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This work applies the computer vision “You only look once” (YOLO) algorithm to extract bounding boxes around dislocations in weak‐beam dark‐field transmission electron microscopy (WBDF TEM) images of semiconductor thin films. A formula is derived to relate the sum of the relative heights of the bounding boxes to the dislocation densities in the films. WBDF TEM images reported in the literature and taken from our α‐Ga2O3 samples are divided into train, evaluation, and test datasets. Different models are trained using the train dataset and evaluated using the evaluation dataset to find the best confidence values, which are used to select the best model based on the performance against the test data set. For α‐Ga2O3 thin films, dislocation density output by this model is on average ≈58% of those estimated by the traditional Ham method. A factor of 4/π may contribute to the systematic underestimation of the model versus the Ham method. [ABSTRACT FROM AUTHOR]

Manganese stabilizes the austenite and reduces the stacking fault energy in face‐centered cubic (FCC) structures, promoting the formation of hexagonal and cubic structures. This study investigates the phase transformation and mechanical behavior of extremely low‐carbon FeCrMn austenitic stainless AISI 201 steel subjected to cold drawing in three stages, ultimately reaching a true strain of 0.93. The objective is to examine the transformation from the parent FCC structure to hexagonal close‐packed and body‐centered cubic structures as a combination of transformation‐induced plasticity and twinning‐induced plasticity phenomena. X‐ray diffraction and electron backscatter diffraction analyses are utilized to investigate phase transformations under significant plastic deformation and the crystallographic orientation relationships between phases. Additionally, tensile and hardness tests are performed to assess the impact of plastic deformation on mechanical properties. Results indicate that a true strain of 50% is optimal for balancing strength and ductility in CrMnFe austenitic stainless steel. After applying a true strain of ε1 = 26.7%, yield strength (YS) increases by 192% and ultimate tensile strength (UTS) by 35%, while elongation reduces by 41%. With a further strain of ε2 = 57.5%, YS increases by 71% and UTS by 44%, but elongation drastically decreases by 94%. Applying the final strain of ε3 = 94.0%, YS and UTS only increase by 3% and 2%, respectively, while elongation further reduced by 40%. These findings suggest that a true strain of 50% shall be considered the maximum reduction for maintaining a balance between strength and ductility in CrMnFe austenitic stainless steel. [ABSTRACT FROM AUTHOR]

The influence of different heat‐treatment conditions on the microstructure evolution and mechanical properties of cold‐rolled Fe–7.69Mn–2.76Al–0.12C (wt%) steel is investigated using various techniques, including scanning electron microscopy, transmission electron microscopy (TEM), and X‐ray diffraction. In the results, it is shown that the stability of austenite is significantly affected by different partitioning temperatures. It is found that factors such as dislocation density, temperature, and grain size collectively influence partitioning behavior. In the observations, it is revealed that when the partitioning temperature is raised from 120 to 180 °C, the dislocation density of the face centered cubic phase within the test steel decreases from 10.38 × 1016 to 7.67 × 1016 m−2. Additionally, within specimens exhibiting higher dislocation densities, carbon element diffusion is more uniform. During the experiment, the poor stability of the austenite is found to be susceptible to stress‐inducing the phase of the martensitic transformation, and the type of the martensite transformation affects the deformation process and the performance of submission behavior. In the observations under TEM, a phenomenon where variations in dislocation density within individual austenite grains may lead to differing stability across grain regions is revealed, thereby triggering localized martensitic phase transformations. [ABSTRACT FROM AUTHOR]

The microstructural evolution and underlaying deformation behavior of Ti–6Al–4V alloy fabricated by laser powder bed fusion (LPBF‐Ti64) during room (RT) and cryogenic (CT: −150 °C) temperature are systematically investigated. LPBF‐Ti64 exhibits significantly higher strength at CT (yield strength [YS]: 1339 MPa, ultimate compressive strength [UCS]: 1982 MPa) as compared to RT (YS: 1013 MPa, UCS: 1690 MPa) with reduction in fracture strain. To probe the deformation mechanisms responsible for exceptionally higher strength at CT, LPBF‐Ti64 alloy is compressed to 10% strain at both temperatures and the underlaying strengthening mechanisms are investigated. Dislocation strengthening is emphasized to be the major strengthening mechanisms (1166/1359 MPa for RT‐10 and CT‐10, respectively). The significantly higher dislocation density at CT (3.73 ×$\times$ 1016 m−2 for CT‐10 and 2.9 ×$\times$ 1016 m−2 for RT‐10) is attributed to the restricted dislocation movement at low temperature, whereas dislocation nucleation during further deformation will multiply the dislocations interlocking and pinning, hence resulting in higher strength. [ABSTRACT FROM AUTHOR]

