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The temperature and rate sensitivity of the flow stress in ultrafine-grained (UFG) copper prepared by equal-channel angular hydroextrusion was studied. Tensile tests and stress relaxation tests were carried out in the temperature range of 77 to 295 K. It was established that, with increasing temperature, the flow stress decreases monotonously, and the activation volume reaches a maximum of ∼190b3 at a temperature of ∼(200 ± 20) K. The results of the experiment are discussed in terms of two thermally activated plastic deformation mechanisms, namely the intersection of "forest" dislocations and dynamic recovery, which are capable of significantly distorting stress relaxation kinetics in UFG copper. [ABSTRACT FROM AUTHOR]

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 3He 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. [ABSTRACT FROM AUTHOR]