Heterogeneous plastic deformation and crack passivation behaviors influenced by phase morphology in Mg-Li alloys

Magnesium-lithium (Mg-Li) alloys, known for their high specific strength, are highly attractive for lightweight structural applications. For dual-phase Mg-Li alloy, the small adjustment of Li content will result in a huge difference in the mechanical properties and microstructures. In this study, the effects of lithium (Li) addition on grain refinement and deformation mechanism were investigated in dual-phase Mg-Li alloys. In situ tensile testing combined with digital image correlation (DIC) analysis reveals that refinement of α-phase grains enhances plastic deformation and reduces strain localization. The α1 phases in Mg-8Li alloys, with an average width of 52.8 μm, are more susceptible to yielding than the α2 and α3 phases, which have average widths of 1.34 μm and 0.66 μm, respectively. In contrast, Mg-9Li avoids coarse α1 phases, resulting in delayed yield. Additionally, cracks in β phases of Mg-9Li alloys are consistently obstructed by the α2 and α3 phases, enhancing toughness by preventing direct shear fractures. Substantial α-phase grain refinement dissipates stress concentrations at crack tips, promoting crack tip passivation and a shift to fully ductile fracture behavior. The insights gained from these results will improve our understanding of the deformation behavior and guide the development of toughened dual-phase heterostructured Mg-based alloys.