Kinetics of phase transformations in Ti, Zr and Hf under static and dynamic pressures

Abstract

Based on the available experimental data, the isothermal and the athermal kinetics of the α → ω transformation in group IV transition metal elements Ti, Zr and Hf under static and dynamic pressures have been analysed. Theoretical calculations of hysteresis (or onset of bulk athermal transformation) indicate that in contrast to Ti and Zr, the ω-polymorph of Hf cannot be retained in a metastable form at room pressure. The isothermal kinetics observed under static pressures in Ti and Zr can be understood either on the basis of a bulk isothermal-athermal mechanism or on the basis of a nucleation process where the activation barrier decreases rapidly with increasing pressure. Analysis of shock-wave studies of these metals indicate that quantitative studies of ω-fraction vs. peak pressure are required to determine the athermal martensitic start pressure, P M s α → ω , under shock and for its comparison with the static data.