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Nucleation is the first step in a phase transformation, whereby a grain of a different phase forms within a volume of the parent phase. Fluctuations in density, composition, and structure are always occurring at the atomic level within a substance. Often these fluctuations lead to structures that are more thermodynamically stable than the parent phase. However, there is a barrier to the formation of new phases, since energy is required to create the surface between the new phase and the parent phase. Below a certain critical size, fluctuations will disappear due to the energy penalty of creating new surface. Above this critical size, the driving free energy per volume will dominate, causing the transformed region to grow. Nucleation occurs when the atomic clusters pass through this region, after which the overall energy for tranformation is positive and the nuclei are favored to grow.
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