A model for the precipitate transformation of Mg-Si-rich clusters into Mg5Si6 β" in Al-Mg-Si aluminum alloys

A model is developed that describes the kinetics of precipitate transformations in the course of natural and artificial aging of Al alloys containing Mg and Si additions. In our approach, the disordered Mg-Si-rich clusters, which form during natural aging in the highly supersaturated Al matrix, can directly transform into the monoclinic Mg5Si6 (beta ''), without prior dissolution of the clusters and independent nucleation of beta '' in the Al matrix. The transformation rate is evaluated with classical nucleation theory (CNT), assuming that the clusters represent an infinitely large matrix phase in which the beta '' precipitates can nucleate. The adapted CNT model is described, and the basic features of the precipitate transformation are discussed in a parameter study. The model can also account for the observation that, during natural aging, the parent clusters occur in a variety of Mg to Si ratios, all of which have a characteristic probability of either transforming into the beta '' phase or dissolving.