Study On Long Period Stacking Ordered Phase Microstructure And Mechanical Properties Of Mg-Ni-Y Alloys

As LPSO can effectively strengthen magnesium alloys,it has become a hot research spot in recent years.Mg-Ni-Y is an important ternary system in LPSO-reinforced magnesium alloys.However,the phase structure of LPSO in this system is still disagreeable,and the effects of LPSO contents on the mechanical properties of the alloys is also lacking of research.This study uses alloying method,the micro structure and phase transformation of LPSO in Mg-Ni-Y alloys were investigated by SEM,EDS,DSC and TEM.Completing the partial tenery isothermal sections of Mg-Ni-Y system in the Mg-rich region,Mg-Ni-Y alloys with different LPSO contents were designed to study the effects of deformation recrystallization and LPSO contents on the mechanical properties of Mg-Ni-Y alloys.The main findings are as follows:(1)A variety of LPSO phases can be formed in Mg-Ni-Y alloys.The compositions of the alloys are different,and the steady-state LPSO phase structure is also different.Mg-rich Mg97.0Ni0.9Y1.2 and Mg96.4Ni1.2Y2.4 and Mg95.8Ni1.5Y2.7 alloys only contain 18R in as-cast state,the steady-state phase structure is also 18R;with Ni,Y elemental content increased,the as-cast Mg89.8Ni4.5Y5.7 alloy contains 14H and 18R,and the steady-state phase structure is 14H;further increasing the Ni,Y elenetal content,as-cast Mg84.4Ni7.1Y8.5 alloy contains 10H and 18R,While the steady-state phase structure is 18R.(2)Combined with SEM,TEM,EDS and other analysis results,the isothermal phase diagram of magnesium-rich Mg-Ni-Y alloy at 400? and 500? has been improved.There are three three-phase equilibrium regions in the 400? isothermal phase diagram of Mg-rich region:(Mg)+Mg24Y5+18R,14H+Mg2Ni+(Mg),18R+Mg2Ni+MgNi4Y,one two-phase equilibrium region:18R+(Mg).There are two three-phase equilibrium regions in the 500? isothermal phase diagram of Mg-riched regions:(Mg)+Mg24Ys+18R,14H+MgNi4Y+(Mg),two two-phase equilibrium regions:18R+(Mg)and 18R+MgNi4Y.(3)According to the Mg-Ni-Y phase diagram,the Mg97.0Ni0.9Y1.2 and Mg96.4Ni1.2Y2.4 and Mg95.8Ni1.5Y2.7 alloys with the mass percentage of LPSO content 10%,20%and 30%are designed respectively to study the effects of extrusion temperature and LPSO content on the mechanical properties.The results show that under different extrusion temperatures,the LPSO content increases while increasing the strenth.The yield strength of Mg95.8Ni1.5Y2.7 alloy with LPSO content of 30%can reach 252 MPa and the tensile strength is 344 MPa.In addition,LPSO enhances the dynamic recrystallization temperature of the alloy.It was found that the complete dynamic recrystallization temperatures of Mg-Ni-Y alloys with LPSO content of 1 0%,20%and 30%were 300?,330? and 360? respectively.After recrystallization,the mechanical properties of Mg95.8Ni1.5Y2.7 alloy with LPSO content of 30%:yield strength ?s=243 MPa,tensile strength ?b=347 MPa,elongation ?=14%.