A dipole model for the cross-slip of screw dislocations in fee metals

Abstract

An attempt is made to reconcile observations and theories of cross-slip with recent models of cyclic plasticity and the fatigue endurance limit of crystals. According to this view, the stress required to separate two screw dislocations of opposite signs, which are just on the point of mutual annihilation by cross-slip, controls both the stress τ_III at the end of linear (stage II) hardening and the saturation stress [sgrave]_sat in the cyclic stress-strain curve. It is shown, in confirmation of conclusions by many earlier researchers, that direct annihilation of the dislocations cannot account for the temperature dependence of the stress, although it might account for observations on silver and copper at liquid-helium temperature. Based on the work of Hirsch, a model of stress-aided migration of jogs is proposed, which seems able to account for the observations if the activation energy for the migration is about 0.1 eV in copper, nickel and aluminium. A by-product of the model is a possible reinterpretation of the Bordoni peaks in the damping of face-centred metals in terms of jog migration.