PHASE-RELATIONS AND THERMODYNAMICS OF THE SYSTEM FE-CR-O IN THE TEMPERATURE-RANGE OF 1600-DEGREES-C TO 1825-DEGREES-C (1873 TO 2098-K) UNDER STRONGLY REDUCING CONDITIONS

Equilibrium relations involving alloy and oxide phases in the system Fe-Cr-O were determined in the temperature range from 1600-degrees-C to 1825-degrees-C (1873 to 2087 K). Compositions of coexisting alloy and spinel phases were established as a function of oxygen pressure by equilibrating liquid Fe-Cr alloys with iron chromite (Fe3-xCr(x)O4) solid solutions at 1600-degrees-C and 1700-degrees-C. Combinations of these experimental data and thermodynamic calculations were used to construct composition-oxygen pressure diagrams for the system at 1600-degrees-C and 1700-degrees-C. Additional runs for selected mixtures were made at still higher temperatures (1700-degrees-C to 1825-degrees-C), and thermodynamic parameters were derived for spinel-containing phase assemblages at temperatures up to 1865-degrees-C. The spinel phases occurring in the present system are typically in the high-chromium range of the solid-solution series Fe3O4-Cr3O4, i.e., in the range between stoichiometric iron chromite (FeCr2O4) and Cr3O4. The activities of the various oxide components of the spinel solid solution at 1600-degrees-C were calculated from experimentally determined parameters for co-existing alloy and spinel phases, as well as by statistical-mechanical modeling of the same spinel solid solution based on crystal-chemical considerations. The agreement between the two sets of results was excellent. Temperature variation of parameters characterizing the univariant equilibria spinel + Cr2O3 + alloy and spinel + alloy + liquid oxide was established. The univariant curves were found to display temperature maxima of 1715-degrees-C +/- 5-degrees-C and approximately 1865-degrees-C, respectively. In analogy with relations in the Cr-O system, the increase in divalent chromium of the liquid oxide phase with decreasing oxygen potential was identified as the main cause of the sharp decrease in liquidus temperatures of chromites in contact with Fe-Cr alloys of high Cr contents.