Nitrogen diffusion in vacancy-rich ferrite and austenite, from first principles to applications

Abstract

This work contains a systematic study of the diffusion of nitrogen in Ferrite (α-Fe) and Austenite (γ-Fe) from first principles, using a robust multiscale model which combines Density Functional Theory (DFT) and Kinetic Monte Carlo (KMC). Both ferromagnetic BCC Fe and non-magnetic FCC Fe are considered using DFT to drive a diffusion model, which shows strong agreement with experimental diffusion data in literature. Further, quantified predictions are calculated for nitrogen diffusion in iron crystals which are vacancy-rich. It was found that an extended diffusion coefficient of nitrogen can be expressed as a function of nitrogen and vacancy concentration by fitting polynomial coefficients. These are calculated within the 100∘C<T<1538∘C temperature range, and 0.1at.%<cN<1at.% nitrogen concentration range. Moreover, the error of extrapolating the diffusion coefficient outside of the calculated nitrogen concentration range was found to be small at some temperatures. Such insights in vacancy-rich crystals may be useful to nitriding manufacturers, as enhanced diffusion models are an important factor in improving existing processes and avoiding common manufacturing problems such as the egg-shell-effect.