Methodological aspects of calculations of the thermodynamic factor in interdiffusion

Abstract

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