Modeling dynamic phase transitions in Ti and Zr

Both Ti and Zr exhibit phase transitions from the alpha (hcp) to the omega phase at pressures of a few GPa. In addition, the Hugoniot of Zr shows a second phase transition at 23 GPa. We have developed multiphase equations of state for these metals based on ultrasonic, static compression, and shock data. The second transition in Zr is consistent with a phase diagram in which the high-temperature and high-pressure bcc phases are a single continuous phase. Time-resolved experiments using plate impact and continuous magnetic loading are compared to simulations to investigate the kinetics of these phase transitions. Strong kinetic effects are observed in the alpha - omega transition in both metals, with the dynamic phase transition observed at pressures well above the equilibrium phase boundary. Data on Zr samples of varied purity are consistent with a strong reduction of the transformation rate by impurities.