The multiple orientation relationships and morphology of β' phase in Al-Mg-Si-Cu alloy

The orientation relationships (ORs), segregation behavior and morphologies of β' precipitate in an over-aged Al-Mg-Si-Cu alloy are systematically characterized by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Six different ORs and two morphologies, i.e. rod- and lath-like are revealed for β' precipitates, and Cu segregation at the β′/α-Al interface is observed in all these precipitates. The rod-like β' precipitate has multiple β'-angles ranging from 6.1° to 14.1° and non-uniform Cu segregation at the β′/α-Al interface, while the lath-like β' precipitate has a constant β'-angle of 0° and a periodic Cu segregation. These different ORs are explained to be attributable to the rotation of QP lattice, a near-hexagonal network of Si columns formed within β' precipitates, which causes different lattice matching of β′ with α-Al lattice. These findings provide new insights in controlling the precipitation hardening and mechanical properties of this type of alloys.