Dislocation Transmission Research Articles

EBSD Analysis of Orientation Gradients Developed near Grain Boundaries

The local misorientation and orientation gradient development near grain boundaries (GBs) are analyzed in a deep drawing quality steel sheet (AKDQ) subjected to interrupted tensile tests in a notched sample. The microstructure is studied using electron backscatter diffraction (EBSD) with subgrain-level spatial resolution. The evolution of misorientation accumulation for particular GBs was traced in grains located in different zones inside the notch, identifying the effective area of influence of GBs inside the neighboring grains. A local study was performed, and the evolution in misorientation development near GB was investigated. The results show a low correlation between GB width and sharpness of the orientation gradient with the mesoscopic strain, but instead orientation gradients between GB zones and the interior of the grains were observed with increasing strain. The increase in severity observed in some GBs can be related to dislocation pile up development, which reduces the permeability of a boundary to dislocation transmission.

Correlations between dislocation density evolution and spall strengths of Cu/Ta multilayered systems at the atomic scales: The role of spacing of KS interfaces

The deformation and failure (spallation) behavior of Cu/Ta multilayered systems with Kurdjumov–Sachs (KS) orientation relationship is investigated at the atomic scales under shock loading conditions. Molecular dynamics (MD) simulations investigate the role of spacing between KS interfaces on the nucleation, evolution and interaction of defect structures (dislocations) in the Cu/Ta multilayered microstructures with layer thicknesses ranging from 3 nm to 47 nm. The shock compression response and failure response is investigated using the computed values of the Hugoniot elastic limit (HEL) and the spall strengths, respectively. KS interfaces serve as strong barriers to dislocation propagation and transmission across the interface and the spacing of the interfaces is observed to influence the spall behavior. The variation of the spall strength values suggests a critical interface spacing of 6 nm, below which the spall strength of the multilayered microstructure is observed to be lower than that for single-crystal Cu for the same loading conditions. The correlations between the temporal evolution of densities of various types of dislocation at the spall planes for the various microstructures and the resulting values of the spall strengths provide a clear rationale for why a microstructure results in increased/decreased spall strength values for the multiphase system.

Strengthening mechanisms and deformability of nanotwinned AlMg alloys

Abstract

Atomistic insight into the dislocation nucleation at crystalline/crystalline and crystalline/amorphous interfaces without full symmetry

Misfit dislocations at bimetal interfaces play a decisive role in determining various deformation behaviors by carrying the shear sliding, serving as a barrier for dislocation transmission and a source of dislocation nucleation. However, when the interface does not possess the distinct feature of misfit dislocations, the nucleation mechanism of lattice dislocations at the interfaces cannot be simply quantified by previously developed atomistic mechanisms based on characteristic misfit dislocations. Using crystalline/crystalline interfaces with a large lattice mismatch and crystalline/amorphous interfaces without local symmetry as prototypes, we show for the first time that the dislocation nucleation at such interfaces is attributable to the localized strain heterogeneities by modifying the volumetric and shear strain components at the atomic level to mechanically respond to different loadings. Using atomic strain tensor analysis, we found that in-plane localized shearing plays a critical role in the emission of lattice dislocations from interfaces, while the corresponding normal components of the volumetric strain tensor will dominate the character of the nucleated lattice dislocation by modifying the atomic excess volume at the interface to overcome the barrier to dislocation nucleation. Further exploration of various crystalline/amorphous interfaces by varying the chemical composition of the amorphous side indicates that chemical heterogeneity may substantially change the strain heterogeneity by forming a different clustered structure at the interface, resulting in the preferred choice of nucleation sites at the boundary regions that can be defined as nano shear traces (NSTs). These results provide a foundation to investigate the effects of strain and chemical heterogeneities in order to provide a realistic explanation of interface mediated deformation mechanisms and an efficient solution to tune interface dominated plasticity.

