Dislocation Dynamics Analysis Research Articles

Atomistic analysis of temperature-dependent dislocation dynamics in Ni3Al-based intermetallic alloys

In this study, we investigate the dynamics of superlattice edge dislocations composed of partial dislocations, complex stacking faults, and antiphase boundaries in binary and ternary Ni-based alloys (γ’ phase) with the L12 structure. In the case of Ni3Al, the dislocation velocity decreases with increasing temperature at low applied shear stress, mainly owing to the phonon drag effect. Meanwhile, in the case of ternary alloys constructed by exchanging Ni atoms with Co atoms in Ni3Al, the effects of temperature on dislocation motion are more complex. At low applied shear stress, the dislocation motion is disturbed by solute Co (i.e., solid-solution strengthening). In this case, the temperature promotes dislocation motion because the potential energy barrier dominates the thermally activated dislocation dynamics. In contrast, when the applied shear stress increases, the temperature suppresses dislocation motion owing to the phonon drag effect. The drag effect also increases with increasing Co concentration. The origin of the energy barrier due to solute atoms at low applied shear stress can be explained by the interaction between the partial dislocation and solute Co. Meanwhile, the origin of the drag effect on the superlattice dislocation dynamics is discussed on the basis of atomic vibrational motions and wavy dislocation behaviors. In this study, we propose and construct a map showing the dominant effects on complex superlattice dislocation dynamics. The map has energy-barrier-dominated, phonon-drag-dominated, and sound velocity-dominated regions, suggesting that the dominant factor changes depending on the applied shear stress, temperature, and strength of the solute–dislocation interaction.

Dislocation dynamics analysis of the strengthening induced by shearable and non-shearable precipitates in the presence of dislocation pile-ups

The effect of dislocation pile-ups on the critical resolved shear stress necessary to overcome a regular distribution of circular precipitates has been analyzed by means of dislocation dynamics simulations. Both precipitates that can and cannot be sheared by dislocations have been considered. Hardening increases with the number of dislocations in the pile-up in the case of impenetrable precipitates due to the back-stress while shearable precipitates are eventually cut by dislocations if enough Orowan loops are piled-up against the precipitate. Analytical expressions for the critical resolved shear stress to overcome both impenetrable and shearable precipitates were developed based on the concept of an effective diameter as a function of the number of dislocations in the pile-up and of the friction stress to shear the precipitate.

Dislocation dynamics analysis influence of precipitate shape in dislocation-precipitate interactions

Dislocation dynamics analysis influence of precipitate shape in dislocation-precipitate interactions

Dislocation dynamics analysis of fracture behavior of microcracks near a macrocrack

In this study, the fracture behavior of a microcrack near a macrocrack tip was analyzed by dislocation dynamics using both the uncoupled model, in which the microcrack and macrocrack are considered separately, and the coupled model, in which they are considered within the same model, to evaluate the effect of the macrocrack on the fracture toughness value. The coupled model allows analysis in which microcrack, and dislocations emitted from microcrack tips are directly affected by stresses from macrocracks and dislocations emitted from macrocrack tips. The validity of each model was examined by comparing the results obtained from the uncoupled and coupled analyses.

Analysis of Basal Plane Dislocation Dynamics in PVT-Grown 4H-SiC Crystals during High Temperature Treatment

