Achieving excellent superplasticity of Mg-7Zn-5Gd-0.6Zr alloy at low temperature regime

Mg-7Zn-5Gd-0.6Zr (wt%) alloy strengthened with quasicrystal phase (I-Mg₃Zn₆Gd phase) is prepared through hot extrusion and subsequent heat treatments. The low temperature (range from 25 °C to 250 °C) superplastic deformation behavior of the as-extruded, aging treated (T5) and solution and aging treated (T6) alloys are investigated. The results reveal that a superior superplastic elongation of 863% is obtained at 250 °C and strain rate of 1.67 × 10^-3 s^-1 and the elongation of this alloy increases with the increasing tensile temperature. Detailed microstructural analyses show that I-Mg₃Zn₆Gd phase and W-Mg₃Gd₂Zn₃ phase are crushed into small particles during extrusion. A high density of nanoscale I-phase precipitates after T5 treatment. Dynamic recrystallization occurs in as-extruded Mg-7Zn-5Gd-0.6Zr alloy. The T5-treated Mg-7Zn-5Gd-0.6Zr alloy shows a relatively weak basal texture intensity, a large number fraction of high angle boundaries and a very finer grain structure (3.01 μm). During superplastic deformation, the nanoscale I-phase is slightly elongated and the microstructure is still equiaxed grains. The superplastic mechanism of the alloy is grain boundary sliding (GBS) accommodated by dislocation movement and static recrystallization. The cavity nucleation at the nanoscale I-phase/α-Mg matrix boundaries or grain boundaries and the cavity stringer formation leads to final fracture.