This research involves conducting unidirectional axial thermal compression experiments on high‐strength titanium‐clad steel plates (TCSPs) at different temperatures and strain rates. Thermally compressed samples are evaluated for macromorphology, microstructure, macroscopic texture, and dislocation density. Deformation consistency is observed in α‐titanium under certain conditions, but not in β‐titanium. Pearlite is the main deformation microstructure near the steel interface, expanding with temperature. Striped grain clusters are observed in the α‐titanium near the interface, enhancing deformation uniformity; furthermore, the prevalence of striped grains increases with higher strain rates. However, dynamic recrystallization in β‐titanium hinderes deformation consistency. The fiber presence of texture <111>//ND ensures consistent deformation consistency, whereas the plate texture {0001} <101¯$\bar{1}$0> in the α‐titanium encourages it. During the deformation process at a consistent rate of strain, it is observed that both matrices initially exhibit a rise, followed by a decline in dislocation density as the temperature increases, peaking at 850 °C. Dislocation density rises as strain rate increases, while temperature remaines constant. At temperatures exceeding 850 °C, bonding strengths decrease with deformation rate and remain comparable. Bonding strengths decreases at 800 °C due to intermetallic compound formation. [ABSTRACT FROM AUTHOR]

The high crystal quality and low dislocation densities of aluminum nitride (AlN) grown on flat and nano-patterned sapphire substrate that are synthesized by the metal-organic chemical vapor deposition (MOCVD) method are essential for the realization of high-efficiency deep ultraviolet light-emitting diodes. The micro-strains of 0.18 × 10−3 cm−2 for flat substrate AlN and 0.11 × 10−3 cm−2 for nano-patterned substrate AlN are obtained by X-ray diffractometer (XRD). The screw and edge dislocation densities of samples are determined by XRD and transmission electron microscope (TEM), and the results indicate that the nano-patterned substrates are effective in reducing the threading dislocation density. The mechanism of the variation of the threading dislocation in AlN films grown on flat and nano-patterned substrates is investigated comparatively. The etch pit density (EPD) determined by preferential chemical etching is about 1.04 × 108 cm−2 for AlN grown on a nano-patterned substrate, which is slightly smaller than the results obtained by XRD and TEM investigation. Three types of etch pits with different sizes are all revealed on the AlN surface using the hot KOH etching method. [ABSTRACT FROM AUTHOR]

The mechanical behavior of parts is significantly affected by the material’s internal defective structure and its evolution. The paper aims to build a complex physically based mathematical model for describing the behavior of metals in the deformation and destruction process. The main deformation mechanisms of metals and alloys are considered. The mechanism and criterion for the microcrack nucleation, as well as a method for microcracks describing, are outlined. The structure and main relations of the developed model are presented, including a description of the most significant mechanisms carriers evolution implemented at each structural-scale level. A submodel of the evolution of dislocation densities during deformation due to such mechanisms as the new dislocations generation and opposite dislocations annihilation on close slipping systems is described. The algorithm for implementing the model and the results of modeling the dislocation structure evolution are presented. The multi-level approach based on the crystal plasticity and the introduction of internal variables is found to be sufficiently effective for describing both the propagation and nucleation of microcracks in metals.

