Study of the precipitate evolution mechanism in an Al-Mg-Si-Cu alloy with Si-rich and low Cu composition

The precipitation behavior of a typical Si-rich low Cu Al-Mg-Si-Cu alloy (AA6022 alloy) was thoroughly investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atom probe tomography (APT) and first-principles calculations. It was revealed that the slight Cu addition significantly alters the precipitate sequence of the Al-Mg-Si alloy, resulting in a distinct sequence compared with the Cu-free and high Cu alloys. Low amounts of Cu are incorporated in the clusters and the β″ phase in the under- and peak-aged stages. During over-aging, fragmentation and hybridization of different unit cells or substructures linked to the intermediate stage of the β″ → B′/Q′ transformation is observed. The B′/Q′ composite precipitate is shown to be the dominant precipitate in the over-aged condition, while other precipitates, i.e., β', U1 and U2, commonly displayed in the Si-rich Cu-free alloys, are suppressed. During subsequent aging, a B′/Q composite precipitate with a core-shell structure is formed due to the segregation of Cu atoms at the precipitate/matrix interface. The Cu segregation is shown to be beneficial for stabilizing the precipitate interface structure. The proposed altered precipitation reaction of the Si-rich low Cu Al-Mg-Si-Cu alloy is: SSSS → atomic clusters → GP zones → β″ → B′/Q′ → B′/Q + Si. • The β', U1 and U2 phases are suppressed after slight addition of Cu. • The B′/Q′ composite precipitate is the dominant precipitate formed in over-aged stage. • An altered precipitation reaction of the Si-rich low Cu Al-Mg-Si-Cu alloy is proposed.