deformation and fracture of single-crystal and sintered polycrystalline silicon carbide produced by cavitation

NASA Technical Reports Server (NTRS)

Miyoshi, Kazuhisa; Hattori, Shuji; Okada, Tsunenori; Buckley, Donald H.

1987-01-01

An investigation was conducted to examine the deformation and fracture behavior of single-crystal and sintered polycrystalline SiC surfaces exposed to cavitation. Cavitation erosion experiments were conducted in distilled water at 25 C by using a magnetostrictive oscillator in close proximity (1 mm) to the surface of SiC. The horn frequency was 20 kHz, and the double amplitude of the vibrating disk was 50 microns. The results of the investigation indicate that the SiC (0001) surface could be deformed in a plastic manner during cavitation. Dislocation etch pits were formed when the surface was chemically etched. The number of defects, including dislocations in the SiC (0001) surface, increased with increasing exposure time to cavitation. The presence of intrinsic defects such as voids in the surficial layers of the sintered polycrystalline SiC determined the zones at which fractured grains and fracture pits (pores) were generated. Single-crystal SiC had superior erosion resistance to that of sintered polycrystalline SiC.

Deformation and fracture of single-crystal and sintered polycrystalline silicon carbide produced by cavitation

NASA Technical Reports Server (NTRS)

Miyoshi, Kazuhisa; Hattori, Shuji; Okada, Tsunenori; Buckley, Donald H.

1989-01-01

An investigation was conducted to examine the deformation and fracture behavior of single-crystal and sintered polycrystalline SiC surfaces exposed to cavitation. Cavitation erosion experiments were conducted in distilled water at 25 C by using a magnetostrictive oscillator in close proximity (1 mm) to the surface of SiC. The horn frequency was 20 kHz, and the double amplitude of the vibrating disk was 50 microns. The results of the investigation indicate that the SiC (0001) surface could be deformed in a plastic manner during cavitation. Dislocation etch pits were formed when the surface was chemically etched. The number of defects, including dislocations in SiC (0001) surface, increased with increasing exposure time to cavitation. The presence of intrinsic defects such as voids in the surficial layers of the sintered polycrystalline SiC determined the zones at which fractured grains and fracture pits (pores) were generated. Single-crystal SiC had superior erosion resistance to that of sintered polycrystalline SiC.

Crystal plasticity modeling of irradiation growth in Zircaloy-2

NASA Astrophysics Data System (ADS)

Patra, Anirban; Tomé, Carlos N.; Golubov, Stanislav I.

2017-08-01

A physically based reaction-diffusion model is implemented in the visco-plastic self-consistent (VPSC) crystal plasticity framework to simulate irradiation growth in hcp Zr and its alloys. The reaction-diffusion model accounts for the defects produced by the cascade of displaced atoms, their diffusion to lattice sinks and the contribution to crystallographic strain at the level of single crystals. The VPSC framework accounts for intergranular interactions and irradiation creep, and calculates the strain in the polycrystalline ensemble. A novel scheme is proposed to model the simultaneous evolution of both, number density and radius, of irradiation-induced dislocation loops directly from experimental data of dislocation density evolution during irradiation. This framework is used to predict the irradiation growth behaviour of cold-worked Zircaloy-2 and trends compared to available experimental data. The role of internal stresses in inducing irradiation creep is discussed. Effects of grain size, texture and external stress on the coupled irradiation growth and creep behaviour are also studied and compared with available experimental data.

Defect analysis of the LED structure deposited on the sapphire substrate

NASA Astrophysics Data System (ADS)

Nie, Qichu; Jiang, Zhimin; Gan, Zhiyin; Liu, Sheng; Yan, Han; Fang, Haisheng

2018-04-01

Transmission electron microscope (TEM) and double-crystal X-ray diffraction (DCXRD) measurements have been performed to investigate dislocations of the whole structure of the LED layers deposited on both the conventional (unpatterned sapphire substrate, UPSS) and patterned sapphire substrates (PSS). TEM results show that there exists a dislocation-accumulated region near the substrate/GaN interface, where the dislocation density is much higher with the UPPS than that with the PSS. It indicates that the pattern on the substrate surface is able to block the formation and propagation of dislocations. Further analysis discloses that slope of the pattern is found to suppress the deposition of GaN, and thus to provide more spaces for the epitaxially lateral overgrowth (ELO) of high temperature GaN, which significantly reduces the number of the initial islands, and minimizes dislocation formation due to the island coalescence. V-defect incorporating the threading dislocation is detected in the InGaN/GaN multi-quantum wells (MQWs), and its propagation mechanism is determined as the decrease of the surface energy due to the incorporation of indium. In addition, temperature dependence of dislocation formation is further investigated. The results show that dislocation with the screw component decreases monotonously as temperature goes up. However, edge dislocation firstly drops, and then increases by temperature due to the enhanced thermal mismatch stress. It implies that an optimized range of the growth temperature can be obtained to improve quality of the LED layers.

Structure and energetics of extended defects in ice Ih

NASA Astrophysics Data System (ADS)

Silva Junior, Domingos L.; de Koning, Maurice

2012-01-01

We consider the molecular structure and energetics of extended defects in proton-disordered hexagonal ice Ih. Using plane-wave density functional theory (DFT) calculations, we compute the energetics of stacking faults and determine the structure of the 30∘ and 90∘ partial dislocations on the basal plane. Consistent with experimental data, the formation energies of all fully reconstructed stacking faults are found to be very low. This is consistent with the idea that basal-plane glide dislocations in ice Ih are dissociated into partial dislocations separated by an area of stacking fault. For both types of partial dislocation we find a strong tendency toward core reconstruction through pairwise hydrogen-bond reformation. In the case of the 30∘ dislocation, the pairwise hydrogen-bond formation leads to a period-doubling core structure equivalent to that seen in zinc-blende semiconductor crystals. For the 90∘ partial we consider two possible core reconstructions, one in which the periodicity of the structure along the core remains unaltered and another in which it is doubled. The latter is preferred, although the energy difference between both is rather small, so that a coexistence of both reconstructions appears plausible. Our results imply that a mobility theory for dislocations on the basal plane in ice Ih should be based on the idea of reconstructed partial dislocations.

Elastic strain relaxation in interfacial dislocation patterns: II. From long- and short-range interactions to local reactions

NASA Astrophysics Data System (ADS)

Vattré, A.

2017-08-01

The long- and short-range interactions as well as planar reactions between two infinitely periodic sets of crossing dislocations are investigated using anisotropic elasticity theory in face- (fcc) and body- (bcc) centered cubic materials. Two preliminary cases are proposed to examine the substantial changes in the elastic stress states and the corresponding strain energies due to a slight rearrangement in the internal dislocation geometries and characters. In general, significant differences and discrepancies resulting from the considered cubic crystal structure and the approximation of isotropic elasticity are exhibited. In a third scenario, special attention is paid to connecting specific internal dislocation structures from the previous cases with non-equilibrium configurations predicted by the quantized Frank-Bilby equation for the (111) fcc and (110) bcc twist grain boundaries. The present solutions lead to the formation of energetically favorable dislocation junctions with non-randomly strain-relaxed configurations of lower energy. In particular, the local dislocation interactions and reactions form equilibrium hexagonal-shaped patterns with planar three-fold dislocation nodes without producing spurious far-field stresses.Numerical application results are presented from a selection of cubic metals including aluminum, copper, tantalum, and niobium. In contrast to the fcc materials, asymmetric dislocation nodes occur in the anisotropic bcc cases, within which the minimum-energy paths for predicting the fully strain-relaxed dislocation patterns depend on the Zener anisotropic factor with respect to unity. The associated changes in the dislocation structures as well as the removal of the elastic strain energy upon relaxations are quantified and also discussed.

Effect of strain rate and dislocation density on the twinning behavior in Tantalum

DOE PAGES

Florando, Jeffrey N.; El-Dasher, Bassem S.; Chen, Changqiang; ...

2016-04-28

The conditions which affect twinning in tantalum have been investigated across a range of strain rates and initial dislocation densities. Tantalum samples were subjected to a range of strain rates, from 10 –4/s to 10 3/s under uniaxial stress conditions, and under laser-induced shock-loading conditions. In this study, twinning was observed at 77K at strain rates from 1/s to 103/s, and during laser-induced shock experiments. The effect of the initial dislocation density, which was imparted by deforming the material to different amounts of pre-strain, was also studied, and it was shown that twinning is suppressed after a given amount ofmore » pre-strain, even as the global stress continues to increase. These results indicate that the conditions for twinning cannot be represented solely by a critical global stress value, but are also dependent on the evolution of the dislocation density. Additionally, the analysis shows that if twinning is initiated, the nucleated twins may continue to grow as a function of strain, even as the dislocation density continues to increase.« less

The core structure and recombination energy of a copper screw dislocation: a Peierls study

DOE PAGES

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

2017-05-19

The recombination process of dislocations is central to cross-slip, and transmission through Σ3 grain boundaries among other fundamental plastic deformation processes. Despite its importance, a detailed mechanistic understanding remains lacking. In this paper, we apply a continuous dislocation model, inspired by Peierls and Nabarro, complete with an ab-initio computed -surface and continuous units of infinitesimal dislocation slip, towards computing the stress-dependent recombination path of both an isotropic and anisotropic Cu screw dislocation. Under no applied stress, our model reproduces the stacking fault width between Shockley partial dislocations as predicted by discrete linear elasticity. Upon application of a compressive Escaig stress,more » the two partial dislocations coalesce to a separation of ~|b|. Upon increased loading the edge components of each partial dislocation recede, leaving behind a spread Peierls screw dislocation, indicating the recombined state. We demonstrate that the critical stress required to achieve the recombined state is independent of the shear modulus. Rather the critical recombination stress depends on an energy difference between an unstable fault energy (γτ) and the intrinsic stacking fault energy (γτ-γisf). We report recombination energies of ΔW = 0.168 eV/Å and ΔW = 0.084 eV/Å, respectively, for the Cu screw dislocation within isotropic and anisotropic media. Finally, we develop an analytic model which provides insight into our simulation results which compare favourably with other (similar) models.« less

The core structure and recombination energy of a copper screw dislocation: a Peierls study

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

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

The recombination process of dislocations is central to cross-slip, and transmission through Σ3 grain boundaries among other fundamental plastic deformation processes. Despite its importance, a detailed mechanistic understanding remains lacking. In this paper, we apply a continuous dislocation model, inspired by Peierls and Nabarro, complete with an ab-initio computed -surface and continuous units of infinitesimal dislocation slip, towards computing the stress-dependent recombination path of both an isotropic and anisotropic Cu screw dislocation. Under no applied stress, our model reproduces the stacking fault width between Shockley partial dislocations as predicted by discrete linear elasticity. Upon application of a compressive Escaig stress,more » the two partial dislocations coalesce to a separation of ~|b|. Upon increased loading the edge components of each partial dislocation recede, leaving behind a spread Peierls screw dislocation, indicating the recombined state. We demonstrate that the critical stress required to achieve the recombined state is independent of the shear modulus. Rather the critical recombination stress depends on an energy difference between an unstable fault energy (γτ) and the intrinsic stacking fault energy (γτ-γisf). We report recombination energies of ΔW = 0.168 eV/Å and ΔW = 0.084 eV/Å, respectively, for the Cu screw dislocation within isotropic and anisotropic media. Finally, we develop an analytic model which provides insight into our simulation results which compare favourably with other (similar) models.« less

Doping and compensation in Al-rich AlGaN grown on single crystal AlN and sapphire by MOCVD

NASA Astrophysics Data System (ADS)

Bryan, Isaac; Bryan, Zachary; Washiyama, Shun; Reddy, Pramod; Gaddy, Benjamin; Sarkar, Biplab; Breckenridge, M. Hayden; Guo, Qiang; Bobea, Milena; Tweedie, James; Mita, Seiji; Irving, Douglas; Collazo, Ramon; Sitar, Zlatko

2018-02-01

In order to understand the influence of dislocations on doping and compensation in Al-rich AlGaN, thin films were grown by metal organic chemical vapor deposition (MOCVD) on different templates on sapphire and low dislocation density single crystalline AlN. AlGaN grown on AlN exhibited the highest conductivity, carrier concentration, and mobility for any doping concentration due to low threading dislocation related compensation and reduced self-compensation. The onset of self-compensation, i.e., the "knee behavior" in conductivity, was found to depend only on the chemical potential of silicon, strongly indicating the cation vacancy complex with Si as the source of self-compensation. However, the magnitude of self-compensation was found to increase with an increase in dislocation density, and consequently, AlGaN grown on AlN substrates demonstrated higher conductivity over the entire doping range.

Theory of electron-phonon-dislon interacting system—toward a quantized theory of dislocations

NASA Astrophysics Data System (ADS)

Li, Mingda; Tsurimaki, Yoichiro; Meng, Qingping; Andrejevic, Nina; Zhu, Yimei; Mahan, Gerald D.; Chen, Gang

2018-02-01

We provide a comprehensive theoretical framework to study how crystal dislocations influence the functional properties of materials, based on the idea of a quantized dislocation, namely a ‘dislon’. In contrast to previous work on dislons which focused on exotic phenomenology, here we focus on their theoretical structure and computational power. We first provide a pedagogical introduction that explains the necessity and benefits of taking the dislon approach and why the dislon Hamiltonian takes its current form. Then, we study the electron-dislocation and phonon-dislocation scattering problems using the dislon formalism. Both the effective electron and phonon theories are derived, from which the role of dislocations on electronic and phononic transport properties is computed. Compared with traditional dislocation scattering studies, which are intrinsically single-particle, low-order perturbation and classical quenched defect in nature, the dislon theory not only allows easy incorporation of quantum many-body effects such as electron correlation, electron-phonon interaction, and higher-order scattering events, but also allows proper consideration of the dislocation’s long-range strain field and dynamic aspects on equal footing for arbitrary types of straight-line dislocations. This means that instead of developing individual models for specific dislocation scattering problems, the dislon theory allows for the calculation of electronic structure and electrical transport, thermal transport, optical and superconducting properties, etc, under one unified theory. Furthermore, the dislon theory has another advantage over empirical models in that it requires no fitting parameters. The dislon theory could serve as a major computational tool to understand the role of dislocations on multiple materials’ functional properties at an unprecedented level of clarity, and may have wide applications in dislocated energy materials.

Bridgman-type apparatus for the study of growth-property relationships - Arsenic vapor pressure-GaAs property relationship

NASA Technical Reports Server (NTRS)

Parsey, J. M.; Nanishi, Y.; Lagowski, J.; Gatos, H. C.

1982-01-01

A precision Bridgman-type apparatus is described which was designed and constructed for the investigation of relationships between crystal growth parameters and the properties of GaAs crystals. Several key features of the system are highlighted, such as the use of a heat pipe for precise arsenic vapor pressure control and seeding without the presence of a viewing window. Pertinent growth parameters, such as arsenic source temperature, thermal gradients in the growing crystal and in the melt, and the macroscopic growth velocity can be independently controlled. During operation, thermal stability better than + or - 0.02 C is realized; thermal gradients can be varied up to 30 C/cm in the crystal region, and up to 20 C/cm in the melt region; the macroscopic growth velocity can be varied from 50 microns/hr to 6.0 cm/hr. It was found that the density of dislocations depends critically on As partial pressure; and essentially dislocation-free, undoped, crystals were grown under As pressure precisely controlled by an As source maintained at 617 C. The free carrier concentration varied with As pressure variations. This variation in free carrier concentration was found to be associated with variations in the compensation ratio rather than with standard segregation phenomena.

Comparison of the quality of single-crystal diamonds grown on two types of seed substrates by MPCVD

NASA Astrophysics Data System (ADS)

Zhao, Yun; Guo, Yanzhao; Lin, Liangzhen; Zheng, Yuting; Hei, Lifu; Liu, Jinlong; Wei, Junjun; Chen, Liangxian; Li, Chengming

2018-06-01

Microwave plasma chemical vapor deposition (MPCVD) was used to grow single-crystal diamonds on two types of single-crystal diamond seed substrates prepared by high-pressure, high-temperature (HPHT) and chemical vapor deposition (CVD) methods. The quality of diamonds grown on the different seed substrates was compared. Fluorescence characteristics showed that the sectors of the HPHT seed substrates were obviously partitioned. Raman and absorption spectra showed that the CVD seed substrate produced higher-quality crystals with fewer nitrogen impurities. X-ray topography showed that the HPHT seed substrate had obvious growth sector boundaries, inclusions, dislocations, and stacking faults. The polarization characteristics of HPHT seed substrate were obvious, and the stress distribution was not uniform. When etching HPHT and CVD seed substrates using the same parameters, the etching morphology and extent of different growth sectors of the two substrates differed. Although extended defects were inevitably formed at the interface and propagated in the CVD layer, the dislocation density of a 1 mm-thick CVD layer grown on a CVD seed substrate was only half that of a 1 mm-thick CVD layer grown on an HPHT seed substrate. Therefore, the use of CVD seed substrate enabled the growth of a relatively higher-quality CVD single-crystal diamond.

Cross-scale MD simulations of dynamic strength of tantalum

NASA Astrophysics Data System (ADS)

Bulatov, Vasily

2017-06-01

Dislocations are ubiquitous in metals where their motion presents the dominant and often the only mode of plastic response to straining. Over the last 25 years computational prediction of plastic response in metals has relied on Discrete Dislocation Dynamics (DDD) as the most fundamental method to account for collective dynamics of moving dislocations. Here we present first direct atomistic MD simulations of dislocation-mediated plasticity that are sufficiently large and long to compute plasticity response of single crystal tantalum while tracing the underlying dynamics of dislocations in all atomistic details. Where feasible, direct MD simulations sidestep DDD altogether thus reducing uncertainties of strength predictions to those of the interatomic potential. In the specific context of shock-induced material dynamics, the same MD models predict when, under what conditions and how dislocations interact and compete with other fundamental mechanisms of dynamic response, e.g. twinning, phase-transformations, fracture. In collaboration with: Luis Zepeda-Ruiz, Lawrence Livermore National Laboratory; Alexander Stukowski, Technische Universitat Darmstadt; Tomas Oppelstrup, Lawrence Livermore National Laboratory. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Effects of solutes on dislocation nucleation from grain boundaries

DOE PAGES

Borovikov, Valery; Mendelev, Mikhail I.; King, Alexander H.

