Molecular Statics Simulations Research Articles

Atomic-scale investigation of interactions between Cu solute and defects in Zr alloys

The effects of dilute Cu solute on dislocations and grain boundaries in Zr alloys have been investigated at the atomic scale by atom probe tomography measurements, molecular dynamics, and molecular statics simulations. We demonstrate that Cu can segregate at dislocations and form nanoclusters in Zr alloys to retard the dislocation motion at high-stress creep. The solid solution strengthening of Cu is stronger than Sn and Nb at the same concentration and 673 K. Moreover, Cu is inclined to segregate at grain boundaries in Zr alloys, which is expected to reduce the Coble creep rate. As such, minor Cu alloying is an effective strategy to tailor the properties of dislocations and grain boundaries for enhancing the creep performance of Zr alloys.

Negative poisson’s ratio behavior of Al4Li9 alloy

The negative Poisson’s ratio behavior in three-dimensional crystalline materials is a novel phenomenon. Deepening the research into the theoretical mechanisms that govern the negative Poisson’s ratio behavior offers valuable insights for the design and application of advanced auxetic materials. In this study, we construct the atomic model of Al4Li9 alloy based on experimental results and investigate the direction-dependent mechanical properties using the molecular statics simulations and density functional theory calculations. Our theoretical investigation reveals that a unique charge response within the analogous honeycomb structural unit in the Al4Li9 alloy results in a distinct exhibit of the significant negative Poisson’s ratio property during deformation.

Stress states and point defect capture radii of dislocation a-loops and c-loops in alpha-zirconium

Improving the predictive capability of microstructure evolution in irradiated α-zirconium requires a thorough understanding of the interaction rates between mobile defects and the existing, and evolving, microstructure. One of the parameters necessary to calculate these rates is the defect capture radii of dislocation loops. In order to better quantify this parameter, atomistic binding energy maps of point defects to interstitial a-loops, vacancy a-loops, and vacancy c-loops have been constructed using large-scale molecular statics simulations. Point defect capture radii have been correlated to the decay of binding energies with increasing distance using two criteria: spontaneous and thermal drift capture. Considering the spontaneous capture criteria, SIAs exhibit biased capture to all dislocation loops. However, when thermal drift capture is considered, the local stress state of dislocation loops imparts an inherent bias for the capture of same-type defects (i.e. vacancies to vacancy loops and vice versa). This phenomenon may drive the co-existence of vacancy and interstitial a-loops in irradiated α-Zr microstructure, an aspect that is poorly captured by most current models. The capture radii reported here will directly improve the predictive capability of microstructure evolution modeling in neutron-irradiated α-Zr.

Stress-Strain and Resistance-Strain Hysteresis in Single-Wall Carbon Nanotube Films for Stretchable Battery Electrodes

The need to protect the vulnerable and limited resources that are naturally available while meeting the growing energy demand of the world makes the sustainable development of renewable energy of paramount importance. To address these needs, a new type of battery electrode has been developed at Honda Research Institute (HRI), based on free-standing single-wall carbon nanotube (SWNT) films containing active battery materials as dispersed nanoparticles. The SWNT films provide both mechanical support and electrical conductivity, removing the need for metal electrodes (increasing the energy density of the battery) and binder materials (making the process environmentally friendly).Under cyclic strain, the SWNT-based electrodes show hysteretic behaviors in both stress-strain and electrical resistance-strain relations. The electrical resistance-strain behavior shows a different trend from the previously observed behavior on 2D SWNT films (on elastic substrate). We hypothesize that this change can be attributed to the differences in the SWNT network structures in the 2D and 3D films. To understand the microscopic origin of the hysteresis, we performed coarse-grained molecular statics (CGMS) simulations of the SWNT film. The simulations show that the intrinsic Van der Waals interaction between SWNTs is responsible for the observed robust mechanical properties of the electrodes. In addition, our study also reveals that the sparsity of the 3D morphology results in the current being carried across the simulation box only by a few paths. As a consequence, the resistance hysteresis is governed entirely by the change in the power-carrying capacity of the network due to the breaking and formation of contacts between the SWNTs during loading-unloading. The qualitative nature of the resistance-strain hysteresis can thus be attributed to the rapid formation of inter-nanotube contacts, as the nanotube bundles (elongated by initial loading) buckle during unloading.

