Just as carbon, nitrogen is one of the most effective solid solution strenghteners in steel. Especially in austenitic stainless steels it is commonly used as cheap alloying element which at the same time enhances the corrosion resistance, particularly in Cl-containing environments. Although the Fe-N phase diagram is rather well-known and looks quite similar to the Fe-C phase diagram, certain aspects remain uncertain today. In particular, the kinetics of transformations during cooling from the high temperature austenite are not well described. This includes the formation and kinetics of metastable displacive or non-displacive phases (‘martensite-like’ or ‘bainite-like’) but also the stability of the Fe4N phase. This phase can be compared to the classic Fe3C or cementite in the Fe-C diagram, for which it is known that two related metastable carbides exist (-carbide: hexagonal close-packed with Fe2-3C stoichiometry, and Hägg carbid: monoclinic with a stoichiometry of Fe5C2). It is therefore reasonable to assume that metastable nitrides can be formed as well, for example during tempering of the transformation product N-containing austenite. Finally, it is known that the nucleation of Fe3C is fairly difficult and it is difficult to precipitate cementite in steels with less than 0.01% C. No consistent data is available on the kinetics of Fe4N formation.

Goal The goal of this project is to obtain a better image of the phases that form during cooling from the high-temperature austenite, from an experimental as well as from a computational point of view (which of both aspects is to be emphasized, depends on the interest and background of the student, and two students can work together on complementary aspects). Microstructure evolution, morphology changes and hardness evolution will be determined by dilatometer testing at various cooling rates, on binary Fe-N alloys with various N-contents. Quantum simulations will be used to rationalize these observations, and in particular to understand the differences between Fe-C and Fe-N model systems. In particular, the potential stability of metastable nitride phases will be adressed. As such, a better understanding is to be obtained on the transformation behaviour in the Fe-N binary system, which is essential for future product developments.