Microstructural characterization and mechanical response of the extruded ZK61 magnesium alloy under dynamic and quasi-static loading

The microstructural characterization and mechanical behavior of the extruded ZK61 alloy were examined under dynamic and quasi-static loading at 623 K. The microstructure showed that an adiabatic shear band (ASB, ~10 μm in width) was formed under dynamic loading at 3.6 × 10³ s⁻¹ due to severe local plastic deformation and it was composed of ultra-fine grains (~0.46 μm). By comparison, the shear deformation of quasi-static loading was located in an area (~200 μm in width) with equiaxed grains (~4 μm). The texture analysis of the shear regions showed a stable orientation that all the (0002) basal plane was parallel to the shear direction (SD) and the orientation mainly aggregated near 〈10−13〉//SD, while a weak peak around 〈0001〉//SD appeared with a strain rate of 1.0 × 10⁻³ s⁻¹. The main recrystallization mechanism during quasi-static deformation was discontinuous dynamic recrystallization (DDRX) which nucleated at the grain boundaries by absorbing surrounding high-density dislocations. However, the formation of the DRXed grains within the ASB during dynamic deformation was attributed to rotational dynamic recrystallization (RDR) that the sub-grain boundaries tilted nearly 30° to generate new grains driven by the grain boundary energy. For mechanical responses, the stress-strain curve of dynamic loading showed high yield stress and long working-hardening stage, while the strain-hardening and thermal-softening in quasi-static curves reached a dynamic balance.