Control of shear band dynamics in Cu50Zr50 metallic glass by introducing amorphous-crystalline interfaces

The effects of interfaces in crystalline Cu/amorphous Cu50Zr50 on shear banding dynamics are investigated using molecular dynamics simulations. The results show that the introduction of amorphous-crystalline interfaces can effectively promote the homogeneous distribution of shear transformation zones (STZs). We found that the high potential energy combined with the defects of the interfaces promote the nucleation of STZs. Further, the amorphous-crystalline interfaces are transition regions that mediate the transmission of dislocations in the crystallites into STZs in the metallic glasses. The accumulation of STZs and dislocations at the interfaces provide the sources for the enhanced plastic deformation in nanocomposites. These results allow us to determine the fundamental atomic structure of amorphous-crystalline interfaces in nanocomposites.

Studying dislocation-induced shielding effect on the crack-tip in polycrystal by discrete dislocation dynamics

The dislocation-induced shielding effect on the crack-tip depends on not only the number of dislocations but also the distance of dislocations from the crack-tip. The former is closely related to the nucleation of dislocation but the latter to the grain size and the obstruction of grain boundaries (GBs) to dislocations. In this paper, a discrete dislocation dynamic framework was further developed with a crack-tip dislocation emission scheme especially introduced, and then a series of discrete dislocation dynamic simulations were performed to study the shielding effect of dislocations on the crack-tip in poly-crystals. The different roles of the dislocation nucleation from the Frank-Read sources (DNFR) and dislocation emission from the crack-tip (DECT) in the shielding effect were studied, which indicates the DECT mechanism plays more important role compared with the DNFR one for the cases considered here. The shielding effect of dislocation is dominated by the grain size as well as the energy barrier for dislocation transmission across GB. With the increase of grain size, the shielding effect of dislocations on the crack-tip increases significantly, showing strong grain size effect. Moreover, with the increase of the GB energy barrier for dislocation transmission, the shielding effect of dislocations on the crack-tip first increases and then reaches its peak and finally decreases, showing an interesting mountain-shaped dependence on the GB energy barrier. This mountain-shaped dependence is gradually flattened with the increases of grain size. These interesting results enrich our understanding of the dislocations-induced shielding effect on the crack-tip in the poly-crystals.

On the nature of twin boundary-associated strengthening in Fe-Mn-C steel

We unravel the nature of twin boundary-associated strengthening in Fe-Mn-C twinning-induced plasticity steel (TWIPs) by micro-pillar compression tests. Dislocation interactions with a coherent twin boundary and their role on strain hardening were investigated. The results indicate that twin-matrix bundles dynamically introduced by deformation twinning and their interaction with dislocations are required for strengthening Fe-Mn-C TWIPs, while single coherent twin boundaries enable dislocation transmission. Correlative studies on orientation dependent deformation mechanisms, detailed dislocation-twin boundary interactions, and the resulting local stress-strain responses suggest that twin boundary-associated strengthening is primarily caused by the reduction of the mean free dislocation path in nano-twinned microstructures.

Dislocation interactions and crack nucleation in a fatigued near-alpha titanium alloy

Dislocation interactions at the crack nucleation site were investigated in near-alpha titanium alloy Ti-6242Si subjected to low cycle fatigue. Cyclic plastic strain in the alloy resulted in dislocation pile-ups in the primary alpha grains, nucleated at the boundaries between the primary alpha and the two-phase regions. These two phase regions provided a barrier to slip transfer between primary alpha grains. We suggest that crack nucleation occurred near the basal plane of primary alpha grains by the subsurface double-ended pile-up mechanism first conceived by Tanaka and Mura. Superjogs on the basal 〈a〉 dislocations were observed near the crack nucleation location. The two phase regions showed direct transmission of a3 dislocations between secondary alpha plates, transmitted through the beta ligaments as a[010], which then decompose into (a/2)〈111〉 dislocation networks in the beta. The beta ligaments themselves do not appear to form an especially impenetrable barrier to slip, in agreement with the micropillar and crystal plasticity investigations of Zhang et al.

