Effects of Ti addition on the microstructure and mechanical properties of Al–Zn–Mg–Cu–Zr alloy

The effects of Ti addition (0.1 wt%) on the microstructure evolution and mechanical properties of an Al–7.6Zn–2.6Mg–2.0Cu–0.1Zr alloy were investigated during solidification, extrusion, solution treatment, and aging. The addition of Ti reduces the grain size and degree of phase agglomeration during solidification, thereby improving the strength and ductility of the as-cast alloy. The extrusion forms a bimodal structure consisting of fine dynamically recrystallized (DRXed) grains and coarse elongated unDRXed grains. Ti induces the refinement of the η-phases that enhances dynamic recrystallization (DRX) by particle-stimulated nucleation, resulting in reduced strength of the as-extruded alloy. The dissolution of the η-phase occurs in the initial stages of the solution treatment, followed by coarsening of the η-phase. The solution treatment causes static recrystallization and grain growth, increasing the grain size. The addition of Ti decreases the size and increases the number density of the L1₂ precipitate by modifying the chemical composition of (Al,Zn)₃Zr into (Al,Zn)₃(Zr,Ti), and causes the formation of an Al₁₈Mg₃Ti₂ phase during the solution treatment. The addition of Ti also enhances heterogeneous nucleation of η-Mg(Zn,Cu,Al)₂ at the Al₁₈Mg₃Ti₂ interface and grain boundary but has a negligible effect on the formation of nanoprecipitates (GPII zone, η') in the matrix. The improved strength and ductility of the solution-treated and aged alloys are attributed to the higher number density of the fine L1₂-(Al,Zn)₃(Zr,Ti) precipitates and grain refinement, as well as the finer η-phases acting as cracking sites.