Precipitation in Al-Cu-Mg(-Zn) alloys

The alloy systems Al–Cu–Mg (2xxx) and Al–Zn–Mg (7xxx) are hardened by metastable nano-precipitates formed during heat treatment. The precipitate phases of the two systems are very different: 2xxx alloys have GPB-zones and S phases, which are needle-shaped, and have their main growth and coherency directions along Al. In 7xxx alloys, the equilibrium η phase and its precursors grow mainly as plates parallel to {111}Al planes. In a context of formation enthalpy of the precipitates and diffusion of the different solute elements, it is interesting to find out if the two situations can coexist in the same material. To address this, we have investigated the evolution of hardness, conductivity and presence of the relevant precipitate types during artificial ageing of Al–Cu–Mg(–Zn) alloys. Transmission electron microscopy was used for determining the precipitate microstructure in under- and over-aged conditions.