A theoretical model is suggested which identifies processes whose competition controls a minimum saturation grain size achievable in ultrafine-grained materials and Gum-metals (special titanium alloys) processed by severe plastic deformation. Within this model, typical values of the minimum grain size in various materials are estimated. The estimation is based on (i) the representations that grain refinement in deformed metals occurs through evolution of ensembles of lattice dislocations, and (ii) the experimentally established fact that grain growth in ultrafine-grained metals under severe plastic deformation is typically controlled by stress-induced migration of grain boundaries. Obtained estimates of minimum grain sizes are well consistent with corresponding experimental data.