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We study the effects of uniaxial pressure on the thermal conductivity between two nanoparticles using atomistic simulation. While the system is compressed, we analyze the evolution of contact area, the relative density, and the dislocation density. Lattice thermal conductivity is calculated by non-equilibrium molecular dynamics simulations at several stages of the compression. Despite the increment of dislocation defects, thermal conductivity increases with pressure due to the increase in relative density and contact radius. The behavior of the contact radius is compared with the Johnson–Kendall–Roberts (JKR) model. While there is good agreement at low strain, after significant plasticity, signaled by the emission of dislocations from the contact region, the discrepancy with JKR grows larger with the dislocation density. The results for thermal conductivity show good agreement with previous studies at zero strain, and a theoretical model is used to accurately explain its behavior vs strain-dependent contact radius. Both the Kapitza resistance and thermal resistance decrease with strain but with very different evolution. Simulations of a bulk sample under uniaxial strain were also carried out, allowing for a clear distinction between the role of compressive stress, which increases the conductivity, vs the role of dislocations, which decrease the conductivity. For the NP system, there is the additional role of contact area, which increases with stress and also modifies conductivity. An analytical model with a single free parameter allows for a description of all these effects and matches both our bulk and NP simulation results. [ABSTRACT FROM AUTHOR]

The detailed study of the effect of the initial microstructure on its evolution under hydrostatic compression before, during, and after the irreversible α → ω phase transformation and during pressure release in Zr using in situ x-ray diffraction is presented. Two samples were studied: one is plastically pre-deformed Zr with saturated hardness and the other is annealed. Phase transformation α → ω initiates at lower pressure for a pre-deformed sample but for a volume fraction of ω Zr, c > --> 0.7 , a larger volume fraction is observed for the annealed sample. This implies that the proportionality between the athermal resistance to the transformation and the yield strength in the continuum phase transformation theory is invalid; an advanced version of the theory is outlined. Phenomenological plasticity theory under hydrostatic loading is outlined in terms of microstructural parameters, and plastic strain is estimated. During transformation, the first rule is suggested, i.e., the average domain size, microstrain, and dislocation density in ω Zr for c < 0.8 are functions of the volume fraction, c of ω Zr only, which are independent of the plastic strain tensor prior to transformation and pressure. The microstructure is not inherited during phase transformation. Surprisingly, for the annealed sample, the final dislocation density and the average microstrain after pressure release in the ω phase are larger than for the severely pre-deformed sample. The results suggest that an extended experimental basis is required for the predictive models for the combined pressure-induced phase transformations and microstructure evolutions. [ABSTRACT FROM AUTHOR]

One of the most important issues related to the strength of metals is the strain rate sensitivity of the flow stress. In this study, an analytical model of the flow stress as a function of strain rate is derived theoretically. The model can reproduce the strain rate sensitivity of the flow stress of copper over a wide range of strain rates (up to 109 s−1) quantitatively. Our theoretical derivations indicate that the strain rate sensitivity of the flow stress, especially that above 103 s−1, is a result of both the variation of the dislocation mobility mechanism with stress and the particular stress dependence of dislocation density but is not a result of each single mechanism. In particular, the stress dependence of the dislocation density and the initial dislocation density are critical to the quantitative relation of the flow stress–strain rate at high strain rate and the strain rate threshold, under which the upturn of the flow stress occurs, respectively. Moreover, experiments with copper of different initial dislocation densities at moderate and high strain rate are performed. The strain rate threshold of the flow stress upturn observed in the experiments grows considerably as initial dislocation density increases, which is in accordance with theoretical prediction by our model. [ABSTRACT FROM AUTHOR]

Recently, high entropy alloy (HEA) has become a research hotspot as a new candidate structural material in nuclear reactors due to its good irradiation resistance in swelling and hardening. Focusing on the temperature effect of irradiation damage, this work investigated the influence of irradiation temperature on dislocation evolution and irradiation hardening of HEAs. CoCrFeMnNi HEA was irradiated by high-energy Fe ions at room temperature and 500 °C. It was found that dense small dislocations were produced in the damage attenuation region (i.e., the tail of the ion range) of HEAs after irradiation at room temperature, whereas the irradiation-induced dislocations could not be observed in the damage attenuation region when the irradiation temperature was increased to 500 °C. For the small-sized dislocations, dissociation may occur more easily than long-range migration in HEAs (such as CoCrFeNi systems) due to the inhibition of defect migration and the decrease in defect binding energy, and this order is reversed in pure metals (such as Ni, W). Therefore, at 500 °C irradiation, small dislocations in the damage attenuation region of CoCrFeMnNi HEAs were dissociated before migrating to deeper regions, thereby resulting in the depth of dislocation distribution smaller than the stopping and range of ions in matter-calculated damage stopping depth, unlike the phenomenon in pure metals where dislocations migrated to regions exceeding the calculated depth. In addition, the dislocation density of CoCrFeMnNi HEAs decreased significantly due to the promotion of dissociation and merging of dislocations by elevated temperatures, and the hardening after 500 °C irradiation was less than that after room temperature irradiation. [ABSTRACT FROM AUTHOR]

Recently, deep-ultraviolet (DUV) light-emitting devices have attracted attention for various applications. GaN/AlN superlattices have emerged as a promising alternative for achieving high-efficiency DUV emission. To fabricate superlattices with high crystal quality and abrupt interfaces, we have utilized face-to-face-annealed sputter-deposited AlN template substrates characterized by a flat surface and low dislocation density. Furthermore, radio-frequency plasma-assisted molecular beam epitaxy with in situ reflection high-energy electron diffraction monitoring was employed for the growth process. The growth of the superlattices follows a specific sequence. Step 1: AlN growth, Step 2: conversion of Al droplets to AlN, Step 3: GaN growth, and Step 4: evaporation of Ga droplets. This study explored the impact of GaN thickness on the GaN/AlN superlattice. The GaN thickness was linearly controlled by changing the duration of Step 3. This approach allowed for the growth of a flat GaN layer up to 1 monolayer (ML) and achieved superlattices with abrupt interfaces. Single-peak cathodoluminescence (CL) emission at 240–245 nm was observed from the superlattices, with the peak shift toward longer wavelengths as the GaN thickness increased. In contrast, quantum dot-like GaN islands were generated with a thickness of over 1 ML, induced by compressive strain. Superlattices with thicker GaN exhibited broad CL emission with multiple peaks. However, the AlN barrier layer reduced the surface roughness and maintained abrupt interfaces within the superlattices. Therefore, to obtain sharp single-peak UV emission from GaN/AlN superlattices, the growth sequence should be controlled to obtain flat GaN layers without dots. [ABSTRACT FROM AUTHOR]

