Shockley Partials Research Articles

Recordings and Analysis of Atomic Ledge and Dislocation Movements in InGaAs to Nickelide Nanowire Phase Transformation.

The formation of low resistance and self-aligned contacts with thermally stable alloyed phases is a prerequisite for realizing reliable functionality in ultrascaled semiconductor transistors. Detailed structural analysis of the phase transformation accompanying contact alloying can facilitate contact engineering as transistor channels approach a few atoms across. Original in situ heating transmission electron microscopy studies are carried out to record and analyze the atomic scale dynamics of contact alloy formation between Ni and In0.53 Ga0.47 As nanowire channels. It is observed that the nickelide reacts on the In0.53 Ga0.47 As (111) || Ni2 In0.53 Ga0.47 As (0001) interface with atomic ledge propagation along the Ni2 In0.53 Ga0.47 As [101¯0] direction. Ledges nucleate as a train of strained single-bilayers and propagate in-plane as double-bilayers that are associated with a misfit dislocation of b→=2c3[0001]. The atomic structure is reconstructed to explain this phase transformation that involves collective gliding of three Shockley partials in In0.53 Ga0.47 As lattice to cancel out shear stress and the formation of misfit dislocations to compensate the large lattice mismatch in the newly formed nickelide phase and the In0.53 Ga0.47 As layers. This work demonstrates the applicability of interfacial disconnection (ledge + dislocation) theory in a nanowire channel during thermally induced phase transformation that is typical in metal/III-V semiconductor reactions.

Deformation mechanisms during large strain deformation of high Mn TWIP steel

The evolution of texture and microstructure has been studied up to very large strains (ɛt≈3.9) on a twinning induced plasticity (TWIP) steel. Different micromechanisms have been envisaged during different deformation regimes. At early stage of deformation, the mechanism is normal slip dominated, while deformation twining starts at ~20% rolling and increases up to ~40% deformation level. Further deformation takes place by macroscopic shear banding (SBs). A Brass (Bs) type texture forms and strengthens throughout the deformation, which has been simulated by visco-plastic self consistent (VPSC) modelling. The results of VPSC simulations correlate the relative contributions of normal slip, Shockley partials and twin activities at different strain levels, which were observed by microstructural investigation. The Lankford parameter is estimated from texture, indicate that above 70% deformation level, the material develops very high in-plane anisotropy.

Deformation twinning behaviors of the low stacking fault energy high-entropy alloy: An in-situ TEM study

The twinning mechanism in an FeCoNiCrAl0.1 high-entropy alloy was investigated by in-situ transmission electron microscopy. Deformation twins were commonly found on the conjugate plane and were produced by twinning dislocation gliding on every adjacent (111) plane layer by layer. The twinning dislocations were Shockley partials and were generated from the cross-slip.

Effects of Cr on stacking-fault energy and damping capacity of FeMn

Effects of Cr on the stacking-fault energy (SFE) and damping capacity were investigated by experimental and theoretical methods. The effect of Cr on SFE was thermodynamically calculated; the damping capacity, phase composition, microstructure and corrosion resistance were experimentally measured. The results showed that Cr increases the SFE of Fe17MnxCr which resulted in the decay of damping capacity; the increase in α’ martensite with the increasing Cr content above 6% pins the movement of Shockley partials’ dislocation and thus makes the damping capacity even worse; the optimum Cr content of Fe17MnxCr to improve the corrosion resistance in water vapour with a lesser decrease in damping capacity is about 6%.

Simultaneous twinning nucleation mechanisms in an Fe–Mn–Si–Al twinning induced plasticity steel

Two simultaneous twinning nucleation mechanisms were observed in transmission electron microscopy (TEM) during low strain rate tensile deformation of a Fe–27Mn–2.5Si–3.5Al twinning induced plasticity (TWIP) steel at −40 °C. Deformation twinning took place on the conjugate (11¯1) plane through the activation of pole and three–layer mechanisms producing Shockley twinning dislocations having the same Burgers vector. Secondary twinning mechanisms were not identified. The active twinning mechanisms were influenced by the stacking fault energy (SFE) of the deformed austenite, which as estimated using electron and X-ray diffraction was ∼15 mJ/m2 and 20 mJ/m2 for 2% and 5% strains, respectively. This low SFE is believed to hinder cross–slip of twinning Shockley partials so that the stair–rod cross–slip twinning mechanism was not favored. The selection of twinning mechanisms by the microstructure is explained through critical twinning stress, the incidence of intrinsic and extrinsic stacking faults, as well as proper a2[11¯0] dislocations having a screw component that serve as suitable pole dislocations. The active twinning mechanism is plausibly not contributing markedly to the strain hardening of TWIP steels but the SFE and other hardening mechanisms are seemingly more prevalent.

