Abstract
{101̅2} twinning is an important deformation mechanism of Mg alloys and, in fact, the most common twinning mode in all hexagonal metals. Under appropriate loading, these twins grow pervasively, consuming large volume fractions of the material while the migrating twin boundaries (TBs) undergo extensive interactions with dislocations. As such, the deformation behavior of the twins can dominate the subsequent stress-strain response of the polycrystalline aggregate. The present work focuses on investigating the interactions between basal dislocations within the surrounding matrix and (101̅2) TBs, with the ultimate aim of understanding the influences these interactions have on defects within the twin and on TB migration. The crystallography-based notion that 2 matrix dislocations with [a1] or [a3] Burgers vector can react to form a single dislocation with [c-a2] or [c+a2] Burgers vector within a (101̅2) twin and a residual twinning dislocation at the TB was confirmed, by post-mortem transmission electron microscopy (TEM) analyses. The twins are shown to contain abundant non-basal [c] and dislocations and basal I1 stacking faults (SF) in the vicinity of the TB.
To further elucidate the dislocations transformation process, in situ tension experiments were performed in a TEM. The impingement of gliding dislocations on a static, unloaded TB was observed to induce local twin growth, with profuse basal I1 SF in the wake of the advancing interface. In combination with molecular dynamics simulations, this evidence permitted the following unit process to be formulated. Each [a] dislocation in the matrix transforms to a sessile partial dislocation, which is trailed by an I1 SF in the twin, with a residual dislocation left behind at the interface. As such, this individual reaction is a generalization of the classic Basinski mechanism, which was originally formulated for face centered cubic metals. The subsequent transformation of a second [a] dislocation can lead to termination of the SF. The net Burgers vector of the two sessile partial dislocations is , and the residual interfacial defects add to a unit twinning dislocation (disconnection). The observed twin growth was quantitatively evaluated and could be attributed completely to the dislocations transformation.
Finally, a formation mechanism for perfect dislocations was further proposed, with supporting evidence provided by additional in situ TEM results. TB migration is identified as a necessity and creates a thermodynamically favorable condition for the constriction of partial dislocations to form a single, perfect dislocation with large Burgers vector. The characteristic defects in the (101̅2) twins and the formation mechanisms reported in this study will contribute to the development of physically-based crystal plasticity models and the full field modeling of the twinning dominated mechanical response of hexagonal metals.
