SURFACE COMPOSITION OF ALLOYS.

In equilibrium, the composition of the surface of an alloy will, in general, differ from that of the bulk. The broken-bond model is applicable to alloys with atoms of virtually equal size. If the heat of alloy formation is zero, the component of lower heat of atomization is found enriched in the surface. If both partners have equal heats of sublimination, the surface of a diluted alloy is enriched with the minority component. Size effects can enhance or weaken the electronic effects. In general, lattice strain can be relaxed by precipitating atoms of deviating size on the surface. Two-phase alloys are described by the 'cherry model', i. e. , one alloy phase, the 'kernel' is surrounded by another alloy, the 'flesh', and the surface of the outer phase, the 'skin' displays a deviating surface composition as in monophasic alloys. In the presence of molecules capable of forming chemical bonds with individual metal atoms, 'chemisorption induced surface segregation' can be observed at low temperatures, i. e. , the surface becomes enriched with the metal forming the stronger chemisorption bonds.