Phase transformation diagrams continuous-cooling

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. 

Fig. 7.12 Schematics of (a) tool position vs. time, (b) thermal cycle with superimposed continuous cooling transformation curve, and (c) pseudobinary phase diagram. Positions a" through "i" on the diagrams correspond to Fig. 7.13(a) through (f) and are used to describe microstructural evolution in the stir zone for friction stir welding on Ti-6AI-4V.

It is important to note that the treatments relating to the kinetics of phase transformations in Section 10.3 are constrained to the condition of constant temperature. By way of contrast, the discussion of this section pertains to phase transformations that occm with changing temperatirre.This same distinction exists between Sections 10.5 (Isothermal Transformatiorr Diagrams) and 10.6 (Continuous-Cooling Transformation Diagrams). 

In summary, isothermal and continuous-cooling transformation diagrams are, in a sense, phase diagrams in which the parameter of time is introduced. Each is experimentally determined for an alloy of specified composition, the variables being temperature and time. These diagrams allow prediction of the microstructure after some time period for constant-temperature and continuous-cooling heat treatments, respectively. 

The phase diagram that Beaudry et al. established is shown in fig. 65. The boundary for the transformation from the hep solid solution to the bcc form goes through a minimum at 22at% Sc and 800°C as determined by differential thermal analysis data taken at 5 at% intervals. The thermal analysis established the existence of the continuous series of bcc solid solutions. The shape of the miscibility gap was established by metallographic examination of annealed alloys quenched from various temperatures. Anomalies were noted in the heating and cooling curves between... 

In this discussion of the microstructural development of iron-carbon alloys, it has been assumed that, upon cooling, conditions of metastable equilibrium have been continuously maintained that is, sufficient time has been allowed at each new temperature for any necessary adjustment in phase compositions and relative amounts as predicted from the Fe-FejC phase diagram. In most situations these cooling rates are impracti-cally slow and unnecessary in fact, on many occasions nonequilibrium conditions are desirable. Two nonequilibrium effects of practical importance are (1) the occurrence of phase changes or transformations at temperatures other than those predicted by phase boundary lines on the phase diagram, and (2) the existence at room temperature of nonequilibrium phases that do not appear on the phase diagram. Both are discussed in Chapter 10. 

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