Some Observations on Fatigue in Copper Single Crystals

Abstract

The surface features and dislocation substructures present in copper single crystals subjected to slow cyclic plastic shear strains of ± 1% are reported and discussed. Thin-film electron microscopy has revealed that at no time during the hardening stage are dislocations grouped into a uniform, three-dimensional cell structure; they are arranged in dislocation walls on the glide plane, the plane normal to the primary b, and a plane at ∼ 45° to both these. After cycling at saturation the crystal breaks down into two regions, one exhibiting a misorientated, three-dimensional cell structure and the other showing an intensification of the regular structure generated during the hardening stage. No difference was observed between the dislocation structure at the specimen surface and that in the interior. From optical and scanning electron microscopy studies of the surface it is postulated that slip at saturation occurs preferentially in one of the two regions observed. The marked similarity between the regular structure described and the structure of persistent slipbands, seen in conventional low-stress fatigue tests, suggests that these regions carry most of the saturation strain.