Unusual nucleation and growth of γ′ iron nitride upon nitriding Fe-4.75 at.% Al alloy

The influence of substitutionally dissolved Al in ferritic Fe-4.75 at.% Al alloy on the nucleation and growth of gamma' iron nitride (Fe4N1-x) was investigated upon nitriding in NH3/H-2 gas mixtures. The nitrided specimens were characterised employing optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis and X-ray diffraction. As compared to the nitriding of pure ferrite (alpha-Fe), where a layer of gamma' develops at the surface, upon nitriding ferritic Fe-4.75 at.% Al an unusual morphology of gamma' plates develops at the surface, which plates deeply penetrate the substrate. In the diffusion zone, nano-sized precipitates of gamma' and of metastable, cubic (NaCl-type) AlN occur, having, with the ferrite matrix, a Nishiyama-Wassermann orientation relationship and a Bain orientation relationship, respectively. The gamma' plates contain a high density of stacking faults and fine epsilon iron nitride (Fe2N1-z) precipitates, although the formation of epsilon iron nitride is not expected for the employed nitriding parameters. On the basis of dedicated nitriding experiments it is shown that the unusual microstructural development is a consequence of the negligible solubility of Al in gamma' and the obstructed precipitation of the thermodynamically stable, hexagonal (wurtzite-type) AlN in ferrite.