The melting and phase relations of Fe-Ni and Fe-Ni-Si alloys up to 100 GPa

The existence of density deficit in the Earth's core has been proposed based on comparison of seismic data with the density of FeNi alloys, and Si is considered to be one of the major candidates for the light elements in the core (e.g., Birch, 1952). The phase relations in the Fe-Si system have been studied by some authors, and several polymorphs such as hcp phase, fcc phase, B2 phase, and bcc phase have been reported to date in the Fe-Si alloy system (e.g., Lin et al., 2002; Kuwayama and Hirose, 2004; Asanuma et al., 2008; Kuwayama et al., 2009). However the stability fields of these phases are not consistent among these studies. Kuwayama et al. (2009) reported that the stable phases changed with compositions from hcp + fcc phases for Fe-6.4 wt% Si to hcp + B2 phases for Fe-9.9 wt% Si at 65 GPa. On the other hand, Asanuma et al. (2008) reported that the stability field of hcp and fcc phases extended up to about 115 GPa for Fe-3.4 wt% Si. In this study we determined the phase and melting relations in Ni bearing Fe-4.0 wt% Si alloys. The melting and phase relations of Fe-Ni and Fe-Ni-Si alloys were studied using a double sided laser-heated diamond anvil cell up to about 100 GPa. The starting alloys are Fe-4.8 wt% Ni-4.0 wt% Si and Fe-5.2 wt% Ni alloys. The melting temperatures of Fe-4.8 wt% Ni-4.0 wt% Si and Fe-5.2 wt% Ni alloys were determined up to 70 GPa and 100 GPa respectively based on a discontinuity in a laser power and temperature relations during heating. Observation of the quenched texture was also conducted to determine melting in some runs. Both melting curves of Fe-Ni-Si and Fe-Ni alloys are very close with each other up to about 100 GPa and can be fitted to Simon's equation T_M = T₀ ((P_M - P₀)/a + 1)^1/c, where P₀ = 0, T₀ = 1741 K, a = 67 × 39 GPa, and c = 1.2 × 0.5 for Fe-Ni-Si alloys, and P₀ = 0, T₀ = 1791 K, a = 85 × 22 GPa, and c = 1.1 × 0.2 for Fe-Ni alloys. The phase relations of Fe-4.8 wt% Ni-4.0 wt% Si alloys were studied by in situ X-ray diffraction in the pressure and temperature ranges of 50-100 GPa and 1500-3200 K. The hcp phase was observed in all pressure and temperature conditions studied. Coexistence of hcp and fcc phases were observed in some P-T conditions and the stability field of the coexistence was narrower than that of the Fe-Si system reported by Asanuma et al. (2008).