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We discuss the dependence of self- and interdiffusion coefficients ontemperature and composition for two prototypical binary metallic melts, Al-Niand Zr-Ni, in molecular-dynamics (MD) computer simulations and themode-coupling theory of the glass transition (MCT). Dynamical processes thatare mainly entropic in origin slow down mass transport (as expressed throughself diffusion) in the mixture as compared to the ideal-mixing contribution.Interdiffusion of chemical species is a competition of slow kinetic modes witha strong thermodynamic driving force that is caused by non-entropicinteractions. The combination of both dynamic and thermodynamic effects causesqualitative differences in the concentration dependence of self-diffusion andinterdiffusion coefficients. At high temperatures, the thermodynamicenhancement of interdiffusion prevails, while at low temperatures, kineticeffects dominate the concentration dependence, rationalized within MCT as theapproach to its ideal-glass transition temperature $T_c$. The Darken equationrelating self- and interdiffusion qualitatively reproduces theconcentration-dependence in both Zr-Ni and Al-Ni, but quantitatively, thekinetic contributions to interdiffusion can be slower than the lower boundsuggested by the Darken equation. As temperature is decreased, the agreementwith Darken's equation improves, due to a strong coupling of all kinetic modesthat is a generic feature predicted by MCT.

Diffusion and Interdiffusion in Binary Metallic Melts