Effects Of Ho On Microstructure And Properties Of Mg-Y-Zn Alloy

The special demand of material lightweight,Outstanding features,energy saving and environmental protection in the automotive,aircraft manufacturing and other industries makes the rare earth magnesium become an advanced modern metal materials.There is no doult that a new series of rare earth magnesium alloy development will push the application of magnesium alloy materials promotion go ahead.At present,Mg-RE-Zn based alloys with long-period stacking structure?LPSO?is a hot topic in the field of magnesium alloys,because of their excellent mechanical properties.This paper choose Mg-Y-Zn alloys which have been reported widely as the basis and designed Mg-3Y-2Zn alloy.Then the rare earth Ho,which is close to the nature of rare earth Y,was selected as an additive element to study the effect of Ho on the microstructure and properties of Mg-Y-Zn alloy.In this paper,the experimental cast alloy obtained by vacuum induction melting,and through homogenization,hot-rolled process prepared the rolled alloy.At first,the as-cast Mg-xHo-3Y-2Zn?x=0,1.5,3,6?alloys were studied in the experiment,and the results show that as the content of Ho increases,they gradually changed from dendrite to equiaxed crystal.In these alloys,a phase composition included?-Mg matrix phase,X-Mg₁₂Y?Ho?Zn phase,a small amount of W-Mg₃Y?Ho?₂Zn₃ phase and Mg₂₄Y?Ho?₅phase.With increasing Ho content,the volume fraction of Mg₁₂Y?Ho?Zn phase with long period stacking structure?LPSO?in the alloy gradually increased.In addition,LPSO phases in the as-cast Mg-3Y-2Zn and Mg-3Ho-3Y-2Zn alloys were all 6H.What's more,the strength of alloys at room temperature have been promoted with Ho content increased,elongation increased first and then decreased.Mg-3Ho-3Y-2Zn alloy had a better mechanical propertie.The ultimate tensile strength?UTS?,tensile yield strength?TYS?,and elongation for Mg-3Ho-3Y-2Zn alloy were 168.2 MPa,90.4 MPa and 11.0%,respectively.At high temperature,Mg-xHo-3Y-2Zn?x=3,6?had the same strength as that at room temperature when the content of Ho was higher,and the tensile strength at high temperature was better.In as-cast alloys,the corrosion resistance of Mg-6Ho-3Y-2Zn was better,while the corrosion resistance of Mg-3Ho-3Y-2Zn was worse.Through the research on the rolled alloy Mg-xHo-3Y-2Zn?x=0,1.5,3,6?,it is found that the structure type of LPSO phase has been changed in the rolled alloy.Among Mg-3H-3Y-2Zn alloys,there were two types of 18R and 14H LPSO phase at the same time.With Ho content increased,the tensile strength of Mg-xHo-3Y-2Zn firstly increased and then decreased at room temperature,the elongation decreased gradually,while the tensile strength at high temperature gradually increased,but the elongation at break also decreased.In the rolled alloy,the corrosion resistance of Mg-6Ho-3Y-2Zn was better,while the corrosion resistance of Mg-3Ho-3Y-2Zn was worse.In order to further understand the effect of Ho on the microstructure and properties of Mg-Y-Zn alloys,two alloys for Mg-1.5Ho-1.5Y-1Zn and Mg-6Y-2Zn were designed and compared with Mg-3Ho-3Y-2Zn which had good properties.The results show that the as-cast Mg-1.5Ho-1.5Y-1Zn alloy was dominated by dendrite.With the increase of the total content of rare earth elements Ho,Y and Zn in the alloy,the alloy was transformed into equiaxed.In addition,the volume fraction of LPSO phase in Mg-3Ho-3Y-2Zn increased,compared with Mg-1.5Ho-1.5Y-1Zn alloy.It is noted that the as-cast and rolled Mg-3Ho-3Y-2Zn alloys had better room temperature and high-temperature tensile properties than Mg-1.5Ho-1.5Y-1Zn.With 3 wt.%of Y replaced by Ho,LPSO phase content in the alloy decreased.The as-cast and as-rolled Mg-3Ho-3Y-2Zn alloys also had better room-temperature and high-temperature tensile properties than Mg-6Y-2Zn.