Equilibration processes in surfaces of the binary alloy Fe-Al

The scenario of geometrical relaxation or reconstruction known for the surfaces of elemental solids is enriched by surface segregation in the case of compound surfaces. Both phenomena are consequences of the mere creation of the surface in which atoms in a certain slab assume a new positional and stoichiometric equilibrium state. With emphasis on segregation we describe the equilibration processes involved using data evaluated from Auger electron spectroscopy and quantitative electron diffraction available for the binary alloy Fe1-xAlx for various values of x and different surface orientations. A model is proposed according to which thermally activated processes restore equilibrium between the top layer and a subsurface atomic slab below much faster than between this slab and the bulk of the sample. Whilst the top layer is always Al enriched, the chemical order of the slab is dictated by the phase diagram, i.e. there is local equilibrium according to the actual stoichiometry. So, a transient structure for a certain Al bulk concentration corresponds to the equilibrium structure of a sample with lower bulk Al content. Surface orientation-dependent features can be easily interpreted via the different arrangements of nearest and next-nearest neighbours. By Al segregation, also precipitates can develop with the surface acting as a nucleation centre, again consistent with the phase diagram and local equilibrium. The features found may be extended to other alloys with one of its constituents segregating to the surface.

Duke Scholars

Author Volker Blum Thomas Lord Department of Mechanical Engineering and Materia ...