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Precise prediction of phase diagrams in molecular dynamics simulations is challenging due to the simultaneous need for long time and large length scales and accurate interatomic potentials. We show that thermodynamic integration from low-cost force fields to neural network potentials trained using density-functional theory (DFT) enables rapid first-principles prediction of the solid–liquid phase boundary in the model salt NaCl. We use this technique to compare the accuracy of several DFT exchange–correlation functionals for predicting the NaCl phase boundary and find that the inclusion of dispersion interactions is critical to obtain good agreement with experiment. Importantly, our approach introduces a method to predict solid–liquid phase boundaries for any material at an ab initio level of accuracy, with the majority of the computational cost at the level of classical potentials. [ABSTRACT FROM AUTHOR]

Understanding how nanoparticles deform under compression not only is of scientific importance but also has practical significance in various applications such as tribology, nanoparticle-based probes, and the dry grinding of raw materials. In this study, we conducted compression tests on model brittle glassy nanoparticles using molecular dynamics simulations. We found that during the early stages of plastic deformation, shear bands formed in a similar pattern regardless of the nanoparticle size. However, as the deformation continued, dominant cracks emerged in large nanoparticles while being suppressed in smaller ones. This size-dependent brittle-to-ductile transition can be explained by a simple model based on Griffith's theory. We also investigated the effect of the surface stress state on fracture using thermally tempered nanoparticles. We observed that the presence of compressive surface stress strengthened the nanoparticle by suppressing crack formation, even when a pre-notch was present. On the other hand, tensile surface stress had the opposite effect. Interestingly, nanoparticles with both tensile and compressive surface stress promoted shear deformation, which could potentially compromise the mechanical performance of tempered glass despite delayed crack formation. [ABSTRACT FROM AUTHOR]

Mechanical properties of glassy nanowires have been intensively investigated recently by both nanomechanical experiments and atomic-level simulations. Unfortunately, there exists a huge gap in the strain rate of the nanomechanical tests between experiments and simulations, which makes it difficult to compare results even for the same material system. Using accelerated atomistic simulations based on a self-learning metabasin escape algorithm, here, we report the tensile mechanical properties of amorphous Stillinger–Weber silicon nanowires with different intrinsic ductility under strain rates ranging from 1010 to 10−1 s−1. It is found that both brittle and ductile glassy silicon nanowires display weakened strength with a decreasing strain rate, in agreement with the cooperative shear model. Moreover, as the strain rate decreases, the amount of plasticity remains unchanged for the brittle nanowires, yet it decreases for the ductile ones. Such deteriorated plasticity in ductile glassy nanowires is caused by enhanced strain localization at low strain rates. Lastly, we show that via the distance matrix of nonaffine displacement, a more hierarchical potential energy landscape is responsible for the higher strain localization propensity in ductile silicon glassy nanowires. [ABSTRACT FROM AUTHOR]

Figures 5-8 show density and Si/Al coordination change during the compression-decompression cycle with and without a shear stress component using the two sets of parameters. In the original paper, the short-range interaction parameters for the Si-Li pair in the SHIK potential[1] were used by mistake for the Si-Na pair in the sodium aluminosilicate (NAS) glass. Si and Al coordination changes during the compression-decompression cycle in NAS without [(a) and (b)] and with shear [(c) and (d)] using the two sets of short-range interaction parameters. [Extracted from the article]

Experimental studies have shown that glass systems with high boron content exhibit superior crack resistance under sharp contact loading. However, the underlying mechanism is still not fully understood. In this context, we carried out classical molecular dynamics simulations on sodium aluminosilicate and sodium aluminoborate systems to investigate the effect of boron on the response of glass to nanoindentation. A rigid V-shaped indenter is used to indent the glass sample with a fixed loading rate, during which the indenter interacts with the glass via a repulsive force field. The indenter angle and tip radius are varied to study the effect of indenter sharpness, as what has been done in experiments. These simulated nanoindentation tests reveal how the stress/strain field and the glass structure evolve with deformation underneath the indenter. It was found that a large number of boron atoms in the plastic zone change from three- to fourfold coordination during the loading process, and most of them revert back to the threefold coordination state during the unloading process. Our study shows that this "reversible" boron coordination change plays a critical role in increasing the damage resistance of glass. [ABSTRACT FROM AUTHOR]

Metallic glasses with secondary crystalline phase have been reported to either enhance or degrade the global plasticity. Here, we employed molecular dynamics simulation to investigate a generic glass-forming system, in which crystal-glass composites can be obtained via in situ crystallization. Ex situ composite samples were also constructed by embedding perfect crystals into monolithic glasses in order to delineate the precise influences of the composite microstructure. Our simulations demonstrate both toughening and embrittlement effects due to nanocrystallization. The double-edged-sword role of nanocrystals is mainly controlled by elasticity mismatch between the crystalline phase and the glass matrix. The effects of volume fraction and the size of nanocrystals can be understood in the perspective of cumulative probability of shear bands formation and propagation being interfered by the presence of nanocrystals. [ABSTRACT FROM AUTHOR]

We report a heuristic approach to measure the fracture energy of model metallic glasses using molecular dynamics simulation. Specifically, we adopted the Rivlin-Thomas method, simplified by Suo et al., which is applicable even with the presence of plastic flow. We further modified the testing condition with semi-rigid holders in our molecular simulations, to avoid unintended fracture near the holders. This method was first applied in measuring the fracture energy of a brittle model glass, which agrees well with direct K I C and J I C measurements (both measurements are independent of the crack size). Furthermore, the fracture energy values of a family of model metallic glasses, ranging from brittle to ductile (BTD), were measured. The Poisson's ratio-fracture energy (v-G, or v-G/2γ, normalized by the surface energy) relation obtained here exhibits a BTD transition at a critical Poisson's ratio of 0.31–0.32, consistent with experimental results. [ABSTRACT FROM AUTHOR]

