| Mg-Gd-Y alloys have been recognized as promising light structural materials for the aerospace due to their low density, high specific elastic modulus and high specific strength. However, as the stiffness requirement of the newly developed spacecraft is higher and higher, the low Young’s modulus (45GPa) restricts the extensive application of these alloys. In order to solve this problem, the second particle Mg2Si with high Young’s modulus was introduced by alloying in the thesis. The microstructure and mechanical properities of the studied alloys were investigated by using optical microscopy (OM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile and other testing means. The relationship among composition, processing, microstructure and property was investigated. Main conclusions were listed in the following:(1) The effects of Si addition on microstructure and mechanical properties of Mg-8Gd-4Y-Nd-Zr alloy was investigated. Si addition obviously reduced the liquidity of the Mg-8Gd-4Y-Nd-Zr alloy melt, deteriorated the castability, coarsened the as-cast grains, decreased the strength, but improved the Young’s modulus, the proper content of Si element in the alloy should be under1wt.%. The properties of the prepared extruded-T5Mg-8Gd-4Y-Nd-Si-Zr alloy were as follows: ultimate strength σb=392MPa, yield strength σ0.2=347MPa, elongation δ=2.7%and Young’s modulus E=51.0GPa. Compared with the Mg-8Gd-4Y-Nd-Zr alloy, Si addition increased the Young’s modulus, but decreased the ultimate strength and yield strength. The increase in Young’s modulus was attributed to the formation of Mg2Si and (RE+Si)-rich particles with high Young’s modulus. While the decrease in strength was ascribed to the formation of the (RE+Si)-rich particles, which consumed plenty of rare earth atoms in the matrix and weakened the age-hardening effect during ageing.(2) The effects of Zn addition on microstructure and mechanical properties of the Mg-Gd-Y-Nd-Si alloy was investigated. The novel Mg-RE alloys owning good combination of high Young’s modulus, high strength and excellent castability were prepared. Zn addition obviously improved the liquidity of the melt and the castability of the alloys. The mechanism is that Zn reacted with Mg and RE in the melt, generated plenty of new phases containing Zn and changed the structure of melt. The tensile properties of the extruded-T5Mg-8Gd-4Y-Nd-3Zn-3Si alloy were as follows:ultimate strength ab=345MPa, yield strength ao.2=317MPa, elongation δ=6.5%and Young’s modulus E=58.5GPa. The Mg-10Gd-4Y-Nd-3Zn-3Si alloy were as follows:ultimate strength ab=390MPa, yield strength σ0.2=343MPa, elongation8=4.6%and Young’s modulus E=58.1GPa.(3) The modification effect of Gd, Y and Nd on Mg2Si particles in the hypereutectic Mg-3wt.%Si alloy were investigated, which laid the foundation for optimizing the composition of the Mg-RE(Gd, Y, Nd)-Si alloys. Proper Gd, Y and Nd addition could effectively modify and refine the primary Mg2Si in the Mg-3wt.%Si alloy. The optimal modification effect was obtained when the Gd, Y and Nd content were1.0,0.8and1.0wt.%, respectively. The morphology of the primary Mg2Si transformed from coarse dendrite into fine polygon, meanwhile, the average size was refined from65μm to10μm,13μm and10μm, respectively. However, the primary Mg2Si became coarse again when the Gd, Y and Nd content exceeded2.0,1.2and3.0wt.%, corresponding to the over modification effect. |
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