METHODOLOGICAL ASPECTS OF CALCULATIONS OF THE THERMODYNAMIC FACTOR ININTERDIFFUSION

Self-diffusion can be studied in binary systems for which appropriate radioisotopes for both constituents of the alloy are available. With r espect to the Fe-Al system suitable radioisotopes are available for th e Fe component (e.g. Fe-59) but not for Al. In the framework of the Bo ltzmann-Matano method further insight into the diffusion behaviour of Fe-Al intermetallics can be gained from interdiffusion experiments. Th e interdiffusion, coefficient is related via the modified Darken equat ion to the tracer diffusivities of the constituents. The thermodynamic factor phi entering the Darken equation is proportional to the second derivative of the Gibbs free energy, G, of the alloy on the molar fra ction of one of the components. These relations can be used to deduce the tracer diffusivity of Al provided that the tracer diffusivity of F e, the interdiffusion coefficient and the thermodynamic factor are kno wn. The mixing energy, which enters G; could be calculated on the basi s of the thermodynamical data or by means of non-empirical calculation s. We suggest for the latter ab initio calculations the use of modern non-empirical methods.