Development of bimodal grain-structured Al-Zn-Mg-Cu-Zr alloys for strength-ductility synergy via microalloying with Hf and Sc

The effect of the combined addition of Sc and Hf on the microstructure and properties of high-strength Al-Zn-Mg-Cu-Zr alloys were systematically investigated. The multiscale particles are acquired after homogenization, with the network-structured coarse AlZnMgCu(Hf) and W-AlCuSc phases distributed along grain boundaries and fine W, Al3M and Al-Sc-Zr-rich particles located within the grains. After simple rolling and solute annealing, bimodal grain structures can be tailored via the collaborative effect from the linearly distributed coarse phases resulting in recrystallized coarse grain (CG) bands and the submicron particles inhibiting grain growth to form fine grain (FG) zones. As Sc gradually replaces Hf, the proportion of submicron particles increases, and the area fraction of CG bands decreases significantly from 88% to 28%. The alloy containing ∼50% CG bands shows an increase in elongation of > 23% with almost no loss in strength compared to the others. The similar strength is attributed to balance tradeoffs among the nanoprecipitates, secondary Al3M dispersoids, and grain structure. The coupling of the intergranular fracture in CG bands and the shear fracture in FG zones contributes to excellent ductility. This finding offers meaningful insights into the collaboration between multiscale particles and grain structure, and provides a simple way to obtain bimodal grain-structured Al-Zn-Mg-Cu alloys with excellent strength-ductility synergy.