Binary alloys, surface segregation

Muller, S., Stohr, M., and Wieckhorst, O. (2006) Structure and stability of binary alloy surfaces segregation, relaxation, and ordering from first-principles calculations. Appl. Phys. A, 82, 415. 

Statistical thermodynamic descriptions of these transitions in substitutional alloys have been developed for the cases of both binary and ternary alloys , using a simple nearest neighbor bond model of the surface segregation phenomenon (including strain energy effects). Results of the model have been evaluated here using model parameters appropriate for a Pb-5at%Bi-0.04at%Ni alloy for which experimental results will be provided below. However, the model can be applied in principle to the computation of equilibrium surface composition of any ternary solution. 

Small particles of binary alloys have been investigated in detail in static EXAFS experiments, but if information about the dynamic behavior of the alloy composition and the segregation phenomena is desired, time-resolved combined EXAFS/XRD studies are necessary. Figure 18 shows the atomic structure of a small binary particle of a Ni-Au alloy as predicted from Monte Carlo simulations (60). Ni and Au do not form a miscible alloy in the bulk but can form a stable alloy at the surface. The structural and chemical changes that occur when such particles are exposed to different... 

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]. 

Fig.5. Calculated solute surface concentrations for Ni-8at%Al-4at%Cu(l 11) thick solid lines - FCEM, thin solid lines - the BW-type approximation. Dashed-dotted lines - solute surface concentrations for the binary alloy Ni-8at%Al(lll) and Ni-4at%Cu(l 11) surfaces calculated in the FCEM approximation. Note the enhancement of Cu segregation induced by ternary alloying and short-range order effects.

The presence of one main component at the surface of a binary alloy can be the consequence of a strong segregation of this component towards the surface layer. The driving forces for such a surface segregation are [6, 7] ... 

Let us first consider, as a striking example, the comparison of two alloys for which a very large surface segregation is expected (to be of the same order) but for which the misfit is very different (see Table 4). These are PdsPtQs and Pd5Ni95 binary alloys. 

Surface oxidation behavior is particularly important with regard to the use of glassy metals in as-quenched state. A general observation made with binary alloys is that the more electropositive constituent of the alloy tends to segregate to the surface upon oxidation. This procedure can occur already at lower temperature, and consequently the surface of freshly prepared alloys is likely to be covered with a thin layer of oxides of this constituent. This phenomenon has certainly contributed to controversy with regard to the catalytic properties of glassy metal surfaces, since in many of the earlier investigations little care was taken of this behavior and authors tacitly assumed that the surface composition resembles the bulk composition of the quenched materials. 

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

Surface segregation of binary alloys is ofren described [46] in the form... 

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]... 

For dilute binary alloys involving a dilute solid solution of solute B in solvent A, surface segregation Xs is given in terms of Xb and the free energy of segregation per mole of solute, AG in a Langmuir-McClean type expression [32). 

Surface Segregation On thermodynamic grounds, prior to any ion bombardment, a homogeneous binary alloy in thermal equilibrium (a well annealed sample completely free of surface contamination) will, in general, exhibit top monolayer segregation, i.e.,... 

Teraoka Y (1990) Surface relaxation effects on surface segregation and order-disorder transition temperatures of binary alloys. Surf Sci 238(1-3) L453-L456... 

Materials science (microelectronics analysis, surface layers, multilayers, PIXE channeling of dopants in crystals, depth profiling, binary alloys, impurities deposited in nuclear fusion devices, magnetic relaxation in nanocrystalline iron, insulating materials, radiation-induced segregation, superconductors, catalysts, and diffusion studies). 

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