The mechanisms of the high-strain-rate superplastic deformation of Al-Mg-based alloy

•The superplastic deformation mechanisms are studied for a novel fast-forming Al alloy.•Strain-induced evolution of the strain rate sensitivity and grain structure are studied.•The step-by-step evolutions of the focused-ion-beam-gridded surfaces are studied.•Grain-boundary sliding contributes about 40% at high strain rate deformation.•High intragranular strain leads to dynamic recrystallization and helps with grain refining.