A theoretical model is proposed that describes the mechanism of hardening of ultrafine-grained aluminum, obtained by severe plastic torsion deformation, after low-temperature annealing. In the framework of the model, hardening is realized due to the sequential transformation of the grain-boundary dislocation structure. In particular, plastic deformation occurs through the emission of lattice dislocations from triple junctions of grain boundaries containing pile-ups of grain-boundary dislocations, the subsequent sliding of lattice dislocations in the bulk of the grain, and the formation of walls of grain-boundary dislocations climbing along opposite grain boundaries. The energy characteristics and critical stresses for the emission of lattice dislocations are calculated. The theoretical dependences of the flow stress on the plastic deformation are plotted, which show good qualitative and quantitative agreement with experimental data.

A theoretical model which describes a micromechanism of plasticity enhancement in an ultrafine-grained Al-Cu-Zr alloy after annealing and additional deformation is suggested. Within the framework of the model, it was shown that nanoprecipitates of the secondary phase Al2Cu in the grain boundaries become the effective sources of the lattice dislocations in the presence of a large number of the grain boundary dislocations. The theoretical dependences of the flow stress on the degree of the plastic deformation demonstrate good qualitative and quantitative agreement with the experimental data. The emission of the lattice dislocations from nanoprecipitates provides higher plasticity compared to the emission of the lattice dislocations from the triple junctions of the grain boundaries.

A theoretical model which describes the mechanism of pore dissolution at grain boundaries in ultrafine-grained materials during the ageing annealing is suggested. Within the framework of the model, pore dissolution occurs due to the emission of vacancies and the climb of grain-boundary dislocations along the grain boundary towards the pore. It is shown that in this case there is a significant decrease in the total energy of the system. The results of the model are in good agreement with the available experimental observations of pore dissolution during annealing of ultrafine-grained Al-Zr alloy.