Compound formation capability

Miedema s model and parameters and prediction of compound formation capability. A semi-empirical approach to the evaluation of the compound formation capability and of the heats of formation of alloys was proposed by Miedema and co-workers. This resulted in a model which became very popular and, especially because there was scarcity of experimental data, was frequently used in the evaluation, even if approximate and several times incorrect, of the formation enthalpies. The model suggested for energy effects in alloys is well known essentially it is based on the definition of two parameters. By assigning two coordinates... 

In Figs. 2.22 and 2.23 all the binary combinations are mapped as a function of the Mendeleev numbers of the two elements involved. The compound formation capability is represented in Fig. 2.22 by means of a few codes, whereas in Fig. 2.23 an indication is given of the thermal stability of the intermediate phases. To this end, values correlated to the so-called Raynor Index (Raynor 1972, 1974) are coded in this figure. 

Figure 2.22. Compound formation capability in binary systems. The different element combinations are mapped on Mendeleev number coordinates and those systems are indicated in which the formation of intermediate phases has been observed (either from the liquid or in the solid state). Blank boxes indicate systems for which no certain data are available. Notice that the compound-forming alloys are crowded in a region corresponding to a large difference in the Mendeleev numbers of the elements involved (for instance, basic metals with semi-metals).

Some aspects of the mentioned relationships have been presented in previous chapters while discussing special characteristics of the alloying behaviour. The reader is especially directed to Chapter 2 for the role played by some factors in the definition of phase equilibria aspects, such as compound formation capability, solid solution formation and their relationships with the Mendeleev Number and Pettifor and Villars maps. Stability and enthalpy of formation of alloys and Miedema s model and parameters have also been briefly commented on. In Chapter 3, mainly dedicated to the structural characteristics of the intermetallic phases, a number of comments have been reported about the effects of different factors, such as geometrical factor, atomic dimension factor, etc. on these characteristics. 

Miedema s theory and structural information. The Miedema model for energy effects in alloys, presented in 2.2.1.3 has been very useful in an evaluation, albeit approximate, of the formation enthalpies and in the prediction of compound formation capability. For an example of the application and limits of this model, see the comments on the thermochemistry of the Laves phases reported in 3.9.3. However notice that the general usefulness of the Miedema approaches has diminished with time, both for its inherent approximation and for... 

Moving towards the other groups of the Periodic Table, after a gap in the hydrogen compound formation capability, a new class of hydrides is encountered. 

Phase diagrams of alkali metal alloys. The pattern of the intermetallic reactivity of these metals is shown in Fig. 5.6, where the compound formation capability with the different elements is summarized. 

Figure 5.6. Compound formation capability in the binary alloys of alkali metals. The different elements, the binary combinations of which with Li, Na, K, Rb, Cs are considered, are identified by their positions in the Periodic Table. No rehable data have been found about the stable equilibrium phases in the Na-P and Cs-As systems compound formation is, however, probable.

Figure 5.7. Binary compound formation capability of Ca, Sr, Ba and of Eu and Yb. Those elements are marked for which compounds with the mentioned divalent metals are known.

R-Me andAn-Me alloys. A summary of the alloying behaviour of the 3rd group metals with special attention to the compound formation capability is shown in Fig. 5.14. For the lanthanides two examples are shown La and Gd, the behaviour of which may be considered to give a reasonable first approximation description of the general intermetallic reactivity pattern of the lanthanides. For the actinides the reactivity schemes are shown for Th, U and Pu for the alloys of the other metals of this series, only a few data are available. 

Figure 5.14. Compound formation capability in the binary alloys of Sc, Y, light trivalent lanthanides (as exemplified by La), heavy trivalent lanthanides (exemplified by Gd) and of the actinides (exemplified by Th, U and Pu). The different partners of the 3rd group metals are identified by their position in the Periodic Table. Notice that a sharper subdivision between compound-forming and not forming metals will result from a shifting of Be and Mg from their position in the 2nd group towards the 12th group (see 5.12.3). The behaviour of the divalent lanthanides Eu and Yb is shown in Fig. 5.7 where it is compared with that of the alkaline earth metals.

Figure 5.17. Compound formation capability in the binary alloys of Ti, Zr, Hf. Notice the reactivity pattern similar to that shown by the metals of previous groups. No compound formation with metals of the first groups, except Be, lanthanides included.

Figure 5.20. Compound formation capability in the binary alloys of V, Nb, Ta.

Figure 5.33. Compound formation capability in the binary alloys of Be, Mg, Zn, Cd, Hg. For a comparison A1 and Ca patterns are also shown.

Considering the overall compound formation capability of the various elements of the 13th group we notice a certain number of analogies between them, such as the compound formation with the metals at the left side of the Periodic Table (with the exception of A1 with the alkali metals), including lanthanides and actinides. 

In the conclusion of this chapter and on the basis of the data and schemes shown in the preceding paragraphs, we summarize some regularities observed in the description of the alloying behaviour shown by the different binary alloys. Giving special attention to the compound formation capability, the following points, even though qualitative, are underlined ... 

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