2016-12-27

When grain sizes are reduced to the nanoscale, grain boundaries (GB) become the dominant sources of the dislocations that enable plastic deformation. Here, we present the first molecular dynamics (MD) study of the effect of substitutional solutes on the dislocation nucleation process from GBs during uniaxial tensile deformation. A simple bi-crystal geometry is utilized in which the nucleation and propagation of dislocations away from a GB is the only active mechanism of plastic deformation. Solutes with atomic radii both larger and smaller than the solvent atomic radius were considered. Although the segregation sites are different for the two cases, bothmore » produce increases in the stress required to nucleate a dislocation. MD simulations at room temperature revealed that this increase in the nucleation stress is associated with changes of the GB structure at the emission site caused by dislocation emission, leading to increases in the heats of segregation of the solute atoms, which cannot diffuse to lower-energy sites on the timescale of the nucleation event. These results contribute directly to understanding the strength of nanocrystalline materials, and suggest suitable directions for nanocrystalline alloy design leading toward structural applications.« less

Microstructurally Based Cross-slip Mechanisms and Their Effects on Dislocation Microstructure Evolution in fcc Crystals

DTIC Science & Technology

2015-01-01

still necessary. One such model that could bridge this gap is discrete dis- location dynamics ( DDD ) simulations, in which both the time- and length-scale...limitations from atomic simulations are greatly reduced. Over the past two decades, two-dimen- sional (2D) and three-dimensional (3D) DDD methods have...dislocation ensem- bles according to physics-based rules [27–34]. The physics that can be incorporated in DDD simulations can range http://dx.doi.org

Recent Results from Epitaxial Growth on Step Free 4H-SiC Mesas

NASA Technical Reports Server (NTRS)

Neudeck, Philip G.; Trunek, Andrew J.; Spry, David J.; Powell, J. Anthony; Du, Hui; Skowronski, Marek; Bassim, Nabil D.; Mastro, Michael A.; Twigg, Mark E.; Holm, Ronald T.;

Role of dislocations and carrier concentration in limiting the electron mobility of InN films grown by plasma assisted molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Tangi, Malleswararao; De, Arpan; Shivaprasad, S. M.

2018-01-01

We report the molecular beam epitaxy growth of device quality InN films on GaN epilayer and nano-wall network (NWN) templates deposited on c-sapphire by varying the film thickness up to 1 μm. The careful experiments are directed towards obtaining high mobility InN layers having a low band gap with improved crystal quality. The dislocation density is quantified by using high resolution X-ray diffraction rocking curve broadening values of symmetric and asymmetric reflections, respectively. We observe that the dislocation density of the InN films grown on GaN NWN is less than that of the films grown on the GaN epilayer. This is attributed to the nanoepitaxial lateral overlayer growth (ELOG) process, where the presence of voids at the interface of InN/GaN NWN prevents the propagation of dislocation lines into the InN epilayers, thereby causing less defects in the overgrown InN films. Thus, this new adaptation of the nano-ELOG growth process enables us to prepare InN layers with high electron mobility. The obtained electron mobility of 2121 cm2/Vs for 1 μm thick InN/GaN NWN is comparable with the literature values of similar thickness InN films. Furthermore, in order to understand the reasons that limit electron mobility, the charge neutrality condition is employed to study the variation of electron mobility as a function of dislocation density and carrier concentration. Overall, this study provides a route to attaining improved crystal quality and electronic properties of InN films.

Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) Simulations of the Molecular Crystal alphaRDX

DTIC Science & Technology

2013-08-01

potential for HMX / RDX (3, 9). ...................................................................................8 1 1. Purpose This work...6 dispersion and electrostatic interactions. Constants for the SB potential are given in table 1. 8 Table 1. SB potential for HMX / RDX (3, 9...modeling dislocations in the energetic molecular crystal RDX using the Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) molecular

Defect levels of semi-insulating CdMnTe:In crystals

NASA Astrophysics Data System (ADS)

Kim, K. H.; Bolotinikov, A. E.; Camarda, G. S.; Hossain, A.; Gul, R.; Yang, G.; Cui, Y.; Prochazka, J.; Franc, J.; Hong, J.; James, R. B.

2011-06-01

Using photoluminescence (PL) and current deep-level transient spectroscopy (I-DLTS), we investigated the electronic defects of indium-doped detector-grade CdMnTe:In (CMT:In) crystals grown by the vertical Bridgman method. We similarly analyzed CdZnTe:In (CZT:In) and undoped CdMnTe (CMT) crystals grown under the amount of same level of excess Te and/or indium doping level to detail the fundamental properties of the electronic defect structure more readily. Extended defects, existing in all the samples, were revealed by synchrotron white beam x-ray diffraction topography and scanning electron microscopy. The electronic structure of CMT is very similar to that of CZT, with shallow traps, A-centers, Cd vacancies, deep levels, and Te antisites. The 1.1-eV deep level, revealed by PL in earlier studies of CZT and CdTe, were attributed to dislocation-induced defects. In our I-DLTS measurements, the 1.1-eV traps showed different activation energies with applied bias voltage and an exponential dependence on the trap-filling time, which are typical characteristics of dislocation-induced defects. We propose a new defect-trap model for indium-doped CMT crystals.

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

NASA Technical Reports Server (NTRS)

Chen, QI; Liu, Hao-Wen

1988-01-01

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

A combined optical, SEM and STM study of growth spirals on the polytypic cadmium iodide crystals

NASA Astrophysics Data System (ADS)

Singh, Rajendra; Samanta, S. B.; Narlikar, A. V.; Trigunayat, G. C.

2000-05-01

Some novel results of a combined sequential study of growth spirals on the basal surface of the richly polytypic CdI 2 crystals by optical microscopy, scanning electron microscopy (SEM) and scanning tunneling microscopy (STM) are presented and discussed. Under the high resolution and magnification achieved in the scanning electron microscope, the growth steps of large heights seen in the optical micrographs are found to have a large number of additional steps of smaller heights existing between any two adjacent large height growth steps. When further seen by a scanning tunneling microscope, which provides still higher resolution, sequences of unit substeps, each of height equal to the unit cell height of the underlying polytype, are revealed to exist on the surface. Several large steps also lie between the unit steps, with heights equal to an integral multiple of either the unit cell height of the underlying polytype or the thickness of a molecular sheet I-Cd-I. It is suggested that initially a giant screw dislocation may form by brittle fracture of the crystal platelet, which may gradually decompose into numerous unit dislocations during subsequent crystal growth.

Dislocation Processes and Frictional Stability of Faults

NASA Astrophysics Data System (ADS)

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

2011-12-01

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

Leakage current analysis for dislocations in Na-flux GaN bulk single crystals by conductive atomic force microscopy

NASA Astrophysics Data System (ADS)

Hamachi, T.; Takeuchi, S.; Tohei, T.; Imanishi, M.; Imade, M.; Mori, Y.; Sakai, A.

2018-04-01

The mechanisms associated with electrical conduction through individual threading dislocations (TDs) in a Na-flux GaN crystal grown with a multipoint-seed-GaN technique were investigated by conductive atomic force microscopy (C-AFM). To focus on individual TDs, dislocation-related etch pits (DREPs) were formed on the Na-flux GaN surface by wet chemical etching, after which microscopic Pt electrodes were locally fabricated on the DREPs to form conformal contacts to the Na-flux GaN crystal, using electron beam assisted deposition. The C-AFM data clearly demonstrate that the leakage current flows through the individual TD sites. It is also evident that the leakage current and the electrical conduction mechanism vary significantly based on the area within the Na-flux GaN crystal where the TDs are formed. These regions include the c-growth sector (cGS) in which the GaN grows in the [0001 ] direction on top of the point-seed with a c-plane growth front, the facet-growth sector (FGS) in which the GaN grows with {10 1 ¯ 1 } facets on the side of the cGS, the boundary region between the cGS and FGS (BR), and the coalescence boundary region between FGSs (CBR). The local current-voltage (I-V) characteristics of the specimen demonstrate space charge limited current conduction and conduction related to band-like trap states associated with TDs in the FGS, BR, and CBR. A detailed analysis of the I-V data indicates that the electrical conduction through TDs in the cGS may proceed via the Poole-Frenkel emission mechanism.

Radiation Tolerant Interfaces: Influence of Local Stoichiometry at the Misfit Dislocation on Radiation Damage Resistance of Metal/Oxide Interfaces

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

Shutthanandan, Vaithiyalingam; Choudhury, Samrat; Manandhar, Sandeep

To understand how variations in interface properties such as misfit-dislocation density and local chemistry affect radiation-induced defect absorption and recombination, we have explored a model system of CrxV1-x alloy epitaxial films deposited on MgO single crystals. By controlling film composition, the lattice mismatch with MgO was adjusted so that the misfit-dislocation density varies at the interface. These interfaces were exposed to irradiation and in situ results show that the film with a semi-coherent interface (Cr) withstands irradiation while V film, which has similar semi-coherent interface like Cr, showed the largest damage. Theoretical calculations indicate that, unlike at metal/metal interfaces, themore » misfit dislocation density does not dominate radiation damage tolerance at metal/oxide interfaces. Rather, the stoichiometry, and the precise location of the misfit-dislocation density relative to the interface, drives defect behavior. Together, these results demonstrate the sensitivity of defect recombination to interfacial chemistry and provide new avenues for engineering radiation-tolerant nanomaterials.« less

Method for reducing or eliminating interface defects in mismatched semiconductor epilayers

DOEpatents

Fitzgerald, Jr., Eugene A.; Ast, Dieter G.

1992-01-01

The present invention and process relates to crystal lattice mismatched semiconductor composite having a first semiconductor layer and a second semiconductor growth layer deposited thereon to form an interface wherein the growth layer can be deposited at thicknesses in excess of the critical thickness, even up to about 10.times. critical thickness. Such composite has an interface which is substantially free of interface defects. For example, the size of the growth areas in a mismatched In.sub.0.05 Ga.sub.0.95 As/(001)GaAs interface was controlled by fabricating 2-.mu.m high pillars of various lateral geometries and lateral dimensions before the epitaxial deposition of 3500.ANG. of In.sub.0.05 Ga.sub.0.95 As. The linear dislocation density at the interface was reduced from >5000 dislocations/cm to about zero for 25-.mu.m lateral dimensions and to less than 800 dislocations/cm for lateral dimensions as large as 100 .mu.m. The fabricated pillars control the lateral dimensions of the growth layer and block the glide of misfit dislocations with the resultant decrease in dislocation density.

Method for reducing or eliminating interface defects in mismatched semiconductor eiplayers

DOEpatents

Fitzgerald, Jr., Eugene A.; Ast, Dieter G.

1991-01-01

The present invention and process relates to crystal lattice mismatched semiconductor composite having a first semiconductor layer and a second semiconductor growth layer deposited thereon to form an interface wherein the growth layer can be deposited at thicknesses in excess of the critical thickness, even up to about 10x critical thickness. Such composite has an interface which is substantially free of interface defects. For example, the size of the growth areas in a mismatched In.sub.0.05 Ga.sub.0.95 As/(001)GaAs interface was controlled by fabricating 2-.mu.m high pillars of various lateral geometries and lateral dimensions before the epitaxial deposition of 3500.ANG. of In.sub.0.05 Ga.sub.0.95 As. The linear dislocation density at the interface was reduced from >5000 dislocations/cm to about zero for 25-.mu.m lateral dimensions and to less than 800 dislocations/cm for lateral dimensions as large as 100 .mu.m. The fabricated pillars control the lateral dimensions of the growth layer and block the glide of misfit dislocations with the resultant decrease in dislocation density.

Method for reducing or eliminating interface defects in mismatched semiconductor epilayers

DOEpatents

Fitzgerald, E.A. Jr.; Ast, D.G.

1992-10-20

The present invention and process relates to crystal lattice mismatched semiconductor composite having a first semiconductor layer and a second semiconductor growth layer deposited thereon to form an interface wherein the growth layer can be deposited at thicknesses in excess of the critical thickness, even up to about 10[times] critical thickness. Such composite has an interface which is substantially free of interface defects. For example, the size of the growth areas in a mismatched In[sub 0.05]Ga[sub 0.95]As/(001)GaAs interface was controlled by fabricating 2-[mu]m high pillars of various lateral geometries and lateral dimensions before the epitaxial deposition of 3500 [angstrom] of In[sub 0.05]Ga[sub 0.95]As. The linear dislocation density at the interface was reduced from >5000 dislocations/cm to about zero for 25-[mu]m lateral dimensions and to less than 800 dislocations/cm for lateral dimensions as large as 100 [mu]m. The fabricated pillars control the lateral dimensions of the growth layer and block the glide of misfit dislocations with the resultant decrease in dislocation density. 7 figs.

The Correlation Between Dislocations and Vacancy Defects Using Positron Annihilation Spectroscopy

NASA Astrophysics Data System (ADS)

Pang, Jinbiao; Li, Hui; Zhou, Kai; Wang, Zhu

2012-07-01

An analysis program for positron annihilation lifetime spectra is only applicable to isolated defects, but is of no use in the presence of defective correlations. Such limitations have long caused problems for positron researchers in their studies of complicated defective systems. In order to solve this problem, we aim to take a semiconductor material, for example, to achieve a credible average lifetime of single crystal silicon under plastic deformation at different temperatures using positron life time spectroscopy. By establishing reasonable positron trapping models with defective correlations and sorting out four lifetime components with multiple parameters, as well as their respective intensities, information is obtained on the positron trapping centers, such as the positron trapping rates of defects, the density of the dislocation lines and correlation between the dislocation lines, and the vacancy defects, by fitting with the average lifetime with the aid of Matlab software. These results give strong grounds for the existence of dislocation-vacancy correlation in plastically deformed silicon, and lay a theoretical foundation for the analysis of positron lifetime spectra when the positron trapping model involves dislocation-related defects.

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

NASA Astrophysics Data System (ADS)

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

2017-07-01

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

20 CFR 663.200 - What are intensive services for adults and dislocated workers?

Code of Federal Regulations, 2014 CFR

2014-04-01

... 20 Employees' Benefits 4 2014-04-01 2014-04-01 false What are intensive services for adults and..., DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Intensive Services § 663.200 What are intensive services for adults and dislocated workers? (a...

20 CFR 663.200 - What are intensive services for adults and dislocated workers?

Code of Federal Regulations, 2012 CFR

2012-04-01

... 20 Employees' Benefits 4 2012-04-01 2012-04-01 false What are intensive services for adults and..., DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Intensive Services § 663.200 What are intensive services for adults and dislocated workers? (a...

20 CFR 663.800 - What are supportive services for adults and dislocated workers?

Code of Federal Regulations, 2014 CFR

2014-04-01

... 20 Employees' Benefits 4 2014-04-01 2014-04-01 false What are supportive services for adults and..., DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Supportive Services § 663.800 What are supportive services for adults and dislocated workers...

20 CFR 663.200 - What are intensive services for adults and dislocated workers?

Code of Federal Regulations, 2013 CFR

2013-04-01

... 20 Employees' Benefits 4 2013-04-01 2013-04-01 false What are intensive services for adults and..., DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Intensive Services § 663.200 What are intensive services for adults and dislocated workers? (a...

20 CFR 663.800 - What are supportive services for adults and dislocated workers?

Code of Federal Regulations, 2013 CFR

2013-04-01

... 20 Employees' Benefits 4 2013-04-01 2013-04-01 false What are supportive services for adults and..., DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Supportive Services § 663.800 What are supportive services for adults and dislocated workers...

20 CFR 663.800 - What are supportive services for adults and dislocated workers?

Code of Federal Regulations, 2012 CFR

2012-04-01

... 20 Employees' Benefits 4 2012-04-01 2012-04-01 false What are supportive services for adults and..., DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Supportive Services § 663.800 What are supportive services for adults and dislocated workers...

20 CFR 663.300 - What are training services for adults and dislocated workers?

Code of Federal Regulations, 2012 CFR

2012-04-01

... 20 Employees' Benefits 4 2012-04-01 2012-04-01 false What are training services for adults and..., DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Training Services § 663.300 What are training services for adults and dislocated workers...

20 CFR 663.300 - What are training services for adults and dislocated workers?

Code of Federal Regulations, 2014 CFR

2014-04-01

... 20 Employees' Benefits 4 2014-04-01 2014-04-01 false What are training services for adults and..., DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Training Services § 663.300 What are training services for adults and dislocated workers...

20 CFR 663.300 - What are training services for adults and dislocated workers?

Code of Federal Regulations, 2013 CFR

2013-04-01

... 20 Employees' Benefits 4 2013-04-01 2013-04-01 false What are training services for adults and..., DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Training Services § 663.300 What are training services for adults and dislocated workers...

Selective heteroepitaxy on deeply grooved substrate: A route to low cost semipolar GaN platforms of bulk quality

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

Tendille, Florian, E-mail: florian.tendille@crhea.cnrs.fr; Vennéguès, Philippe; De Mierry, Philippe

2016-08-22

Semipolar GaN crystal stripes larger than 100 μm with dislocation densities below 5 × 10{sup 6} cm{sup −2} are achieved using a low cost fabrication process. An original sapphire patterning procedure is proposed, enabling selective growth of semipolar oriented GaN stripes while confining the defects to specific areas. Radiative and non-radiative crystalline defects are investigated by cathodoluminescence and can be correlated to the development of crystal microstructure during the growth process. A dislocation reduction mechanism, supported by transmission electron microscopy, is proposed. This method represents a step forward toward low-cost quasi-bulk semipolar GaN epitaxial platforms with an excellent structural quality which will allowmore » for even more efficient III-nitride based devices.« less

Unstable plastic deformation of ultrafine-grained copper at 0.5 K

NASA Astrophysics Data System (ADS)

Isaev, N. V.; Grigorova, T. V.; Shumilin, S. E.; Polishchuk, S. S.; Davydenko, O. A.

2017-12-01

We investigate the relation between the strain-hardening rate and flow instability of polycrystalline Cu-OF deformed by tension at a constant rate in a liquid 3He atmosphere. The microstructure of the ultrafine-grained crystal, obtained by the equal-channel angular hydro-extrusion method, was varied by annealing at recovery and recrystallization temperatures and was monitored by x-ray diffraction. It is shown that that the flow instability, manifesting itself as macroscopic stress serrations on the tension curve, appears at a threshold tension sufficient for activation of a dynamic recovery that leads to a decrease of the strain-hardening coefficient. We discuss the effect of grain size and the initial dislocation density on the evolution of the dislocation structure that determines the scale and the statistical properties of the flow instability in the investigated crystals at low temperature.

