Microstructural evolution and superplasticity in an Mg-Gd-Y-Zr alloy after processing by different SPD techniques

Mg-Gd-Y-Zr alloys have attracted much attention recently due to their ability to exhibit stable fine-grained microstructures and their potential superplastic behavior. However, the microstructure and superplasticity of these alloys processed by severe plastic deformation (SPD) methods remain less understood. In this work, the microstructure and superplastic behavior of an Mg-5Gd-4Y-0.4Zr (GW54) alloy were investigated after processing using extrusion and the SPD processes of equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). Microstructural characterization by transmission electron microscopy and electron backscattered diffraction showed that nano-sized grains of ~72±5 nm were obtained after 8 HPT turns whereas the grain sizes were about ~4.6±0.2 and ~2.2±0.2 µm after extrusion and 4 ECAP passes, respectively. Shear punch tests revealed that the optimum temperature for superplasticity is 623 K for the HPT samples and 723 K for the ECAP and extrusion samples, at which the strain rate sensitivities were measured as about 0.42±0.05, 0.46±0.05 and 0.50±0.05 for the extrusion, ECAP and HPT samples, respectively, and the corresponding activation energies were about 117, 101 and 110 kJ/mol for these three processing conditions. These results suggest that grain boundary sliding controlled by grain boundary diffusion is the dominant mechanism of deformation at the optimum temperatures for superplastic flow.