Ordered alloys and amorphous materials

Ordered alloys and amorphous materials 5.3.1 Ordered alloys... 

As pointed out by Stephens and Goldman (State University of New York at Stony Brook). Quasicrystals are neither uniformly ordered like crystals nor amorphous like glasses. Many features of quasicrystals cun be explained, but their atomic structure remains to be described fully. See also Aluminum Alloys and Engineered Materials... 

Quasicrystals are solid materials exhibiting diffraction patterns with apparently sharp spots containing symmetry axes such as fivefold or eightfold axes, which are incompatible with the three-dimensional periodicity associated with crystal lattices. Many such materials are aluminum alloys, which exhibit diffraction patterns with fivefold symmetry axes such materials are called icosahedral quasicrystals. " Such quasicrystals " may be defined to have delta functions in their Fourier transforms, but their local point symmetries are incompatible with the periodic order of traditional crystallography. Structures with fivefold symmetry exhibit quasiperiodicity in two dimensions and periodicity in the third. Quasicrystals are thus seen to exhibit a lower order than in true crystals but a higher order than truly amorphous materials. 

The second type of impurity, substitution of a lattice atom with an impurity atom, allows us to enter the world of alloys and intermetallics. Let us diverge slightly for a moment to discuss how control of substitutional impurities can lead to some useful materials, and then we will conclude our description of point defects. An alloy, by definition, is a metallic solid or liquid formed from an intimate combination of two or more elements. By intimate combination, we mean either a liquid or solid solution. In the instance where the solid is crystalline, some of the impurity atoms, usually defined as the minority constituent, occupy sites in the lattice that would normally be occupied by the majority constituent. Alloys need not be crystalline, however. If a liquid alloy is quenched rapidly enough, an amorphous metal can result. The solid material is still an alloy, since the elements are in intimate combination, but there is no crystalline order and hence no substitutional impurities. To aid in our description of substitutional impurities, we will limit the current description to crystalline alloys, but keep in mind that amorphous alloys exist as well. 

A completely novel approach to technical electrolysis for anodic oxygen evolution from alkaline solution is the use of amorphous metals, i.e. chilled melts of nickel/cobalt mixtures whose crystallization is prevented by the addition of refractory metals like Ti, Zr, B, Mo, Hf, and P (46-51). For this type of material, enhanced catalytic activity in heterogeneous catalysis of gas-phase reactions has been observed (51). These amorphous metals are shown to be more corrosion resistant than the respective crystallized alloys, and the oxides being formed at their surfaces often exhibit a higher catalytic activity than those formed on ordered alloys, as shown by Kreysa (52-54). 

In order to complete this review, a brief overview of magnetic materials other than oxides is presented in this chapter. Soft and hard metallic alloys are discussed first instead of a detailed account of the numerous alloy systems, this overview focuses on the mechanisms for obtaining specific microstructures, which, in turn, lead to a precise control of anisotropy in soft materials, and to coercivity in hard materials. These discussions include examples of classic alloys, as well as the recently developed soft amorphous alloys and the impressive supermagnets with extremely high coercive fields. 

A similar behaviour is found for the isomer shift at a 3d atom site (fig. 93b). The broken line connects the IS values in Fe metal with those in Th Fe, (Viccaro et al., 1979). In these cases, too, the IS values in the amorphous alloys (full circles) do not behave extraordinarily. It seems therefore that in order to understand the differences in magnetic properties between amorphous and crystalline materials, one has to look for a mechanism other than that of a reduced charge transfer. Further experimental evidence refuting charge transfer effects as the main origin for the moment reduction upon alloying will be discussed in the next section, dealing with photoemission experiments. 

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