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

NASA Astrophysics Data System (ADS)

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

1997-03-01

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

CFD Growth of 3C-SiC on 4H/6H Mesas

NASA Technical Reports Server (NTRS)

Neudeck, Philip G.; Trunek, Andrew J.; Spry, David J.; Powell, J. Anthony; Du, Hui; Skowronski, Marek; Huang, XianRong; Dudley, Michael

2006-01-01

This article describes growth and characterization of the highest quality reproducible 3C-SiC heteroepitaxial films ever reported. By properly nucleating 3C-SiC growth on top of perfectly on-axis (0001) 4H-SiC mesa surfaces completely free of atomic scale steps and extended defects, growth of 3C-SiC mesa heterofilms completely free of extended crystal defects can be achieved. In contrast, nucleation and growth of 3C-SiC mesa heterofilms on top of 4H-SiC mesas with atomic-scale steps always results in numerous observable dislocations threading through the 3C-SiC epilayer. High-resolution X-ray diffraction and transmission electron microscopy measurements indicate non-trivial in-plane lattice mismatch between the 3C and 4H layers. This mismatch is somewhat relieved in the step-free mesa case via misfit dislocations confined to the 3C/4H interfacial region without dislocations threading into the overlying 3C-SiC layer. These results indicate that the presence or absence of steps at the 3C/4H heteroepitaxial interface critically impacts the quality, defect structure, and relaxation mechanisms of single-crystal heteroepitaxial 3C-SiC films.

MaRIE first experiments summaries version: May 9, 2010

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

Sarrao, John L

2010-01-01

A predictive understanding of microstructure-based heterogeneity and its consequences for materials damage & failure and phase transformation initiation is presently lacking. Most metallic materials used in applications are polycrystalline aggregates - individual single crystals separated by grain boundaries. Most of these materials are either metallic alloys or contain impurities. In either case, there is spatial variability in their chemical composition. These materials also contain dislocations which will be distributed in some way throughout the individual grains and increase in density with deformation and typically form dislocation sub-cell arrangements - producing spatial distribution in dislocation density. Many materials also produce twinmore » or slip band structures with deformation which produce further heterogeneity within individual crystals. The objective of this first experiment is to probe the physics of dynamic solid-solid phase transformation and damage at length scales approaching those at which they nucleate in order to gain a detailed understanding of this process and the influence real material microstructure has on these events. These experiments would simultaneously be simulated by the appropriate modeling tools to further develop these predictive tools and to assist in our interpretation of experimental results.« less

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

NASA Astrophysics Data System (ADS)

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

2018-07-01

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

Surface morphology and dislocation characteristics near the surface of 4H-SiC wafer using multi-directional scanning transmission electron microscopy.

PubMed

Sato, Takahiro; Orai, Yoshihisa; Suzuki, Yuya; Ito, Hiroyuki; Isshiki, Toshiyuki; Fukui, Munetoshi; Nakamura, Kuniyasu; Schamp, C T

2017-10-01

To improve the reliability of silicon carbide (SiC) electronic power devices, the characteristics of various kinds of crystal defects should be precisely understood. Of particular importance is understanding the correlation between the surface morphology and the near surface dislocations. In order to analyze the dislocations near the surface of 4H-SiC wafers, a dislocation analysis protocol has been developed. This protocol consists of the following process: (1) inspection of surface defects using low energy scanning electron microscopy (LESEM), (2) identification of small and shallow etch pits using KOH low temperature etching, (3) classification of etch pits using LESEM, (4) specimen preparation of several hundred nanometer thick sample using the in-situ focused ion beam micro-sampling® technique, (5) crystallographic analysis using the selected diffraction mode of the scanning transmission electron microscope (STEM), and (6) determination of the Burgers vector using multi-directional STEM (MD-STEM). The results show a correlation between the triangular terrace shaped surface defects and an hexagonal etch pit arising from threading dislocations, linear shaped surface defects and elliptical shaped etch pits arising from basal plane dislocations. Through the observation of the sample from two orthogonal directions via the MD-STEM technique, a basal plane dislocation is found to dissociate into an extended dislocation bound by two partial dislocations. A protocol developed and presented in this paper enables one to correlate near surface defects of a 4H-SiC wafer with the root cause dislocations giving rise to those surface defects. © The Author 2017. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

A molecular dynamics investigation into the mechanisms of subsurface damage and material removal of monocrystalline copper subjected to nanoscale high speed grinding

NASA Astrophysics Data System (ADS)

Li, Jia; Fang, Qihong; Liu, Youwen; Zhang, Liangchi

2014-06-01

This paper investigates the mechanisms of subsurface damage and material removal of monocrystalline copper when it is under a nanoscale high speed grinding of a diamond tip. The analysis was carried out with the aid of three-dimensional molecular dynamics simulations. The key factors that would influence the deformation of the material were carefully explored by analyzing the chip, dislocation movement, and workpiece deformation, which include grinding speed, depth of cut, grid tip radius, crystal orientation and machining angle of copper. An analytical model was also established to predict the emission of partial dislocations during the nanoscale high speed grinding. The investigation showed that a higher grinding velocity, a larger tip radius or a larger depth of cut would result in a larger chipping volume and a greater temperature rise in the copper workpiece. A lower grinding velocity would produce more intrinsic stacking faults. It was also found that the transition of deformation mechanisms depends on the competition between the dislocations and deformation twinning. There is a critical machining angle, at which a higher velocity, a smaller tip radius, or a smaller depth of cut will reduce the subsurface damage and improve the smoothness of a ground surface. The established analytical model showed that the Shockley dislocation emission is most likely to occur with the crystal orientations of (0 0 1)[1 0 0] at 45° angle.

Cyclic deformation of NI/sub 3/(Al,Nb) single crystals at ambient and elevated temperatures

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

Bonda, N.R.

Cyclic tests were performed on Ni/sub 3/(Al,Nb) (..gamma..' phase) single crystals by using a servo-hydraulic machine under fully reversed plastic strain control at a frequency of 0.1-0.2 Hz at room temperature, 400/sup 0/C and 700/sup 0/C. Since the monotonic behavior is orientation dependent, three orientations were studied. Asymmetry in tensile and compressive stresses was observed in the cyclic hardening curves of specimens tested at these temperatures and they were discussed with regard to the model suggested by Paider et al for monotonic behavior. The stress levels in the cyclic stress-strain curves (CSSC) at room temperature depended on orientation and cyclicmore » history. No CSSCs were established at 400/sup 0/C and 700/sup 0/C. The deformation in cyclic tests at small plastic strain amplitudes was found to be different from that in monotonic tests in the microplastic regions in which the deformation is believed to be carried by a small density of edge dislocations. But in cyclic deformation, to and from motion of dislocations trap the edge dislocations into dipoles and therefore screw dislocations will be forced to participate in the deformation. Cracks on the surfaces of specimens tested at room temperature and 400/sup 0/C were found to be of stage I type, whereas at 700/sup 0/C, they were of stage II type.« less

Dual-phase steel sheets under cyclic tension-compression to large strains: Experiments and crystal plasticity modeling

NASA Astrophysics Data System (ADS)

Zecevic, Milovan; Korkolis, Yannis P.; Kuwabara, Toshihiko; Knezevic, Marko

2016-11-01

In this work, we develop a physically-based crystal plasticity model for the prediction of cyclic tension-compression deformation of multi-phase materials, specifically dual-phase (DP) steels. The model is elasto-plastic in nature and integrates a hardening law based on statistically stored dislocation density, localized hardening due to geometrically necessary dislocations (GNDs), slip-system-level kinematic backstresses, and annihilation of dislocations. The model further features a two level homogenization scheme where the first level is the overall response of a two-phase polycrystalline aggregate and the second level is the homogenized response of the martensite polycrystalline regions. The model is applied to simulate a cyclic tension-compression-tension deformation behavior of DP590 steel sheets. From experiments, we observe that the material exhibits a typical decreasing hardening rate during forward loading, followed by a linear and then a non-linear unloading upon the load reversal, the Bauschinger effect, and changes in hardening rate during strain reversals. To predict these effects, we identify the model parameters using a portion of the measured data and validate and verify them using the remaining data. The developed model is capable of predicting all the particular features of the cyclic deformation of DP590 steel, with great accuracy. From the predictions, we infer and discuss the effects of GNDs, the backstresses, dislocation annihilation, and the two-level homogenization scheme on capturing the cyclic deformation behavior of the material.

Properties of high quality GaP single crystals grown by computer controlled liquid encapsulated Czochralski technique

NASA Astrophysics Data System (ADS)

Kokubun, Y.; Washizuka, S.; Ushizawa, J.; Watanabe, M.; Fukuda, T.

1982-11-01

The properties of GaP single crystals grown by an automatically diameter controlled liquid encapsulated Czochralski technique using a computer have been studied. A dislocation density less than 5×104 cm-2 has been observed for crystal grown in a temperature gradient lower than 70 °C/cm near the solid-liquid interface. Crystals have about 10% higher electron mobility than that of commercially available coracle controlled crystals and have 0.2˜0.5 compensation ratios. Yellow light emitting diodes using computer controlled (100) substrates have shown extremely high external quantum efficiency of 0.3%.

Potassium-cobalt sulphate crystal growth assisted by low frequency vibrations

NASA Astrophysics Data System (ADS)

Sadovsky, A.; Ermochenkov, I.; Dubovenko, E.; Sukhanova, E.; Bebyakin, M.; Dubov, V.; Avetissov, I.

2018-02-01

Single crystals of K2Co(SO4)2·6H2O were grown from solution using the temperature reduction method enhanced by the axial low frequency vibration control technique (AVC-technique). Physical modeling of heat-mass transfer in solution under the AVC action was performed. The growth rate of the AVC grown crystal was found to be twice that of the crystal grown under natural convection conditions. Analysis of spectral characteristics (absorption and Raman spectra) as well as structural properties (dislocation density and microhardness) of the grown crystals showed the significant superiority of the AVC technique for the growth of K2Co(SO4)2·6H2O crystals.

Effect of strain rate and dislocation density on the twinning behavior in tantalum

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

Florando, Jeffrey N., E-mail: florando1@llnl.gov; Swift, Damian C.; Barton, Nathan R.

2016-04-15

The conditions which affect twinning in tantalum have been investigated across a range of strain rates and initial dislocation densities. Tantalum samples were subjected to a range of strain rates, from 10{sup −4}/s to 10{sup 3}/s under uniaxial stress conditions, and under laser-induced shock-loading conditions. In this study, twinning was observed at 77 K at strain rates from 1/s to 10{sup 3}/s, and during laser-induced shock experiments. The effect of the initial dislocation density, which was imparted by deforming the material to different amounts of pre-strain, was also studied, and it was shown that twinning is suppressed after a givenmore » amount of pre-strain, even as the global stress continues to increase. These results indicate that the conditions for twinning cannot be represented solely by a critical global stress value, but are also dependent on the evolution of the dislocation density. In addition, the analysis shows that if twinning is initiated, the nucleated twins may continue to grow as a function of strain, even as the dislocation density continues to increase.« less

Ground based experiments on the growth and characterization of L-Arginine Phosphate (LAP) crystals

NASA Technical Reports Server (NTRS)

Rao, S. M.; Cao, C.; Batra, A. K.; Lal, R. B.; Mookherji, T. K.

1991-01-01

L-Arginine Phosphate (LAP) is a new nonlinear optical material with higher efficiency for harmonic generation compared to KDP. Crystals of LAP were grown in the laboratory from supersaturated solutions by temperature lowering technique. Investigations revealed the presence of large dislocation densities inside the crystals which are observed to produce refractive index changes causing damage at high laser powers. This is a result of the convection during crystal growth from supersaturated solutions. It is proposed to grow these crystals in a diffusion controlled growth condition under microgravity environment and compare the crystals grown in space with those grown on ground. Physical properties of the solutions needed for modelling of crystal growth are also presented.

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

NASA Astrophysics Data System (ADS)

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

2018-02-01

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

20 CFR 663.145 - What services are WIA title I adult and dislocated workers formula funds used to provide?

Code of Federal Regulations, 2014 CFR

2014-04-01

... 20 Employees' Benefits 4 2014-04-01 2014-04-01 false What services are WIA title I adult and... TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One...

20 CFR 663.150 - What core services must be provided to adults and dislocated workers?

Code of Federal Regulations, 2013 CFR

2013-04-01

... 20 Employees' Benefits 4 2013-04-01 2013-04-01 false What core services must be provided to adults... ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One-Stop Delivery...

20 CFR 663.145 - What services are WIA title I adult and dislocated workers formula funds used to provide?

Code of Federal Regulations, 2013 CFR

2013-04-01

... 20 Employees' Benefits 4 2013-04-01 2013-04-01 false What services are WIA title I adult and... TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One...

20 CFR 663.150 - What core services must be provided to adults and dislocated workers?

Code of Federal Regulations, 2012 CFR

2012-04-01

... 20 Employees' Benefits 4 2012-04-01 2012-04-01 false What core services must be provided to adults... ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One-Stop Delivery...

20 CFR 663.145 - What services are WIA title I adult and dislocated workers formula funds used to provide?

Code of Federal Regulations, 2012 CFR

2012-04-01

... 20 Employees' Benefits 4 2012-04-01 2012-04-01 false What services are WIA title I adult and... TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One...

20 CFR 663.150 - What core services must be provided to adults and dislocated workers?

Code of Federal Regulations, 2014 CFR

2014-04-01

... 20 Employees' Benefits 4 2014-04-01 2014-04-01 false What core services must be provided to adults... ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services Through the One-Stop Delivery...

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

PubMed

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

2015-06-01

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

Time-Dependent Mechanical Response of APbX 3 (A = Cs, CH 3NH 3; X = I, Br) Single Crystals [The Dynamic Mechanical Properties of Lead-Halide Perovskite Single Crystals are Independent of A-site Cation Chemistry

DOE PAGES

Reyes-Martinez, Marcos A.; Abdelhady, Ahmed L.; Saidaminov, Makhsud I.; ...

2017-05-02

The ease of processing hybrid organic–inorganic perovskite (HOIPs) films, belonging to a material class with composition ABX 3, from solution and at mild temperatures promises their use in deformable technologies, including flexible photovoltaic devices, sensors, and displays. To successfully apply these materials in deformable devices, knowledge of their mechanical response to dynamic strain is necessary. The authors elucidate the time- and rate-dependent mechanical properties of HOIPs and an inorganic perovskite (IP) single crystal by measuring nanoindentation creep and stress relaxation. The observation of pop-in events and slip bands on the surface of the indented crystals demonstrate dislocation-mediated plastic deformation. Themore » magnitudes of creep and relaxation of both HOIPs and IPs are similar, negating prior hypothesis that the presence of organic A-site cations alters the mechanical response of these materials. Moreover, these samples exhibit a pronounced increase in creep, and stress relaxation as a function of indentation rate whose magnitudes reflect differences in the rates of nucleation and propagation of dislocations within the crystal structures of HOIPs and IP. In conclusion, this contribution provides understanding that is critical for designing perovskite devices capable of withstanding mechanical deformations.« less

Preparation of c-axis perpendicularly oriented ultra-thin L10-FePt films on MgO and VN underlayers

NASA Astrophysics Data System (ADS)

Futamoto, Masaaki; Shimizu, Tomoki; Ohtake, Mitsuru

2018-05-01

Ultra-thin L10-FePt films of 2 nm average thickness are prepared on (001) oriented MgO and VN underlayers epitaxially grown on base substrate of SrTiO3(001) single crystal. Detailed cross-sectional structures are observed by high-resolution transmission electron microscopy. Continuous L10-FePt(001) thin films with very flat surface are prepared on VN(001) underlayer whereas the films prepared on MgO(001) underlayer consist of isolated L10-FePt(001) crystal islands. Presence of misfit dislocation and lattice bending in L10-FePt material is reducing the effective lattice mismatch with respect to the underlayer to be less than 0.5 %. Formation of very flat and continuous FePt layer on VN underlayer is due to the large surface energy of VN material where de-wetting of FePt material at high temperature annealing process is suppressed under a force balance between the surface and interface energies of FePt and VN materials. An employment of underlayer or substrate material with the lattice constant and the surface energy larger than those of L10-FePt is important for the preparation of very thin FePt epitaxial thin continuous film with the c-axis controlled to be perpendicular to the substrate surface.

Induction of optical vortex in the crystals subjected to bending stresses.

PubMed

Skab, Ihor; Vasylkiv, Yurij; Vlokh, Rostyslav

2012-08-20

We describe a method for generation of optical vortices that relies on bending of transparent parallelepiped-shaped samples fabricated from either glass or crystalline solid materials. It is shown that the induced singularity of optical indicatrix rotation leads in general to appearance of a mixed screw-edge dislocation of the phase front of outgoing optical beam. At the same time, some specified geometrical parameters of the sample can ensure generation of a purely screw dislocation of the phase front and, as a result, a singly charged canonical optical vortex.

Investigation of Strain Aging in the Ordered Intermetallic Compound beta-NiAl. Ph.D. Thesis Final Contractor Report

NASA Technical Reports Server (NTRS)

Weaver, Mark Lovell

1995-01-01

The phenomenon of strain aging has been investigated in polycrystalline and single crystal NiAl alloys at temperatures between 300 and 1200 K. Static strain aging studies revealed that after annealing at 1100 K for 7200 s (i.e., 2h) followed by furnace cooling, high purity, nitrogen-doped and titanium-doped polycrystalline alloys exhibited continuous yielding, while conventional-purity and carbon-doped alloys exhibited distinct yield points and Luders strains. Prestraining by hydrostatic pressurization removed the yield points, but they could be reintroduced by further annealing treatments. Yield points could be reintroduced more rapidly if the specimens were prestrained uniaxially rather than hydrostatically, owing to the arrangement of dislocations into cell structures during uniaxial deformation. The time dependence of the strain aging events followed at t(exp 2/3) relationship suggesting that the yield points observed in polycrystalline NiAl were the result of the pinning of mobile dislocations by interstitials, specifically carbon. Between 700 and 800 K, yield stress plateaus, yield stress transients upon a ten-fold increase in strain rate, work hardening peaks, and dips in the strain rate sensitivity (SRS) have been observed in conventional-purity and carbon-doped polycrystals. In single crystals, similar behavior was observed; in conventional-purity single crystals, however, the strain rate sensitivity became negative resulting in serrated yielding, whereas, the strain rate sensitivity stayed positive in high purity and in molybdenum-doped NiAl. These observations are indicative of dynamic strain aging (DSA) and are discussed in terms of conventional strain aging theories. The impact of these phenomena on the composition-structure-property relations are discerned. Finally, a good correlation has been demonstrated between the properties of NiAl alloys and a recently developed model for strain aging in metals and alloys developed by Reed-Hill et al.

