Pre-precipitate clusters and precipitation processes in Al-Mg-Si alloys

The solute clusters and the metastable precipitates in aged Al-Mg-Si alloys have been characterized by a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). After longterm natural aging, Mg-Si co-clusters have been detected in addition to separate Si and Mg atom clusters. The particle density of beta " after 10 h artificial aging at 175 degrees C varies depending on pre-aging conditions, i.e. pre-aging at 70 degrees C increases the number density of the beta " precipitates, whereas natural aging reduces it. This suggests that the spherical GP zones formed at 70 degrees C serve as nucleation sites for the beta " in the subsequent artificial aging, whereas co-clusters formed at room temperature do not. Atom probe analysis results have revealed that the Mg:Si ratios of the GP zones and the beta " precipitates in the alloy with excess amount of Si are 1:1, whereas those in the Al-Mg2Si quasi-binary alloy are 2.1. Based on these results, the characteristic two-step age-hardening behavior in Al-Mg-Si alloys is discussed. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.