Molecular Dynamics Simulations Of Nanoindentation Research Articles

Simulation of copper nanoindentation using LAMMPS - atomic to micro-scale

This study explores the mechanical properties of copper at the nanoscale through molecular dynamics simulations of nanoindentation using LAMMPS. Focusing on atomic-scale interactions, the research investigates the relationship between indentation depth and pressure, considering factors like indenter shape and application rate. The simulations reveal complex material behaviors diverging from classical Hertzian contact theory, highlighting the intricacies of nano-scale mechanical responses. The study contributes significantly to the understanding of copper’s mechanical properties and the application of computational simulation in material science.

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.

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.

Statistical modeling of the effect of chemical inhomogeneity on incipient plasticity in complex concentrated alloys

In recent years, complex concentrated alloys (CCAs), also referred to as medium- or high-entropy alloys, have attracted substantial research interest due to their excellent mechanical properties including high strength, ductility, and toughness. It is known that the chemical inhomogeneity of CCAs gives rise to spatial variations in local properties such as the generalized stacking fault energy (GSFE) surface, which in turn affect their mechanical properties, but how such an inhomogeneity affects dislocation nucleation and incipient plasticity remains largely unknown and unexplored. Here, we develop a physics-informed statistical model for incipient plasticity in CCAs by combining elasticity theory for dislocation nucleation and statistical modeling of nanoindentation. Our model connects a material's fundamental properties to the statistics of incipient plasticity and is validated by the excellent agreement with the statistical data from molecular dynamics simulations of nanoindentation of CrCoNi CCA samples. By accounting for the spatial variation in the local generalized stacking fault energy surface in CCAs, our model captures the key difference in the nanoindentation-induced incipient plasticity response of CCAs compared with a conventional metal (fcc Cu) and also reproduces the trends across CCA samples with different degrees of short-range ordering. Our model also reveals a critical length scale for the underlying GSFE fluctuations which controls the overall statistics of incipient plasticity during nanoindentation of CCAs, which reflects the critical loop size of the underlying dislocation nucleation mechanism.

Molecular Dynamics Simulation on Nanoindentation of M50 Bearing Steel.

M50 bearing steel has great potential for applications in the field of aerospace engineering, as it exhibits outstanding mechanical and physical properties. From a microscopic point of view, bearing wear originates from the microscopic region of the contact interface, which usually only contains hundreds or even several atomic layers. However, the existing researches seldom study the wear of M50 bearing steel on the microscopic scale. This study explored the atomic-scale modeling method of M50 bearing steel. Then molecular dynamics simulations of nanoindentation on the M50 bearing steel model were carried out to study the size effect of the mechanical behaviors. The simulation results show that with the change in the radius of the diamond indenter in the nanoindentation simulation, the calculated nanohardness decreases. According to the size effect, when the indentation radius is 200 nm, the hardness obtained by the simulation is about 9.26 GPa, and that of the M50 sample measured by the nanoindentation is 10.4 GPa. Then nanoindentation simulations were carried out at different temperatures. The main bearings of aero-engines generally work at 300-500 degrees Celsius. When the simulated temperature was increased from 300 K to 800 K, the hardness of the model decreased by 15%, and the model was more prone to plastic deformation. In this study, a new molecular dynamics modeling method for M50 bearing steel was proposed, and then nanoindentation simulation was carried out, and the nanoindentation experiment verified the correctness of the model. These results are beneficial to the basic understanding of the mechanical performance of M50 bearing steel.

Study on the elastoplastic deformation behavior and dislocation evolution of 4H-SiC film

The elastoplastic deformation behavior and dislocation evolution of 4H-SiC film are systematically investigated by molecular dynamics simulation of nanoindentation and the generalized stacking fault energy (GSFE) analysis. It is found that “pop-in” event occur in the process of the film deformation, which is the critical point for the elastic–plastic transition of the film. At lower load, to release the deformation energy and shear stress within the film, the film undergoes elastic deformation along the direction of easy deformation. As the load continues to increase, the film shifts to plastic deformation. The dislocations in the plastic deformation region are mainly full dislocations, and the deformation is dominated by the propagation of full dislocations as the depth increases. Partial dislocations are more likely to be nucleated at deeper depth. The GSFE analysis reveals that the film, after being subjected to external load, preferentially activates the dislocation with a Burgers vector of 1/3〈112¯0〉. The dislocation is then dissociated into two partial dislocations, namely leading and trailing partial dislocations. Meanwhile, the slip of 1/3〈11¯00〉 dislocation can also occur on the base plane.

