Superplastic behavior and microstructure evolution of a fine-grained Mg-Nd-Zn-Zr alloy processed by hot extrusion

In this study, the starting materials of the fine-grained Mg-Nd-Zn-Zr alloy with a grain size of ∼2.78 μm were prepared by hot extrusion with an extrusion ratio of 19.63 at 400 °C. The superplastic behavior and microstructure evolution of the fine-grained extruded Mg-Nd-Zn-Zr alloy was studied in the temperature ranges of 350 °C–500 °C and the strain rate ranged from 1 × 10−2 s−1 to 5 × 10−4 s−1. Results showed that the maximum elongation of 1015.50% was obtained at 450 °C with 5 × 10−4 s−1, and high strain rate superplasticity (HSRS) was achieved at 400 °C and 450 °C with 1 × 10−2 s−1. Moreover, theoretical calculations revealed that deformation mechanisms were grain boundary slip (DBS), m = 0.42–0.52 at 350 °C–450 °C, and solute drag creep (SDC), m = 0.37 at 500 °C. Meanwhile, there were regulated by grain boundary diffusion or lattice diffusion. In the last stage of deformation, due to grain growth and significant strain, plenty of geometrically necessary dislocations (GNDs) and {10–12} twins were generated in the grain, resulting in stress concentration. Because the stress concentration could not be eliminated in time, cavities were formed at the second phase particles and trigeminal grain boundaries. Various types of fracture, such as ductile fracture, microvoid accumulation fracture, and so on, resulted from the growth and connection of cavities.