Effect of the Addition of High Li Concentration on the Microstructure and Mechanical Properties of Al–Mg–Si Alloys with Different Mg Contents

Abstract

In the present work, the microstructural characteristics and mechanical properties of Al–1.5 Mg–0.6Si and Al–3.0 Mg–0.6Si alloy containing 3 wt% Li were investigated by optical microscopy (OM), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), time of flight–secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), and mechanical performance testing. The addition of Li reduces the density of the base alloy by up to 8.4%. The residual second phases contain Mg and Si in the hot-rolled condition, but the Mg/Si atomic ratio decreases after quenching, which means that Li substitute some of the Mg and convert Mg2Si into a (Mg, Li)2Si phase during solution treatment. The results of SIMS observations confirm this. The high Mg-containing alloy has a more rapid hardening response compared to the low Mg-containing alloy. TEM observation reveals that the δ′-Al3Li\u2009+\u2009β′′-Mg2Si dual phases can be observed in the high Mg-containing alloy after aging for 100 h at 170 °C. The higher Mg content enhances the precipitation of the δ′ phase, which results in the high Mg-containing alloy having a larger average diameter size of δ′ particles and wider δ′-precipitate-free zones (δ′-PFZs). The mechanical properties are significantly improved with the elastic modulus increasing by more than 16.5%. However, the existence of large second phases and wide δ′-PFZs in Li-containing alloys is detrimental to their ductility; as a result, their elongation is much lower than that of the base alloy.