Peritectoidal decomposition

Melting point peritectoid decomposition ° peritecdc decomposition eutectoid formation ... 

The composition of any member of the series varies within narrow but observable limits depending upon the temperature and oxygen pressure and upon the direction of preparation (hysteresis). The most common feature of the phase diagram in the CeOx and PrOx systems shows a peritectoidal decomposition of members of the homologous series to at least one disordered phase... 

Rouanet, 1971), or to undergo a peritectoid decomposition rather than an order-disorder transformation. Thus fig. 28.18 is a redrawn version of fig. 28.16 incorporating these proposed features. 

The foregoing classification is not without ambiguity. For example, it is common practice to call the reaction A - B +C° (see Fig. 6-1) induced by decreasing the temperature a phase transformation. The similar (peritectoid) reaction C = a+fi (Fig. 12-2) induced by a temperature increase, however, is named a decomposition reaction. In addition, the isothermal reaction AO = A+j02, which occurs if the intensive variable fio2 is decreased so that AO decomposes, is called a metal oxide reduction. It is thus categorized as a genuine heterogeneous solid state reaction (the... 

Figure 4.1 shows these diffusion couples below (left) and above (right) the decomposition temperature of the , phases.10 It is obvious that the phase bands of the , phases are present between the fi and 6 phases the left column of the microstructures while it is absent in the right column showing the presence of an invariant phase reaction. The accordingly invariant temperatures are given in Table 4.1. For the Nb-C system the formation of a phase band was most difficult to observe because the peritectoid temperature is rather low and the carbon diffusivity is slow. 

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