Nanoindentation Simulations Research Articles

Cholla cactus wood (Cylindropuntia imbricata): Hierarchical structure and micromechanical properties

The Cholla cactus is a species of cacti that survives in arid environments and produces a unique mesh-like porous wood. In this article, we present a comprehensive investigation on the hierarchical structure and micromechanical properties of the Cholla cactus wood. Multiple approaches consisting of X-ray tomography, scanning electron microscopy, scanning probe microscopy, nanoindentation, and finite element simulations were used to gain insight into the structure, property, and design principles of the Cholla cactus wood. The microstructure of the Cholla cactus wood consists of different components, including vessels, rays, and fibers. In the present study, we quantitatively describe the structure of each of these wood components and their likely functions, both from the perspective of biological and mechanical behavior. Nanoindentation experiments revealed for the first time that the cell walls of the fibers exhibit stiffness and hardness higher than those of rays. Furthermore, the idea of making porous, thin-walled cylinders was abstracted from the design of vessel elements, and the structures inspired by this principle were studied in tensile and torsional loading conditions using finite element simulations. Finite element simulations revealed that the utilization of a larger volume of material to carry the load leads to an increase in toughness of these structures, and thus suggested that the pores should be architected to maximize the distribution of load. Statement of significanceThe Cholla cactus wood possess a unique hierarchical structure that enables it to thrive in arid environments. Our correlative microscopy approach reveals incredible strategies that individual wood components exhibit to enable the survival of Cholla cactus in extreme environments. The present work quantifies the microstructure and mechanical properties of this very interesting natural system. We further investigate a design principle inspired by the vessel elements, one of the wood components of Cholla cactus, using finite element simulations. The study presented here advances our understanding of the structural significance of Cholla cactus and potentially other desert plants and will further help design architected structural materials.

A micromechanical nested machine learning model for characterizing materials behaviors of bulk metallic glasses

In the present work, a novel micromechanical data-driven Machine Learning (ML) framework was proposed to characterize material parameters in bulk metallic glasses (BMGs) using nanoindentation simulations with Berkovich and spherical tips. A vast collection of data on material behavior in BMGs during nanoindentation was compiled, utilizing a series of Drucker-Prager model coefficients. The predictive model was constructed using a nested machine learning algorithm, which incorporated hyper-parameters tuning and a principal model. This nested configuration optimized the architecture of a deep neural network, acting as the key estimator for BMG material properties. The outcomes indicated that the ML model proficiently predicted critical material properties, including compressive yield strength, elastic modulus, flow stress ratio, and Poisson ratio. Notably, the ML model exhibited superior accuracy in predicting elastic modulus and compressive strength, suggesting a robust correlation with flexural elasticity and compressive strength. Furthermore, the study highlighted the significance of input feature weight functions, as they strongly influenced the ML model's performance. Each output target's dependence on the individual proportion of input features contributed to the model's adaptability in handling variations in material properties. Finally, the findings of this work contribute to a deeper understanding of the relationships between input features and material properties, facilitating improved predictions of BMG behavior.

Multiscale modeling of nanoindentation and nanoscratching by generalized particle method

Most concurrent multiscale methods that expand atomistic region by continuum domain suffer from inconsistent material constitutive properties, which affect the integrity of model in the interface of atomistic and continuum domains. In this paper, the Generalized Particle (GP) method is employed to simulate nanoindentation and nanoscratching of a single-crystal aluminum sample. The main advantage of the GP method lies in its ability to extend the simulation model while maintaining consistent atomic properties across all scales. Coarsening of the atomic domain has been conducted through two-scale and three-scale GP model. The results showed a strong consistency between the results of full atomic simulations and those achieved through the GP method for both nanoindentation and nanoscratch simulations. Also, wave reflections were not seen at the interfaces. The study revealed that an increase in the number of distinct particle domains led to a reduction in the accuracy of multiscale simulations.

