Grain-boundary shear-migration coupling. II. Geometrical model for general boundaries

Grain boundary-mediated plasticity is now a well-established phenomenon, especially in fine-grained or nanocrystalline metals. It has been described in several models, but most of these apply to very exclusive configurations such as symmetrical grain boundaries, or grain boundaries that possess a specific coincidence orientation relationship. In real polycrystals, grain boundaries have random orientations, and the current models cannot account for the shear associated with their migration (see part I of this study [1]). The present work presents a model of shear-migration coupling in which grain boundaries do not have specific orientation relationships. It consists in defining couples of shear and rotation values, able to transform one lattice orientation into another, regardless of the type of boundary that separate them. This purely geometrical model can be seen as a generalized formulation of the existing shear coupling theories. No long-range diffusion is involved, but very localized atomic shuffling is generally necessary in the core of mobile interfacial dislocations.