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Some low-coordination materials, including water, silica, and silicon, exhibit polyamorphism, having multiple amorphous forms. However, the microscopic mechanism and kinetic pathway of amorphous-amorphous transition (AAT) remain largely unknown. Here, we use a state-of-the-art machine-learning potential and local structural analysis to investigate the microscopic kinetics of AAT in silicon after a rapid pressure change. We find that the transition from low-density-amorphous (LDA) to high-density-amorphous (HDA) occurs through nucleation and growth, resulting in non-spherical interfaces that underscore the mechanical nature of AAT. In contrast, the reverse transition occurs through spinodal decomposition. Further pressurisation transforms LDA into very-high-density amorphous (VHDA), with HDA serving as an intermediate state. Notably, the final amorphous states are inherently unstable, transitioning into crystals. Our findings demonstrate that AAT and crystallisation are driven by joint thermodynamic and mechanical instabilities, assisted by preordering, occurring without diffusion. This unique mechanical and diffusion-less nature distinguishes AAT from liquid-liquid transitions. The mechanism of amorphous-amorphous transitions is highly debated. Here, the authors use molecular dynamics simulations to reveal transitions via nucleation-growth or spinodal decomposition, resembling a thermodynamic phase transition but influenced by mechanics. [ABSTRACT FROM AUTHOR]

Recently, Ta/Cu nanocomposites have been widely used in therapeutic medical devices due to their excellent bioactivity and biocompatibility, antimicrobial property, and outstanding corrosion and wear resistance. Since mechanical yielding and any other deformation in the patient's body during treatment are unacceptable in medicine, the characterization of the mechanical behavior of these nanomaterials is of great importance. We focus on the microstructural evolution of Ta/Cu nanocomposite samples under uniaxial tensile loading conditions at different strain rates using a series of molecular dynamics simulations and compare to the reference case of pure Ta. The results show that the increase in dislocation density at lower strain rates leads to the significant weakening of the mechanical properties. The strain rate-dependent plastic deformation mechanism of the samples can be divided into three main categories: phase transitions at the extreme strain rates, dislocation slip/twinning at lower strain rates for coarse-grained samples, and grain-boundary based activities for the finer-grained samples. Finally, we demonstrate that the load transfer from the Ta matrix to the Cu nanoparticles via the interfacial region can significantly affect the plastic deformation of the matrix in all nanocomposite samples. These results will prove useful for the design of therapeutic implants based on Ta/Cu nanocomposites. [ABSTRACT FROM AUTHOR]

Twin boundary (TB) plays an important role in the deformation process of materials. In this paper, molecular dynamics (MD) simulation was used to investigate the nanoindentation deformation behaviour of single-crystal FeCoCrNiCu high entropy alloy (SC-HEA) and nano-twinned FeCoCrNiCu high entropy alloy (NT-HEA) with different twin spacings. It is found that the main characteristic of plastic deformation of SC-HEA is the dislocation loop emission. The dislocation movement and distribution of NT-HEA are very different from that of SC-HEA. We found that partial dislocation slip parallel to the twin boundary (PSPTB) and twin partial slip (TPS) can lead to alloy softening. The hindrance of the TB causes the dislocation to slip within a single layer (known as confined layer slip, CLS), which strengthens the material. In the process of nanoindentation, the softening and strengthening mechanisms are constantly competing. When the twin spacing is larger than 1.23 nm, CLS dominates the competition with the hardening mechanism, and the hardness of the material increases with the decrease of the twin spacing. When the twin spacing is less than 1.23 nm, the dominant mechanism of plastic deformation changes to the softening mechanism controlled by TPS, and the hardness thus decreases as twin spacing increases. [ABSTRACT FROM AUTHOR]

