Optimizing Zn content for balanced strength-toughness in Sc/Zr-modified Al-Zn-Mg alloys

The effects of Zn content (Zn = 5.4, 5.7, and 6.8 wt%) on the microstructure and mechanical properties of the Al-Zn-Mg-Sc-Zr alloys were investigated. During the rolling deformation, the presence of Al 3 (Sc, Zr) dispersoids exerts a strong pinning effect, which contributes to the preservation of a fibrous structure and predominantly fine sub-grains for the three alloys. After solution treatment, a higher Zn content leads to an increased number of coarse particles. These particles, in turn, induce particle-stimulated nucleation, thereby increasing the degree of recrystallization and decreasing the density of geometrically necessary dislocations in the alloys. After aging, the recrystallized grains can trigger wide precipitate-free zones and coarser grain boundary precipitates, which can induce intergranular fracture. Consequently, although an elevated Zn content promotes aging precipitation behavior and strengthens the alloy, it concurrently compromises impact toughness by increasing the number of high-angle grain boundaries and raising the risk of intergranular fracture. Therefore, a favorable balance between strength and toughness is achieved at a Zn content of 5.72 wt%, with the alloy exhibiting an ultimate tensile strength of 565 MPa, a yield strength of 522 MPa, an elongation of 11.2 %, and an impact toughness of 30.6 J/cm 2 .