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.

AZ31 Mg alloy plates with bimodal grain structures were fabricated via conventional extrusion under varying temperatures and speeds to investigate the mechanisms governing dynamic recrystallization (DRX) and texture evolution. Although all samples exhibited similar DRXed grain sizes (5.0–6.5 µm) and fractions (76%–80%), they developed distinct c-axis orientations and mechanical properties. The P1 sample (350 °C, 0.1 mm/min) exhibited the lowest yield strength (~192 MPa) but the highest elongation (~18.2%), whereas the P3 sample (400 °C, 0.6 mm/min) showed the highest yield strength (~241 MPa) and the lowest elongation (~14.2%). The P2 sample (400 °C, 0.1 mm/min) demonstrated intermediate behavior (~226 MPa, ~17.7%). These variations were primarily attributed to differences in c-axis orientations, particularly their alignment with respect to the normal direction (ND) and their slight deviation from the extrusion direction (ED). Microstructural analysis revealed that distinct DRX mechanisms were activated under different extrusion conditions. P1 predominantly exhibited twinning-induced dynamic recrystallization (TDRX) and continuous dynamic recrystallization (CDRX), whereas P3 primarily showed CDRX and discontinuous dynamic recrystallization (DDRX). These DRX mechanisms, in combination with the activated slip systems governed by the evolving local stress state, collectively contributed to orientation rotation and texture development. During the early stage of extrusion, tensile strain along the ED promoted basal <a> slip, rotating the c-axes toward the ND. As deformation progressed, compressive strain along the ND became dominant. In P1, basal <a> slip remained active, aligning the c-axes along the ND and forming a smaller angle with the ED. In contrast, P3 exhibited predominant pyramidal <c + a> slip, resulting in a pronounced deviation of the c-axes from the ND and a slightly larger angle relative to the ED. The P2 sample exhibited a transitional texture state between those of P1 and P3.

This study elucidates the non-thermal mechanism of dislocation density reduction in a Mg-Y-Nd-Gd-Zr alloy under continuous electropulsing (6.67–15 A/mm²) at ultra-low temperatures (−150 °C to −196 °C) through tripartite characterization and first-principles analysis. Electron backscatter diffraction (EBSD) reveals a 15.2 % decrease in geometrically necessary dislocation (GND) density with increasing current, while X-ray line profile analysis (XLPA) confirms the inverse correlation between current intensity and overall defect density. Transmission electron microscopy (TEM) directly visualizes the dissolution of entangled dislocation clusters into isolated lines under high-current treatment (15 A/mm²), corroborating the statistical trends. First-principles calculations demonstrate that localized charge accumulation at defect sites reduces Mg vacancy formation energy by up to 2.8 %, lowering lattice resistance to dislocation glide. This charge-state-dependent vacancy proliferation provides a mechanistic link between electron flow and dislocation annihilation. The reduction of vacancy formation energy is a significant factor in the electron-induced dislocation evolution effect at ultra-low temperatures. These findings provide direct evidence for electron-induced dislocation annihilation mechanisms independent of Joule heating, advancing the understanding of electroplasticity in hexagonal close-packed alloys, and providing a novel approach for rapid, non-oxidative microstructural and property tuning of magnesium alloys.

Magnesium, being the lightest structural metal, faces limitations in alloy development due to its inherently low elastic modulus. Therefore, this study develops high-performance, high-modulus Mg-15Gd-8Y-xAl-0.3Mn (wt.%) (x = 6, 8, 10) alloys and investigates their microstructure and mechanical properties. The findings indicate that the alloys primarily consist of Al₂RE and α-Mg phases, with both the amount and size of Al₂RE phase increasing as the Al content rises. After extrusion, both the grains and the Al₂RE phase are refined. The increased modulus of the alloys is mainly due to the formation of the high-modulus Al₂RE phase. When the Al content is 6%, 8%, and 10%, the modulus of the alloys is 51.8 GPa, 53.8 GPa, and 56.1 GPa, respectively. Additionally, the Al₂RE and Mg₅RE phases can jointly regulate the microstructure and mechanical properties of the alloys. As the Al content increases, the amount of Al₂RE phase increases, consuming the rare earth elements in the alloy and reducing the nano-precipitated Mg₅RE phase. Consequently, with the increase in Al content, the recrystallization rate increases, and the recrystallized grains become larger. When the Al content is 6%, the alloy exhibits a bimodal structure with the smallest recrystallized grains, resulting in the highest yield strength of 341 MPa. When the Al content is 8%, the alloy has a fine, fully recrystallized structure, leading to a relatively high elongation of 9.1%. These findings provide valuable insights for designing high-modulus magnesium alloys with optimized yield strength and elongation for structural applications.

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.

High thermal conductivity and high strength Mg-1.5Mn-2.5Ce alloy with a tensile yield strength of 387.0 MPa, ultimate tensile strength of 395.8 MPa, and thermal conductivity of 142.1 W/(m·K) was successfully fabricated via hot extrusion. The effects of La and Ce additions on the microstructure, thermal conductivity, and mechanical properties of the Mg-1.5Mn alloy were investigated. The results indicated that both the as-extruded Mg-1.5Mn-2.5La and Mg-1.5Mn-2.5Ce alloys exhibited a bimodal grain structure, with dynamically precipitated nano-scale α-Mn phases. In comparison with La, the addition of Ce enhanced the dynamic precipitation more effectively during hot extrusion, while its influence on promoting the dynamic recrystallization was relatively weaker. The high tensile strength obtained in the as-extruded Mg-1.5Mn-2.5RE alloys can be attributed to the combined influence of the bimodal grain structure (with fine dynamic recrystallized (DRXed) grain size and high proportion of un-dynamic recrystallized (unDRXed) grains), dense nano-scale precipitates, and broken Mg₁₂RE phases, while the remarkable thermal conductivity was due to the precipitation of Mn-rich phases from the Mg matrix.

The addition of effective nucleating particles in the melt to achieve grain refinement has become the most widely used method for the casting industries. In this study, a novel GNP@MgO particle with a nanocomposite structure was prepared by utilizing an in-situ reaction of the carbon source gas with Mg melt. The results showed that the particles can significantly reduce the average grain size of Mg-9Al alloy from 130.4 µm to 13.1 µm, and achieve an ultra-high grain refinement efficiency of 90%. The refinement mechanisms are that the Al₄C₃ phase can act as a heterogeneous nucleation site for α-Mg grains due to the orientation relationship as (001)_Al4C3//(002)_Mg. Meanwhile, the particle distribution model shows that the velocity of MgO particles is much higher than the growth rate of α-Mg grains. Therefore, it is pushed to the vicinity of grain boundaries during solidification, effectively limiting the growth of α-Mg grains. The remarkable grain refinement effect was achieved through the synergistic modulation of Al₄C₃ and MgO particles. This work may provide new insight into designing high efficiency grain refiners for Mg-Al alloys.