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

Abstract

The influence of substitutionally dissolved Al in ferritic Fe-4.75 at.% Al alloy on the nucleation and growth of γ́ iron nitride (Fe 4N 1-x) was investigated upon nitriding in NH 3/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 (α-Fe), where a layer of γ́ develops at the surface, upon nitriding ferritic Fe-4.75 at.% Al an unusual morphology of γ́ plates develops at the surface, which plates deeply penetrate the substrate. In the diffusion zone, nano-sized precipitates of γ́ 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 plates γ́ contain a high density of stacking faults and fine iron nitride (Fe 2N 1-z) precipitates, although the formation of 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 γ́ and the obstructed precipitation of the thermodynamically stable, hexagonal (wurtzite-type) AlN in ferrite. © 2012 Copyright Taylor and Francis Group, LLC.