The solution quenching process is a critical heat treatment process for aluminum alloys to obtain better strength and a homogeneous super solid solution. In this paper, the mechanical properties of Al-5%Cu-0.4%Mn in the as-quenched state are tested. The alloy, designated as ZL205 (Chinese standard), has a chemical composition similar to a 2xxx series aluminum alloy and is used for constructing large thin wall components. The ZL205A alloy shows good performance under loading at room temperature while losing its toughness and exhibiting tensile brittleness, which is unexpected at elevated temperatures (especially at 300 °C). After observing the fracture sections of ZL205A at room temperature and at 300 °C, it may be concluded that one possible reason leading to this phenomenon may be the formation of the T phase at grain boundaries. Such a hypothesis is validated and discussed with the help of SEM observations.