Research On The Hot Deformation Behaviour And Extrusion Technology Of Mg-Zn-Y-Mn-Ti Magnesium Alloy Reinforced By Long Period Stacking Ordered Structure

As the lightest metal structure material,magnesium alloy has a series of advantages such as low density,high specific strength and specific stiffness,so it has a broad application prospect in aerospace,automobile,electronics and other industries.However,its low absolute strength and poor plasticity limit its wide application in structural parts.In recent years,Mg-Zn-Y alloys have attracted great attention due to their unique microstructure and excellent mechanical properties.The long-period stacking ordered structure（LPSO phase）is a very effective strengthening phase in Mg-Zn-Y alloys and can improve the mechanical properties of the alloys at room and high temperatures.As early as2001,Kawamura et al.prepared a Mg₉₇Zn₁Y₂（at.%）magnesium alloy with long-period stacking ordered structure by rapid solidification powder metallurgy（RS/PM）.The alloy was reported to exhibit a high yield strength of 610 MPa and a relatively high tensile ductility of 5%at room temperature.However,this method has the disadvantages of high production cost and difficult to prepare block materials,so its wide application in industry is limited.Moreover,the mechanical properties of Mg-Zn-Y alloy decreases severely when using conventional casting technology.To improve the mechanical properties of Mg-Zn-Y alloys containing LPSO phase,the effects of different Zn/Y atomic ratios on the microstructure and mechanical properties of Mg-Zn-Y-Mn alloy were firstly studied,and the Mg₉₄Zn_2.5Y_2.5Mn₁（at.%）alloy exhibited the best mechanical properties.Then the Mg₉₄Zn_2.5Y_2.5Mn₁（at.%）was alloyed by Ti to optimize the composition of the alloy.The optimized Mg_93.5Zn_2.5Y_2.5Mn₁Ti_0.5.5 alloy was treated by solid-solution and the hot compression tests were carried out to establish the constitutive equation,the kinetics of DRX volume fraction and hot processing map of the solution-treated Mg_93.5Zn_2.5Y_2.5Mn₁Ti_0.5.5 alloy.Finally,the extrusion technology（equal channel angular extrusion,direction extrusion,unequal channel angular extrusion）of the solution-treated Mg_93.5Zn_2.5Y_2.5Mn₁Ti_0.5.5 alloy were studied by means of numerical simulation and experimental verification based on the hot compression data.The experimental results are as follows:（1）The composition of second phase in Mg-Zn-Y-Mn alloy is related to the Zn/Y atomic ratio.The alloy consists ofα-Mg,I phase（Mg₆Zn₃Y）and W phase（Mg₃Zn₃Y₂）when Zn/Y ratio is 3.0;α-Mg and W phase（Mg₃Zn₃Y₂）when Zn/Y ratio is 1.5;α-Mg,W phase（Mg₃Zn₃Y₂）and LPSO phase(Mg₁₂ZnY)When Zn/Y ratio is equal or lesser than 1.0.And when Zn/Y is equal to 1.0,the Mg-Zn-Y-Mn alloy exhibits the best mechanical properties and creep resistance.（2）The as-cast Mg-Zn-Y-Mn-Ti alloy is mainly composed ofα-Mg,block18R LPSO phase(Mg₁₂ZnY)and fishbone-like W phase（Mg₃Zn₃Y₂）,and the addition of Ti can effectively refine the grain size and promote the formation of LPSO phase.The tensile and compressive strengths first increase and then decrease with the increase of Ti content,When the Ti content is 0.3 at.%,the alloy shows the highest strength;However,when the Ti content is 0.5 at.%,the alloy shows the lowest yield asymmetry;These changes are contributed to the combined effects of grain refinement,increased of LPSO phase and reduced axial ration（c/a）.（3）Aftersolutiontreatment,thefishbone-likeWphasein Mg_93.5Zn_2.5Y_2.5Mn₁Ti_0.5.5 alloy can transform into the spherical W phase,Besides,partial of the block 18R LPSO phase along the grain boundaries change to the fine lamellar 14H LPSO phase distributed in the Mg matrix.The spheroidization of W phase and the formation of 14H LPSO phase in the Mg matrix can effectively improve the mechanical properties of the alloy.The mechanical properties is optimized after 40 h solution treatment.The yield strength,tensile strength and elongation are 165 MPa,265 MPa and 11.5%,respectively.（4）The solution-treated Mg_93.5Zn_2.5Y_2.5Mn₁Ti_0.5.5 alloy shows dynamic recrystallization softening mechanism during hot deformation.The flow stress of solution-treated Mg_93.5Zn_2.5Y_2.5Mn₁Ti_0.5.5 alloy decreases with the increase of deformation temperature or the decrease of strain rate.The volume fraction of dynamic recrystallization and grain size increase with the increase of deformation temperature or the decrease of strain rate.The suitable region for hot working of the alloy is(380⁴50℃,0.001⁰.01 s^-1).（5）The simulation results show that the tensile stress near the inner corner of the extruded specimen is completely eliminated after the modification of equal channel angular extrusion die,which makes the whole specimen always in the state of compressive stress during the process of deformation.And the equivalent strain of one pass is slightly improved,the modification of die will not reduce the effect of grain refinement.The experimental results show that the yield strength and tensile strength of the alloy first increase and then decrease with the increase of extrusion pass,but the elongation continuously increases.The alloy shows the best comprehensive mechanical properties after the extrusion of six passes.The yield strength is 251 MPa,the tensile strength is340 MPa,and the elongation is 26.1%.（6）The simulation results show that the equivalent stress,equivalent strain and extrusion force decrease with the increase of extrusion temperature during direction extrusion,but the distribution of equivalent strain is more uniform.With the increase of extrusion speed,the equivalent stress and extrusion force increase,while the equivalent strain first increases and then decreases.With the increase of extrusion ratio,the equivalent stress,equivalent strain and extrusion force increase,but the sample appeared the phenomenon of difficult extrusion at the extrusion ratio of 25.The experimental results show that the yield strength and tensile strength of the alloy first increase and then decrease,while the elongation increases with the increase of extrusion temperature.When extruded at 400℃,the alloy exhibits the best mechanical properties;the yield strength is315 MPa,the tensile strength is 412 MPa and the elongation is 17.8%.（7）The simulation results show that the equivalent stress,equivalent strain and extrusion force decrease with the increase of extrusion temperature during unequal channel angular extrusion,and the distribution of equivalent strain is more uniform.With the increase of extrusion speed,the equivalent stress and extrusion force increase,while the equivalent strain first increases and then decreases.With the increase of extrusion ratio,the equivalent stress,equivalent strain and extrusion force increase,but the sample appeared the phenomenon of difficult extrusion at the extrusion ratio of 16.The experimental results show that the yield strength and tensile strength of the alloy first increase and then decrease with the increase of extrusion temperature,while the elongation of the alloy increases continuously.The specimen extruded at 400℃shows the best comprehensive mechanical properties.The yield strength is 278 MPa,the tensile strength is 375 MPa,and the elongation is 22.8%.