Structural metallic materials with interfaces of immiscible materials provide opportunities to design and tailor the microstructures for desired mechanical behavior. Metallic microstructures with plasticity contributors of the FCC and BCC phases show significant promise for damage-tolerant applications due to their enhanced strengths and thermal stability. A fundamental understanding of the dynamic failure behavior is needed to design and tailor these microstructures with desired mechanical responses under extreme environments. This study uses molecular dynamics (MD) simulations to characterize plasticity contributors for various interface microstructures and the damage evolution behavior of FCC/BCC laminate microstructures. This study uses six model Cu/Ta interface systems with different orientation relationships that are as- created, and pre-deformed to understand the modifications in the plasticity contributions and the void nucleation/evolution behavior. The results suggest that pre-existing misfit dislocations and loading orientations (perpendicular to and parallel to the interface) affect the activation of primary and secondary slip systems. The dynamic strengths are observed to correlate with the energy of the interfaces, with the strengths being highest for low-energy interfaces and lowest for high-energy interfaces. However, the presence of pre-deformation of these interface microstructures affects not only the dynamic strength of the microstructures but also the correlation with interface energy. [ABSTRACT FROM AUTHOR]

This work elucidates the effect of interface orientation, loading mode, and crystallography on the deformation mechanisms of Cu-Nb–multilayered nanocomposites. Molecular dynamics simulations of deformation behavior of accumulative roll-bonded Cu-Nb–multilayered nanocomposites (MNCs) were performed at room temperature conditions and at a constant strain rate under iso-stress and iso-strain conditions. Interface deformation mechanisms involving nucleation of partial dislocation at the interface and gliding in the Cu layer were observed under iso-stress and iso-strain conditions. Uniaxial stress–strain curves were analyzed for tension and compression under iso-stress and iso-strain loading conditions. The stress–strain plots were explored to understand the elastic, yield, and post-yielding behavior of Cu-Nb MNCs. Under compression with interface orientation normal to the loading direction, twin nucleated via gliding of partial dislocations. Under tension in the iso-stress case, only slip-assisted deformation was observed. Conversely, the deformation behavior under compression and tension was via slip and twinning, respectively, for iso-strain conditions. [ABSTRACT FROM AUTHOR]

The role of immiscible Cu/Mo interfaces on the formation of helium (He) bubbles in ion-irradiated Cu/Mo 5 nm multilayers is examined. Interfaces significantly enhance the critical He concentration above which bubbles, approximately 1 nm in diameter, are detected via through-focus imaging in a transmission electron microscope. He-to-vacancy ratio affects the formation and distribution of He bubbles. The diameter of He bubbles in Cu appears to be slightly larger than that in Mo. [ABSTRACT FROM AUTHOR]

The frictional behavior between self-mated diamond-like carbon (DLC) films is investigated systematically by molecular dynamics (MD) simulation, analyzing the role of atomic configuration evolution in the sliding process and focusing on the intrinsic causes of different frictional behaviors. According to the different sliding speeds, there are two ways of friction evolution: sawtooth-type stick–slip friction and smooth friction. The results show that the evolution of atomic configuration plays an important role in the variation of friction force. The periodic shear deformation and interface fracture evolution mode exhibited by the system controls the friction variation at low velocities, leading to sawtooth-type stick–slip frictional behavior. However, in the process of high-speed sliding, the stable sliding interface leads to the stable number of C-C covalent bonds at the interface, which makes the system show smooth friction behavior. At this time, the friction is mainly controlled by the interfacial C-C bonds and is influenced by the graphitization phase transition. The research work in this paper can deepen the understanding of the internal mechanism of frictional behavior of diamond-like carbon films. [ABSTRACT FROM AUTHOR]

The predictions of scaling laws for the structure and properties of defect clusters are generally limited to small defect clusters in their ground-state configurations. We investigated the size and geometrical configuration dependence of nano-sized defect clusters in niobium (Nb) using molecular dynamics. We studied the structure and stability of large clusters of size up to fifty defects for vacancies and one hundred defects for interstitials, as well as the role of helium and metastable configurations on the stability of these clusters. We compared three different interatomic potentials in order to determine the relative stability of these clusters as a function of their size and geometrical configurations. Additionally, we conducted a statistical analysis to predict the formation and binding energies of interstitial clusters as a function of both their size and configuration. We find that the size dependence of vacancy and interstitial clusters can be approximated by functional forms that account for bulk and surface effects as well as some considerations of elastic interactions. We also find that helium and metastable configurations can make vacancy and interstitial clusters thermally stable depending on the configuration. Our parameterized functional forms for the formation and binding energies are valid for a very broad range of defect sizes and configurations making it possible to be used directly in a coarse-grained modeling strategy such as Monte Carlo, cluster dynamics or dislocation dynamics which look at defect accumulation and evolution in microstructures. [ABSTRACT FROM AUTHOR]

