Study On Solution Aging Behavior And Properties Of Magnesium Alloy With LPSO

As a special structural phase of magnesium alloy,LPSO phase had caused widespread concern in academic circles due to its superiority on the promote mechanical properties of the alloy.In this paper,the size and morphology of the secondary phase in the Mg-Zn-Y alloy were controlled by means of alloy composition design,heat treatment and so on.The magnesium alloy reinforced by LPSO phase with good mechanical properties was obtained.At the same time,we studied the effect of solid solution and aging treatment on microstructure and performance of Mg-Zn-Y alloy by means of OM,SEM,TEM,XRD and relative performance tests.The aging kinetics equation of LPSO phase based on the theory of phase transformation is obtained.The results are as follows:The microstructure of Mg100-3x ZnxY2x(x=1.5,1,0.5)alloy consists of dendritic α-Mg matrix and secondary phase at grain boundaries.With the increase of Zn and Y content,the grain size is decreasing.There is only 18R-LPSO phase at grain boundaries of Mg97Zn1Y2 alloy and Mg98.5Zn0.5Y1 alloy.The second phase of Mg95.5Zn1.5Y3 alloy is composed of W phase and 18R-LPSO phase.Meanwhile,W phase and LPSO phase are parallel to each other,the orientation relationship is [111]W//[11—20]LPSO.The solidification process of these alloys was analyzed by DTA.The results show that α-Mg first formed in Mg97Zn1Y2 alloy and Mg98.5Zn0.5Y1 alloy at 640℃.As the temperature dropped to 550℃,LPSO phase was formed due to eutectic reaction occurs at the grain boundary.But for Mg95.5Zn1.5Y3 alloy,when the temperature dropped to 460℃,W phase will be generated at the grain boundary.The microstructure of the alloy greatly influences the comprehensive properties of the alloy.With the increasing content of Zn and Y,the hardness and strength of the alloy are also increasing.At the same time,higher Zn and Y content can induce the formation of W phase in the microstructure of Mg95.5Zn1.5Y3 alloy.The developed W phase,which is advantage to the plasticity of the alloy,result in the slight decrease in the plasticityAfter the solid solution treatment of Mg95.5Zn1.5Y3 alloy at 520℃,the dendrite is fused and the grain size amplified.The fishbone like W phase in the grain boundary is mostly dissolved into the matrix,and the residual W phase is presented in the form of particles.The LPSO phase is partially solid solution at this temperaturethe because of LPSO phase transformation temperature is much higher than that of the W phase transformation temperature.In addition,the residual LPSO phase at the grain boundary gradually changesfrom choroid shape to strip shape and rod shape.When the solution temperature is 500℃,the effect of solid solution is not obvious.If the solution temperature is too high,it will lead to the increase of the grain size and the presence of remelting balls,the alloy is in the state of over-burning.The hardness test showed that the solid solution treatment can improve the hardness of the alloy,but the hardness of the alloy decreases slightly when the alloy is in the state of over-burning.The optimum solid solution process parameters of Mg95.5Zn1.5Y3 alloy is 530℃/6h after a series of experiments of solution temperature and solution time.In the initial period of aging treatment of the Mg95.5Zn1.5Y3 alloy,a small amount of lamellar 14H-LPSO phase precipitates in the matrix.The volume fraction of 14H-LPSO phase in the matrix increases with the time extended,and the 14H-LPSO phase grows along the base surface and the cylinder,gradually coarsening under the action of the temperature field.Substantial precipitation of 14H-LPSO phase in the matrix can improve the hardness of the alloy.The aging conductivity data of Mg95.5Zn1.5Y3 alloy were fitted by using the Machi’s criterion and Avrami’s kinetic equation.The phase transition kinetics equation and the conductivity equation of the alloy are respectively f(28)1-exp(-0.2705t0.6368)and0.6368s(28)0.000167(10)0.0832(1-exp(-0.2705t)).The comparison of the calculated values with the experimental values shows that the two equations can describe the aging process of Mg95.5Zn1.5Y3 alloy.The corrosion resistance and corrosion mechanism of Mg100-3xZnx Y2x(x = 1.5,1,0.5)cast alloys were studied.The corrosion resistance of Mg97Zn1Y2 alloy was the best and the corrosion resistance of Mg98.5Zn0.5Y1 alloy was the worst.The fishbone like W phase is detrimental to the corrosion resistance of the alloy,which makes the Mg95.5Zn1.5Y3 alloy corrosion resistance decreased.After the solid solution treatment,the corrosion resistance of Mg95.5Zn1.5Y3 alloy promoted due to a large number of Zn and Y atoms were dissolved into the matrix,the volume fraction of W phase decreased,LPSO rod-shaped.It is advantageous to improve the corrosion resistance of the alloy by using a large amount of fine 14H-LPSO phase precipitated in the matrix and short time aging.However,with the increase of aging time,the 14H-LPSO phase in the matrix is coarsening and the corrosion resistance of the alloy decreases.