Shock wave study of the α ⇄ ε phase transition in iron

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Abstract
Plate impact experiments producing plane waves of up to 40 GPa (400 kbar) peak stress were performed using Armco iron specimens and impactors. Highly accurate time-resolved measurements of the resulting free-surface velocities of the specimens were obtained with the VISAR laser interferometer instrumentation system. The free-surface velocity profiles provide new information concerning the rate effects associated with the α → ε polymorphic phase transition at 13 GPa, the material strength and release wave speeds at 10 and 40 GPa, and the stress level at which the iron reverts back to the α phase on unloading. A strong magnetic field was found to produce no measurable change in the phase-transition stress. The accuracy of the ``factor-of-2'' assumption relating free-surface velocity to particle velocity in iron was also evaluated experimentally.