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hase Diagram Evaluations: Section II

C-Fe-Mn (Carbon-Iron-Manganese) V. Raghavan

The early review of this ternary system by [1988Ray] presented a liquidus projection, isothermal sections at 1100, 1000, 900, 800 and 600 °C and isothermal sections for the Fe-rich region at 700, 675, 650 and 600 °C. The isothermal sections were for the stable equilibrium with graphite. An update by [1994Rag] reviewed the thermodynamic analysis of [1990Hua] and the high pressure equilibria studied by [1990Put]. Recently, a new thermodynamic assessment of the system was carried by [2011Dju], which included new ab initio calculations of the enthalpies of formation of metastable iron carbides in the optimization.

Binary Systems The Fe-C phase diagram [1992Oka] is in the form of a double diagram, corresponding to metastable equilibrium with cementite (Fe3C) (D011, Fe3C-type orthorhombic) or stable equilibrium with graphite (A9, hexagonal). The peritectic formation of austenite (Fe-based fcc solid solution c) is followed by the eutectic reaction, which yields austenite and cementite (or graphite). The eutectoidal decomposition of austenite yields ferrite (Fe-based bcc solid solution a) and cementite (or graphite). The Mn-C phase diagram [Massalski2] depicts the following intermediate phases: e (13.524.5 at.% C; hexagonal; stable between 1308 and 990 °C), Mn23C6 (D84, Cr23C6-type cubic), Mn3C (stable between 1052 and 971 °C; Fe3C-type orthorhombic), Mn5C2 (monoclinic, C2/c) and Mn7C3 (D101, Cr7C3-type orthorhombic). There are no intermediate phases in the Fe-Mn system. A thermodynamic description of Fe-Mn was presented by [1989Hua] and more recently by [2004Wit].

and a = 1 for fcc, A12 and A13 structures, 0.5 for hcp and 3 for bcc. The metallic carbides were described as follows. M23C6: (Fe,Mn)20(Fe,Mn)3(C)6, M3C: (Fe,Mn)3(C), M5C2: (Fe, Mn)5(C)2, and M7C3: (Fe,Mn)7(C)3. In the case of M23C6, an approximately uniform distribution of Mn in the first two sublattices was assumed. The key literature data on phase equilibria and thermodynamic properties were listed, indicating those used in the optimization. Among the recent data, the work of [2003Kim] on the thermodynamics of C in liquid Fe-Mn alloys and of [2004Kim] on the effect of C on the c M e transformation were included in the optimization. The optimized parameters were listed. [2011Dju] carried out the calculations for the stable equilibrium with graphite (gr), cementite (Fe3C) being a metastable phase in the Fe-C system. Two isothermal sections computed by [2011Dju] at 1100 and 600 °C are shown in Fig. 1 and 2. When compared with the computed sections of [1990Hua], the main difference is that the liquid phase calculated by [1990Hua] at 1100 °C within the ternary field is absent in the present results, see Fig. 1. The computed liquidus projection is shown in Fig. 3 [2011Dju]. There are three ternary peritectic reactions on the surface: P1 (1529 °C), P2 (1458 °C) and P3 (1442 °C) and three U-type transition reactions: U1 (1524 °C), U2 (1419 °C) and U3 (1392 °C). Five vertical sections wer