AbstractAdding small amounts (<1wt-%) of both magnesium and silver to an aluminium alloy containing about 4 wt-% Cu causes precipitates with a hexagonal structure (Ω-phase) to form on {111} planes of the aluminium lattice. Precipitation ofθ′on {100} planes may also occur, the relative proportions of the two types of precipitate being dependent on the levels of magnesium and silver, e.g. ~0·7 wt-% of each element almost entirely suppressesθ′formation. Even whenθ′does form in parallel withΩ-phase, on prolonged aging it tends to dissolve in favour ofΩgrowth. Using an X-ray technique to establish foil thickness, the relative amounts ofΩandθ′precipitate have been measured as a function of aging time, analysis of the data showing that growth is diffusion controlled with an activation energy of 136±15 kJ mol−1.MST/648