Amorphous continuous cooling transformation

Fig. 60. Schematic illustration of a two-stage continuous cooling transformation (C.C.T.) behavior for Al-rich amorphous alloys and two kinds of cooling curves.

For a number of applications, particularly those associated with conditions of continuous cooling or heating, equilibrium is clearly never approached and calculations must be modified to take kinetic factors into account. For example, solidification rarely occurs via equilibrium, amorphous phases are formed by a variety of non-equilibrium processing routes and in solid-state transformations in low-alloy steels much work is done to understand time-temperature-transformation diagrams which are non-equilibrium in nature. The next chapter shows how CALPHAD methods can be extended to such cases. 

Lead oxide (PbO) exists in two modifications (polymorphs) (1) red tetragonal lead oxide (tet-PbO) (also known as a-PbO or litharge) and (2) yellow orthorhombic lead oxide (orthorhomb-PbO) (also known as P-PbO or massicot). Tet-PbO is stable at low temperatures and low pressures. The transition temperature of tet-PbO to orthorhomb-PbO is 486—489 °C and the thermal effect of the transition is 1.35 kJ mol When orthorhomb-PbO is cooled rapidly, it may remain unchanged and continue to exist at low temperatures. Eventually, it is slowly transformed into tet-PbO under external physical action. Lead oxide exists also in amorphous form. The latter s amount depends on the method of manufacture of PbO. 

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