Unstable plastic deformation of ultrafine-grained copper at 0.5K

We investigate the relation between the strain-hardening rate and flow instability of polycrystalline Cu-OF deformed by tension at a constant rate in a liquid 3 He atmosphere. The microstructure of the ultrafine-grained crystal, obtained by the equal-channel angular hydro-extrusion method, was varied by annealing at recovery and recrystallization temperatures and was monitored by x-ray diffraction. It is shown that that the flow instability, manifesting itself as macroscopic stress serrations on the tension curve, appears at a threshold tension sufficient for activation of a dynamic recovery that leads to a decrease of the strain-hardening coefficient. We discuss the effect of grain size and the initial dislocation density on the evolution of the dislocation structure that determines the scale and the statistical properties of the flow instability in the investigated crystals at low temperature. Published by AIP Publishing.