The initiation of plastic flow in copper

Abstract

The continuous shear stress shear strain behavior for tubular polycrystalline specimens of copper and its dilute alloys deformed in torsion at room temperature was studied using a strain resolution of the order of 10 −9. Irreversible plastic deformation began at the smallest measureable stresses. The small but measurable apparent elastic regions observed in all specimens very near zero applied stress are thought to be associated with the small unrecoverable prestrains produced during the handling and gripping operations. These apparent elastic regions were completely absent in specimens deformed following the introduction of fresh, mobile dislocations into their gage section surfaces. The addition of solute concentrations up to 0.1 at % aluminum in copper had little effect upon the movement of these freshly introduced dislocations. Polycrystalline copper strain hardens readily at room temperature and exhibits a discontinuous plastic behavior only following prior straining. The anelastic unloading behavior is also demonstrated for annealed copper specimens. It is suggested that plastic deformation starts by the motion of a few favorably oriented dislocation segments that can glide over great enough distances to form strong new interactions with other dislocations.