What Is a Phase Transformation

Liquid-solid and solid-solid phase transformations are also known as condensed-matter phase transformations. Condensed-matter phase transformations, like other kinetic processes, are driven by thermodynamics. When a region of matter can lower its total free energy by changing its composition, structure, symmetry, density, or any other phase-defining aspect, a phase transformation can occur. 

In most condensed-matter phase transformations, pressure is typically not a chief controlling variable (although it is important in certain circumstances). This is in distinct contrast to the gas-solid processes discussed in the previous chapter, where gas-phase partial pressures typically played a cendal role. 

Collective shearing displacement from old to new atom positions 

Long-range difFusional transport of B atoms to a/P phase boundary 

FIGURE 6.1 (a) Illustration of a diffusionless phase transformation involving a displacive 

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