In this work, AZ31B extruded sheets with mixed-grain microstructures were prepared through extrusion. Samples of mixed-grain microstructure with different morphologies were selected from the AZ31B extruded sheets (referred to as M1 and M2 samples, respectively). The creep tests were performed on these samples at the temperature range of 150–200 ℃, and the stress level range of 50–100 MPa. The creep properties and fracture behavior of AZ31 extruded sheets with mixed-grain microstructures were studied. Results showed that the creep properties of the M2 sample always outperformed that of the M1 sample and M1 and M2 samples’ creep was dominated by dislocation movement. The creep rate of M2 samples (1.5 × 10^-7 ± 1.1 × 10^-10 s^-1) is an order of magnitude lower than that of M1 samples (4.8 × 10^-6 ± 8.1 × 10^-10 s^-1) at 200 ℃ under 50 MPa The high activity of basal slip and softening mechanism in the M1 sample significantly accelerated creep, resulting in a relatively high creep rate. Moreover, the stress concentration within the M1 sample caused by deformation incompatibility, increased the initiation and propagation of voids, ultimately leading to fracture and poorer creep performance. However, the numerous <10 µm fine grains surrounding deformed coarse grains in the M2 sample facilitated better coordination of deformation through dislocation slip, effectively slowing down the initiation of voids during the creep process. Meanwhile, the strain was uniformly distributed within each grain, mitigating stress concentration, inhibiting voids propagation, and contributing to the superior creep resistance of the M2 sample.

Creep aging forming (CAF) is a potential process used to manufacture large integral components of magnesium (Mg) alloys. The selected stress plays a crucial role in creep aging processes but the mechanism by which stress loading method affects creep aging of Mg alloys is still unclear. In this paper, the microstructural evolution of precipitated phases and precipitation-free zones (PFZ) at grain boundaries with different stress loading modes (unstressed, unidirectional tensile stress, and cyclic stress) at 250 ℃ were investigated along with changes in mechanical properties. The results showed that the addition of stress during aging effectively promoted the precipitation of precipitated phases, while unaffecting grain size. Unidirectional tensile stress caused directional growth of β' phase ([1010]), as well as rotation of weave towards the basal plane texture, resulting in namely stress orientation effect. Solute atoms diffused in the direction of tensile stress while vacancies moved perpendicular to the direction of tensile stress, resulting in PFZ at grain boundaries (157.06 nm). By contrast, cyclic stresses led to the growth of β' phase in three directions ([1010], [1100] and [0110]). The solute atoms and vacancies were uniformly distributed in the Mg matrix instead of directional diffusion, effectively reducing the width of PFZ (112.39 nm) at the grain boundary. These features significantly improved the mechanical properties of alloy specimens after cyclic stress creep aging when compared to unidirectional stress creep aging, with yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) enhanced from 171.6 MPa, 305.5 MPa, and 4.4% to 174.8 MPa, 326.3 MPa, and 6.9%, respectively.