Sound wave velocities of fcc Fe–Ni alloy at high pressure and temperature by mean of inelastic X-ray scattering

Knowledge of the 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 cores of the terrestrial planets and the Moon. Here we present a study of a FeNi alloy with 22 at.% of Ni to 72 GPa and 715 K, using inelastic X-ray scattering (IXS) and X-ray powder diffraction from polycrystalline material. The X-ray diffraction (XRD) study revealed stability of the face centred cubic (fcc) over the hexagonal close packed (hcp) phase in the whole investigated pressure–temperature range. The study presents first investigations of elasticity of fcc phase of iron–nickel Fe0.78Ni0.22 alloy. The isothermal equations of state were derived at room temperature and at 715 K (K300 = 162(1) GPa, K ′ 300 = 4.97 ( 1 ) , V300 = 6.89(1) cm3/mol; K715 = 160(1) GPa, K ′ 715 = 4.97 ( 2 ) , V715 = 6.96(1) cm3/mol). Inelastic X-ray measurements allow the determination of the longitudinal acoustic wave velocity VP, and provide, combined with the measured equations of state, the full isotropic elasticity of the material. We found that within experimental errors our data follow the Birchʹs law. We did not observe any significant deviations for fcc Fe0.78Ni0.22 from elastic properties of pure ɛ-iron.