Effects of stress loading mode on microstructures and properties of Mg-9Gd-2Nd-0.5Zr alloy treated by creep aging

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.