Ordered and Disordered Alloys

B. Roessler, D.T. Novick and M B. Bever, Aimealing of the ordered and disordered alloy Cu,Au after cold work, Trans. Met. Soc. AIME 227 985 (1963). 

RELATIVISTIC CALCULATIONS OF PHOTOEMISSION-AND LEED-INTENSITIES FOR ORDERED AND DISORDERED ALLOYS APPLICATION TO CusPt and Cu Pta... 

A considerable difference is found between ordered and disordered alloys containing 50 at. % Ni [55]. The ordered alloy shows much narrower resonance lines because of the more uniform site environments. 

In a study of the superconducting alloys AujBi and AuNba, no significant change in the recoil-free fraction was found at the superconducting transition temperature [107]. Preliminary investigations on copper-gold alloys have revealed a difference in chemical isomer shift between ordered and disordered alloys at each particular composition [108]. 

The electronic properties of elemental metals, ordered and disordered alloys and metal hydrides, and the experimental and theoretical methods to study them are described in Chapter 4.3 by I. Chorkendorf Here we restrict ourselves to a short overview. 

Chris Wolverton and Alex Zunger. Ising-like description of structurally relaxed ordered and disordered alloys. Phys. Rev. Lett., 75 3162-3165,1995. 

The binary alloy Ac,j Beg, Cyt-I-Ci) = 1, shall be studied by the means of supercells containing N atoms with periodic boundary conditions. Each site of the cell can be occupied by a A or a B atom. If the chemical occupations are not considered, the lattice described by the sites within the supercell and their periodic replicas is a simple lattice, with one atom per unit cell. Below, it shall referred to as the geometrical lattice . In order to have a theory flexible enough to deal on equal footing both with ordered and disordered alloys, in principle, one should consider all the different alloy configurations that belong to the... 

Different models for ordered and disordered alloys will be discussed below. The random alloy model can be defined by saying the the occupations of different sites are not statistically correlated, i.e. 

The formulation of the charge excess functional, Eq. (12), and the solution for the local charges, Eq. (25) are well defined both for ordered and disordered alloys. This has been possible because of the introduction of the chemical potential y. due to this, only three of the four constants that characterise the qV linear law s enter in the final solution, Eq. (25). These three quantities together with y, determined by the actual alloy configuration, are equivalent to the original set of four constants. 

B. Urban-Erbil and W. Pfeiler, Ordering and disordering in CuPt-alloys, in. .Ordering and Disordering in... 

Watanabe M, Tsurumi K, Mizukami T, Nakamura T, Stonehart P. 1994. Activity and stability of ordered and disordered Co-Pt alloys for phosphoric acid fuel cells. J Electrochem Soc 141 2659-2668. 

In the previous chapter we looked at some questions concerning solid intermetallic phases both terminal (that is solubility fields which include one of the components) and intermediate. Particularly we have seen, in several alloy systems, the formation in the solid state of intermetallic compounds or, more generally, intermetallic phases. A few general and introductory remarks about these phases have been presented by means of Figs. 2.2-2.4, in which structural schemes of ordered and disordered phases have been suggested. On the other hand we have seen that in binary (and multi-component) metal systems, several crystalline phases (terminal and intermediate, stable and also metastable) may occur. 

The methodology for obtaining the partial atomic volume and its application as a realistic measure of atomic size in metals and alloys has been discussed by Bhatia and Cahn (2005) they illustrated its use as a powerful tool in understanding the behaviour of solid solutions in both ordered and disordered states. 

Figure 3. Mossbauer spectra for ordered and disordered samples of an alloy of 25% rhodium and 75% iron. The disordered sample was quenched and contains retained austenite (17)...

Substitutional solid solutions can have any composition within the range of miscibility of the metals concerned, and there is random arrangement of the atoms over the sites of the structure of the solvent metal. At particular ratios of the numbers of atoms superstructures may be formed, and an alloy with either of the two extreme structures, the ordered and disordered, but with the same composition in each case, can possess markedly different physical properties. Composition therefore does not completely specify such an alloy. Interstitial solid solutions also have compositions variable within certain ranges. The upper limit to the number of interstitial atoms is set by the number of holes of suitable size, but this limit is not necessarily reached, as we shall see later. When a symmetrical arrangement is possible for a particular ratio of interstitial to parent lattice atoms this is adopted. In intermediate cases the arrangement of the interstitial atoms is random. 

The order parameter is essentially a kinematic measure, describing the state of order within a system without any intrinsic reference to what factors drove the system to the state of interest. For example, in thinking about the transition between the ordered and disordered states of an alloy, it is useful to define an order parameter that measures the occupation probabilities on different sublattices. Above the order-disorder temperature, the sublattice occupations are random, while below the critical temperature, there is an enhanced probability of finding a particular species on a particular sublattice. The conventional example of this thinking is that provided by brass which is a mixture of Cu and Zn atoms in equal concentrations on a bcc lattice. The structure can be interpreted as two interpenetrating simple cubic lattices where it is understood that at high temperatures we are as likely to find a Cu atom on one sublattice as the other. A useful choice for the order parameter, which we denote by r], is... 

This chapter is organized as follows. First, in sect. 2, we consider the surfaces of metals. In sect. 2.1 we describe the structure of unreconstructed clean metal surfaces and then proceed, in sect. 2.2, to consider the reconstructed surfaces. The surface structure of ordered and disordered metallic alloys is described in sect. 2.3. In sect. 2.4 we describe the surface structures associated with atomic adsorption on metals and in sect. 2.5 we consider molecular adsorption on metals. The structure of semiconductor surfaces is... 

Order and Disorder in Iron-Aluminium Alloys. A mosl valuable routribution lo Hie X-ray examination of superlaHice structures was made in their study of tlie iron-aluminium alloys by Bradley and Jay in 198 2. The atoms of these metals have diameters which are such that they fall only just within the favourable range for the formation of wide solid solution of aluminium in iron. The lattice of the solvent metal, iron, is in this case body-centred cubic, and eight adjacent lattice units... 

A very similar transformation of the original hep adlayer to a surface alloy coverage with the same Tl-Tl interatomic distances and [V3 x V3]R30° symmetry has been observed in the system Tl/Ag(lll) during extended polarization of the incompletely formed first T1 adsorbate layer. As in the system Pb/Ag(lll), there is strong evidence that the transformations proceed from the boundaries of the peripheral adsorbate-free domains inwards on the terraces. However, in contrast to the system Pb/Ag(lll), the transformed coverages include both ordered and disordered domains, and then-desorption results in the formation of monoatomic pits in the substrate with widths of ca. 3 to 10 nm [3]. These pits diminish and finally vanish within a few minutes by coalescence and lateral displacement, at a rate that can be increased markedly by positive shift of the substrate potential. 

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