The Effect Of Gd On The Microstructure And Properties Of Mg-Zn-Mn-Sn Alloy

In this paper,Mg-8Zn-1Mn-3Sn magnesium alloy as the base alloy,Gd element is added in the base alloy by using the method of alloy modification.The effect of rare earthGd on the microstructure and mechanical properties of Mg-8Zn-1Mn-3Sn magnesium alloy are studied through X-ray fluorescence spectrometry(XRF),optical microscope(OM),scanning electron microscope(SEM),X-ray diffractometer(XRD),energy spectrum analyzer(EDS),Brinell hardness tester and room temperature tensile test.Meantime,the effect of different heat treatment processes on the structure and properties of Mg-8Zn-1Mn-3Sn magnesium alloy are systematically studied.The solid solution treatment process is optimized by comparing the solid solution alloy structure;the effect of solid solution aging treatment on the microstructure and mechanical properties of the alloy in different processes is analyzed.Finally,the effect of ultrasonic treatment on the MgSnGd phase during the solidification of the alloy is systematically discussed.The results show that the rare earthGd has a significant refinement effect on the as-cast Mg-8Zn-1Mn-3Sn magnesium alloy structure.As theGd content increases,the grain size of the Mg-8Zn-1Mn-3Sn-xGd alloy gradually decreases.The structure of as-cast Mg-8Zn-1Mn-3Sn magnesium alloy is mainly composed of?-Mgmatrix,MgZn₂,Mg₇Zn₃,?-Mn,MgZn,Mg₂Sn phase.After addingGd element,a new Mg-Sn-Gd phase is formed in the alloy.At the same time,the Mg-Sn-Gd phase will be formed first during the solidification of the alloy,resulting in a decrease in the content of the secondary phase Mg₂Sn phase.The phase composition of the MgSnGd phase in the alloy changes with the change ofGd content.When the amount of rare earthGd added is less than 1.0%,the morphology of the Mg-Sn-Gd phase of the alloy is needle-like and bulk Shape,and mainly distributed near the grain boundary,a small amount of Mg-Sn-Gd is distributed in the crystal.When the amount of rare earthGd added is greater than 1.0%,there is a MgSnGd phase with an atomic ratio of 1:1:1 in the alloy system.The rare earth phase is mainly distributed in the crystal in the shape of rod and block.After hot extrusion,the dynamic recrystallization of the alloy in the extruded state is sufficient,and the average grain size is about 4?m.This is because the Mg-Sn-Gd phase has a high melting point and good thermal stability,can pin the grain boundaries during hot extrusion and inhibit dynamic recrystallization grain growth,making the recrystallized grain size very small.With the increase ofGd content,the tensile strength and yield strength of the extruded Mg-8Zn-1Mn-3Sn alloy increase first and then decrease.The tensile strength,yield strength and elongation of extruded Mg-8Zn-1Mn-3Sn-0.5Gd are 352MPa,273MPa and 11.4%,respectively.The extruded Mg-8Zn-1Mn-3Sn-0.5Gd alloy was solution treated at 420?for 2h,most of the second phase in the alloy dissolved,and only a small amount of undissolved second phase remained at the grain boundary.After aging treatment at 400?×3h+180?×12h,the tensile strength,yield strength and elongation are 345.5MPa,235MPa and5.4%,respectively.After aging treatment at 420?×2h+180?×12h,the tensile strength,yield strength and elongation of the alloy are 384MPa,337MPa and 1.6%,respectively.The tensile strength and yield strength of the alloy increased significantly,and the elongation of the alloy decreased significantly.Among them,the yield strength of the alloy is the most obvious.Compared with the extruded state,the yield strength is increased by 21.2%,and the tensile strength is increased by 7.8%.The Mg-8Zn-1Mn-3Sn-1.2Gd as-cast alloy has a large-sized MgSnGd phase.The MgSnGd second phase mainly accumulates in the partial area of alloy structure.When the as-cast structure is treated with 500W ultrasound,the amount of large phases of MgSnGd in the alloy decreases,and the MgSnGd ternary phase segregation phenomenon disappear,and the alloy second phase is uniformly distributed.At the same time,most of the bulk MgSnGd second phase changed from large to granular,and some rod-shaped MgSnGd second phase changed from rod to needle.This is because ultrasonic treatment can not only improve the transmission capacity of solute atoms at the solid-liquid interface,reduce the thickness of the solute enrichment layer and thereby reduce the supercooling of the components,while stirring the melt during the solidification of the alloy and accelerating the melt Flow promotes the diffusion of solute atoms in the melt,makes the distribution ofGd and Sn elements uniform and dispersed,and reduces the phenomenon of component segregation so that MgSnGd is evenly distributed in the matrix.The cavitation effect of ultrasonic treatment will have a certain stirring effect on the metal melt,which can break the second phase and dendrites and refine the MgSnGd phase and grains.