Nucleation of extended defects in BCC transition metals

Abstract

Critical thickness of deformation twins in body centered cubic transition metals have been a topic of debate with many computational and experimental studies accepting a three-layer twin thickness based on nucleation from a screw dislocation without proof whereas recent in-situ experiments suggest six-layer thick twin nuclei observed via HRTEM. In this study, we settle this debate by conclusively determining twin nuclei thickness in these metals using atomistic simulations to examine atomic structure and energetics of deformation twins under nonzero finite pure shear stresses. Our study reveals that twins in BCC transition metals nucleate as two-layer thick nuclei under stress as opposed to the three-layer thick twin nuclei proposed in the literature. As they grow and stress is relieved, they will adopt configurations that are much closer to the zero stress stability predictions. This study also reveals that the atomic structures of the twin nuclei also depend on individual BCC transition metal rather than universally maintaining the more commonly acknowledged pure reflection structure.