Design And Fabrication Of High Strength And High Modulus Of Mg-RE-Si Alloys

Abstract:Magnesium alloy is the lightest metal in the practical application of structural materials with low density, which take advantages over strength and specific stiffness, and widely used in aerospace materials.However, as the development of the industral technology and stiffness requirement of the newly spacecraft is higher and higher, the bad elasticity modulus of magnesium alloy obviously restricts the extensive application of these alloys. In order to settle this problem down, we introduced the second particle Mg2Si with high elasticity modulus by alloying in the thesis. Optical microscopy (OM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), tensile method and other testing means were used to investigate the microstructure and mechanical properties of these alloys in the study. And the conclusions were set as below:(1) To design a new high-strength、high-modulus and heat-resistant Mg-Gd-Y-Nd-Si-Zr alloy, Si is added to improve the elastic modulus of the alloy, and the RE content was adjusted to improve the strength of the alloy. Meanwhile we used Thermo-calc Software to calculate phase diagram.(2) Si is added in Mg-8Gd-4Y-Nd-Zr alloy to improve its elasticity modulus. The properties of the prepared extruded-T5Mg-8Gd-4Y-Nd-Si-Zr alloy were as listed:ultimate strength σb=392MPa, yield strength σ0.2=347MPa, elongation δ=2.7%and Young’s modulus E=48.0Gpa; 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 formation of Mg2Si and (RE+Si)-rich particles with high Young’s modulus increase in Young’s modulus in alloy.Meanwhile the formation of the (RE+Si)-rich particles, which decrease plenty of rare earth atoms in the matrix and weakened the age-hardening effect which decrease in strength. (3) Frabricated the final ingredient of the alloy is Mg-4Gd-10Y-4Nd-3Si, and the properties of the prepared extruded-T5alloy were as listed:ultimate strength σb=413M Pa, yield strength σ0.2=367MPa, elongation δ=1.5%and Young’s modulus E=52.0GPa.(4) The morphology of the primary Mg2Si translates from coarse dendrite into fine polygon with increasing Y content. The average size of the primary Mg2Si significantly decreases to about13μm with increasing Y content up to0.8wt.%, and then becomes coarser again with Y content up to1.2wt.%and2.4wt.%. The modification and refinement of the primary Mg2Si is mainly attributed to the poisoning effect. The Mg2Y phase in the primary Mg2Si translates into GdSi compound and Si3Y5compounds as the Y content exceeds3.0wt.%. Therefore, it is reasonable to conclude that the proper Y (0.8wt.%) addition can effectively modify and refine the primary Mg2Si.