Distributed Bragg reflectors (DBRs) based on alternating layers of porous and non-porous GaN have previously been fabricated at the wafer-scale in heteroepitaxial GaN layers grown on sapphire substrates. Porosification is achieved via the electrochemical etching of highly Si-doped layers, and the etchant accesses the n+-GaN layers through nanoscale channels arising at threading dislocations that are ubiquitous in the heteroepitaxial growth process. Here, we show that the same process applies to GaN multilayer structures grown on silicon substrates. The reflectance of the resulting DBRs depends on the voltage at which the porosification process is carried out. Etching at higher voltages yields higher porosities. However, while an increase in porosity is theoretically expected to lead to peak reflectance, in practice, the highest reflectance is achieved at a moderate etching voltage because etching at higher voltages leads to pore formation in the nominally non-porous layers, pore coarsening in the porous layers, and in the worst cases layer collapse. We also find that at the high threading dislocation densities present in these samples, not all dislocations participate in the etching process at low and moderate etching voltages. However, the number of dislocations involved in the process increases with etching voltage. [ABSTRACT FROM AUTHOR]

A fabrication technique for high-quality AlN templates using only metalorganic vapor phase epitaxy, which involves a tiny-pit AlN layer optimized by changing the growth temperature and V/III ratio, is proposed. Combining the tiny-pit AlN layer with the main AlN layer can be beneficial for achieving epitaxial lateral overgrowth and reducing threading dislocation density (TDD). According to transmission electron microscopy observations, the TDD in the AlN layer on the tiny-pit layer was 7.3 × 109 cm–2, and the TDD was further reduced to 2.6 × 108 cm–2 by adopting multiple tiny-pit layers. Tiny voids were observed at the interface between the tiny-pit AlN and main AlN layers, and the radius of curvature of the AlN layer also increased compared with that for a conventional AlN template by reducing stress in the AlN layer. In addition, the N-polar AlN layer was grown using Al-polar tiny-pit AlN layers through polarity inversion on a sapphire substrate with an offcut angle of 2.0°. Consequently, it was found that the Al-polar tiny-pit AlN is highly effective in improving the crystalline quality of the N-polar AlN layer. The surface flatness of the N-polar AlN layer with the tiny-pit layer and polarity inversion retained the conventional N-polar AlN template. [ABSTRACT FROM AUTHOR]

We discuss the structure of threading dislocations in α-Ga2O3 thin films grown on c- and m-plane sapphire substrates. The thickness-dependent threading dislocation density in both films directly affects the electrical properties of the films including carrier concentration and mobility. Two distinct types of threading dislocations are identified for each of the c- and m-plane α-Ga2O3 thin films. The c-plane α-Ga2O3 thin film shows Burgers vectors of 1 / 3 [ 1 1 ¯ 01 ] and 1 / 3 [ 11 2 ¯ 0 ] , while the m-plane α-Ga2O3 thin film displays Burgers vectors of 1 / 3 [ 2 1 ¯ 1 ¯ 0 ] and 1 / 3 [ 1 1 ¯ 01 ]. This paper presents a detailed structure of the threading dislocations in α-Ga2O3, which has been little disclosed thus far mainly due to the difficulty in synthesizing the metastable α-Ga2O3. [ABSTRACT FROM AUTHOR]

Relaxed Si1−xGex layers on Si(001) serve as virtual substrates for strained Si or Ge layers. However, plastically relaxed layers inevitably contain misfit and threading dislocations, negatively affecting devices. Deposition of a SiGe layer on the backside of the substrate introduces a dislocation reservoir at the wafer edge that can reduce the threading dislocation density (TDD) of Si0.98Ge0.02/Si layers, as these preexisting dislocations start gliding toward the wafer center upon reaching the critical thickness. Here, we show that this low-strain system can be used effectively to study dislocation glide kinetics. In agreement with the literature, dislocation glide is a thermally activated process with an activation energy of 2.12–2.16 eV. Near the critical thickness, relaxation is sluggish and inefficient due to the linear dependence of the glide velocity on excess stress. At lower growth rates, dislocations from the edge reservoir are activated in a lower density due to the increase in the critical thickness through partial strain relaxation by already activated dislocations. Contrary to common models, here, the lowest possible growth rate is not essential for minimizing the TDD. Instead, a careful balance between low and high growth rates is beneficial. Overcoming the initial sluggish and inefficient relaxation phase is critical while also avoiding accumulation of strain energy, and, therefore, the activation of dislocation sources. Only in a later stage of buffer growth, the growth rate should be reduced to a minimum. With this method, the TDD of strain relaxed Si0.84Ge0.16 layers is reduced to 7 × 104 cm−2. [ABSTRACT FROM AUTHOR]

Applying graphene (Gr) coatings to high-entropy alloys (HEA) is anticipated to enhance their tribological characteristics. The current understanding of the mechanism by which the Gr/HEA is enhanced at the atomic level is still limited. Molecular dynamics simulations revealed the mechanical behavior and strengthening mechanism of the Gr/AlCoCrFeNi HEA during nanoindentation and nanoscratch. The results demonstrate a substantial increase in the indentation hardness of the Gr/AlCoCrFeNi HEA by about 2.4 times. When Gr changed from a single layer to three layers, it further improved (3.2 times for a double layer and 3.9 times for three layers). At the same time, the friction coefficient is effectively reduced. Furthermore, the elevated in-plane stiffness of the Gr coating leads to an expansion of the effective loading area, resulting in increased Shockley dislocation and stair-rod dislocation density within the Gr/AlCoCrFeNi HEA, thereby amplifying the strain hardening effect and reducing subsurface damage. Qualitative experiments confirmed the excellent wear resistance of the Gr/HEA, and coating Gr increased the width of scratches, effectively confirming our simulation results. These findings provide valuable insights for the development and design of Gr/HEA composite coatings with enhanced mechanical properties. [ABSTRACT FROM AUTHOR]

The Taylor–Quinney coefficient (TQC) stands as a critical parameter intricately linked to the thermomechanical coupling plastic deformation behavior, defined as the ratio of heat generation to plastic work. Combining experimental studies and atomistic simulations, the TQC is discussed in the context of three CoCrNi-based chemically complex medium/high-entropy alloys (CoCrNi, CoCrNiFe, and CoCrNiFeMn). Notably, the obtained TQC of these alloys hovers around 0.7, well below the generally assumed value of 0.9–1 in traditional metals associated with adiabatic shearing. The lower TQC implies that these alloys possess the much better capability of storing energy of cold-work. It is further found that immobile dislocations are the most effective carriers for storing the energy of cold-work. Among these three alloys, CoCrNiFeMn exhibits the highest TQC due to its relatively lower density of immobile dislocations and Lomer–Cottrell locks. [ABSTRACT FROM AUTHOR]

We simulate an alloy of HfNbTaTi mixed in six different proportions and also of the equiatomic system under uniaxial tensile loading at 300 K. Molecular dynamics simulation trajectories are analyzed using radial distribution functions, OVITO, bond-orientational order parameters, and coordination numbers. Equiatomic and the two other alloys (Hf 0.31 Nb 0.23 Ta 0.23 Ti 0.23 and Hf 0.23 Nb 0.31 Ta 0.23 Ti 0.23 ) containing comparable fraction of elements deform similarly through the formation of an amorphous state. Two alloys rich in Nb (Hf 0.17 Nb 0.50 Ta 0.16 Ti 0.17 ) and Ta (Hf 0.17 Nb 0.16 Ta 0.50 Ti 0.17 ) deform similarly resulting in the formation of bcc atoms, which transform to fcc at higher loading. Finally, alloys rich in Hf (Hf 0.50 Nb 0.16 Ta 0.17 Ti 0.17 ) and Ti (Hf 0.17 Nb 0.16 Ta 0.17 Ti 0.50 ) deform resulting in high dislocation densities and hcp atoms. These two hcp-rich alloys also undergo strain hardening. In each mixture during loading, local orientational order of all the different elements changes similarly. Atoms prefer to pair with other atoms than to themselves during tensile loading. [ABSTRACT FROM AUTHOR]

