Influence of strain rate on grain refinement and texture evolution under complex shear stress conditions

The effects of different complex shear stress conditions on grain refinement and texture evolution of Mg-13Gd-4Y-2Zn-0.5Zr alloy were investigated. With increasing strain rate, the average grain size of compression-shear (CS) and compression-torsion (CT) samples are decreased, and the grain size of dynamic recrystallization (DRX) grains is also decreased. This is because that the precipitation number of β phases is increased, and the hindering effect on grain growth can be significantly enhanced. The DRX fractions of CS and CT samples are decreased with increased strain rate. The low DRX fraction at high strain rate is related to the insufficient time for grains to nucleate. The DRX process can be promoted by the PSN mechanism of second phases, and the grain growth can be restricted by the pinning effect. At the same time, the texture strength is enhanced as the strain rate increased. Besides, the kinking degree of lamellar long-period stacking ordered (LPSO) phases is increased. Under complex shear stress conditions, non-basal slip, especially pyramidal slip, is easily activated and the texture is deflected greatly. Compared with the CS samples, CT samples have smaller average grain size, higher DRX fraction, and lower texture strength for a certain strain rate. This is because that the equivalent stress of the CT sample is larger, the stress triaxiality is smaller, so more serious dislocations are piled up near grain boundaries and second phases. At the same time, since CT sample was sheared with torsion, the dislocation movement path can be called “rotational dislocation accumulation”, and the longer distribution path of the CT sample is generated, so more sub-grains and low-angle grain boundaries (LAGBs) are formed. Compared with the CS sample, more huge-angle grain boundaries (HAGBs) and DRX grains are formed from grain boundary to grain interior, so better grain refinement effect is achieved.