Achieving high superplasticity of a traditional thermal–mechanical processed non-superplastic Al–Zn–Mg alloy sheet by low Sc additions

• The superplastic of the Al–Zn–Mg–Sc–Zr alloy subjected to a traditional thermal–mechanical processing was investigated.
• The boundary characteristics and thermal stability of the Al–Zn–Mg–Sc–Zr alloy were examined.
• The deformation mechanism for Al–Zn–Mg–Sc–Zr alloy was analyzed.

The non-superplastic Al–Zn–Mg alloy sheet produced by a simple traditional thermal–mechanical processing can achieve high superplasticity at the temperatures ranging from 450 to 500 °C and the strain rates ranging from 1 × 10−3 to 1 × 10−2 s−1 by low scandium additions in the presence of 0.10% Sc (wt.%). An elongation of 1050% is obtained at 500 °C and 5 × 10−3 s−1. Analyses on the superplastic data reveal that the average values of the strain rate sensitivity and the activation energy of the Al–Zn–Mg–Sc–Zr alloy are about 0.5 and 85 kJ/mol−1, respectively. The microstructural results show that the studied alloy consists of 3.14 μm grains characterized by a high fraction of low angle grain boundaries and strong β-fiber rolling textures. During superplastic deformation, low angle grain boundaries gradually transfer into high angle grain boundaries to sustain grain boundary sliding, and the texture intensity diminishes. Besides, β-fiber rolling textures weaken and cube and random textures are dominant in the superplastic deformed alloy. Superior superplastic ductility of the Al–Zn–Mg–Sc–Zr alloy is ascribed to the coherent 10–20 nm Al3ScxZr1−x particles that strongly retard recrystallization grain growth. Analyses of the superplastic data indicate that grain boundary sliding is the predominant deformation mechanism.