Stability of the work hardened state against stress reversal in copper single crystals

Abstract

The response of work hardened specimens of copper single crystals to the reversal of the loading direction was studied. Specimens were stressed successively in tension and in compression, and the change in dislocation distributions was observed using an etch pit technique. After the reversal of the load, deformation proceeds with approximately the same flow stress and the same hardening rate as the preceding deformation. This observation indicates that the dislocation arrangements responsible for the flow stress are stable and give rise to the same flow stress for both the directions of deformation, and that the arrangements are developed additively irrespective of the loading directions. The most characteristic arrangements of dislocations in the etch pit observations, “streak-like arrays” of pits, are stable against stress reversal and increase in number additively irrespective of the loading directions, which suggests that this arrangement should be closely related to the flow stress. On the other hand, glide polygonization is unstable. The stability of other characteristic dislocation arrangements was also examined. These results have been compared with current models of work hardening. A model assuming highly directional arrangements of dislocations to be responsible for the flow stress, such as the long range stress theory, cannot be compatible with the present observations.