Question

What is precipitate-free zone? How does it form? How to get rid of it? (20 pts)

Answer 1

It is often the case that precipitation does not occur uniformly throughout the microstructure during the heat treatment of a supersaturated phase. Regions in the proximity of a grain boundary are frequently found to be free of precipitates. These precipitate free zones (or PFZ's) occur for two reasons:

1. The most common reason for the formation of PFZ's is that precipitates nucleate heterogeneously on vacancies. 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.
2. The grain boundaries themselves are potent heterogeneous nucleation sites. Particles may nucleate first at these boundaries, thereby removing sufficient solute from the adjacent matrix.

PFZ – Precipitate free zone; a zone near a
5 grain boundary and is free of precipitates.

The precipitate-free zones (PFZ) and grain boundary precipitates are visible in this image. The PFZ's are obviously regions of weakness. However, they have a more serious deterimental effect. The free energy of the PFZ is different from the remaining microstructure, so that corrosion currents arise in the presence of electrolytes (water). This leads to severe and unacceptable attack of the microstructure. This can be avoided by cladding the age-hardened sample with pure aluminium, a procedure adopted for the construction of airframes.