Coupling Inward Diffusion and Precipitation Kinetics; the Case of Nitriding Iron-Based Alloys

Abstract

A model that describes the inward diffusion of an element I into a solid substrate and the simultaneous precipitation of a compound M y I z , with M as the alloying element initially dissolved in the substrate matrix, is presented for the case of nitriding iron-based alloys. The model was developed by coupling the diffusion kinetics and the precipitation (nucleation and growth) kinetics. Additionally, the role of excess nitrogen and the kinetics of ammonia dissociation at the iron surface were incorporated into this coupled model. The model was successfully applied to the case of nitriding an Fe-2.23 at. pct V alloy; the simulation results are in good agreement with the measured data and allow for detailed understanding of the evolution of the nitride precipitates (volume fraction, number density, and size distribution) as a function of both nitriding time and depth in the specimen. The present model exposed the pronounced effects of the precipitation kinetics, of excess nitrogen, and of the surface-reaction kinetics on the overall nitriding kinetics and demonstrated a striking, nonmonotonous change with time of precipitate particle size at a distinct depth in the specimen.

Alloying element nitride stability in iron-based alloys; denitriding of nitrided Fe–V alloys

“…This includes the excess nitrogen dissolved in the ferrite matrix due to the expansion of the ferrite matrix by the precipitate-strain field, the so-called mobile excess nitrogen; 37,38 (iii) adsorbed/segregated nitrogen: due to the usually nano-sized nature of the nitride precipitates and the corresponding, very large, coherent precipitate/matrix interfacial area, considerable nitrogen can be adsorbed at the precipitate–matrix interfaces; this is the so-called immobile excess nitrogen. 37,38 In order to quantitatively describe the nitriding process (see, e.g., Schacherl et al , 38 Meijering, 39 van Landeghem et al , 40 Selg et al , 41 and Jung et al 42 ), knowledge of the respective amounts of excess nitrogen is required. Previous analyses of absorbed nitrogen of types (i–iii) have been performed in Hosmani et al , 17 Podgurski et al , 22 Biglari et al , 25 Jack, 43 Podgurski and Davis, 44 Hosmani et al , 45 Hosmani et al , 46 Jung et al 47 .…”