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

NASA Astrophysics Data System (ADS)

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

1999-06-01

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

Confinement of Screw Dislocations to Predetermined Lateral Positions in (0001) 4H-SiC Epilayers Using Homoepitaxial Web Growth

NASA Technical Reports Server (NTRS)

Neudeck, Philip G.; Spry, Andrew J.; Trunek, Andrew J.; Powell, J. Anthony; Beheim, Glenn M.

2002-01-01

This paper reports initial demonstration of a cantilevered homoepitaxial growth process that places screw dislocations at predetermined lateral positions in on-axis 4H-SiC mesa epilayers. Thin cantilevers were grown extending toward the interior of hollow pre-growth mesa shapes etched into an on-axis 4H-SiC wafer, eventually completely coalescing to form roofed cavities. Each completely coalesced cavity exhibited either: 1) a screw dislocation growth spiral located exactly where final cantilever coalescence occurred, or 2) no growth spiral. The fact that growth spirals are not observed at any other position except the central coalescence point suggests that substrate screw dislocations, initially surrounded by the hollow portion of the pre-growth mesa shape, are relocated to the final coalescence point of the webbed epilayer roof. Molten potassium hydroxide etch studies revealed that properly grown webbed cantilevers exhibited no etch pits, confirming the superior crystal quality of the cantilevers.

Pure climb creep mechanism drives flow in Earth’s lower mantle

PubMed Central

Boioli, Francesca; Carrez, Philippe; Cordier, Patrick; Devincre, Benoit; Gouriet, Karine; Hirel, Pierre; Kraych, Antoine; Ritterbex, Sebastian

2017-01-01

At high pressure prevailing in the lower mantle, lattice friction opposed to dislocation glide becomes very high, as reported in recent experimental and theoretical studies. We examine the consequences of this high resistance to plastic shear exhibited by ringwoodite and bridgmanite on creep mechanisms under mantle conditions. To evaluate the consequences of this effect, we model dislocation creep by dislocation dynamics. The calculation yields to an original dominant creep behavior for lower mantle silicates where strain is produced by dislocation climb, which is very different from what can be activated under high stresses under laboratory conditions. This mechanism, named pure climb creep, is grain-size–insensitive and produces no crystal preferred orientation. In comparison to the previous considered diffusion creep mechanism, it is also a more efficient strain-producing mechanism for grain sizes larger than ca. 0.1 mm. The specificities of pure climb creep well match the seismic anisotropy observed of Earth’s lower mantle. PMID:28345037

X-ray topography using the forward transmitted beam under multiple-beam diffraction conditions

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

Tsusaka, Y., E-mail: tsusaka@sci.u-hyogo.ac.jp; Takano, H.; Takeda, S.

2016-02-15

X-ray topographs are taken for a sapphire wafer with the [0001] surface normal, as an example, by forward transmitted synchrotron x-ray beams combined with two-dimensional electronic arrays in the x-ray detector having a spatial resolution of 1 μm. They exhibit no shape deformation and no position shift of the dislocation lines on the topographs. Since the topography is performed under multiple-beam diffraction conditions, the topographic images of a single diffraction (two-wave approximation condition) or plural diffractions (six-wave approximation condition) can be recorded without large specimen position changes. As usual Lang topographs, it is possible to determine the Burgers vector ofmore » each dislocation line. Because of high parallelism of the incoming x-rays and linear sensitivity of the electronic arrays to the incident x-rays, the present technique can be used to visualize individual dislocations in single crystals of the dislocation density as high as 1 × 10{sup 5} cm{sup −2}.« less

Structural Rheology of the Smectic Phase

PubMed Central

Fujii, Shuji; Komura, Shigeyuki; Lu, Chun-Yi David

2014-01-01

In this review article, we discuss the rheological properties of the thermotropic smectic liquid crystal 8CB with focal conic domains (FCDs) from the viewpoint of structural rheology. It is known that the unbinding of the dislocation loops in the smectic phase drives the smectic-nematic transition. Here we discuss how the unbinding of the dislocation loops affects the evolution of the FCD size, linear and nonlinear rheological behaviors of the smectic phase. By studying the FCD formation from the perpendicularly oriented smectic layers, we also argue that dislocations play a key role in the structural development in layered systems. Furthermore, similarities in the rheological behavior between the FCDs in the smectic phase and the onion structures in the lyotropic lamellar phase suggest that these systems share a common physical origin for the elasticity. PMID:28788123

Predicting plasticity with soft vibrational modes: from dislocations to glasses.

PubMed

Rottler, Jörg; Schoenholz, Samuel S; Liu, Andrea J

2014-04-01

We show that quasilocalized low-frequency modes in the vibrational spectrum can be used to construct soft spots, or regions vulnerable to rearrangement, which serve as a universal tool for the identification of flow defects in solids. We show that soft spots not only encode spatial information, via their location, but also directional information, via directors for particles within each soft spot. Single crystals with isolated dislocations exhibit low-frequency phonon modes that localize at the core, and their polarization pattern predicts the motion of atoms during elementary dislocation glide in two and three dimensions in exquisite detail. Even in polycrystals and disordered solids, we find that the directors associated with particles in soft spots are highly correlated with the direction of particle displacements in rearrangements.

Plastic Deformation of Quartz: Unfinished business?

NASA Astrophysics Data System (ADS)

Paterson, M. S.

2011-12-01

Starting at Harvard in the mid-1930's, David Griggs built a series of high pressure machines for experimental rock deformation. One persistent aim was to achieve the plastic deformation of quartz. Each time he built a new machine for higher pressure and/or temperature, one of the first materials he tested would be quartz. This search went on through a 500 MPa liquid-medium machine at temperatures up to 300°C, then with a gas-medium machine for temperatures up to 800°C, and finally with a solid-medium machine for higher pressures and temperatures. Quartz proved stubbornly resistant to deformation except at extremely high stresses until, finally and somewhat serendipitously, it was found possible to deform quartz at relatively low stresses in the presence of water under special conditions. The breakthrough came in an experiment in a 1500 MPa solid-medium apparatus in which talc was used as pressure medium. At the temperature of the experiment, the talc dehydrated and so released water. Under these conditions, natural quartz proved to be very weak and to readily undergo plastic deformation, a phenomenon that became known as "hydrolytic weakening". Soon after this discovery, it was also found that certain synthetic single crystals could be easily deformed ab initio. These crystals were from a particular set that had been grown rapidly under hydrothermal conditions and had incorporated water during growth. Attempts in our laboratory to weaken crystals in a gas-medium apparatus at around 300 MPa by cooking dry quartz in the presence of added water were all unsuccessful, although we could deform wet synthetic crystals. There was considerable speculation about a role of high pressure in promoting hydrolytic weakening, but the dilemma was eventually clarified by electron microscope studies by Fitz Gerald and coworkers. These studies showed that crystals that had been subjected to high pressure and temperature in the solid-medium apparatus were extensively microcracked, presumably due to non-hydrostatic stresses generated from the solid medium during raising the pressure, thus evidently promoting the ingress of water. From our gas-medium experiments it would seem that both the solubility and the rate of diffusion of water-related species in dry quartz must be very low, such that at 300 MPa and around 500 - 1000 °C the water penetrates less than a few micrometres in the course of a few hours. Thus the sluggishness of diffusion and the low equilibrium solubility of water-related species in quartz probably explain the failure to achieve hydrolytic weakening in the gas-medium apparatus. However, the documentation of these properties remains inadequate. The initial, and still current, Frank-Griggs hypothesis for the origin of hydrolytic weakening is that the water plays a role in the breaking of the covalent silicon-oxygen bonds as a dislocation is propagated. It is a corollary that the dislocation must be saturated with water or that the water must migrate with the dislocation as it moves. Heggie and Jones have done a number of ab initio calculations on the role of the water in the migration of dislocations in quartz which support the idea that the motion of dislocations is aided by the presence of water-related species in the dislocation core.

Influence of in-situ deposited SiNx interlayer on crystal quality of GaN epitaxial films

NASA Astrophysics Data System (ADS)

Fan, Teng; Jia, Wei; Tong, Guangyun; Zhai, Guangmei; Li, Tianbao; Dong, Hailiang; Xu, Bingshe

2018-05-01

GaN epitaxial films with SiNx interlayers were prepared by metal organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates. The influences of deposition times and locations of SiNx interlayers on crystal quality of GaN epitaxial films were studied. Under the optimal growth time of 120 s for the SiNx interlayer, the dislocation density of GaN film is reduced to 4.05 × 108 cm-2 proved by high resolution X-ray diffraction results. It is found that when the SiNx interlayer deposits on the GaN nucleation islands, the subsequent GaN film has the lowest dislocation density of only 2.89 × 108 cm-2. Moreover, a model is proposed to illustrate the morphological evolution and associated propagation processes of TDs in GaN epi-layers with SiNx interlayers for different deposition times and locations.

Identifying Defects with Guided Algorithms in Bragg Coherent Diffractive Imaging

DOE PAGES

Ulvestad, A.; Nashed, Y.; Beutier, G.; ...

2017-08-30

In this study, crystallographic defects such as dislocations can significantly alter material properties and functionality. However, imaging these imperfections during operation remains challenging due to the short length scales involved and the reactive environments of interest. Bragg coherent diffractive imaging (BCDI) has emerged as a powerful tool capable of identifying dislocations, twin domains, and other defects in 3D detail with nanometer spatial resolution within nanocrystals and grains in reactive environments. However, BCDI relies on phase retrieval algorithms that can fail to accurately reconstruct the defect network. Here, we use numerical simulations to explore different guided phase retrieval algorithms for imagingmore » defective crystals using BCDI. We explore different defect types, defect densities, Bragg peaks, and guided algorithm fitness metrics as a function of signal-to-noise ratio. Based on these results, we offer a general prescription for phasing of defective crystals with no a prior knowledge.« less

Diffusive and martensitic nucleation kinetics in solid-solid transitions of colloidal crystals

NASA Astrophysics Data System (ADS)

Peng, Yi; Li, Wei; Wang, Feng; Still, Tim; Yodh, Arjun G.; Han, Yilong

2017-05-01

Solid-solid transitions between crystals follow diffusive nucleation, or various diffusionless transitions, but these kinetics are difficult to predict and observe. Here we observed the rich kinetics of transitions from square lattices to triangular lattices in tunable colloidal thin films with single-particle dynamics by video microscopy. Applying a small pressure gradient in defect-free regions or near dislocations markedly transform the diffusive nucleation with an intermediate-stage liquid into a martensitic generation and oscillation of dislocation pairs followed by a diffusive nucleus growth. This transformation is neither purely diffusive nor purely martensitic as conventionally assumed but a combination thereof, and thus presents new challenges to both theory and the empirical criterion of martensitic transformations. We studied how pressure, density, grain boundary, triple junction and interface coherency affect the nucleus growth, shape and kinetic pathways. These novel microscopic kinetics cast new light on control solid-solid transitions and microstructural evolutions in polycrystals.

Diffusive and martensitic nucleation kinetics in solid-solid transitions of colloidal crystals

PubMed Central

Peng, Yi; Li, Wei; Wang, Feng; Still, Tim; Yodh, Arjun G.; Han, Yilong

2017-01-01

Solid–solid transitions between crystals follow diffusive nucleation, or various diffusionless transitions, but these kinetics are difficult to predict and observe. Here we observed the rich kinetics of transitions from square lattices to triangular lattices in tunable colloidal thin films with single-particle dynamics by video microscopy. Applying a small pressure gradient in defect-free regions or near dislocations markedly transform the diffusive nucleation with an intermediate-stage liquid into a martensitic generation and oscillation of dislocation pairs followed by a diffusive nucleus growth. This transformation is neither purely diffusive nor purely martensitic as conventionally assumed but a combination thereof, and thus presents new challenges to both theory and the empirical criterion of martensitic transformations. We studied how pressure, density, grain boundary, triple junction and interface coherency affect the nucleus growth, shape and kinetic pathways. These novel microscopic kinetics cast new light on control solid–solid transitions and microstructural evolutions in polycrystals. PMID:28504246

Identifying Defects with Guided Algorithms in Bragg Coherent Diffractive Imaging

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

Ulvestad, A.; Nashed, Y.; Beutier, G.

In this study, crystallographic defects such as dislocations can significantly alter material properties and functionality. However, imaging these imperfections during operation remains challenging due to the short length scales involved and the reactive environments of interest. Bragg coherent diffractive imaging (BCDI) has emerged as a powerful tool capable of identifying dislocations, twin domains, and other defects in 3D detail with nanometer spatial resolution within nanocrystals and grains in reactive environments. However, BCDI relies on phase retrieval algorithms that can fail to accurately reconstruct the defect network. Here, we use numerical simulations to explore different guided phase retrieval algorithms for imagingmore » defective crystals using BCDI. We explore different defect types, defect densities, Bragg peaks, and guided algorithm fitness metrics as a function of signal-to-noise ratio. Based on these results, we offer a general prescription for phasing of defective crystals with no a prior knowledge.« less

20 CFR 663.610 - Does the statutory priority for use of adult funds also apply to dislocated worker funds?

Code of Federal Regulations, 2013 CFR

2013-04-01

... 20 Employees' Benefits 4 2013-04-01 2013-04-01 false Does the statutory priority for use of adult... TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE... priority for use of adult funds also apply to dislocated worker funds? No, the statutory priority applies...

20 CFR 663.610 - Does the statutory priority for use of adult funds also apply to dislocated worker funds?

Code of Federal Regulations, 2012 CFR

2012-04-01

... 20 Employees' Benefits 4 2012-04-01 2012-04-01 false Does the statutory priority for use of adult... TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE... priority for use of adult funds also apply to dislocated worker funds? No, the statutory priority applies...

20 CFR 663.610 - Does the statutory priority for use of adult funds also apply to dislocated worker funds?

Code of Federal Regulations, 2014 CFR

2014-04-01

... 20 Employees' Benefits 4 2014-04-01 2014-04-01 false Does the statutory priority for use of adult... TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE... priority for use of adult funds also apply to dislocated worker funds? No, the statutory priority applies...

Modeling elasticity in crystal growth.

PubMed

Elder, K R; Katakowski, Mark; Haataja, Mikko; Grant, Martin

2002-06-17

A new model of crystal growth is presented that describes the phenomena on atomic length and diffusive time scales. The former incorporates elastic and plastic deformation in a natural manner, and the latter enables access to time scales much larger than conventional atomic methods. The model is shown to be consistent with the predictions of Read and Shockley for grain boundary energy, and Matthews and Blakeslee for misfit dislocations in epitaxial growth.

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

NASA Astrophysics Data System (ADS)

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

2016-12-01

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

Critical Supersaturation for Ice Crystal Growth: Laboratory Measurements and Atmospheric Modeling Implications

NASA Astrophysics Data System (ADS)

Magee, N.; Moyle, A.; Lamb, D.

2003-12-01

An improved understanding of ice crystal growth, particularly at low temperatures, is much in demand for the advancement of numerical modeling of atmospheric processes. Cirrus models must contend with the complexity of ice crystals growing in cold temperatures, low pressures, low supersaturations, and with multiple nucleation mechanisms. Recent observations have allowed increasingly realistic parameterizations of cirrus ice crystal microphysics, but these observations need to be supplemented by a fundamental understanding of growth processes affecting low-temperature crystals. Several experimental studies have demonstrated that certain ice crystals require a minimum "critical" supersaturation before exhibiting detectable growth. These crystals are presumed to be essentially defect-free, preventing vicinal hillock growth at the site of crystal dislocations. In the case of crystal growth by spiral dislocation, advancement of faces begins as soon as supersaturation is present. The finding of conditional critical supersaturations have analogies in other materials (metals, semiconductors, potassium dihydrogen phosphate) and are thermodynamically predicted given a two-dimensional nucleation growth mechanism. Previous measurements have determined the critical supersaturation for ice as a function of temperature and crystallographic face from 0 to --15° C with extrapolation to --30° C. For both basal and prism faces, critical supersaturation is seen to increase with decreasing temperature, suggesting that low-temperature, low-supersaturation processes are most likely to be affected by this critical contingency. We present laboratory results to verify and extend prior critical supersaturation measurements using a novel approach for supersaturation generation, control, and measurement. The crystals are grown on the tip of a fine glass fiber ( ˜10 microns in diameter) under varying conditions of temperature, pressure, and saturation. Supersaturation is generated when a pre-saturated airflow passes over a coil of ice warmed by electrical resistance upstream from the growing crystal. Supersaturation is determined by a system of differential thermocouples calibrated to sulfuric acid drop size measurements. Measurements follow those made in earlier studies, but also extend to temperatures of --45° C, mimicking conditions found in some high altitude clouds.

Modification of the crystal structure of gadolinium gallium garnet by helium ion irradiation

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

Ostafiychuk, B. K.; Yaremiy, I. P., E-mail: yaremiy@rambler.ru; Yaremiy, S. I.

2013-12-15

The structure of gadolinium gallium garnet (GGG) single crystals before and after implantation by He{sup +} ions has been investigated using high-resolution X-ray diffraction methods and the generalized dynamic theory of X-ray scattering. The main types of growth defects in GGG single crystals and radiation-induced defects in the ion-implanted layer have been determined. It is established that the concentration of dislocation loops in the GGG surface layer modified by ion implantation increases and their radius decreases with an increase in the implantation dose.

Etude par émission acoustique de la dynamique des dislocations pendant la déformation cyclique de polycristaux d'aluminium

NASA Astrophysics Data System (ADS)

Slimani, A.; Fleischmann, P.; Fougères, R.

1992-06-01

The cyclic plasticity of 5N polycrystalline aluminium have been studied at room temperature by measuring the continuous acoustic emission (A.E.) due to dislocations movements in the metal. In this study, original data have been obtained in the understanding of continuous A.E. sources. In comparison with classical interpretation given in the literature, the fact that dislocations are arranged according to a dislocation cell structure from the first cycle has been included in the analysis of the results. From this, it has been shown that the amplitude of the A.E. signal is not directly connected with the plastic strain rate prescribed to the fatigue sample and that the probability density function of dislocation loops created during the cycling can be determined. La plasticité cyclique de l'AI 5N polycristallin a été étudiée à la température ambiante à partir de mesures d'émission acoustique continue (E.A.). L'application de la technique de l'E.A. nous a permis d'obtenir des données originales quant aux mécanismes sources d'E.A. Par rapport aux interprétations classiques de la littérature, nous avons fait intervenir le fait que, dès les premiers cycles, une structure cellulaire de dislocations est établie. Nous montrons que l'amplitude du signal d'E.A. n'est plus liée directement à la vitesse de déformation plastique macroscopique. A partir de cette donnée, l'analyse des résultats d'E.A. permet d'obtenir des informations sur la fonction distribution des boucles de dislocations créées au cours de la déformation cyclique.

