  In order to probe the mechanism of solid-solid phase transformations, we have
applied ultrafast shock wave compression (120 picosecond duration) and
ultrashort (130 femtosecond) x-ray diffraction at the Linac Coherent Light
Source (LCLS) to probe the compression-induced phase transition pathway in
zirconium. Surprisingly, rather than transform from alpha-Zr to the more
disordered hex-3 equilibrium omega-Zr phase, in its place we find the formation
of a non-equilibrium body-centered cubic (bcc) metastable intermediate.
Theoretically hypothesized for several decades, this bcc intermediate state has
now been found to be dynamically stabilized under uniaxial loading at
sub-nanosecond timescales. Molecular dynamics simulations of shock-wave
propagation in zirconium predict this transformation via the dynamical
intermediate state. In contrast with longer timescale experiments 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 transformation at extreme conditions.
