It was recalled that 1/3<10•0> partial dislocations played a crucial role in the plastic deformation of sapphire. During deformation at high temperatures, basal slip (1/3<11•0>{00•1}) required the lowest critical resolved shear stress. The 1/3<11•0> perfect dislocations underwent dissociation (probably restricted to the dislocation core) to give 1/3<10•0> and 1/3<01•0> half-partial dislocations. These partials glided on an electrically neutral so-called motion plane within a puckered cation array. The 1/3<10•0> partial also acted as the twinning partial when basal twinning occurred at 600 to 1000C. Twinning occurred when a pinned screw partial, sweeping over the same motion plane, made a complete loop of a micro-twin and then cross-slipped onto the next available motion plane to start twin-thickening. New transmission electron microscopic evidence confirmed several predictions of a new model for basal twinning. Prism-plane slip (1/3<10•0>{1¯2•0}) was the preferred slip system at temperatures below about 600C, in spite of the very large Burgers vector (0.822nm) of the <10•0> dislocation. This occurred because the latter dislocation dissociated into 3 co-linear 1/3<10•0> partials which were separated by 2 relatively low-energy stacking faults. The stacking-fault energy in sapphire was much lower on prism planes than on basal planes. The motion plane for prism-plane slip lay between 2 puckered O layers, but also permitted dislocation motion with no net change.

Slip and Twinning Dislocations in Sapphire (α-Al2O3). A.H.Heuer, K.P.D.Lagerlöf, J.Castaing: Philosophical Magazine A, 1998, 78[3], 747-63