Creep behavior and dynamic precipitation of highly heat-resistant Mg alloy with low rare-earth content

Achieving high creep resistance in low rare-earth (RE) containing Mg alloys is quite difficult due to the limited precipitation density. Here we reported a low-RE-alloyed Mg alloy (RE = 4 wt %) with good creep resistance, which is superior to most heat-resistant Mg benchmark alloys. Our creep-resistant alloy is characterized by nano-spaced stacking faults (SFs) in grain interiors and thermally stable Mg ₂RE phases at grain boundaries. Both intragranular SFs and grain-boundary Mg ₂ RE phase remain the unharmed structures, with no obvious decomposition or coarsening after the creep at 200 and 220 ºC for 100 h. During creep, a large amount of dynamic precipitation such as discrete β particles and basal γ’’ phases was introduced, which plays a crucial role in resisting the creep deformation within grains. As a result, nano-spaced SFs, thermally stable Mg ₂ RE skeletons at grain boundaries, and dynamic precipitation of β and γ’’ phases are responsible for the enhanced creep resistance of this alloy. After creep, the alloy exhibits abnormally high stress exponents ( n = 9.2 at 200ºC and n = 9.3 at 200ºC) and stress-dependent activation energies of 105–153 kJ/mol at 90–120 MPa. High stress exponents can be normalized to be 3–4 using the threshold stress approach or Garofalo`s sinh relation, suggesting that dislocation climb acts as the operative creep mechanism. Finally, we proposed a strategy to develop highly creep-resistant low-RE-alloyed Mg alloys via constructing thermally stable microstructures at grain boundaries and interiors.