The role of atomistic processes in growth of Cu–Ni metallic/bimetallic nanoparticles

Controlling the morphology of non-noble bimetallic nanocrystals can provide an excellent opportunity to improve performance and activity in catalytic reactions. Although several studies have focused on the overall macroscopic description of the synthesis process, identifying the leading factors in a typical crystal growth process at the atomic scale is still challenging. Here we report the results of atomic scale calculations on the shape evolution of bimetallic Cu–Ni nanoparticle growth using molecular static and dynamic simulations. Our calculations show that statistical analysis of space and time characteristics of single atom diffusion mechanisms and their energy barriers provide sound guidance for fabricating end products with specific shapes and architecture in a growth process.

Coarse-grained modeling for predicting the piezoresistive response of CNT-elastomer nanocomposite

Significant attention has been paid to developing highly flexible and highly stretchable strain sensors due to the increasing demand for wearable devices such as motion-capturing devices and health-monitoring devices. Especially, carbon nanotube (CNT) network-based elastomeric sensors have been studied extensively for their unique strong piezoresistive response under large deformation. Despite its importance for the facile design of sensors, the effect of length and volume fraction of CNT on the piezoresistivity over a large strain range has not been fully uncovered. In this study, by combining coarse-grained molecular statics (CGMS) simulations and efficient percolation network analysis, we investigate the piezoresistive response of the CNT network for a wide range of the length and volume fraction and visualized the CNT network topology to understand the mechanism behind the piezoresistivity response. Based on the set of calculations, we obtain the design map of stretchability and sensitivity for the CNT-elastomer nanocomposite sensors over a wide range of design parameters of CNT, which can be used to fabricate the strain sensor with a desired performance.

Characterization and Simulation of Nanoscale Catastrophic Failure of Metal/Ceramic Interfaces.

The catastrophic failure of metal/ceramic interfaces is a complex process involving the energy transfer between accumulated elastic strain energy and many types of energy dissipation. To quantify the contribution of bulk and interface cohesive energy to the interface cleavage fracture without global plastic deformation, we characterized the quasi-static fracture process of both coherent and semi-coherent fcc-metal/MgO(001) interface systems using a spring series model and molecular static simulations. Our results show that the theoretical catastrophe point and spring-back length by the spring series model are basically consistent with the simulation results of the coherent interface systems. For defect interfaces with misfit dislocations, atomistic simulations revealed an obvious interface weakening effect in terms of reduced tensile strength and work of adhesion. As the model thickness increases, the tensile failure behaviors show significant scale effects-thick models tend to catastrophic failure with abrupt stress drop and obvious spring-back phenomenon. This work provides insight into the origin of catastrophic failure at metal/ceramic interfaces, which highlights a pathway by combining the material and structure design to improve the reliability of layered metal-ceramic composites.

Atomistic simulation study of the structure, segregation and stability of grain boundaries in the U-Zr metallic fuel

Uranium-zirconium (U-Zr) metallic fuel is considered as fuel candidate for sodium-cooled fast reactors due to its compatibility with sodium, intrinsic passive safety characteristics and higher burnup capability compared to the oxide systems. In the present study, we performed molecular dynamics (MD) and static simulations to investigate the stability, cohesive and structural properties of U-Zr grain boundaries, with 10 and 20 at.% of Zr content. The effect of the Zr composition, temperature and atomic configurations were investigated, and the fracture mechanism highlighted. The cohesive energies of special low energy GBs indicate that the intergranular fracture mechanism plays a significant role in the brittle regime. Moreover, we investigated the role of the atomic configurations in determining the GB properties of the alloys. Indeed, for a given misorientation angle, we found that different atomic configurations do not only yield different energies, but also different strain distribution, atomic coordination and different net expansions (free volume at the boundary). In general, we observed that Zr atoms tend to segregate in low energy configurations. We also observed a systematic increase in GB and surface energies with the Zr concentration, in which the surface energies are systematically higher than the corresponding GB energies. The reported findings highlight important effects of the atomic configurations on properties of investigated GB structures. Therefore, it clearly indicates that the atomic configurations of GBs must be correctly determined for the accurate modeling of the alloys properties.

