Short-Range Order in Metal Alloys

Instead, characteristic atomic chains formed by the same chemical species can be observed. This phenomenon is called short-range order (SRO) and may heavily influence the energetics of the alloy surface and consequently its stability. A quantitative description of metal alloy surfaces has to take such substitutional ordering effects into account One even has the possibility to quantify SRO by the so-called ordering parameters, which will be discussed in the next section. 

Unfortunately, SRO is temperature dependent Consequently, temperature becomes an extremely important parameter in surface segregation. We will see that in experiment the annealing temperature chosen may be essential for the resulting properties. For some alloys, the annealing temperature necessary to reach a thermodynamical equilibrium lies far beyond the melting temperature. So if we compare measured and predicted data, one has to make sure that parameters such as temperatures are treated correctly in order not to compare apples and oranges. 

Surface segregation takes place in practically all metal alloys and is controlled by the chemical equilibrium between the near-surface layers and the bulk. Consequently, a successful theoretical description of this phenomenon demands a consideration of both bulk and surface properties in order to understand correlations between segregation profile, atomic structure, SRO, and temperature. For this reason, the basics of the alloy s bulk properties have to be discussed (Section 11.2) before considering the surfaces and their experimental (Section 11.3.1) as well as theoretical characterizations (Sections 11.3.2 and 11.3.3). In Section 11.3, we will introduce the methods that are in general applied to alloy surfaces. Special focus will be on a very new ab initio-based description that allows for a direct prediction of the segregation profile and the mentioned correlated parameters. This concept will then be applied to two different classes of alloy phases an intermetallic compound and a disordered alloy. The last example will demonstrate which possible effects will take place if an adsorbate comes to the surface. Besides changes in the atomic position of the surface atoms (the so-called adsorbate-induced surface reconstruction), 

Materials scientists often refer to disordered alloys as solid solutions. This has nothing to do with the liquid phase but just stands for the fact that there is a certain solubility of B atoms in the A matrix. Following the classical rales of Hume-Rothery et al. [6, 7] about the formation of solid solutions, we distinguish between the so-called extensive solid solutions (solubility of B atoms in A higher than 5%) and restricted solid solutions (solubility of B atoms in A smaller than 5%). 

The mixing enthalpy or the formation enthalpy is the energy necessary to mix Cu and Zn atoms on a common lattice. Mathematically, the per-atom formation enthalpy Hf(or) of a compound A Bj crystalhzing in a certain structure a is 

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