Dislocation Clusters Research Articles

Stress analysis and dislocation cluster generation in silicon crystal with artificial grain boundaries

We conducted a dislocation and stress analysis on various grain boundaries (GBs) using silicon ingots that contained artificial GBs to permit systematic comparison of experimental and analytical results. Through photoluminescence imaging, we found that the number of dislocation clusters generated around the 〈110〉-oriented GBs was significantly higher than those around the 〈100〉-oriented GBs. The stress analysis revealed that this difference is linked to the maximum shear stress around the GB. However, there were some GBs where dislocation cluster generation was not observed despite the presence of high shear stress. For most of these GBs, the direction of the maximum shear stress in the 12 slip system of silicon crystal was found to be oblique downward to the growth direction, which appears to inhibit dislocation propagation.

Development and morphological analysis of the zone refining process for high purity germanium

A highly efficient purification process has been developed to enhance the purity of germanium (Ge) up to 13 N from an initial 4N purity. Hall effect measurements demonstrated that part of the zone refined Ge (ZRGe) bars exhibited p-type conductivity and high purity with a net charge carrier concentration of around 4–9 × 1010 cm−3. Noteworthy observations include a rise in carrier concentration (1011 to 1012 cm−3) at a very low zone refining (ZR) velocity of 1 mm/min, which is attributed to the diffusion of impurities from the container into the Ge melt. The purification efficiency was enhanced by optimizing the ZR velocity to around 4–5 mm/min. Purification was carried out at different process steps using graphite and quartz containers. Subsequent microstructural examinations, conducted in conjunction with wet-chemical etching of the sample unveiled the presence of various grains, grain boundaries (GBs), and low & high-density of dislocation clusters: These observations were made in samples that had undergone zone refining under various gas atmospheres including argon, nitrogen and hydrogen. Electron back scattered diffraction (EBSD) analysis reveals the characteristics of multiple grains and grain boundaries (GBs) of Ge refined in argon and nitrogen atmosphere, including low-angle GBs, high-angle GBs with coincide site lattice (CSL) Σ3, Σ9, Σ27a, and random angle GBs. In contrast, the EBSD results of Ge refined in hydrogen atmosphere shows larger grains with only a few low-angle grain boundaries. The materials refined in argon and nitrogen show a lower carrier lifetime (10–30 μs) owing to the presence of various high angle grains boundaries as well as low purity. Conversely, zone refining in hydrogen resulted in the carrier lifetime of around 50 μs in polycrystalline Ge wafers with larger grains.

A customised novel hybrid post-treatment process achieved excellent mechanical properties in additively manufactured Haynes 230 alloy

ABSTRACT It is challenging to maintain decent plasticity while increasing strength in laser powder bed fusion (LPBF) manufactured Haynes 230 alloy. The fundamental issue of both inadequate affected depth and deteriorated plasticity exists for laser shock peening (LSP). To address this problem, the treatment method of heat treatment plus laser shock peening (HT-LSP) was proposed. After heat treatment, dispersive M23C6 carbides were precipitated, and dislocation was predominately decreased inside grains but increased at grain boundaries. The junction of three imperfect dislocations was identified at M23C6 after HT-LSP treatment, indicating the novel phenomenon of twin formation via the polar-axis mechanism of dislocation proliferation, given the inhibition of intergranular transfer of dislocations and adequate clean rooms without dislocation clusters inside grains. Grain rotation occurred after HT-LSP derived from dynamic recrystallisation and twin distortion. The hardness along depth direction of the sample demonstrated the increased effective affected depth of LSP from 900 to 1400 μm after heat treatment. The shear strength of the HT-LSP sample was increased to 662 MPa. The elongation of the sample reaching 13.4% was also higher than that with LSP. The mechanical performance improvement was mainly due to the gradient twin layer, carbides’ precipitation and high-density dislocation.

