ON BASAL SLIP AND BASAL TWINNING IN SAPPHIRE (ALPHA-AL2O3) .2. 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. I t is shown that deformation twinning is a natural outgrowth of the pre sent model of slip when the material is deformed in a particular regim e of temperature and stress. The model is based on the glide of a lead ing 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 s hear, eta(1), = [10 (1) over bar 0], and lying in the twin/matrix inte rface.