Microstructure and superplasticity of the as-cast Mg-9Al-1Zn magnesium alloy after high-ratio differential speed rolling

The direct application of high-ratio differential speed rolling (HRDSR) on the cast microstructure of Mg-9Al-1Zn (AZ91) alloy resulted in significant refinement of grain size and divorced eutectic β-phase with semi-network structure. Nanosized β-phase particles (50-100 nm) precipitated at grain boundaries and within grains from the Al-rich α-phase, which formed as a result of fragmentation and decomposition of the divorced eutectic β-phase during severe plastic deformation by HRDSR. The HRDSR-processed AZ91 alloy exhibited excellent superplasticity at low temperatures below 573 K. The optimum superplasticity (with a maximum tensile elongation of 830%) was achieved at 573 K at 1 × 10−3 s−1. Beyond 573 K, serious deterioration in superplasticity occurred due to a loss of thermal stability of grain size caused by the dissolution of the fine β-phase particles into the matrix. Models that can explain the high-temperature flow behavior of the ultrafine grained AZ91 alloy were proposed.