Mg-Li alloys with high lithium concentrations possess a lightweight body-centered cubic (BCC) matrix structure (β-Li). Interspersed eutectics (primarily the reticulated I-phase) often form along phase boundaries (PBs) and grain boundaries (GBs) which strengthen the alloy but cause the loss of ductility due to the brittle behavior of I-phase. By modifying the Li content, we fabricated the (β+α) biphase Mg-Li alloy in which the α-Mg phase with a hexagonal close-packed structure (HCP) is embedded in β-Li matrix, significantly increasing interface density. The high-density interfaces mitigate the distribution and dimension of the I-phase along GBs and PBs. The alloy exhibits enhanced ductility (elongation (EL) = 17.8 %) compared with the alloy without the α-Mg phase (EL = 5.1 %). Structural characterizations unveil the strengthening mechanism of the nanoscale B2 (Li, Mg)₃Zn-type precipitates in conjunction with the microscale I-phase. The (Li, Mg)₃Zn nanophases augment the yield and ultimate tensile strength of the alloy without a discernible compromise in ductility, predominantly due to gliding dislocations cutting through the precipitates. In contrast, the microscale I-phase presents a formidable barrier to dislocation motion, facilitating dislocation pileups at interfaces and culminating in diminished ductility across the interface. In-situ stretching techniques were employed to scrutinize the microstructural evolution of alloys during tensile deformation, elucidating that the deformation compatibility of alloys correlates with the average size of the I-phase and their distribution along GBs and PBs. Corresponding to the orientation relationship (OR) between the α-Mg and β-Li phases {110}Li//{0001}Mg and <111>Li //<1120>Mg, the slip continuity between α-Mg and β-Li on plane pairs of {123}Li-{1122}Mg and {112}Li-{1122}Mg assures the deformation compatibility through facilitating the deformation across interfaces. Simultaneously, during the stretching process, the dispersed I-phase instigates the emergence of sporadic microcracks, indicating gradual damage evolution. These discoveries offer novel insights into achieving exceptional strength-ductility amalgamations in Mg-Li alloys through microstructural adjustments.

Currently, the hierarchical structure is one of the most effective means to enhance the strength and plasticity of metal materials, since the strain localization can be effectively delayed by the coordination of the unique microstructure. In this study, a hierarchical structure of Mg-15Gd-1Zn-0.4Zr (GZ151K) alloys containing grain, twin, and precipitation structural units was prepared by ultrasonic surface rolling process (USRP) and recrystallization annealing (RU). The results showed that the stress gradient generated by USRP formed a twin gradient structure, which will activate the twin-assisted precipitation (TAP) effect and twin-induced recrystallization (TIR) effect during RU. Then, the twin gradient structure transformed into a twin-precipitation gradient structure, and finally into a hierarchical structure with grain-twin-precipitation as the increasement of recrystallization degree. Besides, the dual gradient structure with twin and precipitation structural units had the highest strength and microhardness owing to the precipitation strengthening. However, the hierarchical structure with grain, twin, and precipitation structural units exhibited the most excellent combination of strength and plasticity under grain refinement and precipitation strengthening.