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

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

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

2014-12-08

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

Dislocations Accelerate Oxygen Ion Diffusion in La 0.8Sr 0.2MnO 3 Epitaxial Thin Films

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

Navickas, Edvinas; Chen, Yan; Lu, Qiyang

Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO 3 (LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO 3 and SrTiO 3 substrates. The lattice mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced bymore » dislocations, especially in the LSM films on LaAlO 3. Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO 3. In conclusion, the diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk.« less

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

NASA Astrophysics Data System (ADS)

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

2018-03-01

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

Dislocations Accelerate Oxygen Ion Diffusion in La0.8Sr0.2MnO3 Epitaxial Thin Films

PubMed Central

2017-01-01

Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO3 (LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO3 and SrTiO3 substrates. The lattice mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced by dislocations, especially in the LSM films on LaAlO3. Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO3. The diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk. PMID:28981249

Use of hydrogen etching to remove existing dislocations in GaN epitaxial layers

NASA Astrophysics Data System (ADS)

Yeh, Yen-Hsien; Chu, Chung-Ming; Wu, Yin-Hao; Hsu, Ying-Chia; Yu, Tzu-Yi; Lee, Wei-I.

2015-08-01

In this paper, based on the anisotropic nature of hydrogen (H2) etching on GaN, we describe a new approach to the removal of threading dislocations in GaN layers. The top surfaces of c-plane (Ga-face) and a-plane GaNs are considered stable in H2; therefore, H2 etches only crystal imperfections such as dislocation and basal plane stacking fault (BSF) sites. We used H2 to etch undoped c-plane GaN, n-type c-plane GaN, a-plane GaN, and an InGaN/GaN multiple quantum well structure. Several examinations were performed, indicating deep cavities on the c-plane GaN samples after H2 etching; furthermore, gorge-like grooves were observed on the a-plane GaN samples. The deep cavities on the c-plane GaN were considered the etched dislocation sites, and the gorge-like grooves on the a-plane GaN were considered the etched BSF sites. Photoluminescence measurements were performed and the results indicated that the H2-etched samples demonstrate superior optoelectronic properties, probably because of the elimination of dislocations.

Dislocations Accelerate Oxygen Ion Diffusion in La 0.8Sr 0.2MnO 3 Epitaxial Thin Films

DOE PAGES

Navickas, Edvinas; Chen, Yan; Lu, Qiyang; ...

2017-10-05

Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO 3 (LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO 3 and SrTiO 3 substrates. The lattice mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced bymore » dislocations, especially in the LSM films on LaAlO 3. Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO 3. In conclusion, the diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk.« less

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

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

Vinogradov, A.; Laboratory of Hybrid Nanostructured Materials, NITU MISiS, Moscow 119490; Yasnikov, I. S.

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 commonmore » 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.« less

Symmetric Tilt Grain Boundaries of Synthetic Forsterite Bicrystals

NASA Astrophysics Data System (ADS)

Heinemann, S.; Wirth, R.; Dresen, G.

2002-12-01

{ indent1.5em skip0ex Structure and transport properties of grain boundaries in rocks are still poorly understood. For example, grain boundary diffusivity and mobility depend on orientation, and they are different for low and high angle grain boundaries. The transition from low to high angle grain boundaries in rock-forming minerals is not studied in detail, but a high angle grain boundary is commonly defined by a lattice misorientation of >10°-15°. To investigate the physical properties of olivine grain boundaries we produced a series of synthetic forsterite bicrystals with symmetric tilt grain boundaries by direct bonding [1,2]. For each bicrystal two oriented synthetic forsterite single crystals were joined at room temperature and annealed at 400°C for one week. All bicrystals were cut in two parts and one part was annealed further at 1650°C for 48h. The tilt axis of the boundary in the synthesized bicrystals is parallel to the a direction, and the tilt angles of the series range from 9° to 21°. Specimens were prepared for investigations in the transmission electron microscope (TEM) using focused ion beam (FIB) technique. High-resolution TEM investigations of symmetric tilt grain boundaries reveal dislocation arrays between undisturbed crystal regions in samples annealed at 400°C and 1650°C. This suggests that bonding of bicrystals occurred already below or at 400°C. The burgers vectors of the dislocations are parallel to c. Dislocation cores do not overlap up to a tilt angle of 21°. This indicates that for forsterite small angle grain boundaries exist up to tilt angles of 21°. The dislocation model of small angle grain boundaries can be applied and the observed dislocation spacings d are related to tilt angle θ and burgers vector length b by Franks formula [3]: d = b/(2sin(2/θ )) ~ b/θ . With tilt angles increasing from 9° to 21° the dislocation spacing decreased. Using Frank's equation and the observation that dislocations do not overlap at a misorientation of 21° allows to estimate the maximum radius of the dislocation cores, r0<1.4b.} {skip0ex \\small [1] Heinemann S et al. (2001) Phys Chem Minerals 28, 685 [2] Heinemann S et al. (2002) Beih Eur Mineral 14(1), 66 [3] Frank FC (1951) Pittsburgh Symposium on the Plastic Deformation of Solids. 150}

Reconfigurable interactions and three-dimensional patterning of colloidal particles and defects in lamellar soft media

PubMed Central

Trivedi, Rahul P.; Klevets, Ivan I.; Senyuk, Bohdan; Lee, Taewoo; Smalyukh, Ivan I.

2012-01-01

Colloidal systems find important applications ranging from fabrication of photonic crystals to direct probing of phenomena typically encountered in atomic crystals and glasses. New applications—such as nanoantennas, plasmonic sensors, and nanocircuits—pose a challenge of achieving sparse colloidal assemblies with tunable interparticle separations that can be controlled at will. We demonstrate reconfigurable multiscale interactions and assembly of colloids mediated by defects in cholesteric liquid crystals that are probed by means of laser manipulation and three-dimensional imaging. We find that colloids attract via distance-independent elastic interactions when pinned to the ends of cholesteric oily streaks, line defects at which one or more layers are interrupted. However, dislocations and oily streaks can also be optically manipulated to induce kinks, allowing one to lock them into the desired configurations that are stabilized by elastic energy barriers for structural transformation of the particle-connecting defects. Under the influence of elastic energy landscape due to these defects, sublamellar-sized colloids self-assemble into structures mimicking the cores of dislocations and oily streaks. Interactions between these defect-embedded colloids can be varied from attractive to repulsive by optically introducing dislocation kinks. The reconfigurable nature of defect–particle interactions allows for patterning of defects by manipulation of colloids and, in turn, patterning of particles by these defects, thus achieving desired colloidal configurations on scales ranging from the size of defect core to the sample size. This defect-colloidal sculpturing may be extended to other lamellar media, providing the means for optically guided self-assembly of mesoscopic composites with predesigned properties. PMID:22411822

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

NASA Astrophysics Data System (ADS)

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

2018-01-01

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

Translation effects on vertical Bridgman growth and optical, mechanical and surface analysis of 2-phenylphenol single crystal

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

Sadhasivam, S., E-mail: sadha.phy1@gmail.com; Perumal, Rajesh Narayana

2-phenylphenol optical crystals were grown in cone ampoules using vertical Bridgman technique. Single crystal of 2-phenylphenol with 150 mm length has been grown. The inclination on the conical part of the ampoule reduces the growth defects in the 2-phenylphenol single crystal. The lattice parameters and structure studied using single crystal X-ray diffraction method. 2-phenylphenol single crystal belongs to orthorhombic space group Fdd2. The micro translation rate affects crystal growth of 2-phenylphenol crystal was studied. The translation rate dependent defects present in the crystal were investigated by transmittance, indentation and etching characterizations. The dislocation induced indentation crack lengths variations were studied. Etchmore » pits and striations observed for the selective etchants furnish significant information on growth aspects and degree of defect present in the crystal.« less

Effect of orientation on deformation behavior of Fe nanowires: A molecular dynamics study

NASA Astrophysics Data System (ADS)

Sainath, G.; Srinivasan, V. S.; Choudhary, B. K.; Mathew, M. D.; Jayakumar, T.

2014-04-01

Molecular dynamics simulations have been carried out to study the effect of crystal orientation on tensile deformation behaviour of single crystal BCC Fe nanowires at 10 K. Two nanowires with an initial orientation of <100>/{100} and <110>/{111} have been chosen for this study. The simulation results show that the deformation mechanisms varied with crystal orientation. The nanowire with an initial orientation of <100>/{100} deforms predominantly by twinning mechanism, whereas the nanowire oriented in <110>/{111}, deforms by dislocation plasticity. In addition, the single crystal oriented in <110>/{111} shows higher strength and elastic modulus than <100>/{100} oriented nanowire.

Defect-induced solid state amorphization of molecular crystals

NASA Astrophysics Data System (ADS)

Lei, Lei; Carvajal, Teresa; Koslowski, Marisol

2012-04-01

We investigate the process of mechanically induced amorphization in small molecule organic crystals under extensive deformation. In this work, we develop a model that describes the amorphization of molecular crystals, in which the plastic response is calculated with a phase field dislocation dynamics theory in four materials: acetaminophen, sucrose, γ-indomethacin, and aspirin. The model is able to predict the fraction of amorphous material generated in single crystals for a given applied stress. Our results show that γ-indomethacin and sucrose demonstrate large volume fractions of amorphous material after sufficient plastic deformation, while smaller amorphous volume fractions are predicted in acetaminophen and aspirin, in agreement with experimental observation.

Analysis of plastic deformation in silicon web crystals

NASA Technical Reports Server (NTRS)

Spitznagel, J. A.; Seidensticker, R. G.; Lien, S. Y.; Mchugh, J. P.; Hopkins, R. H.

1987-01-01

Numerical calculation of 111-plane 110-line slip activity in silicon web crystals generated by thermal stresses is in good agreement with etch pit patterns and X-ray topographic data. The data suggest that stress redistribution effects are small and that a model, similar to that proposed by Penning (1958) and Jordan (1981) but modified to account for dislocation annihilation and egress, can be used to describe plastic flow effects during silicon web growth.

Kinetic processes for plastic deformation of olivine in the Poyi ultramafic intrusion, NW China: Insights from the textural analysis of a 1700 m fully cored succession

NASA Astrophysics Data System (ADS)

Yao, Zhuo-sen; Qin, Ke-zhang; Xue, Sheng-chao

2017-07-01

The ubiquitous presence of undulose extinction and subgrain boundaries in olivine crystals is commonly perceived as originating in the mantle, however these plastic deformation features are also well developed in the Poyi ultramafic intrusion, NW China. In this case, olivine was deformed through kinetic processes in a crustal magma chamber, rather than by deformation processes in the upper mantle. Moreover, accumulation and textural coarsening were critical to the characteristics of crystal size distributions (CSDs) of olivines in the Poyi intrusion. The axial deformational compaction of crystal mush was revealed by virtue of other quantitative textural analyses (e.g., spatial distribution patter, alignment factor and aspect ratio). Additionally, based on the contrast of density between crystal matrix and interstitial melt, adequate stress was generated by the km-scale crystal framework in Poyi body ( 2-11 MPa) which triggered the distortion of grain-lattice in olivine. The deformation mechanisms of olivine primarily are dislocation creep and dislocation-accommodated grain boundary sliding (DisGBS), while diffusion creep is subsidiary. This study has revealed various kinetic processes in a magmatic system by first demonstrating the genetic relationship between mineral deformation and axial compaction of crystal mush while highlighting the uncertainty of employing the deformation features of olivine in peridotite xenoliths as an indicator for a mantle origin. In contrast to the olivine populations of xenocrysts that underwent fragmentation during ascent, the deformed primitive olivines in compaction exhibit a distinct shortage of small grains, which is conducive to delimiting these two types of deformed grains.

Three-dimensional imaging of dislocation dynamics during the hydriding phase transformation

DOE PAGES

Ulvestad, A.; Welland, M. J.; Cha, W.; ...

2017-01-16

Crystallographic imperfections can significantly alter material properties and responses to external stimuli, including solute induced phase transformations and crystal growth and dissolution . Despite recent progress in imaging defects using both electron and x-ray techniques, in situ three-dimensional imaging studies of defect dynamics, necessary to understand and engineer nanoscale processes, remains challenging. Here, we report in situ three-dimensional imaging of defect dynamics during the hydriding phase transformation of individual palladium nanocrystals by Bragg Coherent Diffractive Imaging (BCDI) . During constant pressure experiments, we observed that the phase transformation begins after the nucleation of dislocations in large (300 nm) particles. Themore » 3D dislocation network shows that dislocations are close to the phase boundary. The 3D phase morphology resolved by BCDI suggests that the hydrogen-rich phase is more similar to a spherical cap on the hydrogen-poor phase than the core-shell model commonly assumed. We substantiate this conclusion using 3D phase field modeling and demonstrate how phase morphology affects the critical size for dislocation nucleation. We determine the size dependence of the transformation pressure for large (150-300 nm) palladium nanocrystals using variable pressure experiments. Our results reveal a pathway for solute induced structural phase transformations in nanocrystals and demonstrate BCDI as a novel method for understanding dislocation dynamics in phase transforming systems at the nanoscale.« less

A preliminary review of organic materials single crystal growth by the Czochralski technique

NASA Astrophysics Data System (ADS)

Penn, B. G.; Shields, A. W.; Frazier, D. O.

1988-09-01

The growth of single crystals of organic compounds by the Czochralski method is reviewed. From the literature it is found that single crystals of benzil, a nonlinear optical material with a d sub 11 value of 11.2 + or - 1.5 x d sub 11 value of alpha quartz, has fewer dislocations than generally contained in Bridgman crystals. More perfect crystals were grown by repeated Czochralski growth. This consists of etching away the defect-containing portion of a Czochralski grown crystal and using it as a seed for further growth. Other compounds used to grow single crystals are benzophenone, 12-tricosanone (laurone), and salol. The physical properties, growth apparatus, and processing conditions presented in the literature are discussed. Moreover, some of the possible advantages of growing single crystals of organic compounds in microgravity to obtain more perfect crystals than on Earth are reviewed.

A preliminary review of organic materials single crystal growth by the Czochralski technique

NASA Technical Reports Server (NTRS)

Penn, B. G.; Shields, A. W.; Frazier, D. O.

1988-01-01

The growth of single crystals of organic compounds by the Czochralski method is reviewed. From the literature it is found that single crystals of benzil, a nonlinear optical material with a d sub 11 value of 11.2 + or - 1.5 x d sub 11 value of alpha quartz, has fewer dislocations than generally contained in Bridgman crystals. More perfect crystals were grown by repeated Czochralski growth. This consists of etching away the defect-containing portion of a Czochralski grown crystal and using it as a seed for further growth. Other compounds used to grow single crystals are benzophenone, 12-tricosanone (laurone), and salol. The physical properties, growth apparatus, and processing conditions presented in the literature are discussed. Moreover, some of the possible advantages of growing single crystals of organic compounds in microgravity to obtain more perfect crystals than on Earth are reviewed.

Potential productivity benefits of float-zone versus Czochralski crystal growth

NASA Technical Reports Server (NTRS)

Abe, T.

1985-01-01

Efficient mass production of single-crystal silicon is necessary for the efficient silicon solar arrays needed in the coming decade. However, it is anticipated that there will be difficulty growing such volumes of crystals using conventional Czochralski (Cz) methods. While the productivity of single crystals might increase with a crystal diameter increase, there are two obstacles to the mass production of large diameter Czochralski crystals, the long production cycle due to slow growth rate and the high heat requirements of the furnaces. Also counterproductive would be the large resistivity gradient along the growth direction of the crystals due to impurity concentration. Comparison between Float zone (FZ) and Cz crystal growth on the basis of a crystal 150 mm in diameter is on an order of two to four times in favor of the FZ method. This advantage results from high growth rates and steady-state growth while maintaining a dislocation-free condition and impurity segregation.

An acoustic emission study of plastic deformation in polycrystalline aluminium

NASA Technical Reports Server (NTRS)

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

1979-01-01

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

Study of microdefects and their distribution in dislocation-free Si-doped HB GaAs by X-ray diffuse scattering on triple-crystal diffractometer

NASA Astrophysics Data System (ADS)

Charniy, L. A.; Morozov, A. N.; Bublik, V. T.; Scherbachev, K. D.; Stepantsova, I. V.; Kaganer, V. M.

1992-03-01

Microdefects in dislocation-free Si-doped (n = (1-3) × 10 18cm-3) HB GaAs crystals were studied by X-ray diffuse scattering measured with the help of a triple-crystal diffractometer. The intensity of the diffuse scattering as well as the isointensity contours around different reciprocal lattice points were analysed. A comparison of the measured isointensity contours with the theoretically calculated ones showed that the microdefects detected are interstitial dislocation loops with the Burgers vectors b = {1}/{2}<110 #3862;; lying in the planes #38;{110} and {111}. The mean radius of the dislocation loops R0 was determined using the wave vector q0 alpha; R-10 corresponding to the transmition point where the Huang diffuse scattering I( q) alpha q-2 ( q < q0) changed to the asymptotic scattering I( q) alpha q-4 ( q #62 q0). The analysis of a D-shaped cross-sectional (111) wafer cut from the end part of the HB ingot showed that R0 changed smoothly along the [ overline211] symmetry axis of the wafer. The highly inhomogeneous "new-moon"-like distribution of the non-dislocational etch-pits was also obtained. The maximal loop radius obtained at the edges of the wafer, R 0 = 1 μm, corresponds to the wafer area enriched with etch-pits and the minimal one, R 0 = 0.3 μm, corresponds to the bound of the new-moon-like area denuded from etch-pits. Microdefects of a new type were detected in the denuded area. These microdefects consist of nuclei, 0.1 μm in radius, and an extended atmosphere of interstitials. The minimal microdefect radius in the centre of the wafer corresponds to the maximum local value of the lattice parameter a = 5.655380 Å, and the minimum local value a = 5.65372 Å was obtained at the wafer edges enriched with microdefect-related etch-pits. Absolute X-ray diffuse intensity measurements were used for microdefect concentration determination. Normalization of I( q) was based on the comparison of the Huang intensity with the thermal diffuse scattering intensity which is predominant for the wave vector q å R-10. The microdefect concentration determined in this way appeared to be 4 × 10 9 cm -3 at the edges of the wafer and 4 × 10 11 cm -3 at the centre of the new-moon-like etch-pit denuded zone. The number of interstitial atoms forming dislocation loops is shown to be the same across the area of the wafer and equal to 10 16 cm -3.