Unraveling the effects of interface atomic structures on the mechanical properties is a key step toward the elaborate design of Ti2AlN/TiAl composite with excellent performance. However, the impact of different interface atomic structures upon the mechanical properties of Ti2AlN/TiAl composite, which is extremely important from the perspective of material design, remains poorly understood and essentially unquantified so far. In this research work, molecular dynamics simulations of Ti2AlN(0001)/TiAl(111) coherent interface and Ti2AlN(101¯3)/TiAl111 incoherent interface system under parallel-to-interface compression are carried out. It is found that these two types of interface systems show different compressive deformation behaviors due to significantly different interface-dislocation interactive mechanisms. The compressive ultimate strengths of Ti2AlN(0001)/TiAl(111) coherent interface and Ti2AlN(101¯3)/TiAl111 incoherent interface systems are comparable, but the ductility of incoherent interface system is obviously higher than that of coherent interface system. This is because the incoherent interface can simultaneously serve as the source for dislocation nucleation and the barrier for dislocation motion, and thus plays a dual role of softening and hardening in the compressive deformation. Therefore, it can be expected that tuning the interface with Ti2AlN(101¯3)/TiAl111 incoherent atomic structure can contribute to increasing the compressive ductility of Ti2AlN/TiAl composite without lowering its strength. [ABSTRACT FROM AUTHOR]

Making glasses ductile at room temperature is a daunting challenge, but has been shown to be feasible in recent years. We explain the plastic flow from the standpoint of the flexibility available in the amorphous structure: imparting flexibility into the structure facilitates bond switching needed to mediate shear transformations to carry strain. This structure-property correlation is demonstrated using molecular dynamics simulation data. The flexibility can be improved via ultrafast quench or rejuvenation. In particular, the flexibility volume parameter offers a quantitative metric to explain the flexibility and deformability, even for glasses where the commonly cited free volume is not applicable. This Perspective demonstrates using examples and models that it is the flexibility rather than the excess volume that can be tuned to facilitate plastic flow and ductility in glassy materials. [ABSTRACT FROM AUTHOR]

Seeking nuclear materials that possess a high resistance against particle radiation damage is a long-standing issue. In this work, to reveal the radiation effect on the metallic glasses (MGs), the atomistic structural evolution of MGs induced by particle radiation is investigated, by performing a molecular dynamics simulation. It is found that radiation-induced vacancy-like defects appear in the MG structural model during the collision cascades. However, these defects are transient and unstable that they are fully annihilated. In particular, free volumes which are specific structural characteristics in MGs can annihilate these transient defects. In addition, there is a rearrangement of free volumes that large free volumes change into small ones and are apt to distribute homogeneously in the amorphous model after structural relaxation, so that the problems of radiation-induced structural instability and energy imbalance are solved. This work will shed light on evaluating the structural stability of MGs under particle radiation. [ABSTRACT FROM AUTHOR]

In this paper, the creep behavior of nanocrystalline Ni having bimodal grain structure is investigated using molecular dynamics simulation. Analysis of structural evolution during the creep process has also been performed. It is observed that an increase in size of coarse grain causes improvement in creep properties of bimodal nanocrystalline Ni. Influence of bimodality (i.e., size difference between coarse and fine grains) on creep properties are found to be reduced with increasing creep temperature. The dislocation density is observed to decrease exponentially with progress of creep deformation. Grain boundary diffusion controlled creep mechanism is found to be dominant at the primary creep region and the initial part of the secondary creep region. After that shear diffusion transformation mechanism is found to be significantly responsible for deformation as bimodal nanocrystalline Ni transforms to amorphous structure with further progress of the creep process. The presence of <0, 2, 8, 5>, <0, 2, 8, 2 >, and <0, 1, 10, 2 > distorted icosahedra has a significant influence on creep rate in the tertiary creep regime according to Voronoi cluster analysis. [ABSTRACT FROM AUTHOR]

We present an orthogonal tight-binding (TB) potential model for W/Cu binary systems. This model can reasonably predict the electronic structures, elastic properties, and thermodynamics properties of W/Cu systems. Furthermore, by performing the TB Monte Carlo simulations and the TB molecular dynamics simulations, we find that (1) the W(110) surface in the fusion reactor exhibits pre-melting behaviors, (2) W and Cu atoms in a W/Cu binary system prefer to form single element domains, and (3) the interface between a W domain and a Cu domain degrades the transport property of the heat in a W/Cu system significantly. [ABSTRACT FROM AUTHOR]