The epitaxial growth of transparent p‐AlGaN‐based ultraviolet‐A (UVA), light‐emitting diodes (LEDs) may solve the problems of UVA light absorption through the GaN buffers and p‐GaN contact layers at (326–341 nm)‐band emission, respectively. Herein, first, an idea of conventional n‐AlGaN buffer layer (BL) and n‐AlGaN electron source layer (ESL) for the suppression of threading dislocation density (TDD) and enhancement of internal quantum efficiency (IQE) of UVA emitters, using low‐pressure metalorganic vapor‐phase epitaxy (LP‐MOVPE) is attempted. As a result, the total‐TDDs is reduced from ≈ 3 × 109 cm−2 to ≈ 1 × 109 cm−2 in the n‐AlGaN ESL of a 326 nm‐band UVA multiquantum‐wells (MQWs), and IQE is also improved from 30% to 52% at room temperature (RT). Second, an idea of Si‐doped n‐AlGaN Superlattices (SLs)‐based BL, using LP‐MOVPE is challenged. Subsequently, a record IQE of 56% at RT and high crystal quality in 341 nm‐band UVA MQWs are observed. Finally, using a well thickness ≈ 2 nm in SLs‐based UVA MQWs, the light power and external quantum efficiency (EQE), respectively, are remarkably enhanced from 3.5 mW and 0.5% to 7.5 mW and 1.4% on wafer in 341 nm‐Band UVA LED. The perspective for the improvements of UVA emitter's performances is also discussed. [ABSTRACT FROM AUTHOR]

Microstructural internal lengths play an important role on the local and macroscopic mechanical behaviors of steels. In this study, the dislocation density gradients near grain boundaries in a ferritic steel are investigated using SEM/EBSD together with instrumented nanoindentation for undeformed and pre-deformed aluminum-killed steels (Al-k) at 3% and 18% tensile plastic strains. The effect of the distance to grain boundaries on Geometrically Necessary Dislocations (GND) densities is first determined by analyzing orientation gradients from 2D-EBSD maps. Then, nanohardness measurements are performed in the vicinity of grain boundaries. Data analyses show a clear correlation between the spatial gradients of GND density and the ones of nanohardness. Using a mechanistic model, the total dislocation densities are estimated from the measured nanohardness values. From both GND and total dislocation density profiles, the value of an internal length, denoted λ, is estimated from the analysis of dislocation density gradients near grain boundaries. At the end, the capabilities of 2D-EBSD and nanoindentation methods to assess this value are discussed. [ABSTRACT FROM AUTHOR]

Medium manganese (Mn) transformation‐induced plasticity (TRIP) steel, with nominal composition of Fe–9Mn–1.5Si–1Al–0.05C, is intercritically annealed after water quenching (WQ) or air quenching (AQ) from austenitization. The results indicate that the lower density of dislocations and a small amount of retained austenite are found in AQ specimen compared with WQ specimen by X‐ray diffraction (XRD). The higher density of dislocations significantly affects the formation of immovable dislocations and promotes martensitic recovery during the intercritical annealing (IA) process, which ultimately causes yield point elongation and a lower yield strength in WQ‐620 sample during the tensile test, respectively. In contrast, AQ‐620 specimen relying on an active TRIP effect displays a continuous deformation behavior, slightly better yield strength, tensile strength, and total elongation, simultaneously. [ABSTRACT FROM AUTHOR]

A quantitative analysis of the surface and subsurface damage to gallium nitride (GaN) substrate from precise mechanical polishing with diamond abrasives and characterizations of their removal by the chemical mechanical polishing (CMP) process with colloidal silica slurry was performed. The cross-sectional transmission electron microscopy showed that the highly-defected regions were approximately 200 nm from the surface. The cathodoluminescence (CL) imaging showed that the subsurface damage layers were a cluster of extremely high-density networks of line-shaped defects. The depth-resolved and CMP-process-time-resolved CL imaging sensitively detected that while the high defect density subsurface damage layers were approximately 200 nm from the surface some of the subsurface damage reached over 1 μm in depth. Using the high-resolution x-ray diffraction, the CMP process was shown to decrease the dislocation densities, enlarge the correlation length, and release the compressive stress in the crystals induced by the mechanical polishing process. Improvement in these crystal properties was found to occur by removing the highly-defected surface regions. • Subsurface damage (SSD) depth in GaN substrate was quantified. • Cross-sectional transmission electron microscopy showed 200 nm SSD layers. • Cathodoluminescence imaging indicated cluster of line-shaped defects in SSD regions. • Dislocation densities were decreased by chemical mechanical polishing (CMP) process. • CMP process was shown to release compressive stress. [ABSTRACT FROM AUTHOR]