MD simulation study on the microstructural evolution of single-crystal Fe with pre-existing defects by Cr ion implantation

In this study, we present the molecular dynamics simulation of metal ion implantation, investigating the strengthening mechanism from a microstructural perspective. A single crystal Fe with pre-existing defects is constructed based on actual dislocation density and the effects of Cr ion implantation are investigated. Additionally, nanoindentation simulations are conducted to analyze the mechanical properties of workpieces with and without ion implantation. Effects of implantation energy and dose on the microstructure and hardness of workpieces are analyzed. The results indicate that (i) ion implantation introduces FeCr bonds with higher bond energy while reducing the original FeFe bond energy, (ii) increasing implantation energy and dose corresponds to a greater number of point defects and a higher amorphous structures proportion, (iii) the microstructure transformed by ion implantation promotes dislocation nucleation during nanoindentation, (iv) ion implantation enhances workpiece hardness due to the dual effects of interstitials impeding dislocation motion and the heightened mutual obstruction attributed to a higher dislocation density. Notably, the hardness initially enhances with increasing implantation energy and dose, followed by a slight decrease. The computational approach in this paper can serve as a valuable tool for studying the microstructural evolution of metal ion implantation by molecular dynamics simulations.

Indentation Depth-Dependent Hardness of Metal-Organic Framework Crystals: The Effect of Local Amorphization Induced by Indentation.

The hardness of metal-organic frameworks (MOFs) is an important mechanical property metric measuring their resistance to the permanent plastic deformation. The hardness of most MOFs measured from nanoindentation experiments usually exhibits the similar unique indentation depth dependence feature, the mechanism of which still remains unclear. In order to explain the effect of the indentation depth on the hardness of MOFs, we conducted nanoindentation simulations on HKUST-1 by using reactive molecular dynamics simulations. Our simulations reveal that the HKUST-1 material near the indenter can transform from the parent crystalline phase to a new amorphous phase due to the high pressure generated, while its counterpart far from the indenter remains in the crystalline phase. By considering the crystalline-amorphous interface in the energy analysis of MOFs, we derived an analytical expression of the hardness at different indentation depths. It is found that the interface effect can greatly increase the hardness of MOFs, as observed in nanoindentation simulations. Moreover, the proposed analytical expression can well explain the indentation depth-dependent hardness of many MOF crystals measured in nanoindentation experiments. Overall, this work can provide a better understanding of the indentation depth dependence of the hardness of MOFs.

Silicon phase transitions in nanoindentation: Advanced molecular dynamics simulations with machine learning phase recognition

Closing the gap between experiments and simulations in the investigation of high-pressure silicon phase transitions calls for advanced, new-generation modeling approaches. By exploiting massive parallelization, we here provide molecular dynamics (MD) simulations of Si nanoindentation based on the Gaussian Approximation Potential (GAP). Results are analyzed by exploiting a customized Neural Network Phase Recognition (NN-PR) approach, helping to shed light on the phase transitions occurring during the simulations. Our results show that GAP provides a realistic description of silicon phase transitions. With the support of NN-PR method, the formation mechanism and stability of high-pressure phases are comprehensively studied. Additionally, we also show how simulations based on the less demanding and widely-used Tersoff potential are still useful to investigate the role played by the indenter tip modeling. However, high-pressure phases obtained with GAP are more consistent with observations made in nanoindentation experiments, removing a spurious phase that is shown by Tersoff simulations. This behavior is explained on the base of relative phase stability with comparison with Density Functional Theory (DFT) calculations. This work provides insight into the application of state-of-the-art Machine Learning (ML) methods on nanoindentation simulations, enabling further understanding of the phase transition mechanisms in silicon.

Interface interaction between high viscosity asphalt and aggregate: A multi-scale study based on experiments and molecular dynamics simulation

