Peritectic type

Figure 2.13. Building blocks of binary phase diagrams examples of three-phase (invariant) reactions. In the upper part the general appearance, inside a phase diagram, of the two types of invariant equilibria is presented, that is, the so-called 1 st class (or eutectic type) and the 2nd class (or peritectic type) equilibria. In the lower part the various invariant equilibria formed by selected binary alloys for well-defined values of temperature and composition are listed. In the Hf-Ru diagram, for instance, three 1 st class equilibria may be observed, 1 (pHf) — (aHf) + HfRu (eutectoid, three solid phases involved), 2 L — (3Hf + HfRu (eutectic), 3 L —> HfRu + (Ru) (eutectic).

However, the situation is different if one considers the total transformation, including the solidus and peritectic type reactions where substantial solid state difflision is needed to obtain complete equilibrium. Unless very slow cooling rates are used, or some further control mechanism utilised in the experiment, it is quite common to observe significant undercooling below the equilibrium temperature of transformation. The following sections will briefly describe determinations of phase diagrams where non-isothermal techniques have been successfully used, and possible problems associated with non-equilibrium effects will be discussed. 

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. 

Investigation data on peritectic reaction of austenite formation from the liquid and the 6-ferrite (a-ferrite + a— -y) in the ternary systems C-Fe-X (X = Mo, W, Cr, Si, A1 and others) have been summarized by [2001Kal]. It was proposed to name all steels undergoing peritectic reaction the peritectic type steels. 

The section Ce2Se3-SnSe (fig. 16) is a quasi-binary section of the ternary system Ce-Sn-Se and belongs to a peritectic type with limited solid solutions based on SnSe (Murguzov et al. 1986). The parameters of the peritectic point are 70 mol% SnSe and 870 5°C. The ternary compound Ce2SnSe is formed according to the reaction... 

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