Effect of He+ fluence on surface morphology and ion-irradiation induced defect evolution in 7075 aluminum alloys

NASA Astrophysics Data System (ADS)

Ni, Kai; Ma, Qian; Wan, Hao; Yang, Bin; Ge, Junjie; Zhang, Lingyu; Si, Naichao

2018-02-01

The evolution of microstructure for 7075 aluminum alloys with 50 Kev helium ions irradiation were studied by using optical microscopy (OM), scanning electron microscopy (SEM), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The fluences of 1 × 1015, 1 × 1016 and 1 × 1017 ions cm-2 were selected, and irradiation experiments were conducted at room temperatures. The transmission process of He+ ions was simulated by using SRIM software, including distribution of ion ranges, energy losses and atomic displacements. Experimental results show that irradiated pits and micro-cracks were observed on irradiation sample surface, and the size of constituent particles (not including Mg2Si) decreased with the increasing dose. The x-ray diffraction results of the pair of peaks is better resolved in irradiated samples might indicate that the stressed structure consequence due to crystal defects (vacancies and interstitials) after He+ implantation. TEM observation indicated that the density of MgZn2 phase was significantly reduced after helium ion irradiation which is harmful to strength. Besides, the development of compressive stress produced a large amount of dislocation defects in the 1015 ions cm-2 sample. Moreover, higher fluence irradiation produced more dislocations in sample. At fluence of 1016 ions cm-2, dislocation wall formed by dislocation slip and aggregation in the interior of grains, leading to the refinement of these grains. As fluence increased to 1017 ions cm-2, dislocation loops were observed in pinned dislocation. Moreover, dislocation as effective defect sink, irradiation-induced vacancy defects aggregated to these sinks, and resulted in the formation of helium bubbles in dislocation.

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

NASA Astrophysics Data System (ADS)

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

2016-12-01

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

High-speed collision of copper nanoparticle with aluminum surface: Molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Pogorelko, Victor V.; Mayer, Alexander E.; Krasnikov, Vasiliy S.

2016-12-01

We investigate the effect of the high-speed collision of copper nanoparticles with aluminum surface by means of molecular dynamic simulations. Studied diameter of nanoparticles is varied within the range 7.2-22 nm and the velocity of impact is equal to 500 or 1000 m/s. Dislocation analysis shows that a large quantity of dislocations is formed within the impact area. Overall length of dislocations is determined, first of all, by the impact velocity and by the size of incident copper nanoparticle, in other words, by the kinetic energy of the nanoparticle. Dislocations occupy the total volume of the impacted aluminum single crystal layer (40.5 nm in thickness) in the form of intertwined structure in the case of large kinetic energy of the incident nanoparticle. Decrease in the initial kinetic energy or increase in the layer thickness lead to restriction of the penetration depth of the dislocation net; formation of separate dislocation loops is observed in this case. Increase in the initial system temperature slightly raises the dislocation density inside the bombarded layer and considerably decreases the dislocation density inside the nanoparticle. The temperature increase also leads to a deeper penetration of the copper atoms inside the aluminum. Additional molecular dynamic simulations show that the deposited particles demonstrate a very good adhesion even in the case of the considered relatively large nanoparticles. Medium energy of the nanoparticles corresponding to velocity of about 500 m/s and elevated temperature of the system about 700-900 K are optimal parameters for production of high-quality layers of copper on the aluminum surface. These conditions provide both a good adhesion and a less degree of the plastic deformation. At the same time, higher impact velocities can be used for combined treatment consisting of both the plastic deformation and the coating.

Growth and characterization of Cadmium Thiosemicarbazide Bromide crystals for antibacterial and nonlinear optical applications

NASA Astrophysics Data System (ADS)

Thomas Joseph Prakash, J.; Martin Sam Gnanaraj, J.

2015-01-01

Semiorganic nonlinear optical crystals of Cadmium Thiosemicarbazide Bromide was grown by slow evaporation solution growth technique. The unit cell parameters were estimated by subjecting the crystals to single crystal X-ray diffraction. The grown crystals were subjected to Powder X-ray diffraction for analyzing the crystalline nature of the sample. FTIR studies reveal the functional groups and the optical characters were analyzed by UV-Vis spectral studies. Mechanical stability of the sample was assessed by Vicker's micro hardness test. The presence of surface dislocations was identified by chemical etching technique. Antibacterial study was carried out against ACDP declared harmful pathogens. SHG efficiency of CTSB crystal was tested using Nd: YAG laser and it was found to be ∼1.8 times that of potassium dihydrogen phosphate.

Epitaxial GaN layers formed on langasite substrates by the plasma-assisted MBE method

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

Lobanov, D. N., E-mail: dima@ipmras.ru; Novikov, A. V.; Yunin, P. A.

2016-11-15

In this publication, the results of development of the technology of the epitaxial growth of GaN on single-crystal langasite substrates La{sub 3}Ga{sub 5}SiO{sub 14} (0001) by the plasma-assisted molecular-beam epitaxy (PA MBE) method are reported. An investigation of the effect of the growth temperature at the initial stage of deposition on the crystal quality and morphology of the obtained GaN layer is performed. It is demonstrated that the optimal temperature for deposition of the initial GaN layer onto the langasite substrate is about ~520°C. A decrease in the growth temperature to this value allows the suppression of oxygen diffusion frommore » langasite into the growing layer and a decrease in the dislocation density in the main GaN layer upon its subsequent high-temperature deposition (~700°C). Further lowering of the growth temperature of the nucleation layer leads to sharp degradation of the GaN/LGS layer crystal quality. As a result of the performed research, an epitaxial GaN/LGS layer with a dislocation density of ~10{sup 11} cm{sup –2} and low surface roughness (<2 nm) is obtained.« less

A multi-scale model of dislocation plasticity in α-Fe: Incorporating temperature, strain rate and non-Schmid effects

DOE PAGES

Lim, H.; Hale, L. M.; Zimmerman, J. A.; ...

2015-01-05

In this study, we develop an atomistically informed crystal plasticity finite element (CP-FE) model for body-centered-cubic (BCC) α-Fe that incorporates non-Schmid stress dependent slip with temperature and strain rate effects. Based on recent insights obtained from atomistic simulations, we propose a new constitutive model that combines a generalized non-Schmid yield law with aspects from a line tension (LT) model for describing activation enthalpy required for the motion of dislocation kinks. Atomistic calculations are conducted to quantify the non-Schmid effects while both experimental data and atomistic simulations are used to assess the temperature and strain rate effects. The parameterized constitutive equationmore » is implemented into a BCC CP-FE model to simulate plastic deformation of single and polycrystalline Fe which is compared with experimental data from the literature. This direct comparison demonstrates that the atomistically informed model accurately captures the effects of crystal orientation, temperature and strain rate on the flow behavior of siangle crystal Fe. Furthermore, our proposed CP-FE model exhibits temperature and strain rate dependent flow and yield surfaces in polycrystalline Fe that deviate from conventional CP-FE models based on Schmid's law.« less

Cyclic softening in annealed Zircaloy-2: Role of edge dislocation dipoles and vacancies

NASA Astrophysics Data System (ADS)

Sudhakar Rao, G.; Singh, S. R.; Krsjak, Vladimir; Singh, Vakil

2018-04-01

The mechanism of cyclic softening in annealed Zircaloy-2 at low strain amplitudes under strain controlled fatigue at room temperature is rationalized. The unusual softening due to continuous decrease in the phenomenological friction stress is found to be associated with decrease in the resistance against movement of dislocations because of the formation and easy glide of pure edge dislocation dipoles and consequent decrease in friction stress from reduction in the shear modulus. Positron annihilation spectroscopy data strongly support the increase in edge dislocation density containing jogs, from increased positron trapping and increase in annihilation lifetime.

Manipulating crystallization with molecular additives.

PubMed

Shtukenberg, Alexander G; Lee, Stephanie S; Kahr, Bart; Ward, Michael D

2014-01-01

Given the importance of organic crystals in a wide range of industrial applications, the chemistry, biology, materials science, and chemical engineering communities have focused considerable attention on developing methods to control crystal structure, size, shape, and orientation. Tailored additives have been used to control crystallization to great effect, presumably by selectively binding to particular crystallographic surfaces and sites. However, substantial knowledge gaps still exist in the fundamental mechanisms that govern the formation and growth of organic crystals in both the absence and presence of additives. In this review, we highlight research discoveries that reveal the role of additives, either introduced by design or present adventitiously, on various stages of formation and growth of organic crystals, including nucleation, dislocation spiral growth mechanisms, growth inhibition, and nonclassical crystal morphologies. The insights from these investigations and others of their kind are likely to guide the development of innovative methods to manipulate crystallization for a wide range of materials and applications.

Influence of crystal orientation on the formation of femtosecond laser-induced periodic surface structures and lattice defects accumulation

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

Sedao, Xxx; Garrelie, Florence, E-mail: florence.garrelie@univ-st-etienne.fr; Colombier, Jean-Philippe

2014-04-28

The influence of crystal orientation on the formation of femtosecond laser-induced periodic surface structures (LIPSS) has been investigated on a polycrystalline nickel sample. Electron Backscatter Diffraction characterization has been exploited to provide structural information within the laser spot on irradiated samples to determine the dependence of LIPSS formation and lattice defects (stacking faults, twins, dislocations) upon the crystal orientation. Significant differences are observed at low-to-medium number of laser pulses, outstandingly for (111)-oriented surface which favors lattice defects formation rather than LIPSS formation.

Stress-dislocation interaction mechanism in low-temperature thermo-compression sintering of Ag NPs

NASA Astrophysics Data System (ADS)

Wang, Fuliang; Tang, Zikai; He, Hu

2018-04-01

The sintering of metal nanoparticles (NPs) has been widely studied in the field of nanotechnology, and low-temperature sintering has become the industry standard. In this study, a molecular dynamics (MD) model was established to study the sintering behaviour of silver NPs during low-temperature thermo-compression. Primarily, we studied the sintering process, in which the ratio of neck radius to particle radius (x/r) changes. Under a uniaxial pressure, the maximum ratio in the temperature range 420-425 K was 1. According to the change of x/r, the process can be broken down into three stages: the neck-formation stage, neck-growth stage, and neck-stability stage. In addition, the relationship between potential energy, internal stress, and dislocation density during sintering is discussed. The results showed that cycling internal stress played an important role in sintering. Under the uniaxial pressure, the stress-dislocation interaction was found to be the major mechanism for thermo-compression sintering because the plastic deformation product dislocation intensified the diffusion of atoms. Also, the displacement vector, the mean square displacement, and the changing crystal structure during sintering were studied.

Studies of O18 impurity trapping at interstitial dislocation loops in ion implanted Fe (1 1 0) by ion channeling and ab initio calculations

NASA Astrophysics Data System (ADS)

Mathayan, Vairavel; Kothalamuthu, Saravanan; Gnanasekaran, Jaiganesh; Balakrishnan, Sundaravel; Panigrahi, Binaykumar

2018-01-01

The O18 and self ions are implanted at same depth in Fe (1 1 0) crystal and annealed to study the oxygen trapping under excess self interstitial defects. Rutherford backscattering spectrometry, nuclear reaction analysis and channeling measurements have been performed to determine the lattice site position of O18. The presence of dislocation loops is confirmed by energy-dependent dechanneling parameter measurements. From the tilt angular scans of Fe and O18 signals along 〈1 0 0〉, 〈1 1 0〉 axes, O18 is found to be displaced 0.2 Å from tetrahedral towards octahedral interstitial site in O18. Similar lattice site location of oxygen with the displacement of 0.37 Å is predicted by density functional theory calculations for the interaction of oxygen with 〈1 0 0〉 interstitial dislocation loop structure. Our results provide strong evidence on oxygen trapping at interstitial dislocation loops in the presence of excess interstitial defects in iron.

Single versus successive pop-in modes in nanoindentation tests of single crystals

DOE PAGES

Xia, Yuzhi; Gao, Yanfei; Pharr, George M.; ...

2016-05-24

From recent nanoindentation experiments, two types of pop-in modes have been identified: a single pop-in with a large displacement excursion, or a number of pop-ins with comparable and small displacement excursions. Theoretical analyses are developed here to study the roles played by indenter tip radius, pre-existing defect density, heterogeneous nucleation source type, and lattice resistance on the pop-in modes. The evolution of dislocation structures in earlier pop-ins provides input to modeling a stochastic, heterogeneous mechanism that may be responsible for the subsequent pop-ins. It is found that when the first pop-in occurs near theoretical shear stress, the pop-in mode ismore » determined by the lattice resistance and tip radius. When the first pop-in occurs at low shear stress, whether the successive pop-in mode occurs depends on how the heterogeneous dislocation nucleation source density increases as compared to the increase of the total dislocation density. Lastly, the above transitions are found to correlate well with the ratio of indenter tip radius to the mean spacing of dislocation nucleation sources.« less

Hot Spots from Dislocation Pile-up Avalanches

NASA Astrophysics Data System (ADS)

Armstrong, Ronald; Grise, William

2005-07-01

The model of hot spots developed at dislocation pile-up avalanches has been employed to explain both: greater drop- weight heights being required to initiate chemical decomposition of smaller crystals [1]; and, the susceptibility to shear banding of energetic and reference inert materials, for example, adiabatic shear banding in steel [2]. The evidence for RDX (cyclotrimethylenetrinitramine) is that few dislocations are needed in the pile-ups thus providing justification for assessing dynamic pile-up release on a numerical basis for few dislocation numbers [3]. For release from a viscous obstacle, previous and new computations lead to a local temperature plateau occurring at the origin of pile-up release [4], in line with the physical concept of a hot spot. [1] R.W. Armstrong, C.S. Coffey, V.F. DeVost and W.L. Elban, J. Appl. Phys. 68 (1990) 979. [2] R.W. Armstrong and F.J. Zerilli, Mech. Mater. 17 (1994) 319. [3] R.W. Armstrong, Proc. Eighth Intern. Seminar: New Trends in Research of Energetic Materials, April 19- 21, 2005, Pardubice, CZ. [4] W.R. Grise, NRC/AFOSR Summer Faculty Fellowship Program, AFRL/MNME, Eglin Air Force Base, FL, 2003.

20 CFR 663.165 - How long must an individual be in core services in order to be eligible for intensive services?

Code of Federal Regulations, 2014 CFR

2014-04-01

... EMPLOYMENT AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services...

20 CFR 663.165 - How long must an individual be in core services in order to be eligible for intensive services?

Code of Federal Regulations, 2013 CFR

2013-04-01

... EMPLOYMENT AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services...

20 CFR 663.165 - How long must an individual be in core services in order to be eligible for intensive services?

Code of Federal Regulations, 2012 CFR

2012-04-01

... EMPLOYMENT AND TRAINING ADMINISTRATION, DEPARTMENT OF LABOR (CONTINUED) ADULT AND DISLOCATED WORKER ACTIVITIES UNDER TITLE I OF THE WORKFORCE INVESTMENT ACT Delivery of Adult and Dislocated Worker Services...

Phase-field-crystal model for ordered crystals

NASA Astrophysics Data System (ADS)

Alster, Eli; Elder, K. R.; Hoyt, Jeffrey J.; Voorhees, Peter W.

2017-02-01

We describe a general method to model multicomponent ordered crystals using the phase-field-crystal (PFC) formalism. As a test case, a generic B2 compound is investigated. We are able to produce a line of either first-order or second-order order-disorder phase transitions, features that have not been incorporated in existing PFC approaches. Further, it is found that the only elastic constant for B2 that depends on ordering is C11. This B2 model is then used to study antiphase boundaries (APBs). The APBs are shown to reproduce classical mean-field results. Dynamical simulations of ordering across small-angle grain boundaries predict that dislocation cores pin the evolution of APBs.

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

PubMed

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

2016-08-01

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

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

Ulvestad, A.; Welland, M. J.; Cha, W.

Crystallographic imperfections can significantly alter material properties and responses to external stimuli, including solute induced phase transformations and crystal growth and dissolution . Despite recent progress in imaging defects using both electron and x-ray techniques, in situ three-dimensional imaging studies of defect dynamics, necessary to understand and engineer nanoscale processes, remains challenging. Here, we report in situ three-dimensional imaging of defect dynamics during the hydriding phase transformation of individual palladium nanocrystals by Bragg Coherent Diffractive Imaging (BCDI) . During constant pressure experiments, we observed that the phase transformation begins after the nucleation of dislocations in large (300 nm) particles. Themore » 3D dislocation network shows that dislocations are close to the phase boundary. The 3D phase morphology resolved by BCDI suggests that the hydrogen-rich phase is more similar to a spherical cap on the hydrogen-poor phase than the core-shell model commonly assumed. We substantiate this conclusion using 3D phase field modeling and demonstrate how phase morphology affects the critical size for dislocation nucleation. We determine the size dependence of the transformation pressure for large (150-300 nm) palladium nanocrystals using variable pressure experiments. Our results reveal a pathway for solute induced structural phase transformations in nanocrystals and demonstrate BCDI as a novel method for understanding dislocation dynamics in phase transforming systems at the nanoscale.« less

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

NASA Astrophysics Data System (ADS)

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

2016-11-01

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

Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops

PubMed Central

Peterman, Emily M.; Reddy, Steven M.; Saxey, David W.; Snoeyenbos, David R.; Rickard, William D. A.; Fougerouse, Denis; Kylander-Clark, Andrew R. C.

2016-01-01

Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 μm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe microscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The 207Pb/206Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages. PMID:27617295

Asymmetric, compressive, SiGe epilayers on Si grown by lateral liquid-phase epitaxy utilizing a distinction between dislocation nucleation and glide critical thicknesses

NASA Astrophysics Data System (ADS)

O'Reilly, Andrew J.; Quitoriano, Nathaniel

2018-01-01

Uniaxially strained Si1-xGex channels have been proposed as a solution for high mobility channels in next-generation MOSFETS to ensure continued device improvement as the benefits from further miniaturisation are diminishing. Previously proposed techniques to deposit uniaxially strained Si1-xGex epilayers on Si (0 0 1) substrates require multiple deposition steps and only yielded thin strips of uniaxially strained films. A lateral liquid-phase epitaxy (LLPE) technique was developed to deposit a blanket epilayer of asymmetrically strained Si97.4Ge2.6 on Si in a single step, where the epilayer was fully strained in the growth direction and 31% strain-relaxed in the orthogonal direction. The LLPE technique promoted the glide of misfit dislocations, which nucleated in a region with an orthogonal misfit dislocation network, into a region where the dislocation nucleation was inhibited. This created an array of parallel misfit dislocations which were the source of the asymmetric strain. By observing the thicknesses at which the dislocation network transitions from orthogonal to parallel and at which point dislocation glide is exhausted, the separate critical thicknesses for dislocation nucleation and dislocation glide can be determined.

