Piling-up of Dislocations beneath a Surface Film

Abstract

THE concept of a surface film obstructing the egress of dislocations from a crystal and thereby causing them to pile-up beneath the film has proved useful in explaining the results of several metallurgical experiments^1,2. However, direct evidence for such a phenomenon has been lacking. During an investigation to study some aspects of the effects of surface coatings—oxides, metallic and surface-active films, etc.—on the mechanical behaviour of materials, several examples of piled-up groups of dislocations have been observed. Fig. 1 shows a pile-up of edge dislocations beneath a surface coating in lithium fluoride. The coating was formed by reaction of the crystal with pure magnesium oxide powder at 600° C.; the dislocations were revealed by the techniques developed by Gilman and Johnston³.