N-Atom Doping of ω–Fe, α–Fe, and γ–Fe Compounds: A First-Principle Study

Recently, a new phase, ω–Fe, has been observed in martensitic substructures, providing a new path for studying the position and evolution of nitrogen in high-nitrogen steels. In this paper, the density functional method was used to investigate the thermodynamic and dynamic stability of N atoms in the phases of ω–Fe, α–Fe, and γ–Fe in martensite, as well as the influence of magnetic order on them. The calculated results show that in the pure Fe phases, ferromagnetic α–Fe is a stable phase both in thermodynamics and dynamics. ω–Fe and γ–Fe are most stable in ferrimagnetism and show dynamic stability, while in ferromagnetic state they are unstable in both thermodynamics and dynamics. N-atom doping of 25% (Fe3N) makes γ–Fe and ω–Fe thermodynamically and dynamically stable in ferromagnetic state. However, a higher N content is not conducive to the stability of ω–Fe and γ–Fe. The electronic structure shows that as the content of N atoms becomes higher than 25%, the 2p orbitals of N atoms move towards the Fermi level and become more dispersed, resulting in a large contribution of the density of states at the Fermi level. In addition, N atoms are not conducive to the stability of α–Fe, as they relax to the structure of γ–Fe at 25% N content (Fe3N), while α–Fe in higher N contents (Fe3N2 and FeN) relaxes to the structure of ω–Fe correspondingly. Obviously, N tends to stabilize in the ω and γ phases in martensite, and our study provides a new clue for the formation mechanism of nitrides and martensitic transformation in Fe–N alloys.