Secondary ageing in an aluminium alloy 7050

Secondary precipitation takes place in alloy 7050 at 65 °C after underageing at 130 °C and quenching (T6I4-65 temper) and results in a significantly increased number density of the η′ platelets, the precipitates also formed in the T6 temper. The modified microstructure results in tensile properties comparable to that of the T6 temper, but with significantly improved fracture toughness. A combined transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) study has shown that secondary ageing at 65 °C results in evolution of the GPI zones formed during underageing into the η′ phase. Ageing at 65 °C alone results in the formation of GPII zones, which provide lesser strengthening than the η′ platelets. The DSC study revealed six exothermic reactions corresponding to the formation of six different types of precipitate during the DSC scan.