Dual mechanisms of grain refinement in a FeCoCrNi high-entropy alloy processed by high-pressure torsion

An equiatomic FeCoCrNi high-entropy alloy with a face-centered cubic structure was fabricated by a powder metallurgy route, and then processed by high-pressure torsion. Detailed microscopy investigations revealed that grain refinement from coarse grains to nanocrystalline grains occurred mainly via concurrent nanoband (NB) subdivision and deformation twinning. NB–NB, twin–NB and twin–twin interactions contributed to the deformation process. The twin–twin interactions resulted in severe lattice distortion and accumulation of high densities of dislocations in the interaction areas. With increasing strain, NB subdivision and interactions between primary twins and inclined secondary stacking faults (SFs)/nanotwins occurred. Secondary nanotwins divided the primary twins into many equiaxed parts, leading to further grain refinement. The interactions between secondary SFs/nanotwins associated with the presence of Shockley partials and primary twins also transformed the primary twin boundaries into incoherent high-angle grain boundaries.

Correlation of Etch Pits and Dislocations in As-grown and Thermal Cycle-Annealed HgCdTe(211) Films

This paper reports observations of the different types of etch pits and dislocations present in thick HgCdTe (211) layers grown by molecular beam epitaxy on CdTe/Si (211) composite substrates. Dislocation analysis for as-grown and thermal cycle-annealed samples has been carried out using bright-field transmission electron microscopy. Triangular pits present in as-grown material are associated with a mixture of Frank partials and perfect dislocations, while pits with fish-eye shapes have perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector. The dislocations beneath skew pits are more complex as they have two different crystallographic directions, and are associated with a mixture of Shockley partials and perfect dislocations. Dislocation analysis of samples after thermal cycle annealing (TCA) shows that the majority of dislocations under the etch pits are short segments of perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector while the remainder are Shockley partials. The absence of fish-eye shape pits in TCA samples suggests that they are associated with mobile dislocations that have reacted during annealing, causing the overall etch pit density to be reduced. Very large pits with a density ∼2×103 cm−2 are observed in as-grown and TCA samples. These defects thread from within the CdTe buffer layer into the upper regions of the HgCdTe layers. Their depth in as-grown material is so large that it is not possible to locate and identify the underlying defects.

Electronic structure study of screw dislocation core energetics in Aluminum and core energetics informed forces in a dislocation aggregate

We use a real-space formulation of orbital-free DFT to study the core energetics and core structure of an isolated screw dislocation in Aluminum. Using a direct energetics based approach, we estimate the core size of a perfect screw dislocation to be ≈ 7 |b|, which is considerably larger than previous estimates of 1–3 |b| based on displacement fields. The perfect screw upon structural relaxation dissociates into two Shockley partials with partial separation distances of 8.2 Å and 6.6 Å measured from the screw and edge component differential displacement plots, respectively. Similar to a previous electronic structure study on edge dislocation, we find that the core energy of the relaxed screw dislocation is not a constant, but strongly dependent on macroscopic deformations. Next, we use the edge and screw dislocation core energetics data with physically reasonable assumptions to develop a continuum energetics model for an aggregate of dislocations that accounts for the core energy dependence on macroscopic deformations. Further, we use this energetics model in a discrete dislocation network, and from the variations of the core energy with respect to the nodal positions of the network, we obtain the nodal core force which can directly be incorporated into discrete dislocation dynamics frameworks. We analyze and classify the nodal core force into three different contributions based on their decay behavior. Two of these contributions to the core force, both arising from the core energy dependence on macroscopic deformations, are not accounted for in currently used discrete dislocation dynamics models which assume the core energy to be a constant excepting for its dependence on the dislocation line orientation. Using case studies involving simple dislocation structures, we demonstrate that the contribution to the core force from the core energy dependence on macroscopic deformations can be significant in comparison to the elastic Peach–Koehler force even up to distances of 10–15 nm between dislocation structures. Thus, these core effects, whose origins are in the electronic structure of the dislocation core, can play an important role in influencing dislocation-dislocation interactions to much larger distances than considered heretofore.

