Measurement of competing pathways in a shock-induced phase transition in zirconium by femtosecond diffraction

TL;DR

This study uses femtosecond diffraction and simulations to reveal that zirconium undergoes a phase transition via multiple competing pathways simultaneously, challenging the traditional single-path model.

Contribution

It provides the first direct in situ observation of multiple transformation pathways in zirconium during a shock-induced phase transition.

Findings

Zirconium transitions via three pathways simultaneously.

Presence of a broad diffuse background during the transition.

Validation of diffuse signals with large-scale molecular dynamics simulations.

Abstract

The traditional picture of solid-solid phase transformations assumes an ordered parent phase transforms into an ordered daughter phase via a single unique pathway. Zirconium and its prototypical phase transition from hexagonal close-packed (hcp) to simple hexagonal (hex-3) structure has generated considerable controversy over several decades regarding which mechanism mediates the transformation. However, a lack of in situ measurements over the relevant atomistic timescales has hindered our ability to identify the true pathway. In this study, we exploit femtosecond X-ray diffraction coupled with nanosecond laser compression to give unprecedented insights into the complexities of how materials transform at the lattice level. We observe single-crystal zirconium changing from hcp to a hex-3 structure via not one but three competing pathways simultaneously. Concurrently, we also observe a broad diffuse background underlying the sharp Bragg diffraction during the transition. We corroborate our observation of the diffuse signal with multimillion-atom molecular dynamics simulations using a machine-learned interatomic potential. Our study demonstrates that the traditional mechanistic view of transitions may fail for even an elemental metal and that the mechanisms by which materials transform are far more intricate than generally thought.