Correlation of surface segregation with bulk diffusion for binary metal alloys

Previous treatments of surface segregation in metal alloys have all failed in some systems, due either to inadequacies in the theories or in the thermochemical data. A new model is proposed here using only alloy diffusion data. Predicted surface compositions agree quantitatively with ISS measurements and qualitatively AES results.     

Study of the segregation phenomena on the surface of binary alloys and steels in oxygen environment

The elemental composition of the surfaces of Pd₂₅V₇₅, Pd₄₄V₅₆, and Cr₂₀Ni₈₀ binary alloys and 40Kh13 instrumental steel exposed for a long time in air has been investigated by X-ray photoelectron spectroscopy (SPECS system, monochromatic radiation AlK_α line). The composition of the samples was also investigated as a function of the heating temperature. The features of the dependences of the concentrations of the main components of binary metal systems on the sample processing temperature are explained within the segregation theories of ternary systems of highly diluted solutions.     

Electronic theory of the chemisorption effect on surface segregation in transition-metal alloys

The tight-binding Ising model generalized for a system consisting of a transition-metal alloy with chemisorbed adatoms is used to calculate the effect of hydrogen chemisorption on surface segregation in a Cu---Ni alloy. The simple model of chemisorption and of the band structure of the alloy is assumed. The charge-neutrality condition is incorporated into the model.     

Physical and mathematical model of the surface segregation in binary alloys of transition metals

The surface segregation in binary alloys of transition metals based on platinum and palladium with different surface-face orientations are simulated via the electron-density functional method. The concentrations of the surface-active components of the binary alloys and displacements of the surface ion planes thereof are calculated self-consistently. The influence of surface segregation and lattice relaxation on the surface energy and electron work function on the surface of alloys of transition metals with various concentrations is investigated. The simulation results are compared with experimental data.     

Kinetics of surface segregation in alloys

The kinetics of surface segregation in ordering alloys are studied with the help of a multilayer model in slabs of different thicknesses. The time evolution of the concentrations of atomic layers perpendicular to the (100) planes of an AB-type BCC ordering alloy (CoFe) are calculated. As a result of the competition between the surface segregation of A atoms and ordering in the bulk, a metastable configuration with two anti-phase boundaries (APB) inside the slab, with odd numbers of layers, was obtained if we started from an ordered initial state with B atoms on the free surfaces (B-termination). The effect of the temperature was also investigated for a slab with 41 layers and from the calculation of the free energies of the metastable and stable states it was shown that the metastable state can exist below T₁ = 0.95T_c, where T_c is the critical temperature of the order-disorder transition in bulk. It was obtained that T₁ decreases with decreasing thickness of the slab, while the difference of free energies of the metastable and stable states slightly increases. The effect of the slab thickness is a typical size effect: the time necessary to reach the steady state decreases with decreasing number of layers.     

Surface segregation in binary alloys: A semiempirical approach

The preferential segregation concentration of one component in a binary alloy can be estimated, as a function of depth from the surface, from relative X-ray photoemission signal intensities. The details of this semiempirical calculation are outlined. In addition, common approximations applied to these calculations as well as typical experimental errors are analyzed to assess the effects upon the resulting calculated segregation profiles. The semiempirical analysis has been applied to previously published data for Cu₁₇N₈₃ and Fe₇₂Cr₂₈ binary alloys.     

Electronic theory of surface segregation in CuNi alloys

A calculation of the surface composition of CuNi alloys based on their electronic properties is proposed. Without any adjustable parameters, the theory predicts a surface enrichment in Cu quite consistent with the experimental results. Bulk and surface local densities of states are also given.     

Charge and spin structure factors in random binary alloys

The charge and spin structure factors, that are important in the x-ray and neutron scattering, have been examinated in random binary alloys, in the framework of the Coherent Potential Approximation (CPA). The spin structure factor shows a characteristic feature, due to the disorder, which in transition metal alloys is expected to give a more contracted neutron form factor.     

A Monte Carlo study of surface segregation in alloys

The Monte Carlo method has been applied to the study of surface segregation in a multi-layer, regular solution model of alloy surfaces. Three different alloy configurations have been investigated: semi-infinite slabs, thin films and small particles. The results show that the alloy component with the lowest surface energy tends to segregate to the first three or four surface atom layers and that segregation is greater in clustering alloys than in ordering alloys. Furthermore, segregation is more pronounced in low coordination surfaces, as evidenced by a comparison of {110} and {100}-oriented surfaces of fcc alloys. The degree of surface segregation in thin films and small particles (in the particle size range studied) tends to be smaller than in semi-infinite slabs, because of mass conservation constraints, and decreases with decreasing film thickness and particle size. The results obtained are contrasted with previous calculations and possible avenues for improving surface segregation models are discussed.     

A method for the study of surface segregation in multicomponent alloys

A simple algorithm for the determination of segregation profiles in multicomponent systems based on a mean field formalism and a quantum approximate method for the energetics is introduced. The method is described and applied to two ternary systems, concentrating on the changes in segregation patterns relative to the corresponding binary cases.     

Indium-indium pair correlation and surface segregation in InGaAs alloys

In-In pair correlations and In surface segregation in In xGa 1-xAs alloys are studied by first-principles total-energy calculations. By calculating the substitution energy of a single In atom, we find that the near-surface energetics explains the observed In segregation on InGaAs(001)-beta2(2x4) surfaces. Indium surface segregation further enhances the In site selectivity, thus the long-range ordering. We find that the [110] and [001] In-In pair correlations are repulsive and nearly isotropic in bulk but are highly anisotropic near the (001) surface. The sign of the [110] In-In interaction energies vs the distance from the surface is oscillatory. These findings explain the recent puzzling cross-sectional x-STM results.     

Tight-binding Ising-model study of chemisorption effects on surface segregation in transition-metal alloys

The tight-binding Ising model is generalized for the system consisting of a transition-metal alloy with chemisorbed adatoms. The effect of chemisorption on the surface segregation is calculated within the scope of a semi-infinite linear chain model for hydrogen chemisorbed on a Cu-Ni alloy. The charge-neutrality condition is incorporated into the model and its effect on the calculated surface segregation is demonstrated.     

ToF atom-probe fim investigation of surface segregation in dilute alloys

A time-of-flight atom-probe field ion microscope was successfully employed for the first time to investigate surface segregation in dilute alloys. We were able to achieve a single atomic layer resolution and obtained the absolute concentrations of alloy species of each surface layer. Cr atoms are found to segregate to the surface of Stainless Steel 410 whereas no segregation of the minority species was found for Pt-8% W and Pt-5% Ru. The first layer concentration of Cr in the {110} plane of Stainless Steel 410 at 500°C was found to be 38.5 ± 12.5% and 63.4 ± 10.2% respectively for samples with near surface layers Cr average concentration of 6.3 ± 2.1% and 11.9 ± 2.5%. The heat of segregation of Cr to the {110} plane of Stainless 410 was found to be 3.43 and 3.92 kcal/mole respectively from the two sets of data. The data also gives the difference in surface tensions between iron and chromium at this plane to be 269 and 282 erg/cm² respectively. Segregation studies on the {012} plane as well as on a grain boundary of Stainless Steel 410 were also done. In some cases, though the first surface layer is enriched with Cr in Stainless Steel 410, the near surface layers show a depletion of Cr. In Pt-8% W and Pt-5% Ru, our concentration depth profiles with a single atomic layer resolution show no segregation or depletion of the minority species either for the top layers or for the near surface layers.