Abstract
Knowledge of the elastic properties and equations of state of iron and iron alloys are of fundamental interest in Earth and planetary sciences as they are the main constituents of telluric planetary cores. Here, we present results of X-ray diffraction measurements on a ternary Fe–Ni–Si alloy with 5 wt% Ni and 5 wt% Si, quasi-hydrostatically compressed at ambient temperature up to 56 GPa, and under simultaneous high pressure and high temperature conditions, up to 74 GPa and 1750 K. The established pressure dependence of the c/a axial ratio at ambient temperature and the pressure–volume–temperature (P–V–T) equation of state are compared with previous work and literature studies. Our results show that Ni addition does not affect the compressibility and axial compressibility of Fe–Si alloys at ambient temperature, but we suggest that ternary Fe–Ni–Si alloys might have a reduced thermal expansion in respect to pure Fe and binary Fe–Si alloys. In particular, once the thermal equations of state are considered together with velocity measurements, we conclude that elements other than Si and Ni have to be present in the Earth’s inner core to account for both density and seismic velocities.
Highlights
Different lines of evidence, from the analysis of meteorite compositions and Earth’s differentiation models to the comparison of shock compression measurements with seismic observations, have put forward the notion that iron (Fe) is the main constituent of the Earth’s core [1,2]
Based on results obtained at high pressure and ambient temperature, Ni alloying seems to marginally affect the compressibility curves [6,7,8] and compressional sound velocities [9,10,11] of pure Fe
A comparison of the obtained results with previous work indicates that Ni does not affect the compressibility and axial compressibility of Fe–Si alloys at ambient temperature, but we suggest that Ni-bearing Fe–Si alloys might have a reduced thermal expansion
Summary
From the analysis of meteorite compositions and Earth’s differentiation models to the comparison of shock compression measurements with seismic observations, have put forward the notion that iron (Fe) is the main constituent of the Earth’s core [1,2]. The density mismatch in the case of the solid inner core is less than that of the outer core, and though the fact that pure Fe is too dense in respect to seismic models has been well-established, the accurate quantification of this density deficit is still an object of active research (e.g., [4,5]). Based on results obtained at high pressure and ambient temperature, Ni alloying seems to marginally affect the compressibility curves [6,7,8] and compressional sound velocities [9,10,11] of pure Fe. Possible effects of
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