Abstract <p>A theoretical model is proposed to describe the micromechanism of plastic deformation in an ultrafine-grained Al-Mg-Zr alloy structured by high-pressure torsion (HPT) with grain boundary segregations of Mg atoms formed during HPT. In the model, plastic deformation is realized due to the emission of lattice dislocations from triple junctions of grain boundaries (GBs), which contain arrays of extrinsic grain boundary dislocations that are pinned by Mg atoms segregated at GBs. These segregations act as obstacles to gliding of extrinsic grain boundary dislocations, thus hindering the formation of dislocation pile-ups near the triple GB junctions and reducing the stress concentration at them, which leads to significant strengthening of the alloy. This model is used to calculate the yield strength of the ultrafine-grained Al-Mg-Zr alloy after HPT and after additional thermomechanical treatment consisting of low-temperature annealing and slight deformation by HPT. An increase in the alloy plasticity due to such thermomechanical treatment is discussed. The proposed model agrees well with the available experimental data.</p>