It is well known that severe plastic deformation (SPD) leads to considerable structure refinement in the processed materials. The obtained nanostructured materials are characterized by equiaxial grains of small size and high-angle grain boundaries. These features are responsible, in particular, for their outstanding mechanical properties .
However, there is still no unified theoretical approach to describe mechanisms which control the structure refinement at SPD. To assist solving this important problem, in the present work it is suggested to use a physically-based energetic criterion for grain subdivision which is formulated in terms of disclination theory. This model is based on the assumption that during SPD grain splits into a number of substructure elements in order to adjust the total system energy by decreasing the local contribution related to individual grains. Using disclination approach allowed to calculate the energy of a probe grain caused by its deformation under given deformation conditions. The grain believed to be divided in a number of subgrains if the split configuration possessed less energy than initial one. The capabilities of the proposed model for the structure refinement description (decrease in grain size and misorientation growth) are demonstrated for the case of deformation by equal-channel angular pressing. It is planned to implement the developed criterion with self-consistent viscoplastic model  in order to describe deformation behavior and microstructure evolution during SPD (equal-channel angular pressing).
 R.Z.Valiev, R.K.Islamgaliev, I.V.Alexandrov, Bulk nanostructured materials from severe plastic deformation, Prog. Mater. Sci., vol. 45, 103-189 (2000).
 I. J. Beyerlein, R. Lebonsohn, Carlos Tome, Modeling of texture and microstructural evolution in the equal channel angular process, Mater. Sci. Engng A. 345, 122-138 (2003).