The α→ϵ phase transition in iron at strain rates up to ∼109 s−1

We have used a table-top scale laser to dynamically compress iron at strain rates in excess of 109 s−1. Using an embedded ultrafast interferometer, we have measured corresponding free surface histories with a time resolution of approximately 10 ps. We have analyzed the surface histories using a method that accounts for nonsteady wave propagation and time-dependent material behavior. We show that at these strain rates, the α→ϵ polymorphic transition begins within 100 ps after an initial very large (∼10 GPa) and mostly elastic compression and appears largely complete within a similar time thereafter. The corresponding deviatoric stress before the transition begins can exceed 3 GPa, while the transition stress itself is up to 25 GPa, nearly twice the value measured at low strain rates. We use these results to propose a systematic variation with loading time of the normal-stress/relative-volume curve followed by iron during rapid compression.