Precipitation sequence of various kinds of metastable phases in Al-1.0mass% Mg2Si-0.4mass% Si alloy

The precipitation of three new types of metastable phases, i.e., TYPE-A, TY PE-B and TYPE-C, with different crystal structures from the beta' phase is proposed from our research on the change in crystal structures and formatio n sequence of metastable phases during the aging of the Al-1.0mass% Mg2Si-0 .4mass% Si alloy by a combination of analytical high resolution electron mi croscopy and energy dispersive X-ray spectroscopy. The sequence of their fo rmation is explained as follows. First, precipitation of the beta' phase an d TYPE-B precipitate, then beta' dissolution into the matrix and degradatio n of the TYPE-B precipitate. Finally, predominant precipitation of the meta stable TYPE-A precipitate. The TYPE-C precipitate appeared heterogeneously in the over-aged condition. (C) 2000 Kluwer Academic Publishers.