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.