Solid solution hardening of sapphire (α-Al2O3)

Sapphire crystals doped with Cr and Ti (the latter as Ti3+ or Ti4+) have been deformed by basal slip at temperatures between 1300 and 1700 °C. For the isovalent solutes Cr3+ and Ti3+, the concentration dependence of the yield stress can be explained in terms of solution hardening theories based on obstacles causing symmetric strain fields due mainly to the solute size difference. For the aliovalent solute Ti4+, the solution hardening rate is much higher and can be explained in terms of the interaction of dislocations with charge-compensating defect complexes; it is shown that the elastic (tetragonal distortion) interaction is considerably more important than the electrostatic dipole interaction.