Dislocation Generation Mechanisms Research Articles

Effects of pulsed laser processing on structural evolution of diamonds - A molecular dynamics and experimental study

The pulsed laser processing has been used for machining ultrahard diamonds. However, structural evolutions, along with microscopic mechanisms, have hardly been clarified. In the present study, a molecular dynamics model for describing the pulsed laser processing of diamonds was developed, and the calculation results were verified through corresponding experiments, by using the typical nanosecond pulsed laser processing. At first, the temporal evolution of the temperature field was calculated, establishing the correlation between the temperature distribution and material ablation phenomena. Besides, intricacies of dislocation generation mechanism were elaborately explained, indicating that stress waves played prominent roles in formation and propagation of dislocations. In order to understand the phase transition phenomena, including the graphitization, vaporization and solidification, variations in coordination numbers based on calculations, together with Raman and XPS characterizations on diamond samples before and after laser processing, were examined and compared. Moreover, different effects of the pulsed laser processing on polycrystalline and monocrystalline diamonds were compared. It was found that the polycrystalline diamond shows lower rate of temperature fall and lower velocity of stress waves. This study provides valuable insights into the microscopic mechanisms governing pulsed laser processing of diamonds, thereby broadening the scope for optimizing laser machining processes.

Multicrystalline Informatics Applied to Multicrystalline Silicon for Unraveling the Microscopic Root Cause of Dislocation Generation.

A comprehensive analysis of optical and photoluminescence images obtained from practical multicrystalline silicon wafers is conducted, utilizing various machine learning models for dislocation cluster region extraction, grain segmentation, and crystal orientation prediction. As a result, a realistic 3D model that includes the generation point of dislocation clusters is built. Finite element stress analysis on the 3D model coupled with crystal growth simulation reveals inhomogeneous and complex stress distribution and that dislocation clusters are frequently formed along the slip plane with the highest shear stress among twelve equivalents, concentrated along bending grain boundaries (GBs). Multiscale analysis of the extracted GBs near the generation point of dislocation clusters combined with ab initio calculations has shown that the dislocation generation due to the concentration of shear stress is caused by the nanofacet formation associated with GB bending. This mechanism cannot be captured by the Haasen-Alexander-Sumino model. Thus, this research method reveals the existence of a dislocation generation mechanism unique to the multicrystalline structure. Multicrystalline informatics linking experimental, theoretical, computational, and data science on multicrystalline materials at multiple scales is expected to contribute to the advancement of materials science by unraveling complex phenomena in various multicrystalline materials.

Analytic model of dislocation density evolution in fcc polycrystals accounting for dislocation generation, storage, and dynamic recovery mechanisms

An analytic model of the evolution of dislocation density in fcc polycrystals is described. The evolution equations approximately account for most known dislocation storage, dynamic recovery, and dislocation generation mechanisms in fcc polycrystals. Specifically, the model incorporates network (forest) and grain boundary storage, mobile-network and mobile–mobile annihilation, screw–screw annihilation via athermal and thermal single cross-slip, generation by double cross-slip (Koehler mechanism, including dipole formation), Frank-Read sources, grain boundary nucleation, and mobile–immobile dislocation nucleation due to shock loading. Single cross-slip is assumed to proceed through the Friedel–Escaig (FE) mechanism; the corresponding activation energy is calculated using a modified FE model. The activation energy for double cross-slip is calculated for the first time by extending the FE model. The exact evolution equations are integro-differential equations, and as such are difficult to implement in a code; hence, the evolution equations are simplified by making several approximations. Preliminary results on copper are presented, including comparisons to experimental data.

A synchrotron X-ray topography study of crystallographic defects in ScAlMgO4 single crystals

ScAlMgO4 is a promising material that can be used to fabricate lattice-matched substrates for the epitaxial growth of GaN and InGaN. To improve its crystal quality, we used synchrotron X-ray topography to study the dislocations in a 50.8-mm-diameter ScAlMgO4 single crystal grown via the Czochralski method. Dislocation character was analyzed in terms of Burgers vectors by comparing dislocation contrasts recorded using six g-vectors that represented the m+c, a+c, and c-type crystal planes. The central area of the substrate, which corresponded to the portion grown vertically under the seed crystal, was characterized by millimeter-sized domains bound by mixed-type dislocation arrays with both in-plane and c-axis Burgers vectors. Basal plane dislocations (BPDs) typically formed an arrow-like shape with three branches. Growth striations with alternating high- and low-dislocation-density areas were observed in the diameter-expanding area and high-dislocation-density areas were covered with tangled BPD networks. The mechanisms of dislocation generation, multiplication, and reaction are discussed based on their line directions and dislocation character.

