Competing pathways during a shock-induced phase transition in Zr

Shock compression can cause materials to undergo structural transformations and form new phases of matter with novel material properties. Determining the phase transition mechanism using the orientation relations (ORs) between ambient and high-pressure phases have been typically gleaned from examinations of recovered samples. But the results of such efforts are often complicated by issues such as post-shock annealing and wave interactions. The ductile alpha to brittle omega transition in group IV metals (Zr, Ti) has been studied extensively. Despite decades of study, the pathway by which the high-pressure omega phase is formed remains controversial with several ORs reported in literature. Here we present in-situ x-ray diffraction measurements which provide a snapshot of the active alpha to omega ORs during nanosecond shock-compression of a single crystal [0001] Zr sample. Our data shows that the transformation proceeds by three concurrent and distinct mechanisms which compete for dominance. Our results indicate the alpha to omega transformation is more complex than previously understood, with shear stress playing a critical role in determining the favorability of a particular mechanism. The insights gathered in our study point to a new intricate picture of how materials transform under extreme conditions and take steps to reconcile the discordant results of previous experimental and theoretical investigations.