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Phase Diagram Evaluations: Section II

Fe-Ir-Pt-S (Iron-Iridium-Platinum-Sulfur) V. Raghavan

The phase relations of this quaternary system were partially characterized at 1100 and 1000 °C by [2000Mak].

Binary Systems For brief descriptions of the Fe-Ir, Fe-S, and Ir-S systems, see the Fe-Ir-S review in this issue. The Fe-Pt and Pt-S phase diagrams are briefly covered under Fe-Pt-S. Ir and Pt [both face-centered-cubic (fcc)] form a continuous solid solution over a wide temperature range. A miscibility gap appears at lower temperatures, where two fcc phases (Ir-rich and Pt-rich) are present [Massalski2].

Ternary Systems Updates on the Fe-Ir-S and Fe-Pt-S systems appear in this issue. The Fe-Ir-Pt and Ir-Pt-S systems do not appear to have been investigated.

Quaternary Phase Equilibria With starting materials of Fe (15-20 ppm metallic impurities), 99.98% Pt, 99.995% Ir and 99.999% S, [2000Mak]

melted 112 samples in evacuated glass tubes with annealing times of 4-5 weeks. Compositions in the S rich region were not investigated. The quenched samples were examined by optical microscopy and electron probe microanalysis. The partial phase relations at 1100 and 1000 °C were presented in two ways. Ternary alloy compositions, in equilibrium with the various sulfide phases (Fe1−xS, PtS, Ir2S3, the ternary phase ␶ originating from the Fe-Ir-S system and the two sulfide melts L1 and L2 from the Fe-S system), were plotted on the Fe-Ir-Pt plane. Projections were also made on this plane of the coexisting compositions for two-phase and three-phase equilibria and of the tie-tetrahedra for fourphase equilibria. No new ternary or quaternary phase was found. The ternary alloy phases in equilibrium with the sulfide phases at 1100 and 1000 °C are shown Fig. 1 and 2. At 1100 °C (Fig. 1), a major part of the field is occupied by the fcc solid solution ␥, with a miscibility gap at the Pt end between ␥ and (Pt). The composition ranges (in at.% Pt) of the phases along the Fe-Pt side are ␥ (0-43), FePt (45.8-58), FePt3 (60-77), and (Pt) (80-100), with little or no solubility of S. The maximum solubilities of Ir in FePt, FePt3 and (Pt) are 24.0, 29.3 and 5.1 at.% respectively. The approximate projections of the sulfide phase fields on the Fe-Ir-Pt plane are indicated in Fig. 1 [2000Mak].

Fig. 1 Fe-Ir-Pt-S composition of alloy phases in equilibrium with sulfide phases at 1100 °C [2000Mak]

Journal of Phase Equilibria and Diffusion Vol. 25 No. 3 2004

289

Section II: Phase Diagram Evaluations

Fig. 2 Fe-Ir-Pt-S composition of alloy phases in equilibrium with sulfide phases at 1000 °C [2000Mak]

At 1000 °C (Fig. 2), the area occupied by the fcc solid solution ␥ reduces significantly as compared with that at 1100 °C in Fig. 1. The composition ranges (in at.% Pt) of the phases along the Fe-Pt side are ␥ (0-38), FePt (41-57.5), FePt3 (60.5-77), and (Pt) (82-100). The maximum solubilities of Ir in FePt, FePt3, and (Pt) are ∼17, 23.6, and 2.2 at.% respectively. The approximate projections of the sulfide phase fields on the Fe-Ir-Pt plane are indic