Influence from Extrusion Parameters on High Strain Rate and Low Temperature Superplasticity of AZ Series Mg-Based Alloys

The most simple and feasible one-step hot-extrusion method with high extrusion ratio from 40 to 166 was applied to process fine-grained AZ series Mg alloys into bar, sheet and tube products. The stable grain size d refined by dynamic crystallization (DRX) during hot-extrusion process can be described quantitatively by the Zener-Holloman parameter (Z-parameter). For AZ31, the relationship between d and Z is represented by lnd=27-0.95lnZ.  The yield stress (YS) and ultimate tensile stress (UTS) at room temperature and the superplastic elongation at elevated temperature in AZ series alloys basically show an increasing trend with decreasing grain size. For example, the superior HSRSP up to ∼1000% at 10^-2 s^-1, as well as LTSP up to ∼600% at 200-300°C, can be achieved in the AZ31 bars (∼2 μm) or AZ91 sheets (∼2.5 μm). Fine-grained microstructures and high fractions (∼88%) of high angle boundary (HAB, >15°) are mainly responsible for the superior HSRSP or LTSP.