Superplasticity of fine-grained Mg-xGd alloys processed by multi-directional forging

Microstructural development and superplastic behavior of Mg-xGd (x = 1, 2, 3 wt%) alloys were investigated after the multi-directional forging (MDF) process. Microstructural observations of the extruded materials revealed non-homogeneous structures with bimodal grain sizes, consisting of coarse elongated grains resulting from deformation, surrounded by fine grains formed by partial dynamic recrystallization. The grain sizes of the recrystallized regions in the Mg-1Gd, Mg-2Gd and Mg-3Gd alloys were 5.6, 3.9 and 3.1 mu m, respectively. After six passes of the MDF process, the bimodal grain structure disappeared as a result of complete dynamic recrystallization and almost homogeneous microstructures with the grain sizes of 2.5, 2.2 and 1.9 mu m were obtained in the Mg-1Gd, Mg-2Gd and Mg-3Gd alloys, respectively. The smaller fraction of the recrystallized region in the extruded Mg-3Gd alloy and the smaller grain size in the MDF processed condition imply that more pronounced recrystallization retardation occurs through solute drag mechanism at the highest Gd content. Superplasticity was assessed by measuring the strain rate sensitivity index (m-value), through shear punch testing (SPT) in a temperature range of 623-698 K at various strain rates. After six passes of MDF, peak m-values of 0.45 and 0.46 were obtained at 648 K for Mg-1Gd and Mg-2Gd alloys, respectively, while for the Mg-3Gd alloy the peak m-value of 0.48 was achieved at a lower temperature of 623 K. According to the m-value of about 0.5 and the corresponding activation energies of 101-114 kJ/mol, the governing superplastic deformation mechanism was determined as grain boundary sliding (GBS) controlled by grain boundary diffusion.