The plastic deformation of sapphire was studied, under hydrostatic confining pressures, at temperatures which were below the ambient-pressure brittle-ductile transition temperature. Samples which were oriented for prism-plane slip (type-I) were deformed via dislocation slip at temperatures which were as low as 200C. Samples which were oriented for basal slip (type-II) could be plastically deformed at temperatures of as low as 400C, but exhibited a more complicated deformation behavior in that the sample orientation also allowed for the activation of basal twinning and 2 of the 3 rhombohedral twin systems. The temperature dependence of the critical resolved shear stress, ln[] = ln[o] - BT, for basal slip was significantly greater than that for prism-plane slip (Bbasal > Bprism) and caused the latter system to be the easy-slip system below about 600C. Basal slip was the easy-slip system at high temperatures. Type-II samples deformed mainly via basal twinning rather than via rhombohedral twinning and basal slip. The differing temperature dependences of  for basal and prism-plane slip were attributed to the details of the dislocation core structure. Prism-plane dislocations, with a Burgers vector of 0.822nm, could dissociate into 3 co-linear partials (with a Burgers vector of 0.274nm) that were separated by relatively low-energy stacking faults. The equivalent dissociation of basal dislocations (Burgers vector of 0.476nm) produced 2 non co-linear partial dislocations that were separated by a relatively high-energy stacking fault. It was concluded that the dissociation of basal dislocations was most likely to be restricted to the dislocation core. This was reflected by a higher Peierls stress at low temperatures, for basal slip as compared with prism-plane slip.

K.Peter, D.Lagerlöf, A.H.Heuer, J.Castaing, J.P.Rivière, T.E.Mitchell: Journal of the American Ceramic Society, 1994, 77[2], 385-97