In-situ high-resolution transmission electron microscopy investigation of grain boundary dislocation activities in a nanocrystalline CrMnFeCoNi high-entropy alloy

Abstract Relaxation activities of grain boundary dislocations in a nanocrystalline CrMnFeCoNi high-entropy alloy were investigated using in-situ high-resolution transmission electron microscopy. 1/3 Frank dislocations at a low-angle grain boundary were relaxed by climbing or emitting Shockley partials that produced stacking faults slightly narrower than the theoretically predicted width. The extended dislocation structures were quite stable since no any change of the Shockley partial position or the stacking fault width was induced under further e-beam irradiation. These behaviours indicate easy activation of Shockley partials and strong barriers for dislocation motion in the alloy, which explains well the excellent deformability and strengthening effect of the alloy. In contrast, Frank dislocations at a twin boundary maintained a compact core structure, but were accompanied with a rotation process of the adjacent grain.

Mobility of dissociated mixed dislocations under an Escaig stress

In FCC metals, the structure of a mixed dislocation core consists of two Shockley Partials which have different screw and edge character. The interactions between the partial dislocations can influence the stacking fault width (SFW). The SFW can also be manipulated by controlling the non-glide component of the total shear stress within the glide plane, commonly referred to as the ‘Escaig stress’, or τe. Molecular dynamics simulations were used to reproduce the dynamic behaviour of the atomistic core and stacking fault of a moving mixed 30° dislocation in copper and with several magnitudes of τe stress. Results showed that the must be relatively large to cause a significant effect on the SFW and that once the SFW is changed it also has a corresponding effect on the drag coefficient for a dislocation moving at steady-state. The reduction of the SFW, to the extent that the partial dislocations come within close proximity (i.e., partially merge into an imperfect full dislocation), changed the linear curve-fit of the stress–velocity curve and could be associated with a ‘quasi-Peierls barrier’ effect. The SFW was also shown to change under a pure glide stress without the addition of a τe stress when the velocity approached the supersonic limit, and caused an increase of the SFW in one direction and a reduction of the SFW in the other direction. This result demonstrates a unique characteristic of mixed dislocations which cannot be observed in the traditionally representative core structures of pure screw and pure edge dislocations because of their perfect symmetry. Surprisingly, the effects of the glide stress on the SFW were similar in magnitude to the effects of , and also were found to have a corresponding effect on the drag coefficient. The unique characteristics observed are summarised in terms of a unified equation for the force equilibrium at steady-state, which considers the drag coefficient of the two partials as independent functions of the velocities. This relationship is rationalised in terms of a drag force imbalance between the two partials of a mixed 30° dislocation, caused by a slightly higher drag force in the ‘mildly non planar’ 0° partial than in the 60° partial.

Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi

The tensile properties of CrCoNi, a medium-entropy alloy, have been shown to be significantly better than those of CrMnFeCoNi, a high-entropy alloy. To understand the deformation mechanisms responsible for its superiority, tensile tests were performed on CrCoNi at liquid nitrogen temperature (77 K) and room temperature (293 K) and interrupted at different strains. Microstructural analyses by transmission electron microscopy showed that, during the early stage of plasticity, deformation occurs by the glide of 1/2<110> dislocations dissociated into 1/6<112> Shockley partials on {111} planes, similar to the behavior of CrMnFeCoNi. Measurements of the partial separations yielded a stacking fault energy of 22 ± 4 mJ m−2, which is ∼25% lower than that of CrMnFeCoNi. With increasing strain, nanotwinning appears as an additional deformation mechanism in CrCoNi. The critical resolved shear stress for twinning in CrCoNi with 16 μm grain size is 260 ± 30 MPa, roughly independent of temperature, and comparable to that of CrMnFeCoNi having similar grain size. However, the yield strength and work hardening rate of CrCoNi are higher than those of CrMnFeCoNi. Consequently, the twinning stress is reached earlier (at lower strains) in CrCoNi. This in turn results in an extended strain range where nanotwinning can provide high, steady work hardening, leading to the superior mechanical properties (ultimate strength, ductility, and toughness) of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi.