Evolution Behavior of Mixed Higher Order Optical Vortex–Edge Dislocations Propagating Through Atmospheric Turbulence

We model the effect of the off-axis distance of edge dislocation on the transmission of mixed higher order optical vortex–edge dislocations in atmospheric turbulence. We find that when mixed dislocations beams propagate through atmospheric turbulence, the higher order optical vortex will break down, and the topological charge is conserved in the process; an edge dislocation evolves into a pair of optical vortices with the opposite topological charge. With the increase of the propagation distance, one of the optical vortices that are broken down and one of the optical vortices that are evolved will annihilate. And the bigger the off-axis distance of edge dislocation is, the bigger the annihilation distance of optical vortices is. The results illustrate that as the off-axis distance of edge dislocation increases, the transmission of mixed dislocations propagating through atmospheric turbulence becomes better.

A Model of Enhanced Strain Rate Sensitivity in Nanocrystalline and Ultrafine-Grained Metals

Abstract A model is suggested that describes enhanced strain rate sensitivity of nanocrystalline and ultrafine-grained metals. Within the model, plastic deformation of such metals incorporates dislocation transmission across grain boundaries (GBs) in the stress fields of dislocation pileups, the emission of individual dislocations from GBs as well as GB sliding accommodated by GB dislocation climb and/or Coble creep. The model predicts a strong increase in the strain rate sensitivity and a decrease in the activation volume with decreasing grain size, in accord with experimental data.We also considered the effect of GB sliding and Coble creep on the anomalous dependence of the activation volume on temperature observed in nanocrystalline Ni. It is demonstrated that although an account for GB sliding and Coble creep leads to the appearance of cusps in the temperature dependence of the activation volume, these mechanisms alone cannot be responsible for the observed anomalous dependence of the activation volume on temperature.

Dislocation-dominated non-crystallographic rotation in shear bands of Cu/Au nanolayered composites

Shear band, a kind of unstable plasticity behavior, limits deformability of nanoscale materials, however the basic mechanism for such plasticity instability is still not understood well. Here, we found that the plastic deformation within the shear band of 25 nm-Cu/Au nanolayered composites is mainly contributed by the dislocation-dominated non-crystallographic rotation of the Cu/Au interface without destroying the crystallographic orientation. Homogeneous dislocation transmission across the transparent interface led to the non-crystallographic rotation to accommodate the geometrical requirement. The crystallographic relationship and structure in the interface zone can keep intact until the interface zone in the heavily-thinned layers becomes into amorphization.

A versatile digitally-graded buffer structure for metamorphic device applications

Exploring more effective buffer schemes for mitigating dislocation deficiencies is the key technology towards higher performance metamorphic devices. Here we demonstrate a versatile metamorphic grading structure consisting of 38-period alternated multilayers of In0.52Al0.48As and In0.82Al0.18As on InP substrate, thicknesses of which in each period were gradually varied in opposite directions from 48.7 and 1.3 nm to 1.3 and 48.7 nm, respectively, akin to a digital alloy. Both preferentially dislocation nucleation and blocking of threading dislocation transmission are observed near the In0.82Al0.18As/In0.52Al0.48As interfaces, which help relax the strain and lower the residual defect density. A 2.6 μm In0.83Ga0.17As pin photodetector is fabricated on this pseudo-substrate, attaining a low dark current density of 2.9 × 10−6 A cm−2 and a high detectivity of 1.8 × 1010 cmHz1/2W−1 at room temperature, comparable with the states of the art that on linearly-graded buffer layers. These results indicate such digitally-graded buffer structures are promising for enhancing performances of metamorphic devices, and can be easily generalized to other lattice-mismatched material systems.

Slip transfer across γ-TiAl lamellae in tension

This work identifies the mechanisms governing dislocation transmission across lamellar interfaces in a Ti-45Al-2Nb-2Mn (at.%) + 0.8(vol%) TiB2 (Ti4522XD) alloy. With this aim, a microgrip and a microtensile specimen were designed and fabricated using a focused ion beam (FIB), to test micrometer-sized specimens in tension. The deformation mechanisms acting in a microtensile specimen of a Ti4522XD alloy were investigated through electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results revealed that domain boundaries (order variant) and lamellar interfaces (order variant, true twin and pseudo-twin) are relatively transparent to dislocation propagation, but they present significant differences in the geometrical alignment between the incoming and outgoing slip systems. While for order variant interfaces, the incoming and outgoing slip systems are perfectly aligned, true twin and pseudo twin interfaces require a change in slip plane and direction, which leaves residual dislocations with Burgers vectors lying on the interface plane. The analysis of the slip transfer mechanisms revealed that the outgoing slip system is governed more by the maximum geometrical alignment, than the applied resolved shear stress.

