Enlarged Zn Content Improves Fatigue Crack Propagation Resistance of Two Al-Zn-Mg-Cu Alloys with Multiple Aging Tempers

Fatigue crack propagation is closely associated to the chemical composition of Al-Zn-Mg-Cu alloys. In this work, two Al-Zn-Mg-Cu alloys with a variation of zinc content was investigated and multiple aging treatments were exerted on them and a same regime was selected for further fatigue analysis by tensile property tests. The corresponding precipitation characteristics and fracture surface were observed. The results showed that the alloy with lower zinc content (LZ alloy) possessed an inferior strength value compared with the alloy with higher zinc content (HZ alloy) under the same three stage aging treatments while the elongation had no obvious difference. In contrast, the LZ alloy had a higher fatigue crack propagation rate than the HZ alloy. The observation of fracture surface also proved it. The precipitation observation demonstrated that both had GP zones and η' phase, which possessed the majority. Quantitative analysis of precipitates exhibited that the HZ alloy had a larger proportion of large size precipitates than the LZ alloy. The mechanism of the interaction between dislocation and precipitate was employed to elaborate the difference.