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At the initial strain rate of 10-2 s-1,superior superplastic ductility of> 800%can be obtained at a wide temperature range(475~525 ℃)by adding 0.25 Sc and 0.10 Zr into a simple thermo-mechanical processed non-superplastic Al-5.40Zn-1.98Mg alloy(wt.%).Furthermore,the new Al-Zn-Mg-Se-Zr alloy has a maximum superplasticity of 1520%and can successfully achieve hemi-ellipsoidal bulge forming at 500 ℃and 10-2s-1,which makes the microalloyed alloy commercially attractive for fabricating complex large-volume high-strength aerospace components.The microstructure,texture and deformation flow behaviour of Al-Zn-Mg and Al-Zn-Mg-Sc-Zr alloys were investigated,comparatively.Compared with the coarse and recrystallized grains of Al-Zn-Mg alloy,the Al-Zn-Mg-Sc-Zr alloy consists of ultrafine sub-grains in the size of about 0.7 μm and strong β-fiber rolling texture.During deformation,the grains of Al-Zn-Mg alloy further grow up,whereas for the Sc-enriched alloy,the low angle grain boundaries gradually transfer into high angle grain boundaries and the grains gradually orientate randomly.At the true strains increase from 0.25 to 0.69,strain rate sensitivity factor decreases from 0.29 ± 0.03 to 0.19 ± 0.03,and deformation activation energy increases from 112 kJ/mol to 121 kJ/mol in Sc-free alloy.However,for the Sc-enriched alloy,when the true strains increase from 0.25 to 1.10,strain rate strain rate sensitivity gradually increases from 0.33 ± 0.03 to 0.45 ± 0.05,and deformation activation energy decreases from 107 kJ/mol to 84 kJ/mol.The established deformation constitutive equations and microstructural results reveal that dislocation viscous glide/climb creep is dominant in the hot deformation of Al-Zn-Mg alloy.However,for Al-Zn-Mg-Sc-Zr alloy,dislocation viscous glide creep mechanism only manifests at the working hardening stage,and grain boundary sliding mechanism plays a prominent role at dynamic softening stage.Sc and Zr microalloying mechanism for successfully achieving excellent forming can be mainly ascribed to the effective Zener drag force from coherent Al3ScxZr1-x nano-particles with L12 structure,which can effectively refine initial grains,inhibit grain growth and homogenize grain structure during hot deformation.