The pull-off strength, boiling peeling ratio, adhesion work, stripping work, interaction energy, nanoindentation resistance force, and hydrogen bond distribution were analyzed based on laboratory tests and molecular dynamics simulation. The interface interaction between the original, aged, or regenerated high-viscosity asphalt (HVA) and the acid or alkali aggregate component in the aqueous or anhydrous environment was clarified from multi-scales of macro, micro, and nano scales. The results show that the pulling strength has an obvious positive correlation with the interfacial adhesion work, which can evaluate the interfacial adhesion state with water. The boiling peeling ratio has an obvious positive correlation with the interface stripping work, which can evaluate the interfacial adhesion state without water. The ratio of adhesion work to stripping work can characterize the interface water-stripping resistance. In nanoindentation simulation, the vertical resistance force of the indenter characterizes the interaction between HVA and the aggregate indenter. The interfacial adhesion state deteriorates with the deepening of HVA aging. Further addition of a high viscosity modifier (HVM) in the regenerated HVA effectively restores the interfacial adhesion state. The higher temperature causes worse interfacial adhesion, and the hydrogen bond ratio, adhesion, and water-stripping resistance of the interface between the HVA and alkaline aggregate components are much better than those between the HVA and acid aggregate components. Van der Waals force plays a leading role in interfacial adhesion. With the deepening of aging, the oxygen-containing functional groups such as carbonyl and sulfoxide in HVA increase, in which it is easy for oxygen atoms to form hydrogen bonds with water, so the water-stripping resistance of the interface decreases.

Grain refinement induced by grain boundary segregation in FeNiCrCoCu high-entropy alloys using molecular dynamics simulation of nanoindentation

Atomistic simulation of nanoindentation is performed to investigate the effect of Cu segregation at grain boundaries (GBs) on the mechanical properties of FeNiCrCoCu high-entropy alloys. Initially, a combined Monte Carlo and Molecular dynamics simulation technique is adopted to generate the models with Cu segregation, and further the cases of random Cu distribution are considered for the purpose of comparision. The dislocation density, shear strain and GB behavior in the process of nanoindentation are analyzed in detail. The results reveal that the Cu segregation at GBs leads to superiority in hardness, strength, and stiffness. Dislocation analysis shows that the sessile dislocation density is higher at the initial state for the segregation sample, while dislocation nucleation and movements are suppressed so that dislocation-mediated plasticity is weakened. Furthermore, microrotation analysis during the nanoindentation process indicates that segregation of Cu pins the GBs, thereby inhibiting GB rotation and consequently weakening GB-mediated plastic deformation. In this case, grain refinement is observed, and in good agreement with experiment results. Overall, this study provides insights into the influence of GB segregation of Cu on the mechanical properties and deformation response of FeNiCrCoCu HEAs, and highlights the importance of GB composition for tailoring high-strength materials.

Adhesion Performance of Rubber Modified Asphalt in Chip Seal: A Molecular Dynamic Study.

Chip seals are widely used for asphalt pavement maintenance, yet the understanding of the interaction between asphalt and aggregates embedded in the asphalt layer remains limited. This paper aims to quantify the interaction between asphalt and aggregate at the microscope level to better understand their adhesion performance in chip seals. Rubber-modified and neat asphalt models are established and verified based on various parameters, including density, viscosity, solubility, glass-transition temperature (Tg), and cohesive energy density (CED). Subsequently, nanoindentation simulation is employed to analyze the adhesion force and interface stress between aggregates and asphalt, considering different embedded depths and pull-off speeds. The adhesion energy between asphalt and silica is also calculated. The results indicate that rubber-modified asphalt exhibits lower density, CED, solubility parameters, and Tg while having higher viscosity than neat asphalt. The adhesion force and interface stress display a quadratic relationship with embedded depths and pull-off speeds. Furthermore, the bond between rubber-modified asphalt and silica is stronger than that between neat asphalt and silica. These findings advance the comprehension of asphalt-aggregate adhesion in chip seals and offer insights for optimizing chip seal design through molecular simulation, thereby potentially enhancing asphalt pavement performance.

