Research On Microstructure Evolution And Mechanical Properties Of Mg-(Gd,Er)-Zn-Zr Alloy

As the lightest metal structure material, magnesium is widely used in electric device, biomedical, auto industry, aerospace and other engineering fields. Furthermore, magnesium is regarded as “Green engineering material in 21 st century”. In present study, Mg-Gd-Er-Zn-Zr alloys with varied Er/Gd ratio are investigated under different manufactured. Also, thire microstructure and mechanical properties are observed and tested, respectively. The study focuses on the lattice structure transformation and morphology of the secondary phase. Meanwhile, the influence of microstructure and secondary phase on mechanical propertie is analysed. In particularly, the formation mechanism of secondary phase in the as-cast alloys with varied Er/Gd ratio is investigated. The evolution of morphology and transformation of lattice structure during the solid solution process are investigated. The precipitation process and lattice structure evlolution of the precipitated phase during the aging process are observed, and the influence of Er/Gd ratio on the precipitation sequence of the alloy were analysed. The influence of Long period stacking faults order（LPSO） structure on dynamic recrystallazation process is discussed. Moreover, the strengthen mechanism of as-cast, as-solution, peak-aged and as-extruded alloy is also discussed. The research results indicate that:The secondary phases in as-cast Mg-14Er-1Zn-0.6Zr and Mg-14Gd-1Zn-0.6Zr alloy are 18R-LPSO structure and（Mg,Zn）3Gd phase, respectively. With the decreasing of Er/Gd ratio in the alloys, the morphology of secondary phase transforms from block shape to net shape distribution; when the Er/Gd ratio>2.5, the18R-LPSO structure existed in the alloy; when the Er/Gd ratio=2.5, the（Mg,Zn）3Gd phase begains to form in the as-cast alloy; with the Er/Gd ratio<2.5, the secondary phase in the alloy is mainly（Mg,Zn）3RE phase. The content of Er and Gd elements of secondary phase is increasing with those of the as-castalloy; the UTS and YTS values increase with decreasing of Er/Gd ratio. Meanwhile, elongation decreases with decreasing of Er/Gd ratio; because of the lower HVs value of LPSO structure, the LPSO structure has positive deformed effective during the deformation process, results a great effect on elongation;（Mg,Zn）3RE phase shows obvious effect on strengthenning of the as-cast alloys. However, because its net-shape distribution, the elongation was decreased.During the solid solution process, the 18R-type to 14H-type transformation of LPSO structure has been happened, the product is block LPSO structure, which distributed at the grain boundary. The lamellar LPSO structure precipitated in the matrix, which extend from grain boundary to inner grain; after the solid solution treatment, the block and lammellar LPSO structure co-existed in the Mg-7Gd-Er-1Zn-0.6Zr alloy. The formation of block LPSO structure relates to the Er element in the alloy.During the isothermal aging process, the evolution of precipitated phase is influenced by the Er/Gd ratio: there is no obviously peak age state when the Er/Gd ratio≥1, the precipitated phase is hard to precipitate from the matrix; the precipitation sequence of Mg-7Er-7Gd-1Zn-0.6Zr alloy stop at the early stage, in which the β′ could not formed, and showed a weaker aging effect;when the Er/Gd ratio≤2.5, alloys have great aging hardning effect, the precipitation process includes pre-aged, peak-aged and over-aged stages. The precipitation sequence of Mg-10Gd-4Er-1Zn-Zr alloy at 225 oC is SSSS�'β′′�'β′�'β1;the precipitated phase in the peak aged state alloy is β′ phase. With the appearance of β1 phase, the alloy shows over-aged behavior. At 275 oC, the transformation of β1 to β is observed, which isbenefites from the high misfit of them. The orientation of precipitated phase and Mg-matrix could be confirmed as: the orientation of β′′ phase and Mg-matrix is[0001]β″//[0001]Mg and(1010}β″//（1010）Mg; the orientation of β′ phase and Mg-matrix is: [001]β′//[0001]Mg and [010]β′//[1010]Mg,（100）β′//（1120）Mg; the orientation of β1 phase and Mg-matrix is: [110]β1//[0001]Mg and（112）β1//（1100）Mg。In the as-extruded alloy, block LPSO structure has positive effect on dynamic recrystallization; lemaellar LPSO structure hindered the dynamic recrystallization. Lamellar LPSO structure mainly strengthen the matrix. Due to the existence of lamellar LPSO structure, the alloy has good performance of mechanical properties below the dynamic recrystallization temperature; because of the block LPSO structure distributed at the grain boundary, the block LPSO structure shows better mechanical properties when temperature rise above the dynamic recrystallization temperature. All the extruded alloy shows great aging hardening effect, compared to that in the as-solutioned alloy, the peak-aged time of extruded alloy is shoter. It was caused by the defect, which are formed during extrusion process. The precipitation behavior is effected by Er/Gd ratio,when the Er/Gd ratio≤1, the alloy shows great aging hardening effect.In the as-cast and peak aged alloy, the LPSO structure is not the main strengthening phase. However, in the extruded alloy, the dynamic recrystallization is hindered by the lemallar LPSO structure, lead to the formation of the {0002}<101₀> fiber texture in the extruded alloy, which could enhance the UTS and YTS. The LPSO structure is kinked during the extrusion process, which could transfer the load from matrix, and enhanced the elongation of the alloy.