The break-up of dislocation dipoles in plastically deformed samples of undoped and 30ppmMgO-doped sapphire was monitored by using conventional transmission electron microscopic techniques. The dislocation dipoles broke up into prismatic dislocation loops in a sequential process during annealing. That is, the dislocation loops were pinched off at the end of a dislocation dipole. This pinch-off process was controlled mainly by pipe diffusion, and the pipe-diffusion coefficients at 1300 to 1500C were estimated by monitoring the kinetics of the dipole break-up process. The pipe-diffusivity in undoped material could be described by:

D (m2/s) = 8.1 x 10-3 exp[-4.5(eV)/kT]

The pipe-diffusion kinetics in MgO-doped material were determined at 1250 and 1300C, and were some 6 times faster than those in undoped sapphire. Climb dissociation of dislocations which constituted perfect dipoles in sapphire was common. The annihilation of one set of partials could result in the formation of faulted dipoles, which could pinch-off to form faulted dislocation loops. The pipe diffusivity for faulted dipoles in the undoped material was determined at 1300 and 1350C, and was to be 4 to 10 times higher than that for perfect dipoles.

Determination of Pipe Diffusion Coefficients in Undoped and Magnesia-Doped Sapphire (α-Al2O3) - a Study Based on Annihilation of Dislocation Dipoles. X.Tang, K.P.D.Lagerlöf, A.H.Heuer: Journal of the American Ceramic Society, 2003, 86[4], 560-5