Defect-dependent nitride surface layer development upon nitriding of Fe–1 at.% Mo alloy

Abstract

Upon nitriding of binary Fe–1 at.% Mo alloy in a NH₃/H₂ gas mixture under conditions (thermodynamically) allowing γ′-Fe₄N_{1– x} compound layer growth (nitriding potential: 0.7 atm^−1/2 at 753 K (480 °C) – 823 K (550 °C)), a strong dependency of the morphology of the formed compound layer on the defect density of the specimen was observed. Nitriding of cold-rolled Fe–1 at.% Mo specimens leads to the formation of a closed compound layer of approximately constant thickness, comparable to nitriding of pure iron. Within the compound layer, that is, in the near-surface region, Mo nitrides are present. The growth of the compound layer could be described by a modified parabolic growth law leading to an activation energy comparable to literature data for the activation energy of growth of a γ′-Fe₄N_{1− x} layer on pure iron. Upon low temperature nitriding (i.e. ⩽793 K (520 °C)) of recrystallized Fe–1 at.% Mo specimens, an irregular, ‘needle-like’ morphology of γ′-Fe₄N_{1− x} nucleated at the surface occurs. This γ′ iron nitride has an orientation relationship (OR) with the matrix close to the Nishiyama–Wassermann OR. The different morphologies of the formed compound layer can be interpreted as consequences of the ease or difficulty of precipitation of Mo as nitride as function of the defect density.

Keywords:

• gas nitriding
• binary Fe–1 at.% Mo alloys
• nitride layer development
• morphology
• kinetics

Acknowledgements

We are grateful to Mrs. S. Haug (Max Planck Institute for Intelligent Systems) for electron probe microanalysis and to Dipl.-Ing. P. Kress (Max Planck Institute for Intelligent Systems) for assistance with the nitriding experiments.

Notes

1. This contrasts with what is observed for γ′-layer growth on pure iron, where growth of γ′ in the first stage of nitriding, where no closed layer of γ′ nitride occurs at the surface, is relatively fast, as the nitrogen follows a short-circuit path through the ferrite matrix, by-passing diffusion through the already formed γ′ nuclei (diffusion of N through ferrite is much faster than through γ′ nitride) [16].

2. In quasi-steady-state diffusion, at a given time, the diffusive flux through the growing γ′ layer, J_N ^{( r ′)} , is constant throughout the γ′ layer and its instantaneous value depends only on the actual layer thickness. This approximation is valid because of the very small composition range of γ′ iron nitride and the large concentration difference between the γ″ layer and the γ′-iron substrate, as confirmed by numerical calculations [21].