Segregation of bulk constituents (either impurities or alloy components) physically and chemically alters the properties of a surface, which in turn determine the properties of materials for use in technological applications. The engineering of surface properties by controlled surface modification may be performed by the controlled segregation of bulk constituents.

   Of recent fundamental interest to this group has been the study of the surface atomic structure of alloys after segregation or cosegregation of alloy constituents. In particular, we have studied the site competition behavior of the "tramp" elements sulfur and arsenic to the surface of Fe-9%W(100). Sulfur and arsenic compete for the available c(2x2) sites, leading to a total coverage near 1/2 monolayer. This overlayer induces a significant expansion of the first metal-to-metal interlayer spacing away from the value for the clean Fe(100) surface. Using multiple angular scattering spectra, it is possible to determine the W concentration near the surface on a layer-by-layer basis.

   Another alloy system studied was Fe-15%Cr(100). Due to strong attractive interactions, chromium and carbon impurity can segregate to the surface together (cosegregation). Alternatively, when the C content exceeds the bulk solubility limit, chromium-carbide precipitates may form. In this system, the surface acts as a nucleation center for precipitation of thin CrC layers. This is of interest since there is no stable bulk phase of CrC. The previously hypothesized rocksalt structure for this surface compound was clearly verified using MEIS, since the transition from the bulk bcc(100) surface to the epitaxed rocksalt structure requires a dramatic 41% interlayer expansion.

a) Illustration of the epitaxial relationship given by (100)bcc II (100) rocksalt ; (001)bcc II (011) rocksalt . The C and metal atoms are open and closed circles, respectively. C atoms reside at interstitial sites within the bulk of the alloy, lying at the face centers of the conventional bcc unit cell. The ideal surface rocksalt compound is formed upon filling all of the possible C sites at the surface, followed by expansion of the interlayer spacings by a factor of x2 terminated bcc(100) and ideal epitaxed rocksalt(100) interlayer.    b) Side views of the (100) and (110) scattering planes for a bcc(100) surface. Atoms out of the plane of the page are shaded. The various channeling and blocking configurations used in this work are indicated. ‘‘I’’ and ‘‘II’’ indicate configurations in the (100) and (110) scattering planes, respectively.