Calculation of elastic constants of embedded-atom-model potentials in the NVT ensemble.

A method for the calculation of elastic constants in the NVT ensamble using molecular dynamics (MD) simulation with a realistic many-body embedded-atom-model (EAM) potential is studied in detail. It is shown that, in such NVT MD simulations, the evaluation of elastic constants is robust and accurate because it gives the elastic tensor in a single simulation which converges using a small number of time steps and particles. These results highlight the applicability of this method in (i) the calculation of local elastic constants of nonhomogeneous crystalline materials and (ii) the calibration of interatomic potentials, as a fast and accurate alternative to the common method of explicit deformation, which requires a set of consistent simulations at different conditions. The method is demonstrated for the calculation of the elastic constants of copper in the temperature range of 0-1000 K, and results agree with the target values used for the potential calibration. The various contributions to the values of the elastic constants, namely, the Born, stress fluctuation, and ideal gas terms, are studied as a function of temperature.

Stochastic Model of Breakdown Nucleation under Intense Electric Fields.

A plastic response due to dislocation activity under intense electric fields is proposed as a source of breakdown. A model is formulated based on stochastic multiplication and arrest under the stress generated by the field. A critical transition in the dislocation population is suggested as the cause of protrusion formation leading to subsequent arcing. The model is studied using Monte Carlo simulations and theoretical analysis, yielding a simplified dependence of the breakdown rates on the electric field. These agree with experimental observations of field and temperature breakdown dependencies.

Irradiation induced grain boundary flow--a new creep mechanism at the nanoscale.

A new mechanism of irradiation enhanced creep is proposed for nanocrystalline materials. It derives from local relaxations within the grain boundaries as they absorb point defects produced by irradiation. The process is studied by inserting point defects into the grain boundaries and following the materials response by molecular dynamics. Calculated creep compliances are found in good agreement with those measured in dilute nanocrystalline Cu-W alloys [Tai, K.; Averback, R. S.; Bellon, P.; Ashkenazy Y. Scr. Mater.2011, 65, 163]. The simulations provide a direct link between irradiation induced creep in nanocrystalline materials with radiation-induced viscous flow in amorphous materials, suggesting that grain boundaries in these materials can be treated as an amorphous phase. We provide a simple analytic model based on this assumption that reproduces the main features of the observed creep rates, a linear dependence on stress, inverse dependence of grain size, a weak dependence on temperature, and a reasonable estimate of the absolute creep rate.

Solidification velocities in deeply undercooled silver.

We measure the solidification velocity of pure Ag as a function of undercooling temperature from the melting point (Tm=1235 K) to 0.6Tm using ultrafast, pump-probe laser experiments. The thickness of the liquid layer, while it solidifies, is measured using optical third harmonic generation. We show that velocity reaches a maximum value at 0.85Tm, and then remains nearly constant with additional undercooling. These results contradict simple collision-limited models, but they are in good agreement with molecular dynamics simulations presented here, which show that the crystallization velocity depends weakly on temperature from 0.85Tm to less than approximately 0.1Tm.