Phase Transformations

Most phase transformations do not occur instantaneously but begin by the formation of numerous small particles of the new phase(s), which increase in size until the transformation has reached completion.

The progress of a phase transformation can be broken down into two distinct stages: nucleation and growth.

This shows the nucleation of very small particles (only a few hundred atoms) which increase in size during cooling, resulting in the disappearance of the parent phase.

Homogeneous vs Heterogeneous nucleation

Solidification of a solid solution alloy is heterogeneous – more energetically favourable for nucleation on pre-existing surfaces or interfaces.

Homogeneous Nucleation and Gibbs “free” Energy

Gibbs “free” energy is energy available to do work.

A spontaneous phase transformation will only occur when the Gibbs free energy has a negative value: ΔG < 0.

ΔG = ΔH – T ΔS

ΔG = 0: equilibrium

ΔG > 0: non-spontaneous

ΔG < 0: spontaneous

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