Nitrogen Diffusion in Vacancy-Rich Ferrite and Austenite, from First Principles to Applications

This work contains a systematic study of the diffusion of nitrogen in Ferrite (α Fe, BCC) and Austenite (γ Fe, FCC) from first principles, using a robust multi scale model which combines Density Functional Theory (DFT) and Kinetic Monte Carlo (KMC). Both ferromagnetic BCC and non-magnetic FCC iron 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. In particular, 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 < 1500◦C temperature range, and 0.01 at.% < cN < 10.0 at.% nitrogen concentration range. 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.