Stress-dependent activation entropy in thermally activated cross-slip of dislocations

TL;DR

This paper investigates the large activation entropy observed in dislocation cross slip, revealing that anharmonic effects like thermal softening and vibrational modes are responsible, which has implications for understanding thermally activated processes in solids.

Contribution

The study identifies the physical origin of the anomalously large activation entropy in dislocation cross slip as anharmonic effects, bridging the gap between MD simulations and transition state theory.

Findings

Anharmonic effects cause large activation entropy in cross slip.

Thermal softening and vibrational modes are key factors.

Results are relevant for a wide range of thermally activated processes.

Abstract

Cross slip of screw dislocations in crystalline solids is a stress-driven thermally activated process essential to many phenomena during plastic deformation, including dislocation pattern formation, strain hardening, and dynamic recovery. Molecular dynamics (MD) simulation has played an important role in determining the microscopic mechanisms of cross slip. However, due to its limited timescale, MD can only predict cross-slip rates in high-stress or high-temperature conditions. The transition state theory can predict the cross-slip rate over a broad range of stress and temperature conditions, but its predictions have been found to be several orders of magnitude too low in comparison to MD results. This discrepancy can be expressed as an anomalously large activation entropy whose physical origin remains unclear. Here we resolve this discrepancy by showing that the large activation entropy results from anharmonic effects, including thermal softening, thermal expansion, and soft vibrational modes of the dislocation. We expect these anharmonic effects to be significant in a wide range of stress-driven thermally activated processes in solids.