Slip transfer across phase boundaries in dual phase titanium alloys and the effect on strain rate sensitivity

Dislocation transmission through α/β phase boundaries in titanium alloys is studied using integrated crystal plasticity (CP) and discrete dislocation plasticity (DDP) modelling techniques, combined with experimental micro-pillar compression test results. Direct dislocation transmission together with the nucleation of new dislocations ahead of a pile-up at an α/β interface, termed indirect slip transfer, are both assessed and their role in controlling microstructure-dependent strain rate sensitivity considered. A critical shear stress criterion for direct slip transfer across an α/β interface in Ti-6242 has been established by capturing the local slip penetration through the phase boundary using CP and DDP comparisons with experimental two phase micro-pillar compression. The competition between direct and indirect slip transfer has been investigated using a single Frank-Read source DDP model. Direct slip transfer is found to occur only under specific conditions which have been quantified. The strain rate sensitivity of dual phase titanium alloys is demonstrated to depend on average pile-up size which is significantly influenced by α/β morphology.

Unraveling the discrepancies in size dependence of hardness and thermal stability in crystalline/amorphous nanostructured multilayers: Cu/Cu-Ti vs. Cu/HfO2.

Crystalline/amorphous interfaces (CAIs) confer outstanding mechanical properties on crystalline/amorphous nanostructured multilayers (C/ANMs), which are widely used in micro/nanodevices, because their unique interfacial structure possesses high strain compatibility. In this study, Cu/X (X = Cu-Ti, HfO2) C/ANMs with equal layer thicknesses (h) were comparatively investigated in terms of size-dependent hardness (H) and thermal stability to uncover the fundamental difference(s) between Cu/Cu-Ti and Cu/HfO2. It was found that both as-deposited Cu/Cu-Ti and Cu/HfO2 C/ANMs exhibited a maximum hardness at a critical thickness of h ∼30 nm, which was caused by a transition from confined dislocation gliding to dislocation transmission across the interface. Specifically, the Cu/Cu-Ti C/ANMs exhibited annealing hardening, whereas the Cu/HfO2 C/ANMs exhibited annealing softening associated with a minimum softening at h ∼ 30 nm, which was closely correlated with their thermal stability. In comparison with monolithic amorphous X thin films, the glassy X nanolayers in the present Cu/X C/ANMs exhibited reduced thermal stability and a trend that smaller sizes led to higher stability. The underlying mechanism of the size-dependent crystallization behavior of X nanolayers is discussed in terms of the constraining effects of the interface. These findings provide deep insights into the design of Cu/metallic-glass and Cu/ceramic-glass C/ANMs with optimal performance.

A review of slip transfer: applications of mesoscale techniques

In this review article, we present and discuss recent mesoscale modeling studies of slip transmission of dislocations through biphase interfaces. Specific focus is given to fcc/fcc material systems. We first briefly review experimental, atomistic, and continuum-scale work that has helped to shape our understanding of these systems. Then several mesoscale methods are discussed, including Peierls–Nabarro models, discrete dislocation dynamics models, and phase field-based techniques. Recent extensions to the mesoscale mechanics technique called phase field dislocation dynamics are reviewed in detail. Results are compiled and discussed in terms of the proposed guidelines that relate composite properties to the critical stress required for a slip transmission event.

Differentiating between intergranular and transgranular fracture in polycrystalline aggregates

The competition between intergranular (IG) and transgranular (TG) fracture in fcc polycrystalline aggregates with physically representative GB misorientation distributions comprised of random low-angle, random high-angle, and coincident site lattice (CSL) GBs has been investigated. Physically-based critical conditions for IG fracture, due to the formation of dislocation pileups, and TG fracture, due to the propagation of cracks on cleavage planes, were coupled to a dislocation-density-based crystal plasticity formulation and a computational fracture scheme for crack branching to investigate how dislocation–GB interactions influence dislocation transmission, pileup formation, and local failure modes. The predictions indicate that aggregates with a large fraction of random and CSL high-angle GBs are dominated by IG fracture, as low GB transmission leads to extensive dislocation-density pileup formation and localized stress accumulations that induce IG fracture. Aggregates with a majority of low-angle GBs are dominated by TG failure, which is consistent with experimental observations. This investigation provides a fundamental understanding of the physical mechanisms governing IG and TG fracture in polycrystalline aggregates.

