Interactive transformation mechanisms of multiple metastable precipitates in a Si-rich Al-Mg-Si alloy

The precipitate evolution mechanism of the Al-Mg-Si alloys with a Si-rich composition have been a long-term challenge due to the complex transformation paths among multiple precipitates. In the present work, the interactive transformation process of multiple metastable precipitates in a Si-rich Al-Mg-Si alloy were thoroughly studied by atomic resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), in-situ heating TEM and first-principles calculation. It was revealed that the β″ phase transforms simultaneously to various types of precipitates in the over-aged stage. Two different evolution paths are proposed. Firstly, the β″ phase transforms directly to the U1, U2 and β′ phases. Secondly, the U2 phase, as an intermediate phase, transforms to the U1, β′ and B′ phases during the prolonging aging. The formation and evolution of substructures, including the QP lattice and Al–Si columns, are the key for structural transformation of precipitates. During the severe over-aging, the evolution of precipitates is governed by the dissolution of small precipitates (β′ and B′ phases) and the growth of the large ones (U1 phase). These results provide scientific basis for both property improvement of the commercial Al-Mg-Si alloys and development of new 6xxx aluminum alloys.