It was recalled that sapphire was weak at intermediate temperatures because rhombohedral twins could propagate under a low shear stress. The introduction of a dislocation forest improves the strength of sapphire; dislocations were obstacles to twin propagation. The increase in shear stress with increasing dislocation density was measured. The hardening was essentially due to the energy change associated with the incorporation of dislocations in the twins. The microstructure observed by optical microscopy was compatible with the current understanding of the mechanism for twin propagation.

Hardening of Rhombohedral Twinning in Sapphire (α-Al2O3) by Basal Slip Dislocations. J.Castaing, A.Munoz, A.D.Rodriguez: Philosophical Magazine, 2002, 82[7], 1419-31