| Summary: | The effects of combined Ti and Zr addition on microstructures and mechanical properties are systematically investigated in Al–Zn–Mg–Cu alloy by microstructure characterizations and a physical-based model. The results show that combined addition of Ti and Zr can promote the precipitation of nano L12 Al3(Ti,Zr) dispersoids, while primary D022 Al3Ti and Al3(Ti,Zr) phases are formed during solidification when Ti addition is over 0.2 wt%. As compared to the 0Ti alloy, the ductility and toughness is enhanced markedly by 0.1–0.2 wt%Ti addition since the volume fraction of nano L12 Al3(Ti,Zr) dispersoids is increased. Both the strength and ductility are significantly decreased when Ti addition is more than 0.35 wt%. The strain hardening behavior and fractured morphology analyses suggest that the deterioration of mechanical properties is mainly due to the localized recrystallization and cracks caused by coarse primary phases, which cause the higher dislocation dynamic recovery rate. Combined addition of minor Ti and Zr elements may provide a simple approach to improve toughness and simultaneously reduce cost in developing high-strength Al alloys.
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