Tailoring the stability of iron carbides to enhance the mechanical performances of Fe–C–Mn–Si alloys

The Fe–C–Mn–Si alloys are promising advanced high strength steels (AHSSs) with their performances greatly influenced by iron carbides precipitated in quenching and partitioning process. To date, the structural characteristics and physicochemical properties of iron carbides, particularly the non-stoichiometric carbides like η and ε phases, still remain elusive. In the present work, the atomic structures, phase stability, mechanical properties, and temperature-dependent thermodynamic properties of potential iron carbides precipitated in Fe–C–Mn–Si alloys, including η-Fe₂C-Pnnm, ε-Fe₂C-P6₃/mmc, ε-Fe_nC-P6₃22, θ-Fe₃C-Pnma and χ-Fe₅C₂-C2/c, are investigated by performing the first-principles calculations based on density functional theory. It turns out that the hexagonal Fe_2.4C and Fe₃C carbides have the same space group of P6₃22, with their local atomic structure being featured by the C-centered CFe₆ principal cluster with coordination number of six. Thus, these two phases are regarded as the same ε iron carbide. The orthorhombic η-Fe₂C and θ-Fe₃C phases are more stable than the hexagonal ε-Fe₂C and ε-Fe_nC (n = 2.4, 3) carbides, resulting in a potential phase transition from ε to η and/or θ. Analysis on stabilities indicate that the C atoms in iron carbides prefer to occupy the ordered octahedral and prismatic interstices. In comparison with the ε-Fe_nC (n = 2.4, 3) carbides, which is stiffer at low temperatures, the precipitation of the θ-Fe₃C and η-Fe₂C carbides can benefit to strengthen the Fe–C–Mn–Si alloys at elevated temperatures. The present work provides an important insight for understanding the atomic structures and thermodynamic properties of iron carbides along with their contribution to the mechanical performances of AHSSs.