Atomistic simulations of nanoindentation on nanoglasses: Effects of grain size and gradient microstructure on the mechanical properties

The use of nanoglass (NG) microstructures has been shown as an effective strategy to improve the ductility of rather brittle metallic glass alloys. To evaluate the effects of grain size and gradient design on the mechanical properties of NGs, we perform molecular dynamics simulations of nanoindentation on Cu64Zr36 NGs. We consider samples with uniform 3 and 7 nm grain sizes as well as gradient microstructures with gradient grain sizes varying from 3 to 7 nm. The results show that the deformation mechanism in NGs with small grain sizes is dominated by the activation and evolution of multiple shear transformation zones. Increasing grain sizes enhances the elastic modulus and hardness at the cost of reduced abrasion resistance. Though the average grain size at the indentation surface plays a crucial role in the deformation behavior of gradient NG models, their strain localization and plastic deformation states are affected by the grain sizes far from the indenter. Desired combinations of mechanical properties can be realized by different NG microstructure designs.

Molecular Dynamics Simulation of Nanoindentation of Nb-Zr Alloys with Different Zr Content

To understand the nanomechanical behaviors of the Nb-based alloys with Zr addition at room/high temperature, the molecular dynamics simulations of nanoindentation are conducted. In this work, the load-unload displacement curve, hardness, and dislocation characteristics of Nb-Zr alloys with varying Zr content ranging from 0 to 5 wt.% are studied. The simulation results are found to closely agree with the experimental one at 1 wt.%, therefore showing the reliability of the simulation. Moreover, considering distinct responses of alloys to different service temperature, the high-temperature nanoindentation are performed. The effects of Zr addition on the mechanical deformation under both temperatures are compared. The same phenomenon is found such that the optimum concentration range yielding the greatest hardness is 1–3 wt.%. The elastic modulus of NbZr alloy improves with elevated concentration at room temperature, while the hardness at higher temperature exhibits the opposite trend. This is attributed to the higher amplitude of atomic vibrations at high temperatures, which is more likely to deviate atoms from their equilibrium positions and weaken the pinning effect under external loading. Therefore, we believe that our studies on the nanomechanical mechanisms of materials at room/high temperature will provide an effective way for the alloying optimization design.

Deformation evolution of Cu/Ta nanoscale multilayer during nanoindentation by a molecular dynamics study

Molecular dynamics simulation of nanoindentation is performed to study the plastic deformation evolution of Cu/Ta and Ta/Cu nanoscale multilayers with effects of layer thickness and interface orientation relationship. The results reveal that the plastic deformation mechanism is determined by both the intrinsic property of monolayer and the interface characteristic, and the mechanical property has an important dependence on Ta deformation. The interface could strongly interact with and adsorb lattice dislocations, acting as the primary emission source of dislocations due to high stress concentration. The Cu/Ta interface exhibits a strong blocking effect obstructing dislocation traversing interface, while the Ta/Cu interface could transmit deformation stress to activate Cu layer deformation. The effect of interface orientation indicates that the propagation of crystal defects depends on slip system of monolayer, and the dislocation evolution in Ta layer shows a two-step mechanism: the V-shaped structure formation, and its cusp and root segments triggering the subsequently extended dislocations. Contrarily, the high-energy OT orientation interface causes random defect structures. The indentation hardness reveals two critical thicknesses (70 and 160 Å for Ta/Cu multilayer, 40 and 160 Å for Cu/Ta multilayer), which strongly depend on the size of indenter and indentation depth at nanoscale. This work provides an important understanding of the resistance of nanostructured materials to mechanical deformation for designing wear-resistant coatings and structural nanocomposites.

Atomistic insights into the deformation mechanism of a CoCrNi medium entropy alloy under nanoindentation

Understanding the fundamental of contact-induced behavior in metals is critical for the studies of associated mechanical properties. Although nanocontact plasticity in the conventional pure metals or solutions have been well established, such theoretical frameworks may break down in the recently emerging medium entropy alloys (MEAs), owing to their unique disordered feature and inevitable chemical fluctuations. Here combined with crystal defect theories, we investigate the nucleation and evolution of dislocations in CoCrNi MEA during nanoindentation. Molecular dynamics simulations of nanoindentation are carried out to analyze the effects of composition inhomogeneity and temperature on the defect behavior. In contrast to the usual homogeneous nucleation criterion in pure metals, heterogeneous dislocation nucleation is preferred to occur at Cr-rich clusters in CoCrNi MEA at a lower indentation depth. Although the local composition fluctuation can facilitate the nucleation of dislocations, these partials are pinned significantly by the local atomic sites with a high level of CoCr chemical short-range order. Moreover, nanotwin and phase-transformation are promoted at low temperature, due to the low stacking fault energy and enhanced glide resistance. In particular, the formation and evolution of the distorted prismatic dislocation loop are visualized at high temperature. The new phenomena reported herein could provide a basis for tuning local composition and atomic arrangement to obtain excellent mechanical properties in medium or even high entropy alloys.

