Formation of intermediate phases in alloy systems

A different, more complex, alloying behaviour is illustrated in a number of figures (Figs. 2.18-2.21) where several examples of alloy diagrams are presented which are typically characterized by the formation, through a range of different mechanisms, of intermediate phases. 

In the field of solid-state chemistry an important group of substances is represented by the intermetallic compounds and phases. In binary and multi-component metal systems, in fact, several crystalline phases (terminal and intermediate, stable and metastable) may occur. A few introductory remarks about these substances will be presented in relation to the mentioned figures. 

Simple schematic phase diagrams of binary alloy systems are shown in Fig. 2.18 in which the formation of one intermediate solid phase may be noticed. In these 

Ag with stoichiometry Pti7Agi5 is formed via a solid state reaction (Pt) + (Ag) — Pt17Ag15. 

In the Au-Bi system the compound Au2Bi is stable in a restricted range of temperature only it is formed by a peritectic reaction (371°C) and, at a lower temperature (116°C), it is decomposed according to the eutectoidal reaction Au2Bi — (Au) + (Bi). In the Zn-Te system, finally, we have the congruently melting compound ZnTe. In this system a miscibility gap in the liquid state may also be noticed. 

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