Numerical simulations of the evolution of mobile and immobile dislocations in ceramics under applied pressure in dry pressing at a relatively low temperature are performed in order to study the possibility of production of all-dislocation-ceramics of solid electrolytes, which are expected to have extremely high ionic conductivity without dendrite formation because the diameter of a dislocation pipe is considerably larger than the distance between neighboring dislocations. The present numerical simulations are only for the densification process by plastic deformation of grains under high pressure under the assumption that the compaction of particles by their rearrangement is completed beforehand. By the plastic deformation, new dislocations are generated inside the grains. The required total dislocation density of about 1017 m−2 seems to be achievable under some conditions of dry pressing according to the present numerical simulations. Very short ultrasound irradiation at the beginning of the dry pressing sometimes considerably increases the dislocation density, while for other cases, it even considerably decreases the dislocation density due to enhanced annihilation of mobile dislocations by ultrasound. [ABSTRACT FROM AUTHOR]

Extending the storage-recovery model, we propose a new strengthening model, premised on detailed evolution laws for both mobile and immobile dislocations, for metals under moderate to intense loading. These dislocation density evolution laws include the multiplication, storage under the effect of dislocation junctions, release of pinned dislocations, and annihilation by cross-slip. The storage-release description is derived from a simplified depiction of the probability distribution function of the dislocation length in dislocation networks. Although the model requires only few parameters to characterize the evolution of dislocation densities, remarkable agreement is found with available experimental data. From a theoretical study of the long-time behavior of the model, analytical expressions are provided to easily extract most of these parameters from experimental stress–strain curves in the quasi-static regime, whereas the parameter that governs the strength of the release process is adjusted from dynamic tensile tests. Their values so determined for polycrystalline tantalum allow the model to reproduce experimental plate-impact data with a very good match. [ABSTRACT FROM AUTHOR]

Annealing Mg-implanted homoepitaxial GaN at temperatures above 1400 °C eliminates the formation of inversion domains and leads to improved dopant activation efficiency. Extended defects, in the form of inversion domains, contain electrically inactive Mg after post-implantation annealing at temperatures as high as 1300 °C (one GPa N2 overpressure), which results in a low dopant activation efficiency. Triple-axis x-ray data reveal that implant-induced strain is fully relieved after annealing at 1300 °C for 10 min, indicating that strain-inducing point defects formed during implantation have reconfigured and inversion domains are formed. However, annealing at temperatures of 1400–1500 °C (one GPa N2 overpressure) eliminates the presence of the inversion domains. While residual defects, such as dislocation loops, still exist after annealing at and above 1400 °C, chemical analysis at multiple dislocation loops shows no sign of Mg segregation. Meanwhile, an overall decreasing trend in the dislocation loop density is observed after annealing at the higher temperatures and longer times. Additionally, once inversion domains are formed and the samples are cooled to room temperature, they are shown to dissolve with subsequent annealing above 1400 °C. While such defects have been observed before, the important finding that such defects can be dissolved with a short, higher temperature step is key. Earlier work [Breckenridge et al., J. Appl. Phys. Lett. 118, 022101 (2021)] addressing electrical measurements of these types of samples showed that annealing at 1400 °C leads to a dopant activation efficiency that is an order of magnitude higher than that observed at 1300 °C. This work complements earlier work by identifying the inversion domains, which incorporate Mg, and points to the benefits, in terms of defect density and p-type dopant activation, of using higher temperature (>1400 °C) annealing cycles to activate Mg in GaN, even if the Mg-containing inversion domains had been formed during lower temperature annealing. [ABSTRACT FROM AUTHOR]

Three sets of heteroepitaxial diamond crystals grown under nominally identical process conditions on Ir/YSZ/Si(001) substrates have been studied with respect to structural defects, chemical purity, and x-ray induced photoconductivity (PC). The dislocation density that varied systematically between 10 7 and 10 9 cm − 2 had a minor influence on dark conductivity and photoconductive gain G. In contrast, the substitutional nitrogen (N N ) and substitutional boron (N B ) defects, which are both present at very low concentrations (≤ 1 ppb), turned out to be the crucial factors controlling the electrical behavior. Small differences between both resulted in variations of the photocurrents by up to 5 orders of magnitude. The maximum in G of 0.75 × 10 4 was measured in the sample with highest dark conductivity. It could be explained conclusively within our model calculations on gain formation by N N ≤ N B . For low gain samples, we found N N > --> N B . However, the measured G values were far from theoretical predictions. This indicates a dominating role of additional traps. After x-ray switch-off, persistent photoconductivity (PPC) was observed in high gain samples. It was attributed to an energetic barrier hampering the recharging of nitrogen atoms by hole capture. As a possible source, strain fields generated by dislocations are suggested. [ABSTRACT FROM AUTHOR]

Creep behavior of an as-received, normalized, and tempered 9Cr-1Mo (P 91) steel and its secondary short-term normalized and short-term tempered counterpart at temperatures of 550 °C, 600 °C, and 650 °C, at stresses in the range of 80–260 MPa, has been studied. The secondary heat treatment leads to insignificant coarsening of the prior austenite grains but refines the martensitic lath sub-grains and the grain boundary M23C6 carbides and enhances the dislocation density compared to that of the as-received alloy. Tensile creep tests reveal that the creep life of the secondary heat-treated alloy is several folds higher than that of the as-received alloy. For both alloys, considering the effect of threshold stresses at different temperatures, an effective stress exponent neff of 4.5–5.5, and activation energies of 316.4 and 258.3 KJ mol−1, it indicates that the steady-state creep relaxation mechanism involves dislocation climb and annihilation. Microstructural characterization, before and after the creep tests, reveals that lath sub-grains and M23C6 precipitates in both alloys undergo significant coarsening, with the former leading to microstructural instability at the onset of the tertiary stage of creep. Deformation mechanisms in the steady-state regime are described as a balance between processes that impede and facilitate dislocation motion in the sub-grain interior. The interplay between energy minimization-induced sub-grain coarsening and Zener pinning of the boundaries by M23C6 is discussed in the context of microstructural instability-induced failure. [ABSTRACT FROM AUTHOR]

