Understanding of the thermodynamics of the iron-nitrogen system is a basis for systematically influencing the outcome of nitriding treatments of iron-based alloys and of other metallurgical processes involving Fe and N. Thereby, the thermodynamics of the Fe-N system, in particular, reflects the intrinsic properties of the involved phases, e.g. the differently ordered versions of fcc and hcp based solid solutions and corresponding intermediate phases (labelled gamma and epsilon, together with primes for the ordered versions). These are strongly affected by their composition-dependent magnetic properties including changes from low-spin to high-spin states for the iron, which are associated with characteristic composition-dependent variations of the atomic volume. Due to the latter, the phase equilibria in the Fe-N system are significantly modified with increasing pressures. These was studied by post mortem analysis of Fe-N alloys treated in an multi anvil apparatus [1,2, unpublished] and thermodynamic analysis using the CALPHAD method [3]. Most characteristically this includes the disappearance of the high-volume gamma'-Fe4N phase at about 5 GPa.
The phase equilibria are also characteristically influenced by partially substituting Fe by other metallic elements as it is the case in many technical alloys. These elements may lead to formation of hardening nitride precipitates and/or they can dissolve in the iron nitrides. The general thermodynamics of such systems is discussed. Moreover, the example of Fe-Cu alloys is presented, where Cu appears to characteristically stabilize the gamma'-Fe4N phase [4].
[1] M. H. Wetzel et al., 801 (2019) 438
[2] M.H. Wetzel et al., Materials 14 (2021) 3963
[3] M.H. Wetzel et al., J. Alloys Compd. 914 (2022) 165304
[4] A. Leineweber et al., Metals 12 (2022) 619