Investigation of damage mechanisms related to microstructural features of ferrite-cementite steels via experiments and multiscale simulations

Spheroidized ferrite-cementite steel (SFC) is susceptible to micro-defects owing to the mismatched deformation between the ferrite and cementite during cold forming, which seriously deteriorates the fatigue life. However, the damage mechanism related to microstructural features in the SFC steel during cold deformation has not been fully investigated, hindering control of the microstructure and forming process. This study presents a method coupled with experiments and multiscale simulations to research the damage mechanisms and the dependence of microstructure features under uniaxial tension in SFC steel. In situ tensile test revealed three damage mechanisms in SFC steel under uniaxial tension: cementite cracking, ferrite/cementite interface debonding, and ferrite cracking. In particular, the final fracture was mainly dominated by the ferrite/cementite interface debonding. In multiscale simulations, the macroscale tensile and nanoindentation simulations were carried out to obtain the strain history and mechanical properties of the mesoscale simulations, respectively. The mesoscale IP-based RVE and CP-based unit cells simulations were performed to capture the driving forces from the three damage mechanisms and the dependence of microstructural features, respectively. The nanoscale molecular dynamics simulation was conducted to identify the model parameters of cohesive zone model (CZM) in the mesoscale simulations. The numerical results indicated that particle-cracking void occurred when the internal von Mises stress exceeded the ultimate strength of the cementite particles. Interface-debonding void nucleated when the load on the particle was insufficient to resist its strength. Matrix-cracking should be attributed to the stress concentration caused by the dislocation pile-up or the crossing of multiple slip bands. In addition, interfacial debonding were more likely to occur in the stable orientations and high-angle grain boundaries of ferrite matrix. Compared with a circular cementite particle, an elliptical particle was more prone to cause interfacial debonding. Also, the ferrite/ferrite grain boundary showed earlier interfacial debonding perpendicular to the loading direction than parallel to the loading direction.

Investigation on the crystal plasticity of α phase Ti-6Al-4V based on nano-indentation simulation and experiment

Ti-6Al-4V parts are usually hypothesized as isotropic mechanical properties during micro-machining operation. However, the crystal anisotropic mechanical property is an influential factor in the machining of crystals with different sizes and orientations. The hypothesis causes distinct variation in predicting machining force, and consequently the surface integrity cannot be exactly predicted in the micro-machining. Actually, Ti-6Al-4V parts are pseudo isotropy composed of grains with different orientations, elastic modulus, and hardness. Simultaneously, the initiation mode of plastic slip system varies with the grain orientation when the crystal is subjected to an applied load. It is challengeable to evaluate the effect of crystal orientation on its macroscopic properties, either by traditional experimental methods or macroscopic finite element analysis. Nano-indentation provides one novel method to investigate the plastic deformation of selected grains with specific orientations to characterize their mechanical response properties. This research aims to investigate the plastic behavior of α phase Ti-6Al-4V with different orientation grains by means of nano-indentation experiment and crystal plastic finite element method. In the study, single crystals of α phase are classified into “hard grains” (θ < 30°), “soft grains” (θ > 60°) and “normal grains” (30° < θ < 60°) by defining the crystal inclination angle (θ). The elastic modulus and hardness of hard grains are 9% and 28% larger than those of soft grains, respectively. This difference in mechanical properties depends on the slip system that dominates the plastic deformation behavior. The plastic deformation of soft grains is dominated by prismatic slip (48%), while hard grains are accommodated to plastic deformation by basal slip (51%). The average total cumulative shear strain of the soft grains is double that of the hard grains at a maximum load of 10 mN. Meanwhile, the sensitivity analysis is performed to evaluate the effects of the three sets of < a > slip systems and the two sets of < c + a > slip systems on plastic deformation with different grain orientations. The research contributes to the understanding of the plastic deformation mechanism in micro-machining and the exactly predicting the surface integrity generated from micro-machining.

Microstructural evolution and analysis of Zr–Co/Cu–Al glassy composites and crystallized alloys via indentation and MD simulations

