Atomic processes of shear-coupled migration in {112} twins and vicinal grain boundaries in bcc-Fe

Tilt {112} grain boundaries (GBs) in bcc metals perform shear-coupled grain-boundary migration by the creation and glide of disconnections. Disconnection dipoles may be created at the pristine GB at high stresses or may be generated at the core of a GB dislocation that acts as a source of disconnections. We characterize this source in terms of its Burgers vector, denoted (b) over right arrow (1/-1), and describe the mechanism that allows the source to move conservatively with the GB. The (b) over right arrow (1/-1) grain-boundary dislocation is created, for instance, during the absorption of a crystal dislocation by the {112} grain boundary. In addition, (b) over right arrow (1/-1) accommodates {112} vicinal grain boundaries that are formed by segments of {112} planes separated by (b) over right arrow (1/-1) grain-boundary dislocations. The presence of these (b) over right arrow (1/-1) dislocations facilitates the conservative displacement of both the pristine and the vicinal GBs. We show that the creation of disconnections is the key for the absorption of edge and screw dislocations by the GB and the drag of mixed dislocations by the GB during its migration. These conservative processes are efficient ways to accommodate plastic deformation by the growth and shrink of {112} twins, and shear-coupled motion of the {112} GB and its vicinal GBs.