We designed catalytic nanorotors and investigated the rotational motion and energy conversion efficiency using reactive molecular dynamics in two dimensions. First, a two-arm nanorotor was constructed by decorating a slender beam with catalysts asymmetrically on its two long edges, while fixing the beam center as the rotational axis. Autonomous rotation was observed for the two-arm nanorotor immersing in a fuel environment. Here fuel molecules undergo exothermic combination reaction facilitated by the catalysts. It was found that the angular velocity increases with the catalyst coverage parabolically, while the rotary nanomotor efficiency stays roughly constant. These observations are consistent with a single-collision-momentum-transfer-based propulsion model. Furthermore, multi-arm nanorotors (up to eight arms) were constructed by carving radially distributed arms followed by decorating catalysts. For multi-arm nanorotors, both the angular velocity and the efficiency decrease as the number of arms increases. These behaviors contradict the aforementioned model, which are likely due to the deceleration from secondary collisions between products and the nanorotor arms. Our simulation results show that the optimal design for a nanorotor that maximizes its angular velocity and the motor efficiency is a two-arm nanorotor with nearly full coverage of catalysts. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulations, model Zr50Cu40Al10 metallic glass (MG) nanopillars were subjected to simulated irradiation processes followed by uniaxial compression tests. As the intensity or dosage of irradiation increases, the plastic deformation mode of the MG nanopillars transits from localized shear banding to homogeneous shear flow. The suppression of shear banding in MG nanopillars is due to irradiation-induced structural disordering. Furthermore, a correlation is found between the average potential energy of MG nanopillars and their deformation modes, common to both irradiation processing and thermal processing. Our results imply that the homogeneous shear flow observed in experimental MG nanopillars carved by focused ion beam may be due to irradiation damage instead of size effect. [ABSTRACT FROM AUTHOR]

We report compression-compression fatigue simulations on model metallic-glass nanowires up to a 4% maximum compressive strain and 100 cycles using the molecular dynamics method. The distribution and irreversibility of the fatigue-induced deformations are characterized by the local shear strain and rise of the system temperature, respectively. Furthermore, the system potential energy, structural signatures in terms of icosahedral centers, and radial distribution functions are obtained to identify structural transformations during cyclic loading. No noticeable changes on the aforementioned isotropic structural signatures have been found during fatigue tests. Based on a binary view of the constituting local structures (stable clusters and floppy clusters) in metallic glasses, we attribute the irreversible deformation to inter-conversion of those floppy clusters, which leads to a constant composition of stable and floppy clusters. Finally, uniaxial compression tests were carried out on the cyclically loaded samples. The mechanical behaviors of fatigue-tested samples are rather similar to the original sample. Upon cyclic loading, unlike macroscopic samples, a metallic-glass nanowire is free of permanent structural damage, and therefore does not exhibit deteriorated mechanical behaviors. The excellent fatigue-resistance behavior of a metallic-glass nanowire might be related to its defect-free structure (no stress concentrators). It appears that defects may play a central role in fatigue damages in experimental metallic-glass systems. [ABSTRACT FROM AUTHOR]

In this paper, we establish the nonperturbative Anderson localization for a class of one-frequency quasiperiodic block operators with long-range interactions. [ABSTRACT FROM AUTHOR]

A simulation study demonstrates that localization can arise as the result of the breakdown of stable quasicrystal-like atomic configurations. Samples produced at elevated quench rates and via more energetic processes contain a lower fraction of such configurations and exhibit significantly less pronounced localization and shorter spacing between bands. In the samples produced by the lowest quench rates, localization is accompanied by the amorphization of material with initially quasicrystal-like medium range order. This result is of particular significance in light of recent experimental evidence of local quasicrystal order in the most stable of the bulk metallic glasses. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of the self-propulsion of a model nanometer-scale motor by reactive molecular dynamics simulations. The nanomotor is decorated with catalysts on only one side that promotes exothermic reactions of the surrounding fuel. Unidirectional drift of the nanomotor is observed that is superimposed on its Brownian motion. The motor response upon the application of external loads is also investigated and the thermodynamic efficiency is calculated. It is shown that the propulsion of our nanomotor can be understood by a momentum transfer model which is akin to rocket propulsion.

A mimetic porous carbon model is generated using quench molecular dynamics simulations that reproduces experimental radial distribution functions of activated carbon. The resulting structure is composed of curved and defected graphene sheets. The curvature is induced by nonhexagonal rings. The quench conditions are systematically varied and the final porous structure is scrutinized in terms of its pore size distribution, pore connectivity, and fractal dimension. It is found that the initial carbon density affects the fractal dimension but only causes a minor shift in the pore size distribution. On the other hand, the quench rate affects the pore size distribution but only causes a minor shift in the fractal dimension.

We present simulations of a model molecular solid of nitrogen cubane subject to thermal agitation and mechanical shock. A new approach, a reactive state summation potential, has been used to model nitrogen cubane dissociation. At elevated temperatures, the system decomposes to N(2) mixed with a small amount of oligomeric nitrogen. When subject to shock loading the system detonates above some critical threshold after which a shock front is self-sustained by the energy release from chemical reactions at a constant intrinsic speed. This is the first example of a fully three-dimensional atomic simulation of a chemically-sustained detonation. The spatial confinement of the shock front results in longer chain intermediates than in the case of thermal decomposition, suggesting that shock intermediates can be structurally very different from the same material subject to comparable temperatures and pressures.