Stability, structure and electronic properties of γ-Fe23C6 from first-principles theory

First-principles’ calculations of GGA and GGA + U type have been performed for γ-Fe23C6, a complex iron carbide with 116 atom in the unit cell. GGA results were found to be in better agreement with experimental data than GGA + U results. Various occupancies for Wyckoff positions and corresponding magnetic orderings have been explored. Our calculations reveal that the crystal structure is composed of a framework of strongly linked Fe atoms, and additional stabilizing Fe and C atoms positioned in cavities. The local electronic and magnetic properties vary strongly among the non-equivalent Fe sites in γ-Fe23C6. The lattice parameters of γ-Fe23C6 match those of austenite well. Surprisingly, pure γ-Fe23C6 is found to be more stable than commonly occurring θ-Fe3C cementite. Moreover, the calculations show low vacancy energy (about 0.37 eV) for Fe at 4a sites in γ-Fe23C6. Conditions of formation and factors hampering the formation of γ-Fe23C6 in steel manufacturing processes are discussed.