Twinning and fracture in zinc single crystals

Abstract

Tensile and fatigue tests have been performed on zinc single crystals oriented with the basal plane parallel to the stress axis. In tensile tests twinning occurred at lower stresses at 20 or 77°K than at room temperature. {1122} slip, which had preceded twinning, was prominent on room temperature specimens, but was observed only rarely on 77°K specimens, and never on 20°K cases. All fractures were associated with twinning. In push-pull fatigue tests no specimens failed at 20 or 77°K if they survived the initial stress build-up. It appeared that fracture occurred when the twinning stress was reached in a tensile half-cycle, the compression half-cycles having little effect. At room temperature, however, failures could be produced during steady cyclic stressing. Detailed observations showed that neither tensile nor fatigue failures could be attributed to a fracture mechanism, involving the junction of twin tips. which was proposed by previous investigators. In an Appendix a description is given of a twinning dislocation mechanism which is consistent with the present observations. The specimen geometry precluded gauge length basal slip, and ‘longitudinal’ cracks (i.e. in the matrix basal plane) did not occur. It is suggested that this supports a previous hypothesis that such longitudinal cracks are nucleated by pyramidal-basal slip interactions rather than a mechanism involving twins.