On Basal slip and basal twinning in sapphire (α-Al 2O 3)—II. A new model of basal twinning

Based on a new model for basal slip in sapphire developed in Part I of this series of papers, the formation of a basal twin is considered. It is shown that deformation twinning is a natural outgrowth of the present model of slip when the material is deformed in a particular regime of temperature and stress. The model is based on the glide of a leading partial dislocation on a basal slip plane and double-cross-slip to the next-but-one slip plane. The model predicts a Type-II twin, i.e. a twin related to the matrix by a 2-fold axis along the direction of shear, η 1 = [101¯0〉, and lying in the twin/matrix interface.