B2-containing bulk metallic glass (BMG) composites are still attractive due to their unique deformation mechanism and positive effect on work-hardening and ductility. As stated in review [1], the competition between B2 phase formation, retention of amorphous matrix and intermetallic compounds depends on several parameters, such as alloy composition and cooling rate. For a given BMG composite containing B2 phase, the glass matrix, B2 phase, and brittle compounds generally coexist. As a result, the relationship between microstructures such as atomic diffusion and micro-indentations must be investigated. In this work, nano-indentation, micro-hardness, and molecular dynamics (MD) simulations were performed on two Zr–Co/Cu–Al BMG composites with certain differences in amorphous matrix and crystalline phases such as their distribution, volume fraction, or types. The results indicated that the (1) formation of B2 phase, and precipitation of brittle compounds substantially differed; (2) the atomic transfer in these indented positions can be observed in simulated experiments and line scanning images (energy dispersive spectroscopy) for alloy compositions; (3) the degree of atomic diffusion in BMG composites functioned as the depth of nano-indentation via MD simulations; (4) the differences in nano-deformation via MD simulations can be observed for Zr–Cu alloys with various crystals; (5) the micro-indentation may stimulate transformation or B2 phase formation in localized regions upon indented stress; (6) the simulated results and experimental outcomes were mutually corroborated. These results also give a hint that atomic diffusion and phase formation in multiple mingled microstructures really existed, but they were very complicated and different in localized areas. This research is important and provides a clue for further exploration of the relationship between deformation and atomic diffusion.

Self–ion irradiation of high purity iron: Unveiling plasticity mechanisms through nanoindentation experiments and large-scale atomistic simulations

Ion irradiation may enhance material hardness through crystal defect nucleation and reorganization. In this study, we examine the nanomechanical behavior of high-purity iron samples, comparing the response of pristine specimen to those that have been self–irradiated with 5 MeV ions at 300∘C. We utilize spherical nanoindentation to investigate the nanomechanical response, and we focus on the comprehensive modeling of the self–irradiation effects in high-purity iron through large-scale molecular simulations. Transmission electron microscopy is used in the irradiated regions, at various depths below the nanoindentation imprint, to analyze the nucleation of dislocation networks and the plastic deformation mechanisms at room temperature. Large scale novel molecular dynamics simulations are conducted to simulate overlapping collision cascades reaching an irradiation dose with defect density similar to experiments, followed by nanoindentation simulations that display qualitative agreement to experiments. We find that irradiated sample requires higher critical load for the transition from elastic to plastic deformation due to interaction of dislocation lines with the dislocation loops and point defects formed during the irradiation, leading to hardening.

The Influence of Crystallographic Orientation and Grain Boundary on Nanoindentation Behavior of Inconel 718 Superalloy Based on Crystal Plasticity Theory

The crystal plasticity finite element method (CPFEM) is widely used to explore the microscopic mechanical behavior of materials and understand the deformation mechanism at the grain-level. However, few CPFEM simulation studies have been carried out to analyze the nanoindentation deformation mechanism of polycrystalline materials at the microscale level. In this study, a three-dimensional CPFEM-based nanoindentation simulation is performed on an Inconel 718 polycrystalline material to examine the influence of different crystallographic parameters on nanoindentation behavior. A representative volume element model is developed to calibrate the crystal plastic constitutive parameters by comparing the stress-strain data with the experimental results. The indentation force-displacement curves, stress distributions, and pile-up patterns are obtained by CPFEM simulation. The results show that the crystallographic orientation and grain boundary have little influence on the force-displacement curves of the nanoindentation, but significantly influence the local stress distributions and shape of the pile-up patterns. As the difference in crystallographic orientation between grains increases, changes in the pile-up patterns and stress distributions caused by this effect become more significant. In addition, the simulation results reveal that the existence of grain boundaries affects the continuity of the stress distribution. The obstruction on the continuity of stress distribution increases as the grain boundary angle increases. This research demonstrates that the proposed CPFEM model can well describe the microscopic compressive deformation behaviors of Inconel 718 under nanoindentation.