In this study, the estimated stress, strain, and crystallite sizes of different Mn-doped ZnO nanoparticles were calculated using the Williamson-Hall method and compared with the values obtained from the Debye-Scherrer formula. Moreover, defects, and magnetic properties of Mn-doped ZnO nanoparticles at different concentrations were investigated. The sol-gel method was used to synthesize nanoparticles. The X-ray diffraction and Rietveld analysis results confirm that the desired structure is formed and that no secondary phase is present up to an Mn concentration of x = 0.2. In and out of plane lattice parameters, cell volumes, bond length, atomic locality, and dislocation density (δ) were clarified. The grain size of the concentration-dependent samples was provided by scanning electron microscope. Photoluminescence (PL) spectra exhibited ultraviolet emission along with a broad band encompassing violet, blue, and red regions, attributed to defect-related and excitonic emissions. These emissions were notably influenced by synthesis conditions and doping elements and ratios. Electron spin resonance properties of the concentration-dependent samples were analyzed to figure out the g-factor through line widths of pike-to-pike (ΔHPP) of ESR spectra. Mn-doped ZnO nanoparticles exhibited ferromagnetism at room temperature. [ABSTRACT FROM AUTHOR]

The austenitic stainless steel undergo warm rolling, followed by cryogenic treatment. The warm‐rolled and cryogenically treated (WR‐CT) sample exhibits superior strength and ductility compared to the warm‐rolled (WR) sample, achieving a yield strength of 949 MPa and an elongation of ≈56.2%. The enhanced strength is attributed to the higher dislocation density, additional grain refinement, and the presence of martensite in the WR‐CT sample. The WR and WR‐CT samples exhibit similar deformation mechanisms. In the initial stage, dislocation slip predominantly governs the deformation. During the intermediate stage, the transformation‐induced plasticity (TRIP) effect dominates the deformation mechanism. In the final stage, nearly all austenite transforms into martensite, and deformation mainly occurs via dislocation slip within the martensite phase. However, in the WR‐CT sample, the TRIP effect is delayed during the stable processing stage (≈8–23%) due to the higher mechanical stability of austenite, resulting from further grain refinement and higher dislocation density. Additionally, the coordinated deformation of strain‐induced martensite with varying orientations contributes to delayed necking, achieving an optimal balance between strength and ductility. [ABSTRACT FROM AUTHOR]

Tin (Sn)‐based perovskites have emerged as promising alternatives to lead (Pb)‐based perovskites in thin‐film photovoltaics due to their comparable properties and reduced toxicity. Strains in perovskites can be tailored to modulate their optoelectronic properties, but mechanisms for the effects of strains on Sn‐based perovskite films and devices are unrevealed and corresponding strain engineering is unexplored. Herein, a strain engineering strategy is developed through incorporating 4‐fluorobenzylammonium halide salts (FBZAX, X = I, Br, Cl) into the perovskite precursor to regulate the strain effects in resultant Sn‐based perovskite films. It is found that a moderate level of compressive strain achieved by FBZABr alleviates the dislocations within perovskites to enhance carrier transport and reduces the defect density to prolong carrier lifetime. These improvements enable a champion efficiency exceeding 14% of Sn‐based perovskite solar cells with excellent operational stability. [ABSTRACT FROM AUTHOR]

The growing demand for lightweight and high-strength materials in the aerospace and automotive industries, as well as the need for highly conductive materials such as heat sinks, electrodes and integrated circuits, has fueled the exploration of innovative composites. Metal matrix composites (MMCs) reinforced with graphene offer a promising solution, combining the inherent properties of metals with the unique characteristics of graphene. However, the fabrication of MMCs reinforced with graphene poses several challenges such as poor wettability of graphene within the metal matrix, a non-uniform distribution of graphene and graphene clustering. Various fabrication methods have been used to address these challenges; among them, high-pressure torsion (HPT) is a promising solution due to the introduction of a fine- or even nanograined structure with well-distributed graphene within the matrix through severe shear deformation. Grain boundary strengthening, Orowan bypassing due to the presence of non-shearable graphene particles, stress transfer to the reinforcements and the inherent properties of graphene can also enhance the mechanical properties of the graphene-containing MMCs produced by HPT. On the other hand, HPT negatively affects the electrical conductivity of the metal matrices by increasing the dislocation density and the number of grain boundaries. Nevertheless, graphene can also enhance the electrical conductivity of the composite by endowing the metal matrix with its π electrons. A current comprehensive examination of the literature provides valuable insights into the development of graphene-reinforced metal matrix composites fabricated by HPT and gives additional information on their potential applications. [ABSTRACT FROM AUTHOR]

This study proposes a creep‐life prediction model based on a back‐propagation artificial neural network (BP‐ANN). The model predicted the minimum creep rate by optimizing microstructure characteristics, including martensite lath width, sub‐grain, precipitates (M23C6 and Laves phases), and dislocation density. The BP‐ANN‐based predicted creep life of 9%Cr heat‐resistant steel is compared with the traditional time‐temperature life prediction method and the machine learning support vector regression model. The results show that the BP‐ANN model has better accuracy and performance. The % error between the predicted and the experimental creep life is 5% to 10%, primarily distributed in the two‐factor band line (2% to 5%). The BP‐ANN model gives the lowest mean absolute error (0.9926), root mean square error (29.411), average absolute relative error (4.7%), and multiple coefficient of determination (R2 0.96693). The results show that the BP‐ANN model improved the creep fracture life prediction of 9%Cr heat‐resistant steel. [ABSTRACT FROM AUTHOR]

Electromagnetic forming is now increasingly being applied to 7075-T6 high-strength aluminium alloy. To address the issues of uneven deformation and low forming limit of 7075-T6 high-strength aluminum alloy in the electromagnetic forming process, a two-step electromagnetic forming method with a field shaper and electric pulse treatment composite forming method was proposed. This new method combines the advantages of high-rate forming and the electroplastic effect, which significantly improves the deformation uniformity and forming limit of the material. In comparison with quasi-static stamping, the forming height and forming limit of the two-step electromagnetic forming method increased by 25.9% and 27.2%, respectively. Furthermore, increases of 48.4% in forming height and 54.4% in forming limit were observed with the two-step electromagnetic forming and electric pulse treatment composite process. Following electric pulse treatment, the entangled dislocations in the formed material were released, the dislocation density decreased, and the texture strength was improved. The maximum temperature of the sheet reached 169.9℃ upon the application of a 10kV pulse current, indicating that electrical and thermal effects are the primary factors influencing the change in material properties. [ABSTRACT FROM AUTHOR]

