(100) SURFACE SEGREGATION IN CU-NI ALLOYS

Atomistic simulations of segregation to the (100) free surface in Ni-Cu alloys have been performed for a wide range of temperatures and compositions within the solid-solution region of the alloy phase diagram. In addition to the surface-segregation profile, surface structures, free energies, enthalpies, and entropies were determined. These simulations were performed within the framework of the free-energy simulation method, in which an approximate free-energy functional is minimized with respect to atomic coordinates and atomic-site occupation. For all alloy bulk compositions (0. 05 less-than-or-equal-to C less-than-or-equal-to 0.95) and temperatures (400 less-than-or-equal-to T less-than-or-equal-to 1000 K) examined, Cu segregates strongly to the surface and Ni segregates to the planes just below the surface. The width of the segregation profile is limited to approximately three atomic planes. The resultant segregation profiles are shown to be in good agreement with an empirical segregation theory. A simpler method for determining the equilibrium segregation in terms of the properties of unrelaxed pure Ni and pure Cu surface data is proposed and shown to be more accurate than the existing empirical segregation analyses. The surface thermodynamic properties depend sensitively on the magnitude of the surface segregation. The enthalpy, entropy of segregation, and the change in the interlayer spacing adjacent to the surface are shown to vary linearly with the magnitude of the surface segregation.