Effect of grain boundary segregation of Co or Ti on cyclic deformation of aluminium bi-crystals

There exist many factors affecting fatigue behavior of metallic materials, among them the effect of grain boundary (GB) segregation of alloying elements remains largely unexplored. To exploit this factor, an atomistic simulation is performed to investigate the cyclic behavior of Al bi-crystals with GB segregation of Co or Ti during their mode I cyclic loading perpendicular to GB plane. Results are given in comparison with the case of pure Al bi-crystal. Detailed analysis of the structure evolution during cyclic loading for the considered bi-crystals is presented. It is found that during the cycling, plastic deformation of the pure Al bi-crystal occurs through perfect dislocation sliding followed by twinning, while the GB segregations inhibit the formation of both perfect dislocations and twins in an Al matrix leading to decrease in ductility of the bi-crystals and intergranular crack propagation rate. The cyclic loading of the samples with GB segregation is accompanied with a generation of plenty of Shockley partials. Both the alloying elements in GBs can improve the fatigue lifetime of the Al bi-crystals. The ability of GB segregations in bi-crystalline and polycrystalline Al to retard the dislocation emission from GBs that has been first theoretically revealed in the study could be successfully used to improve fatigue characteristics of metals, especially nanocrystalline metals with a dense GB network.

Resolving individual Shockley partials of a dissociated dislocation by STEM

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.

Observation of double Shockley stacking fault expansion in heavily-nitrogen-doped 4H-SiC using PL technique

The expansion of double-Shockley stacking faults (DSFs) in 4H-SiC was investigated by a photoluminescence (PL) imaging technique. To observe DSFs nondestructively, heavily-nitrogen-doped epilayers were prepared to be used as specimens and the PL technique was applied for the observation. The size and shape of the DSFs in the epilayer were clearly distinguished in the two dimensional PL images, although it is normally difficult to observe luminescence from DSFs in substrates grown by the sublimation method. The nucleation and expansion of the DSFs were tracked in the course of successive high-temperature annealing. Dislocation velocities of Shockley partials along the edge of the DSFs were evaluated from the PL observations. Activation energy for the dislocation glide was estimated from the temperature dependence of the velocities. The nucleation sites of the DSFs are also discussed.

4H-SiCエピ層中の転位

Properties and behavior of dislocations in 4H-SiC are presented based on classical theory of dislocations. 1) A new symbol which is an extension of the famous Frank symbol is introduced to explain complex stacking sequence in SiC polytypes. 2) A basal plane dislocation (BPD) is dissociated into two Shockley partials. Polarity of BPD is described. 3) Principles to assign Burgers vectors and polarity to the individual Shockley partials are derived. 4) Threading screw dislocations (TSD) can be dissociated into four partials, thereby reducing the total energy considerably. 5) Deviation of threading dislocations (TD) from the exact [0001] orientation is reasoned from the viewpoint of energy consideration. 6) Transition from BPD to threading edge dislocations (TED) is explained by taking into account effects of the crystal surface. All of behavior can be explained quite well in terms of classical theory of dislocations.

Deformation mechanisms of Al0.1CoCrFeNi at elevated temperatures

Deformation mechanisms of a high-entropy alloy with a single face-centered-cubic phase, Al0.1CoCrFeNi, at elevated temperatures are studied to explore the high temperature performances of high-entropy alloys. Tensile tests at a strain rate of 10−4s−1 are performed at different temperatures ranging from 25 to 700°C. While both yield strength and ultimate tensile strength decrease with increasing temperature, the maximum elongation to fracture occurred at 500°C. Transmission electron microscopy characterizations reveal that, at both 25 and 500°C, most of deformation occurs by dislocation glide on the normal face-centered-cubic slip system, {111}〈110〉. In contrast, numerous stacking faults are observed at 600 and 700°C, accompanied by the decreasing of dislocation density, which are attributed to the motion of Shockley partials and the dissociation of dislocations, respectively. According to the Considere's criterion, it is assumed that the dissociation of dislocations and movement of Shockley partials at higher temperatures significantly decreases the work hardening during tensile tests, promoting the early onset of necking instability and decreasing the high-temperature ductility.

In-situ transmission electron microscopy study of oxygen vacancy ordering and dislocation annihilation in undoped and Sm-doped CeO2 ceramics during redox processes