Based on the method of continuous distribution of dislocations and relevant theories of fracture mechanics, the effect of the distribution of hydrogen atoms at the crack tip (CT) on the emission of dislocations from kinked cracks under far-field uniform tensile stress is studied. The results show that hydrogen atoms can promote the emission and migration of CT dislocation, which leads to the increase of dislocation density (severe dislocation plugging), the increase of energy release rate, and the decrease of elastic zone size (dislocation-free zone) at CT. In addition, the sequence of initiation of grain boundary (GB) microcracks caused by dislocation emission at both ends of the whole crack (horizontal section and kinked section) and the propagation mechanism of a crack in a kinked section are analyzed. The results show that the critical tensile stress of GB microcrack initiation decreases with the increase of hydrogen atom radius rh and the decrease of hydrogen atom position xh , indicating that hydrogen atoms promote the initiation of GB microcrack initiation. At the same time, the existence of hydrogen atoms not only promotes the initiation of GB microcrack, but also accelerates the expansion of kinked cracks, which may eventually lead to the convergence of the main crack and the microcrack, resulting in the failure and destruction of materials. Therefore, the theoretical research in this paper has made a new understanding of the mechanism of hydrogen fracture of crystal materials. Practical Applications: In this paper, the effects of hydrogen atoms at CT on dislocation emission at CT, crack initiation at GB, and the size of the dislocation-free zone were studied by establishing a crack dislocation model and using the distributed dislocation method. The results show that the accumulation of hydrogen atoms at the CT promotes the emission of dislocation at the CT, leads to the serious plugging of dislocation near the GB, promotes the initiation of microcracks and the expansion of kinked cracks, and leads to the reduction of the size of the dislocation-free zone at the CT. This work provides new information on the microscopic fracture mechanics of hydrogen on materials. On the one hand, it can better predict and prevent the fracture of materials, so as to improve the safety of engineering structures. On the other hand, it can help explain some microscopic experimental phenomena. [ABSTRACT FROM AUTHOR]

Dislocation evolution in multilayered Ti/Nb composites processed by accumulative roll bonding under uniaxial tension was quantitatively evaluated by high-energy X-ray diffraction. The composites with different heterogeneous microstructures show distinct dislocation activities in Ti, which majorly contributes to their different ductility. For the composite in which extensive shear bands have formed before loading, the stable fraction of 〈c+a〉 dislocations and the suppressed activation of 〈c〉 dislocations contribute to its uniform deformation. We emphasize that tracking the dynamic evolution of dislocations is fundamental for understanding ductility of the multilayers, whereas the increased 〈c+a〉 dislocations found by post-mortem analysis do not necessarily dominate. IMPACT STATEMENT: Dynamic evolution of dislocations, rather than only dislocation densities obtained by post-mortem analysis, is found to explain ductility of multilayered composites, which can be tailored by adjusting heterogeneous microstructures. [ABSTRACT FROM AUTHOR]

The high-nitrogen stainless bearing steel 30Cr15Mo1VN, possessing excellent tensile strength (~ 2466 MPa) and impact toughness (~ 130.3 J), was manufactured by pressurized induction melting and pressurized electroslag remelting (PIM + PESR) duplex process. Herein, the inclusion characteristics and element segregation of as-cast ingots, as well as the precipitate characteristics, retained austenite (RA) distribution were systematically investigated to clarify the effect of PESR on tensile and impact properties. Compared with PIM ingot, the lower quantity and larger spacing of inclusions in PIM + PESR ingot were beneficial to improving toughness. Besides, the dendrite segregation originating from solidification inherited to tempered steels and changed the multiphase structure and toughening mechanism. First, the lighter segregation (C, N, Cr, etc.) was induced by the high cooling rate, directional solidification, and short-time homogenization during PESR process, obtaining the higher contents of precipitates and RA in the PIM + PESR ingot. Second, the smaller precipitates and more RA were uniformly distributed in tempered PIM + PESR steel by alleviating segregation, obtaining better interface and matrix plasticity. Third, the dislocation densities of martensite and RA were increased by the greater precipitation pinning effect after PESR, and the uniform area ratios of close-packed and Bain groups were obtained, effectively inhibiting the propagation of secondary crack. Finally, the smaller strength difference between RA and martensite owing to lighter segregation after PESR, alleviated strain localization at phase interfaces and accommodated plastic deformation of matrix, thus, significantly enhancing the strength and toughness of the PIM+PESR steel. [ABSTRACT FROM AUTHOR]

AbstractA novel analytical technique based on singular integral operators (SIO) is presented to study the strip electric-magnetic polarization saturation (EMPS) model in magneto-electro-elastic (MEE) solids with two unequal collinear cracks. Unknown semipermeable crack-face conditions and saturated zone lengths are evaluated using iterative techniques and the obtained new analytical solutions for dislocation densities. Numerical studies of fracture parameters are conducted with respect to variations in electrical loading, inter crack space distance and crack-length ratio. The accuracy and efficacy of the SIO method are validated against published results for cases with two equal collinear cracks and center crack. [ABSTRACT FROM AUTHOR]

The study aims to explore high‐carbon steel's behavior during cold drawing, focusing on the link between microstructural changes, texture, and mechanical properties at different strain levels. Using X‐ray diffraction, scanning electron microscopy–electron backscatter diffraction, micro‐ and nanoindentation, and wear analysis, findings show that grain size and interlamellar spacing decrease as strain increases, and pearlitic colonies become more compact and aligned. This refinement leads to significant hardening, from 333.8 HV in the wire rod to 470.4 HV at 2.05 strain, aligning with the Hall–Petch law. The <110> fiber texture develops linearly, and a higher dislocation density is noted at lower strain rates, with low‐angle grain boundaries increasing from 65.2% to 76.5%. The cold drawing process is thus divided into two phases, marked by a shift from low‐angle grain boundaries dominance and a notable reduction in wear rate compared to severely deformed states. [ABSTRACT FROM AUTHOR]

The tensile deformation behavior of pure titanium (Ti) gradient nanograined (GNG) is investigated based on molecular dynamics simulation. The effects of strain rate and temperature on the mechanical properties as well as deformation mechanism of GNG α‐Ti are systematically investigated. The increase of strain rate rises the tensile strength and decreases the dislocation density and stacking faults in the gradient structure of the material. This is because at higher strain rates, the time available for dislocation nucleation and proliferation is insufficient, delaying the critical point for plastic deformation. Additionally, as temperature increases, both the elastic modulus and tensile strength decrease. This is attributed to enhanced atomic activity and grain boundary migration, which reduce the material's ability to resist dislocation motion. Notably, at elevated temperatures, the material exhibits improved work‐hardening capacity due to significant grain boundary distortion, despite the overall reduction in strength. These findings provide a deeper understanding of the strain rate and temperature‐dependent mechanisms in GNG α‐Ti, which are crucial for optimizing its mechanical performance in practical applications. [ABSTRACT FROM AUTHOR]

This study investigates the impact of machining processes on the electrochemical behavior of Ti6AL4V alloy dental implants immersed in a simulated biological solution (SBF). Four distinct implant samples were crafted under varying machining conditions, and their electrochemical responses were assessed through potentiodynamic polarization (PDP) curves and electrochemical impedance spectroscopy (EIS) at room temperature. The exploration extends to the calculation of corrosion kinetics parameters via Tafel extrapolation and impedance analyses. Notably, the S750-Q0.01 sample exhibited the lowest corrosion current density (I_corr = 1.3166 µA/cm2) and the highest polarization resistance (Rp = 616 Ohm cm2), indicating superior corrosion resistance. In contrast, the S750-Q0.12 sample showed the highest dislocation density (1.037), signifying notable microstructural alterations. Electrochemical impedance results further revealed that samples machined at higher spindle speeds and lower cutting depths, such as S1000-Q0.01, exhibited higher resistance to corrosion with Rp values reaching 4396 Ohm cm2. Bioactivity analysis through Ca/P ratios demonstrated that the S1000-Q0.01 sample formed the most bioactive apatite layer with a Ca/P ratio close to hydroxyapatite standards. Phase analysis was carried out using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The outcomes revealed distinct corrosion behaviors in the SBF solution for machined implants under different conditions, signifying the significant influence of machining variations on microstructural changes and implant performance. [ABSTRACT FROM AUTHOR]

