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

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 degrees C and the strain rates ranging from 1 x 10 (3) to 1 x 10 (2) s (1) by low scandium additions in the presence of 0.10% Sc (wt.%). An elongation of 1050% is obtained at 500 degrees C and 5 x 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 mu m grains characterized by a high fraction of low angle grain boundaries and strong beta-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, beta-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. (C) 2015 Elsevier B.V. All rights reserved.