Expanded austenite, which was formed byr N-plasma immersion ion implantation or low-energy nitriding of austenitic stainless, was characterized by a high N content of up to 20at%, plus unusually rapid diffusion which, in general, could not be described by using a single diffusion coefficient. The concentration-dependent diffusivity was calculated here for several experimental parameters and steel alloys. Two mathematical simplifications of the general diffusion theory, which were justifiable for physical reasons, helped the solution of the equations. Firstly, a constant surface concentration was assumed, in spite of a constant N flux into the surface. Secondly, only mobile N atoms in a stationary steel matrix were considered. It was thereby possible to solve the Boltzmann-Matano equation and to obtain the concentration-dependent diffusion coefficient. In all cases, a step-like behavior was found; with a high value for high N contents and a low value for low ones.

Concentration Dependent Nitrogen Diffusion Coefficient in Expanded Austenite Formed by Ion Implantation. S.Mändl, B.Rauschenbach: Journal of Applied Physics, 2002, 91[12], 9737-42