Linear stability analysis of a γ′-Fe4N nitride layer growing in pure iron

In nitriding processes of steels, corrugations are commonly observed at the interface between nitride layers and the ferritic matrix. These corrugations have been explained as the consequence of the destabilization of the interface. We have performed a linear stability analysis in the flavour of the Mullins–Sekerka analysis on the interface separating a growing nitride layer in pure iron. This analysis shows that the corrugations cannot be due to a Mullins–Sekerka type instability, because the interface is absolutely stable, the composition field and the interfacial energy being both stabilizing. We conclude by a discussion on the role of the stress fields in the appearance of the corrugations.