Study on Microstructure and Properties of the LPSO Reinforced Mg Alloys Fabricated with Ultrasonic Treatment and Squeeze Casting

Author:Yang Xiong

Supervisor:wu shu sen

Database:Doctor

Degree Year:2019

Download:50

Pages:159

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As the lightest metal structural materials,magnesium alloys are widely used in automotive industry,aerospace and electronic products.However,some factors,such as the low strength and difficult plastic processing,of magnesium alloys limit their wider application.Therefore,it is very important to develop new high-performance magnesium alloys,improve the forming and processing technology to further expand the application scope of magnesium alloys.Long Period Stacking Ordered Structure(LPSO)in magnesium alloys has excellent strengthening effect on magnesium alloys due to its special atomic arrangement characteristics.At present,little research has been done on the structure of the as-cast LPSO-reinforced magnesium alloys.The solidified behavior and the technology and mechanism on controlling the structure in different alloys have not been studied perfectly.Especially,the control of the size,quantity and distribution of LPSO structure in the alloys with low RE(rare earth)elements content has never been studied systematically.In this paper,Mg-Ni-Y and Mg-Zn-Y alloys with LPSO structure were were chosen as the object.The effects of alloys’ composition,ultrasonic treatment process and squeeze casting process on solidified structure and mechanical properties of the alloys were systematically studied.The effects of Ni/Y ratio on the microstructure and mechanical properties of Mg99.0-xNixY1.0 alloy with low Y content were studied.The results show that when the Ni/Y ratio is less than 0.5,the alloy consists of α-Mg,LPSO and Mg24Y5 phases;when the Ni/Y ratio is 0.5,the alloy mainly consists of α-Mg and LPSO phases;when the Ni/Y ratio is more than 0.5,the alloy mainly consists of α-Mg,LPSO structure and Ni-rich eutectic structure.When the Ni/Y ratio is 0.5,the volume fraction of LPSO structure in the alloy is the highest,and the coexistence of 18 R type and a small amount of 14 H type LPSO structure is observed in the as-cast alloy.The tensile strength,yield strength and elongation of the alloy are 204 MPa,80 MPa and 8.0%,respectively.In addition,Mg100-3xTMxY2x(x=0.5,1.0,2.0 at.%,TM=Ni,Zn)alloys at a fixed Ni/Y ratio were prepared.The volume fraction of LPSO structure in the alloys increased significantly,but the mechanical properties of the alloys did not improve significantly.Because of the difference of electronegativity between Ni and Zn,the volume fraction of LPSO in Mg-Ni-Y alloy is higher and the mechanical properties are better when the content of Ni and Zn is the same.For the Mg98.5Ni0.5Y1.0 alloy at the optimum Ni/Y ratio of 0.5,the microstructure and mechanical properties of the alloy were improved by ultrasonic treatment(UT).The results show that the grain size of a-Mg in Mg98.5Ni0.5Y1.0 alloy is the smallest and the structure of LPSO is refined obviously when the ultrasonic power is 1200 W.UT promotes the nucleation of α-Mg grains and uniform distribution of solute atoms,which is the main reason for the refinement of phases in the alloys.When the ultrasonic power is 1200 W,the alloy with UT exhibited the best mechanical properties,and its tensile strength,yield strength and elongation increased by 9.31%,15% and 45.5% respectively,compared with those of the alloy without UT.The better strengthening effect of the fine LPSO structure after UT is the important reasons for the enhancement of alloys’ mechanical properties.In addition,UT leads to the changes of solute atoms’ solid solubility and then promotes the precipitation of MgNi4 Y phase in the Mg-LPSO dual-phase Mg98.5Ni0.5Y1.0 alloy.The MgNi4 Y phase is distributed near the LPSO structure.The effects of squeezing pressure on the microstructures and mechanical properties of Mg98.5Ni0.5Y1.0 and Mg97Zn1Y2 alloys at the state of semi solid were studied.The results show that the α-Mg grains and LPSO structure are further refined after squeezeing,and the properties of the alloy are improved,compared with the conventional casting.At the same time,squeeze casting(SC)can increase the solid solubility of solute atoms in the Mg matrix,resulting in the changes in phases at grain boundaries.For the Mg98.5Ni0.5Y1.0 alloy,SC promotes the precipitation of Mg2 Ni phase on grain boundary,while for Mg97Zn1Y2 alloy,SC promotes the precipitation of strip YZn3 phase with higher hardness than LPSO structure.The YZn3 phase locates inside the bulk LPSO structure,which will weaken the strengthening effect of LPSO structure on the alloy.With the increase of SC pressure,the number of Mg2 Ni phase and YZn3 phase in the two alloys increases gradually.Considering the effect of SC on grain refinement and the factors promoting the formation of harmful phases,the optimum squeeze pressure is 100 MPa for the alloy fabricated by SC.When the pressure is 100 MPa,the tensile strength,yield strength and elongation of Mg97Zn1Y2 alloy are 252 MPa,157 MPa and 11.6%,respectively,which are increased by 9.6%,20.8% and 43.2% compared with the alloy fabricated by conventional casting.The effects of Al content on the microstructure and mechanical properties of Mg98.5Ni0.5Y1.0 alloy were studied.The newly formed phase containing Al was identified as Al2 NiY phase by TEM.The Al element added to Mg98.5Ni0.5Y1.0 alloy does not participate in the formation of LPSO structure.The volume fraction of LPSO structure in the alloy decreases due to the consumption of Ni and Y elements by Al2 NiY phase.The Al2 NiY phase exhibits a radial structure and its lamellar branches extend from the center to the matrix.Both of the Al2NiY-LPSO and Al2NiY-matrix interfaces are is non-coherent.The microhardness of Al2 NiY phase is higher than that of LPSO structure and a-Mg matrix.During the deformation process,Al2 NiY phase will split the matrix,which is not conducive to the mechanical properties of the alloy.After UT,the coarse Al2 NiY phase in the alloy with 0.5 at.% Al is refined to short flakes and distributes evenly in the alloy.The tensile strength and elongation of the alloy reach 187 MPa and 7.9% respectively,which are 21.4% and 105.7% higher than those of the untreated alloy.It can be seen that the mechanical properties of the alloy can be enhanced significantly by the change of the morphology of Al2 NiY phase.The effects of Sr content on the microstructure and mechanical properties of Mg98.5Ni0.5Y1.0 alloy were studied.The results show that Sr element can promote the formation of stacking faults and increase the number of stacking faults significantly.The high density stacking faults in the Mg98.5Ni0.5Y1.0 alloys are beneficial to the transformation from 18R-LPSO structure to 14H-LPSO structure.Therefore,the 14 H structure with big size was found in the as-cast Mg98.4Ni0.5Y1.0Sr0.1 alloy.Some solute atoms(Ni and Y)are consumed in high density stacking faults.With the increase of Sr content,the number of stacking faults increases and the volume fraction of bulk LPSO structure on grain boundaries decreases.When Sr content reaches 0.2 at.%,stacking faults occur inner the a-Mg grains and Mg-Sr compounds occur at grain boundaries.In addition,with the increase of Sr content,the strength of the alloy does not change significantly,but the elongation of the alloy increases firstly and then decreases.When Sr content is 0.1 at.%,the elongation of the alloy reaches 11.09%,which is 38.6% higher than that of the alloy without Sr addition.This is mainly due to the excellent deformation properties of the lamellar structure in the alloy.Therefore,the addition of Sr element can effectively improve the plasticity of the alloy.