Superior low-temperature superplasticity in fine-grained ZK60 Mg alloy sheet produced by a combination of repeated upsetting process and sheet extrusion

Fine-grained magnesium alloy sheets are potential candidates for superplastic forming applications. In the present study, the ZK60 Mg alloy sheet was obtained by a combination of repeated upsetting (RU) process, as a severe plastic deformation method, and subsequent forward sheet extrusion. Performing extrusion on the specimens processed by 1 and 5 passes of RU resulted in sheets with average grain sizes smaller than 10 μm, and a basal texture component. The sheet produced by 5 passes of RU and subsequent extrusion (denoted as A5RE) showed low-temperature superplasticity with the strain rate sensitivity index (m-value) of 0.57 and 0.62 at 523 K and 573 K, respectively. The microstructural characterization revealed that a fully recrystallized (VDRX = 96%) fine-grained microstructure containing a large volume fraction of high-angle grain boundaries (HAGBs) of 80.5%, together with the presence of thermally stable secondary phase particles, are the main reasons for achieving the superplasticity in the A5RE condition. The m-value greater than 0.5 and activation energy of 106 kJ/mol calculated for this condition indicate the grain boundary sliding (GBS), accommodated by grain boundary diffusion, as the dominant deformation mechanism during superplastic forming.