The kinetics of the ω to α phase transformation in Zr, Ti: Analysis of data from shock-recovered samples and atomistic simulations

An open question in the kinetics of martensitic transformation is the microscopic mechanism responsible for the evolution of a new phase. We have analyzed data from shocked recovered samples that monitor the volume fraction of the ω phase in α-Zr as a function of time under isothermal conditions in the temperature range 430-545 K. Our results show that the effective activation energy strongly depends on the peak pressures of shock compression. In addition, we confirm that the orientation relationship in Zr for this reverse transformation is consistent with the original suggestion by Silcock in Ti for a direct α → ω martensitic transition. Combined with large-scale molecular dynamics simulations, we find that the difference in the effective activation energy, which is related to the nature of the isothermal kinetics, is controlled by heterogeneous nucleation from defects in the microstructure.