Ceria (CeO2) based ceramics have been widely used for many applications due to their unique ionic, electronic, and catalytic properties. Here, we report our findings in investigating into the redox processes of undoped and Sm-doped CeO2 ceramics stimulated by high-energy electron beam irradiation within a transmission electron microscope (TEM). The reduced structure with oxygen vacancy ordering has been identified as the CeO1.68 (C-Ce2O3+δ) phase via high-resolution TEM. The reduction of Ce4+ to Ce3+ has been monitored by electron energy-loss spectroscopy. The decreased electronic conductivity of the Sm-doped CeO2 (Sm0.2Ce0.8O1.9, SDC) is revealed by electron holography, as positive electrostatic charges accumulated at the surfaces of SDC grains under electron beam irradiation, but not at CeO2 grains. The formation of the reduced CeO1.68 domains corresponds to lattice expansion compared to the CeO2 matrix. Therefore, the growth of CeO1.68 nuclei builds up strain inside the matrix, causing annihilation of dislocations inside the grains. By using in-situ high-resolution TEM and a fast OneView camera recording system, we investigated dislocation motion inside both CeO2 and SDC grains under electron beam irradiation. The dislocations prefer to dissociate into Shockley partials bounded by stacking faults. Then, the partials can easily glide in the {111} planes to reach the grain surfaces. Even the Lomer-Cottrell lock can be swept away by the phase change induced strain field. Our results revealed the high mobility of dislocations inside CeO2 and SDC grains during their respective redox processes.

Self-twinning in solid-state decomposition

Twinning in face-centered cubic materials has received increasing attention owing to its favorable contributions to materials properties. While the models designed to account for twin nucleation have been mainly concerned with deformation twinning under external stress, addressing autocatalytic nucleation of twin (self-twinning) remains a challenge. Here, we report experimental evidences and a dislocation-based model demonstrating that a new source for twinning (self-twinning) operatives during solid-state decomposition. The initial stage of self-twinning is a stress relaxation process to relieve the local stress concentration caused by pile-up of interfacial dislocations at the precipitate-matrix interface. Dynamical two-beam analyses on dislocations configurations showed that three types of partial dislocations were involved in self-twinning: two Shockley partials, 16[112] on primary (111¯) plane and 16[112¯] on conjugate (111) plane; a stair-rod dislocation of 16[110] connecting primary and conjugate planes. We further show that the thermally-activated cross-slip of Shockley partials with aid of stair-rod dislocation plays a crucial role in self-twinning process.

Size effect, critical resolved shear stress, stacking fault energy, and solid solution strengthening in the CrMnFeCoNi high-entropy alloy.

High-entropy alloys (HEAs) comprise a novel class of scientifically and technologically interesting materials. Among these, equatomic CrMnFeCoNi with the face-centered cubic (FCC) structure is noteworthy because its ductility and strength increase with decreasing temperature while maintaining outstanding fracture toughness at cryogenic temperatures. Here we report for the first time by single-crystal micropillar compression that its bulk room temperature critical resolved shear stress (CRSS) is ~33–43 MPa, ~10 times higher than that of pure nickel. CRSS depends on pillar size with an inverse power-law scaling exponent of –0.63 independent of orientation. Planar ½ < 110 > {111} dislocations dissociate into Shockley partials whose separations range from ~3.5–4.5 nm near the screw orientation to ~5–8 nm near the edge, yielding a stacking fault energy of 30 ± 5 mJ/m2. Dislocations are smoothly curved without any preferred line orientation indicating no significant anisotropy in mobilities of edge and screw segments. The shear-modulus-normalized CRSS of the HEA is not exceptionally high compared to those of certain concentrated binary FCC solid solutions. Its rough magnitude calculated using the Fleischer/Labusch models corresponds to that of a hypothetical binary with the elastic constants of our HEA, solute concentrations of 20–50 at.%, and atomic size misfit of ~4%.

Behavior of crystal defects in synthetic type-IIa single-crystalline diamond at high temperatures under normal pressure

The behavior of dislocation lines (DLs) and stacking faults (SFs) in synthetic type-IIa single-crystalline diamond at high temperatures under normal pressure has been investigated. After annealing the diamond at 1500°C for 60min in pure N2 atmosphere, straight DLs were bent to converge to fewer curved dislocation bundles, so that some of the stacking faults were extinct while new DLs appeared at the edges of the removed SFs. These results indicate that SFs in the diamond examined belong to the Shockley type, and that the Shockley partials changed to a perfect dislocation. From this result, the following generation mechanism has been proposed for SFs in diamond. On one hand, because [112] dislocations in the (111) growth sector are contained in the slip plane labelled as (1̅1̅1), one perfect dislocation tends to be split into two Shockley partials and a SF when an appropriate stress is applied. On the other hand, the angle between the {111} slip plane and the direction of bundled dislocations in the (001) growth sector is as high as 54.7°, so that a perfect dislocation can hardly slip into partial dislocations. Thus, SFs exist only in the (111) growth sector of type IIa diamond.