Layer-growth kinetics on gaseous nitriding of pure iron: Evaluation of diffusion coefficients for nitrogen in iron nitrides

Abstract

Models were derived for monolayer and bilayer growth into a substrate in which diffusion of the solute governs the growth kinetics, as in gas-solid reactions, for example. In the models, the composition dependence of the solute diffusivity in the phases constituting the layers was accounted for by appropriate definition of an effective diffusion coefficient for a (sub)layer. This effective diffusion coefficient is the intrinsic diffusion coefficient weighted over the composition range of the (sub)layer. The models were applied for analyzing the growth kinetics of a γ′-Fe₄N_1-x monolayer on an α-Fe substrate and the growth kinetics of an ε-Fe₂N_1-z/γ′-Fe₄N_1-x bilayer on an α-Fe substrate, as observed by gaseous nitriding in an NH₃/H₂-gas mixture at 843 K. The kinetics of layer development and the evolution of the microstructure were investigated by means of thermogravimetry, layer-thickness measurements, light microscopy, and electron probe X-ray microanalysis (EPMA). The effective and self-diffusion coefficients were determined for each of the nitride layers. The composition dependence of the intrinsic (and effective) diffusion coefficients was established. Re-evaluating literature data for diffusion in γ′-Fe₄N_1-x on the basis of the present model, it followed that the previous and present data are consistent. The activation energy for diffusion of nitrogen in γ′-Fe₄N_1-x was determined from the temperature dependence of the self-diffusion coefficient. The self-diffusion coefficient for nitrogen in ε-Fe₂N_1-z was significantly larger than that for γ′-Fe₄N_1-x. This was explained qualitatively, considering the possible mechanisms for interstitial diffusion of nitrogen atoms in the close-packed iron lattices of the ε and γ′ iron nitrides.