The occurrence of dislocations in crystals grown from themelt

Abstract

Single crystals of aluminium were grown by the Czochralski method at various rates. The crystals were carrot-shaped, about 4 cm long, tapering from a diameter of about 3 mm at the top to about 0·1 mm at the bottom. X-ray diffraction micrographs were taken of these specimens by means of Lang's technique, which is capable of revealing the dislocation structure in the interior of a crystal. It was found that the density of dislocations present in an as-grown crystal increases with both increasing diameter and rate of growth. For sufficiently small diameters (below about 0·3 mm) it was possible to grow crystals free of detectable dislocations. These experimental results are consistent with the idea that the majority of the initial dislocations are produced by the collapse of vacancy discs that are nucleated when a sufficient supersaturation of vacancies is reached in the crystal. Theoretical calculations of the supersaturation of vacancies that arise during crystal growth were made by considering the diffusion of these imperfections in a specimen cooled from the melting temperature. It is shown that the range of diameters below which the vacancy supersaturation is too small to nucleate a dislocation is of the same order of magnitude as the diameters of crystals found that contain no detectable dislocations.