Ultrafine-grained Mg-Zn-Y-Zr alloy with remarkable improvement in superplasticity

•Ultrafine-grained structures fabricated by severe plastic deformation was proposed.•Refining of recrystallized grains and uniform diespersed precipitates were obtained.•The superplastic enlogation of 642% was achieved at 450 °C.•Friction stir processing is feasible to construct superplasticity Mg-Zn-Y-Zr alloys. Ultrafine-grained structure in Mg-Zn-Y-Zr alloy was achieved towards extraordinary superplasticity induced by friction stir processing. Grain refinement and homogeneous dispersion of precipitates were promoted by the coupled thermo-mechanical effect. The ultra-fined grains size of 1.9 ± 0.4 μm and the superplastic elongation of 642% were obtained. Grain boundary sliding was considered as the predominant superplastic deformation mechanism in ultrafine-grained materials.