Microstructural evolution and enhanced superplasticity in friction stir processed Mg-Zn-Y-Zr alloy

The extruded Mg-Zn-Y-Zr plate was subjected to friction stir processing (FSP). FSP resulted in significant breakup and dispersion of bulky W-phase particles and remarkable grain refinement, thereby substantially enhancing superplasticity. Maximum superplasticity of 635% was achieved at 450 °C and a relatively high strain rate of 3 × 10^-3 s^-1. By comparison, the as-extruded sample did not exhibit superplasticity. Grain boundary sliding was identified to be the primary deformation mechanism in the FSP Mg-Zn-Y-Zr by superplastic data analyses and surfacial morphology observations. Furthermore, the superplastic deformation kinetics of the FSP Mg-Zn-Y-Zr is significantly faster than that of equal channel angular pressed (ECAP) magnesium alloys under both as-ECAP and annealing conditions.