Nanoindentation of Nanoglasses Tested by Molecular Dynamics Simulations: Influence of Structural Relaxation and Chemical Segregation on the Mechanical Response

We present molecular dynamics simulations of nanoindentation in order to investigate the effects of segregation and structural relaxation on the mechanical properties of Cu64Zr36 nanoglasses prepared by particle consolidation and long-time annealing. Our analysis of load-displacement curves shows that the effective elastic modulus of nanoglasses is lower than that of their homogeneous metallic glass counterpart. This is mainly because of the defective short-range order present in the glass-glass interface, but to a lesser extend due to chemical inhomogeneities. Structural relaxation obtained by long-time annealing (500 ns) at 0.8 Tg leads to a shift from a homogeneous deformation to a mix of homogeneous deformation and shear bands. The obtained hardness values of annealed nanoglass are comparable to those of homogenous glass samples, but significantly higher as compared to juvenile as-prepared nanoglass samples. The results are discussed in the context of recent nanonindentation experiments.

Response of an amorphous/crystalline interface to nanoindentation: an atomistic study

We perform molecular dynamics simulations of nanoindentation in bilayer systems composed of a Cu64.5Zr35.5 metallic glass and a pure Cu crystalline sample. Both the cases where the glass and where the crystal is top are studied. We find a strong asymmetry in the response of the glass-top and the crystal-top system. The crystal-top system is the weakest one. Here, dislocations nucleate at the interface and also induce considerable strain in the glass layer beneath; hence the interface acts to reduce the plastic activity in the top crystal layer. In contrast, the glass-top system is the hardest system of the four systems studied. Shear bands form considerably earlier in the glass-top system than in a pure glass. The interface hinders shear band propagation; it thus causes a deflection of the load exerted by the indenter and hardens the material. Dislocations in the bottom crystal can be generated only when the indenter has penetrated the interface.

Molecular dynamics simulation of nanoindentation in Al and Fe: On the influence of system characteristics

Indentation is one of the most widely used methods to determine the characteristics of metallic materials. Molecular dynamics (MD) simulations are a powerful tool to investigate the material behavior at the atomistic scale during nanoindentation and to get insights into the understanding of the plastic deformation of a material. However, the optimal choice of parameters for the simulation is not clear, and there is a large body of literature with strongly different simulation configurations. In this study, we systematically investigate the effect of the choice of interaction potentials, substrate size, indenter size and indentation velocity on hardness and defect generation during nanoindentation of face-centered cubic and body-centered cubic materials. As prototypical materials we consider aluminum and iron single crystals. Our sensitivity analysis reveals that the thickness of the substrate has a large influence on the measured hardness by MD simulations, while the impact of the indentation velocity is small. The results are useful for researchers planning to use MD to study the nanoindentation process using highly expensive potentials such as ReaxFF and COMB3 or small indenter velocities. This article aims to provide a guideline for designing nanoindentation simulations using MD.

Mechanical degradation due to vacancies produced by grain boundary corrosion of steel

Ductile alloys fail in corrosive environments by intergranular stress corrosion cracking, through interactions between mechanical and chemical processes that are not yet understood. We investigate formation and mechanical effects of metal defects produced by grain boundary corrosion of low-alloy pipeline steel, at conditions of high susceptibility to stress corrosion cracking in the absence of hydrogen evolution. Nanoindentation measurements show local softening near corroded grain boundaries, indicated by significantly reduced critical loads for dislocation nucleation. Molecular dynamics simulations of nanoindentation of bulk iron showed that metal vacancies and not interstitial hydrogen atoms explain the observed critical load reduction. Both the dislocation activation volume and dislocation activation energy for vacancy-charged samples are found to be nearly one-half of that for a hydrogen charged samples. Quantitative agreement with experimentally measured indentation response was found for vacancy concentrations equivalent to the bulk silicon concentration in the steel, suggesting that vacancies originate from oxidation of reactive silicon solute atoms at grain boundaries. The results help explain the chemical mechanism of formation of vacancy defects that may participate in grain boundary degradation in the absence of hydrogen embrittlement environment.

Effects of Langevin friction and time steps in the molecular dynamics simulation of nanoindentation

ABSTRACT In the simplest form of molecular dynamics (MD), the system evolves according to Newton's equations of motion with its total energy conserved. Moreover, to model systems at constant temperatures, a modified set of equations of motion, called a thermostat, can be used. Langevin thermostat with one parameter called the Langevin friction coefficient is a popular thermostat, but problems may arise when it is applied to nonequilibrium systems without tuning the friction coefficient. Additionally, the size of MD time step also plays a pivotal role in determining accuracy and efficiency. In this study, we consider the effects of time step and Langevin friction on MD simulations. First, we evaluate the effectiveness of the Langevin thermostat for different combinations of time step and friction coefficient. Second, nanoindentation of Nickel crystals are carried out for five different combinations of Langevin friction and time steps. The simulation results reveal noticeable differences in mechanical properties as the dynamics changes from a constant energy to constant temperature simulations with increasing friction coefficients. The structural characteristics are also quantified using the common neighbour analysis, centrosymmetry parameter and dislocation analysis, all of which show a similar trend of more atoms being deformed with increasing friction.

