Orientation dependence of plasticity and transformation kinetics in Zirconium at higher shock pressure*

We simulated the high pressure phase transformation in zirconium using molecular dynamics with a new EAM potential fitted using force matching. Under quasistatic ("Hugoniostat") conditions the transformation from α-ω follows the phase diagram, as expected. However under directly simulated shock conditions the structure transforms from α-β (hcp-bcc) rather than the expected ω phase. Assuming sudden, isotropic pressurization we trace this to a different phase transformation mechanism (so-called TAO1) in which bcc appears as an intermediate state, and under pressure the barrier hcp-bcc is lower than bcc-ω. The assumption of isotropic loading requires significant plasticity, which we also simulated and found to be of marginal validity. Plasticity depends on the crystal orientation and in the absence of ideal plasticity, so does the transformation path and, indeed, the observed high pressure phase. We have determined the orientational dependence of the transformation kinetics between hcp-ω-bcc and hcp-bcc at higher shock pressure.