The Fe–Mn enthalpy phase diagram from first principles

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Density-functional theory-based total-energy calculations have been performed in order to ascertain the heats of formation of the binary phases Fe1–xMnx in the bcc, fcc, and hcp structure types. Each structure type and composition range of the solid solutions have been critically assessed with respect to their magnetic ground state and the course of the molar volume as a function of the stoichiometry. All theoretical energy data have been combined into an enthalpy–composition phase diagram, to be critically compared with an analogous plot derived from the CALPHAD method. The two radically different methods result in very similar diagrams. Noticeable deviations do occur as well, particularly for highly unstable regions where the real magnetic ground state is a matter of debate.