Driving Forces for Transformation Temperature and Composition

In order for a phase transformation to occur, a driving force must be present. For most of the condensed-matter phase transformations discussed in this chapter, the driving force is supplied by a change in temperature or composition. Temperature and composition are two of the primary processing knobs that materials engineers have at their disposal to manipulate the structure and property of materials for various applications. 

FIGURE 6.3 Example temperature and composition-induced phase transformations in the Cu-Ag binary eutectic alloy system. 

LIQUID-SOLID AND SOLID-SOLID PHASE TRANSFORMATIONS 

As with all reaction free-energy changes, this AGy can be expressed in terms of A//y and A5y as 

Since AHy and ASy do not vary much with temperature, we can approximate the free energy change for the transformation at any arbitrary temperature T Tg as 

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