Solids, binary systems equilibrium with other phases

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. 

The ternary systems of these kinds of metals with boron reveal a more complex structure because of the presence of many other ternary phases denoted to as tp-and co-phases. The stoichiometries of these solid solutions thereof. Other ternary phases have the composition and M M B2,v, e.g., TaNiB2, Mo2peB4, and MosCoB. As an example, an isothermal section of the B-Co-Mo system is shown in Fig. 27 in which both the x- and the (p-phases are linked with Co as the binder [128], However, in systems with Fe replacing Co, a <,c-phase does not exist. Hence co is in equilibrium with liquid metal and is thus likely to form a cermet material with Fe like the x-phase Mo2Fei3Bs (Fig. 28). Phase compositions located in the pseudo-binary equilibria with a metal can easily be pressureless liquid phase sintered at temperatures between 1500°C and 1700°C. Wear-resistant parts have been developed from Mo2FeB2-Fe cermets with Ni or Cr additives [129-131, 307]. Figure 29 presents an isothermal section of the Ni-Ta-B system at 950°C [126] with three ternary phases where only X is in equilibrium with metallic Ni. 

Since we have only one non-stoichiometric solid phase, we will approach the study of this system by quasi-chemistiy of structure elements. However, this approach presents some difficulties. Indeed, hydrated salts are relatively conplex solids with at least three principal components the anion (itself often complex), the cation, and water. If salt admits several limiting hydrates, water molecules are not all equivalent. All these complexities require a simplification of the representation of solid. With this intention, we consider hydrated solids as pseudo-binary (see section 2.4.1) of which one of the components is the water concerned with dehydration and the other component is the skeleton of anhydrous salt or incorporates possible n molecules of water not implicated in the equilibrium under study. We will disregard specific defects related to the skeleton and thus take into account the following structure elements ... 

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