Degenerate transition pathways for screw dislocations: implications for migration

TL;DR

This paper investigates the transition pathways of screw dislocations in bcc metals, revealing that common atomistic simulation methods may produce unphysical results due to landscape anomalies, impacting our understanding of dislocation migration.

Contribution

It identifies the cause of unphysical transition pathways in NEB simulations, highlighting the role of monkey saddles in the energy landscape of screw dislocation migration.

Findings

NEB method predicts double-hump energy barriers inconsistent with Ab Initio results.

Monkey saddles in the energy landscape cause unphysical transition pathways.

Implications for screw dislocation motion under stress are discussed.

Abstract

In body-centred-cubic (bcc) metals migrating 1/2<111> screw dislocations experience a periodic energy landscape with a triangular symmetry. Atomistic simulations, such as those performed using the nudged-elastic-band (NEB) method, generally predict a transition-pathway energy-barrier with a double-hump; contradicting Ab Initio findings. Examining the trajectories predicted by NEB for a particle in a Peierls energy landscape representative of that obtained for a screw dislocation, reveals an unphysical anomaly caused by the occurrence of monkey saddles in the landscape. The implications for motion of screws with and without stress are discussed.