Formula omitted] dislocations in [formula omitted] twins in Mg: A kinematic and energetic requirement

Abstract

Consideration of published TEM and molecular dynamics studies of interactions between matrix dislocations and {101¯2} twin boundaries (TB) in hexagonal close packed metals inspires a hypothesis regarding the formation of dislocations with <c+a> Burgers vectors inside the twins. It was previously reported that when basal <a> dislocations from the matrix are swept by an advancing TB, they are converted into partial dislocations bounding basal I1 stacking faults (SFs) within the twin. In the present work, we show that this dislocation configuration near TB renders it energetically favorable for a pair of partial dislocations to constrict into a <c+a> dislocation within the twin. New in situ TEM observations confirm this hypothesis by revealing long <c+a> dislocation dipoles terminated by the TB at one end and by I1 SFs within the twin at the other end. Finally, the resulting near-screw <c+a> dislocation dipoles are observed to be glissile, enabling self-annihilation in some circumstances. Thus, rather than requiring a high energy dislocation pile-up to press two <a> dislocations through a TB into a single <c+a> dislocation, it is demonstrated that <c+a> dislocations naturally form, within {101¯2} twins in Mg, as TB migrates into a matrix, which inevitably contains dislocations with <a> Burgers vectors. These observations help to explain how the twins themselves may subsequently deform and why they may be particularly vulnerable to plastic, shear localization.