Superplasticity behaviors of Al-Zn-Mg-Zr cold-rolled alloy sheet with minor Sc addition

A refined microstructure of Al-Zn-Mg-Sc-Zr alloy sheet was produced by simple hot and cold rolling to an average grain size of 3 µm. Experiments were completed in electro-fluid servo-fatigue tester and results were investigated by means of optical microscope (OM), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Superplastic deformation was conducted and superplastic ductility of ≥200% was achieved at a testing temperature range from 425 ºC to 500 ºC and relative high strain rate range of 1×10−3 s−1~1×10−1 s−1. The maximum elongation of 539% was obtained at 500 ºC and 1×10−2 s−1. In addition, the scanning electron microscopy (SEM) and transmission electron microscope (TEM) analyses showed that the presence of Al3 (Sc, Zr) particles in pinning grain boundaries and dislocations had a great influence on the superplastic deformation. The analyses of superplastic test data calculated out the coherent strain rates sensitivity parameter of 0.43 and the average activation energy of 143.762 kJ/mol. The data interpreted that the dominant deformation mechanism was grain boundary sliding controlled by lattice self-diffusion.