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A theoretical treatment of the fundamental phenomenon of phase instability of nanocrystalline alloys under intensive plastic deformation are performed. It is shown within the framework of a diffusion model that decomposition of nanocrystalline alloys is related to the existence of powerful sources of nonequilibrium vacancies that acting during deformation. Irreversibility of decomposition is defined by conservation a uni-directional vacancies flux between, for instance, triple junctions of grains in nanocrystals. The effect is observed when certain relationships are fulfilled between the characteristic times of vacancy flux existence, the homogenizing time and the time of life of a stable nanocrystalline structure. The evolution of substructure in the course of deformation may change the nature of transformation: back from decomposition to formation of a solid solution.