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Nucleation
is a process that must occur before a new phase can form in a system undergoing
a phase transition. If the process takes place away from any boundaries
or defects it is known as homogeneous nucleation. Nucleation at a surface,
interface, dislocation or other defect in the material is known as heterogeneous
nucleation. Heterogeneous nucleation is favored because it requires a lower
free energy change to form the initial stable nucleus, that can then grow
to generate the new phase. The diagram illustrates some nucleation sites
that occur in a solid state transformation process. |
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