| Magnesium alloy has a prospect of widely appolication as lightweight structural material. The magnesium alloy has poor plastic deformability compared with aluminum and steel due to its close-packed hexagonal lattice, which increases the difficulty of plastic processing and producing cost. In recent years,the research and development of magnesium alloys focus on improving working plasticity and high strength by means of new alloy phase design and new deformation process development etc. of the wrought magnesium alloy. Mg-Zn magnesium alloy becomes a research hotspot for its high strength and good plasticity. However, the widely applied Mg-Zn magnesium alloy nowadays is high strength magnesium alloy with high Zn content, of which the plastic deformation capacity is poor.In order to develop the high plastic wrought magnesium alloy, the paper studied the Superiority of adding rare earth element Y of Mg-Zn-Zr magnesium alloy with low Zn content. The casting billets with different contents of Y were prepared by means of adding Mg-Y master alloy into Mg-2.0Zn-0.3Zr alloy, The effects of element Y on the microstructures and mechanical properties of the as-cast ZK20 magnesium alloy were investigated. The process of homogenization annealing was optimized. The effects of adding rare earth element Y on the microstructures and mechanical properties of the extrude ZK20 magnesium alloy treated by homogenization annealing were investigated. The extrusion process of the alloy ZK20+0.9%Y was optimized. The results are as following:When Y content varied from 0.9% to 1.9%( wt), the microstructure becomes finer and the intercrystalline intermetallic compound showed a continuous thin fishing net. When Y content increased up to 3.7%(wt), the compound appears discontinuous thick fishing net. The tensile strength of the alloy improved when the Y content increased from 1.9% to 5.8%(wt). The preferable plasticity with a maximal percent elongation of 24.8% is obtained when the alloy has 0.9% Y content, which has less W-phase than the alloy of 1.9% Y. When the Y content of the alloy is 3.7%(wt), the mass of W-phase decreased compared to that of 1.9%(wt) Y due to the present of X-phase, while the intercrystalline compound became discontinuous thick fishing net arrangement, which lead to improve plasticity of the alloy. The alloy with 3.7% (wt) Y content has better comprehensive mechanical properties than others, with the tensile strength of 232 MPa, the yield strength of 124 MPa and the percent elongation of 23.5%. The Y element adding could lead to an increase of flow stress and deformation resistance, The alloy with 3.7%(wt) Y content has a flow stress of 85.7 MPa.The W-phase disappeared in ZK20+3.7%Y alloy, under the conditions of 420℃×12h homogenization annealing. After hot extrusion deformation, the microstructure became finer than ZK20 alloy.The strength was significantly improved for the disappearance of composition segregation and recrystallization. The strength was increased with the increase of the Y content. As a result, the alloy with 5.8%(wt) Y content had a tensile strength of 279 MPa and elongation of 28.1%. When the Y content was over 3.7%(wt), the elongation increased with the increase of the X-phase content. After hot extrusion under conditions of 330℃, the ZK20+0.9%Y alloy had the yield strength of 322 MPa and the percent elongation of 21.9%.
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