Mechanical properties and microstructural evolution in a superlight Mg-7.28Li-2.19Al-0.091Y alloy fabricated by rolling

Abstract

To improve its strength and ductility, a novel microduplex Mg-7.28Li-2.19Al-0.091Y alloy was designed; its sheets 2 mm in thickness were fabricated by casting, homogenization, and rolling. Mechanical properties and microstructural evolution were investigated by tensile tester, hardness tester, optical microscope, transmission electron microscopy (TEM), X-ray diffractometer (XRD), and scanning electron microscopy (SEM). Tensile tests reveal that the ultimate tensile strength of 299 MPa and elongation of 15.7% were demonstrated in the cold-rolled alloy; the ultimate tensile strength of 211 MPa and elongation of 26.5% were demonstrated in the annealed alloy. Hardness tests indicate that the microhardness of α-Mg phase reaches HV 85. TEM examination reveals that high-density dislocations, dislocation pile-up, dislocation tangle, and interaction between dislocations and particles result in dislocation strengthening and second phase strengthening in the cold-rolled alloy. TEM and XRD results show that α-Mg, β-Li, AlLi, MgLi2Al, and Al2Y phases exist in the cold-rolled alloy while α-Mg, β-Li, MgLi2Al, and Al2Y phases exist in the annealed alloy. SEM observation shows that the fracture mode in the cold-rolled state is a ductile and brittle mixed fracture, and the fracture mode in the annealed state is a ductile fracture. The elongations in both cases are consistent with the fracture morphologies. Strengthening mechanisms were analyzed via crystal defect strengthening models. A strengthening sequence was proposed in this alloy as follows: dislocation strengthening > solid solution strengthening > grain boundary strengthening > lattice friction strengthening > second phase strengthening.