Influence of Cross-Correlations on Interdiffusion in Liquid Alloys

Abstract

Diffusion in the melt impacts on the formation and evolution of microstructure during solidification. Al-Cu has been established as a model system on which numerous studies were performed regarding the solidification behaviour, its thermophysical properties and the structure of the melt. Furthermore, Al-Cu exhibits pronounced X-ray contrast, facilitating the application of in situ measurement techniques. We present accurate measurements of interdiffusion in Al-Cu melts. Data are obtained by a combination of X-ray radiography with a linear shear-cell on ground and under microgravity conditions. This method avoids disturbances due to the melting process, free surfaces or buoyancy-driven convection. Additional interdiffusion experiments in a classical shear-cell further verify the reliability of the measurement results. In a binary alloy, self- and interdiffusion coefficients are connected by the thermodynamic factor, the concentrations and a factor which describes the influence of cross-correlations. We used self-diffusion coefficients from quasi-elastic neutron scattering to determine this cross-correlation factor. It is shown that its influence is weak in Al-rich Al-Cu melts. The results are in good agreement to ab inito molecular dynamic simulations.