Lang topography of dislocations in cadmium

A method is described of preparing oriented single crystals of cadmium sufficiently perfect for x-ray transmission topography. The method, involving carefully controlled cooling, is thought to have general applicability as a means of achieving low dislocation densities in metals.The main observations of importance are of large (∼50 μm) edge dislocation loops in the basal plane. Contrast effects in 1010 and 1012 reflections indicate that certain loops are of pure edge type with 〈c〉 vectors, others being of 〈c + a〉 type; it is believed that these are the result of vacancy condensation on pre-existing dislocations. Certain loops are observed to shrink over a period of days at room temperature, and the kinetics of this process are consistent with known data on self-diffusion.Other observations more briefly discussed are (i) bars of contrast attributed to unresolved stacks of 〈a〉 vector loops, (ii) contrast effects in sections parallel to (1010), (iii) comparison of dislocation densities estimated by topography and by etch-pitting, (iv) ageing effects in foils, including loop growth in certain areas and a long-term deterioration in crystal quality attributed to small fatigue strains.