Impact of deformation faceting on [formula omitted] and [formula omitted] embryonic twin nucleation in hexagonal close-packed metals

Abstract

A crystal boundary under stress may develop pile-ups of disconnections. The augmented distortion field around the pile-up may relax by forming a low-energy boundary, but one which does not require a significant change in misorientation. In deformation faceting, the change in misorientation is mediated by an interfacial wedge disclination dipole, while disconnection content persists if the new facet preserves the incompatibility of the original disconnection steps. Any disconnection gliding on one facet may enter and glide along the adjacent facet (cross-facet), provided it transforms across the disclination barrier into a mobile defect. {101¯2} and {101¯1} are special twin boundaries in hexagonal close-packed metals that can convert slip dislocations into disconnections which tend to pile-up and relax via facet nucleation. Thus, these twins may facet into other mobile boundaries which enable cross-faceting as a way to absorb and overcome slip barriers without sacrificing much of their mobility. In this paper, we provide a detailed characterization of the low-energy boundaries, disclinations and disconnection transformations that may separately intervene in {101¯2} and {101¯1} twin faceting, with extensions to {101¯3} twinning. We further show that this faceting promotes formation of bulk twin embryos and facilitates their maturity into large dimensions appropriate for lenticular twin propagation. We apply the trichromatic complex concept, which we recently introduced for {101¯2} twin faceting.