On the Oscillatory Segregation Behavior of Cr Near Low Index Ni Surfaces

Abstract

Ni-Cr alloys exhibit oscillatory segregation behaviors at low index surfaces, in which the preferred segregation species changes from Ni in the first layer to Cr in the second layer. In many dilute alloy systems, this oscillatory pattern is attributed to the elastic release of stresses in the local lattice around the segregating solute or impurity atom. These stresses are mostly thought to originate from mismatches in the atomic size of the solute and host atoms. In Ni-Cr alloys, however, an appreciable mismatch in atomic size is not present, leading to questions about the origins of the oscillatory behavior in this alloy. Using density functional theory, we have modelled the segregation of a single Cr atom in the  (100)  and  (111)  surfaces of fcc Ni, an alloy which exhibits this oscillatory behavior. We show that the negative energy correlates directly with the amount of charge assigned to the Cr atom through a Bader charge analysis. We find that as the Ni is able to strip valence charge from the Cr, the nearest neighbor Ni atoms contract slightly around the Cr decreasing its atomic volume. The amount charge transfer, and thus highest positive charge on the Cr atom, occurs when Cr is in the second layer of the surface and where the system exhibits a oscillating negative segregation energy. When in the second layer, the Cr atom also displaces upwards towards the electron dense interlayer region between the first and second surface layers.