The deformation processes in 4H-SiC crystals are of particular interest because they generate mostly basal plane dislocations (BPDs) which are implicated in the degradation or premature breakdown of SiC power devices [1]. Fundamental understanding of the generation and multiplication of BPDs is critical for design of growth strategies to control the density of BPDs during PVT growth of 4H-SiC crystals. Recently, we reported on the direct observation of thermal gradient induced motion of BPDs through in-situ synchrotron X-ray topography imaging of PVT-grown 4H-SiC wafers subject to high temperature treatment [2]. The multiplication of BPDs by the operation and interactions of multiple Frank-Read sources leading to formation of complex configurations was reported. We have analyzed the series of images recorded during the in-situ experiment in detail to deduce the Burgers vectors and core structures of the moving dislocation segments and measure their individual velocities. Dislocations with Burgers vector along the off-cut 11-20 direction were found to be the only dislocations undergoing deformation during heat treatment. Sample was imaged after heat treatment and the segments of dislocations used for velocity measurements were found to be either pure screw or 60° dislocations comprised of Si cores. Using the kink-diffusion model, the activation energies for dislocation motion have been estimated from the velocity data for each of these dislocation types and found to be 2.21 eV for 60° and 3.28eV for pure screw segments respectively. These values are in good agreement with the macroscopic studies of yielding of semiconductor crystals during high temperature compression and indentation experiments [3,4]. Quantitative expression of the temperature dependent critical resolved shear stress required for dislocation motion has been derived from this analysis.

Analysis of Basal Plane Dislocation Dynamics in 4H-SiC Crystals during High Temperature Treatment

Direct observation of thermal gradient induced motion of basal plane dislocations by in-situ synchrotron X-ray topography imaging of PVT-grown 4H-SiC wafers subject to high temperature treatment has provided an opportunity to analyze the movement of dislocations. Dislocations with Burgers vector along the off-cut [11-20] direction were found to be the only dislocations involved in deformation during heat treatment and the segments of dislocations used for velocity measurements were found to be either pure screw comprised of both Si-and C-core partials or 60° dislocations comprised of purely Si cores. Using the kink-diffusion model, the activation energies for dislocation motion have been calculated from the velocity data for each of these dislocation types and found to be 3.28eV for pure screw and 2.21eV for 60° dislocation segments, respectively.

Analysis of Basal Plane Dislocation Dynamics in PVT-Grown 4H-SiC Crystals during High Temperature Treatment

Fundamental understanding of the generation and multiplication of basal plane dislocations (BPDs) in PVT-grown 4H-SiC crystals is critical for design of growth strategies to control their densities during PVT growth of 4H-SiC crystals. Direct observation of thermal gradient induced motion of basal plane dislocations by in-situ synchrotron X-ray topography imaging of PVT-grown 4H-SiC wafers subject to high temperature treatment has provided an opportunity to analyze the movement of dislocations. Dislocations with Burgers vector along the off-cut [11-20] direction were found to be the only dislocations involved in deformation during heat treatment and the segments of dislocations used for velocity measurements were found to be either pure screw comprised of both Si- and C- core partials or 60° dislocations comprised of purely Si cores. Using the kink-diffusion model, the activation energies for dislocation motion have been estimated from the velocity data for each of these dislocation types and found to be 2.21 eV for 60° and 3.28 eV for pure screw segments, respectively. These values are in good agreement with the macroscopic studies of yielding of semiconductor crystals during high temperature compression and indentation experiments. Quantitative expression of the temperature dependent critical resolved shear stress required for dislocation motion has been derived from this analysis.

Dislocation Dynamics Analysis of the Interaction Mechanism between G-phase and Dislocation in Spinodally Decomposed Fe-Cr Alloys

Dislocation Dynamics Analysis of the Interaction Mechanism between G-phase and Dislocation in Spinodally Decomposed Fe-Cr Alloys

Continuum dislocation dynamics-based grain fragmentation modeling

This paper proposes a grain fragmentation modeling approach that couples continuum dislocation dynamics analysis with a crystal-plasticity framework. The proposed model investigates the microstructural features of FCC metals subjected to severe plastic deformation (SPD) processes. Several aspects of the deformation process were considered in this model, including texture evolution, statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) densities evolution, and grains fragmentation and its effect on the overall mechanical response. The proposed model was applied to a reference volume element (RVE) in which the grains are distributed and assigned an initial position. Within the model, each grain has the ability to split into 1024 new smaller grains, which subsequently leads to strain hardening and grain refinement. The latter was modeled by accounting for the grain-grain interaction, for which the concept of the GNDs is incorporated into the mean free path of the dislocations. GNDs were assumed to be induced by grain boundaries that restrict the free deformation of a grain and result in an increase of stresses leading to the grain size reduction. Our grain fragmentation hypothesis was based on the Tóth et al. (2010) lattice curvature assumption [Tóth, L.S., Estrin, Y., Lapovok, R., Gu, C., 2010. A model of grain fragmentation based on lattice curvature. Acta Mater. 58, 1782–1794]. The grain refinement procedure was triggered when the misorientation threshold between subgrains was exceeded. The model parameters were calibrated using torsion tests of pure copper material. The simulation results give reliable predictions of the crystallographic texture, the evolution of dislocation density, and the final grain size based on available experimental data.

