Asymmetry and deformation mechanism in stress relaxation of 7B50 alloy

Stress relaxation behavior, deformation mechanism, and microstructure evolution of retrogression and reaging 7B50 aluminum alloy (7B50-T7751) have been experimentally investigated with different stress directions in this study. The stress relaxation aging tests with various initial stress levels and directions were carried out at 140 degree celsius for up to 16 h. Room temperature tensile tests and transmission electron microscopy (TEM) observation tests were performed subsequently. The results show that the stress relaxation behavior under tension and compression exhibits significant asymmetry. The stress relaxed and stress relaxation rates under tension are relatively higher as compared to compression, as smaller and more dispersed dislocations under tensile stress reduce the threshold stress. The stress direction contribute the opposite effect on strength evolution during stress relaxation. Furthermore, the stress relaxation mechanism of the material has been analyzed based on creep stress exponent and threshold stress. In the low-stress range, dislocation climbing controls the deformation mechanism under tensile stress, while dislocation slip under compressive stress. As the stress increases, dislocation slip controls the deformation mechanism under tensile stress, while grain boundary slip under compressive stress. A physically-based stress relaxation constitutive model considering the dislocation theory and stress relaxation mechanism is established. The predicted results of the model are in good agreement with the experimental data, indicating that the model can effectively describes stress relaxation behavior under complex stress conditions.