Simulated nanoindentation into single-phase fcc Fe_{x}Ni_{1-x} alloys predicts maximum hardness for equiatomic stoichiometry

We investigate by molecular dynamics simulation the mechanical behavior of concentrated alloys under nanoindentation for the special example of single-phase fcc Fe_{x}Ni_{1-x} alloys. The indentation hardness is maximum for the equiatomic alloy, x=0.5. This finding is in agreement with experimental results on the strength of these alloys under uniaxial strain. We explain this finding with the increase of the unstable stacking fault energy in the alloys towards x=0.5. With increasing Fe content, loop emission from the plastic zone under the indenter becomes less pronounced and the plastic zone features a larger fraction of screw dislocation segments; simultaneously, the length of the dislocation network and the number of atoms in the stacking faults generated in the plastic zone increase. However, the volume of twinned regions in the plastic zone is highest for the elemental solids and decreases for the alloys. This feature is explained by the fact that twinning proceeds by the glide of dislocations on adjacent parallel lattice planes; this concerted motion is less efficient in the alloys. Finally, we find that surface imprints show increasing pile-up heights with increasing Fe content. The present results will be of interest for hardness engineering or generating hardness profiles in concentrated alloys.

Response of mechanical properties and subsurface damage in β-SiC to temperature and crystal plane during nanoindentation simulation

A series of theoretical calculations based on molecular dynamics (MD) was carried out to investigate the ceramic material mechanical response of β-SiC. Importantly, the effect of temperature and crystal plane on material mechanical performance, correlative mechanisms of subsurface damage behavior, and otherness in the subsurface layer as well as explored and analyzed at nanoindentation process. MD calculation results show that the ambient temperature rises can promote SiC material dislocation generation, nucleation, and propagation, which leads to the degree of damage in the subsurface layer increases and material brittle failure occurs because of the concentration of internal stress in ceramic material. Moreover, it was discovered that dislocation ring was a microscopic phenomenon of the transformation from elastic deformation to plastic deformation of SiC, and the dislocation ring nucleus, dislocation ring growth, dislocation ring propagation, dislocation ring brittle fracture are the main plastic deformation characteristics of SiC during nanoindentation simulation. In addition, with the increase of temperature, the damage degree of subsurface layer is intensified. The direct driving force could induce partial phase transformation occurs for β-SiC material from cubic sphalerite structure to the hexagonal wurtzite structure. Moreover, the majority of amorphous atoms generated at low temperatures would decrease with increasing temperature. The results of this work can be to enrich understand the details information on dynamic microstructure evolution process, and the corresponding subsurface damage behavior at the atomic scale when materials are subjected to external working loads and extreme temperature.

Analysis of transverse elastic modulus and compressive deformation mechanism of cellulose crystals based on molecular dynamics

Cellulose nanocrystals (CNCs) have been widely used as reinforcing agents for composite materials due to their excellent mechanical properties. CNC composites are inevitably deformed by external forces during use. In order to reduce the damage of CNC composites in application and improve their service life, their mechanical properties and deformation mechanisms need to be studied in depth. In this study, cellulose Iβ crystals were selected for molecular dynamics simulation of nanoindentation to predict the transverse elastic modulus of cellulose crystals. The simulation results were compared with existing nanoindentation tests to verify the simulation accuracy. The impact of the loading depth, loading speed, and radius of the spherical indenter on elastic modulus and surface deformation of a system was explored. The results show that the loading depth and loading speed of the indenter have weak influence on the elastic modulus, and the small radius of the indenter (1 nm) will lead to the distortion of the elastic modulus. The compression deformation mechanism of cellulose crystal at the molecular level was analyzed by analyzing changes in cellulose crystal indentation morphology, molecular potential energy and hydrogen bond number during compression simulation. As the indenter radius and loading depth increase, the deformation degree of cellulose crystal also increases, whereas the loading speed within the 50–150 m/s range does not significantly influence on the deformation degree of cellulose crystal. These results may guide the production of nanocomposites with cellulose crystal reinforcements, which can be useful for preventing material damage and enhancing material efficiency.

Heterogeneity of microstructures in a Cu–Zr based amorphous alloy composite reinforced by crystalline phases

