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 beta '' phase transforms simultaneously to various types of precipitates in the over-aged stage. Two different evolution paths are proposed. Firstly, the beta '' phase transforms directly to the U1, U2 and beta ' phases. Secondly, the U2 phase, as an intermediate phase, transforms to the U1, beta ' 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 (beta ' 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.

Automated summary

The evolution mechanism of multiple metastable precipitates in Si-rich Al-Mg-Si alloy was studied using high resolution microscopy and first-principles calculation. It was found that the beta'' phase transforms simultaneously to various types of precipitates in the over-aged stage, with two different evolution paths proposed. The formation and evolution of substructures are key factors in the transformation of precipitates.