The nucleation of precipitates: The system Al-Zn-Mg

The morphology and distribution of precipitates in an Al-Zn-Mg alloy have been studied by transmission electron microscopy with particular attention being paid to the effect of lattice defects on the nucleation process. The apparently homogeneous nucleation of precipitates within the grains has been shown to be largely controlled by the concentration and distribution of vacant lattice sites. A semiquantitative theory has been developed for the formation of precipitate free zones at grain boundaries and the results have been used to develop a model of nucleation by vacancy/solute atom clusters. The most important feature of the model is that the dispersion of precipitates is very sensitive to quench procedure and the very early stages of ageing. It is thought that the results may be useful in the explanation of quench sensitivity, delay time before ageing, rate of heating to the ageing temperature and trace element effects in alloys.