Wear and neutron shielding resilience of titanium-hexagonal boron nitride coatings against extreme lunar radiation and thermal cycles

Ti6Al4V and Al6061 are aerospace alloys used in lunar structural components and rovers owing to their high specific strength. However, they deteriorate rapidly when subjected to demanding conditions of the lunar environment, such as extreme thermal cycles, solar particle radiation, and abrasive regolith. Cryo-milled powders were employed to deposit hBN-reinforced protective titanium coatings through plasma spray (atmospheric-APS and vacuum-VPS) with 2 and 10 vol% of hBN on Ti6Al4V and Al6061 substrates. These coatings were evaluated for durability under extreme lunar-like conditions, including thermal cycling, electron radiation, and synergistic (electron radiation-thermal cycle) exposure. The coatings exhibited radiation-induced hardening, resulting in a substantial increase in their microhardness ~20–40 % compared to virgin condition. Bright-field transmission electron microscopy (TEM) analysis reveals the presence of high dislocation densities, black dot defects, and dislocation clusters, providing evidence of the severe impact of synergistic environmental stresses. Ball-on-disk tribological tests were conducted in the presence of JSC-1 A lunar regolith simulant to evaluate the wear performance of coatings. Compared to conventional Ti6Al4V substrate, 50 %, 90 %, and 70 % reductions in wear volume were observed in the Ti/2 vol% hBN coatings in thermal-only, radiation-only, and synergistic environments. Neutron shielding tests indicate that the synergistic coatings offer higher neutron shielding performance by up to 28 % compared to virgin coatings, attributed to improved neutron capture by 10B isotopes. Upon a comprehensive assessment and ranking of multiple coatings under varying conditions, the VPS Ti/2 vol% hBN coating emerges as the top choice for protection against extreme radiation and thermal cycles for future lunar explorations.

Strength-ductility synergy of deposited Al–Mg-Sc alloy with a homogeneous phase distribution brought by heat treatment

Post-processing is crucial for achieving high mechanical performance in deposited Al–Mg-Sc alloys. However, simultaneously enhancing strength and ductility is challenging. This study analyzed the effects of aging and solution + aging treatments on precipitation behavior and tensile properties, obtaining the alloy with excellent mechanical performance. The results indicated that aging treatment precipitated fine Al3Sc phases to enhance strength, while residual primary Al6(Fe, Mn) phases led to poor ductility due to dislocation clusters. Solution + aging treatment refined large-sized Al6(Fe, Mn) phases, and formed dense secondary Al3Sc phases to delay dislocation clustering during tensile, thereby achieving the better performance in strength-ductility synergy. Compared to the deposited alloy, ultimate tensile strength and elongation of alloy prepared through solution + aging increased by 17 % and 13 %, respectively. This study offers new insights into enhancing mechanical performance of aluminum alloys and expands the engineering applications of Al–Mg-Sc alloys.

A Comparative Study of Methods for Calculating the Dislocation Density in GaN-on-Si Epitaxial Wafers.

A series of characterization methods involving high-resolution X-ray diffraction (HR-XRD), electron channel contrast imaging (ECCI), cathodoluminescence microscopy (CL), and atomic force microscopy (AFM) were applied to calculate the dislocation density of GaN-on-Si epitaxial wafers, and their performance was analyzed and evaluated. The ECCI technique, owing to its high lateral resolution, reveals dislocation distributions on material surfaces, which can visually characterize the dislocation density. While the CL technique is effective for low-density dislocations, it is difficult to accurately identify the number of dislocation clusters in CL images as the density increases. The AFM technique analyzes surface dislocation characteristics by detecting surface pits caused by dislocations, which are easily affected by sample and probe conditions. A prevalent method for assessing the crystal quality of GaN is the rocking curve of HR-XRD (ω-scan), which calculates the dislocation density based on the FWHM value of the curves. By comparing the above four dislocation characterization methods, the advantages and limitations of each method are clarified, which also verifies the applicability of DB=β29b2 for GaN-on-Si epitaxial wafers. This provides an important reference value for dislocation characterization in GaN-on-Si materials. The accuracy evaluation of dislocation density can truly and reliably reflect crystal quality, which is conducive to further optimization. Furthermore, this study can also be applied to other heterogeneous or homogeneous epitaxial materials.

