J. du Plessis

A model for surface segregation in multicomponent alloys—part I: Equilibrium segregation

Abstract This paper presents the first part of a consistent reanalysis of the description of interfacial segregation in multicomponent systems in terms of macroscopic thermodynamical properties. It is shown that an analytical function for the surface tension may be found and that the equilibrium surface coverages are identified with minima of this function. The biliary and ternary segregation equations are then derived from the surface tension function using the regular solution model. Theoretical and experimental results, such as hysteresis effects and discontinuous transitions in the surface coverage vs temperature curves, are explained in terms of the behaviour of the surface tension function.

A model for surface segregation in multicomponent alloys—part III: The kinetics of surface segregation in a binary alloy

Abstract The kinetics of surface segregation in a binary alloy is described by setting up and solving a system of rate equations using the change in Gibbs free energy between adjacent cells as atoms are interchanged between these cells. It is shown that this description is consistent with both the thermodynamical approach for the calculation of equilibrium surface coverages and the kinetic approach solving Ficks equations. The effect of the different parameters is discussed explaining results from the literature.

Non-equilibrium surface segregation of silicon in Fe-6.3at%Si(110)

Abstract The non-equilibrium segregation of silicon from the bulk to the (110) surface of a Fe-6.3at%Si single crystal was monitored in the temperature range 484 to 536°C by means of AES. Two distinct processes were observed. Initially a diffusion process controlling the initial surface enrichment of Si up to a temporary equilibrium value of about 15% was observed. This value corresponds to a 7 × 1 LEED pattern observed in similar crystals. Subsequently a two-dimensional nucleation process starts allowing more atoms onto the surface. Activation energies for both mechanisms were determined and are respectively 170 ± 20 and 480 ± 60 kJ mol .

The kinetics of weak surface segregation

The kinetics of surface segregation for small values of the surface enrichment factor α is investigated in terms of a model using the difference in chemical potential as driving force for the atomic fluxes. It is first shown that this model is equivalent to the Fick description of diffusion for high α and it is then found that the segregation kinetics is faster than the t12 dependence predicted b the semi-infinite solution of Ficks equations for low values of α. The difference is explained in terms of the additional driving force due to the segregation energy ΔG.

Sn bulk-to-surface diffusion in a Cu(111)(Sn) single crystal

Abstract The kinetics of the bulk-to-surface Sn diffusion in a dilute Cu(Sn)(111) single crystal had been investigated in the temperature range 678 to 794 K using Auger electron spectroscopy and low energy electron diffraction. It is shown that a sputter time correction is essential to obtain an accurate estimate of the diffusion coefficient D . After deriving the segregation equation for limited surface site accessibility, the modified Darken equations are used, with sputter time corrections, to determine the diffusion coefficient D and the segregation energy ΔG from the kinetic data. The data and fits are characterised by a high degree of accuracy and the fit parameters have the following values: D 0 = (7 ± 6) × 10 −6 m 2 /s, E = 168 ± 5 kJ/mol and ΔG = 76 ± 3 kJ/mol.

Linear programmed heating in surface segregation measurements

Abstract A new method of determining bulk diffusion parameters from surface segregation measurements is proposed whereby the sample temperature is increased linearly with time. The advantages of this method are discussed and a simple procedure is given which may be used to obtain the pre-exponential factor D 11 and the activation energy E from a single segregation run.

The segregation of carbon to the (110) surface of a Fe-10at% Si single crystal

Abstract The kinetics of the surface segregation of carbon to the (110) surface of a Fe-10at%Si single crystal was studied in the temperature range 350–430°C by Auger electron spectroscopy. The results could not be explained by diffusion models. However, it was found that the rate determining process was graphite island growth on the surface of the crystal to attain a maximum coverage of 40%. The activation energy for the process is (230±20) kJ/mol and the hexagonal constant for the graphite overlayer is 0.245±0.002 nm. These results are in contrast with segregation results obtained for a Fe-6at%Si single crystal where bulk diffusion was the rate limiting process and a maximum carbon coverage of 12% was observed.

SURFACE STRUCTURE AND COMPOSITION OF NIAL(100) BY LOW-ENERGY ION SCATTERING

The atomic structure of the (100) surface of NiAl was investigated by low‐energy ion scattering spectrometry (LEISS) using 1000 eV He+ ions as well as alkali ion impact collision ion scattering spectrometry (ALICISS), using 1450 eV Na+ ions. The ALICISS results were analyzed with a computer program based on a model using hitting probability concepts. The LEISS results showed that the top layer consisted mainly of Al with an Al/Ni ratio of 76/24 and the second layer entirely Ni. The ALICISS results could be best explained by a vertical relaxation of 25% (Δd12=−0.36 A) between the first and second layers and a relaxation of 15% (Δd24=−0.43 A) between the second and fourth layers. These results are discussed in terms of published data from the literature.

Kinetics near the discontinuous surface transition in the Cu(Ag)(111) binary segregating system

Abstract The kinetics of the segregating element silver in the system Cu(111)(Ag) was measured at temperatures close to where the discontinuous transition in the silver surface concentration versus temperature occurs. The seemingly complex kinetic behaviour of two-step diffusion and anomalous segregation processes could be fitted using only the surface segregation energy Δ G , the surface interaction parameter Ω S and the bulk diffusion coefficient D parameters.

A model for surface segregation in multicomponent alloys—part II: Comment on other segregation analyses

Abstract The Guttmann and other macroscopic thermodynamical analyses of surface segregation are discussed in terms of a model presented by the authors. Several inconsistencies in these descriptions are identified and discussed. The correct criteria for the occurrence of discontinuous transitions in the segregation isotherm and surface coverage vs temperature curves are presented.