A first-principles reassessment of the Fe-N phase diagram in the low-nitrogen limit

Abstract Nitriding of steels has been widely used for almost a century. However, insight in two important precipitating phases for low concentration through-thickness nitriding is still lacking, hindering further development of the process. Due to their metastable nature, manufacturing large homogeneous samples of Fe 4 N and Fe 16 N 2 is very challenging. Consequently, measuring thermodynamic properties, such as heat capacity and free energy, has proven difficult at best. In this work, we have calculated those thermodynamic properties using density-functional theory (DFT) for Fe 4 N, Fe 16 N 2 and ferrite with nitrogen in solid solution. This information is a prerequisite to improve the accuracy of larger-scale modeling approaches of iron nitrides. We used the free energies to construct the temperature/concentration phase diagram for low nitrogen concentrations from 0 K to 865 K . Both the range of metastability for Fe 16 N 2 and the nitrogen solvus confirm the experimental data. On the other hand, it was concluded that the experimental Curie temperature for Fe 16 N 2 is severely underestimated because of the thermodynamic instability above 400 K .