Effect of Sc/Zr ratio on the microstructure and mechanical properties of new type of Al-Zn-Mg-Sc-Zr alloys

The rare earth scandium (Sc) as a microalloying element has attracted an increasing interest in aluminum alloys for achieving excellent mechanical properties. Combining with zirconium, high strength and low price Al-Sc alloys are expected. The effects of Sc and Zr on the grain refinement, recrystallization resistance and precipitation hardening were investigated in new type of Al-Zn-Mg-Sc-Zr alloys by rolling, annealing and aging processes. Scandium addition into the Al-Zn-Mg alloys can efficiently refine the grain size and increase recrystallization resistance, especially together with zirconium addition. The maximum value of the yield-to-tensile strength (627 MPa/667 MPa) was obtained with 0.2Sc/0.4Zr ratio of the alloy after solution-aging treatment. The additional strengthening of the alloys is attributed to the grain refinement and the precipitation-strengthening effect of Al3Sc, Al3Zr or Al3(Sc, Zr) in the proper ratio of Sc/Zr during aging.