Surface segregation in an Al–4.2 at% Ag alloy

Abstract

The equilibrium surface composition of a polycrystalline Al–4.2 at% Ag alloy has been measured as a function of temperature using Auger electron spectroscopy. These results indicate that silver segregates to surfaces with an apparent enthalpy of segregation of about −38.5 kJ/mol. The equilibrium average surface composition of the alloy has been calculated as a function of surface orientation using the Lee–Aaronson discrete lattice plane model. A modified regular solution model originally due to Alexander et al. was used to represent the interactions between Al and Ag atoms. While this model successfully described the relevant portion of the Al–Ag phase diagram and the faceting behavior of GP zones in Al rich Al–Ag alloys, it predicted no surface segregation of Ag in the present studies. These results are in contrast to the results obtained experimentally. Possible reasons for the lack of agreement are considered. Finally, the surface segregation of silver measured by AES was compared with that predicted by the Gibbs adsorption isotherm, wherein the latter calculation made use of the results of ultra high vacuum measurements of the bulk composition dependence of the surface tension of α Al–Ag alloys (to be reported elsewhere). Qualitative agreement was found between the two experimental methods, both indicating enrichment of Ag at the free surfaces.