Studies on tensile behaviour and microstructural evolution of UFG Mg-4Zn-4Gd alloy processed through hot rolling

The effect of rolling at elevated temperature on mechanical behaviour of bulk Mg-4Zn-4Gd alloy was investigated. The alloy was subjected to solution treatment at 673K for 24h, followed by flat and cross rolling at elevated temperature (723K). The microstructure of ultrafine grains (less than ~1µm size) was successfully achieved from solutionized sample with grain size ~78µm after the rolling process, which has significantly influenced the mechanical behaviour of the deformed Mg alloy. The yield strength (YS), ultimate tensile strength (UTS), elongation-to-failure (%E) and hardness of a rolled specimen have enhanced maximum up to the value of 276.3 ± 13MPa, 341.3 ± 7MPa, ~26.1% and 74.2 ± 2HV, respectively. The improvement in mechanical properties of processed Mg alloys due to the combined effect of grain refinement and secondary phase (W-phase) distribution into the Mg matrix upon the repetitive rolling process. In addition, the improvement in tensile properties is explained by correlating the dislocation density (ρ), scanning electron (SEM) and transmission electron (TEM) micrographs. The developed intrinsic property of rolled specimens is substantiated by tensile fractography obtained from SEM equipped with energy dispersive X-ray spectroscopy (EDS). Various other microstructural features were also revealed by TEM and X-ray diffraction (XRD) analysis. The anisotropy of various rolled Mg-4Zn-4Gd specimens was also elucidated on the basis of tensile results.

Shock-induced stacking fault pyramids in Ni/Al multilayers

The formations of stacking fault tetrahedra in {111}fcc/{111}fcc multilayers have been studied extensively, but the researches related to {001}fcc/{001}fcc interfaces can rarely be found in the literature. Using molecular dynamics (MD) simulations, the shock-induced dislocation structure in Ni/Al multilayers was studied in this article, the stacking fault pyramids (SFPs) initiated from Ni/Al interface was firstly observed, and the corresponding mechanism was explored. It was shown that the Shockley partial dislocations dissociated from the rectangular misfit dislocations along Ni/Al interface are first emitted into the Al interlayer and subsequently intersect with each other to form SFPs. With the propagation of the shock front in the multilayers, two adjacent SFPs interact with each other and form Lomer-Cottrell locks. The dislocation sheet composed of SFPs and Lomer-Cottrell locks can act as a barrier to dislocation transmission. The effect of shock piston velocity on the nucleation site of dislocation was also studied. It was shown that dislocations would originate from the interface at a low piston speed (<1.1km/s), and appear at the shock front if the piston speed exceeds 1.2km/s.

Edge dislocations interacting with a Σ11 symmetrical grain boundary in copper upon mixed loading: A quasicontinuum method study

Using quasicontinuum method (QCM), we study GB migration in the dissociated edge dislocation interacting with a Σ11 symmetrical tilt grain boundary (STGB) upon the mixed-mode of simple shear and tensile loadings. The interaction mechanisms are analyzed with the aid of Burgers vectors conservation equations. It is found that loading state significantly impacts not only the dislocation transmission but also the nucleation of grain boundary dislocations (GBDs). Under mixed loading, GBD dipole nucleation tends to easily occur in comparison to the pure shearing even when some incoming dislocations impinging on GB. Besides, mixed loading could reduce the normal stress resolved on the out-going slip plane in the neighboring grain, which makes the dislocation transmission easier in comparison with the pure shearing loading. Further, a map of the critical shear and normal stresses resolved on the grain boundary (GB) plane at the dislocation-GB interaction site is given to identify dislocation transmission, nucleation of GBD and GBD dipole for the different loading states and the cases of different number of incoming dislocations.