Nonequilibrium phase transformations in alloys under severe plastic deformation

Experimental results and theoretical concepts of anomalous phase transformations in alloys under severe plastic deformation (SPD) are reviewed. The unconventional phase and structural state of alloys that emerges as a result of SPD determines the unique combination of physical and chemical properties that is of interest for various applications in technology. The driving forces and possible mechanisms that implement the anomalous transformations as a function of SPD intensity, alloy composition, and temperature are discussed. We distinguish among these mechanisms two fundamental and qualitatively different ones that are due to: (i) direct ‘mixing’ of atoms in slip bands or (ii) accelerated diffusion on defects under locally alternating thermodynamic conditions and subsequent ‘freezing’ of the nonequilibrium state that is reached. Summarizing the experimental data and theoretical concepts regarding the change in microscopic transformation mechanisms as a function of temperature and/or intensity of treatment, we suggest a diagram of nonequilibrium stationary states attainable during the development of phase and structural instability under SPD. The proposed approach enables the prediction of the structural state of alloys and compounds by controlling thermodynamic and kinetic parameters of the system.