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

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.