Herein, the potential of reformed GaN nanowires (NWs) fabricated by metalorganic chemical vapor deposition (MOCVD) for device‐quality low‐defect density templates and low‐cost alternative to bulk GaN substrates is demonstrated. The effects of epilayer thickness and NW reformation conditions on the crystalline quality and thermal conductivity of the subsequent GaN epilayers are investigated. Smooth surfaces with atomically step‐like morphologies with no spirals are achieved for GaN epilayers on the reformed NW templates, indicating step‐flow growth mode. It is further found that annealing of the NWs at a temperature of 1030 °C in the presence of NH3 and H2, followed by a coalescence done at the same temperature under planar growth conditions, leads to the most efficient screw dislocation density reduction by nearly an order of magnitude. At these optimized conditions, the growth takes place in a layer‐by‐layer fashion, producing a smooth surface with a root mean square (RMS) roughness of 0.12 nm. The highest thermal conductivity of k = 206 W m−1 K−1, approaching the respective value of bulk GaN, is obtained for the optimized 2 μm‐thick GaN layer. The thermal conductivity results are further discussed in terms of the phonon‐dislocation and the phonon‐boundary scattering. [ABSTRACT FROM AUTHOR]

In this study, the behavior of the industrially prominent AISI 1070 pearlitic steel under pressures up to 35 GPa was meticulously explored. Leveraging the 1D integration patterns from the Debye-Scherrer rings, we introduced a groundbreaking method to quantify dislocation densities, providing a novel lens to understand lattice distortions in such widely-used steels. At 5 GPa, the decomposition of cementite became evident, corroborated by changes in lattice parameters and dislocation densities. As the pressure neared to 10 GPa, the emergence of Hägg carbide was observed. The hallmark discovery was the phase transition from BCC to HCP around 20 GPa. This transformation bore witness to further distortions in the HCP structure with mounting pressures. Finally, molecular dynamic simulations revealed a mostly BCC to HCP transformation via an intermediary FCC phase at crystalline defects, elucidating observed orientations and phase transformation pathways. [ABSTRACT FROM AUTHOR]

The analysis of line broadening in X-ray and neutron diffraction patterns using profile functions constructed on the basis of well-established physical principles and TEM observations of lattice defects has proven to be a powerful tool for characterizing microstructures in crystalline materials. These principles are applied in the convolutional multiple-whole-profile (CMWP) procedure to determine dislocation densities, crystallite size, stacking fault and twin boundary densities, and intergranular strains. The different lattice defect contributions to line broadening are separated by considering the hkl dependence of strain anisotropy, planar defect broadening and peak shifts, and the defect dependent profile shapes. The Levenberg–Marquardt (LM) peak fitting procedure can be used successfully to determine crystal defect types and densities as long as the diffraction patterns are relatively simple. However, in more complicated cases like hexagonal materials or multiple-phase patterns, using the LM procedure alone may cause uncertainties. Here, we extended the CMWP procedure by including a Monte Carlo statistical method where the LM and a Monte Carlo algorithm were combined in an alternating manner. The updated CMWP procedure eliminated uncertainties and provided global optimized parameters of the microstructure in good correlation with electron microscopy methods.

Martensitic steels are widely used structural materials with outstanding mechanical properties. Their high strength is provided by the non-diffusional phase transformation of fcc γ into thin lamellar bcc plates during fast cooling. Coherency strains between the fcc and bcc lamellae induce large dislocation densities in the range of 1016 m−2, well above the densities attainable by conventional plastic deformation. Using high resolution X-ray line profile analysis, scanning electron microscopy, and hardness tests we show that during tensile deformation when the active Burgers vectors are within the lath plane the lath-packets work soften. On the contrary, when the active Burgers vectors are oblique to the lath-plane the lath-packets work harden. The softening and hardening processes in the differently oriented lath-packets produce a composite of hard and soft components on the length scale of lath-packet size. The stress–strain response of the alloy is discussed in terms of the different mean free paths and the different annihilation lengths of dislocations in the softened and hardened lath-packets. The relatively good ductility is shown to be produced by the composite microstructure induced by plastic strain.

