Thermodynamics of the solid phases in the system Fe−Mn−C

Abstract

Phase relationships in the Fe−Mn−C system in the temperature range 600 to 1100°C have been studied using metallographic and X-ray methods and the electron microprobe. Isothermal sections of the phase diagram of the system are reported based on the present results and those of earlier investigators. The fcc λ-phase (austenite) containing carbon is stable at all values ofy _Mn=x _Mn/(x _Mn+x _Fe) in the range 890 to 1100°C and in a more restricted composition range at lower temperatures. Its composition under conditions of equilibrium with the carbides (Fe, Mn)₃C, (Fe, Mn)₂₃C₆, ε, and liquid are shown for several temperatures. The free energy of formation of the cementite phase, (Fe, Mn)₃C, at 1000°C, from γ-Fe, γ-Mn (undercooled) and graphite is ΔG ₁₂₇₃=−35,790y _Mn−2760y _Fe+3RT (y _Mn lny _Mn+y _Fe lny _Fe). The data show that the alloyed cementite is essentially and ideal mixture of Fe₃C and Mn₃C,i.e., the metal atoms are distributed at random on the metal sites in the lattice.