Sound velocity and elastic properties of Fe–Ni and Fe–Ni–C liquids at high pressure
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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 (ρ) 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 ρ increased mildly and V P decreased, whereas the effect of C dissolution was to decrease ρ but increase V P. In contrast, alloying with S significantly reduces both ρ and V P. Therefore, the effects of light elements (C and S) and Ni on the ρ 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.
KeywordsSound velocity Density Fe-alloy Liquid High pressure
The authors acknowledge to Y. Suzuki and M. Hoshino for X-ray CT measurements and to T. Sakamaki, S. Kishimoto, M. Tahara, M. Igarashi, Y. Tange, M. Sakurai, N. Funamori, T. Sakaiya, and Y. Kono for their technical supports and discussions. The authors also acknowledge to two anonymous reviewers for their constructive comments. This work is partly supported by Grants-in-Aid for scientific research from the Ministry of Education, Culture, Science, and Sport and Technology of the Japanese Government to H. T. (Nos. 23340159, 26247089, 26610141) and S. U. (No. 23340129). The experiments have been performed under contract of the SPring-8 facility (Proposal Numbers: 2011B1278, 2012A1481, 2012B1177, 2013A1072, 2013A1508, 2013B1174, 2013B1701, 2013B1488, 2014A1161, 2014A1662). This work was partly carried out under the collaborative research program of Geodynamics Research Center, Ehime University.
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