THERMODYNAMICS AND PHASE-RELATIONSHIPS OF TRANSITION METAL-SULFUR SYSTEMS .3. THERMODYNAMIC PROPERTIES OF THE FE-S LIQUID-PHASE AND THE CALCULATION OF THE FE-S PHASE-DIAGRAM

An associated solution model is applied to describe the thermodynamic behavior of Fe-S liquid. This model assumes the existence of 'FeS' species in addition to Fe and S in the liquid. With two solution parameters for each of the binaries Fe-'FeS' and 'FeS'-S, this model accounts for the compositional dependence of the thermodynamic properties of Fe-S liquid from pure Fe to pure S over a wide range of temperature. The binary Fe-S does not contribute significantly to the excess Gibbs energy of the liquid due to the rather small dissociation constant of 'FeS' to Fe and S. Using this model for the liquid phase and a defect thermodynamic model for the pyrrhotite phase, the Fe-S phase diagram is calculated. The calculated diagram is in excellent agreement with the experimental data, accounting for the range of homogeneity of pyrrhotite at all temperatures. Both the thermodynamic and phase diagram data are obtained from the literature. © 1979 American Society for Metals and the Metallurgical Society of Aime.