| Magnesium alloys, one of the lightest metal structural materials, with high specific intensity, high specific rigidity, low density, nice machinability, damping capacity, excellent thermal conductivity and electromagnetic shielding, and recyclability, have been extensively paid attention to being applied in automobile industry during the past decades, in order to reduce fuel consumption and gas emission, which are considered as eco-structural metal material in the 21 st century and attract many researchers.Mg-Si series of magnesium alloys have excellent creep resistant due to the grain boundaries are suppressed by Mg2 Si particles, which exhibits a low density of 1.99×103kg·m-3, high elastic modulus of 120 Gpa, high melting temperature of 1 085 °C, high hardness of 4.5×109 Nm-2 and low thermal expansion coefficient of 7.5×10-6K-1. Generally, the primary Mg2 Si and eutectic Mg2 Si phases in hypereutectic Mg-Si alloys prepared by metallurgy methods are very coarse and are likely to act crack source on loading. In recent years, many new fabrication techniques are excavated such as hot extrusion(HE), rapid solidification(RS), directional solidification(DS) and mechanical alloying(MA) to change the morphology and size of primary Mg2 Si. Although the microstructures of Mg-high Si alloys manufactured by these new methods are refined and the mechanical properties are improved, these new techniques contribute limitedly in refining Mg2 Si phase. Furthermore, the processes are too complex to be controlled. Hence it is necessary to be improved. Modification is usually combined with conventional metallurgy, and the process route is simple, economical, and very popular in the world. Previous investigations indicate that the adding some elements have positive effect on the refinement of Mg2 Si particles and on the improvement of mechanical properties, such as Sb, Ca, KBF4, Y, P and RE.Based on the above statement, the modification effects of ytterbium(Yb), Na3PO4 on AS31, Mg-4Si alloys were investigated by means of X-ray diffraction(XRD), optical microscopy(OM), scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS) analysis in this article. The purpose of this preliminary experimental is to find an effective and simplified casting process route to produce a relatively fine Mg2 Si phase reinforced magnesium alloy. The major study results are as follows:(1) The experimental results indicate that proper amount of Yb(X=0.2, 0.5, wt.%) in the alloys can notably refine the Chinese script shaped eutectic Mg2 Si phases. With 0.2wt.% Yb modification, the ambient tensile strength and elongation of the alloys increased from 195.69 MPa and 8.59% to 207.49 MPa and 10.66%, respectively, however the elevate temperature performances are not altered greatly. With 0.5wt.% Yb introduced, amount of needle-shaped phases Mn2Si2Yb1 are formed, resulting in stress concentration and decreased the mechanical properties.(2) After solid solution treatment, the Chinese script shaped Mg2 Si phases are changed from Chinese script shape to short pole or round shape, and more evident with Yb added, thus the mechanical properties are improved.(3) The morphology of the primary Mg2 Si phase apparently changes from coarse dendrites to fine dispersive polygonal particles and the mean size decreases from 276.6 μm to 7.1 μm,with combined modification of 0.8wt.%Yb and 2.64 wt.%Na3PO4, such morphological evolution results in improvement in the ultimate tensile strength and elongation of the alloys as compared to the base alloy. This may be attributed to the formation of the Yb P particles that acted as the heterogeneous nucleation substrates for the primary Mg2 Si particles, resulting in a refined distribution of these precipitates.(4) The results of XRD examination(Mg-26.44Na3PO4-7.52 Yb and Mg-4Si-26.44Na3PO4-7.52Yb) show that there was no reaction between Si and Yb or Na3PO4. Nevertheless, solo addition of Yb or Na3PO4 into the melt has no real modification effect on the microstructure. Even the grains of primary Mg2 Si particles and α-Mg phases become coarser than that in the unmodified alloy.(5) Separated add Zn or Al into the melt modified with 0.8wt.% Yb+2.64 wt.% Na3PO4 would deteriorate the co-modification effect. Because the Zn could react with Yb hence decreased the content of Yb which would react with Na3PO4. In addditon, Al would react with Mg matrix form Mg17Al12, it will react with Na3PO4 form a new compound that contin Yb and Al. Although, it could act as the core of primary Mg2 Si, it would reduce the content of Yb P. Therefore the co-modification effect was deteriorated. |
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