Hardening of rhombohedral twinning in sapphire (α-Al2O3) by basal slip dislocations

Sapphire is weak at medium temperatures because rhombohedral twins can propagate under low shear stresses tau. The introduction of a dislocation forest improves the strength of sapphire: dislocations are obstacles to twin propagation. We have measured the increase in tau with the increase in dislocation density. The hardening is essentially due to the energy change associated with the incorporation of dislocations in the twins. The microstructure observed by optical microscope is compatible with the current understanding of the mechanism for twin propagation.