Nucleation and growth of precipitates in Al–Cu–Mg–Ag alloys

Abstract

Adding 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 of θ′ 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 suppresses θ′ formation. Even when θ′ does form in parallel with Ω-phase, on prolonged aging it tends to dissolve in favour of Ω growth. Using an X-ray technique to establish foil thickness, the relative amounts of Ω and θ′ 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