Abstract:There are three variants of precipitate in Al-Cu alloy, and these variants are parallel or perpendicular to the external stress direction. As many reserchers assumed that the external stress is parallel or perpendicular to the precipitate, however the external stress may have a certain angle with the precipitate for some aluminum alloys. Therefore, a new model is proposed in this paper with the consideration of the angle between precipitate and stress, which can give a method to judge the stress orientation effect of the precipitate. Transmission electronic microscopy (TEM) and Scanning transmission electronic microscopy (STEM) are employed in this paper to analyze the precipitate in the aged Al-4Cu alloy under applied stress. The results proved that the existence of the stress orientation effect of GP zones and θ″ phase. It is employed the new model to analyze the stress orientation effect and this model can provide a convinient method to prove whether there is stress orientation effect, which can also be applied to other series aluminum alloy.