Mechanical properties and microstructural evolution in a superlight Mg-6.4Li-3.6Zn-0.37Al-0.36Y alloy processed by multidirectional forging and rolling

A novel dual-phase-dominated fine-grained (5.5 μm) Mg-6.4Li-3.6Zn-0.37Al-0.36Y alloy was fabricated by multidirectional forging and rolling (MDFR); its mechanical properties and microstructural evolution were investigated. The ultimate tensile strength of 286 MPa and elongation of 31.8% were demonstrated in this alloy. The Portevin–Le Chatelier effect was observed in cold-rolled alloy. During MDF, the dominant grain refinement mechanisms of the α-Mg phase are mechanically fragmented grain refinement and deformation-induced in-situ dynamic recrystallization (DRX) grain refinement. However, the dominant grain refinement mechanism of the β-Li phase is deformation-induced in-situ DRX grain refinement. In later stage, thermally activated grain coarsening occurs in both phases. X-ray diffraction analyzes reveal the existence of α-Mg and β-Li phases, Al2Y, and MgLi2Al intermetallic compounds in the MDF state and the existence of α-Mg and β-Li phases, Al2Y, Al12Mg17, and MgLiZn intermetallic compounds in the cold-rolled state. Transmission electron microscopy analyzes show that high-density dislocation pile-up, dislocation tangle, subgrains, and MgLiZn particles exist in the cold-rolled alloy and raise the tensile strength. Texture analyzes reveal that strong {0001} basal texture or basal fiber of α-Mg phase exists, but the texture intensities in this alloy are not high. Annealing does not produce new texture component. A Hall-Petch model was established in this alloy: yield strength = 11.83 + 351.89 times minus square root of grain size. The relationship between critical resolved shear stress and resistance to deformation was modeled. Static recrystallization and grain coarsening during annealing were discussed.