Any fracture process ultimately involves the rupture of atomic bonds. Processes at the atomic scale therefore critically influence the toughness and overall fracture behavior of materials. Atomistic simulation methods including large-scale molecular dynamics simulations with classical potentials, density functional theory calculations and advanced concurrent multiscale methods have led to new insights e.g. on the role of bond trapping, dynamic effects, crack-microstructure interactions and chemical aspects on the fracture toughness and crack propagation patterns in metals and ceramics. This review focuses on atomistic aspects of fracture in crystalline materials where significant advances have been achieved over the last ten years and provides an outlook on future perspectives for atomistic modelling of fracture. [ABSTRACT FROM AUTHOR]

Nanostructured metallic material (NMM) composites are a new class of materials that exhibit high structural stability, mechanical strength, high ductility, toughness and resistance to fracture and fatigue; these properties suggest that these materials can play a leading role in the future micromechanical devices. However, before those materials are put into service in any significant applications, many important fundamental issues remain to be understood. Among them, is the question of the strengthening of NMM using second phase particles and if the addition of precipitates will strengthen the structures in the same manner as in bulk crystalline solids. This issue is addressed in this work by performing molecular dynamics simulations on NMM with precipitates of various sizes and comparing the results with the same structure without precipitates. In this view, Cu/Nb bilayer thin films with spherical Nb particles inside the Cu layer were examined using molecular dynamics simulations and show a significant improvement on their mechanical behavior, compared to similar structures without particles. Furthermore, an analytical model is developed that explains the strengthening behavior of an NMM that has precipitates inside one layer. The theoretical results show a qualitative agreement with the finding of the atomistic simulations. [ABSTRACT FROM AUTHOR]

Using density functional theory (DFT), we found that high pressures intrinsically favor twinning in niobium by reducing the thickness of a stable twin. Five empirical interatomic potentials for niobium were considered in molecular dynamics (MD) shock simulations. The results show that two potentials exhibit the experimentally observed twinning behavior. Comparing with DFT under high pressure, we found that these two potentials are capable of reproducing the generalized stacking fault (GSF) curve, but the others predict several artificial metastable states along the GSF curve resulting in an artificial structural transformation. [ABSTRACT FROM AUTHOR]

During the nanofabrication process of polycrystalline materials, the interactions of dislocations in material determine the evolution of subsurface defects. In this paper, the molecular dynamics simulation models of nanocutting polycrystalline copper, which is used to study the relationship between the crystal structure and the cutting force during the cutting process, were established, and the transformation process between grain boundaries and dislocations was studied to get the effects of grain boundary on dislocation slip and stress conduction. The results show that there are obvious rules between cutting force and cutting process and grain boundaries can prevent dislocation slip and shielding stress conduction. The influence of different cutting parameters on the evolution of subsurface defects of workpiece was further analyzed. Finally, the nanoindentation simulations and experiments were carried out to study the influence of cutting parameters on surface mechanical properties of workpiece. It is found that to some extent, the surface hardening effect of the workpiece is remarkable with the cutting depth increase. [ABSTRACT FROM AUTHOR]

In this paper, constant load bending tests were performed on a nanocrystalline Ni nanowire specimen at different deformation temperatures using molecular dynamics simulation to investigate deformation behavior and mechanisms responsible for fracture. The nature of the fracture occurred in this nanowire specimen is found to transit from brittle to ductile as the temperature rises from 500 to 800 K. Also, with an increase in temperature, the fracture strain is increased indicating more plastic deformation prior to fracture. In the case of 500 K and 600 K deformation temperatures, fracture occurred along the shear band due to slip-twin interaction. On the other hand, at comparatively higher deformation temperatures, such as 700 K and 800 K, twinning and detwinning mechanisms are responsible for accommodating large plastic strain before fracture thus imparting plasticity in the specimen. It has also been found that formation and collapse of the stacking fault tetrahedron causes fracture of nanocrystalline Ni nanowire at 800 K. [ABSTRACT FROM AUTHOR]