Properties of Surface Alloys

Scanning tunnelling microscopy (STM) [33, 34] is widely used for investigations of the local atomic structure of surfaces. A probe "tip" is scanned across the surface revealing the positions of individual atoms. With its ability to achieve atomic resolution and, in most cases, distinguish between chemical species, the STM has provided key insights into the nature of alloy formation on surfaces. Both the static and dynamic properties of surface alloys can be probed with the STM. For the system of Pb on Cu, STM measurements were first to show the existence of surface alloy phases unambiguously and identify many of their stmctural properties [20-22, 35]. 

Deeper insights into the thermodynamic properties of surface alloys, together with many examples from experimental and theoretical studies, can be found in Ref [1] and in the various chapters in Ref [95]. A comprehensive overview of surface segregation energies for many host/guest combinations can be found in Refs. [4]... 

We wish to thank R. J. Behm and A. Gro for many fruitful and inspiring discussions about the nature and specific properties of surface alloys. Furthermore, we are grateful to A. Bergbreiter, A. Engstfeld, and R. Rotter for providing figures containing unpublished results and illustrations. 

Edwards e/a/. carried out controlled potential, slow strain-rate tests on Zimaloy (a cobalt-chromium-molybdenum implant alloy) in Ringer s solution at 37°C and showed that hydrogen absorption may degrade the mechanical properties of the alloy. Potentials were controlled so that the tensile sample was either cathodic or anodic with respect to the metal s free corrosion potential. Hydrogen was generated on the sample surface when the specimen was cathodic, and dissolution of the sample was encouraged when the sample was anodic. The results of these controlled potential tests showed no susceptibility of this alloy to SCC at anodic potentials. 

Purpose of this work was to study the structure, phase composition and mechanical properties of surface diffusion layers formed in the preliminary cold deformed a-Fe and Fe-Ni alloys after nitriding. 

The exceptional properties of the alloy are due in no small way to the yttrium component which together with the aluminium forms a stable and firmly bound oxide layer that exhibits excellent resistance to exhaust gas emissions at high temperatures over prolonged periods.( ) At the same time, it provides an ideal surface to receive another coating of metal or metal oxide which, in the context of catalyst applications, is most essential. At the present time most catalytic convertors utilise ceramic substrates which are prone to damage by both mechanical and thermal shock. 

One important class of particulate composites is dispersion-hardened alloys. These composites consist of a hard particle constituent in a softer metal matrix. The particle constituent seldom exceeds 3% by volume, and the particles are very small, below micrometer sizes. The characteristics of the particles largely control the property of the alloy, and a spacing of 0.2-0.3 tim between particles usually helps optimize properties. As particle size increases, less material is required to achieve the desired interparticle spacing. Refractory oxide particles are often used, although intermetallics such as AlFes also find use. Dispersion-hardened composites are formed in several ways, including surface oxidation of ultrafine metal powders, resulting in trapped metal oxide particles within the metal matrix. Metals of commercial interest for dispersion-hardened alloys include aluminum, nickel, and tungsten. 

TABLE 9. Surface and catalytic properties of Ni67B33 alloy catalysts in the hydrogenation of ethylene (423 K, 1 atm, H2/ethylene = 1)... 

It is important to realize that corrosion rates may be controlled by any of several thermodynamic or kinetic properties of the alloy-scale-environment system and not just by surface or interface reactions. The three stages of high temperature oxidation of a metal, shown schematically in Fig. 1, serve as an example (7). The first or transient stage includes initial gas adsorption, two-dimensional oxide nucleation, initial three-dimensional oxide formation and finally, formation of the dominant oxide that will control the oxidation rate in Stage II. Various portions of Stage I have been widely studied using surface analytical techniques, but its duration can be very short and it is usually assumed (not always correctly) that Stage I has little impact on ultimate corrosion properties of the material. 

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