Study of strain evolution mechanism in Ge1−xSnx materials grown by low temperature molecular beam epitaxy

Ge1−xSnx materials with constant and step-graded compositions have been successfully grown on Ge/Si(001) substrates by using low temperature molecular beam epitaxy (LT-MBE). It has been observed that both completely strained and partially relaxed GeSn materials with the same composition could be formed within the same sample, without adjusting any growth parameters. The residual in-plane strain in GeSn changes in a specific pattern from the GeSn/Ge interface towards the surface. The lower section of the GeSn material remains fully strained and free of dislocations, while most of the threading dislocations are located in the upper section of the layer, causing considerable strain relaxation within this section while maintaining the composition unchanged. This behavior could be explained by kinetic roughening and dislocation generation mechanisms at low temperatures and is remarkably different from GeSn materials grown by chemical vapor deposition (CVD), which exhibit a gradual strain relaxation process as growth continues. This work contributes to the fundamental understanding of the strain relaxation mechanisms of GeSn materials grown by MBE, which is instructive for improving the material quality in the future.

Rapid fabrication of complex nanostructures using room-temperature ultrasonic nanoimprinting

Despite its advantages of scalable process and cost-effectiveness, nanoimprinting faces challenges with imprinting hard materials (e.g., crystalline metals) at low/room temperatures, and with fabricating complex nanostructures rapidly (e.g., heterojunctions of metal and oxide). Herein, we report a room temperature ultrasonic nanoimprinting technique (named nanojackhammer) to address these challenges. Nanojackhammer capitalizes on the concentration of ultrasonic energy flow at nanoscale to shape bulk materials into nanostructures. Working at room temperature, nanojackhammer allows rapid fabrication of complex multi-compositional nanostructures made of virtually all solid materials regardless of their ductility, hardness, reactivity and melting points. Atomistic simulations reveal a unique alternating dislocation generation and recovery mechanism that significantly reduces the imprinting force under ultrasonic cyclic loading. As a proof-of-concept, a metal-oxide-metal plasmonic nanostructure with built-in nanogap is rapidly fabricated and employed for biosensing. As a fast, scalable, and cost-effective nanotechnology, nanojackhammer will enable various unique applications of complex nanostructures in optoelectronics, biosensing, catalysis and beyond.

Microstructure and elevated temperature tensile property of Ti–46Al–7Nb-(W,Cr,B) alloy compared with binary and ternary TiAl alloy

The newly developed high-Nb TiAl with a small amount of W, Cr, and B addition prepared by electromagnetic cold crucible has been investigated by comparing with the microstructure and elevated temperature tensile properties of binary and ternary high-Nb TiAl alloy. Results indicate that microstructures of the three alloys are fully lamellar (FL), near fully lamellar (NFL) and near fully lamellar alloy with TiB (NFL + TiB). The dispersed TiB particles in the alloy have the B27 structure and are highly enriched with Nb and W but depleted with Al by the characterization of transmission electron microscopy. The elevated temperature tensile test results show that the high-Nb TiAl alloy with NFL + TiB microstructure exhibits excellent mechanical properties compared with the other two alloys, and the tensile strength and elongation at 800 °C are 648 MPa and 5.73%, respectively. Fracture morphology shows the alloy has a mixed mode of trans-lamellar and inter-lamellar fracture. As the dispersed second phase particles, rod-like TiB can act as the obstacle to crack propagation and dislocation movement. The excellent mechanical properties of the alloy at elevated temperatures are attributed to the combined effects of dislocation block and nanotwins formation during deformation within the γ lamellae. Moreover, the probable generation and propagation mechanism of dislocations and twins in the γ lamellae are further discussed.

