Sound velocity and elastic properties of Fe-Ni and Fe-Ni-C liquids at high pressure

The sound velocity (V (P)) of liquid Fe-10 wt% Ni and Fe-10 wt% Ni-4 wt% C up to 6.6 GPa was studied using the ultrasonic pulse-echo method combined with synchrotron X-ray techniques. The obtained V (P) of liquid Fe-Ni is insensitive to temperature, whereas that of liquid Fe-Ni-C tends to decrease with increasing temperature. The V (P) values of both liquid Fe-Ni and Fe-Ni-C increase with pressure. Alloying with 10 wt% of Ni slightly reduces the V (P) of liquid Fe, whereas alloying with C is likely to increase the V (P). However, a difference in V (P) between liquid Fe-Ni and Fe-Ni-C becomes to be smaller at higher temperature. By fitting the measured V (P) data with the Murnaghan equation of state, the adiabatic bulk modulus (K (S0)) and its pressure derivative (K (S) (') ) were obtained to be K (S0) = 103 GPa and K (S) (') = 5.7 for liquid Fe-Ni and K (S0) = 110 GPa and K (S) (') = 7.6 for liquid Fe-Ni-C. The calculated density of liquid Fe-Ni-C using the obtained elastic parameters was consistent with the density values measured directly using the X-ray computed tomography technique. In the relation between the density (rho) and sound velocity (V (P)) at 5 GPa (the lunar core condition), it was found that the effect of alloying Fe with Ni was that rho increased mildly and V (P) decreased, whereas the effect of C dissolution was to decrease rho but increase V (P). In contrast, alloying with S significantly reduces both rho and V (P). Therefore, the effects of light elements (C and S) and Ni on the rho and V (P) of liquid Fe are quite different under the lunar core conditions, providing a clue to constrain the light element in the lunar core by comparing with lunar seismic data.