SLIP AND TWINNING DISLOCATIONS IN SAPPHIRE (ALPHA-AL2O3)

The 1/3[10 (1) over bar 0] partial dislocation plays a crucial role in the plastic deformation of sapphire (alpha-Al2O3). During deformation at high temperatures, basal slip (1/3(11 (2) over bar 0)(0001)) has t he lowest critical resolved shear stress; 1/3[11 (2) over bar 0] perfe ct dislocations undergo dissociation (which is probably restricted to the dislocation core) to 1/3[10 (1) over bar 0] and 1/3[01 (1) over ba r 0] half-partial dislocations. These partials glide on an electricall y neutral motion plane within a puckered cation array. The 1/3[10 (1) over bar 0] partial also acts as the twinning partial when basal twinn ing occurs at intermediate temperatures, say 600-1000 degrees C. Twinn ing occurs when a pinned screw partial sweeps out on this same motion plane, forms a complete loop of a microtwin and then cross-slips onto the next available motion plane to permit twin thickening. New transmi ssion electron microscopy evidence is presented, confirming several pr edictions of this new model of basal twinning. Prism plane slip ([10 ( 1) over bar 0] {1 (2) over bar 10}) is actually the preferred slip sys tem at lower temperatures (below about 600 degrees C). in spite of the very large Burgers vector of the [10 (1) over bar 0] dislocation, 0.8 22 nm. This occurs because this dislocation dissociates into three col inear 1/3[10 (1) over bar 0] partials, separated by two relatively low -energy slacking faults. (The stacking-fault energy in sapphire is muc h lower on prism planes than on basal planes.) The motion plane for pr ism plane slip is between two puckered oxygen layers but also permits dislocation motion with no net change.