Modern materials often have multicomponent alloys, whose properties are determined by their phase and structural composition formed as a result of previous thermomechanical processing. Therefore, the problem of managing the structural state occupies an important place in the overall strategy of developing new materials. Particular attention is drawn to phase transformations in ultrafine systems, in particular, in alloys subjected to severe plastic deformation. In this case, the sample size becomes an important parameter determining the physical properties of the substance and, in particular, its structural and phase compositions. Despite the considerable interest of researchers, until recently, it was not possible to achieve significant theoretical results in this area. Therefore, the development of modern models for describing structural transformations in heterogeneous systems and their application to ultrafine-grained materials seem promising and relevant.