Anisotropy of equilibrium surface segregation in Ni-9%Al

The segregation to six low- and high-index surface planes in polycrystalline Ni-9%Al solid solution has been studied using scanning AES. For each surface orientation the low-energy Ni(MVV) and Al(LVV) Auger transitions were measured as function of annealing time in the range ∼ 400-850°C. The surface orientations were determined by the electron channeling pattern (ECP) technique. Under clean conditions aluminum has been always found to segregate from the nickel matrix. Its equilibrium concentration was evaluated by quantitative analysis based on pure element standard spectra with a correction factor for matrix effects, and assuming three models of atomic distribution perpendicular to the surface. The results indicate a very weak, but quite anisotropic driving-force for Al segregation. In particular, the highest enthalpies of segregation among the orientations studied were found for the (510) and (310) surfaces, while the lowest value for (100). The aluminum segregation and its anisotropy are discussed in terms of different theoretical models of equilibrium segregation.