Structure and stability of Fe2C phases from density-functional theory calculations

Fe2C plays a crucial role in the precipitation of iron carbides. Jack's structural models for ε-Fe2C, and non-stoichiometric ε-Fe2.4C, are analyzed using first-principles calculations. Several new configurations of ε-Fe2C with even higher stability are found. We show how ε-Fe2C transforms into η-Fe2C, and address the structural relationships with the χ-Fe5C2, θ-Fe3C and Fe7C3 phases. The relative occurrence of these phases in steel, as well as their probable evolution during tempering of quenched steels, is discussed.