Influences of stress triaxiality and forming parameters on microstructural evolution and fracture mechanisms in a Ni–Cr–Mo-based superalloy

Hot tensile features in a Ni–Cr–Mo-based superalloy with different notched types are explored. Changes of microstructure and fracture mechanisms with stress triaxiality and tensile parameters are elucidated. Results manifest that the maximum load for the Ni–Cr–Mo-based superalloy in hot tensile ascends with increasing stress triaxiality. The development/interaction of dislocation clusters and subgrains is sensibly boosted, while the dynamic recrystallization (DRX) formation/coarsening behaviors is suppressed with ascending stress triaxiality or strain rate. Nevertheless, the intense generation/development in substructure and DRX grains appear at higher tensile temperature. The primary fracture mode is the ductile facture closely associated with dimples formation/coalescence and the progression of serpentine gliding. The primary fracture mechanism of Ni–Cr–Mo-based superalloy is the shear fracture characterized by the microporous polymerization. The coalescence/growing up in dimples becomes weaken with reducing stress triaxiality. Additionally, the dominant texture components are the E, F and Copper textures for the Ni–Cr–Mo-based superalloy with notched type in high-temperature tensile. With increasing stress triaxiality, the S texture sensibly increases.

Swift Au+9 ion irradiation-induced defects and alloy complexity effect on the mechanical hardness of NiCoCrFePd HEA and NiCoCrFe MEA

The outstanding radiation damage stability of an NiCoCrFePd high entropy alloy (HEA) as compared to conventional alloys poses the question for the mechanism of an ion–matter interaction. The positron annihilation lifetime spectroscopic and TEM (transmission electron microscopic) measurements are implemented to trace different kinds of defects produced by 120 MeV Au+9 ion irradiation and their evolution as a function of ion fluence. The variation of lifetimes and corresponding intensities with the ion fluence indicates the formation of dislocation-type defects at a lower ion fluence and vacancy clusters at a higher ion fluence caused by coalescence or agglomeration of dislocation defects. Formation of different types of defects in turn modulates the strain development inside the crystal. Additionally, the HR-TEM investigation of NiCoCrFePd HEA also exhibits the formation of dislocation and vacancy clusters with the average size of vacancy clusters increases from ∼2.9 ± 0.1 to ∼3.8 ± 0.1 nm with the increases in the ion fluence. Surprisingly, the average defect cluster size in NiCoCrFePd HEA is suppressed compared to NiCoCrFe MEA, thereby showing the enhanced radiation stability on Pd incorporation due to the high defect recombination caused by reduced thermal conductivity and high lattice distortion. Nano-indentation measurement shows that the radiation hardening behavior of the NiCoCrFePd HEA responded slowly owing to its damage suppression property as compared to the NiCoCrFe MEA. Additionally, softening behavior also appeared at an early fluence in NiCoCrFe MEA compared to the NiCoCrFePd HEA signifying its excellent resistance to defect accumulation.

TEM investigation on microstructures of age-hardened 2014 Al alloy forged by cold biaxial alternate forging

In this study, microstructures of age-hardened 2014 Al alloy forged by cold biaxial alternate forging was investigated by transmission electron microscope. And also, the forming limits of age-hardened 2014 Al alloy were examined using both conventional compression test and biaxial alternate forging. As a result of compression test, it showed a perfect plastic behavior with no visible change in stress after 27% in strain and eventually, the curve fluctuated as a shear crack occurs after 51% strain. However, it was possible to impose very large strains on the 2014-T6 Al alloy workpieces through the biaxial alternate forging of up to 4 passes. The effective strain was possibly accumulated to 356% and 204% as the maximum and average values, respectively. The results of transmission electron microscope indicated that the high density dislocations were distributed after 3 passes. After 4 passes, the distribution of more increased dislocations was observed and band-shaped dislocation clustering appeared.