In single-crystal plasticity, latent hardening between the different slip systems, coming from the interaction of dislocations, strongly influences the mechanical response under prescribed deformation. In this work, the selection of activated systems and their potential segregation in separated zones, taking place in a way which minimizes strain hardening due to system interactions, is investigated analytically for ductile FCC crystals. The crucial role of the respective intensity of the different interactions, estimated by dislocation dynamics simulations, is emphasized and, among them, the very strong collinear interaction between cross slip systems. The evolution of loading paths along lattice axes from symmetric to non-symmetric system activation, which reduces the number of systems, is triggered by uniform instability modes of the initial deformation framework. In the same trend, the segregation of systems in disconnected parts of the crystal, which tends to dissociate primarily strongly interacting pairs, is initiated by plane wave instability modes. Moreover, the introduction of gradient plasticity effects in the form of back stresses functions of the dislocation density tensor, not only introduces an internal length scale for the segregation process, but also brings a selection among the modes in a way which limits the induced incompatibility of the plastic strain field and, hence, the geometrically necessary dislocation (GND) density accumulation. [ABSTRACT FROM AUTHOR]

Specimens of cold-rolled zirconium were tensile-deformed along the rolling (RD) and the transverse (TD) directions. The stress-strain curves revealed a strong texture dependence. High resolution X-ray line profile analysis was used to determine the prevailing active slip-systems in the specimens with different textures. The reflections in the X-ray diffraction patterns were separated into two groups. One group corresponds to the major and the other group to the random texture component, respectively. The dislocation densities, the subgrain size and the prevailing active slip-systems were evaluated by using the convolutional multiple whole profile (CMWP) procedure. These microstructure parameters were evaluated separately in the two groups of reflections corresponding to the two different texture components. Significant differences were found in both, the evolution of dislocation densities and the development of the fractions of < a > and < c + a > type slip systems in the RD and TD specimens during tensile deformation. The differences between the RD and TD stress-strain curves are discussed in terms of the differences of the microstructure evolution. [ABSTRACT FROM AUTHOR]

A new time-integration algorithm is presented for a dislocation-density-based crystal plasticity model. This includes a monolithic iterative scheme in which stresses were solved simultaneously with dislocation densities based on the fully backward Euler method for the integration of the constitutive model. Furthermore, the heuristic convergence criterion based on the slip resistance, instead of dislocation densities, together with the stresses is employed to circumvent stability issues. A closed form of the consistent tangent moduli is also derived. The constitutive models and time-integration procedures have been implemented as a user-subroutine UMAT of a finite element program Abaqus. Representative comparisons between the numerical predictions and experimental data are then given to verify the robustness and stability of the current implementation of the dislocation-density-based crystal plasticity model. It is demonstrated that finite element calculations reproduce well the anisotropic hardening response of differently oriented pure aluminum single crystals under tension and the variation of pile-up patterns according to different initial crystallographic orientations during nanoindentation. Furthermore, the numerical procedure was used to predict the deformation response of the aluminum multicrystalline specimen under simple shear. Non-uniform deformation fields by the digital image correlation method and surface profiles by the shadow Moiré method were well reproduced by numerical predictions. In addition, the effect of the initial crystallographic orientation and the interaction effects among different crystals on the deformation response of the multicrystal are addressed. [ABSTRACT FROM AUTHOR]

During the deformation of low carbon steel by cold-rolling, dislocations are created and stored in grains depending on local crystallographic orientation, deformation, and deformation gradient. Orientation dependent dislocation densities have been estimated from the broadening of X-ray diffraction lines measured on a synchrotron beamline. Different cold-rolling levels (from 30% to 95% thickness reduction) have been considered. It is shown that the present measurements are consistent with the hypothesis of the sole consideration of screw dislocations for the analysis of the data. The presented evolutions show that the dislocation density first increases within the α fiber (={hkl}<110>) and then within the γ fiber (={111}). A comparison with EBSD measurements is done and confirms that the storage of dislocations during the deformation process is orientation dependent and that this dependence is correlated to the cold-rolling level. If we assume that this dislocation density acts as a driving force during recrystallization, these observations can explain the fact that the recrystallization mechanisms are generally different after moderate or large strains. [ABSTRACT FROM AUTHOR]

