Observation of a metastable intermediate during solid-solid phase transformation in response to rapid compression

TL;DR

This study reveals a metastable bcc intermediate during zirconium's solid-solid phase transition under ultrafast compression, emphasizing the importance of metastability and kinetics in extreme conditions.

Contribution

First direct observation of a metastable bcc intermediate in zirconium during rapid compression using ultrafast x-ray diffraction and shock experiments.

Findings

Discovery of a non-equilibrium bcc intermediate state

Validation of molecular dynamics predictions of the transformation pathway

Highlighting the role of metastability in phase transformation kinetics

Abstract

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.