A novel mathematical-based approach to predict the high-speed deformation behavior of cryodeformed materials with varying stacking fault energies

Cryodeformed (CR) materials have complex microstructures containing nano-twins, dislocation clusters, tangles, and others, depending on the stacking fault energy (SFE). Such unique microstructure makes them suitable for several engineering applications under a high strain rate environment. The high-speed deformation (HSD) behavior of CR materials is difficult to understand due to their complex microstructure. The present work is focused on developing a novel mathematical modeling method to understand the high strain rate deformation of CR materials with varying SFE. The model equation correlates the microstructure-based variables to deformation-based variables like stress, strain, and strain rate. High-speed deformation was conducted using a crossbow test. Transmission electron microscopy (TEM) was conducted on these materials to validate the model output.

Effect of helium ion irradiation on the microstructure, mechanical properties and surface morphology of Inconel 625 alloy

Inconel 625 alloy is mainly used for reactor pressure vessel, fuel cladding, nuclear fuel elements and cryogenic channels for cooling high temperature superconductors in Tokamaks. This study focuses on deciphering the impact of helium ion irradiation on the properties of Inconel 625 alloy. Helium-ions interaction with material leads to blistering, void formation, swelling, and degradation of mechanical properties. To understand these effects, the samples of Inconel 625 alloy were irradiated by helium-ion fluence beams at three different helium-ion beam fluence (1 × 1013, 1 × 1014, 1 × 1015 He/cm2). A decrease in grain size was observed from 34.4 to 19.0 nm with an increase in the ion fluence. According to the results, the residual strain is increased, from 1.8 to 43.0 micro-strain with the increase in helium-ion fluence, however structure is relaxed with decrease in micro-strain at higher helium-ion fluence (1 × 1015 He/cm2) and also there is a prominent increase in grain size. The surface morphology as observed by scanning electron microscope (SEM) illustrates the fuzz formation at low fluence (1 × 1013 He/cm2), while samples irradiated with high helium ion fluence (1 × 1015 He/cm2) showed grain growth, voids and surface blistering (deep valleys). An increase in hardness value at low helium-ion beam fluence is observed due to the presence of large population of defects. However, at higher helium-ion beam fluence, a decrease in hardness value is observed. This decrease might be attributed to the dislocation clustering and grain growth.

Investigation on butterfly white etching area formation mechanism and crack source at different stages in M50 bearing steel

Rolling contact fatigue tests with different stresses and cycles have been conducted to study the formation mechanism and crack source of butterfly white etching area (WEA) in different stages of M50 bearing steel. The results suggest that the structural transformation is prior to cracks. The initial microstructure is the nanocrystalline α-Fe formed by dislocation assisted carbon migration. With the extension of cycles, unlike the previous studies, the butterfly-origin carbide is refined into Mo2C and M7C3 nanocrystalline by the accumulated dislocation clusters. Subsequently, the increase of interfacial energy drives the metal elements diffusion and thus the formation of amorphous structures. A new mechanism called diffusion-assisted dislocation has been proposed for butterfly WEA formation.

One-sided friction stir welding of 1565сhM alloy plates

The results of experimental studies of the features of the structure of butt joints of 1565chM alloy plates with a thickness of 25 mm obtained by one-sided friction stir welding are presented. It is established that the structure of the mixing zone (weld metal) is a dynamically and primarily recrystallized region. The grain size in the weld metal varies depending on the distance from the front surface of the weld from 5.16 μm (upper part of the joint) to 2.93 μm (root part of the joint). The separation of intermetallic compounds in the mixing zone for Al—Mg system alloys helps to reduce the average grain size. In the bodies of grains at the level of half the thickness of the plate, there are both chaotically distributed dislocations and dislocation clusters mainly in the form of dislocation walls. In this case, the scalar density of dislocations at this level exceeds the values of the scalar density of dislocations in the upper and root parts of the weld. In the mixing zone of the joints of the 1565chM alloy plates the microstructure at different levels of horizontal sections along the thickness of the plate is very homogeneous, which indicates the mechanism of layer-by-layer transfer, which is macroscopic.

