Viscosity and melting temperature of iron-silicon alloys at high pressure and temperature

The high pressure melting boundaries of Fe-9wt% Siand Fe-17wt% Si were investigated at pressures up to 14 GPa and temperatures up to 1850°C by means of quench texture analyses. Differences in the quench textures were used to identify if the samples were melted, partially melted or unmelted. The results from this work are consistent with those of previous works that used different methods, as seen by using a respective fit line for both compositions. The continuous increase of melting temperature (Tm) with pressure for Fe-17wt% Si indicates a constant solid phase below the liquidus. The discontinuous increase of Tm for Fe-9wt% Si, with a kink at ∼ 8 GPa, indicates a triple point. Combining these results with previous studies, a solid phase boundary separates bcc and bcc+fcc and extends from the liquidus. This boundary extension to a second triple point separating bcc, bcc+fcc, and bcc+hcp is consistent with thermodynamic modeling and has similar phase behavior to Fe. The Kraut-Kennedy melting law and Lindemann-Vinet method were applied to extrapolate the Tm of Fe9wt% Si and Fe-17wt% Si to the Earth's outer core conditions. The results suggest a large decrease of Tm relative to Tm of pure Fe, which supports the existence of Si in the Earth's outer core. In spite of the small melting temperature depression effect at low pressures up to ∼40 GPa, the melting temperature results of this study cannot exclude the existence of Si in the cores of Mars, Mercury, Io and Ganymede.;Key words. Fe-Si alloy, liquid viscosity, melting temperature, high pressure, high temperature, phase transition, outer core, activation energy, activation volume, quench texture;Viscosity experiments of liquid Fe-17wt% Si were carried out up to 7 GPa at APS and SPring-8 synchrotron facilities by using the in situ X-ray radiography falling-sphere method, with a probe sphere made of a Pt or Re core and a ruby mantle. Viscosity values were measured to range from 15 mPa.s to 93 mPa.s at pressures from 1.8 GPa to 7 GPa and temperatures from 1300°C to 1550°°C. The activation energy for viscous flow of 55.5 kJ/mol from ambient pressure studies was applied to deduce the activation volume of viscous flow of 6.3 cm3/mol in the experimental pressure range of this study. The viscosity of Fe-17wt% Si increases with pressure below 5.4 GPa and appears to remain constant at the melting boundary in the small pressure range studied above 5.4 GPa. This can be explained by the presence of different flow units using hole theory and the free volume model of viscous flow, whereas a pre-requisite for the semi-empirical theory explaining constant viscosity along the melting boundary is that the flow unit remains the same. The viscosity of Fe-17wt% Si was extrapolated to be 25 mPa.s at the Earth's core mantle boundary. This low value of viscosity supports small-circulation convective flow in the Earth's outer core.