FINITE SIZE EFFECTS ON THE THERMODYNAMICS OF Cu-Ni ALLOYS: {100} AND {110} THIN FILMS

A series of atomistic simulations of Cu_xNi_1-x {100} and {110} thin films were performed as a function of Cu-concentration, temperature, and thickness in order to determine the effects of film thickness on the surface concentration and the surface free energy. The surface free energies of these films were found to differ significantly from those of the corresponding flat surfaces of bulk samples. Very thin films exhibit surface entropies ΔS which have an opposite sign to those for the bulk (positive surface entropy). These films undergo a transition from negative to positive surface entropy with increasing film thickness, composition and/or temperature. The unusual behavior of the surface free energy of thin films may be accounted for by the surface energy dependence on the surface concentration and the limited solute availability in these systems. The surface segregation in these thin films may be accurately described by the theory presented in Ref. [3]. Using the prediction of the surface composition and a simulation of a bulk flat surface it is possible to accurately predict the (surface) free energy of thin films as a function of film thickness using only properties of a bulk sample.