The microstructure and mechanical properties of ultrafine-grain (UFG) commercial-grade Al obtained by equichannel angular pressing (ECAP) are study in the temperature range 4.2–295K. Transmission electron microscopy and x-ray diffraction methods are used to show that as the number of passes increases, the grain size decreases, the grain shape becomes increasingly equiaxial, and the dislocation density inside a grain and the character of the intergrain boundaries change. An increase of the coherent scattering region and a decrease of the level of microdeformations indicate that pressing decreases the total density of imperfections of the crystal structure inside grains. As temperature decreases, the yield stress, plasticity, and strain hardening rate of UFG and coarse-grain polycrystals increase substantially. The deformation of UFG polycrystals at 4.2K becomes unstable (abrupt). The temperature dependences of the yield stress σy(T) of UFG and coarse-grain polycrystals, where the form of these dependences is characteristic for thermally activated detachment of dislocations from short-range potential barriers, are studied. The differences observed in the dependences σy(T) for UFG polycrystals are explained by a change in the nature of such barriers and the mechanism by which glide dislocations overcome them. The dependences of the yield stress σy and the coefficient of strain hardening θ on the grain size d are obtained. It is determined that the Hall–Petch relation describes the function σy(d) in the temperature interval 4.2–295K. The Hall–Petch coefficient increases as temperature decreases. The function θ(d) is monotonically decreasing at 295 and 77K but θ is independent of d at 4.2K. The experimental data are discussed within the framework of existing notions about the influence of the microstructure produced by ECAP on the evolution of the dislocation density during subsequent deformation.
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.
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