Investigation on Microstructure and Fatigue Behavior of Al-5Zn- 2Mg High Strength Aluminum Alloy with T5 Heat Treatment

The low cycle fatigue property of Al-5Zn-2Mg high strength aluminum alloy with T5 heat treatment was tested, and the fatigue damage behaviors were observed by OM, SEM and TEM. The results show that the rolling direction of Al-5Zn-2Mg high strength aluminum alloy is distinct. The separations, such as inclusions, mostly distribute along the rolling direction. The η′ (MgZn2) transitional strengthening phases are precipitated in Al-5Zn-2Mg aluminum alloy, and mostly distributed in grain boundary. The appearance of strengthening phase is ellipsoid. Its diameter of short axis is about 20 nm, and the diameter of long axis is about 40 nm. The cycle fatigue life is 4.22×106 cycle in R=0.1, f=8 Hz, and σmax=127-210 MPa. The fatigue crack initiates in surface of Al-5Zn-2Mg aluminum alloy sample, while the fatigue crack come into being and propagate along the rolling direction. The fatigue stress causes the loose microstructure. There have the fatigue striations and shell texture in fatigue crack propagation zone.