Entropic Effects on the Rate of Thermally Activated Dislocation Cross-Slip in FCC Nickel

Dislocation cross-slip is a thermally activated process and a key mechanism responsible for many important phenomena in metal plasticity, including dislocation multiplication, pattern formation, dynamic recovery, and strain hardening. So far, the efforts on predicting the cross-slip rate from atomistic models has been focused on determining the activation enthalpy of cross-slip under stress, while the entropic effects were usually neglected. We show that the rate predicted based only on activation enthalpy underestimates the cross-slip rate by 4-orders of magnitude when compared to direct molecular dynamics (MD) simulations. We further show that the harmonic transition state theory (HTST) can still be used for accurate prediction of cross slip rate in comparison with MD, provided that a set of corrections are made to account for anharmonic effects such as soft modes (in the example case of FCC nickel). The result is method that successfully predicts the cross-slip rate over a much wider range of stress and temperature conditions than accessible by direct MD simulations. We find that thermal expansion and thermal softening effects contribute significantly to the activation entropy and the cross-slip rate; this effect is linked to a non-negligible local volume expansion during cross-slip under stress.