Orientation effect of alpha-to-epsilon phase transformation in single-crystal iron

Abstract

The dynamic response of iron, especially the phase transformation from the ambient body-centered-cubic (bcc) up-phase to the hexagonal-closed packed (hcp) -phase, has been studied extensively in the last 60 years due to its importance in industry and its role as a main constituent of Earth. Recently, this topic has attracted a lot of attention in the aspects of the kinetic characteristics and mechanism of the shock-induced phase transition, including orientation-, temperature-, time- and strain rate-dependences. But only a few data have been published on the crystal orientation effect. The systematic experimental results to identify the predictions of the non-equilibrium molecular dynamics (NEMD) simulation are still lacking. For this reason, we study the shock responses of the [100], [110] and [111] orientated iron single crystals by using a three-independent-sample method in one shot. Unlike previously reported [001] single-crystal iron, a clear three-wave structure consisting of a PEL wave (elastic wave), a P1 wave (plastic wave) and a P2 wave (phase transition wave) is observed in the measured wave profiles for all single-crystal iron samples. The elastic-plastic transition process is in accordance with the numerical simulation of dislocation-based constitutive model for visco-plastic deformation. It is found that the values of Hugoniot elastic limit HEL ((111)/(HEL) (110)/(HEL) (100)/(HEL)) are greater than 6 GPa and dependent on the initial crystal orientation. Such a high yield strength is consistent with the nanosecond X-ray diffraction of [001] single-crystal iron where the uniaxial compression of the lattice has been observed at a shock pressure of about 5.4 GPa. Moreover, the onset pressures PPT for the phase transition are obtained to be 13.890.57 GPa, 14.530.53 GPa and 16.050.67 GPa along the [100], [110], and [111] directions, respectively. Based on these results, it is concluded that the crystal orientation effect of PPT is consistent with the reported NEMD calculations. However, the measured values are lower. In addition, the transition strain-ratio of singlecrystal iron is found to be higher than that of polycrystalline iron, reflecting the influence of the transformation kinetics (i.e., transformation kinetics coefficient) on the wave profile evolution. Our observations indicate that the strong coupling between plasticity and phase transition in single crystal iron might be a key point for understanding the origin of the phase transition and also for ending the controversy of metastable -phase. The fine multi-wave profiles also provide an important experimental reference for improving the phase field modeling of shock-induced phase transition.