Diffusion phenomena, phase and microstructure formation during nitriding of FeAl40

Nitriding of FeAl40 (Fe–40 at% Al) intermetallic high-temperature alloy allows improving its anti-wear properties. Such nitriding leads to the formation of an outer white layer composed of iron nitrides and an inner diffusion layer with precipitated AlN. In the present work, gaseous nitriding was performed at 590 °C over a wide range of nitriding potentials and treatment times. The specimens have been investigated by scanning electron microscopy supplemented by electron backscatter diffraction, electron probe microanalysis, transmission electron microscopy and X-ray diffraction. These measurements reveal the microstructure evolution and form the basis for explaining the observed layer growth kinetics due to internal nitriding involving inwards diffusion of N and immediate precipitation of lamellar AlN. The compressive stress induced by N uptake leads to previously reported out-diffusion of preferably Fe developing the outer white layer, but also to diffusion of Fe to grain boundaries oriented parallel to the surface developing Fe-rich wave-like features. It is argued that the high interface density in the diffusion layer associated with the lamellar AlN obviously facilitates these diffusive processes, since Fe volume diffusion is not sufficiently rapid to account for the observed diffusion lengths. With increasing nitriding potentials also appreciable regions of the diffusion layer transform to iron nitrides. Notably, the ε-iron nitrides develop in the lamellar AlN-containing regions, whereas the Fe-rich wave-like features transform to γ′-iron nitrides. It is likely the presence of Al and/or microstructural constraints that prevent the formation of the γ′-iron nitrides in the case of the lamellar AlN-containing regions.