Effect of alkali metal impurities and cerium modification on the fatigue behaviour of 8090 alloy sheets

In the present study, experiments were carried out to investigate the fatigue behaviour of 8090 alloy sheets with different concentrations of alkali metal impurities and Ce alloying addition. Test results show that, with increasing total content of Na and K in 8090 alloy sheets, the fatigue life increases and the fatigue crack propagation rates decrease, in particular in the regions of the shorter fatigue life and higher fatigue crack propagation rates. In the process of the solution heat treatment of the alloy sheets in the as-rolled condition, the alkali metal impurities tend to accelerate the recrystallization of the matrix phase and to form a non-uniform microstructure, which results in the intergranular propagation of fatigue cracks in 8090 alloy sheets. If the total content of the alkali metal impurities is below a certain limit, the microalloying addition of 0.05% Ce in 8090 alloy sheets can reduce the detrimental effects of Na and K on the fatigue life and the fatigue crack propagation rates. The reason behind this may be that the alloying addition of a minor amount of Ce element could delay the recrystallization and decrease the grain size in recrystallized zones of the alloy sheets.