Dislocation substructure changes with stress reversal in polycrystalline OFHC Cu

Abstract

The rearrangement of dislocations in a deforming metal during a stress reversal is thought to contribute to a lowering of the yield stress in the reverse direction. Part of the stress-strain curve of an annealed polycrystalline OFHC Cu specimen, deformed first to -118 MPa in compression and then in tension, is shown in fig.1. The workhardening rate, θp = dσ/dεp where σ is the true stress and εp the true plastic strain, is plotted against |σ| in fig.2. The compressive workhardening rate, for stresses beyond -118 MPa (point A) where the reversal was made, was obtained from a second specimen that was deformed entirely in compression. Figures 1 and 2 reveal the following: (a) the reverse proportional limit (σr) is ∽40 MPa lower than the “forward” peak stress (σf), and it requires ∽1% εp before the reverse flow stress achieves a value equal to the previous foreward maximum; (b) θp-reverse is initially higher than θp-forward, then falls below, and eventually becomes equal to θp-forward at a much higher stress than the peak forward stress.