The structure and energy of symmetric tilt grain boundaries in tungsten

Considering the limitations of experiments and the necessity for improving the radiation resistance of materials based on the grain boundary (GB) engineering, more complete results on underlying GB properties are not yet available, and the correlations between such properties remain to be figured out. Here, we constructed a systematical database of the basic properties of 113 distinct symmetric tilt grain boundaries (STGBs) with 〈100〉, 〈110〉, and 〈111〉 axes in tungsten, including the GB energy, work of separation, excess volume and a single vacancy/interstitial segregation energy, calculated by molecular statics simulations with six interatomic potentials. Then, the correlations between these parameters were systematically investigated. We found that although the magnitude of GB properties yielded by these potentials is different, the correlations between these parameters are consistent with each other. According to the analysis of dislocations in these STGB's stable structures, we divided these STGBs into two categories, namely GBs consisting of discrete and overlapping dislocations (DDGBs vs. ODGBs), and identified the dislocation type, i.e., 〈100〉, 〈110〉, and 1/2〈111〉 dislocations, and threshold angles. For DDGBs, the work of separation and excess volume are negatively and positively proportional to the GB energy, respectively, while the minimal vacancy/interstitial segregation energy is close to a constant value for the STGBs consisting of the same dislocations. For ODGBs, there is no obvious correlation between these parameters. In addition, we found that the main contribution to the excess volume is within 5 Å around the GB, and the minimal vacancy/interstitial segregation energy of 〈110〉 STGBs is lower than 〈100〉 and 〈111〉 STGBs, showing a greater capture ability for vacancies/interstitials. We also confirmed that, in Fe and W, STGBs with the same crystallography have strongly correlated energies, which can be scaled by the cohesive energy or different shear moduli, depending on the chosen potential.

Moiré engineering in 2D heterostructures with process-induced strain

We report deterministic control over a moiré superlattice interference pattern in twisted bilayer graphene by implementing designable device-level heterostrain with process-induced strain engineering, a widely used technique in industrial silicon nanofabrication processes. By depositing stressed thin films onto our twisted bilayer graphene samples, heterostrain magnitude and strain directionality can be controlled by stressor film force (film stress × film thickness) and patterned stressor geometry, respectively. We examine strain and moiré interference with Raman spectroscopy through in-plane and moiré-activated phonon mode shifts. Results support systematic C3 rotational symmetry breaking and tunable periodicity in moiré superlattices under the application of uniaxial or biaxial heterostrain. Experimental results are validated by molecular statics simulations and density functional theory based first principles calculations. This provides a method not only to tune moiré interference without additional twisting but also to allow for a systematic pathway to explore different van der Waals based moiré superlattice symmetries by deterministic design.

Influence of kinetic effect on interaction between edge dislocation and irradiated dislocation loops in BCC Tantalum

