The Bauschinger effect, work-hardening and recovery in dispersion-hardened copper crystals

Abstract

Single crystals of copper containing dispersions of small Al₂O₃ particles have been deformed in tension and subsequently in compression at 77°K and the nature of the stress-strain curves and the dislocation structure have been investigated. As predicted, a large Bauschinger effect is observed, but this is substantially reduced by annealing at 295°K prior to reverse straining, and effectively disappears after an anneal at 423°K. The number of Orowan loops produced during the tensile deformation is calculated from the magnitude of the Bauschinger effect, and from the amount of recovery in tension. These analyses suggest that at 77°K the number of Orowan loops around particles increases with increasing strain, reaching a limit of ∼3–5 loops after a few percent strain, further dislocation-particle interactions producing primary prismatic loops by cross-slip. The effect of these processes on the flow stress is calculated and Orowan loops are found to contribute substantially to the work-hardening, particularly during the first few per cent strain. The static recovery effect, and the strong temperature dependence of the work-hardening rate of these alloys, are both interpreted in terms of the climb and disappearance of Orowan loops by a process of pipe diffusion.