Molecular dynamics simulations of competing phase-transition mechanisms in dynamically compressed zirconium

The nature of the pressure-induced α → ω (hcp → hex-3) phase transition in zirconium is arguably the most complex and contentious of any elemental metal. Consensus had emerged that the transition is realized via one of two displacive mechanisms, depending on the nature of the compression path. However, recent dynamic-compression experiments and large-scale atomistic simulations have cast doubt on this simplistic picture. We present classical molecular dynamics simulations of single-crystal zirconium shock-compressed along [0001] using a machine-learning-class potential. The transition is predicted to proceed primarily by a modified version of the Usikov-Zilberstein mechanism, whereby the high-pressure ω phase nucleates heterogeneously at the boundaries between grains of an intermediate β (bcc) phase. However, we also observe (1) a second, inequivalent displacive mechanism competing with the first, whose relative prevalence is in fair agreement with diffraction-based measurements, and (2) pockets of material wherein the transition is realized via disordered, diffusive motion. These simulations thus predict a complex, multiple-pathway picture of an elementary solid-solid phase transition that leaves in its wake a rich post-shock microstructure worthy of further study.