TEM investigations of two alloys isothermally heat treated at 175°C and 260°C show how Cu additions to the Al–Mg–Si system affect precipitation. Both alloys had a solute content Mg + Si = 1.3 at.%, 0.127 at.% Cu, but with Mg/Si 0.8 and 1.25. Cu-containing Guinier-Preston (GP) zones and three types of Q′ precursors are identified as most common phases at peak-hardness conditions, whereas β″ accounts for maximum 30% of the total number of precipitates. The precursors have needle (L and S precipitates) or plate (C precipitate) morphologies. They consist of different arrangements of Al, Mg and Cu atoms on a grid defined by triangularly arranged Si planes parallel with and having the same period as {100} Al planes. The Si grid is composed of nearly hexagonal sub-cells of a = b = 4.05 Å, c = 4.05 Å. The Cu arrangement on the grid is often disordered in the needle precursors. The plate precursor is ordered, with a monoclinic unit cell of a = 10.32 Å, b = 8.1 Å, c = 4.05 Å, γ = 101°.

The composition of ${\beta }^{″}$ precipitates in an Al–Mg–Si alloy has been investigated by atom probe tomography, ab initio density functional calculations, and quantitative electron diffraction. Atom probe analysis of an Al-0.72% Si-0.58% Mg (at. %) alloy heat treated at $175°\text{C}$ for 36 h shows that the ${\beta }^{″}$ phase contains $\sim 20\text{at}\text{.}\%$ Al and has a Mg/Si-ratio of 1.1, after correcting for a local magnification effect and for the influence of uneven evaporation rates. The composition difference is explained by an exchange of some Si with Al relative to the published ${\beta }^{″}{\text{-Mg}}_5{\text{Si}}_6$ structure. Ab initio calculations show that replacing the ${\text{Si}}_3$ -site by aluminum leads to energetically favorable compositions consistent with the other phases in the precipitation sequence. Quantitative electron nanodiffraction is relatively insensitive to this substitution of Al by Si in the ${\beta }^{″}$ -phase.