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Phase Diagram Evaluations: Section II

Fe-Mn-S (Iron-Manganese-Sulfur) V. Raghavan

The previous review of this system [1988Rag] presented a schematic liquidus surface, four isothermal sections at 1600, 1300, 1100, and 800 °C mainly for the Fe-FeS-MnSMn region, a pseudobinary section along the FeS-MnS join, the same section in the presence of Fe, a liquidus projection near the Fe corner, and a reaction scheme. Two thermodynamic treatments of the Fe-FeS-MnS-Mn region have been reported recently [1998Mie, 2000Oht].

Binary Systems The Fe-Mn phase diagram [1982Kub1] contains no intermediate phases. A continuous solid solution denoted ␥ forms between fcc Fe and ␥Mn. There are two intermediate phases in the Fe-S system [1982Kub2]. The monosulfide pyrrhotite Fe1−xS (NiAs type hexagonal) is stable at Fedeficient (S-rich) compositions with a range of 50-55 at.% S. Fe1−xS with 52 at.% S melts congruently at 1188 °C. In the Fe-FeS region, the solidification is through a eutectic reaction at 988 °C. In the FeS-S region, a monotectic reaction at 1082 °C yields Fe1−xS of 54.2 at.% S and a sulfurrich liquid (S)l. At 743 °C, cubic FeS2 (pyrite) forms peritectically and undergoes a transition to the orthorhombic form (marcasite) at 425 °C. The phase relations below 350 °C in the pyrrhotite region are complex with the occurrence of several ordered forms. The only intermediate phase of the Mn-S system, MnS, has the NaCl type cubic structure and melts at 1655 °C [Massalski2]. The system is characterized by the presence of a miscibility gap between Mn-rich and MnS-rich liquids, with the monotectic temperature at 1570 °C and the final eutectic solidification near the Mn-end.

Thermodynamic Assessments The thermodynamic treatments of this system by [1998Mie] and [2000Oht] pertain to the Fe-FeS-MnS-Mn region of the system. The solubility of FeS in cubic MnS is up to 79 mol% [1988Rag]. This phase, labeled (Mn,Fe)S by [1988Rag], is called the Q phase after [1937Vog, 1998Mie, 2000Oht]. The solubility of MnS in hexagonal Fe1−xS is only 8 mol% [1988Rag]. This phase is denoted P [1937Vog, 1998Mie, 2000Oht]. [1998Mie] treated the liquid phase on the basis of a two sublattice model (Fe,Mn)(S,Va), where Va stands for vacancies. The two sublattice model was also applied to the sulfide solid solution (Fe,Mn)S, omitting the vacancies. The solid phases, fcc, bcc, and the cubic forms of Mn, were considered as substitutional solutions. The description of the Fe-FeS system was basically from [1984Din], with a reassessment of the interaction parameters using the SGTE phase stability data. The description of the Mn-MnS system by [1976Sta] was used by [1998Mie]. For the Fe-Mn sys-

Fig. 1 Fe-Mn-S computed liquid miscibility gap near the Fe corner at 1600 °C [1998Mie]

tem, the assessment of [1989Hua] was adopted. The ternary interaction parameters LFe,Mn:S and LFe,Mn:S,Va were evaluated by [1998Mie]. The computed results were presented as the FeS-MnS pseudobinary system, the same system in the presence of Fe, the liquidus projection, the liquid miscibility