About the modelling of the surface tension of liquid Ag-Au alloys

The surface tension of liquid Ag-Au alloys has been calculated by different theoretical approaches. A lens type phase diagram of the Ag-Au system and a close similarity of its pure components in terms of the Hume-Rothery empirical factors (atomic radius, crystal structure, valence difference) indicate that so called perfect solution model, as the most simple one, could be used for the calculation of the surface tension. The other two macroscopic models available in the literature, i.e. a classic thermodynamic model based on the Butler equation and the Quasi-Chemical Approximation (QCA) for the regular solutions, developed in the framework of statistical mechanics combined with Quasi Lattice Theory (QLT) have been also used to calculate the surface tension isotherms of Ag-Au melts. Molecular dynamics simulations using an embedded-atom model potential and the mechanistic route were carried out to compute the pressure tensor and the surface tension of liquid Ag-Au alloys, thus aiming to bridge the gap between thermodynamic models for phase diagrams and atomistic simulation. In addition, the surface effects on the microscopic functions are also discussed. The structural characteristics of Ag-Au melts are expressed in terms of the two microscopic functions, i.e. the concentration fluctuations in the long-wavelength limit and the Warren-Cowley short-range order parameter. The results obtained are compared among them as well as with the corresponding literature data. Some discrepancies between the experimental and theoretical data were found.