The roles of grain boundary dislocations and disclinations in the nucleation of {10(1)over-bar12} twinning

Consider a magnesium bicrystal created from two single crystal lattices referred to as black and white. The bicrystal abuts a basal plane of the black crystal with a first-order prismatic plane of the white crystal. At zero stress, the boundary relaxes by developing coherent terraces, while two types of disconnection dipoles and a misfit dislocation above each dipole nucleate, in combination, to remove the attendant long-range elastic strains and maximize local coherency. When the bicrystal is stretched normal to its semicoherent interface, stress motivates the misfit dislocation at the far side of the boundary to glide and annihilate at the triple point. The removal of this bounding misfit dislocation allows the positive step of the disconnection dipole to conservatively move away. The negative step of this dipole, however, remains sessile at the grain boundary as it cannot glide conservatively. This sessile step facilitates nucleation of a new disconnection dipole. The first process of disconnection dipole widening then repeats, and the new residual negative step coalesces with the first one to form a novel disconnection of double step height. The whole process cycles once more, and two of these double-height disconnections pile-up at the boundary. The pile-up quickly relaxes into a coherent boundary along the {10 (1) over bar2} plane; a {10 (1) over bar2} twin facet thus nucleates, and a disclination dipole arises to bound the twin facet at the two junctions. Soon thereafter, the lower disclination, at the opposite side from where the misfit was removed, begins emitting twinning disconnections toward the upper disclination. These twinning disconnections caused their source junction to recede in favor of an overwhelming lengthening of the twin facet. All twinning disconnections end up moving through the upper disclination, thereby undergoing a dislocation transformation event. The new transformed disconnection glides freely along the remaining basal-prismatic boundary segment, and enables this boundary to be dragged with the propagating twin facet. This steady stage proceeds until the entire bicrystal transforms in to a single twinned crystal. This paper examines the fundamentals of these twin nucleation mechanisms at the grain boundary on the basis of the interfacial defect theory advanced by Pond and co-workers [1,2]. It introduces an important twin nucleation mechanism with dynamics of twin faceting in relation to an important asymmetric low-energy boundary in hexagonal close-packed materials. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.