Time-dependence of the alpha to epsilon phase transformation in iron

Iron was ramp-compressed over timescales of 3 ≤ t(ns) ≤ 300 to study the time-dependence of the α→ε (bcc→hcp) phase transformation. Onset stresses (σ α → ε) for the transformation ∼14.8-38.4 GPa were determined through laser and magnetic ramp-compression techniques where the transition strain-rate was varied between 10⁶ ≤ μ α → ε(s^-1) ≤ 5 ×10⁸. We find σ α → ε = 10.8 + 0.55 ln (μ α → ε) for μ α → ε < 10 ⁶/s and σ α → ε = 1.15 (μ α → ε) 0.18 for μ α → ε > 10⁶/s. This μ response is quite similar to recent results on incipient plasticity in Fe [Smith, J. Appl. Phys. 110, 123515 (2011)] suggesting that under high rate ramp compression the α → ε phase transition and plastic deformation occur through similar mechanisms, e.g., the rate limiting step for μ > 10⁶/s is due to phonon scattering from defects moving to relieve strain. We show that over-pressurization of equilibrium phase boundaries is a common feature exhibited under high strain-rate compression of many materials encompassing many orders of magnitude of strain-rate.