The chemical bath deposition method (CBD) was used to create thin films of calcium zirconate on a glass substrate. After that, the samples were annealed for an hour at three different temperatures: 100∘C, 200∘C, and 300∘C. The structural parameters of the samples like lattice strain, dislocation density and crystallite size were calculated from XRD analysis. The lattice strain was found to be very low. The crystallite size confirms the nano-size of the synthesized sample. From UV analysis, the optical energy direct bandgap was calculated in the range of 3.98–4.16eV. A higher annealing temperature of 300∘C causes the thin film’s bandgap to diminish, confirming the calcium zirconate thin film’s broad bandgap characteristics. From SEM analysis, morphology, and adhesive nature was analyzed, and from AFM analysis, the grain size of the sample was found to be in 300nm. [ABSTRACT FROM AUTHOR]

Plasma-enhanced chemical vapor deposition (PECVD) was used to obtain several graphite nanowall (GNW)-type films at different deposition times on silicon and copper to achieve various thicknesses of carbonic films for the development of electrochemical sensors for the detection of anthracene. The PECVD growth time varied from 15 min to 30 min to 45 min, while scanning electron microscopy (SEM) confirmed the changes in the thickness of the GNW films, revealing a continuous increase in the series. X-ray diffraction (XRD) analysis revealed that the crystallinity of the GNW film samples increased with increasing crystallite size and decreasing dislocation density as the deposition time increased. Electrochemical characterization of the GNW-based electrodes indicated that the electroactive area and heterogeneous electron transfer rate constant were greater for the GNW 45 min film in the carbonic material series. We present the transfer of GNW films on flexible polyethylene substrates for achieving flexible electrochemical sensors for further use in anthracene determination. The flexible GNW-based electrodes were investigated using differential pulse voltammetry (DPV) in the presence of anthracene. The results showed that the highest sensitivity in anthracene detection was provided by the sensor with the GNW film obtained after 45 min of PECVD growth. The optimization of the GNW film thickness for the development of flexible electrochemical sensors on polyethylene substrates represents a successful approach for enhancing the electrochemical performance of carbonic materials. [ABSTRACT FROM AUTHOR]

5356 aluminum alloy welding wire is widely used in automotive, aerospace, and marine, due to its excellent corrosion resistance, high strength-to-weight ratio, and excellent weldability. The properties of aluminum alloys are primarily determined by their microstructure. This study investigates the microstructure evolution, mechanical properties, and texture of 5356 aluminum alloy welding wire produced using the continuous casting direct rolling (CCDR) method. The results show that continuous rolling led to an increased generation of dislocations in the matrix, and dynamic recovery plays an important role in reducing dislocation density. As a result of dynamic recovery, the microstructures of 3RPs consist of extensive deformed grains and numerous substructures. The continuous casting direct rolling textures are characterized to be cube and shear textures. After 10 rolling processes, the 5356 aluminum alloy welding wire shows a highest tensile strength of 365 MPa with a lowest elongation of 8.8%. This research provides theoretical guidance to produce high-performance 5356 aluminum alloy welding wires, paving the way for their more efficient and reliable applications. [ABSTRACT FROM AUTHOR]

In this research, different Co2+ doped ZnO nanoparticles (NPs) were hydrothermally synthesized by an environmentally friendly, sustainable technique using the extract of P. capillacea for the first time. Co-ZnO was characterized and confirmed by FTIR, XPS, XRD, BET, EDX, SEM, TEM, DRS UV–Vis spectroscopy, and TGA analyses. Dislocation density, micro strains, lattice parameters and volume of the unit cell were measured using XRD results. XRD suggests that the average size of these NPs was between 44.49 and 65.69 nm with a hexagonal wurtzite structure. Tauc plot displayed that the optical energy bandgap of ZnO NPs (3.18) slowly declines with Co doping (2.96 eV). Near complete removal of the ciprofloxacin (CIPF) antibiotic was attained using Green 5% of Hy-Co-ZnO in the existence of visible LED light which exhibited maximum degradation efficiency (99%) within 120 min for 30 ppm CIPF initial concentration. The photodegradation mechanism of CIPF using Green Hy-Co-ZnO NPs followed the Pseudo-first-order kinetics. The Green Hy-Co-ZnO NPs improved photocatalytic performance toward CIPF for 3 cycles. The experiments were designed using the RSM (CCD) method for selected parameters such as catalyst dosage, antibiotic dosage, shaking speed, and pH. The maximal CIPF degradation efficiency (96.4%) was achieved under optimum conditions of 39.45 ppm CIPF dosage, 60.56 mg catalyst dosage, 177.33 rpm shaking speed and pH 7.57. [ABSTRACT FROM AUTHOR]

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]

Orientation-dependent substrates provide effective platforms for achieving α-Ga2O3 with low dislocation densities, whereas the associated strain and dislocation dynamics have not been fully explored. Herein, we investigated the evolution of growth mode, interfacial strain, and dislocation propagation in the α-Ga2O3 epitaxial layer with various orientations, grown by the halide vapor-phase epitaxy. Strain tensor theory and geometric phase analysis indicate that the m-plane α-Ga2O3 epitaxial layer exhibits the lowest misfit tensile strain, measured at εxx = 1.46% and εyy = 1.81%, resulting in the lowest edge dislocation density. The m-plane lattice exhibits an inclination of 33.60°, while the c-plane lattice is horizontally aligned and the a-plane lattice oriented perpendicularly. The orientation-dependent growth significantly influences stress relaxation through the generation of misfit dislocations, originating from either basal or prismatic slip. Edge dislocations, induced by misfit dislocations, favor the c-axis, remaining well confined within the in-plane interfacial layer of the m-plane α-Ga2O3, leading to reduced low edge dislocation density in the subsequent thick epitaxial layer. These findings shed light on the epitaxial dynamics of α-Ga2O3 heteroepitaxy, paving the way for the development of high-performance power devices. [ABSTRACT FROM AUTHOR]

In this investigation, the effects of different deformation passes—namely, single pass and multipass—on the microstructural evolution and deformation mechanisms in the Ti–6.5Al–2Zr–1Mo–1V alloy are analyzed. It is observed that following a single‐pass hot compression, the extent of lamellar α phase spheroidization enhances as the temperature increases. During multipass hot compression, there is a gradual reduction in the size and concentration of equiaxed α phase, alongside an increase in spheroidization. Dislocation density escalates to 15.88%, while the proportion of high‐angle grain boundaries (HAGBs) diminishes to 75.24%. Static recrystallization occurring during the holding process facilitates dislocation annihilation. The dynamic phase transformation mechanism manifests through interfacial permeation at the primary α phase. Strain localization at the boundaries or sub‐boundaries of the primary α phase, which exhibit minimal curvature, induces elevated shear stress, thereby promoting the shearing of the primary α phase and reducing its presence. Texture components predominantly observed are <‐12‐10>//Z0 and <0001>//Z0, transitioning from <‐12‐10> to <0001> with increasing strain. [ABSTRACT FROM AUTHOR]

