Excellent superplasticity and deformation mechanism of Al–Mg–Sc–Zr alloy processed via simple free forging

A refined microstructure of Al–Mg–Sc–Zr alloy with an average grain size of ~3.7 μm and a portion of high angle boundaries of 69.2% was produced by free forging. Excellent superplastic ductility of ≥500% was achieved at a wide temperature range of 450~500 °C and relatively high strain rate range of 1×10−3~5×10−2 s−1 in the Al–Mg–Sc–Zr alloy. A maximum elongation of 1593% was obtained at 475 °C and 1×10−3 s−1. Moreover, the electron back scattered diffraction (EBSD) and the transmission electron microscopy (TEM) analyses showed that the excellent superplasticity can be attributed to the high fraction of high angle grain boundaries and the presence of Al3(Sc,Zr) dispersoids in the Al–Mg–Sc–Zr alloy microstructure. The analyses on the superplastic data revealed the presence of threshold stress, the coefficient of strain rate sensitivity of 0.5, and an activation energy of 83.9 kJ/mol–1. It indicated that the dominant deformation mechanism was grain boundary sliding. Based on this notion, a constitutive equation for Al–Mg–Sc–Zr alloy has been developed.