Effects of the α-ε phase transition on wave propagation and spallation in laser shock-loaded iron

Despite extensive research work on the a-e phase transition occurring in shock-loaded iron, the kinetics of this transformation remain largely unknown. Here, we present time-resolved free surface velocity measurements in iron foils of thicknesses ranging from 150 to 520 mu m subjected to laser shocks of peak pressure of about 130 GPa and duration of about 3 ns. The records show an elastic precursor followed by a plastic front, but the double wave structure that is usually associated with the phase change does not appear over such short propagation distances. The measured profiles are compared to the predictions of one-dimensional simulations involving rate-dependent descriptions of both twinning and phase transitions. Such comparisons provide an estimate of a time constant governing the transformation kinetics, which is found to strongly condition the evolution and attenuation of the pressure pulse during its propagation. The spall stress evaluated from the records is shown to be significantly higher after the alpha-epsilon-alpha cycle. Metallurgical observations of the recovered samples confirm both the phase transition and the spall damage inferred from the velocity profiles. Finally, they show the very clear change in fracture surface morphology to the so-called smooth spall expected above the phase transformation.