Activation entropy of dislocation glide in body-centered cubic metals from atomistic simulations

TL;DR

This study calculates the activation entropy of dislocation glide in BCC metals using machine learning potentials, revealing it remains largely constant and challenging previous models that suggested high variability.

Contribution

It introduces machine learning interatomic potentials for iron and tungsten to accurately compute activation entropy, providing a more reliable understanding of dislocation dynamics in BCC metals.

Findings

Activation entropy is largely constant across temperature and stress.

Dislocation transitions occur via harmonic oscillations between Peierls valleys.

Thermally-activated yield stress model aligns well with experimental data.

Abstract

The activation entropy of dislocation glide, a key process controlling the strength of many metals, is often assumed to be constant or linked to enthalpy through the empirical Meyer-Neldel law-both of which are simplified approximations. In this study, we take a more direct approach by calculating the activation Gibbs energy for kink-pair nucleation on screw dislocations of two body-centered cubic metals, iron and tungsten. To ensure reliability, we develop machine learning interatomic potentials for both metals, carefully trained on dislocation data from density functional theory. Our findings reveal that dislocations undergo harmonic transitions between Peierls valleys, with an activation entropy that remains largely constant, regardless of temperature or applied stress. We use these results to parameterize a thermally-activated model of yield stress, which consistently matches experimental data in both iron and tungsten. Our work challenges recent studies using classical potentials, which report highly varying activation entropies, and suggests that simulations relying on classical potentials-widely used in materials modeling-could be significantly influenced by overestimated entropic effects.