Herein, an alternative way of achieving site‐specific mechanical properties is explored—the hybridization of a directed energy deposition technology with a secondary deformation process (hammer peening (HP)) which acts between deposited layers. By applying the peening in a selective manner, microstructure and hence mechanical properties can be locally varied. Microstructural characterization reveals recrystallization in the HP‐induced deformation zone. The columnar grains of as‐built regions with a grain size of ≈26 μm are transformed into a recrystallized zone with equiaxed grains having a size of ≈8 μm. There is also a highly strain‐hardened region below this recrystallized zone where the dislocation density is more than two times higher than in the as‐built condition. Subsequent tensile testing reveals that these microstructural zones corresponded to local enhancement in tensile strength normal to the build direction. The strengthening mechanisms are identified as Hall–Petch and dislocation (Taylor) strengthening, and their relative contributions are studied. The local strength enhancement comes at the expense of ductility in the build direction, which is studied via finite element modeling and attributed to strain localization into non‐strengthened areas. The results from this work show the possibility of achieving site‐specific properties via interlayer processing. [ABSTRACT FROM AUTHOR]

A laboratory‐scale hot‐rolled Ti–Mo–V–Nb steel with 1 GPa tensile strength is produced, and its microstructure and tensile properties are characterized using advanced analysis techniques and uniaxial tensile testing. A Gleeble 3800 thermomechanical simulator is used to determine a process window for the thermomechanical controlled processing (TMCP) procedure. Although the simulated TMCP specimens are fully ferritic at coiling temperatures (CT) of 590 and 630 °C, the bainitic and mixed (bainitic + ferritic) microstructure is formed in the hot‐rolled steels. The variation in the microstructure causes variations in the dislocation density through the sheet thickness, which significantly reduces the steel's ductility properties, whereas a 16% elongation is achieved with the fully bainitic microstructure. Another significant difference between the simulated TMCP and hot‐rolled specimens is the precipitation behavior. No nanosized interphase‐precipitated (IP) carbides are formed in the hot‐rolled steel during the austenite‐to‐ferrite phase transformation, although the formation of the nanosized spherical IPs is observed within the polygonal ferrite grains of the simulated TMCP specimens at the CT of 630 °C. Relatively coarse (5–20 nm) spherical (V,Mo,Ti,Nb)C carbides do not strongly affect the tensile properties of the hot‐rolled Ti–Mo–V–Nb steel. The results show that the dislocation and grain boundary strengthening mainly contribute to the strength properties of this steel. [ABSTRACT FROM AUTHOR]

III–V compound semiconductors, such as indium arsenide (InAs), are crucial in the fields of electronics and optoelectronics due to their unique properties, including a narrow bandgap, high electron mobility, low effective electron mass, and outstanding optoelectronic performance. However, the lattice constant mismatch of 7.2% between InAs and gallium arsenide (GaAs) leads to a high dislocation density in the InAs epilayer, typically around 109 cm−2 or higher when InAs is directly grown on GaAs (001). A common traditional approach to reduce material defect density involves using a compositionally graded buffer structure, which requires a thickness of several micrometers. This paper presents an innovative method for growing device-quality InAs epilayers on GaAs (001) substrates through an InAs coalesced quantum dot buffer (CQDB) layer. Remarkably, the InAs CQDB method requires a thickness of only several tens of nanometers, leading to significant improvements in film quality. Utilizing the InAs CQDB layer allows for a substantial reduction in dislocation density from 1.8 × 109 to 3.6 × 107 cm−2. The findings of this study provide a crucial research foundation for addressing lattice mismatch challenges in epitaxial growth, particularly for those investigating methods to integrate different materials in advanced device applications. [ABSTRACT FROM AUTHOR]

This paper systematically investigates the effect of temperature on lattice friction stress and develops a thermally activated dislocation slip mechanism. The findings reveal that temperature has a significant impact on dislocation slip behavior. At low temperatures, the contribution of thermal activation is minimal, leading to dislocation slip in an extended mode and resulting in higher lattice friction stress. In contrast, at high temperatures, the effect of thermal activation increases sharply, causing dislocations to slip in a full mode, thereby reducing lattice friction stress. Additionally, the influence of dislocation density, strain rate, and shear modulus on lattice friction stress is examined. Lattice friction stress decreases as dislocation density increases, but rises with higher strain rates and shear modulus. Overall, the model developed in this work provides a clearer understanding of the dislocation slip mechanism and sheds light on the underlying physical processes governing lattice friction stress. [ABSTRACT FROM AUTHOR]

In this paper, a crystal plasticity finite element method (CPFEM), considering the grain morphology and orientation, as well as the dislocation density, is used to research the tensile deformation behavior of GH4169 based on Electron Backscatter Diffraction (EBSD). The stress, plastic strain, and dislocation density distributions are obtained for different levels of deformation. Results show that the stress, plastic strain, and dislocation density exhibit obvious heterogeneous plastic deformation, and stress concentration and dislocation pileup mainly occurs near grain boundaries. The initial dislocation density mainly affects the stress–strain curve of the material, and it can obviously effect the yield strength but cannot influence the hardening ability of the material. The total dislocation density increases with plastic strain. However, the texture evolution has no evident change with increasing plastic strain, except for the increase of texture content in Cube (001)[100]. [ABSTRACT FROM AUTHOR]

In this article, the homoepitaxial regrowth of AlGaN on AlGaN templates fabricated via chemical mechanical polishing (CMP) on periodically aligned AlN nanopillars is reported on. These templates are lattice relaxed according to X‐Ray diffraction reciprocal space mapping measurements, with a low dislocation density of ≈3 × 108 cm−2. However, the AlGaN surface has hillocks exceeding 104 cm−2. Contrastingly, no hillocks are observed on the homoepitaxially regrown AlGaN on the CMP–AlGaN template and after the UV‐B laser diode (LD) layer structure is crystallized and grown on the homoepitaxially regrown AlGaN. Furthermore, when this wafer is processed for LD fabrication, room‐temperature pulsed oscillation is confirmed in the UV‐B region at a wavelength of 296 nm. The turn‐on voltage and operating voltage also reduce by ≈2 V. [ABSTRACT FROM AUTHOR]

The effect of wet etching on the characteristics of ultraviolet‐B (UV‐B) lasers fabricated on AlGaN templates grown as crystals or wet‐etched AlN nanopillars is compared in detail. Specifically, a tetramethylammonium hydroxide (TMAH) solution is used to form templates with AlN nanopillars of different diameters, followed by determining the dependence of the dislocation density in AlGaN crystals grown on the templates, the threshold power density (Pth) of an optically pumped UV‐B laser, and the threshold current density (Jth) of a UV‐B laser diode (LD) on the extent of etching. It is observed that the pillar diameter decreases almost linearly as the etching time with the TMAH solution increases. The dislocation density of AlGaN crystals grown on AlGaN templates is almost independent of the etching time. Meanwhile, Pth of the optically pumped UV‐B laser is almost halved upon etching with TMAH. Finally, the Jth of the UV‐B LD tends to decrease by 10%–20% upon etching. The optical gain is estimated by optical excitation; wet etching of AlN nanopillars with TMAH solution results in a larger optical gain at the same excitation intensity, indicating that nonradiative recombination centers, such as point defects, are reduced. [ABSTRACT FROM AUTHOR]

