Microstructure and mechanical properties of as-cast ultralight and high strength Mg-10Li-3Al-3Zn-xY alloy with multi-precipitates

Magnesium‑lithium (Mg–Li) alloys have been widely used in various engineering fields due to their ultra-light characteristics. The strengthening of Mg–Li alloys is one of the key factors determining its application and has received extensive attention. In this work, the as-cast ultralight and high strength Mg-10Li-3Al-3Zn-xY (x = 0, 0.5 and 1.0, wt%) alloys with a density of only 1.52 g/cm³ were prepared via vacuum induction melting. The effects of rare earth element yttrium (Y) on the microstructure and mechanical properties of the Mg–Li alloy system were investigated. The results indicated that the microstructures of all the alloys mainly consisted of α-Mg, β-Li, AlLi, and MgLi₂Al phases. The spherical AlLi phase was distributed inside the α-Mg and β-Li matrix. Part of the petal-like MgLi₂Al phase was distributed in the interior of β-Li, and the rest grew along the grain boundaries as long strips. With the addition of Y, Al₂Y phase formed with sharp edges and regular shapes. The Al₂Y phase was mainly distributed inside the α-Mg phase and along the grain boundaries. Meanwhile, the amount of AlLi phase inside the α-Mg phase gradually decreased. The presence of nano-scale lath-shape MgLiZn phase in the β-Li matrix was observed in the HAADF map of the 0.5Y alloy. The continuous growth of MgLi₂Al phase at the grain boundaries weakened its strength. As a consequence, it eventually led to intergranular fracture of the alloy. The 0.5Y alloy showed the highest tensile strength, up to ~244 MPa, and the yield strength was 60.5% higher than that of the 0Y alloy. Solid solution strengthening, precipitate strengthening and grain refinement were thus considered as the primary strengthening mechanisms caused by the addition of Y into the Mg-10Li-3Al-3Zn alloy.