Sound wave velocities of Fe-Ni alloy at high pressure and temperature

Knowledge of high-pressure and high-temperature elasticity of Fe-Ni alloy with low (5-25%) Ni content is crucial for geosciences since it is probably the major component of the core of the Earth, Mars, Mercury, Moon, satellites of Saturn and Jupiter. High-pressure and high-temperature (up to 41 GPa and 700 K) study of FeNi alloy with 22% of Ni was carried out by mean of inelastic X-ray scattering (IXS) from polycrystalline material. Two sets of experiments: at room temperature and at 700 K have been performed. Before and after every measurement (taking about 10 hours) an in-situ 1-D monochromatic X-ray diffraction pattern was collected for volume determination exactly from the sample. X-ray diffraction study revealed stability of fcc over hcp phase in the whole studied P,T range. Isothermal equation of state was derived at room temperature and at 700 K. X-ray inelastic scattering measurements allow to calculate longitudinal acoustic wave velocity VL, that gives, combined with measured equations of state, full isotropic elasticity of the material. We did not observe strong deviations of fcc iron-nickel alloy bulk elasticity from elastic properties of pure hcp iron.