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Based on the nonequilibrium evolution thermodynamics, the structure refinement of metals during severe plastic deformation is investigated. To describe the formation of stationary (limiting) submicrocrystalline or nanocrystalline structures, a two-defect approximation, including the grain boundaries and dislocations, is used. Introduction of the additive noise for the main parameters into the governing equations allowed us to describe the self-consistent behavior of structural defects during the stationary structure formation. Conditions of achieving the stationary state are investigated and possible scenarios of structure refinement modes are determined. Obtained grain boundary density distributions help to estimate the composition of grain structure in the bulk of metal sample quantitatively. The analysis of time evolution of the grain boundaries density shows the presence of correlated fluctuations. The autocorrelation function, describing the frequency characteristics of the structure refinement, is determined.

The special approach within nonequilibrium evolutional thermodynamics is developed. Considering two-defect approximation, which takes into account the densities of grain boundaries and dislocations, this theory describes the evolution of defect subsystems of polycrystalline materials and the formation of stationary submicrocrystalline or nanocrystalline structures. The influence of additive noise of basic parameters allows to describe more accurately the self-consistent behavior of considered structural defects and to provide the possibility of the existence of new fragmentation modes during severe plastic deformation. The phase diagram is constructed. The conditions of simultaneous existence of two limiting structures that correspond to the regime of the phases formation with different grain sizes are found. The distribution of the realization of grain boundaries density helps to estimate quantitatively the composition of grain structure over the volume of metal sample.

Considering nonequilibrium evolutional thermodynamics, the approach is developed permitting to describe the process of metal or alloy fragmentation during severe plastic deformation. As a result of an explicit accounting of the main channels of energy dissipation, the basic Gibbs relation is obtained. The concept of state of strongly nonequilibrium system is introduced by means of the expansion of system's thermodynamic potential over independent variables. Using adiabatic approximation the nature of the basic nonequilibrium variables, the Landau-Khalatnikov equation and thermodynamic potential are determined. Carrying out the investigation of stability loss of steady-states of thermodynamic potential, the phase diagram of fragmentation modes of metallic materials is determined. It is shown that defects densities in formed limiting structures correspond to experimentally observed regularities.

The approach of nonequilibrium evolution thermodynamics earlier offered is developed. It helps to describe the processes of defect formation within the adiabatic approximation. The basic equations system depends on the initial defects distribution (dislocations and grain boundaries). The phase diagram is determined with the domains of the realization of different limiting structure types. The interaction effect of several defect types on the formation of a limiting structure is investigated in terms of the internal energy. The conditions of the formation of two limiting structures are found. The kinetics of the steady-state values establishment of the defects density is investigated within the scope of the adiabatic approximation. The dislocations density change follows the evolution of the grain boundaries density in this approach. It is shown that grain sizes, in limiting structures, decrease with an increase of the elastic strains., Comment: 14 pages, 6 figures