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.

To obtain the Ti_p with different aspect ratios, the Ti_p/Mg-5Zn-0.3Ca composite prepared by semi-solid stir casting was subjected to extrusion at 220℃, 180℃, and 140℃, respectively. Then, the effect of the Ti_p’s aspect ratio on the microstructure, mechanical properties, work hardening and softening behaviors of Ti_p/Mg-5Zn-0.3Ca composites was investigated. The results indicated that the Ti_p could be elongated obviously after low-temperature extrusion, and the aspect ratio of which would reach to 13.7:1 as the extrusion temperature deceased to 140℃. Then the “Ti/Mg” layer-like structure was formed in the Ti_p/Mg-5Zn-0.3Ca composite. Accompanied with the elongation of Ti_p, the dynamic recrystallized grains and dynamic precipitates were both refined significantly, however, the dynamic recrystallization rate changed a little. The elongated Ti_p endowed the Ti_p/Mg-5Zn-0.3Ca composites with better matching of strength and toughness without the sacrifice of elongation and bending strain. Both the work hardening rate and softening rate of Ti_p/Mg-5Zn-0.3Ca composites increased with the increasing aspect ratio of Ti_p. The formation of “Ti/Mg” layer-like structure contributed to the redistribution of strain from large aggregations to a network-like distribution, which effectively suppresses the initiation and propagation of micro-cracks, thus enhancing the plasticity of the Ti_p/Mg-5Zn-0.3Ca composites.

The role of Ca content (0.5, 1.0, 2.0 wt.%) on microstructure, mechanical properties and strain evolution of as-rolled Mg-Al-Ca-Zn-Mn alloy was thoroughly investigated in this work. The results indicate that the primary second phase transformed from the Mg₁₇Al₁₂ phase to the Al₂Ca phase after homogenization, and the amount of Al₂Ca phase increased significantly with increasing Ca content. After hot rolling, the alloys exhibited the typical bimodal microstructure composed of fine dynamic recrystallized (DRXed) grains and coarse elongated un-DRXed grains, the area fraction of the DRXed regions increased with increasing Ca content. Besides, a large number of submicron-sized as well as nano-scaled spherical Mg₁₇Al₁₂ phases dynamically precipitated along the DRXed grain boundaries in all alloys, which promoted the DRX and restricted the grain growth. During rolling deformation, DRX preferentially occurred near the primary second phases and shear bands by the particle stimulated nucleation (PSN) and shear band induced nucleation (SBIN) mechanism in the alloys. The ultimate tensile strength (UTS), yield strength (YS), and elongation to failure (EF) along the rolling direction (RD) of the Mg-8.0Al-1.0Ca-1.0Zn-0.4Mn (wt.%) sheet were 393 MPa, 334 MPa and 8.7%, respectively. Such high strength was mainly attributed to fine DRXed grains, high number density of dynamically precipitated Mg₁₇Al₁₂ phases and strongly textured un-DRXed grains with numerous sub-structures. The reasonable DRX ratio moderated strain localization and thus stabilized tensile deformation, leading to moderate plasticity of the alloy.