It was noted that molecular dynamics simulations had played an important role in revealing the fundamental mechanisms of dislocation nucleation, but that its limited time-scale remained a significant barrier to studying nucleation under experimentally relevant conditions. It was shown here that dislocation nucleation rates could be accurately predicted over a wide range of conditions by determining the activation free energy from umbrella sampling. The data revealed very large activation entropies, which contributed a multiplicative factor of many orders of magnitude to the nucleation rate. The activation entropy at constant strain was caused by thermal expansion, with negligible contribution from the vibrational entropy. The activation entropy at constant stress was significantly larger than that at constant strain, as a result of thermal softening. The large activation entropies were caused by anharmonic effects, showing the limitations of the harmonic approximation widely used for rate estimation in solids. Similar behaviours were expected to occur in other nucleation processes in solids.

Entropic Effect on the Rate of Dislocation Nucleation. S.Ryu, K.Kang, W.Cai: Proceedings of the National Academy of Science, 2011, 108[13], 5174-8