Viscosity of liquid Ag-Cu alloys and the competition between kinetics and thermodynamics

Viscosity of liquid Ag-Cu alloys is determined over the entire compositional range as function of temperature and composition by means of the oscillating cup technique. It is found that the viscosities of the pure elements can be related to their self-diffusion coefficients via the Stokes-Einstein relation. The isothermal viscosities of the alloys investigated agree only reasonably with values predicted by models which are based mainly on thermodynamic principles. In particular, the viscosity isotherm exhibits a concave shape in the Cu-rich concentration range and a convex one in the Ag-rich range. Using a model based on the Stokes-Einstein relation, it can be shown that the behaviour mentioned above corresponds to a transition between two different mechanisms, i.e. a dynamic and a kinetic one. It is evident from the present study, in contrast to what is commonly practised, that the cross correlation term in the Darken equation has to be taken into account.