Influence of temperature on layer growth as measured by in situ XRD observation of nitriding of austenitic stainless steel

Abstract

Investigating the formation of expanded austenite has resulted in several, different models trying to explain the particular diffusion and phase formation behaviour. However, only ex situ information, influenced by cooling and annealing processes of the samples after ion implantation has been available until now. Here, the time and temperature dependent layer growth is reported using in situ XRD measurements obtained from low energy broadbeam nitrogen ion implantation into polycrystalline austenitic stainless steel 304 in the temperature range from 300 to 500°C for a process time of up to 1h. Expanded austenite was observed at all temperatures without any CrN, in agreement with already published lifetime data for this metastable phase. The layer growth was derived from the time evolution of the substrate peak intensity. Using the temperature dependence of the layer growth, an activation energy of nearly 0.8eV was estimated for the nitrogen diffusion. In contrast, a complex behaviour was observed for the lattice expansion and peak width of the expanded peak, indicating additional dynamic annealing during implantation.