Effect of dislocation density on the rhombohedral twinning stress of sapphire (alpha-Al2O3)

The influence of dislocations on the stress to induce rhombohedral twinning has been examined in a-Al2O3 single crystals. Dislocation densities have b een modified by basal slip induced by compression at temperatures between 1 ,000 and 1.300 degreesC, at stresses up to 400 Wa. As a consequence of this pre-deformation, sapphire is more resistant to twinning at 900 degreesC, t he hardening corresponding to a stress increase by a factor of 10. The twin structure is strongly influenced by the dislocations which impede the moti on of twin boundaries; therefore, for high dislocation density very thin tw in lamellae are formed contrary to the case for low density.