Research On Alloying Modification Mechanisms And Corrosion Resistance Of In-situ Synthesized Mg₂Si/Mg-Zn-Si Matrix Composites

Magnesium alloy is the lightest structural material which has been widely used, so it has very important application value and broad application prospects in lots of fields such as automotive, electronics and telecommunications, aerospace and defense, military and so on, in which research on the in-situ magnesium matrix composite by alloying modification is one of the hottest research points.In this paper, the effect of the La-rich rare-earth, Ba single and composite alloying on the microstructure, corrosion resistance and mechanical properties of a high Si content Mg₂Si/Mg-Zn-Si matrix composite was systematically investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), energy dispersive spectroscopy (EDS), image processing software and electronic creep testing machine and theoretical calculation.The main conclusions obtained are as follows:The single element alloying modification results shown that La-rich rare earth and Ba could ectively modify the Mg₂Si phase in the Mg₂Si /Mg-Zn-Si matrix composites. When the adding amount of the elements was 1%, the size of the primary Mg₂Si was 30μm, and the size of the eutectic Mg₂Si was also decreased. The refining mechanisms of La-rich rare-earth and Ba were as follows: La-rich rare-earth as a surfactant, can effectively poisoned the Mg₂Si growth steps and inhibited the growth of the Mg₂Si. At the initial of the solidification, the elements of Ba, Mg, Si formed into BaMg₂Si2 compound, which could be heterogeneous nuclei of the primary Mg₂Si, and finally refined the Mg₂Si phase.La-rich rare earth and Ba addition had combinative effect on the refinement of Mg₂Si. Primary Mg₂Si changed from coarse dendritic shape into fine polygonal particles with the average size of about 20μm. The amount of primary Mg₂Si was also increased. The eutectic Mg₂Si was decreased, and changed into short bars or small particles.The effect of La-rich rare-earth, Ba single element alloying on the corrosion properties of the Mg₂Si/Mg-Zn-Si matrix composites was researched. The results indicatied that the corrosion resistance of the composites was significantly improved. When added 1%RE, the composites'corrosion rate was 0.26g·m^-2·h^-1, reduced by 50%. The corrosion current density was also decreased. With 1%Ba addition, the corrosion rate was 0.326g·m^-2·h^-1, The corrosion current density was 4.741×10^-5J/(A·cm^-2), reduced about 71.5%。By the combinative addition of La-rich rare-earth and Ba, when the amount was the same (0.5%), the Mg₂Si/Mg-Zn-Si matrix composite achieved better corrosion resistance than unit alloying. The corrosion rate reduced by 53%, the corrosion current density decreased nearly an order.The effect of the La-rich rare-earth and Ba combinative addition on the micro-hardness, room temperature and high-temperature tensile properties, high temperature creep properties was also studied. The results indicated that by combinative alloying the composites'average micro-hardness was slightly increased. The standard deviation of hardness was decreased. When the two elements'addition ratio was 1:1, the room and high temperature tensile strength increased 45.7% and 52.4%, respectively, the elongation increased 64.3% and 62.5%. At the same time, the secondary creep rate of the composites decreased nearly 20-fold.