The Phase Diagram of Fe8wt\%Si Alloy Under High-Pressures and High-Temperatures

J F Lin, D L Heinz1, A J Campbell, J M Devine, and G Shen2 (Department of the Geophysical Sciences, 1and James Franck Institute, 2and Consortium for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637; afu@geosci.uchicago.edu)

The high-pressure and high-temperature phase diagram of Fe8wt\%Si alloy has been in-situ studied in a laser-heated diamond anvil cell (LHDAC) up to 69 GPa and 2400 K and in a large-volume press (LVP) up to 18 GPa and 1500 K at GSECARS, APS. A double-sided laser heating, together with a sandwiched sample configuration using NaCl, MgO, or Ar as the thermal insulator and pressure medium, was used in the LHDAC study. A 250-ton LVP using T-cup as the sample assemblage and MgO as the sample capsule was used in the LVP study. The BCC-Fe8wt\%Si transformed to HCP phase under high pressures. HCP phase underwent a HCP-BCC phase transformation in the heating process and, upon further heating, the BCC+HCP phases transformed to FCC+BCC phases in the pressure range of 23 GPa to 69 GPa. Both the HCP-BCC and BCC-FCC transformations have positive slopes while the BCC-FCC boundary is steeper than that of BCC-HCP. The BCC-FCC phase transformation boundary is similar to that of pure iron, and HCP-BCC and the FCC phase. Our study indicates that silicon alloyed with iron can stabilize the BCC phase under high pressures and high temperatures. The LVP experiments also indicate that the BCC phase is the dominant phase in pressures up to 18 GPa and temperatures up to1500 K, which is consistent with the LHDAC observations. Neither the dHCP structure nor the orthorhombic structure proposed for the crystal structure of b-iron under high pressures and high temperatures was observed in this study. The phase diagram and the thermal EOS of HCP-Fe8wt\%Si will be discussed in this paper. Our study indicates that a few weight percent of silicon may significantly change the phase diagram of iron-rich Fe-Si alloys under high pressures and high temperatures. This observation may have profound implications on properties of iron alloyed with light elements under core conditions.