Effect of Li addition on mechanical properties and ageing precipitation behavior of extruded Al−3.0Mg−0.5Si alloy

The effect of Li (2.0 wt%) addition on mechanical properties and ageing precipitation behavior of Al−3.0Mg−0.5Si was investigated by tensile test, dynamic elasticity modulus test, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images. The results show that the tensile strength of the Li-containing alloy can be significantly improved; however, the ductility is sharply decreased and the fracture mechanism changes from ductile fracture to intergranular fracture. The elasticity modulus of the Li-containing alloy increases by 11.6% compared with the base alloy. The microstructure observation shows that the Li addition can absolutely change the precipitation behavior of the base alloy, and δ′-Al3Li phase becomes the main precipitates. Besides, β″-Mg2Si and δ′-Al3Li dual phases precipitation can be visibly observed at 170 °C ageing for 100 h, although the quantity of δ′-Al3Li phase is more than β″-Mg2Si phase. The width of the precipitate-free zone (PFZ) of the Li-containing alloy is much wider at the over-ageing state than the base alloy, which has a negative impact on the ductile and results in the decrease of elongation.