Investigation of Fe-X (X=S, Si, C) Alloys Phase Diagrams at High Pressure and High Temperature using Ultrasonic Interferometry

Abstract:

Light elements have been suggested to be present in the liquid iron cores of the terrestrial planets and moons based on seismological and geochemical observations. It is therefore critical to determine precisely the phase diagrams for Fe-X (X=S, Si, C, O, H, etc.) alloying liquids in order to constrain the light element abundances in planetary cores, and to understand the dynamics and chemical evolution of the planets. Our goal is to refine the phase diagrams of Fe-S, Fe-Si and Fe-C at high pressures. We have successfully determined, at pressures up to 7 GPa, the temperatures of both solidus and liquidus of Fe alloys, with various S, Si and C contents, using ultrasonic wave velocity measurements in multi-anvil apparatus. During the heating of the experiment, we observe the solidus through the disappearance of the P-waves signal when the sample is partially molten, and the liquidus through the appearance of the signal from the liquid when the sample is entirely molten. Those observations have been confirmed during in-situ synchrotron experiments on Fe-5wt%C, Fe-10wt%C, Fe-5wt%Si and Fe-25wt%Si, using the ultrasonic measurements in conjunction with x-ray diffraction at the GSECARS Beamline 13-ID-D. In fact the ultrasonic interferometry allow to determine the solidus and liquidus temperatures more precisely than the x-ray diffraction. Traditional quench experiments used to determine phase relations require a large number of experiments. It is particularly challenging when the phase loop is narrow such as the case of the Fe-Si system. This new off-line in situ technique is a powerful tool to study partially or entirely molten samples without access to a synchrotron radiation source.