Enhanced creep resistance in a heterostructured magnesium alloy through a bimodal grain microstructure

This study investigates the mechanistic effect of grain structure on high-temperature creep behavior of an extruded Mg-2.8Al-1.3Ca-1.1Sm (wt%) alloy. Two distinct microstructures were generated by extrusion under different conditions, a uniform fine-grained microstructure (FGs) and a heterogeneous bimodal microstructure (HGs) composed of fine dynamically recrystallized (DRXed) grains and coarse un-recrystallized (unDRXed) grains. Despite nearly identical room-temperature tensile properties, the HGs alloy showed a 58.8–73.5% reduction in steady-state creep rate in the load range of 40–80 MPa and a 50% longer rupture life at 80 MPa compared with the FGs alloy during creep tests at 150 °C. In the FGs sample, grain boundary sliding occurs at the early stage of the creep test, leading to crack initiation and early fracture. However, in the HGs sample, the coarse unDRXed grains act as a relatively stable load-bearing skeleton, preventing lattice rotation and strain accumulation at grain boundaries. The Al₂Ca precipitates effectively impede basal 〈a〉 dislocation motion and delay the activation of non-basal slip systems, leading to a more uniform distribution of dislocations. The delayer strain localization throughout the creep test, and the resultant improved creep resistance, is caused by suppression of the intra- and intergranular deformation, due to bimodal microstructure with Al₂Ca precipitates.