The role of disconnections in deformation-coupled grain boundary migration

Grain boundary (GB) migration under stress has been recognized in recent years as an important plastic deformation mechanism especially in small-grained materials. It is believed to occur via the motion of disconnections along the interface. However, the origin of these disconnections is a key point for a deeper understanding of this mechanism. In this paper, we consider that GB migration under stress can occur both due to the motion of pre-existing disconnections and due to disconnections resulting from decomposition of lattice dislocations interacting with the GB. High-resolution transmission electron microscopy experiments carried out on an aluminum bicrystal with a Sigma 41 < 0 0 1 >{540} GB indeed confirm the existence of different kinds of disconnections and pure steps prior to deformation. In situ straining experiments performed in the same bicrystal at room and high temperatures reveal the rapid decomposition of lattice dislocations in the GB plane. Theoretical investigation of the possible decomposition reactions shows that different types of disconnections with Burgers vector having both glide and climb components, i.e. parallel and perpendicular to the GB plane, can be produced. Disconnections with a small climb component are likely to move along the GB under stress and induce deformation parallel and perpendicular to the GB plane. Concomitant motion of disconnections with Burgers vectors at right angles to the GB plane is believed to produce GB migration coupled with grain rotation. It is also shown that disconnection interactions in the GB lead preferentially to purely glissile disconnections producing a coupling factor in agreement with the observed coupling factor measured in experiments on macroscopic bicrystals. The idea that shear-coupled GB migration can occur by the continuous feeding of lattice dislocations decomposing in the GB during the migration is also investigated. This process is thought to play a role during recrystallization. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.