Particles may nucleate first at these boundaries, thereby removing sufficient solute from the adjacent matrix. The grain boundaries themselves are potent heterogeneous nucleation sites.A grain boundary is a sink for vacancies so that regions adjacent to the boundary are unable to nucleate the precipitates, even though the matrix may be supersaturated with solute. The most common reason for the formation of PFZ's is that precipitates nucleate heterogeneously on vacancies.These precipitate free zones (or PFZ's) occur for two reasons:
Regions in the proximity of a grain boundary are frequently found to be free of precipitates.
It is often the case that precipitation does not occur uniformly throughout the microstructure during the heat treatment of a supersaturated phase. Precipitate free zones Precipitate Free Zones H.
