Dislocation behaviour in sapphire single crystals

Abstract

Abstract The investigation by x-ray transmission topography of large sapphire crystals, melt-grown by the method of Schmid and Viechnicki (1970), revealed an arrangement of basal dislocations generally described as tangles. Since present theories could not fully account for the tangle formation in sapphire, a mechanism involving dislocation reactions is suggested as a possible explanation. Helices were generated by annealing thin sapphire plates close to the melting temperature. The axes of the helices were found to be parallel to the Burgers vector, i.e. parallel to directions with no preference for any particular one. The helices observed possessed relatively constant pitch and diameter. The average diameter of the helices was ∼ 50 μm. Mechanisms for the formation of cusp-dislocations are discussed. Some of the helical dislocations were found to transform into loops, offering evidence for the loop formation in a material where the collapse of vacancy discs into loops might not occur.