Polymorphic phase transformation rates in shock-loaded potassium chloride

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Abstract
Single crystals of potassium chloride (KCl) were impacted on quartz stress gauges at velocities ranging from 0.0363 to 0.0755 cm/μsec, producing plane shocks with stress amplitudes between 17 and 31 kbar. Stresses greater than 21 kbar are sufficient to induce a polymorphic phase transformation in KCl. For impact stresses above the phase transition point, the theory of continuum mechanics is used to relate measured stress history at the impact interface to transformation rate between the phases. Experimental results show that, when shocked above the measured dynamic transition pressure of 20.8±0.5 kbar, KCl rapidly transforms to a metastable nonequilibrium mixture of phases. Phase transformation to these initial metastable states proceeds at a rate greater than 500 μsec−1 for shock propagation along the 〈100〉 and 〈111〉 directions, based on the magnitude of initial transformation and the minimum response time of the measuring system. The degree of partial transformation is observed to depend only on impact velocity and is independent of crystallographic orientation of the KCl. The observed metastable states are consistent with the postulate that the heat of formation of the nonequilibrium high‐pressure phase is characterized by a specific entropy 3.9 × 10−6 cm2 μsec−2°K−1 higher than the equilibrium high‐pressure phase. Subsequent phase transformation proceeds at a rate that depends on crystallographic orientation of the KCl. When the shock propagates along a 〈111〉 direction, it is observed that transformation proceeds from the metastable state to completion at a rate of 25±5 μsec−1, with final states in agreement with thermodynamic equilibrium calculations. When the shock propagates along a 〈100〉 direction, transformation proceeds at a rate of less than 5 μsec−1. For the 〈100〉 orientation, data are inadequate to allow determination of whether transformation was completed during the experiment, because of uncertainty in late‐time quartz gauge response.

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