Effect of Cu on Nitriding of α-Fe

Nitriding of Fe-1 wt.% Cu and Fe-5 wt.% Cu alloys at 813 K leads to the formation of predominantly the γ′-iron nitride phase (γ′-Fe4N) when using nitriding conditions, which lead to pronounced formation of ε-iron nitride phase (ε-Fe3N1+x) upon nitriding of pure α-Fe. Energy dispersive X-ray analysis reveals that the developing γ′ can attain a Cu content corresponding to that of the base material. In contrast, tiny amounts of ε-nitride that eventually develop contain considerably less Cu. The microstructure implies that the formation of the ε-nitride requires long-range substitutional interdiffusion to achieve the Cu partitioning. These observations were interpreted in terms of a significantly higher solubility of Cu in the γ′ phase than in the ε phase, which is explainable by the phases’ crystal structures. The observations were rationalized in terms of schematic Fe–Cu–N phase diagrams valid for 813 K.