Microstructural properties of CeGdO (x = 0 to 0.3) thin films prepared by pulsed laser deposition technique were studied. The thin films were deposited on Si(100) substrate at a substrate temperature of 973 K at the oxygen partial pressure of 0.2 Pa using KrF excimer laser with energy of 220 mJ. The prepared thin films were characterized by X-ray diffraction, Raman spectroscopy and atomic force microscopy. X-ray diffraction analysis confirmed the polycrystalline nature of the thin films. Crystallite size, strain and dislocation density were calculated. The Raman studies revealed the formation of Ce-O with the systematic variation of peak intensity and full width half maxima depending on concentration of gadolinium dopant. The thickness of the films was estimated using Talystep profiler. The surface roughness was estiamted based on AFM. [ABSTRACT FROM AUTHOR]

Abstract: A novel X-ray diffraction-based technique for grain-by-grain assessment of dislocation density within polycrystals is applied. The technique discriminates dislocation densities of different slip modes, slip systems and dislocation character. Data which was formerly confined to the transmission electron microscope (TEM) is now available from X-ray diffraction (XRD). In addition, there is a profound statistical advantage of the XRD approach over the traditional TEM-based approach. Over 130 grains were analyzed for one sample of CoTi, which would be a Herculean task on the TEM, requiring dozens of samples as well as numerous hours on the microscope and analysis of the images generated for each sample. The present experiments were performed in about 30h per sample and the analysis is semi-automated, involving a Monte-Carlo-type algorithm to determine the dislocation structure best representing the single crystal diffraction peak profiles in a polycrystalline aggregate. Conclusive confirmation of a previously suggested explanation for the anomalous ductility of two CsCl structured intermetallic compounds, CoTi and CoZr, is provided: namely, that hard dislocation modes, with b =〈110〉 and 〈111〉, rarely observed in single crystal experiments, are active in nearly every grain. The results call into question the value of employing single crystal deformation experiments alone to understand the deformation behavior of polycrystalline materials. Further, the results re-emphasize that the uniform stress assumption implicit in Schmid factor analysis is a poor one for materials which are highly anisotropic at the single crystal level. [Copyright &y& Elsevier]

Tension experiments were carried out at room temperature, 473 K and 673 K on AZ31-type extruded magnesium alloy samples. The tensile deformation has almost no effect on the typical extrusion texture at any of the investigated temperatures. X-ray diffraction patterns provided by a high-angular-resolution diffractometer were analyzed for the dislocation density and slip activity after deformation to fracture. The diffraction peaks were sorted into two groups corresponding either to the major or to the random texture components in the specimen. The two groups of reflections were evaluated simultaneously as if the two texture components were two different phases. The dislocation densities in the major texture components are found to be always larger than those in the randomly oriented grain populations. The overwhelming fraction of dislocations prevailing in the samples is found to be of 〈 a〉 type, with a smaller fraction of 〈 c + a〉-type dislocations. The fraction of 〈 c〉-type dislocations is always obtained to be zero within experimental error. [ABSTRACT FROM AUTHOR]

Abstract: Dislocation densities are investigated in a short-fiber-reinforced Al–11wt.% Zn–0.2wt.% Mg metal matrix composite (MMC) with a special focus on regions near the fiber–matrix interfaces. Clear microstructural evidence is provided for the formation of work-hardened zones (WHZs) around fibers during creep using transmission electron microscopy (TEM). The dislocation densities in the WHZs are higher after creep than after squeeze casting, where the plastic strains associated with the thermal stresses that build up during solidification also result in an increased dislocation density close to fibers. The effect of heating and cooling on the dislocation substructure is also considered. The results are discussed in light of previous findings and provide microstructural evidence for the presence of WHZs as predicted by the Dlouhy model of MMC creep. [Copyright &y& Elsevier]

The major objective of this work has been to apply a new compatibility-based fracture theory to the investigation of dynamic failure of polycrystalline metals and alloys. To model the nucleation and propagation of failure surfaces at the microstructural scale, under large deformations and dynamic loading conditions, a general fracture criterion based on the integral law of compatibility is used. This new fracture criterion, was coupled with rate-dependent dislocation-density based crystalline plasticity formulations to elucidate the microstructural mechanisms related to the evolution of intergranular and transgranular failure and to understand how grain sizes and strain-rate sensitivity affect aggregate strength, ductility, and dynamic damage tolerance. It is shown that cracks commonly nucleate at triple junctions and at grain boundaries as intergranular cracks, and that slip bands through grains result in transgranular cracks. [ABSTRACT FROM AUTHOR]

