Immiscibility regions in iron based ferritic solid solutions and their relevance to thermodynamics and kinetics of nitriding

In the process of equilibration of a ferritic iron-based alloy with gaseous nitriding atmosphere, the inwardly diffusing N into the ferritic solid-solution from the gas atmosphere may bring the evolving solid-solution chemistry into immiscible region of the corresponding phase diagram. Distinct kinetic mechanisms of nitriding are operative in different alloy systems, depending on whether the alloy system has a region of immiscibility or not and whether the applied chemical potential of N in the nitriding atmosphere allows the evolving chemistry of the alloy to sample the immiscibility region or not. With this new kind of thermodynamic interpretation, it is now possible to precisely understand the experimental results reported in the literature pertaining to nitrided iron-based binary alloys. Slow kinetics of nitride precipitation in nitrided ferritic Fe-Si and Fe-Al alloys has been attributed to the absence of immiscibility region in ferritic Fe-Si-N and Fe-Al-N systems whereas the slower precipitation of nitrides in ferritic Fe-Mo alloys has been attributed to the implausibility of sampling the immiscibility region of Fe-Mo-N system for typically applied nitriding conditions and Mo contents. Faster kinetics of nitride precipitation in Fe-Ti, Fe-V, Fe-Cr alloys has been attributed to the presence of immiscibility in Fe-Ti-N, Fe-V-N and Fe-Cr-N systems and the applied nitriding conditions allowing sampling of this immiscibility region. Recognising the role of ‘miscibility gap’ in alloy systems to realise rapid kinetics during nitridation treatments opens up a new, unexplored alloy design strategy for the development of steels with favourable nitriding response.