Cellular automata simulation of Nitrogen diffusion in expanded Austenite

The diffusion coefficient of nitrogen in expanded austenite forming on Fe-Cr-Ni austenitic stainless steel under the ingress of nitrogen atoms is strongly concentration dependent. In the present work, the origin of this concentration dependence is explored using 3D cellular automata simulations. The model includes trapping-detrapping of N atoms in short-range order (SRO) octahedral interstices surrounded by Cr atoms and long-range order (LRO) among the nitrogen atoms on the interstitial lattice. The first sites to be occupied by nitrogen are the strong trap sites. Once these sites are and remain occupied, N atoms begin to occupy weaker SRO sites, which enhances the mobility of N atoms in expanded austenite. At even higher N contents, LRO domains form. Only N atoms outside the LRO domains have fewer constraints and higher mobility. As the size of LRO domains increases with nitrogen concentration, the number of movable N atoms decreases, hence reducing their diffusivity.