In this paper, the interaction between an edge dislocation and irradiation <100> sessile self-interstitial atom (SIA) type dislocation loops (DLs) is studied by atomistic simulations in BCC Ta at 0 and 300 K. Both molecular dynamics (MD) and statics (MS) simulations are performed to highlight the dislocation kinetic effect (or dynamic effect) on the critical resolved shear stress (CRSS) for dislocation bypassing the DLs and on the dislocation-SIA DLs interaction mechanisms. The simulation results show that the dislocation-DLs interaction mechanisms are two-fold. When the DL size is lower than a critical value, the moving edge dislocation tends to completely absorb the DLs or fully transform the original DLs into other-type ones. However, when the DL size is higher than the critical value, the moving dislocation tends to absorb or transform the DLs partially or even marginally. Moreover, with increasing applied shear stress, the high-speed dislocation has no sufficient time to absorb SIAs from DLs, which also changes the dislocation-DLs interaction process remarkably. When the dislocation is accelerated to subsonic, an abnormal “pull forward” dislocation configuration may present upon the dislocation bypasses a small DL. More importantly, with the kinetic effect considered, the CRSS for dislocation overcoming the barriers of DLs decreases significantly for the dislocation-DLs interaction mechanisms. In order to appropriately describe the kinetic effect on the CRSS, amended dispersed barrier hardening (DBH) and Bacon-Kocks-Scattergood (BKS) hardening models for the latter dislocation-DLs interaction mechanism are proposed.

Hydrogen-Induced Attractive Force Between Two Partials of Edge Dislocation in Nickel

AbstractHydrogen embrittlement is an important technological problem underpinning failure of many structural elements. It has been extensively investigated in the literatures; however, several open issues remain that prevent a full understanding of this phenomenon. One important issue is the uncertain knowledge of how hydrogen atoms affect the dislocation core structure. Here, by exploring the hydrogen role on the dissociated edge dislocation, we reveal that there exists an additional attractive force between two partials due to the hydrogen atmosphere, which would reduce the equilibrium separation distance. This hydrogen-induced attractive force is quantitatively estimated by means of continuum mechanics. Furthermore, molecular statics simulations also capture the hydrogen-reduced separation distance under varying hydrogen background fractions, qualitatively verifying the theoretical prediction of attractive force. These findings at the atomistic scale will inform the hydrogen embrittlement modeling and experiments, especially on the hydrogen effect on the dislocation glide, climb, dynamics strain ageing, and so on.

Warping caused by surface elasticity in a nanowire under torsion

By combining molecular statics simulations and continuum mechanics-based modeling, we show in this paper that the torsion of a $\ensuremath{\langle}001\ensuremath{\rangle}$ single-crystal copper nanowire with a circular cross section gives rise to a warp, i.e., to a displacement field along the wire axis that renders the cross section nonplanar. This behavior, which is in apparent contradiction with what is predicted by continuum mechanics for an isotropic cylinder, can be well explained if we take into account the elastic response of the wire lateral surface. The latter is characterized by the anisotropy of the surface elastic constants and, more specifically in the case of torsion, by the surface shear constant ${C}_{44}^{S}$ whose strength as a function of the local orientation of the lateral surface is estimated independently from atomistic calculations on slabs presenting different vicinal surfaces. The solution of the torsion problem is then obtained by adopting a semi-inverse method in the framework of the finite strain theory in linear elasticity with Gurtin-Murdoch boundary conditions linking surface stress and bulk stress. It is shown that such an approach is well suited to explain quantitatively the warp obtained in our atomistic simulations and to prove the preponderant role played by the surface elastic constant ${C}_{44}^{S}$.

Insights into the role of Nb segregation on grain boundary structural transition and mechanical response in a Ni–Nb system

The role of Nb segregation on the grain boundary (GB) structural transformation and mechanical response has been studied in a binary Ni–Nb system through atomistic simulation. To investigate the effect of dopant concentration on the GB structural transitions, a Σ5 (310) GB has been created with the help of molecular statics simulation. Thereafter, hybrid Monte Carlo and Molecular dynamics (MD) simulations have been performed to segregate Nb atoms at the Σ5 (310) GB. At lower dopant concentrations, the Nb atoms preferably segregated at the tip of the kite structure. With an increase in dopant concentration and temperature, the Nb atoms segregated at other sites in the GB as well as in the matrix and the GB structure becomes disordered. The effect of doping on the mechanical behavior has been investigated by applying constant strain rate along X-direction through MD. The stress-strain curve of the Ni–Nb system at 300 K revealed an increase in the strength and ductility with the dopant concentration up to a certain limit (0.4 at. % Nb) and decreased thereafter. The superior mechanical response of the Ni-0.4Nb system is attributed to the formation of stacking faults and sessile dislocations during deformation. Stacking faults delay the formation of cracks while sessile dislocations impede the dislocation motion, resulting in higher ductility and strength. Contrarily, nucleation of voids and cracks takes place at a lower extent of strain, resulting in an early fracture in the pure Ni. However, the strength of the doped specimen is found to be lower than the undoped one at 700 K due to the early initiation of cracks in the disordered GB region of the former specimen. The comprehensive understanding of the effect of interfacial segregation on the structural and mechanical behaviour would open up new avenues for alloy design and the development of advanced materials.

