Observation of Fundamental Mechanisms in Compression-Induced Phase Transformations Using Ultrafast X-ray Diffraction

MR Armstrong and HB Radousky and RA Austin and E Stavrou and HX Zong and GJ Ackland and S Brown and JC Crowhurst and AE Gleason and E Granados and P Grivickas and N Holtgrewe and HJ Lee and TT Li and S Lobanov and JT McKeown and B Nagler and I Nam and AJ Nelson and V Prakapenka and C Prescher and JD Roehling and NE Teslich and P Walter and AF Goncharov and JL Belof, JOM, 73, 2185-2193 (2021).

DOI: 10.1007/s11837-020-04535-4

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 alpha-Zr to the more disordered hex-3 equilibrium omega-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.

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