Direct measurements of the α - ϵ transition stress and kinetics for shocked iron

Iron undergoes a polymorphic phase transformation from the α -phase (bcc) to the ϵ -phase (hcp) when compressed to stresses exceeding 13 GPa. Because the epsilon phase is denser than the alpha phase, a single shock wave is unstable and breaks up into an elastic wave, a plastic wave, and a phase transition wave. Examination of this structured wave coupled with various phase transformation models has been used to indirectly examine the transition kinetics. Recently, multimillion-atom molecular dynamics (MD) simulations have been used to examine the shock-induced transition in single crystal iron illustrating an orientation dependence of the transition stress, mechanisms, and kinetics. The objective of the current work was to perform plate impact experiments to examine the shock response of polycrystalline and single crystal iron with nanosecond resolution for impact stresses spanning the α - ϵ transition. The current data reveal an orientation dependence of the transition stress coupled with a transition time that is nonlinearly dependent on the impact stress with a duration ranging from picoseconds to hundreds of nanoseconds. The higher transition stress for iron shocked along the [100] direction is in agreement with the predictions from MD calculations that describe an orientation dependence of the transition stress. However, MD calculations do not capture the complexity of the continuum states achieved or the transition kinetics. Further results and implications are discussed in this paper.