  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.