ScAlMgO4 (SAM) substrates have attracted significant attention as template substrates for fabricating bulk GaN crystals. Radio‐frequency plasma‐assisted molecular beam epitaxy (RF‐MBE) is preferred for the growth of the first GaN layer. However, the metastable zincblende (ZB) phase is mixed on the SAM substrate with a smaller terrace width, coupled with lower growth temperatures in MBE. Herein, the time evolution of GaN growth on SAM substrates is investigated to elucidate the mechanism underlying ZB mixing. ZB‐GaN is generated as stacking faults during the coalescence of the initially grown wurtzite‐ (WZ‐) GaN islands on adjacent SAM terraces, owing to the extraordinarily large step height close to three bilayers of WZ‐GaN. Therefore, high‐temperature growth on a SAM substrate with wider terraces suppresses the generation of the ZB phase in the initial growth stage. ZB‐GaN also decreases with an increase in the growth time. Based on these results, a 940 nm‐thick GaN layer is grown at 750 °C on the SAM substrate with a terrace width of 820 nm. A pure WZ‐GaN layer from the GaN/SAM interface with a flat surface with a root mean square value of below 1 nm and a dislocation density of 7.3 × 109 cm−2 is obtained. [ABSTRACT FROM AUTHOR]

This article is aimed at understanding of the complex design of metalorganic chemical vapour deposition ‐grown InGaN‐based red light‐emitting diode (LED) structure. The contribution of different elements of red LED structure to the stress distribution and threading dislocation density (TDD) evolution is theoretically investigated. For this purpose a self‐consistent modeling of the structure growth process is used, taking into account stress‐modulated indium incorporation, mismatch stress relaxation by threading dislocations and V‐pits, and nucleation of new threading dislocations. The simulation results, consisting of composition, stress, and TDD profiles, are then utilized for modeling of device operation, which allows to analyze contribution of different elements to the heterostructure operation. [ABSTRACT FROM AUTHOR]

In this study, the effect of implementing island growth in an AlN/Al0.1Ga0.9N superlattice on the structural properties of vertical GaN‐on‐Si(111) PIN‐structures is investigated. It is demonstrated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) that islands are formed on‐top of V‐pits present in the AlN nucleation layer and that the island coalescence height can be controlled by the growth temperature. Defect selective etching analyses confirm a noteworthy reduction in the threading dislocation density (TDD), which diminishes from 1.2 × 109 cm−2 ± 7.5 × 107 cm−2 to 8.5 × 108 cm−2 ± 7.3 ×107 cm−2 as the island coalescence height increases from ≈160 nm to ≈450 nm, achieved by increasing the growth temperature. Cross‐sectional transmission electron microscopy (TEM) shows that island growth is particularly favorable for the reduction of a‐type dislocations. As a consequence of the significant reduction of a‐type dislocations in the buffer, stress relaxation during the GaN film growth is reduced, which is supported by in situ wafer curvature measurements and high‐resolution X‐ray diffraction (XRD). Owing to the optimized island growth conditions, thick and crack‐free GaN layers on Si(111) substrates are obtained with an absolute wafer bow of <50 μm. [ABSTRACT FROM AUTHOR]

ScAlMgO4 (SAM) has attracted considerable attention as a substrate for growing InGaN template layers owing to its lattice matching with In0.17Ga0.83N. Furthermore, radio‐frequency plasma‐assisted molecular beam epitaxy (RF‐MBE) can be utilized to grow (In)GaN directly on SAM substrates because the growth is conducted at relatively low temperatures without reactive gases. This prevents the diffusion and desorption of the constituent elements of the SAM substrate. In this study, the substrate‐temperature dependence of the direct growth of InGaN on SAM substrates is investigated through RF‐MBE. In droplets are formed below 600 °C, whereas, the In content of the InGaN crystal decreases rapidly, and the surface roughness increases at ≈650 °C. These results are attributed to the acceleration of In desorption at higher temperatures. The improvement in crystal coherency moderates above 600 °C. Therefore, a nearly lattice‐matched In0.18Ga0.82N film is grown at an optimal temperature of 600 °C by adjusting the group‐III metal fluxes. The resulting film is continuous and relatively flat, with no droplets. The dislocation density with screw component is one order of magnitude lower than that of the lattice‐mismatched InGaN. The dislocation density of the InGaN film is comparable to that of a film grown by metal‐organic vapor‐phase epitaxy. [ABSTRACT FROM AUTHOR]

The development of processes for epitaxial growth of vertical gallium nitride (GaN) drift layers enabling 1.2 kV breakdown voltage on low‐cost sapphire substrates is presented in comparison to GaN bulk substrates. The targeted blocking capability demands drift layers with a thickness of 10 μm and low but controllable n‐type doping. Using a growth rate of 2.5 μm h−1 the concentration of unintentionally incorporated carbon is sufficiently low to adjust the n‐type carrier concentration to ≈1 × 1016 cm−3 for all types of substrates. To assess GaN drift region properties in terms of forward bias conductivity and reverse bias blocking strength, a quasi‐vertical p‐n‐diode structure is utilized. Bow reduction of GaN‐on‐sapphire structures is achieved using a stealth laser scribing process. Breakdown voltages higher than 1600 V and a specific on‐state resistance as low as 0.7 mΩ cm2 are obtained with diodes fabricated on GaN substrates. Similar structures grown on sapphire show breakdown voltages of about 1300 V due to higher levels of current leakage. Comparing different types of substrates, a direct correlation between dislocation density in the drift layer with the leakage current in p‐n diodes is deduced. [ABSTRACT FROM AUTHOR]

This paper deals with Barkhausen noise in Trip steel RAK 40/70+Z1000MBO subjected to uniaxial plastic straining under variable strain rates. Barkhausen noise is investigated especially with respect to microstructure alterations expressed in terms of phase composition and dislocation density. The effects of sample heating and the corresponding Taylor–Quinney coefficient are considered as well. Barkhausen noise of the tensile test is measured in situ as well as after unloading of the samples. In this way, the contribution of external and residual stresses on Barkhausen noise can be distinguished in the direction of tensile loading, as well as in the transversal direction. It was found that the in situ-measured Barkhausen noise grows in both directions as a result of tensile stresses and the realignment of domain walls. The post situ-measured Barkhausen noise drops down in the direction of tensile load due to the high opposition of dislocation density at the expense of the growing transversal direction due to the prevailing effect of the realignment of domain walls. The temperature of the sample remarkably grows along with the increasing strain rate which corresponds with the increasing Taylor–Quinney coefficient. However, this effect plays only a minor role, and the density of the lattice imperfection expressed especially in terms of dislocation density prevails. [ABSTRACT FROM AUTHOR]