The structures of alloys

This section will consider first the structure of pure metals and this will be followed by a description of the structure of alloys. In general, structural features will be discussed in order of increasing size, i.e. starting with atomic features and continuing through microscopic features to macroscopic features. 

The structures of alloys are more complicated than those of pure metals because they are built from atoms of two or more elements with different atomic radii. The packing problem is like that of a storekeeper trying to stack oranges and grapefruit in the same pile. 

All the cation arrays we will discuss in the rest of this paper are either the structures of alloys (including of course the elements) or very simply related to them. They are summarized in Table 3. Some, such as MgO and Th02, are sufficiently well-known to need little comment except perhaps to remark that they represent the structures of a large number of individual compounds. Others (those marked with an asterisk in Table 3, to many of which the same remark applies) are singled out for further discussion because the new description sheds new light on incompletely understood structures or, as in the case of the sulphates, because our approach provides the first satisfactory description of the structure. 

X-ray diffraction has been a part of the physical chemistry laboratory curriculum for a long time, but mostly using the relatively simple powder diffraction technique. However a new experiment introduces the more complex method of single crystal X-ray diffraction (80). Another new experiment uses the technique to investigate the structure of alloys (81). 

The structure of alloys— When two or more metals are melted together in suitable proportions a homogeneous solution often results. On cooling, the homogeneous solid is termed a solid solution, since as in a liquid solution, the atoms are distributed in a random fashion. If the structure of the solid solution is identical with that of one of the components (the solvent) the solution is termed a primary or a solid solution. Primary solid solutions are of two types interstitial solid solutions, in which the atoms of the dissolved substance are situated in the holes between the atoms of the solvent and substitution solid solutions in which the solute atoms have taken the place of solvent atoms in the lattice of the latter. 

Apart from the intrinsic interest of the interatomic distances in metals, it is useful to have a set of radii to refer to when discussing the structures of alloys. Since the c.n. 12 is the most common in metals, it is usual to draw up a standard set of radii for this coordination number. For the metals with ideal close-packed structures the radii are simply one-half the distances between an atom and its twelve equidistant nearest neighbours. Many structures, however, deviate slightly from ideal hexagonal closest packing in such a way that sbt of the neighbours are slightly farther away than the other six, for example. 

The structure of alloys and structural changes at certain temperatures can explain the memory effect of special alloys (see E5.6) like Nitinol , a nickel-titanium alloy [4]. The structure as well as the much greater electric conductivity explains the difference between the copper-gold structures (CuAu and Cu3Au) and... 

W. Hume-Rothery. The Structures of Alloys of Iron, Pergamon, London (1966). 

Tav] Tavadze, F.N., Pirtskhalaishvili, V.A., Nabichvrishvili, M.A., A Study of the Structure of Alloys of the Fe Comer of the Fe-Cr-Mn System (in Russian), Soobsh. Akad. Nauk Gruz. SSR, 49(3), 641-646 (1968) (Phase Diagram, Experimental, Phase Relations, 7)... 

The structure of alloys of lead and thallium. Acta Cryst. 5 (1952) 39—44. (You-Chi Tang and Linus Pauling). SP 53 ... 

Bral] Bradley, A.J., Bragg, W.L., Sykes, C., Researches into the Structure of Alloys , J. Iron Steel Inst (London), 80, 63-156 (1940) (Crys. Structure, Phase Diagram, 22)... 

The object of the present work Is to study the effect of the MA conditions on the structure of alloys and Raney Ni-catalysts made from them as well as on adsorption properties, activity and selectivity of catalysts in liquid-phase hydrogenation reactions. 

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