Solid intermetallic phases formula

As a starting point in the description of the solid intermetallic phases it is useful to recall that their identification and classification requires information about their chemical composition and structure. To be consistent with other fields of descriptive chemistry, this information should be included in specific chemical and structural formulae built up according to well-defined rules. This task, however, in the specific domain of the intermetallic phases, or more generally in the area of solid-state chemistry, is much more complicated than for other chemical compounds. This complexity is related both to the chemical characteristics (formation of variable composition phases) and to the structural properties, since the intermetallic compounds are generally non-molecular in nature, while the conventional chemical symbolism has been mainly developed for the representation of molecular units. As a consequence there is no complete, or generally accepted, method of representing the formulae of intermetallic compounds. 

Figure A 1.52 shows the Ti-Al phase diagram (important for the standard commercial alloy Ti-6% Al-4% V. It shows two peritectic reactions, at each of which liquid reacts with a solid phase to give an intermetallic compound, (a) Ring the peritectics and give the (approximate) chemical formula for the two compounds, (b) Shade all... 

The complex hydride Mg CoH is very similar to Mg FeH. In the binary system of Mg-Co there is no solubility of Co in either solid or liquid Mg and no inter-metallic compound, Mg Co, exists in equilibrium with other phases. However, in contrast to the Mg-Fe system, the intermetallic compound MgCo exists in equili-brium in the Mg-Co binary system (e.g., [14, p. 251]). The theoretical hydrogen capacity of Mg CoH is only 4.5 wt% which is obviously lower than that of Mg FeHg due to the presence of the heavier Co element and one less H atom in the hydride formula. 

Structures of Abnormal Valency or Electron Intermetallic Compounds. We have seen how in many alloy systems the / -. y- and e-phases are based on electron compounds the formula of which differ very widely but which have in common electron atom ratios of 8 2, 21 18 and 7 4. The range of existence of the particular phases is really a range of solid solution in the compound concerned, and this tends to decrease, as it does in primary solid solution, with increase of valency of the second metal. The j3-, y- and -phases have, however, more in common than mere electron concentration, for they have, in addition, the same lattice structure, although the atomic arrangement is usually a purely random one. Thus, the 8 2 / -compound phase is normally body-centred cubic, although it may have a modified cubic structure known as the /3-manganese one the 21 18 y-compound phase, known as the y-brass structure,... 

For some systems, discrete intermediate compounds rather than solid solutions may be found on the phase diagram, and these compounds have distinct chemical formulas for metal-metal systems, they are called intermetallic compounds. For example, consider the magnesium-lead system (Figure 9.20). The compound Mg2Pb has a composition of 19 wt% Mg-81 wt% Pb (33 at% Pb) and is represented as a vertical line on the diagram, rather than as a phase region of finite width hence, Mg2Pb can exist by itself only at this precise composition. 

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