Peritectic melting compound

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. 

Figure 2.36. Au-Sb system. The continuous line represents the (equilibrium) phase diagram containing the peritectic melting AuSb2 compound and a eutectic equilibrium (Es). Dotted lines represent equilibria (and phases) which may be observed on rapid cooling. The sequence of phases (C, AuSb2, it) detected in fast-quenched alloys and their composition ranges are shown in the...

Figure 4. Phase diagram from a two component system with one intermediate compound, 7, which exhibits peritectic melting and displaying limited solid solubility for a and ft.

My own criticism on the theories proposed for superplasticity can be summarized in one word, electrons , or more accurately the lack of electronic consideration. This is similar to the theoretical consideration brought forward in the study of LME (liquid metal embrittlement) described in the previous chapter - no electronic consideration. As shown in Table 2, all superplastic alloys of binary system are found either at eutectic or eutectoid compositions. This is illustrated in Fig. 14 in which a few binary phase diagram involving superplastic alloys are shown. However, the people who made efforts in the formulation of theories did not consider this well-known fact important enough to incorporate into their theory formulation [24], In fact, this observation is so consistent one should ask the question of the special attributes associated with eutectic or eutectoid composition. Or the fact that the intermetallic compounds with superelastic property are all of the peritectic type. It must be emphasized that to this date there is no report of finding superplasticity in congruently-melting compounds. 

In the previous diagrams, points e and / for the congruently-melting compound are both ternary eutectic. On the other hand, for the incongruently-melting compound die point e is the ternary eutectic, whereas the point/is the ternary peritectic. 

An example of a solid compound that does not melt congruently is shown in Fig. 13.4 on the next page. The solid hydrate NaCl-2H20 is 61.9% NaCl by mass. It decomposes at 0 °C to form an aqueous solution of composition 26.3% NaCl by mass and a solid phase of anhydrous NaCl. These three phases can coexist at equiUbrium at 0 °C. A phase transition like this, in which a soUd compound changes into a liquid and a different solid, is called incongruent or peritectic melting, and the point on the phase diagram at this temperature at the composition of the liquid is a peritectic point. 

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