(Invited) Application of 3D in-Situ X-Ray Visualization to Track the Formation of Dislocation Clusters during PVT Growth of SiC

SiC has become the key player among wide bandgap semiconductors for power electronic applications. Since the first description of the physical vapour transport (PVT) growth process of SiC by Tairov and Tsvetkov (J. Crystal Growth, 43, 209(1978)), there has been steady progress in SiC-based crystal growth, epitaxy and device processing. The success of SiC compared to Si is related to its superior material properties such as extremely high electrical breakdown field and high thermal conductivity compared to the standard silicon counterpart. In addition, SiC device processing utilises much of the standard Si processing equipment. A major reason for the success of SiC in power electronic applications compared to other wide bandgap counterparts such as GaN, Ga2O3 and diamond is related to the availability of large diameter SiC wafer materials (150mm = standard, 200mm = developped). Bulk SiC growth is now a very well developed process with comparatively high yields. The extraordinary physical properties also include obstacles related to the strong chemical bonding and complex phase diagram of the material, which pose challenges to the growth process. Therefore, there are still a number of open questions related to the nucleation, progression and termination of the bulk growth process that require fundamental research in materials science and technology.The aim of this presentation is (i) to give an overview of the state-of-the-art PVT growth process and (ii) to discuss a current research topic dealing with the early stage of the growth process and the defect formation that can occur during the initial nucleation of SiC. We have applied 3D in-situ visualisation of the growth process using X-ray computed tomography to visualise island formation on the large seeding area. These data are related to growth process instabilities such as temperature variations during the seeding process and axial doping level changes from the seed to the newly grown crystal. Both process instabilities induce mechanical stress on the SiC lattice and act as sources for dislocation generation and multiplication. We will show a series of growth processes with varying growth parameters that shed light on the initial growth stage of SiC.As the crystal diameter of SiC increases from 150 mm to 200 mm, the results of this study become increasingly important.

A general scheme for point defect sink strength calculation and related machine-learning-based expressions

Irradiation tends to increase the concentration of point defects (PDs) in crystalline materials, whose consecutive interactions with other types of defects, such as dislocation and void, are recognised highly responsible for the characteristic plastic and damaging behaviours of materials under irradiation. Conventional treatments on evaluating the strength of PD sinks see their limitation with strong regularity requirements over the models used for summarising the key underlying microstructural behaviours, where analytical solutions are bound to be the outcome. The present article serves to introduce a general scheme for PD sink strength evaluation, where constraints on solution analyticity are fully resolved with the use of machine learning. In particular, a neural network representation of the PD sink strength due to void/bubble is derived, where PD transportation tendencies against the hydrostatic pressure gradient surrounding a bubble can be considered in details. The treatment is also applied to analyse PD sink strength due to edge dislocation clusters. For nearly uniformly distributed clusters, upon undertaking a two-scale asymptotic strategy, the corresponding sink strength formulation becomes explicit. For randomly distributed dislocations, the sink strength is found to roughly scale with the onsite dislocation density. But for patterned dislocations, such as dislocation dipoles, their sink strength is suggested to vary with the applied load. The machine-learning-based formulation is also compared well with the results obtained by other multiscale methods.

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.

Surface outflow effect on dislocation structures in micrometer-sized metals

Dislocations in metals often multiply and agglomerate into quasi-ordered structures with various morphologies in cyclic loading experiments. Specifically in face-centered cubic metals, the shapes of dislocation clusters change from dislocation bundle structures (called veins) to ladder-like periodic wall structures at a threshold of the applied shear strain that coincides with the fracture initiation threshold. In these ladder-like structures, the interval between adjacent walls is 1 or 2μm, regardless of the material dimensions. The robustness of such micrometer-sized wall intervals implies the impossibility of forming ladder-like structures in micrometer-sized metals. Nonetheless, the kind of dislocation structure that will occur is unclear because of a lack of knowledge about small-scale metal fatigue. In this study, we theoretically speculate the dislocation structures formed in micrometer-sized metals using differential equations that describe the reaction and diffusion of dislocations in the fatigue process. We find that micrometer-sized metals form few-walled structures due to the outflow of dislocations from their free surfaces, even under the parameter conditions under which vein structures form in macroscopic metals. The results indicate that the fracture initiation limit can be decreased by reducing the system size, which contradicts the well-established “smaller is stronger” consensus regarding the mechanics of small-volume single crystals.

