Effect of Sc and Zr additions on grain stability and superplasticity of the simple thermal–mechanical processed Al–Zn–Mg alloy sheet

Effect of scandium and zirconium on grain stability and superplastic ductility in the simple thermal–mechanical processed Al–Zn–Mg alloys was investigated. Tensile testing revealed that the Al–Zn–Mg alloy without Sc and Zr additions showed no superplasticity because of the larger grain size (>10 μm) and the poor stability of the microcrystalline structure during superplastic deformation. However, the Al–Zn–Mg–0.25Sc–0.10Zr (wt%) alloy exhibited excellent superplastic (elongations of ≥500%) at a wide temperature range of 450–550 °C and high strain rate range of 5×10⁻³–5×10⁻² s⁻¹, and the maximum elongation of ~1523% was achieved at 500 °C and 1×10⁻² s⁻¹. Electron back scatter diffraction analysis and transmission electron microscopy results showed that superior superplastic ductility of the Al–Zn–Mg–0.25Sc–0.10Zr alloy can be ascribed to the complete transformation of low angle grain boundaries to high angle grain boundaries due to the occurrence of dynamic recrystallization and the presence of coherent Al₃Sc_xZr_1−x particles that effectively impede the growth of the grains during superplastic deformation. Besides, strong β-fiber rolling textures gradually weakened, and random textures were predominant in the superplastic deformed alloy. Analyses on the superplastic data revealed that the average strain rate sensitivity and the average activation energy of the Al–Zn–Mg–0.25Sc–0.10Zr alloy were ~0.37 and ~84.5 kJ/mol, respectively. All results indicated that the main superplastic deformation mechanism was grain boundary sliding.