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 Fe4N and Fe16N2 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 Fe4N, Fe16N2 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 0K to 865K. Both the range of metastability for Fe16N2 and the nitrogen solvus confirm the experimental data. On the other hand, it was concluded that the experimental Curie temperature for Fe16N2 is severely underestimated because of the thermodynamic instability above 400K.