Substitution alloys

D.D. Johnson and F.J. Pinski, Including charge correlations in calculations of the energetics and electronic structure for random substitutional alloys, Phys. Rev. B 48 11553 (1993). 

All ab initio applications of multiple scattering theory in dilute substitutional alloys rely on the one-to-one correspondence configuration. This holds both for the calculation of transition probabilities [7], represented by Eq. (7), and the electronic structure [8], represented by the Green s function equation [9]... 

Figure 2 The defect corresponding to a migrating atom in a dilute substitutional alloy and the corresponding void reference system.

There is an accelerating trend away from the use of lead-containing solders in contact with potable water. The effects of galvanic corrosion of one of the substitute alloys (Sn3%Ag) in contact with a number of other metals including copper have therefore been studied . The corrosion of tin/Iead alloys in different electrolytes including nitrates, nitric and acetic acids, and citric acid over the pH range 2-6 were reported. The specific alloy Pb/15%Sn was studied in contact with aqueous solutions in the pH range... 

Alloys of metals tend to be stronger and have lower electrical conductivity than pure metals. In substitutional alloys, atoms of the solute metal take the place of some atoms of a metal of similar atomic radius. In interstitial alloys, atoms of the solute element fit into the interstices in a lattice formed by atoms of a metal with a larger atomic radius. 

FIGURE 5.47 In a substitutional alloy, the positions of some of the atoms of one metal are taken by atoms of another metal. The two elements must have similar atomic radii. 

Homogeneous alloys of metals with atoms of similar radius are substitutional alloys. For example, in brass, zinc atoms readily replace copper atoms in the crystalline lattice, because they are nearly the same size (Fig. 16.41). However, the presence of the substituted atoms changes the lattice parameters and distorts the local electronic structure. This distortion lowers the electrical and thermal conductivity of the host metal, but it also increases hardness and strength. Coinage alloys are usually substitutional alloys. They are selected for durability—a coin must last for at least 3 years—and electrical resistance so that genuine coins can be identified by vending machines. 

Figure 4.4 (100) planes of substitutional solid solutions (a) substitutional alloys, typified by Ni-Cu and (b) inorganic oxides, typified by MgO-NiO. 

Ag and Pb-Bi-Ni" alloys The interpretation of the phenomena is somewhat simpler in the case of the substitutional alloys, where both species occupy the same lattice sites. 

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. 

Alloys are metals made by combining two or more elements. Two structural types may be identified substitutional alloys, in which atoms of one... 

When some alloys are formed, atoms of one metal replace atoms of another metal. These alloys are classified as substitution alloys. When a substitution alloy is formed, the atoms of metals forming the alloy are about the same size. Therefore, one kind of atom can fit into a space vacated by another atom. In sterling silver, about 7% of the atoms are copper. These copper atoms are randomly dispersed throughout the metallic crystalline silver atoms. Pewter and brass are other examples of substitution alloys. 

A more rigorous approach to calculating the diffusion coefficients has been adopted by Kikuchi [165], A binary substitution alloy (s = 3) has been considered with the vacancy mechanism of atom migration. He was the first to take account of the temporal correlations and to obtain expressions for the correlation cofactor fc in the non-ideal systems. The derived coefficients satisfy Onsager s reciprocal relations. 

Gyorffy, B.L. (1972). Coherent-potential approximation for a nonoverlapping muffin-tin potential model of random substitutional alloys, Phys. Rev. B 5, 2382-2384. 

Diffusion of atoms or ions in crystalline solids can occur by at least three possible mechanisms, as shown schematically in Figure 2.7. In some solids, transport proceeds primarily by the vacancy mechanism, in which an atom jumps into an adjacent, energetically equivalent vacant lattice site. The vacancy mechanism is generally much slower than the interstitial mechanism (discussed below). Nonetheless, it is thought to be responsible for self-diffusion in all pure metals and for most substitutional alloys (Shewmon, 1989). 

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