Diffusion and Interdiffusion in Binary Metallic Melts

TL;DR

This paper investigates how self- and interdiffusion coefficients in binary metallic melts depend on temperature and composition, using molecular dynamics simulations and mode-coupling theory, revealing the interplay of thermodynamic and kinetic effects.

Contribution

It combines MD simulations and MCT to analyze diffusion in metallic melts, highlighting the temperature-dependent dominance of thermodynamic versus kinetic effects.

Findings

Thermodynamic effects enhance interdiffusion at high temperatures.

Kinetic effects dominate at low temperatures, approaching the glass transition.

Darken's equation qualitatively matches the concentration dependence but underestimates the kinetic slowdown.

Abstract

We discuss the dependence of self- and interdiffusion coefficients on temperature and composition for two prototypical binary metallic melts, Al-Ni and Zr-Ni, in molecular-dynamics (MD) computer simulations and the mode-coupling theory of the glass transition (MCT). Dynamical processes that are mainly entropic in origin slow down mass transport (as expressed through self diffusion) in the mixture as compared to the ideal-mixing contribution. Interdiffusion of chemical species is a competition of slow kinetic modes with a strong thermodynamic driving force that is caused by non-entropic interactions. The combination of both dynamic and thermodynamic effects causes qualitative differences in the concentration dependence of self-diffusion and interdiffusion coefficients. At high temperatures, the thermodynamic enhancement of interdiffusion prevails, while at low temperatures, kinetic effects dominate the concentration dependence, rationalized within MCT as the approach to its ideal-glass transition temperature $T_c$. The Darken equation relating self- and interdiffusion qualitatively reproduces the concentration-dependence in both Zr-Ni and Al-Ni, but quantitatively, the kinetic contributions to interdiffusion can be slower than the lower bound suggested by the Darken equation. As temperature is decreased, the agreement with Darken's equation improves, due to a strong coupling of all kinetic modes that is a generic feature predicted by MCT.