The plastic deformation of AZ31 magnesium alloy under tension at temperatures of 4.2–295K is studied as a function of its microstructure following squeeze casting (SC) and after severe plastic deformation (SPD) by hot rolling and equal-channel angular pressing. SPD reduces the average grain size and creates a texture that favors basal-plane dislocation glide. It is found that plastic deformation becomes unstable (serrated) at temperatures of 4.2–25K and more stress jerks occur in the SPD polycrystal than in the SC alloy. The temperature dependence of the yield stress of the alloy is typical of thermally activated unpinning of dislocations from short-range barriers. The ratio of the yield stresses for the SPD and SC alloys at a given temperature is explained by hardening owing to a reduction in grain size and softening owing to a favorable texture. As the grain size is reduced, the rate of strain hardening of the alloy falls off, but its ductility (strain to fracture) increases because of the texture. The strain rate sensitivity of the alloy for T⩽100K is independent of microstructure and is determined by intersections with forest dislocations. As the temperature is raised over 150–295K the strain rate sensitivity becomes greater owing to activation of dynamic recovery and an enhanced contribution from diffusion processes during plastic deformation of micrograined materials.

This is a study of the effect of microstructure created by severe plastic deformation (SPD) and annealing on strain hardening and jump-like deformation in Al-Li alloys. It is shown that under tension at 0.5 K, SPD processed polycrystals retain a significant strain hardening rate and have high strength and ductility. SPD also simulates unstable (jump-like) flow of the polycrystals owing to dislocation dynamics that shows up as stress jumps in the tension curve. The average amplitude of the jumps increases with strain, while the dislocation amplitude distribution corresponds to collective motion of dislocation avalanches with a distinctive scale. Jump-like deformation is partially suppressed by high-temperature annealing, while the distribution of the jump amplitudes is described by a power law. The relationship established between the coefficient of strain hardening and the average stress jump amplitude suggests a common dislocation dynamic for strain hardening and jump-like deformation at low temperatures. The observed features of low-temperature plastic deformation are treated as a consequence of changes in the grain sizes and density of dislocations owing to SPD and annealing.

The influence of the grain size on the dislocation interaction mechanisms that control the deformation of aluminum in the temperature range 4.2–295K is studied. For this, samples of coarse-grain (CG) and ultrafine-grain (UFG) aluminum obtained by equi-channel angular pressing were deformed by stretching at constant rate ε̇ as well as in the rate-cycling regime along the deformation curve. The effects of temperature on strain hardening and the stain-rate sensitivity of the flow stress of CG and UFG material are compared. It is shown by means of thermal-activation analysis of the experimental data that the dependence of the rate sensitivity parameter m=[∂lnσ∕∂lnε̇]T on the grain size and temperature of the aluminum is explained by a change of the dislocation mechanisms controlling the plastic deformation of the aluminum. In the temperature interval 4.2–40K the plastic deformation of the UFG and CG aluminum is due to a single mechanism of intersection of forest dislocation. In the interval 120–295K for CG and 77–140K for UFG aluminum the increase in the parameter m is explained by the activation of the mechanism of transverse slip of dislocations. At temperatures above 140K the high rate sensitivity of the stress and low rate of strain hardening of UFG aluminum can be due to the activation of the grain-boundary diffusion and grain-boundary slip-through.