Auxeticity of monolayer, few-layer, vdW heterostructure and ribbon penta-graphene.

Using molecular statics simulations, we specifically focus on investigating the negative Poisson's ratio of the monolayer, few-layer, van der Waals, and ribbon penta-graphene. As a result, we provide evidence to show that the Poisson's ratio is the combination of bond stretching and angle rotating mechanism. The auxeticity of monolayer penta-graphene is due to the dominance of bond stretching. However, the significant effect of the angle rotating mechanism causes the enhancement of the in-plane Poisson's ratio of few-layer penta-graphene. Furthermore, the elongation of interlayer bonds results in a negative out-of-plane Poisson's ratio in few-layer penta-graphene. The strong dependence of Poisson's ratio on stacking configuration and number of layers was found. We also show that the van der Waals interaction slightly enhances the auxeticity of heterostructure penta-graphene. Finally, we discuss the significant effects of warped edges on the auxeticity of penta-graphene ribbons.

Energetic and atomic structural analyses of the screw dislocation absorption at tilt grain boundaries in BCC-Fe.

The dislocation-grain boundary (GB) interaction plays an important role in GB-related plasticity. Therefore, an atomistic investigation of the interaction provides a deeper understanding of the strength and fracture of polycrystalline metals. In this study, we investigated the absorption of a screw dislocation with a Burgers vector perpendicular to the GB normal and the corresponding symmetric tilt grain boundaries (STGBs) in BCC-Fe based on molecular static simulations focusing on the STGB-dislocation interaction energy and atomistic structural changes at GB. The STGB-screw dislocation interaction depends on the energetical stability of the STGB against the GB shift along the Burgers vector direction. When the interaction exhibited a large attractive interaction energy, the dislocation dissociation and the GB shift along the Burgers vector direction occurred simultaneously. The interaction energy reveals that the interaction depends on the energetical stability of the STGB in terms of the GB shift in addition to the geometrical descriptor of the GB type, such as the Σ value. The same behavior was also obtained in the reaction when the second dislocation was introduced. We also discuss the screw dislocation absorption and rearrangement of the GB atomistic structure in STGB from an energetic viewpoint.

Microstates and defects of incoherent Σ3 [111] twin boundaries in aluminum

In the last few decades, it has been recognized that a single grain boundary (GB) may exist in several different stable and metastable states, which differ in their atomic structure. However, experimental insights at the atomic structure level are rarely reported. In this study, two different microstates of incoherent Σ3 [111] (112¯) GBs from two different orientation relationships (ORs) in an aluminum thin film grown on sapphire, comprised of slightly different structural units, are reported. The structural units in ORII exhibit hexagonal units while in ORI the hexagonal units are slightly distorted. Molecular statics simulations are utilised to understand the difference in excess properties of both states, which suggest that strain could potentially contribute to the stability of the ORII GB microstate over the ORI microstate. In addition, the atomic structure of the two ORs along the 〈110〉 zone axis reveal the rigid body microscopic translations of different magnitude of {111} planes across the GB experimentally. Furthermore, in the case of asymmetric variants of the same GBs, different types of Σ3 [111] disconnections with Burgers vectors (b1 = 1/6[1¯1¯2]), (b2 = 1/2[1¯01]) and varying step heights (h = 2adsc, 5adsc) are investigated and their implications on the GB mobility are discussed.

