Control of dislocation configuration in sapphire

A method to introduce high density of unidirectional dislocations in sapphire by a "two-stage deformation" technique and a heat-treatment of thin plate is proposed. After deformation of sapphire crystals by basal slip at 1400 degreesC, the samples were further deformed at 1200 degreesC as a second stage. It was found that twinning and cracking normally observed during deformation tests at 1200 degreesC were not observed in the crystals deformed using the two-stage deformation technique, and dislocation densities as high as 1.1 x 10(9)/cm(2) were obtained. This is because the glide dislocations generated at 1400 degreesC promoted slip over twinning in the additional deformation at 1200 degreesC. Subsequently, thin plates containing the high density of dislocations were annealed at 1400 degreesC for 30 min. As a result, it was found that dislocations in the thin plate were able to be straightened across its two opposite surfaces. This is believed to be due to the large image forces acting on the dislocations due to the free surface. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.