Dislocation generation mechanisms in heavily boron-doped diamond epilayers

Doping diamond layers for electronic applications has become straightforward during the last two decades. However, dislocation generation in diamond during the microwave plasma enhanced chemical vapor deposition growth process is still not fully understood. This is a truly relevant topic to avoid for an optimal performance of any device, but, usually, it is not considered when designing diamond structures for electronic devices. The incorporation of a dopant, here boron, into a lattice as close as that of diamond, can promote the appearance of dislocations in the epilayer. The present contribution analyzes the different processes that can take place in this epilayer and gives some rules to avoid the formation of dislocations, based on the comparison of the different dislocation generation mechanisms. Indeed, competitive mechanisms, such as doping atom proximity effect and lattice strain relaxation, are here quantified for heavily boron-doped diamond epilayers. The resulting growth condition windows for defect-free heavily doped diamond are here deduced, introducing the diamond parameters and its lattice expansion in several previously published critical thickness (hc) and critical doping level relationships for different doping levels and growth conditions. Experimental evidence supports the previously discussed thickness-doping-growth condition relationships. Layers with and without dislocations reveal that not only the thickness but also other key factors such as growth orientation and growth parameters are important, as dislocations are shown to be generated in epilayers with a thickness below the People and Bean critical thickness.

Primary creep regeneration in 10%Cr martensitic steel: In-situ and ex-situ microstructure studies

Primary creep regeneration (PCR) is a phenomenon observed during stress-varying/cyclic creep loading conditions where a load reversal might clear the previous strain hardening memory and cause the regeneration of the primary creep regime (i.e. a period of high creep strain rate) upon reloading. In this study, in-situ and ex-situ microstructural examinations, including transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), neutron and synchrotron X-ray diffraction were conducted to better understand the responsible mechanisms of PCR for a 10%Cr martensitic steel at 600 °C. Our experimental evidence indicated that the PCR phenomenon is related to the change of dislocation density due to activation of dislocation generation and recovery mechanisms, formation and relaxation of dislocation pile-ups, as well as bowing/unbowing of dislocation-lines during stress-varying creep loading conditions. These mechanisms could explain the observed creep strain accumulation in the steel during the examined stress-varying creep loading conditions reported in the current and previous studies. The presented mechanistic description of the PCR phenomenon and the reported experimental observations for the microstructural and mechanical parameters can provide a basis for the formulation of physically-based models to describe the creep behaviour of the steel under high-temperature stress-varying creep loading conditions.

Cathodoluminescence Studies of Nanoindented CdZnTe Single Crystal Substrates for Analysis of Residual Stresses and Deformation Behaviour

Nanoindentation-induced residual stresses were analysed on (111) Te face CdZnTe single-crystal substrates in this study. CdZnTe substrates were subjected to nanoindentation using cube corner indenter geometry with a peak load of 10 mN. Loading rates of 1 mN/s and 5 mN/s were used in the experiments, with a holding time of 10 s at peak load. Residual stresses on the indented region were analysed from load-displacement curves and explained using dislocation generation and elastic recovery mechanisms. Residual stresses were found to be of compressive type, just on the indented surface. The slip lines along the slip directions of this material were clearly visible in the FE-SEM images of the indents. Indents and surrounding surfaces were characterized using the Cathodoluminescence (CL) technique. CL mapping of the indented surface revealed the dislocation generation and their propagation behaviour just beneath the indenter as well as in the surrounding surfaces. The dislocations act as non-radiative recombination centres and quench the CL intensity locally. Dark lines were explained as the presence of dislocations in the material. CL mapping analysis shows that both the rosette glide and tetrahedral glide of dislocations are the primary deformation mechanisms present in CdZnTe. A rosette structure was observed in the CL mapping. CL spectra at 300 K of un-deformed CdZnTe show a peak at 810 nm wavelength, which corresponds to near-band-edge emission. After indentation, the CL spectra show the peak intensity at 814 nm and 823 nm wavelengths at the edge of the indents created with a loading rate of 1 mN/s and 5 mN/s, respectively. These peak shifts from 810 nm were attributed to the tensile residual stresses present in the indented material.

The strain-induced martensitic phase transformation of Fe–C alloys considering C addition: A molecular dynamics study

Abstract

New insights into microstructural evolution of epitaxial Ni–Mn–Ga films on MgO (1 0 0) substrate by high-resolution X-ray diffraction and orientation imaging investigations

In this work, a detailed investigation has been performed on hetero-epitaxial growth and microstructural evolution in highly oriented Ni–Mn–Ga (1 0 0) films grown on MgO (1 0 0) substrate using high-resolution X-ray diffraction and orientation imaging microscopy. Mosaicity of the films has been analysed in terms of tilt angle, twist angle, lateral and vertical coherence length and threading dislocation densities by performing rocking curve measurements and reciprocal space mapping. Density of edge dislocations is found to be an order of magnitude higher than the density of screw dislocations, irrespective of film thickness. X-ray pole figure measurements have revealed an orientation relationship of || (1 0 0)MgO; || [0 0 1]MgO between the film and substrate. Microstructure predicted by X-ray diffraction is in agreement with that obtained from electron microscopy and atomic force microscopy. The evolution of microstructure in the film with increasing thickness has been explained vis-à-vis dislocation generation and growth mechanisms. Orientation imaging microscopy observations indicate evolutionary growth of film by overgrowth mechanism. Decrease in coercivity with film thickness has been explained as an interplay between stress field developed due to crystal defects and magnetic domain pinning due to surface roughness.

