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Section II: Phase Diagram Evaluations

Fe-Os-S (Iron-Osmium-Sulfur) V. Raghavan

Very recently, [2002Kar] determined three isothermal sections for this ternary system between 1180 and 900 °C.

Binary Systems A partial Fe-Os phase diagram has been determined experimentally for Fe-rich alloys [1982Kub1]. By analogy with the Fe-Ru phase diagram, a tentative full diagram has been proposed [1982Kub1, 1993Swa]. This tentative diagram shows only solid solutions based on the terminal phases. The work on this ternary system by [2002Kar], however, shows the presence of an intermediate phase of rhombohedral symmetry along the Fe-Os side, as discussed below. There are two intermediate phases in the Fe-S system [1982Kub2]. The monosulfide pyrrhotite Fe1−xS (hexagonal NiAs type) is stable at Fe-deficient (S-rich) compositions with a range of 50-55 at.% S. Fe1−xS at 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 sulfur-rich liquid (S)l. At 743 °C, cubic FeS2 (pyrite) forms peritectically and undergoes a transition to orthorhombic FeS2 (marcasite) at 425 °C. The phase relations below 350 °C in the pyrrhotite region are complex with the occurrence of several ordered forms. In the Os-S phase diagram, a eutectic reaction L ↔ (Os) + OsS2 (cubic pyrite type) occurs at ∼2000 °C and ∼51 at.% S [1992Fis].

0.4320 nm for this phase agree with those listed by [Pearson3] for pure Os (cph). The a and c parameters of the other pair (for the rhombohedral phase) are lower and increase with increasing Os content as shown in Fig. 1. Pending further confirmation of the existence of the rhombohedral phase, the results of [2002Kar] are accepted tentatively and are shown in Fig. 2 and 3. At 1180 °C (Fig. 2), the ␥ phase has a homogeneity range of 0-21.5 at.% Os. The rhombohedral phase (␤) is stable between 27.3 and ∼85 at.% Os. The (Os) phase (hcp) has a range of 98.5-100 at.% Os. The measured S content in all the three alloys is less than 0.1 at.%. Along the Fe-S side, at 1180 °C, there are two sulfide melts L1 and L2 on either side of pyrrhotite (Fe1−xS; Fig. 2). The entire composition range of ␥ and most of the range of ␤ coexist with the S-poor (Fe-rich) sulfide melt L1 (Fig. 2). In the three-phase equilibrium of L1, ␥ and ␤, L1 contains 43.3 at.% S. The Os content of L1 for the end poorest in S is 0-0.02 at.% and at the other end (50 at.% S), it is 0.06-0.1 at.%. In the threephase equilibrium of L1, Fe1−xS and (Os), Fe1−xS contains 0.4 at.% Os. It increases to 0.7 at.% in the three-phase equilibrium of Fe1−xS (53.4 at.% S), (Os) and OsS2. In the three-phase equilibrium of Fe1−xS, L2 and OsS2, OsS2 contains 0.2 at.% Fe and this increases to 1.1 at.% Fe at the three-phase equilibrium of L2, OsS2 and (S)l. In the isothermal section at 1100 °C [2002Kar] not shown here, the triangulation is the same as at 1180 °C. The homogeneity ranges of ␥, ␤, and (Os) are 0-20.2, 26.1-80, and 98.4-100 at