Effect of a Large Height-to-Diameter Ratio Upsetting-Extrusion Process on the Microstructure and Mechanical Properties of Mg-Gd-Y-Zn-Zr Alloys

In this study, Mg-Gd-Y-Zn-Zr alloy bars with the height-to-diameter ratio of 6.5 were deformed using a novel large height-to-diameter ratio upsetting-extrusion process. After the initial billets were upset at 460 degrees C, they were extruded at 460-420 degrees C, respectively, and the true strain of one-pass upsetting-extrusion deformation reached 2.46. The results showed that the 420 degrees C extruded samples had the highest recrystallization fraction of 75.2%. Continuous dynamic recrystallization and discontinuous dynamic recrystallization occurred during upsetting and extrusion, and the lower extrusion temperature favored the precipitation of large particles of Mg5(RE) phase and refined the grains through the particle-stimulated nucleation mechanism, and the average grain size decreased from 93.8 mu m to a minimum of 15.3 mu m. The 440 degrees C extruded samples have the best mechanical properties with yield strength, ultimate tensile strength, and elongation of 260 MPa, 368 MPa, and 13.1%, respectively. Texture strengthening and dislocation strengthening dominate, and the appropriate proportion of coarse grains accommodating deformation has enabled the high elongation. In addition, the anomalous texture of the basal plane perpendicular to the extrusion direction was found in the high recrystallization fraction samples and attributed to the selective growth of recrystallized grains.