The effects of Zr(0, 0.04, 0.10, 0.16 and 0.38wt%) on the microstructures, age precipitation hardening behaviour as well as tensile properties and fracture behaviours at room temperature of Al-Li-Cu-Mg alloys (Li, Cu and Mg concentrations are within the specifie range for commercial 8090 alloy which additionally contains 0.04-0.16 wt% Zr) have been studied. Zr additions inhibit recrystallization and refine grain size dramatically. When aging at 190 ℃, the precipitation of δ'-phase in quenching and early aging stage is retarded by additions of Zr, and cubic metastable Al_3Zr dispersoids act as heterogenuous nucleation sites of δ'-phase, accelerating the coarsening process of δ'phase, as a result, aging process is promoted, and the particle size distributions of δ'-phase change from symmetrical distribution. The coarening behaviour of δ'-phase follows Lifshitz-Slyozov-Wagner (LSW) theory. The widths of PFZ controlled by the characteristics of grain boundaries depend directly on the square root of aging time and become narrow with the increase of Zr contents at the same aging conditions. Minor additions of Zr (0.04wt%) give obvious hardening effect, but the effect on the aging process type is not clear. Increasing Zr contents, the peak-aging time is shortened and the age-hardening effect improved. The optimum Zr contents and solution temperature are 0.10 ～ 0.16wt%Zr and 530 ℃, respectively. The alloys under these conditions show a combined structure of recrystallized (RF<35%) and unrecrystallized, as well as the best combination of strength and ductility. They show transgranular dimple fractography under solution condition and ductile inter-subgranular or transgranular fractography with cleavage fracture characteristics under peak-aged condition. The formation of δ'/βcomplex precipitate, the effect of Zr on the uncleation of S' phase, the effect of grain boundaries on PFZ and the effect of different homogenization and solution treatment on microstructure and properties have also been discussed.
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