Concentration-gradient-driven anisotropic spinodal decomposition kinetics: nitriding of metallic alloys

ABSTRACT

The effect of crystallographically oriented, unidirectional concentration gradients on spinodal decomposition in cubic crystalline solids with elastic and interfacial energy anisotropy is discussed. Phase-field simulations reveal that the kinetics of spinodal decomposition occurring in such systems is dependent on the degree of misorientation between the direction of composition gradient and the preferred crystallographic orientation for growth of spinodal fluctuations; the larger is the misorientation, the slower the kinetics. This phenomenon has been used to explain the well-known grain-orientation-dependent N-uptake kinetics observed during nitriding of metallic alloys. Several plausible causes have been proposed in the literature for the grain-orientation-dependent N-uptake kinetics during nitriding. However, this study reveals that this phenomenon is observed exclusively and without exception in alloy systems having a spinodal instability. The N-uptake kinetics in such systems is known to be dependent on the kinetics of spinodal decomposition. Consequently, anisotropic spinodal decomposition kinetics occurring owing to the presence of a surface-directed N-composition gradient in poly-crystalline metals has been shown to be a more fundamental cause for the phenomenon.

KEYWORDS:

• Spinodal decomposition
• nitriding
• kinetics
• elastic anisotropy
• interfacial energy anisotropy

Acknowledgements

We are grateful to Max Planck Society, Germany for financial support to the Max-Planck Partner Group of the Department of Microstructure Physics and Alloy Design, Max Planck Institute for Iron Research, Dusseldorf, Germany at Indian Institute of Technology Roorkee, Roorkee, India. Authors are also grateful to the Ministry of Human Resource Development, Government of India, for financial support under PMRF.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 Owing to the small N solubility in the solid solution (maximum of 0.4 at. % for pure ferritic iron at 594°C), development of an N-enriched second phase is essential for continuous N uptake into the specimen. Thus, the mechanism of this N-enriched second phase development becomes the rate-governing mechanism for N uptake. For the alloys identified, phase separation by spinodal decomposition is the mechanism for N-enriched second-phase development (on account of kinetic constraints for direct development of equilibrium nitride precipitates).

Additional information

Funding

This work was supported by Max-Planck-Gesellschaft [grant number partner group].