Melting and spall strength of Fe–Ni–Cr alloy measured with Photonic Doppler Velocimetry from free surface velocity in laser shock experiments

The inner core of Earth is composed mainly of solid iron, which is surrounded by a liquid Fe-Ni outer core. The inner-core boundary (ICB) temperature is close to the melting point of Fe at a pressure of 3.3 Mbar. This influences the heat flow, core composition, and magnetic properties of Earth. Despite extensive research, the melting behavior of Fe and its alloys under extreme pressure and temperature conditions remains uncertain.

We conducted experiments at the NLF high-power laser facility at the Soreq NRC using a 351 nm, 5 ns, 450 J laser with a 400 μm spot-size. We introduce a novel application of Photonic Doppler Velocimetry (PDV) to detect the solid-liquid phase transition of a Fe-Ni-Cr (304L) alloy, for laser-driven shock pressure of up to ~7 Mbar, i.e. exceeding the melting at 2-3 Mbar on the Hugoniot. We measure the free surface velocity and shock transit time, and identify the solid-liquid phase transition utilizing several criteria. The results present a clear change in the trend of free-surface velocity versus pressure at the pressure of 2-3 Mbar, consistent with the effect of melting, in agreement with previous studies.

In addition, spall effects are observed in the free surface velocity for a range of shock pressure, from 0.5 Mbar to above the melting pressure. Initial results indicate a correlation between spall strength and melting.

This work advances the study of iron-alloy behavior under extreme conditions, leveraging PDV technology to refine the understanding of melting behavior and equation of state.