Realistic time-scale fully atomistic simulations of surface nucleation of dislocations in pristine nanopillars

TL;DR

This paper employs an accelerated atomistic simulation method to study surface dislocation nucleation in gold nanopillars, revealing how activation free energy varies with temperature and stress, and highlighting the impact of strain-rate on mechanical failure.

Contribution

It introduces a new methodology that extends accelerated dynamics simulations to realistic time scales, enabling detailed analysis of dislocation nucleation under practical conditions.

Findings

Activation free energy depends strongly on stress and temperature.

Failure mechanism remains consistent across strain rates.

Elastic limit varies significantly with strain-rate.

Abstract

We use our recently proposed accelerated dynamics algorithm (Tiwary & van de Walle, 2011) to calculate temperature and stress dependence of activation free energy for surface nucleation of dislocations in pristine Gold nanopillars under realistic loads. While maintaining fully atomistic resolution, we achieve the fraction of a second time-scale regime. We find that the activation free energy depends significantly on the driving force (stress or strain) and temperature, leading to very high activation entropies. We also perform compression tests on Gold nanopillars for strain rates varying between 7 orders of magnitudes, reaching as low as 10^3/s. Our calculations show the quantitative effects on the yield point of unrealistic strain-rate Molecular Dynamics calculations: we find that while the failure mechanism for <001> compression of Gold nanopillars remains the same across the entire strain-rate range, the elastic limit (defined as stress for nucleation of the first dislocation) depends significantly on the strain-rate. We also propose a new methodology that overcomes some of the limits in our original accelerated dynamics scheme (and accelerated dynamics methods in general). We lay out our methods in sufficient details so as to be used for understanding and predicting deformation mechanism under realistic driving forces for various problems.