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Phase Transitions in High Purity Zr Under Dynamic Compression

We present results from ramp compression experiments on high-purity Zr that show the $α\rightarrow ω$, $ω\rightarrow β$, as well as reverse $β\rightarrow ω$ phase transitions. Simulations with a multi-phase equation of state and phenomenological kinetic model match the experimental wave profiles well. While the dynamic $α\rightarrow ω$ transition occurs $\sim 9$ GPa above the equilibrium phase boundary, the $ω\rightarrow β$ transition occurs within 0.9~GPa of equilibrium. We estimate that the dynamic compression path intersects the equilibrium $ω- β$ line at $P= 29.2$ GPa, and $T = 490$ K. The thermodynamic path in the interior of the sample lies $\sim 100$ K above the isentrope at the point of the $ω\rightarrow β$ transition. Approximately half of this dissipative temperature rise is due to plastic work, and half is due to the non-equilibrium $α\rightarrow ω$ transition. The inferred rate of the $α\rightarrow ω$ transition is several orders of magnitude higher than that measured in dynamic diamond anvil cell (DDAC) experiments in an overlapping pressure range. We discuss a model for the influence of shear stress on the nucleation rate. The small fractional volume change $ΔV/V \approx 0.1$ at the $α\rightarrow ω$ transition amplifies the effect of shear stress, and we estimate that for this case shear stress is equivalent to a pressure increase in the range of several GPa. Correcting our transition rate to a hydrostatic rate brings it approximately into line with the DDAC results, suggesting that shear stress plays a significant role in the transformation rate.