Tailoring bimodal grain structure to achieve simultaneous improvement of strength and ductility in magnesium alloys at cryogenic temperatures

Magnesium (Mg) alloys typically suffer from cold brittleness at cryogenic temperatures (CT), where strength significantly increases and ductility decreases with decreasing temperature. This study investigates the improvement of the strength-ductility balance at CT in Mg-3.6Y (wt.%) alloys with a bimodal grain structure, consisting of fine dynamically recrystallized (DRXed) grains and elongated unDRXed grains. The results demonstrate that the sample with ~50% DRXed region fraction achieves a remarkable strength-ductility synergy at CT. Dislocation strengthening in the unDRXed regions and grain boundary strengthening in the DRXed regions increase the tensile yield strength (TYS) by 1.6 times at CT compared to room temperature (RT). Concurrently, activation of {1012} tensile twinning and non-basal slip systems in DRXed regions, including prismatic 〈a〉 and pyramidal I 〈c + a〉 slips, along with abnormal pyramidal slip within unDRXed grains, reduces fracture elongation by only 1% relative to RT. Furthermore, the bimodal grain structure effectively alleviates strain localization through strain partitioning between DRXed and unDRXed grains, leading to the formation of interface-affected zones (IAZs) that promote the accumulation of geometrically necessary dislocations (GNDs) and enhance hetero-deformation-induced (HDI) hardening. At CT, the IAZs become wider and more pronounced, indicating enhanced GND accumulation that promotes stronger strain partitioning and more effective HDI strengthening. This work demonstrates that the bimodal grain structure is an effective approach to overcoming the low-temperature brittleness of Mg alloys, providing valuable insights for the design of high-performance materials for cryogenic applications.