The morphology and orientation relationship variations of Q′ phase in Al–Mg–Si–Cu alloy

•The variations in morphology, OR and lattice parameters of Q′ phase were observed.•The mechanism for the multiple OR was explained.•The Si-network plays an important role in controlling the structure of Q′ phase.

The morphologies and orientation relationships (OR) of the Q′ phase in an over-aged Al––Mg–Si–Cu alloy were characterized by atomic resolution high-angle annular dark-field scanning transmission electron microscopy. Statistical results revealed that the OR of the rod-like Q′ phase with the Al matrix has a variable angle ranging from ~ 3.4° to 15.5° between (112-0)Q′ and (510)α, while the lath Q′ phase has a conventional OR with a constant angle of 11°. Such variations likely to be attributable to the ordering of Si-network in the Q′/β″ phase formed in the peak aging condition.