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

NASA Astrophysics Data System (ADS)

Rezaei Mianroodi, Jaber; Svendsen, Bob

2015-04-01

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

The Weighted Burgers Vector: a new quantity for constraining dislocation densities and types using electron backscatter diffraction on 2D sections through crystalline materials.

PubMed

Wheeler, J; Mariani, E; Piazolo, S; Prior, D J; Trimby, P; Drury, M R

2009-03-01

The Weighted Burgers Vector (WBV) is defined here as the sum, over all types of dislocations, of [(density of intersections of dislocation lines with a map) x (Burgers vector)]. Here we show that it can be calculated, for any crystal system, solely from orientation gradients in a map view, unlike the full dislocation density tensor, which requires gradients in the third dimension. No assumption is made about gradients in the third dimension and they may be non-zero. The only assumption involved is that elastic strains are small so the lattice distortion is entirely due to dislocations. Orientation gradients can be estimated from gridded orientation measurements obtained by EBSD mapping, so the WBV can be calculated as a vector field on an EBSD map. The magnitude of the WBV gives a lower bound on the magnitude of the dislocation density tensor when that magnitude is defined in a coordinate invariant way. The direction of the WBV can constrain the types of Burgers vectors of geometrically necessary dislocations present in the microstructure, most clearly when it is broken down in terms of lattice vectors. The WBV has three advantages over other measures of local lattice distortion: it is a vector and hence carries more information than a scalar quantity, it has an explicit mathematical link to the individual Burgers vectors of dislocations and, since it is derived via tensor calculus, it is not dependent on the map coordinate system. If a sub-grain wall is included in the WBV calculation, the magnitude of the WBV becomes dependent on the step size but its direction still carries information on the Burgers vectors in the wall. The net Burgers vector content of dislocations intersecting an area of a map can be simply calculated by an integration round the edge of that area, a method which is fast and complements point-by-point WBV calculations.

New twinning route in face-centered cubic nanocrystalline metals.

PubMed

Wang, Lihua; Guan, Pengfei; Teng, Jiao; Liu, Pan; Chen, Dengke; Xie, Weiyu; Kong, Deli; Zhang, Shengbai; Zhu, Ting; Zhang, Ze; Ma, Evan; Chen, Mingwei; Han, Xiaodong

2017-12-15

Twin nucleation in a face-centered cubic crystal is believed to be accomplished through the formation of twinning partial dislocations on consecutive atomic planes. Twinning should thus be highly unfavorable in face-centered cubic metals with high twin-fault energy barriers, such as Al, Ni, and Pt, but instead is often observed. Here, we report an in situ atomic-scale observation of twin nucleation in nanocrystalline Pt. Unlike the classical twinning route, deformation twinning initiated through the formation of two stacking faults separated by a single atomic layer, and proceeded with the emission of a partial dislocation in between these two stacking faults. Through this route, a three-layer twin was nucleated without a mandatory layer-by-layer twinning process. This route is facilitated by grain boundaries, abundant in nanocrystalline metals, that promote the nucleation of separated but closely spaced partial dislocations, thus enabling an effective bypassing of the high twin-fault energy barrier.

Atomistic Simulation of Interstitial Dislocation Loop Evolution under Applied Stresses in BCC Iron

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

Long, Xue Hao; Wang, Dong; Setyawan, Wahyu

Evolution of an interstitial 1/2⟨111⟩ dislocation loop under tensile, shear, and torsion stresses is studied with molecular statics method. Under a tensile stress, the dependence of ultimate tensile strength on size of loop is calculated. The formation of small shear loops around the initial prismatic loop is confirmed as an intermediate state to form the final dislocation network. Under a shear stress, the rotation of a loop is observed not only by a change of the habit plane but also through a transformation between a shear and a prismatic loop. Under torsion, a perfect BCC crystal may undergo a BCCmore » to FCC or BCC to HCP transformation. The present work indicates that a 1/2⟨111⟩ loop can delay these transformations, resulting in the formation of micro-crack on the surface.« less

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

NASA Technical Reports Server (NTRS)

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

1987-01-01

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

In situ observation of shear-driven amorphization in silicon crystals

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

He, Yang; Zhong, Li; Fan, Feifei

Amorphous materials have attracted great interest in the scientific and technological fields. An amorphous solid usually forms under the externally driven conditions of melt-quenching, irradiation and severe mechanical deformation. However, its dynamic formation process remains elusive. Here we report the in situ atomic-scale observation of dynamic amorphization processes during mechanical straining of nanoscale silicon crystals by high resolution transmission electron microscopy (HRTEM). We observe the shear-driven amorphization (SDA) occurring in a dominant shear band. The SDA involves a sequence of processes starting with the shear-induced diamond-cubic to diamond-hexagonal phase transition that is followed by dislocation nucleation and accumulation in themore » newly formed phase, leading to the formation of amorphous silicon. The SDA formation through diamond-hexagonal phase is rationalized by its structural conformity with the order in the paracrystalline amorphous silicon, which maybe widely applied to diamond-cubic materials. Besides, the activation of SDA is orientation-dependent through the competition between full dislocation nucleation and partial gliding.« less

Indentation size effects in single crystal copper as revealed by synchrotron x-ray microdiffraction

NASA Astrophysics Data System (ADS)

Feng, G.; Budiman, A. S.; Nix, W. D.; Tamura, N.; Patel, J. R.

2008-08-01

For a Cu single crystal, we find that indentation hardness increases with decreasing indentation depth, a phenomenon widely observed before and called the indentation size effect (ISE). To understand the underlying mechanism, we measure the lattice rotations in indentations of different sizes using white beam x-ray microdiffraction (μXRD); the indentation-induced lattice rotations are directly measured by the streaking of x-ray Laue spots associated with the indentations. The magnitude of the lattice rotations is found to be independent of indentation size, which is consistent with the basic tenets of the ISE model. Using the μXRD data together with an ISE model, we can estimate the effective radius of the indentation plastic zone, and the estimate is consistent with the value predicted by a finite element analysis. Using these results, an estimate of the average dislocation densities within the plastic zones has been made; the findings are consistent with the ISE arising from a dependence of the dislocation density on the depth of indentation.

Effect of microstructure on the coupled electromagnetic-thermo-mechanical response of cyclotrimethylenetrinitramine-estane energetic aggregates to infrared laser radiation

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

Brown, Judith A.; Zikry, M. A., E-mail: zikry@ncsu.edu

2015-09-28

The coupled electromagnetic (EM)-thermo-mechanical response of cyclotrimethylenetrinitramine-estane energetic aggregates under laser irradiation and high strain rate loads has been investigated for various aggregate sizes and binder volume fractions. The cyclotrimethylenetrinitramine (RDX) crystals are modeled with a dislocation density-based crystalline plasticity formulation and the estane binder is modeled with finite viscoelasticity through a nonlinear finite element approach that couples EM wave propagation with laser heat absorption, thermal conduction, and inelastic deformation. Material property and local behavior mismatch at the crystal-binder interfaces resulted in geometric scattering of the EM wave, electric field and laser heating localization, high stress gradients, dislocation density, andmore » crystalline shear slip accumulation. Viscous sliding in the binder was another energy dissipation mechanism that reduced stresses in aggregates with thicker binder ligaments and larger binder volume fractions. This investigation indicates the complex interactions between EM waves and mechanical behavior, for accurate predictions of laser irradiation of heterogeneous materials.« less

Deformed Materials: Towards a Theory of Materials Morphology Dynamics

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

Sethna, James P

This grant supported work on the response of crystals to external stress. Our primary work described how disordered structural materials break in two (statistical models of fracture in disordered materials), studied models of deformation bursts (avalanches) that mediate deformation on the microscale, and developed continuum dislocation dynamics models for plastic deformation (as when scooping ice cream bends a spoon, Fig. 9). Glass is brittle -- it breaks with almost atomically smooth fracture surfaces. Many metals are ductile -- when they break, the fracture surface is locally sheared and stretched, and it is this damage that makes them hard to break.more » Bone and seashells are made of brittle material, but they are strong because they are disordered -- lots of little cracks form as they are sheared and near the fracture surface, diluting the external force. We have studied materials like bone and seashells using simulations, mathematical tools, and statistical mechanics models from physics. In particular, we studied the extreme values of fracture strengths (how likely will a beam in a bridge break far below its design strength), and found that the traditional engineering tools could be improved greatly. We also studied fascinating crackling-noise precursors -- systems which formed microcracks of a broad range of sizes before they broke. Ductile metals under stress undergo irreversible plastic deformation -- the planes of atoms must slide across one another (through the motion of dislocations) to change the overall shape in response to the external force. Microscopically, the dislocations in crystals move in bursts of a broad range of sizes (termed 'avalanches' in the statistical mechanics community, whose motion is deemed 'crackling noise'). In this grant period, we resolved a longstanding mystery about the average shape of avalanches of fixed duration (using tools related to an emergent scale invariance), we developed the fundamental theory describing the shapes of avalanches and how they are affected by the edges of the microscope viewing window, we found that slow creep of dislocations can trigger an oscillating response explaining recent experiments, we explained avalanches under external voltage, and we have studied how avalanches in experiments on the microscale relate to deformation of large samples. Inside the crystals forming the metal, the dislocations arrange into mysterious cellular structures, usually ignored in theories of plasticity. Writing a natural continuum theory for dislocation dynamics, we found that it spontaneously formed walls -- much like models of traffic jams and sonic booms. These walls formed rather realistic cellular structures, which we examined in great detail -- our walls formed fractal structures with fascinating scaling properties, related to those found in turbulent fluids. We found, however, that the numerical and mathematical tools available to solve our equations were not flexible enough to incorporate materials-specific information, and our models did not show the dislocation avalanches seen experimentally. In the last year of this grant, we wrote an invited review article, explaining how plastic flow in metals shares features with other stressed materials, and how tools of statistical physics used in these other systems might be crucial for understanding plasticity.« less

Multiscale Analysis of Structurally-Graded Microstructures Using Molecular Dynamics, Discrete Dislocation Dynamics and Continuum Crystal Plasticity

NASA Technical Reports Server (NTRS)

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

2014-01-01

A multiscale modeling methodology is developed for structurally-graded material microstructures. Molecular dynamic (MD) simulations are performed at the nanoscale to determine fundamental failure mechanisms and quantify material constitutive parameters. These parameters are used to calibrate material processes at the mesoscale using discrete dislocation dynamics (DD). Different grain boundary interactions with dislocations are analyzed using DD to predict grain-size dependent stress-strain behavior. These relationships are mapped into crystal plasticity (CP) parameters to develop a computationally efficient finite element-based DD/CP model for continuum-level simulations and complete the multiscale analysis by predicting the behavior of macroscopic physical specimens. The present analysis is focused on simulating the behavior of a graded microstructure in which grain sizes are on the order of nanometers in the exterior region and transition to larger, multi-micron size in the interior domain. This microstructural configuration has been shown to offer improved mechanical properties over homogeneous coarse-grained materials by increasing yield stress while maintaining ductility. Various mesoscopic polycrystal models of structurally-graded microstructures are generated, analyzed and used as a benchmark for comparison between multiscale DD/CP model and DD predictions. A final series of simulations utilize the DD/CP analysis method exclusively to study macroscopic models that cannot be analyzed by MD or DD methods alone due to the model size.

Investigation of primary static recrystallization in a NiTiFe shape memory alloy subjected to cold canning compression using the coupling crystal plasticity finite element method with cellular automaton

NASA Astrophysics Data System (ADS)

Zhang, Yanqiu; Jiang, Shuyong; Hu, Li; Zhao, Yanan; Sun, Dong

2017-10-01

The behavior of primary static recrystallization (SRX) in a NiTiFe shape memory alloy (SMA) subjected to cold canning compression was investigated using the coupling crystal plasticity finite element method (CPFEM) with the cellular automaton (CA) method, where the distribution of the dislocation density and the deformed grain topology quantified by CPFEM were used as the input for the subsequent SRX simulation performed using the CA method. The simulation results were confirmed by the experimental ones in terms of microstructures, average grain size and recrystallization fraction, which indicates that the proposed coupling method is well able to describe the SRX behavior of the NiTiFe SMA. The results show that the dislocation density exhibits an inhomogeneous distribution in the deformed sample and the recrystallization nuclei mainly concentrate on zones where the dislocation density is relatively higher. An increase in the compressive deformation degree leads to an increase in nucleation rate and a decrease in grain boundary spaces in the compression direction, which reduces the growth spaces for the SRX nuclei and impedes their further growth. In addition, both the mechanisms of local grain refinement in the incomplete SRX and the influence of compressive deformation degree on the grain size of SRX were vividly illustrated by the corresponding physical models.

Inherited Pb isotopic records in olivine antecryst-hosted melt inclusions from Hawaiian lavas

NASA Astrophysics Data System (ADS)

Sakyi, Patrick Asamoah; Tanaka, Ryoji; Kobayashi, Katsura; Nakamura, Eizo

2012-10-01

Dislocation textures of olivine grains and Pb isotopic compositions (207Pb/206Pb and 208Pb/206Pb) of olivine-hosted melt inclusions in basaltic lavas from three Hawaiian volcanoes (Kilauea, Mauna Loa, and Koolau) were examined. More than 70% of the blocky olivine grains in the studied samples have a regular-shaped dislocation texture with their dislocation densities exceeding 106 cm-2, and can be considered as deformed olivine. The size distribution of blocky olivine grains shows that more than 99% of blocky olivines coarser than 1.2 mm are identified as deformed olivine. These deformed olivine grains are identified as antecrysts, which originally crystallized from previous stages of magmatism in the same shield, followed by plastic deformation prior to entrainment in the erupted host magmas. This study revealed that entrainment of mantle-derived crystallization products by younger batches of magma is an important part of the evolution of magnesium-rich Hawaiian magma. Lead isotopic compositions of melt inclusions hosted in the olivine antecrysts provide information of the evolutionary history of Hawaiian volcanoes which could not have been accessed if only whole rock analyses were carried out. Antecryst-hosted melt inclusions in Kilauea and Koolau lavas demonstrate that the source components in the melting region changed during shield formation. In particular, evidence of interaction of plume-derived melts and upper mantle was observed in the earliest stage of Koolau magmatism.

Fast growth of n-type 4H-SiC bulk crystal by gas-source method

NASA Astrophysics Data System (ADS)

Hoshino, Norihiro; Kamata, Isaho; Tokuda, Yuichiro; Makino, Emi; Kanda, Takahiro; Sugiyama, Naohiro; Kuno, Hironari; Kojima, Jun; Tsuchida, Hidekazu

2017-11-01

Fast growth of n-type 4H-SiC crystals was attempted using a high-temperature gas-source method. High growth rates exceeding 9 mm/h were archived at a seed temperature of 2550 °C, although the formation of macro-step bunching caused doping fluctuation and voids in the grown crystal. We investigated a trade-off between growth-rate enhancement and macro-step formation and how to improve the trade-off. By controlling the growth conditions, the growth of highly nitrogen-doped 4H-SiC crystals without the doping fluctuation and void formation were accomplished under a high growth rate exceeding 3 mm/h, maintaining the density of threading screw dislocations in the same level with the seed crystal. The influence of growth parameters on nitrogen incorporations into grown crystals was also surveyed.

Dislocation dynamics simulations of plasticity at small scales

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

Zhou, Caizhi

2010-01-01

As metallic structures and devices are being created on a dimension comparable to the length scales of the underlying dislocation microstructures, the mechanical properties of them change drastically. Since such small structures are increasingly common in modern technologies, there is an emergent need to understand the critical roles of elasticity, plasticity, and fracture in small structures. Dislocation dynamics (DD) simulations, in which the dislocations are the simulated entities, offer a way to extend length scales beyond those of atomistic simulations and the results from DD simulations can be directly compared with the micromechanical tests. The primary objective of this researchmore » is to use 3-D DD simulations to study the plastic deformation of nano- and micro-scale materials and understand the correlation between dislocation motion, interactions and the mechanical response. Specifically, to identify what critical events (i.e., dislocation multiplication, cross-slip, storage, nucleation, junction and dipole formation, pinning etc.) determine the deformation response and how these change from bulk behavior as the system decreases in size and correlate and improve our current knowledge of bulk plasticity with the knowledge gained from the direct observations of small-scale plasticity. Our simulation results on single crystal micropillars and polycrystalline thin films can march the experiment results well and capture the essential features in small-scale plasticity. Furthermore, several simple and accurate models have been developed following our simulation results and can reasonably predict the plastic behavior of small scale materials.« less

Investigations of structural defects, crystalline perfection, metallic impurity concentration and optical quality of flat-top KDP crystal

NASA Astrophysics Data System (ADS)

Sharma, S. K.; Verma, Sunil; Singh, Yeshpal; Bartwal, K. S.; Tiwari, M. K.; Lodha, G. S.; Bhagavannarayana, G.

2015-08-01

KDP crystal grown using flat-top technique has been characterized using X-ray and optical techniques with the aim of correlating the defects structure and impurity concentration in the crystal with its optical properties. Crystallographic defects were investigated using X-ray topography revealing linear and arc like chains of dislocations and to conclude that defects do not originate from the flat-top part of the crystal. Etching was performed to quantify dislocation defects density. The crystalline perfection of the crystal was found to be high as the FWHM of the rocking curves measured at several locations was consistently low 6-9 arc s. The concentration of Fe metallic impurity quantified using X-ray fluorescence technique was approximately 5 times lower in the flat-top part which falls in pyramidal growth sector as compared to the region near to the seed which lies in prismatic sector. The spectrophotometric characterization for plates cut normal to different crystallographic directions in the flat-top potassium dihydrogen phosphate (FT-KDP) crystal was performed to understand the influence of metallic impurity distribution and growth sectors on the optical transmittance. The transmittance of the FT-KDP crystal at 1064 nm and its higher harmonics (2nd, 3rd, 4th and 5th) was determined from the measured spectra and the lower transmission in the UV region was attributed to increased absorption by Fe metallic impurity at these wavelengths. The results are in agreement with the results obtained using X-ray fluorescence and X-ray topography. Birefringence and Mach-Zehnder interferometry show that except for the region near to the seed crystal the optical homogeneity of the entire crystal was good. The laser-induced damage threshold (LDT) values are in the range 2.4-3.9 GW/cm2. The LDT of the plate taken from the flat-top region is higher than that from the bottom of the crystal, indicating that the flat-top technique has good optical quality and is comparable to those reported using rapid growth technique. The results indicate that the structural defects, crystalline quality and impurity concentration have a correlation with the optical properties of the FT-KDP crystal.

