Superplastic behavior and microstructure evolution of a new Al-Mg-Sc-Zr alloy subjected to a simple thermomechanical processing

A new Al-Mg-0.15% Sc-0.10% Zr (wt%) alloy sheet with an average (sub)grain size of ~2.25 µm was processed by a simple thermomechanical processing. Excellent superplastic (elongations of ≥800%) can be achieved at a temperature range of 450-500 °C and a high strain rate range of 1×10−2-1×10−1 s−1. A maximum elongation of ~1579% was obtained at 475 °C and a high strain rate of 5×10−2 s−1. Electron back scatter diffraction analysis and transmission electron microscopy results showed that superior superplastic ductility of the Al-Mg-Sc-Zr alloy can be ascribed to the complete transformation of low angle grain boundaries to high angle grain boundaries due to the occurrence of continuous dynamic recrystallization and the presence of stable coherent Al3ScxZr1−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 parameter and the average activation energy of the Al-Mg-Sc-Zr alloy were ~0.48 and ~84.4 kJ/mol-1, respectively. All results indicated that the main superplastic deformation mechanism was grain boundary sliding.