Research On Mg-Y-Zn-Li Alloys With Long Period Stacking Ordered Structures

Magnesium (Mg) alloys are often considered as potential lightweightstructural alloys for aerospace and transportation because of their high specificstrength and low density. However, the poor mechanical strength and ductilityof Mg alloys have long been impediments to their widescale industrial use. Inrecent years, the long period stacking ordered (LPSO) structure in Mg alloyshas attracted increasing attention due to its own unique microstructure andhigh thermodynamic stability. The LPSO-containing alloys exhibit excellentmechanical properties at room and elevated temperatures.In this paper, Mg-Y-Zn-Li alloys with LPSO structures were developedby conventional cast method. The effect of Li addition, heat treatment andZn/Y ratio on the formation and transformation of LPSO structures as well asthe relationship between microstructures and mechanical properties wereinvestigated systematically. The results demonstrated that:(1) In as-cast Mg94-xY4Zn2Lixalloys, the addition of minor amounts of Liwas effective for the formation of LPSO phases and refinement of grain size. However, the excessive Li addition inhibited the formation of LPSO phases,promoted the formation of (Mg,Zn)24Y5eutectic phases and resulted in thegrain coarsening. When the content of Li was5at.%, the microstructure ofas-cast alloy consisted of α-Mg matrix,18R-LPSO phase and minor(Mg,Zn)24Y5eutectic phase. The alloy showed the finest grain size and bestmechanical properties (Brinell hardness=76.5HB, UTS=189MPa,Elongation=3%).(2) The solid solution time, temperatures and cooling ways can havesignificant effects on microstructure of Mg89Y4Zn2Li5alloy. Thefurnace-cooling was beneficial to the formation of14H-LPSO phase, whilewater-cooling inhibited the formation of14H-LPSO phase.14H-LPSO phaseformed directly through a precipitated behavior in Li-containing alloys, but nottransformed from18R-LPSO phase.(3) After aging treated at200oC for30h, the furnace-coolingMg89Y4Zn2Li5alloy showed the better mechanical properties (UTS=227MPa,Elongation=9.9%) due to the formation of14H-LPSO phase and β＇phase.(4) In Mg-Y-Zn-Li alloys, the variable Zn/Y ratio had great effects onmicrostructures and mechanical properties. When the mole ratio of Zn to Ywas about2, the as-cast Mg92Y1Zn2Li5alloy was mainly composed of α-Mgmatrix and W phase. The alloy exhibited the coarse grain size which degradedthe mechanical properties. When the mole ratio of Zn to Y was about1, theas-cast Mg91Y2Zn2Li5alloy mainly consisted of α-Mg matrix,18R-LPSO phase and W phase. The alloy exhibited the fine grain size which contributedto the optimum combination of mechanical properties (Brinell hardness=67.5HB, UTS=182MPa, Elongation=9.8%).(5) After solution treated at500oC for40h, the secondary phases inMg92Y1Zn2Li5and Mg91Y2Zn2Li5alloys were completely dissolved. Massivespherical MgYZn2phases were dispersively distributed in both of the alloys.However, the spherical MgYZn2phases revealed a more dispersivedistribution in Mg91Y2Zn2Li5alloy. Besides, the fine plate-like14H-LPSOstructure was generated in the alloy. With the effect of14H-LPSO phase andspherical MgYZn2phase, the Mg91Y2Zn2Li5alloy exhibited optimalmechanical properties (Brinell hardness=70.2HB, UTS=206MPa,Elongation=16.9%).