FINITE-SIZE EFFECTS ON THE THERMODYNAMICS OF CU-NI ALLOYS - (100) AND(110) THIN-FILMS

A series of atomistic simulations of CuxNi1-x {100} and {110} thin fil ms were performed as a function of Cu-concentration, temperature, and thickness in order to determine the effects of film thickness on the s urface concentration and the surface free energy. The surface Free ene rgies of these films were found to differ significantly from those of the corresponding flat surfaces of bulk samples. Very thin films exhib it surface entropies Delta S 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 thickne ss, composition and/or temperature. The unusual behavior of the surfac e free energy of thin films may be accounted for by the surface energy dependence on the surface concentration and the limited solute availa bility in these systems. The surface segregation in these thin films m ay be accurately described by the theory presented in Ref. [3]. Using the prediction of the surface composition and a simulation of a bulk f lat surface it is possible to accurately predict the (surface) free en ergy of thin films as a function of film thickness using only properti es of a bulk sample.