Analysis of carbon diffusion in austenite revisited

Tracer carbon diffusion, chemical diffusion and carbon activity data in austenite in terms of a four-frequency interstitial model first formulated by McKee is analysed. The later formulation by Okamura and Allnatt, which has recently been confirmed to be essentially exact, is made use of. In the revised analysis, the general finding of McKee that the increase of both tracer and chemical carbon diffusion coefficients with carbon composition at 1000°C is largely a result of a much higher rate of rotation of an interstitial pair compared with isolated carbon atoms is confirmed. It is found that carbon atoms move almost two times faster as a rotating pair than established originally by McKee. Information on the ratios of the exchange frequencies to other temperatures between 800°C and 1000°C using the available experimental data on the intrinsic diffusivities and the carbon activity is extrapolated.