Dependence of defect susceptibility on microporosity variation in Mg-xLi-3Al-1Zn casting alloys

Abstract

This study quantitatively describes relative contribution of microstructural characteristics and microporosity variation to the tensile properties of Mg-xLi-3Al-1Zn alloys from the perspective of defect susceptibility. The alloys were prepared by melting pure lithium, AZ31 alloy, and additional elements in two crucibles under an Ar atmosphere, and then mixing them to add Li in the range of 2–12 wt%. As the Li content increases, the Li-rich β phase forms along the grain boundaries of the Mg-rich α phase. At approximately 12 wt% Li, the entire matrix transitions to the Li-rich β phase, and the volume density decreases from 1.762 g/cm³ (AZ31 alloy) to a minimum of 1.435 g/cm³. Additionally, the yield strength continuously increased from 67 MPa (AZ31 alloy) to approximately 120 MPa with increasing Li content, while the tensile strength increased from 127 MPa (AZ31 alloy) to a maximum of 144 MPa at 6 wt% Li, and the elongation increased to 19 % at 10 wt% Li before decreasing at higher additions. The fracture mode during tensile deformation clearly depends on the fraction of α phase and β phase, varying with Li addition. However, all tensile properties, including yield strength, are fundamentally influenced by the fractographic porosity based on the fracture surface. The defect susceptibility coefficients on microporosity variation for tensile strength and elongation, and the maximum values achievable under defect-free conditions, are highest at 6 wt% Li and 10 wt% Li, respectively. Furthermore, the relative contributions of microporosity and microstructural components to the defect susceptibility of tensile properties can be described: tensile strength increases in the order of the mixed α+β phase, β phase, and microporosity, while the contribution of the β phase to elongation is relatively lower compared to the mixed α+β phase and microporosity.