Amorphous alloy composites (AACs) comprising reinforcing crystalline phases exhibit far better mechanical properties than their base amorphous alloy counterparts, and Cu–Zr based alloys consisting of B2 phase particles embedded in an amorphous matrix are well-researched AACs in this respect. Traditionally, the B2 particles in Cu–Zr based AACs may appear to be homogeneously crystalline (without any amorphous phase inside), the glassy matrix homogeneously amorphous, and their interfaces atomically sharp. Yet, such a picture would pose difficulties in understanding how crystallization can happen in the glassy matrix, since the crystallized product is denser than but iso-stoichiometric with the matrix. The present work provides insights into this puzzle, by using electron microscopy, nanoindentation and molecular dynamics (MD) simulations to investigate the structures and mechanical properties of the crystalline and amorphous phases, and their interfaces in a Cu62Zr34.5Al3Nb0.5 AAC. Surprisingly, in this AAC, the superficially crystalline particles with tens to hundreds of microns large are found to be heterogeneous at the sub-micron scale, comprising nano-crystallites embedded in a glassy matrix, and the amorphous phase outside also exhibits a gradient in free volume content towards the interface. Smaller particles containing only B2 nano-crystallites are found to be softer than the amorphous matrix, while larger particles are harder than the amorphous phase due to martensitic transformation into the B19’ phase. MD simulations indicate that the B2 phase is marginally more stable than the glassy state, and on crystallization the densification leads to free-volume generation in the surrounding glassy matrix as observed experimentally. Mechanics analysis indicates that the strain energy in the glassy matrix associated with free-volume generation scales with the volume of the crystallization, as would be the crystallization energy itself. Hence, crystallization in the AAC leading to a two-phase structure is a spontaneous but kinetically limiting process arising from the net energy difference between the crystallization energy and free-volume energy produced in the glassy matrix. This work provides a detailed understanding of the heterogeneity in the microstructures in Cu–Zr based AACs, to shed light on the crystallization of the iso-stoichiometric B2 phase from the amorphous phase.

Temperature dependence of mechanical properties and defect formation mechanisms in 3C-SiC: A molecular dynamics study

Nanoindentation simulations of single-crystalline cubic silicon carbide across a temperature range of 300–2000 K are performed using molecular dynamics to elucidate the temperature dependence of mechanical characteristics and lattice defect formation mechanisms. The load–displacement curves obtained by our simulations indicate weak temperature dependence of elastic responses and strong temperature dependence of plastic deformation. We reveal that the critical mean contact pressure, which is a criterion for the plasticity onset decreases with temperature. Meanwhile, plastic deformation at low temperatures starts with the nucleation and expansion of perfect dislocations, whereas the perfect dislocations dissociate into Shockley partial dislocations with stacking faults at high temperatures. After unloading, symmetric impression patterns caused by atomic arrangements on surfaces are observed at 300 K, whereas atomic pile-ups closely related to dislocation movement are formed around impressions at 2000 K. Additionally, the ductile behavior on a (111) surface indent is more affected by temperature than that on a (001) surface.

Effect of indenter radius on mechanical properties of B3-GaN in nanoindentation based on molecular dynamics

Molecular dynamics (MD) simulation of nanoindentation was conducted to study the indentation of sphalerite gallium nitride (B3-GaN) according to different radius spherical indenters. The effect of indenter radius on the evolution of dislocations, atomic displacements, Von Mises stresses, and phase transition of B3-GaN was primarily analyzed. The outcomes of the study indicate that the plastic deformation of B3-GaN crystals may be attributed to the propagation of dislocations and amorphization. The pop-in events of load in the load-depth curve is related to dislocation nucleation, amorphization, and phase transition. As the indenter radius increases, the critical load during the elastic-plastic transformation increases, and the hardness decreases gradually. The Burgers vector of dislocations during indentation is almost all 1/2 <110>, and the main slip directions were along the <110> crystallographic direction family. The magnitude and distribution range of atomic displacements and Von Mises stresses increase with the indenter radius, and this further promotes the nucleation and propagation of dislocations in the slip system, thus intensifying the plastic deformation of B3-GaN crystals. The elastic deformation of B3-GaN crystals may be attributed to amorphization and phase transition. During the elastic deformation phase, increasing the indenter radius promotes the phase transition of B3-GaN to fibrillated gallium nitride (B4-GaN) and enhances the amorphization of B3-GaN in the deformation layer. In addition, the increase in the indenter radius delays the generation of amorphous phases and phase transition. The outcomes of the study help us understand the mechanical properties of B3-GaN along with its potential for use in precision finishing.