ATOM PROBE CHARACTERIZATION OF PREPRECIPITATE CLUSTERS AND PRECIPITATES IN Al-Mg-Si(-Cu) ALLOYS

Pre-precipitation clustering and chemical nature of the metastable precipitates in aged Al-Mg-Si(-Cu) alloys have been investigated by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM) for better understanding of the two-step aging behavior and the effect of excess Si. After a prolonged natural aging, co-clusters of Mg and Si atoms are observed in addition to separate Siand Mgclusters. The ratio of Mg to Si atoms in the co-clusters is approximately 1:1. Spherical G.P. zones are present after pre-aging for 16 h at 70°C, and they contain 20 at.% Mg and 20 at.%Ag. The density of the β′′ that precipitates after 30 min aging at 175°C increases significantly when the specimen is pre-aged at 70°C, suggesting that G.P. zones formed at 70°C serve as nucleation sites for the β′′ in the subsequent 175°C aging. On the other hand, co-clusters formed at room temperature do not serve as nucleation sites for the β′′ precipitates. The mechanism of adverse artificial age hardening response after natural aging is discussed.