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 ( Fe) and Austenite ( 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 temperature range, and 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.