Atomistic structure of Cu-containing β″ precipitates in an Al–Mg–Si–Cu alloy

The β″ precipitates in a peak-aged Al–Mg–Si–Cu alloy were measured with an average composition of 28.6Al–38.7Mg–26.5Si–5.17Cu (at.%) using atom probe tomography. High-angle annular dark-field observations revealed that Cu incompletely substitutes for the Mg1 and Si3 columns, preferentially for one column in each pair of Si3. Cu-free Si columns form a parallelogram-shaped network that constitutes the basis of subsequent precipitates in the system, with a = 0.37 nm, b = 0.38 nm, γ = 113° and c = 0.405 nm.