Origin of variable propensity for anomalous slip in body-centered cubic metals

Many transition metals crystalizing in the body-centered cubic (bcc) structure exhibit anomalous slip on low-stressed planes at low homologous temperatures, which cannot be reconciled with the Schmid law. Specifically, for uniaxial loading in the center of the stereographic triangle, this is manifested by and screw dislocations moving on low-stressed planes. While the anomalous slip is often attributed to non-planar cores of screw dislocations or to the tendency for their networks to glide easily, it remains unclear why it dominates the plastic deformation in some bcc metals, whereas it is weak or even absent in others. Using molecular statics simulations at 0 K, we demonstrate that the anomalous slip in bcc metals is intimately linked with the stability of screw junctions between two intersecting screw dislocations under stress (for example, and screws giving rise to the [100] junction). Our atomic-level studies show that in nearly all bcc metals of the 5th and 6th groups these junctions cannot be broken by the applied stress and the three dislocations can only move on the common plane (in the above example on the plane). On the other hand, these junctions are found to be unstable in alkali metals, tantalum, and iron, where the application of stress results in unzipping of the two dislocations and their further glide on the planes predicted for isolated dislocations. These results also suggest that the experimentally observed increased propensity for the anomalous slip in further stages of plastic deformation may be explained by reduced curvatures of screw dislocations in dense networks.

Twinning and antitwinning in body-centered cubic metals

Deformation twinning in body-centered cubic (BCC) metals occurs by shearing the crystal along {112} planes parallel to 〈111〉 directions. One of these directions (twinning shear) produces a twin, but it is often argued that the opposite sense of shearing (antitwinning shear) does not lead to twin formation. However, recent slip trace and orientational mapping analyses made on many BCC metals after low-temperature plastic deformation show clear evidence of fine misoriented lamellae along the traces of {112} planes sheared in the antitwinning sense. To resolve this controversy, we have utilized molecular statics simulations to determine the energy barriers for uniformly shearing all transition and alkali BCC metals in the twinning and antitwinning sense. The results of these simulations show that twins in transition BCC metals of the 5th and 6th groups can be produced on both types of {112} planes, irrespective of whether the shear is applied in the twinning or the antitwinning sense. However, this is not the case for α-Fe and the BCC structures of the alkali metals Li, Na, and K, where twins are likely to occur only on {112} planes sheared in the twinning sense. Furthermore, we have used TEM diffraction pattern analyses to investigate the characters of misoriented deformation lamellae in Nb and Cr compressed at 77 K, which were found along {112} planes sheared in the antitwinning sense. We demonstrate that these regions constitute regular twins. Similar studies on α-Fe compressed at 77 K prove that twinning in this material takes place exclusively on {112} planes sheared in the twinning sense.

Direct strain correlations at the single-atom level in three-dimensional core-shell interface structures

Nanomaterials with core-shell architectures are prominent examples of strain-engineered materials. The lattice mismatch between the core and shell materials can cause strong interface strain, which affects the surface structures. Therefore, surface functional properties such as catalytic activities can be designed by fine-tuning the misfit strain at the interface. To precisely control the core-shell effect, it is essential to understand how the surface and interface strains are related at the atomic scale. Here, we elucidate the surface-interface strain relations by determining the full 3D atomic structure of Pd@Pt core-shell nanoparticles at the single-atom level via atomic electron tomography. Full 3D displacement fields and strain profiles of core-shell nanoparticles were obtained, which revealed a direct correlation between the surface and interface strain. The strain distributions show a strong shape-dependent anisotropy, whose nature was further corroborated by molecular statics simulations. From the observed surface strains, the surface oxygen reduction reaction activities were predicted. These findings give a deep understanding of structure-property relationships in strain-engineerable core-shell systems, which can lead to direct control over the resulting catalytic properties.