Wire arc additive manufacture is well applied in the fabrication of Al-5.0 Mg products. The investigation of microstructure morphology and phase composition in different regions is not enough. Therefore, the factors of microstructure morphology, dislocations densities and phase composition are worthy to study. Investigations of microstructure morphology demonstrated that the cross-section along the deposition direction has diversified grain size and the diversity increases the dislocation density and decreases the residual stress. The preferred orientation of grain growth analyzed by XRD explained that the decreasing intensity of (200) could improve the homogeneity. EDS results revealed that Al and Al 3 Mg 2 were the main phases. TEM results demonstrated that there was Al 2 Mg formed inside grains and the irregular shapes of precipitations. The temperature gradient in the two regions is the main factor causing the differences in dislocation density, precipitations formation and microstructure diversity [Display omitted] • Diversity in preferred orientation and microstructure is affected by the temperature gradient in different regions. • Al and Al 3 Mg 2 are the main phases and the Al 2 Mg is detected. • High dislocation density resulting from uneven microstructure could reduce the residual stress. [ABSTRACT FROM AUTHOR]

We have studied the use of overshoot graded layers for the control of the dislocation density in mismatched heteroepitaxial layers. Graded ZnSySe1-y structures were grown on GaAs (001) by photoassisted metalorganic vapor-phase epitaxy (MOVPE) and characterized by high-resolution x-ray diffraction (HRXRD). All samples had a uniform top layer of ZnS0.014Se0.986, and various graded layers were incorporated between the substrate and the uniform top layer; these included forward-graded (FG) and reverse-graded (RG) buffers. Some structures incorporated overshoot at the interface with the uniform top layer (FGO and RGO buffers). Among the FG samples, those with overshoot exhibited better crystal quality and lower dislocation densities than those without. This is expected because the mismatched interface between the graded layer and the top ZnS0.014Se0.986 can affect the bending over of threading dislocations for the production of misfit dislocations, indirectly promoting annihilation and coalescence reactions. An overshoot interface with 0.1% mismatch was found to remove 2 x 108 cm-2 dislocations from the top device layer. Overshoot did not reduce the dislocation density in RG structures, but this may be because the sign of the overshoot caused the generation of new dislocations rather than interactions between existing ones. For growing a high-quality device layer with minimal defect density, it appears that steep forward-graded layers with overshoot may be best in this material system. [ABSTRACT FROM AUTHOR]

Abstract: Plastic deformation of Al–Mg alloys were investigated by analyzing the stress–strain curves obtained in uniaxial compression tests at constant loading rate. Experimental results have shown the well-known phenomenon of plastic instabilities – the Portevin–Le Chatelier effect – in these solid solution alloys. Using the characteristics of plastic instabilities – the local stress increment, Δσ, – dislocation densities were calculated and compared with those determined by considering the global strain hardening. The influence of the increasing Mg concentration on the multiplication and the annihilation of dislocations, as well as on the development of the mobile and immobile dislocation densities was also analyzed. [Copyright &y& Elsevier]

The dislocation densities and crystallite size distributions in ball-milled fluorides, MF2 ( M = Ca, Sr, Ba and Cd), of the fluorite structure type have been determined as a function of milling time by X-ray diffraction line-profile analysis. The treatment has been based on the concept of dislocation contrast to explain strain anisotropy by means of the modified Williamson–Hall and Warren–Averbach approaches and a whole-profile fitting method using physically based functions. In most cases, the measured and calculated patterns are in perfect agreement; however, in some specific cases, the first few measured profiles appear to be narrower than the calculated ones. This discrepancy is interpreted as the result of an interference effect similar to that described by Rafaja, Klemm, Schreiber, Knapp & Kužel [ J. Appl. Cryst. (2004), 37, 613–620]. By taking into account and correcting for this interference effect, the microstructure of ball-milled fluorides is determined in terms of dislocation structure and size distributions of coherent domains. A weak coalescence of the crystallites is observed at longer milling periods. An incubation period in the evolution of microstrains is in correlation with the homologous temperatures of the fluorides. [ABSTRACT FROM AUTHOR]