An efficient computational technique for modeling dislocation–precipitate interactions within dislocation dynamics

A new computational technique for modeling dislocation interactions with shearable and non-shearable precipitates within the line dislocation dynamics framework is presented. While shearable precipitates are modeled by defining a resistance function, non-shearable ones are modeled by drawing a comparison between the two well-known Orowan and Frank–Read mechanisms. The precipitates are modeled directly within the dislocation dynamics analysis without the need for any additional numerical methods. Due to low computational cost the method is appropriate for simulation of a high dislocation density interacting with large number of precipitates considering different types and various sizes and resistances. It is also efficient for coupling dislocation dynamics with finite element method in multi-scale frameworks since it does not require the mesh to be consistent with the precipitates geometry.

Numerical simulation of stresses and dislocations in quasi-mono silicon

The Alexander–Haasen model is applied for the analysis of dislocation dynamics in quasi-mono crystalline silicon. Model constants are re-calibrated using stress–strain measurements on small silicon samples under uniaxial compression. It is observed that the activation energy may decrease at low temperatures and the hardening parameter generally increases due to the presence of grown-in dislocation clusters. The calibrated model is applied to an idealized cooling process which allows for a discussion of the basic physical mechanisms leading to residual stresses in quasi-mono ingots. Residual stresses can be reduced by minimizing thermal stresses during the elastic–plastic transition, which was observed approximately between 1100°C and 750°C in the present case.

Dry frictional contact of metal asperities: A dislocation dynamics analysis

Discrete dislocation plasticity simulations are performed to investigate the static frictional behavior of a metal asperity on a large single crystal, in contact with a rigid platen. The focus of this study is on understanding the relative importance of contact slip opposed to plasticity in a single asperity at the micrometer size scale, where plasticity is size dependent.Slip of a contact point is taken to occur when the shear traction exceeds the normal traction at that point times a microscopic friction coefficient. Plasticity initiates through the nucleation of dislocations from Frank-Read sources in the metal and is modeled as the collective motion of edge dislocations.Results show that plasticity can delay or even suppress full slip of the contact. This generally happens when the friction coefficient is large. However, if the flattening depth is sufficiently large to induce nucleation of a large dislocation density, slip is suppressed even when the friction coefficient is very small. This study also shows that when self-similar asperities of different size are flattened to the same depth and subsequently loaded tangentially, their frictional behavior appears size independent. However, when they are submitted to the same contact pressure, smaller asperities slip while larger asperities deform plastically.

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

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 []. 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.

Micromorphic crystal plasticity versus discrete dislocation dynamics analysis of multilayer pile-up hardening in a narrow channel

Size effects in the mechanical behavior of multilayer pile-ups embedded in channel microstructures are investigated in terms of work-hardening, plastic slip and geometrically necessary dislocations (GND) distributions. The mechanical responses with various channel sizes are computed by three-dimensional discrete dislocation dynamics (DDD), micromorphic crystal plasticity (Microcurl) and field dislocation mechanics (FDM). The analysis is first limited to single slip with a slip plane perpendicular to the channel walls. In DDD simulations, it is found that the overall work-hardening is strongly dependent on distance between neighbor slip layers. The size dependence disappears when the neighbor layers are close enough to interact with each other. It is confirmed by direct comparison between DDD simulations and two analytical expressions derived from simplified model of multilayer pile-ups. Distributions of slip and GNDs are presented and analyzed for various channel sizes. The cases of inclined slip plane and of double slip systems in a channel are also considered and investigated. The two alternative crystal plasticity theories, Microcurl and FDM, are then found to reproduce the results of DDD. In particular, quantitative correspondence is found between the Microcurl and DDD results.