Nanoindentation of ZrH2 by molecular dynamics simulation

Classical molecular dynamics simulations of nanoindentation on the (100) and (110) planes using a spherical indenter are performed to investigate the deformation behavior of δ-ZrH2. The Charged Optimized Many Body (COMB) potential is used to describe the interatomic interactions. The effect of the indenter speed and the thickness of the active layer to the nanoindentation are evaluated. We find that the nucleation and movement of {100} <110> dislocations are the main mechanisms of the inelastic deformation during nanoindentation on both the (100) and (110) planes. In addition, the load-displacement curve, hardness and deformation processes extracted from δ-ZrH2 nanoindentation on the (100) and (110) planes are analyzed.

Pile-up and heat effect on the mechanical response of SiGe on Si(0 0 1) substrate during nanoscratching and nanoindentation using molecular dynamics

Molecular dynamics (MD) simulations are performed to study the mechanical behavior of SiGe thin film during nanoindentation and nanoscratching processes. The MD simulations of deposition and annealing process are conducted to create the Si80Ge20 thin film deposited on Si(0 0 1) substrate. MD simulations of nanoindentation and nanoscratching processes are carried out to study the influence of the temperature on deformation behaviors and mechanical properties of this material. Moreover, the influences of the scratching depth, tool velocity, and temperature on the deformation behavior during nanoscratching process are also surveyed. It is found that the elastic and plastic deformations occur and the shear transformation zones (STZs) are independently developed in the contact region. The force, hardness, and Young’s modulus of the Si80Ge20 thin film deposited on the Si substrate decrease as increasing the temperature during the indentation process. The pile-up, friction force, and normal force grow as increasing the scratching depth. As increasing the scratching speed, the pile-up increase while the normal force and the friction force reduce due to the effect of heat and strain rate are more efficient than the effect of the pile-up. As increasing the temperature, the height of pile-up increases while the normal force and the friction force decrease as a result of a higher temperature leads to softening the workpiece because the thermal expansion causes a reduction of the bonding strength of atoms.

Strengthening mechanisms of graphene coatings on Cu film under nanoindentation: A molecular dynamics simulation

Molecular dynamics simulations of nanoindentation are performed to investigate the strengthening mechanisms of graphene coatings on Cu film substrate. It is found that the load bearing capacity of Cu substrate can be evidently improved after covering graphene coatings, and this turns out to be proportional to the number of graphene layers from monolayer to trilayer. The strengthening mechanism of elastic stage mainly results from the stress homogenization effect generated by graphene interface. Moreover, the efficiency of graphene strengthening during plastic stage is much higher than the elastic stage. The interactions between dislocations and graphene coating interface are fully responsible for the strengthening increase of Cu/graphene system in the plastic stage. In addition, a theoretical model to predict the strength of Cu/graphene system depending on the confined layer slip (CLS) model is established.

Molecular dynamics simulations of nanoindentation and scratch in Cu grain boundaries.

The dynamic nanomechanical characteristics of Cu films with different grain boundaries under nanoindentation and scratch conditions were studied by molecular dynamics (MD) simulations. The type of grain boundary is the main factor in the control of the substrate atoms with respect to the size of dislocations since the existence of the grain boundary itself restricts the movement associated with dislocations. In this work, we analyzed the transverse and vertical grain boundaries for different angles. From the simulation results, it was found that the sample with a transverse grain boundary angle of 20° had a higher barrier effect on the slip band as compared to samples with other angles. Moreover, the nanoindentation results (i.e., indentation on the upper area) of the vertical grain boundary showed that the force was translated along the grain boundary, thereby producing intergranular fractures.

Mechanical heterogeneity and its relation with glass-forming ability in Zr-Cu and Zr-Cu-Al metallic glasses

Molecular dynamics simulations of nanoindentation using a spherical indenter were adopted to quantitatively probe the local mechanical heterogeneity (MH) in Zr-Cu and Zr-Cu-Al metallic glasses. Distinct MH at the nanometer scale has been revealed by statistically analyzing the first pop-in stress of different regions in the metallic glasses with an indenter of 4 nm in diameter. More interestingly, it is found that the degree of MH has a close relation to glass-forming ability of alloys, i.e., the smaller MH, the better glass-forming ability. Our findings not only shed light on the intrinsic feature of atomic structures, but also have important implications in understanding the structure-property relationship of MGs.