Development of grain structure during superplastic deformation of an Al–Zn–Mg–Cu–Zr alloy containing Sc

Recent studies have shown that a suitably thermomechanically processed Al–Zn–Mg–Cu–Zr–Sc alloy having an essentially unrecrystallized grain structure may be subjected to superplastic deformation at a temperature and strain rate combination of 475 °C, 1.9 × 10−2 s−1 to obtain a total elongation of 650%. The present work demonstrates that the utilization of two-step strain rate at 425 °C during superplastic deformation of a similarly thermomechanically processed alloy increases the total elongation value to as high as 916%. The beneficial effect of two-step strain rate could further be realized at 350 °C to obtain a total elongation of 438%. Using electron backscattered diffraction, the present work provides a systematic assessment of the changes in (i) percentage recrystallization and (ii) recrystallized grain size with percentage elongation in the material superplastically deformed using both one- and two-step strain rates.