Development of thermodynamic database for high Mn-high Al steels: Phase equilibria in the Fe-Mn-Al-C system by experiment and thermodynamic modeling

Fe-Mn-Al-C system is a core system of high Mn-high Al steels for applications in automotive, cryogenic, electrical steels, etc. In order to provide an efficient tool for alloy/process design, a thermodynamic database of the quaternary system was developed in the present study by experimental phase diagram study coupled with CALPHAD type thermodynamic modeling. All stable phases relevant to steel matrix and precipitates including K-carbide and bcc_B2 (alpha') were considered in the modeling. High temperature equilibrated samples followed by an electron microscopy analysis showed that steel matrix of the austenite (gamma) was observed often to be in equilibrium with the K-carbide. Homogeneous region of the gamma enlarged by increasing Mn content up to 20 mass%, then shrinked by further increasing the Mn content up to 40 mass%. The phase diagram information experimentally determined in the present study and those reported in the literature were utilized in order to model Gibbs energies of the relevant phases. The Compound Energy Formalism (CEF) was used for each solid solution with proper consideration of its crystal structure. In particular, the K-carbide was modeled by a formula (Fe,Mn)(3)(Fe,Mn,Al)(1)(Va,C)(1) and alpha' was modeled by a formula (Fe,Mn,Al)(0.5)(Fe,Mn,Al)(0.5)(Va,C)(3) in order to take into account A2/B2 (alpha/alpha') 2nd order transition, as being important intermetallic precipitates. The Modified Quasichemcial Model in the pair approximation was employed for the liquid phase in order to take into account strong chemical interactions among components. The obtained Gibbs energy equations form a complete thermodynamic database for the Fe-Mn-Al-C system. It can be used to predict alloy phase equilibria of the system, and also can be a part of larger multicomponent steel database. (C) 2015 Elsevier Ltd. All rights reserved.