Medium-energy ion scattering study of arsenic and sulfur segregation to the Fe–9% W(100) surface

Abstract

High-resolution medium-energy ion scattering (MEIS) has been used to investigate the segregation of arsenic and sulfur on the (100) oriented surface of a body-centered cubic Fe–9% W(100) single crystal containing 53 wt-ppm As and 10 wt-ppm S. The sensitivities of Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and MEIS to the presence of As on Fe or Fe–base alloy surfaces are discussed. At temperatures ranging from 800 to 1100°C, both segregants compete for the available surface sites. Arsenic segregation dominates at temperatures around 800°C, while S segregation is most pronounced at temperatures around 1000°C. The total S and As coverage was found to be near 0.45 ML over the entire temperature range. Thermodynamic parameters, such as segregation free energies and interaction energies, have been determined utilizing the regular solution model proposed by Guttmann and McLean. Strong repulsive interactions between the segregants are found. MEIS structure analysis shows the segregated As and S atoms to be arranged 1.22–1.27 Å above the topmost metal layer (depending on surface composition). The metal-to-metal layer distances show an oscillatory relaxation behavior with an expansion of the first-to-second metal layer (+0.05 Å). With As and S on the surface, there is a strong depletion of W in the topmost two metal layers.