SEGREGATION IN MULTICOMPONENT ALLOYS: ORDERED CrN AND CrC SURFACE PHASES ON AFe-15%Cr-C,N(100)SINGLE CRYSTAL

In general the chemical composition of multicomponent alloy surfaces exhibits significant deviations from the bulk composition due to thermally activated segregation processes. Upon cosegregation epitaxially stabilized two-dimensional surface compounds are formed on substrate surfaces of suitable orientation, e.g.CrNonFe-15%Cr-N(100)andCrConFe-15%Cr-C(100). The important role of the epitaxial stabilization manifests itself by the fact that onFe-15%Cr-C(100)crystals the formation of cubicCrCsurface precipitates is possible upon nonequilibrium cosegregation of the constituent components. Such a chromium carbide with rocksalt structure is not stable in the bulk of bccFe-Cr-C alloys.Angle-resolved photoemission of Cr2p3/2, N 1s and C 1s core level photoelectrons is used to determine the structure of theCrNsurface compound and of theCrCsurface precipitate on aFe-15%Cr-C,N(100)single crystal. Both surface phases exhibit a sharp1×1LEED pattern. Polar angle intensity distributions of photoelectrons (x-ray photoelectron diffraction) are recorded in the [001] and [011] azimuths, respectively. ForCrNthe angular intensity distributions exhibit forward scattering peaks for Cr2p3/2but not for N 1s. Most likely, the CrN phase consists of aCrNcompound layer with significant N outward relaxation of about 0.6 Å and a second completedCrlayer. Single scattering cluster (SSC) calculations indicate that theCr-Crinterlayer distance is expanded by about 26% with respect to the bcc lattice. ForCrC, on the other hand,Cr2p3/2and C 1s show both strong forward scattering features in all distributions. TheCrCphase consists of at least three complete compound layers withNaClstructure.