Unusual nucleation and growth of γ′ 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 γ′ iron nitride (Fe₄N_{1− x} ) was investigated upon nitriding in NH₃/H₂ 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 γ′ develops at the surface, upon nitriding ferritic Fe–4.75 at.% Al an unusual morphology of γ′ plates develops at the surface, which plates deeply penetrate the substrate. In the diffusion zone, nano-sized precipitates of γ′ 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 γ′ plates contain a high density of stacking faults and fine ε iron nitride (Fe₂N_{1− z} ) precipitates, although the formation of ε 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 γ′ and the obstructed precipitation of the thermodynamically stable, hexagonal (wurtzite-type) AlN in ferrite.

Keywords:

• Fe–Al alloy
• gas nitriding
• kinetics
• iron nitride
• AlN

Acknowledgements

We are grateful to Mr. J. Koehler and Mr. P. Kress for assistance with the nitriding experiments, Mrs. S. Haug for assistance with the EPMA measurements, Mr. W.-D. Lang for TEM sample preparation, Mrs. S. Kühnemann for SEM investigations and to Dr. A. Leineweber for discussion (all with the Max Planck Institute for Intelligent Systems).