CHEN Tao, ZHAO Luyuan, LI Hui, HUANG Jun, WU Yucheng
Stress controlled fatigue tests at different stress amplitudes, 60%, 70%, 80% and 90% of yield strength (σs) of the tested alloy, respectively, were conducted on 7075-T651 aluminum alloy. The cyclic strain response of the alloy was studied, the fatigue fracture morphology, surface damage morphology and microstructure were observed and the initiation and propagation mechanisms of fatigue crack were analyzed. The results show that micro-scale Al7Cu2Fe particles, nano-scale η' (MgZn2) phase and relatively large sized η (MgZn2) phase were precipitated in the tested alloy. Moreover, fine and sphere-like GP zones with size of 3-10 nm were observed. Under the control of relatively high stress amplitude (80%σs,90%σs), the tested alloy showed a fatigue behavior of first softening then hardening until fracture, while first softening then hardening until becoming stable relatively at low stress amplitudes (60%σs, 70%σs). The tested alloy mainly fractured in a micropore gathered dimple manner. At high stress amplitude, fatigue cracks initiated at the coarse inclusion of Al7Cu2Fe and second phase of MgZn2, with dislocation tangling a lot. Whereas at low stress amplitude, the crack source was at the slightly torn position of the substrate, and the produced dislocation showed shapes of dispersed short or long straight dislocation lines.
