Microstructure evolution and mechanical properties of AZ91D magnesium alloy processed by repetitive upsetting

Repetitive upsetting (RU), a newly developed SPD technique, was employed to process AZ91D magnesium alloy plates at 400 °C for 0, 2, 4 and 8 passes. The microstructure and texture evolution of the plates after RU were investigated and correlated with the mechanical performances at room temperature. Results show that the initial coarse grains are effectively refined, however an abnormal grain coarsening appears after 8 passes of RU. A compression texture and a shear texture, induced by specific local strains, are observed within the RU-processed plates. Both the ultimate tensile strength (UTS) and elongation to fracture of the RU-processed AZ91D alloy are improved with the increase of RU passes, exhibiting a maximum increase of 35.8% and over 75%, respectively. The yield strength (YS) increases steadily in the first 4 RU passes and then decreases slightly after another 4 passes. The mechanical anisotropy is discussed from the perspective of texture and Schmid factor.