CdTe, ZnTe and Cd1-XZnXTe Nanolayers Grown by Atomic Layer Deposition on GaSb and GaAs (001) Oriented Substrates

In this work are presented the results obtained from ZnTe, CdTe and Cd1-xZnxTe growth on GaSb and GaAs (001) oriented substrates by the regimen of Atomic Layer Deposition (ALD). The growths were performed by exposing the substrates alternatively to the elemental vapor sources for the case of ZnTe and CdTe and to the vapors of a solid solution in the case of Cd1-xZnxTe. Ellipsometric measurements showed that the value of the thickness per cycle was around 0.3 nm. That correspond to the value corresponding to a monolayer of ZnTe. As a result of High-resolution X-ray diffraction measurements it can be identified the mechanism of interface dislocation generation in the growth interface as the relaxation of the layers, additional the value of the full width at half maximum (FWHM) result about 300 to 600 arcsec, indicating a large density of dislocation.

Investigations of shock-induced deformation and dislocation mechanism by a multiscale discrete dislocation plasticity model

There are many theoretical and computational investigations devoted to the dislocation mechanism based crystal plasticity under static or quasi-static conditions. However, the dislocation behavior, specifically the nucleation and multiplication of dislocations, remains unclear in crystal under shock loading. In this paper, the dislocation mechanism in single crystalline copper under shock loading is investigated through a multiscale numerical model, which couples discrete dislocation dynamics (DDD) with explicit finite element (FE) analyses. Firstly, since the homogeneous nucleation (HN) of dislocations is considered as the dominant dislocation generation mechanism at extremely high strain rates, the typical parameters of HN are obtained by systematic molecular dynamics (MD) simulations, which include critical shear stress, saturation time, stable density, etc. Then, these parameters are implemented into DDD-FE model by a coarse grained method. It can remarkably improve the computational efficiency without loss of typical dislocation characters. The interactions between shock wave and dislocations are studied in detail. Band-like dislocation walls and their shielding effect on other dislocations are observed during the shock wave propagation. The simulation results are in good agreement with experimental observations. It is also found that both fast HN and avalanche-like dislocation multiplication get involved and lead to the softening of shear stress. Finally, by comparing the dynamic behavior under different impact speeds, a threshold speed around 1000m/s for the dislocation dominant mechanism is proposed from the computations in this work. Beyond this speed, the effect of other defects such as stacking fault and twinning would be prominent.

Elastic-plastic deformation of molybdenum single crystals shocked along [100

To understand the elastic-plastic deformation response of shock-compressed molybdenum (Mo) - a body-centered cubic metal, single crystal samples were shocked along the [100] crystallographic orientation to an elastic impact stress of 12.5 GPa. Elastic-plastic wave profiles, measured at different propagation distances ranging between ∼0.23 to 2.31 mm using laser interferometry, showed a time-dependent material response. Within the experimental scatter, the measured elastic wave amplitudes were nearly constant over the propagation distances examined. These data point to a large and rapid elastic wave attenuation near the impact surface, before reaching a threshold value (elastic limit) of ∼3.6 GPa. Numerical simulations of the measured wave profiles, performed using a dislocation-based continuum model, suggested that {110}⟨111⟩ and/or {112}⟨111⟩ slip systems are operative under shock loading. In contrast to shocked metal single crystals with close-packed structures, the measured wave profiles in Mo single crystals could not be explained in terms of dislocation multiplication alone. A dislocation generation mechanism, operative for shear stresses larger than that at the elastic limit, was required to model the rapid elastic wave attenuation and to provide a good overall match to the measured wave profiles. However, the physical basis for this mechanism was not established for the high-purity single crystal samples used in this study. The numerical simulations also suggested that Mo single crystals do not work harden significantly under shock loading in contrast to the behavior observed under quasi-static loading.

