A Study Of Mg-Sn-Zn Alloys On The Microstructure And Mechanical Properties By Alloying And Deformation

In recent years,the metal products in the practical application of the structure of lightweight and energy-saving emission reduction needs to be intensified,the development of magnesium（Mg）alloy is more and more important.Since magnesium alloys have many advantages,such as high specific strength,high specific stiffness,good conduction and conductivity,good damping and vibration resistance,strong electromagnetic shielding ability,easy casting processing and easy recovery,magnesium alloys playing a more and more important role in the military,communications,aviation,civil and other fields.However,due to the low absolute strength of the material,poor mechanical properties at high temperature,poor plastic deformation ability at ambient temperature and corrosion,Mg alloys are still restricted in application.Compared with the traditional casting magnesium alloy,the deformed magnesium alloy has superior performance in strength,dimension precision and ductility.Researching new deformed magnesium alloys and the production process has become an important topic in the field of magnesium alloy.To address the above problems,based on Mg-Sn-Zn alloy,using rare earth elements y as alloying elements,the alloys of different compositions were fabricated by the induction melting furnace in the argon atmosphere,through X-ray fluorescence spectrometer（XRF）,X-ray diffraction（XRD）,optical microscope（OM）,scanning electron microscope（SEM）,projection electron microscope（TEM）energy dispersive X-ray Spectrometer（EDS）,tensile tester,and other means studied the influence of Y element on Mg-Sn-Zn alloy,and discussed the feasibility and mechanism of the performance improvement of the alloy by extrusion and rolling.The following research has been carried out:the cast alloy ingots were obtained by smelting,the microstructure and mechanical properties of the cast Mg-5Sn-3Zn-xY（x=0,0.2,0.5,0.8,1.0 wt%）alloys were investigated.The microstructure of the cast Mg-Sn-Zn alloys shows obvious dendrite,and the spacing of the dendrite arms was relatively large.After adding Y element,the dendrite structure is obviously weakened,and there is a tendency of dendrite to change into equiaxed crystal.With the increase of Y element,the grain size showed a decreasing trend,but when the Y content exceeded the 0.8wt%,the grain size became larger.The alloy mainly contains Mg₂Sn,MgZn₂.After adding Y the feather-like MgSnY phase emerged,the volume increase with the increase in the Y content,the size gradually become larger and show segregation.In the cast alloy,the Mg2Sn phase and the MgZn2 phase have the tendency of mutual attaching growth,and form the second phase with smaller size,which has a good effect on the alloy performance improvement.When the Y element content is0.5wt%,the comprehensive performance of the cast alloy is better,the ultimate tensile strength is 228,the yield intensity is 68,and the elongation is 18%.The elongation of the cast Mg-Sn-Zn-Y alloys were excellent,all of them can reach the 14-20%,this is due to the existence of fine granular second phase and fine grains.The homogeneous treatment was carried on the cast alloy and the observation of its microstructure found that the grains exhibit equiaxed crystal structure and growing up,and a large number of particles are dissolved into the matrix,but there is still a second phase particle at the grain boundary.The MgSnY phase was remained in the matrix because of its high melting point.Then,the alloy ingots are extruded into plates,the microstructure and mechanical properties of the extruded alloy were analyzed,and the Mg-Sn-Zn-Y alloys going through a relatively complete dynamic recrystallization during the extrusion,showing a homogeneous equiaxed crystal structure.The second phases in the matrix were broken and the quantity increases with the increase of Y content.At ambient temperature,the mechanical properties of the extruded-alloys were better than as-cast alloys,but the elongation decreases greatly.With the increase of the content of Y element,the mechanical properties of extruded alloys were improved.The microstructure and properties of rolled-state alloy were studied by rolling treatment on the extruded-Mg-Sn-Zn-Y alloy sheets with different pass and deformation.The rolling process is divided into ordinary rolling and cross rolling to compare the effects of different processes on the microstructure of the alloy.Compared with the extruded state,the grain of the rolled alloy decreased significantly and stretched along the rolling direction,many twins appeared.The recrystallization occurred more completely with the increase of the rolling pass.The second phase of the alloy is squeezed and broken,showing the streamline distribution,and the cross rolling did not significantly change the distribution streamline,which is mainly determined by the extrusion process.The tensile strength of the rolled alloys is lower than that of the extruded alloys,the elongation decreases with the increase of the tensile strength when the rolling pass increase,and there is no obvious mechanical difference between the cross rolling and the unidirectional rolling.