Determination of Pipe Diffusion Coefficients in Undoped and Magnesia‐Doped Sapphire (α‐Al ₂ O ₃ ): A Study Based on Annihilation of Dislocation Dipoles

The breakup of dislocation dipoles in plastically deformed samples of undoped and 30‐ppm‐MgO‐doped sapphire (α‐Al 2 O 3 ) was monitored using conventional TEM techniques. Dislocation dipoles break up into prismatic dislocation loops in a sequential process during annealing; i.e., dislocation loops are pinched off at the end of a dislocation dipole. This pinch‐off process is primarily controlled by pipe diffusion, and pipe diffusion coefficients at temperatures between 1300° and 1500°C were estimated by monitoring the kinetics of the dipole breakup process. We determined D P U = 8.1( –4.3 +9.1 ) × 10 –3 exp [–(4.5 ± 1.3 eV )/ kT )] m 2 /s for the undoped material. The pipe diffusion kinetics for the MgO‐doped crystal was determined at 1250° and 1300°C and was about 6 times higher than for undoped sapphire. Finally, climb dissociation of the dislocations constituting the perfect dipoles in sapphire is common; annihilation of one set of partials can result in the formation of faulted dipoles, which can pinch off to form faulted dislocation loops. D P U for faulted dipoles in the undoped material was determined at 1300° and 1350°C, and was about 4–10 times higher than for perfect dipoles.