As theoretically hypothesized for several decades in group IV transition metals, we have discovered a dynamically stabilized body-centered cubic (bcc) intermediate state in Zr under uniaxial loading at sub-nanosecond timescales. Under ultrafast shock wave compression, rather than the transformation from α-Zr to the more disordered hex-3 equilibrium ω-Zr phase, in its place we find the formation of a previously unobserved nonequilibrium bcc metastable intermediate. We probe the compression-induced phase transition pathway in zirconium using time-resolved sub-picosecond x-ray diffraction analysis at the Linac Coherent Light Source. We also present molecular dynamics simulations using a potential derived from first-principles methods which independently predict this intermediate phase under ultrafast shock conditions. In contrast with experiments on longer timescale (> 10 ns) where the phase diagram alone is an adequate predictor of the crystalline structure of a material, our recent study highlights the importance of metastability and time dependence in the kinetics of phase transformations.
Armstrong, M.R., Radousky, H.B., Austin, R.A. et al. Observation of Fundamental Mechanisms in Compression-Induced Phase Transformations Using Ultrafast X-ray Diffraction. JOM (2021). abstract
Phase diagram of Zr with the shock Hugoniot and α, β, and ω phase boundaries as reported by Greeff36 and which reflect aggregate gas-gun and diamond anvil cell data from multiple sources. The four shock wave experiments conducted in this study consist of final Rankine–Hugoniot pressures of 13 GPa, 33 GPa, 46 GPa, and 57 GPa as determined from a combination of x-ray diffraction analysis and impedance matching to the adjacent Al standard. The release isentropes for all experiments (as shown for the P = 57 GPa experiment) have a final state outside of the β-Zr field of stability, thus β would not be expected to form upon release. The metastable phase boundary between the hcp α and bcc β phases are calculated from metastable branches of Greeff’s multiphase equation of state and define the region where, if the hex-3 ω phase were absent (or kinetically hindered), the transition from α directly to β would be favored by the bulk Gibbs free energy difference. The dynamically stabilized bcc intermediate state, formed along the α–ω transformation pathway (but never reaching the thermodynamic end-state of the ω phase on the sub-nanosecond timescale in this experiment) is demarcated near 13 GPa, which is the lowest pressure where it has been observed in this study.