Abstract: By using integration by parts to the traditional boundary integral formulation, a traction boundary integral equation for cracked 2-D anisotropic bodies is derived. The new traction integral equation involves only singularity of order 1/r and no hypersingular term appears. The dislocation densities on the crack surface are introduced and the relations between stress intensity factors and dislocation densities near the crack tip are induced to calculate the stress intensity factors. The boundary element method based on the new equation is established and the singular interpolation functions are introduced to model the singularity of the dislocation density (in the order of ) for crack tip elements. The proposed method can be directly used for the 2-D anisotropic body containing cracks of arbitrary geometric shapes. Several numerical examples demonstrate the validity and accuracy of BEM based on the new boundary integral equation. [Copyright &y& Elsevier]

We present a work hardening model with three types of dislocation densities as internal variables. The kinetic equation of state—used to calculate the flow stress—is extended to account for solution hardening by taking solute atoms into account as obstacles for the mobile dislocations. Thus, the model is able to describe the effects of alloy composition on the hardening behavior. Examples are presented on how the model can be used to calculate stress–strain curves for a range of temperatures, strain rates and compositions. Its application to through process modeling is shown. Owing to the use of dislocation densities as internal variables, the model delivers an output that can be directly used as input for subsequent modeling of recrystallization. [Copyright &y& Elsevier]

We investigate degraded GaN-based laser diodes (LDs) on epitaxial lateral overgrown GaN layers in terms of dislocations. Almost all of the threading dislocations that appear in the wing regions are a-type dislocations. Their origins are the lateral extension of dislocations from the seed regions that contingently bend upwards to the episurface. Comparing short-lived LDs and long-lived LDs that have almost the same power consumption, we find that the relative levels of dislocation densities in their respective active layers are different. In the degraded LDs, neither dislocation multiplication from the threading dislocations nor any structural changes of the threading dislocations are observed. This indicates that degradation is not caused by dislocation multiplication at the active layers, which is usually observed in LDs featuring zincblende-based structures. The degradation rate is almost proportional to the square root of the aging time. Our results indicate that degradation is governed by a diffusion process, and a detailed degradation mechanism is proposed. [ABSTRACT FROM PUBLISHER]

We show that an extension of Ananthakrishna’s model to include spatial degrees of freedom produces spatially uncorrelated bands, hopping type and the continuously propagating type with increasing applied strain rate. The velocity of the continuously propagating bands is found to vary linearly with applied strain rate. [Copyright &y& Elsevier]

Strain localization and dislocation microstructure formation are typical features of plastic deformation in metals and alloys. Plastic deformation occurs by the glide of dislocations, and, although dislocation distributions are rather uniform at its onset, they usually become unstable when deformation proceeds and undergo successive transitions towards various types of microstructures such as cells, deformation bands, persistent slip bands, labyrinth structures, etc. This phenomenon is experimentally well documented, but, despite a huge number of theoretical investigations, its modeling is still in its infancy. In the spirit of multiscale modeling, an intermediate step between micro- and macroscopic descriptions, consists in the derivation of mesoscopic rate equations describing the dynamical evolution of dislocation densities. It will be shown on some classical examples why this approach is interesting for the description of mechanical systems undergoing plastic instabilities, and how it may be used to determine the key physical processes for dislocation patterning. It will also be shown that the localization of plastic deformation is a natural consequence of collective behaviors induced by reaction and transport in dislocation populations. [Copyright &y& Elsevier]

A dislocation-density based multiple-slip crystalline formulation and computational schemes have been developed and used for a detailed understanding and accurate prediction of interrelated physical mechanisms that occur on different length scales in fcc polycrystalline aggregates separated by grain boundary interfacial regions of random orientations and distributions. This constitutive framework accounts for the generation, trapping, interaction, and annihilation of mobile and immobile dislocation densities that are generally associated with finite-strain deformation and failure modes in fcc aggregates. Specialized interfacial regions have been introduced to account for dislocation-density and slip transmission, intersection, and blockage at GBs. It is shown that this blockage may result in the formation of pile-ups. [ABSTRACT FROM AUTHOR]