LIX. Elementary structure and slip band formation in aluminium

Summary From electron micrographs of deformed high purity aluminium numerical estimates for the spacing, length and amount of glide of the individual elementary lines have been obtained. It is found that the latter two quantities are compatible with the conception that each line is formed by dislocations emitted from one freely acting source of dislocations. The existing experimental evidence relating to the problem whether or not slip starts at the crystal surfaces is discussed. The evidence suggests that the sources of dislocations might well lie at the surfaces and therefore it is considered possible that the elementary structure is confined to a surface layer. The observed spacing of the steps, in the elementary structure as well as in the slip bands, is comparable with the theoretical critical distance below which rows of dislocations cannot pass each other. This indicates that there are many more possible sources than become active. A mechanism for the development of slip bands out of the elementary structure is proposed. This mechanism entails the annihilation of all dislocations in the immediate neighbourhood of an expanding slip line.