Surface segregation binary alloy systems

Let us use the Gibbs equation to predict surface segregation in binary alloy systems. 

The BFS method has been applied to a variety of problems, ranging from the determination of bulk properties of solid solution fee and bee alloys and the defeet strueture in ordered bee alloys [28] to more speeifie applieations ineluding detailed studies of the strueture and eomposition of alloy surfaees [29], ternary [30] and quaternary alloy surfaees and bulk alloys [31,32], and even the determination of the phase strueture of a 5-element alloy [33]. Previous appheations have foeused on fundamental features in monatomie [26] and alloy surfaces [29] surface energies, reconstructions, surface structure and surface segregation in binary and higher order alloys [34,35] and multilayer relaxations [36,37]. While most of the work deals with metallic systems, the lack of restrictions on the type of system that can be studied translated into the extension of BFS to the study of semiconductors [38]. 

There are several other factors that deserve mention before this subject is left. First, the surface tensions of different crystal planes may vary quite considerably the value is greater for less densely packed planes (e.g. for fcc(lOO) it is less than for fcc(lll), Figure 1.9) because of the smaller number of bonds that need to be broken to create new surface. Preferential segregation at these planes therefore minimises the system s energy, and by extension of this principle atoms of unusually low CN, such as occur at steps and kinks, are particularly favoured sites for segregated atoms. The reader wishing to explore further the question of surface enrichment in binary alloys should consult the classic paper by Williams and Nason. ... 

Recall that grain boundaries are sinks in which impurities or solute might become concentrated. The thermodynamic description of equilibrium segregation involves redistribution of alloyed solute such that the total free energy of the system is minimized. For a two-component system such as a binary alloy, if one considers a dilute solution of component B dissolved as solute in solvent A, the surface excess of B at surfaces per unit area, Fb, is given by [7, 9]... 

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