Electron microscopy and diffraction of synthetic corundum crystals I. Pure aluminium oxide grown by the verneuil process

Abstract

Single crystal aluminium oxide has been chemically thinned with the aid of a jet polishing device and examined in the electron microscope. The behaviour of the dislocations in various plastically deformed crystals has been examined. Tangles of dislocations, commonly found in metals and ionic solids, are absent, and dislocation densities in glide bands are low. Grown-in dislocations apparently participate in the glide process; therefore, they are not strongly pinned by impurities at temperatures permitting plastic deformation. The occurrence of dislocation dipoles is taken as evidence for the existence of cross slipping of basal dislocations at temperatures below those required for slip on {1120} prism planes. No definite evidence for dislocation dissociation has been found and it appears that, normally, such splitting is rare in samples of bulk sapphire. Preliminary studies of dislocation motion in the microscope at temperatures ∼1500°c are reported and discussed.