Intermetallic phases prototypes

Notice that most of the indicated prototypes correspond to structures frequently found not only in intermetallic phases but also in ionic compounds. 

Considering then the phase composition as a significant parameter, we obtain the histogram shown in Fig. 7.1(a) for the distribution of the intermetallic phases according to the stoichiometry of binary prototypes. For instance, the binary Laves phases, the A1B2, Caln2, etc., type phases are all included in the number reported for the 66-67.99 stoichiometry range, even if the real stoichiometry of the specific phase is different, see Fig. 7.1(b). We may note the overall prevalence of phases and, to a certain extent, of structural types, which may be related to simple (1 2, 1 1, 1 3, 2 3, etc.) stoichiometric ratios. 

Table 7.1. Approximate distribution of intermetallic phases among the different structural prototypes ( 8000 binary phases considered), according to the data taken as an example in Villars etal. (1995).

The number and variety of intermetallic phases having more complex structure than the simple ones considered in previous paragraphs is very large. The small groups of prototypes here reported are therefore just a few examples of binary (or ternary) phases having odd or very high stoichiometric ratios. 

Introduction. A number of common structures, ideally corresponding to a 1 1 stoichiometry, are presented in this chapter. Some of them are not specifically characteristic of intermetallic compounds only. The CsCl and NaCl types, for instance, are observed for several kinds of chemical compounds (from typical ionic to metallic phases). Notice that for a number of prototypes a few derivative structures have also been considered and described, underlining crystal analogies and relationships even if with a change in the reference stoichiometry. 

As already mentioned, ternary scandium intermetallics often crystallize in structure types which are superstructures to binary prototypes (table 33, Appendix A). It seems convenient to us to select structure types among them which occur in large groups of elements and their combinations, see table 34. These structure types as well as their binary prototypes are frequently observed among all intermetallics having up to hundreds of known representatives. The atomic size factor has a dominant influence on distribution of atoms in ternary phases. The largest atoms in the structure normally occupy atomic... 

In the case of beryllium, the phase diagrams contain at least one intermetallic compound which has the same stoichiometry (RBeu and AnBen) and the same structure (prototype NaZnu). In the Sc-Be system other intermetallic compounds have been detected. 

By crystallographic transformations one generally means that, when a new phase (product) forms from the old (parent), it bears certain deHnite geometrical relationships. The most widely studied crystallographic transformation is the martensitic transformation, the prototype of which occurs in quenched steels. Actually, martensite (in honor of Professor A. Martens) was the name given by Osmond in 1895 to the microstructure observed in quenched steels, but in more modern times the word martensite designates a transformation mechanism, now known to be associated with many metals, alloys, ceramics, and even some polymers. These transformations occur in a variety of intermetallics, most notably the Hume-Rothery electron compounds as found, for example, in /3-brass of near-equiatomic composition, and many similar alloys of Cu, Ag, and Au. 

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