|Abstract:||A study has been made of the development of microstructure, mechanical properties and oxidation behavior of Mg-Li-Si alloys. In the first instance, an attempt has been made to determine the phase equilibria that occur under near-equilibrium conditions for the case of Mg rich alloys. This has involved the application of optical metallography, x-ray diffraction and differential scanning calorimetry. As a result, the ternary phase diagram for these Mg-rich alloys has been drawn semi-quantitatively. This diagram has been used together with optical and electron metallography to determine the effect of Li on the Mg-Si binary system, as well as understand the development of microstructure in rapidly solidified Mg-Li-Si alloys. Thus, Li is found to reduce the eutectic temperature and also cause the eutectic composition to occur at lower concentrations of Si. A coupled eutectic microstructure has been produced by rapid solidification processing for an hyper-eutectic composition given by Mg-xLi-3Si (where x = 5 to 12 wt.%), and the difference between the behavior of the binary Mg-Si and the ternary Mg-Li-Si alloys has been assessed. The mechanical properties of these alloys have been studied using various types of sample. Thus, rapidly solidified melt-spun ribbons containing coupled eutectic as well as cellular microstructures have been tested, and it has been found, contrary to expectation, that the dispersed phase Mg$\sb2$Si provides sites for crack initiation at the particle/matrix interfaces. In laser surface melted alloys, the effect of Li has been associated with a refinement of grain size. Decohesion along the particle (Mg$\sb2$Si)/matrix interfaces again limits the plastic properties of these alloys. The oxidation behavior of the as-cast Mg-Li and Mg-Li-Si alloys has also been studied. It appears that microstructure has a significant effect on the oxidation behavior of these alloys. It has been proposed that oxidation is taking place at the oxide-metal interface, requiring the diffusion of oxygen through the growing oxide layer.