An ultra-high temperature Pt-15Ir-3Hf-0.5Y alloy with high hardness and tensile strength achieved by hot rolling and ageing treatment

A novel ultra-high temperature Pt-15Ir-3Hf-0.5Y (wt.%) alloy was developed for the first time by highvacuumarc-melting, hot rolling and ageing treatment. The microstructures of the as-cast and hot-rolled and then post-aged samples were investigated. The alloy consists of the (Pt,Ir)5Y phase and matrix. Through transmission electron microscopy atomic resolution imaging, three types of nanoclusters, clusters containing stacking fault, clusters at 1/2 〈011〉 dislocation core and clusters with short-range order were observed in the matrix. The hardness at the ambient temperature of the alloy after hot rolling and ageing treatment was up to 443 HV and the tensile strength at 1500 °C was up to 47.8 MPa. Compared with the solid solution strengthened Pt-Ir binary alloys, the addition of Hf and Y can effectively improve the mechanical properties, which were achieved by introducing the combined effects of nanocluster strengthening and grain boundary strengthening of (Pt,Ir)5Y phase. Our findings provided new insights into designing the Pt-based superalloy with high strength and ductility.

Microstructure evolution mechanisms and a physically-based constitutive model for an Al–Zn–Mg–Cu–Zr aluminum alloy during hot deformation

High-temperature flow features of the Al−Zn−Mg−Cu−Zr aluminum alloy was revealed by hot compression tests. The evolution mechanisms of dislocation clusters, subgrain, and dynamic recrystallization (DRX) grains, are thoroughly explored by EBSD and TEM analysis. Experimental results suggest that the high strain rate can exacerbate dislocation clusters formation, as well as subgrain nucleation/accumulation, inducing the increasing of flow stress. Nevertheless, the noticeable annihilation of substructures, as well as the growth of DRX grains, emerge at the higher temperature, causing the descending of flow stress. Three types of DRX nucleating mechanisms, i.e., discontinuous DRX (DDRX), geometric DRX (GDRX) and continuous DRX (CDRX) are activated in the Al−Zn−Mg−Cu−Zr aluminum alloy during hot compression. Simultaneously, the GDRX often appears at a high compressed temperature or a low strain rate. A physically-based (PB) model is proposed to collaboratively reconstruct true stresses and microstructure evolution features. The estimated values of true stress, DRX fractions and average grain size preferably fit the experimental data, indicating the proposed PB model can precisely catch the thermal compression behaviors and microstructure evolution characteristics of the Al−Zn−Mg−Cu−Zr aluminum alloy.

3D CNN and grad-CAM based visualization for predicting generation of dislocation clusters in multicrystalline silicon

We propose a machine learning-based technique to address the crystallographic characteristics responsible for the generation of crystal defects. A convolutional neural network was trained with pairs of optical images that display the characteristics of the crystal and photoluminescence images that show the distributions of crystal defects. The model was trained to predict the existence of crystal defects at the center pixel of the given image from its optical features. Prediction accuracy and separability were enhanced by feeding three-dimensional data and data augmentation. The prediction was successful with a high area under the curve of over 0.9 in a receiver operating characteristic curve. Likelihood maps showing the distributions of the predicted defects are in good resemblance with the correct distributions. Using the trained model, we visualized the most important regions to the predicted class by gradient-based class activation mapping. The extracted regions were found to contain mostly particular grains where the grain boundaries changed greatly due to crystal growth and clusters of small grains. This technique is beneficial in providing a rapid and statistical analysis of various crystal characteristics because the features of optical images are often complex and difficult to interpret. The interpretations can help us understand the physics of crystal growth and the effects of crystallographic characteristics on the generation of detrimental defects. We believe that this technique will contribute to the development of a better fabrication process for high-performance multicrystalline materials.