The unique hybrid precipitate in a peak-aged Al-Cu-Mg-Ag alloy

The prevalent hardening phase forming in an Al-Cu-Mg-Ag alloy after peak-aging at 150 and 190 °C has been investigated using transmission electron microscopy methods. The precipitate atomic structure was determined. It is a hybrid precipitate (HP) with plate morphology on {111}Al planes, consisting of orthorhombic and hexagonal structural fragments. Density functional theory calculations suggest that the hybridization reduces structural incompatibility of the HP plates with the Al matrix at the broad interfacial boundaries. Incorporation of Cu, Mg and Ag in the bulk HP structure reduces its formation enthalpy.