The inverse hall–petch relation in nanocrystalline metals: A discrete dislocation dynamics analysis

When the grain size in polycrystalline materials is reduced to the nanometer length scale (nanocrystallinity), observations from experiments and atomistic simulations suggest that the yield strength decreases (softening) as the grain size is decreased. This is in contrast to the Hall–Petch relation observed in larger sized grains. We incorporated grain boundary (GB) sliding and dislocation emission from GB junctions into the classical DDD framework, and recovered the smaller is weaker relationship observed in nanocrystalline materials. This current model shows that the inverse Hall–Petch behavior can be obtained through a relief of stress buildup at GB junctions from GB sliding by emitting dislocations from the junctions. The yield stress is shown to vary with grain size, d, by a d1/2 relationship when grain sizes are very small. However, pure GB sliding alone without further plastic accomodation by dislocation emission is grain size independent.

Effect of Plastic Flattening on the Shearing Response of Metal Asperities: A Dislocation Dynamics Analysis

Discrete dislocation (DD) plasticity simulations are carried out to investigate the effect of flattening and shearing of surface asperities. The asperities are chosen to have a rectangular shape to keep the contact area constant. Plasticity is simulated by nucleation, motion, and annihilation of edge dislocations. The results show that plastic flattening of large asperities facilitates subsequent plastic shearing, since it provides dislocations available to glide at lower shear stress than the nucleation strength. The effect of plastic flattening disappears for small asperities, which are harder to be sheared than the large ones, independently of preloading. An effect of asperity spacing is observed with closely spaced asperities being easier to plastically shear than isolated asperities. This effect fades when asperities are very protruding, and therefore plasticity is confined inside the asperities.

Three-dimensional dislocation dynamics analysis of size effects on void growth

Plastic void growth in face-centered cubic single crystals subjected to hydrostatic tension was studied by means of three-dimensional dislocation dynamics. The simulations showed the development of a size effect on the void growth rate of the type “the larger, the faster”. It was found that plastic void growth by dislocation shearing of the voids was inhibited for submicron void sizes due to the lack of active dislocation sources near to the void surface.

A dislocation dynamics analysis of the critical cross-slip annihilation distance and the cyclic saturation stress in fcc single crystals at different temperatures

Cross-slip is treated as a deterministic, mechanically activated process governed by the applied stress, by the interaction force between approaching screw dislocations of the opposite sign and by the line tension related to the persistent slip band channel width. The dislocations are modeled as moving polygons. In the evaluation of the critical cross-slip annihilation distance and the saturation stress in cycling, we accept two assumptions inspired by Brown [Brown L. Philos Mag A 2002;82:1691]: (i) the critical parameter associated with cross-slip is the spreading of the dislocation loop on the cross-slip plane, not the critical formation of a constriction in an extended screw dislocation core. (ii) The saturation stress in cycling is controlled by the stress required to separate two screw dislocations of opposite signs which are just on the point of mutual annihilation by cross-slip. The proposed model predicts the critical annihilation distance and the cyclic saturation stress in agreement with the available experimental data for Cu, Ni and Ag single crystals.

1036 多結晶金属の不連続降伏の転位動力学解析(OS9,OS10 合同ポスターセッション,OS・一般セッション講演)

1036 多結晶金属の不連続降伏の転位動力学解析(OS9,OS10 合同ポスターセッション,OS・一般セッション講演)

1114 微細層状組織内における塑性挙動の離散転位動力学解析(OS11. 材料の組織・強度に関するマルチスケールアナリシス(3),オーガナイズドセッション講演)

1114 微細層状組織内における塑性挙動の離散転位動力学解析(OS11. 材料の組織・強度に関するマルチスケールアナリシス(3),オーガナイズドセッション講演)