Phase transformations and phase stability in nanocrystalline materials

The current state of the researches on diffusionless phase transformations, including allotropic, polymorphous and martensitic transformations, and phase stability are reviewed. The behaviors of phase transformation and phase stability in nanocrystalline materials are markedly affected by the non-equilibrium conditions involved in their preparation, as a result, in this review an ideal demonstrating method of critical size for the stability of a high-temperature phase at low-temperatures is suggested, and the intrinsic conditions of the phase stability are clarified. Our recent experiments exhibit that the reversal transformation temperatures of low-temperature phases in nanocrystalline Co bulk metal and Fe-30Ni wt% alloy are significantly raised up over 800 °C when their grain sizes are smaller than about 15 nm, while in the reported experiments of nanocrystalline particles or films the reversal transformation temperature lowers with decreasing grain size or is independent of grain size. Therefore, the author suggests that more experiments and theories for phase stability in reversal transformation should be performed. The study of grain growth kinetics of nanocrystalline materials, as a basic of investigating phase stability, is another attention aspect.