The study of Li effect on the microstructure and tensile properties of cast Al-Mg2Si metal matrix composite

This study is undertaken to investigate the effect of different concentrations of Li (0.03-0.35%) on the microstructure and tensile properties of Al-15 wt.% Mg2Si in situ metal matrix composite. The results showed that 0.15 wt.% Li addition changes the morphology of primary Mg2Si from irregular or dendritic to polyhedral shape and its average particle size decreases from 32 mu m to 4 mu m. Microstructural observations also depicted that the morphology of the eutectic Mg2Si phase alters from flake-like to coral-like. Further investigations on tensile tests revealed optimum Li level (0.15%) for improving both UTS and elongation values. A study of the fracture surfaces via scanning electron microscope (SEM) revealed a brittle mode of failure in unmodified composite however the addition of 0.15% Li converts the fracture behaviour to ductile. The behaviour is explained in terms of the presence of oxide bifilms in the liquid Al alloy. (C) 2011 Elsevier B. V. All rights reserved.