Evidence for Significant Short-Range Order Effects on Surface Segregation in Ni-Al Solid Solution

Abstract

Aluminum surface segregation in Ni-9% Al as studied by low-energy Auger electron spectroscopy is characterized by quite low equilibrium levels that increase with temperature, in contrast to predictions of Bragg-Williams--type theories. The free-energy cumulant expansion approach, adapted for surface segregation, reveals that short-range order can induce significant suppression of solute segregation, depending mainly on the solvent-solute interaction strength. The diminution of the effect with increasing temperature leads to anomalous increase of equilibrium segregation, in agreement with the Ni-Al experiments.