Entropic Effect on the Rate of Dislocation Nucleation

TL;DR

This paper demonstrates that dislocation nucleation rates in crystalline materials can be accurately predicted by calculating activation free energy using umbrella sampling, highlighting the significant role of activation entropy and anharmonic effects.

Contribution

It introduces a method to predict dislocation nucleation rates over various conditions by determining activation free energy, emphasizing the importance of anharmonic effects and large activation entropies.

Findings

Large activation entropies significantly enhance nucleation rates.

Thermal expansion causes activation entropy at constant strain.

Thermal softening increases activation entropy at constant stress.

Abstract

Dislocation nucleation is essential to our understanding of plastic deformation, ductility and mechanical strength of crystalline materials. Molecular dynamics simulation has played an important role in uncovering the fundamental mechanisms of dislocation nucleation, but its limited time scale remains a significant challenge for studying nucleation at experimentally relevant conditions. Here we show that dislocation nucleation rates can be accurately predicted over a wide range of conditions by determining the activation free energy from umbrella sampling. Our data reveal very large activation entropies, which contribute a multiplicative factor of many orders of magnitude to the nucleation rate. The activation entropy at constant strain is caused by thermal expansion, with negligible contribution from the vibrational entropy. The activation entropy at constant stress is significant larger than that at constant strain, as a result of thermal softening. The large activation entropies are caused by anharmonic effects, showing the limitations of the harmonic approximation widely used in solids. Similar behaviors are expected to occur in other nucleation processes in solids.