Deformation mechanisms of D022 ordered intermetallic phase in superalloys

High-temperature alloys in general and superalloys in particular are strengthened by ordered intermetallic phases that are relatively stable at elevated temperatures. Because of their low symmetry, however, these ordered intermetallic phases have rather complicated deformation mechanisms that are difficult to uncover by experiment alone. In this study we use a combination of ab initio calculation and phase field simulation at the elementary defect level to illustrate how dislocations interact with precipitates of an ordered intermetallic phase, γ″ (D022, tetragonal), the primary strengthening phase in Ni-Nb-Fe-Cr-Ti-Al-Mo alloy (Inconel 718 or IN718 superalloy). A rich variety of new and sophisticated deformation mechanisms are discovered, including a novel mechanism of dislocation generation (accompanied by a spontaneous stacking fault (SF) transition), formation of superlattice intrinsic SF ribbons (SISF-ribbons) and 1/3<112>-type compact super-dislocations, along with ISF shearing and Orowan looping. The predicted deformation microstructures seem to agree with recent electron microscopy observations in IN718. The detailed deformation mechanisms uncovered can be incorporated in constitutive microstructure-property relationships for advanced crystal plasticity modeling and the approach developed can be used to study plastic deformation of other intermetallic phases in different alloy systems.

Stratigraphy of a diamond epitaxial three-dimensional overgrowth using doping superlattices

The selective doped overgrowth of 3D mesa patterns and trenches has become an essential fabrication step of advanced monolithic diamond-based power devices. The methodology here proposed combines the overgrowth of plasma-etched cylindrical mesa structures with the sequential growth of doping superlattices. The latter involve thin heavily boron doped epilayers separating thicker undoped epilayers in a periodic fashion. Besides the classical shape analysis under the scanning electron microscope relying on the appearance of facets corresponding to the main crystallographic directions and their evolution toward slow growing facets, the doping superlattices were used as markers in oriented cross-sectional lamellas prepared by focused ion beam and observed by transmission electron microscopy. This stratigraphic approach is shown here to be applicable to overgrown structures where faceting was not detectable. Intermediate growth directions were detected at different times of the growth process and the periodicity of the superlattice allowed to calculate the growth rates and parameters, providing an original insight into the planarization mechanism. Different configurations of the growth front were obtained for different sample orientations, illustrating the anisotropy of the 3D growth. Dislocations were also observed along the lateral growth fronts with two types of Burger vector: b011¯=12[011¯] and b112=16[112]. Moreover, the clustering of these extended defects in specific regions of the overgrowth prompted a proposal of two different dislocation generation mechanisms.

Metamorphic epitaxial materials

Abstract

Nanoindentation of gold and gold alloys by molecular dynamics simulation

The nanoindentation hardnesses and stacking fault energies (SFE) for pure and alloyed Au are determined from classical molecular dynamics simulations. Rather than a traditional force–displacement dependence that is examined in many previous nanoindentation works, we analyze the hardness vs. force in this study, which shows features that allow us to distinguish defect nucleation processes from hardening processes. During nanoindentation, homogeneously nucleated defects interact to form V-shape lock structures, and finally form four-sided dislocations that are continuously released into the bulk, in a manner similar to the heterogeneous Frank-Read dislocation generation mechanism. Hardness in the alloy system is predicted to be critically controlled by the ease and frequency of nucleation of new defects. Consistent with previous simulation results, the difference of the unstable and stable SFE, rather than the stable SFE along, is found to be closely related to this nucleation process, and thus to hardness.

The Role of Homogeneous Nucleation in Planar Dynamic Discrete Dislocation Plasticity

Homogeneous nucleation of dislocations is the dominant dislocation generation mechanism at strain rates above 108 s−1; at those rates, homogeneous nucleation dominates the plastic relaxation of shock waves in the same way that Frank–Read sources control the onset of plastic flow at low strain rates. This article describes the implementation of homogeneous nucleation in dynamic discrete dislocation plasticity (D3P), a planar method of discrete dislocation dynamics (DDD) that offers a complete elastodynamic treatment of plasticity. The implemented methodology is put to the test by studying four materials—Al, Fe, Ni, and Mo—that are shock loaded with the same intensity and a strain rate of 1010 s−1. It is found that, even for comparable dislocation densities, the lattice shear strength is fundamental in determining the amount of plastic relaxation a material displays when shock loaded.