Thermodynamic Analysis of the Fe-Al-C Ternary System by Incorporating ab initio Energetic Calculations into the CALPHAD Approach

Abstract

A thermodynamic analysis of the Fe-C-Al ternary system has been carried out covering a wide range of temperatures and composition. Special care was taken to the expression of the free energy for the ternary Perovskite carbide phase, Fe 3 AlC (κ), by considering the crystallographic similarity between the K phase and the L12 structure. The free energy was calculated using the (Fe,Al) 3 (Fe,Al) 1 (C,Va) 1 sublattice model, and the K and L12 structures were treated as a continuous solution. Because of the lack of experimental information available, the thermodynamic properties of the E2 1 structure were evaluated using the Full Potential Linearized Augmented Plane Wave (FLAPW) method. The ab initio energetic calculations show that the stable E2 1 structure of the K phase is highly preferred compared with the metastable Fe 3 Al-L1 2 structure. Contour plots of the charge density for the K phase indicate that the bonding between the Fe and C atoms forms in the Fe 3 AlC-E2 1 structure, and that this interaction between the atoms enhances the energetic stability of the K phase. According to our phase diagram calculations, the K phase is in equilibrium with the fcc Fe, B2-type intermetallic compound, as well as the graphite phase. This finding is in good agreement with previous experimental results.