Internal stress assisted nitrogen diffusion in plasma nitrided medical CoCr alloys

•An anomalous diffusion of nitrogen in plasma nitrided medical CoCr alloy is analyzed.•The model considers the stress and diffusion interaction during nitriding process.•Effects of nitrogen diffusivity dependence on internal stresses and CN are analyzed.•The stress assisted diffusion with D(CN) model was applied for CoCr alloys.

The role of internal stress on nitrogen diffusion in medical CoCr alloys during plasma assisted nitriding at low temperature is analyzed by stress assisted diffusion model. The nitrogen diffusion model is based on the concept of the interaction between the compositionally induced internal stress and interstitial nitrogen. From a theoretical point of view, this is mean that the driving force for nitrogen diffusion in CoCr alloys is not only the concentration gradient, but also the gradient of the compositionally induced internal stress. This model was used to predict nitrogen behavior during coupled mass diffusion with stress in plasma nitrided CoCr ASTM F75 and L605 alloys. The calculated nitrogen depth profiles agree very well with experimentally observed ones. Good agreement between simulated and experimental results shows that the stress assisted model can describe the phenomenon of an anomalous diffusion of nitrogen in CoCr alloys. The concentration dependent diffusion coefficients of nitrogen in expanded austenite were determined by fitting the nitrogen experimental profiles in different CoCr alloys (ASTM F75 and L605).