Lattice-Matched Semiconductor Layers on Single Crystalline Sapphire Substrate

NASA Technical Reports Server (NTRS)

Choi, Sang; King, Glen; Park, Yeonjoon

2009-01-01

SiGe is an important semiconductor alloy for high-speed field effect transistors (FETs), high-temperature thermoelectric devices, photovoltaic solar cells, and photon detectors. The growth of SiGe layer is difficult because SiGe alloys have different lattice constants from those of the common Si wafers, which leads to a high density of defects, including dislocations, micro-twins, cracks, and delaminations. This innovation utilizes newly developed rhombohedral epitaxy of cubic semiconductors on trigonal substrates in order to solve the lattice mismatch problem of SiGe by using trigonal single crystals like sapphire (Al2O3) as substrate to give a unique growth-orientation to the SiGe layer, which is automatically controlled at the interface upon sapphire (0001). This technology is different from previous silicon on insulator (SOI) or SGOI (SiGe on insulator) technologies that use amorphous SiO2 as the growth plane. A cubic semiconductor crystal is a special case of a rhombohedron with the inter-planar angle, alpha = 90 deg. With a mathematical transformation, all rhombohedrons can be described by trigonal crystal lattice structures. Therefore, all cubic lattice constants and crystal planes (hkl) s can be transformed into those of trigonal crystal parameters. These unique alignments enable a new opportunity of perfect lattice matching conditions, which can eliminate misfit dislocations. Previously, these atomic alignments were thought to be impossible or very difficult. With the invention of a new x-ray diffraction measurement method here, growth of cubic semiconductors on trigonal crystals became possible. This epitaxy and lattice-matching condition can be applied not only to SiGe (111)/sapphire (0001) substrate relations, but also to other crystal structures and other materials, including similar crystal structures which have pointgroup rotational symmetries by 120 because the cubic (111) direction has 120 rotational symmetry. The use of slightly miscut (less than plus or minus 10 deg.) sapphire (0001) substrate can be used to improve epitaxial relationships better by providing attractive atomic steps in the epitaxial process.

Method of fabricating low-dislocation-density epitaxially-grown films with textured surfaces

DOEpatents

Li, Qiming; Wang, George T

2015-01-13

A method for forming a surface-textured single-crystal film layer by growing the film atop a layer of microparticles on a substrate and subsequently selectively etching away the microparticles to release the surface-textured single-crystal film layer from the substrate. This method is applicable to a very wide variety of substrates and films. In some embodiments, the film is an epitaxial film that has been grown in crystallographic alignment with respect to a crystalline substrate.

Internal friction measurement in high purity tungsten single crystal

NASA Technical Reports Server (NTRS)

Rieu, G. E.

1974-01-01

Internal friction peaks observed after small deformation in high purity tungsten single crystals between liquid helium temperature and 800 K in the frequency range 30-50 KHz, are studied as a function of orientation. An orientation effect is observed in the internal friction spectra due to the creation of internal stresses. The elementary processes related to these peaks are discussed in terms of kink generation and geometric kink motion on screw and edge dislocations in an internal stress field.

Epitaxial Growth of Cubic Crystalline Semiconductor Alloys on Basal Plane of Trigonal or Hexagonal Crystal

NASA Technical Reports Server (NTRS)

Park, Yeonjoon (Inventor); Choi, Sang H. (Inventor); King, Glen C. (Inventor)

2011-01-01

Hetero-epitaxial semiconductor materials comprising cubic crystalline semiconductor alloys grown on the basal plane of trigonal and hexagonal substrates, in which misfit dislocations are reduced by approximate lattice matching of the cubic crystal structure to underlying trigonal or hexagonal substrate structure, enabling the development of alloyed semiconductor layers of greater thickness, resulting in a new class of semiconductor materials and corresponding devices, including improved hetero-bipolar and high-electron mobility transistors, and high-mobility thermoelectric devices.

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

Yang, Yongsoo; Chen, Chien-Chun; Scott, M. C.

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling ‘real’ materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily onmore » average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. The work presented here combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure–property relationships at the fundamental level.« less

Deciphering chemical order/disorder and material properties at the single-atom level.

PubMed

Yang, Yongsoo; Chen, Chien-Chun; Scott, M C; Ophus, Colin; Xu, Rui; Pryor, Alan; Wu, Li; Sun, Fan; Theis, Wolfgang; Zhou, Jihan; Eisenbach, Markus; Kent, Paul R C; Sabirianov, Renat F; Zeng, Hao; Ercius, Peter; Miao, Jianwei

2017-02-01

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling 'real' materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily on average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. This work combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure-property relationships at the fundamental level.

Brittle-viscous deformation of vein quartz under fluid-rich low greenschist facies conditions

NASA Astrophysics Data System (ADS)

Jørgen Kjøll, Hans; Viola, Giulio; Menegon, Luca; Sørensen, Bjørn

2015-04-01

A coarse grained, statically crystallized quartz vein with a random CPO, embedded in a phyllonitic matrix, was studied by optical microscopy, SEM imaging and EBSD to gain insights into the processes of strain localization in quartz deformed under low greenschist facies conditions at the frictional-viscous transition. The vein is located in a high strain zone at the front of an imbricate stack of Caledonian age along the northwesternmost edge of the Repparfjord Tectonic Window in northern Norway. The vein was deformed within the Nussirjavrri Fault Zone (NFZ), an out-of-sequence thrust with a phyllonitic core characterized by a ramp-flat-ramp geometry, NNW plunging stretching lineations and top-to-the SSE thrusting kinematics. Deformation conditions are typical of the frictional-viscous transition. The phyllonitic core formed at the expense of metabasalt wherein feldspar broke down to form interconnected layers of fine, synkinematic phyllosilicates. In the mechanically weak framework of the phyllonite, the studied quartz vein acted as a relatively rigid body deforming mainly by coaxial strain. Viscous deformation, related to the development of a mesoscopic pervasive extensional crenulation cleavage, was accommodated within the vein initially by basal slip of suitably oriented quartz crystals, which produced e.g. undulose extinction, extinction bands and bulging grain boundaries. In the case of misoriented quartz crystals, however, glide-accommodated dislocation creep resulted soon inefficient and led to localized dislocation tangling and strain hardening. In response to 1) hardening, 2) progressive increase of fluid pressure within the actively deforming vein and 3) increasing competence contrast between the vein and the surrounding weak, foliated phyllonitic fault core, quartz crystals began to deform frictionally along specific lattice planes oriented optimally with respect to the imposed stress field. Microfaulting generated small volumes of gouge along intracrystalline microfractures. These fractures were rapidly sealed by nucleation of new grains as transiently over-pressured fluids flushed the deforming system. The new nucleated grains grew initially by solution-precipitation and later by grain boundary migration. They are relatively strain free and show a scattered CPO in resemblance with the host grain, although there is a slight synthetic rotation of the crystallographic axes. Due to the random initial orientation of the vein crystals, strain was thus accommodated differently in the individual crystals, leading to the development of remarkably different microstructures. Crystals oriented optimally for basal slip accommodated strain mainly in a viscous fashion and experienced only minor to no fracturing. Instead, crystals misoriented for basal slip hardened and deformed by pervasive fracturing promoted by the fluid over-pressure and controlled by the orientation of crystallographic planes. Viscous deformation continued after the microfractures sealed, again increasing the fluid pressure. This study indicates the importance of considering shear zones as dynamic systems wherein the activated deformation mechanisms vary transiently in response to the complex temporal and spatial evolution of the shear zone, often in a cyclic fashion.

Selective-area growth of GaN nanocolumns on Si(111) substrates for application to nanocolumn emitters with systematic analysis of dislocation filtering effect of nanocolumns

NASA Astrophysics Data System (ADS)

Kishino, Katsumi; Ishizawa, Shunsuke

2015-06-01

The growth of highly uniform arrays of GaN nanocolumns with diameters from 122 to 430 nm on Si (111) substrates was demonstrated. The employment of GaN film templates with flat surfaces (root mean square surface roughness of 0.84 nm), which were obtained using an AlN/GaN superlattice (SL) buffer on Si, contributed to the high-quality selective-area growth of nanocolumns using a thin Ti mask of 5 nm thickness by rf-plasma-assisted molecular beam epitaxy. Although the GaN template included a large number of dislocations (dislocation density ˜1011 cm-2), the dislocation filtering effect of nanocolumns was enhanced with decreasing nanocolumn diameters (D). Systematic transmission electron microscopy (TEM) observation enabled us to explain the dependence of the dislocation propagation behavior in nanocolumns on the nanocolumn diameter for the first time. Plan-view TEM analysis was performed for nanocolumns with D = 120-324 nm by slicing the nanocolumns horizontally at a height of ˜300 nm above their bottoms and dislocation propagation through the nanocolumns was analyzed by the cross-sectional TEM observation of nanocolumns with D ˜ 200 nm. It was clarified that dislocations were effectively filtered in the bottom 300 nm region of the nanocolumns, the dislocation density of the nanocolumns decreased with decreasing D, and for narrow nanocolumns with D < 200 nm, dislocation-free crystals were obtained in the upper part of the nanocolumns. The dramatic improvement in the emission properties of GaN nanocolumns observed with decreasing diameter is discussed in relation to the decreased dislocation density. The laser action of InGaN/GaN-based nanocolumn arrays with a nanocolumn diameter of 170 nm and a period of 200 nm on Si under optical excitation was obtained with an emission wavelength of 407 nm. We also fabricated red-emitting InGaN-based nanocolumn light-emitting diodes on Si that operated at a wavelength of 652 nm, demonstrating vertical conduction through the AlN/GaN SL buffer to the Si substrate.

Simulation of the temperature distribution in crystals grown by Czochralski method

NASA Technical Reports Server (NTRS)

Dudokovic, M. P.; Ramachandran, P. A.

1985-01-01

Production of perfect crystals, free of residual strain and dislocations and with prescribed dopant concentration, by the Czochralski method is possible only if the complex, interacting phenomena that affect crystal growth in a Cz-puller are fully understood and quantified. Natural and forced convection in the melt, thermocapillary effect and heat transfer in and around the crystal affect its growth rate, the shape of the crystal-melt interface and the temperature gradients in the crystal. The heat transfer problem in the crystal and between the crystal and all other surfaces present in the crystal pulling apparatus are discussed at length. A simulation and computer algorithm are used, based on the following assumptions: (1) only conduction occurs in the crystal (experimentally determined conductivity as a function of temperature is used), (2) melt temperature and the melt-crystal heat transfer coefficient are available (either as constant values or functions of radial position), (3) pseudo-steady state is achieved with respect to temperature gradients, (4) crystal radius is fixed, and (5) both direct and reflected radiation exchange occurs among all surfaces at various temperatures in the crystal puller enclosure.

Study on structural, morphological, optical and thermal properties of guanidine carbonate doped nickel sulfate hexahydrate crystal.

PubMed

Silambarasan, A; Rajesh, P; Ramasamy, P

2015-01-05

The single crystal of guanidine carbonate doped nickel sulfate hexahydrate was grown from solution for ultraviolet filters. The single crystal XRD confirms that the grown single crystal belongs to the tetragonal system with the space group of P4₁2₁2. The crystallinity of the grown crystal was estimated by powder X-ray diffraction studies. The optical transmission and thermal stability of as-grown guanidine carbonate doped nickel sulfate single crystals have been studied. The optical transmission spectrum demonstrates the characteristics of ultraviolet filters. The TG/DTA studies confirm the thermal properties of grown crystals. Thermo-gravimetric analysis showed that the dehydration temperature of the guanidine carbonate doped nickel sulfate crystal is about 100 °C, which is much higher than that of pure nickel sulfate hexahydrate (NSH) crystals which is 72 °C. The growth behaviors and dislocation density were detected under the high resolution XRD and etching studies respectively. Copyright © 2014 Elsevier B.V. All rights reserved.

Bending nanowire growth in solution by mechanical disturbance.

PubMed

Wang, Chao; Wei, Yujie; Jiang, Hongyuan; Sun, Shouheng

2010-06-09

The effect of mechanical disturbance on one-dimensional nanocrystal growth in solution phase is investigated by controlled growth of Au nanowires with and without stirring. While a static growth leads to straight, single-crystal Au nanowires, the mechanic disturbance by stirring tends to bend the nanowire growth, yielding nanowire kinks abundant in various types of crystal defects including dislocations, twin boundaries, and grain boundaries. Mechanical modeling and analysis is introduced to elucidate the nanowire growth mechanisms in these two conditions. The provided fundamental understanding of crystal defect formation at nanoscale could be applied to guide the development of advanced nanomaterials with shape control and unique mechanical properties.

In situ observation of defect annihilation in Kr ion-irradiated bulk Fe/amorphous-Fe 2 Zr nanocomposite alloy

DOE PAGES

Yu, K. Y.; Fan, Z.; Chen, Y.; ...

2014-08-26

Enhanced irradiation tolerance in crystalline multilayers has received significant attention lately. However, little is known on the irradiation response of crystal/amorphous nanolayers. We report on in situ Kr ion irradiation studies of a bulk Fe 96Zr 4 nanocomposite alloy. Irradiation resulted in amorphization of Fe 2Zr and formed crystal/amorphous nanolayers. α-Fe layers exhibited drastically lower defect density and size than those in large α-Fe grains. In situ video revealed that mobile dislocation loops in α-Fe layers were confined by the crystal/amorphous interfaces and kept migrating to annihilate other defects. This study provides new insights on the design of irradiation-tolerant crystal/amorphousmore » nanocomposites.« less

Bread board float zone experiment system for high purity silicon

NASA Technical Reports Server (NTRS)

Kern, E. L.; Gill, G. L., Jr.

1982-01-01

A breadboard float zone experimental system has been established at Westech Systems for use by NASA in the float zone experimental area. A used zoner of suitable size and flexibility was acquired and installed with the necessary utilities. Repairs, alignments and modifications were made to provide for dislocation free zoning of silicon. The zoner is capable of studying process parameters used in growing silicon in gravity and is flexible to allow trying of new features that will test concepts of zoning in microgravity. Characterizing the state of the art molten zones of a growing silicon crystal will establish the data base against which improvements of zoning in gravity or growing in microgravity can be compared. 25 mm diameter was chosen as the reference size, since growth in microgravity will be at that diameter or smaller for about the next 6 years. Dislocation free crystals were growtn in the 100 and 111 orientations, using a wide set of growth conditions. The zone shape at one set of conditions was measured, by simultaneously aluminum doping and freezing the zone, lengthwise slabbing and delineating by etching. The whole set of crystals, grown under various conditions, were slabbed, polished and striation etched, revealing the growth interface shape and the periodic and aperiodic natures of the striations.

21 CFR 890.3665 - Congenital hip dislocation abduction splint.

Code of Federal Regulations, 2011 CFR

2011-04-01

... 21 Food and Drugs 8 2011-04-01 2011-04-01 false Congenital hip dislocation abduction splint. 890.3665 Section 890.3665 Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES PHYSICAL MEDICINE DEVICES Physical Medicine Prosthetic Devices § 890...

21 CFR 890.3665 - Congenital hip dislocation abduction splint.

Code of Federal Regulations, 2014 CFR

2014-04-01

... 21 Food and Drugs 8 2014-04-01 2014-04-01 false Congenital hip dislocation abduction splint. 890.3665 Section 890.3665 Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES PHYSICAL MEDICINE DEVICES Physical Medicine Prosthetic Devices § 890...

21 CFR 890.3665 - Congenital hip dislocation abduction splint.

Code of Federal Regulations, 2012 CFR

2012-04-01

... 21 Food and Drugs 8 2012-04-01 2012-04-01 false Congenital hip dislocation abduction splint. 890.3665 Section 890.3665 Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES PHYSICAL MEDICINE DEVICES Physical Medicine Prosthetic Devices § 890...

21 CFR 890.3665 - Congenital hip dislocation abduction splint.

Code of Federal Regulations, 2013 CFR

2013-04-01

... 21 Food and Drugs 8 2013-04-01 2013-04-01 false Congenital hip dislocation abduction splint. 890.3665 Section 890.3665 Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES PHYSICAL MEDICINE DEVICES Physical Medicine Prosthetic Devices § 890...

Asymptotic self-restabilization of a continuous elastic structure

NASA Astrophysics Data System (ADS)

Bosi, F.; Misseroni, D.; Dal Corso, F.; Neukirch, S.; Bigoni, D.

2016-12-01

A challenge in soft robotics and soft actuation is the determination of an elastic system that spontaneously recovers its trivial path during postcritical deformation after a bifurcation. The interest in this behavior is that a displacement component spontaneously cycles around a null value, thus producing a cyclic soft mechanism. An example of such a system is theoretically proven through the solution of the elastica and a stability analysis based on dynamic perturbations. It is shown that the asymptotic self-restabilization is driven by the development of a configurational force, of similar nature to the Peach-Koehler interaction between dislocations in crystals, which is derived from the principle of least action. A proof-of-concept prototype of the discovered elastic system is designed, realized, and tested, showing that this innovative behavior can be obtained in a real mechanical apparatus.

Ultrastructural studies of synthetic apatite crystals.

PubMed

Arends, J; Jongebloed, W L

1979-03-01

In this paper a survey is given of some ultrastructural properties of synthetic hydroxyapatite. The preparation method by which single crystals with a length in the range of 0.1-3.0mm and a defined purity and stoïchiometry can be produced is given. Two groups of materials are considered in detail: carbonate-rich (greater than 0.1% CO3) and low-carbonate hydroxyapatites. The experiments on carbonate-rich material, being the most interesting from a biological point of view, show that acids attack at an active site in the hexagonal basal-plane of the crystals. Later on the crystals dissolve in the center of the crystal parallel to the c-axis forming tube-like structures. The active site can be protected from dissolution if the crystals are pretreated by EHDP or MFP. A comparison with lattice defect theory shows that most likely dislocations of the "hollow-core" type are responsible for the preferential dissolution.