The aluminium alloy with 4wt% copper has been over aged.
Prolonged heat treatment above 150°C to 200°C has led to the evolution of the stable CuAl2 phase (theta) at the expense of the fine precipitates developed in the age-hardened alloy.
The CuAl2 phase is visible at high magnifications. It does not contribute to the strength of the alloy and the over aged aluminium has poor mechanical properties.
See age hardened aluminium for more information on precipitation in aluminium alloys.