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, Fe3AlC (κ), by considering the crystallographic similarity between the κ 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 κ and L12 structures were treated as a continuous solution. Because of the lack of experimental information available, the thermodynamic properties of the E21 structure were evaluated using the Full Potential Linearized Augmented Plane Wave (FLAPW) method. The ab initio energetic calculations show that the stable E21 structure of the κ phase is highly preferred compared with the metastable Fe3Al-L12 structure. Contour plots of the charge density for the κ phase indicate that the bonding between the Fe and C atoms forms in the Fe3AlC-E21 structure, and that this interaction between the atoms enhances the energetic stability of the κ phase. According to our phase diagram calculations, the κ 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.