The Influence Of LPSO On The Microstructure And Mechanical Properties Of High Rare Earth Magnesium Alloys

Magnesium alloys are playing an increasingly essential role in the field of automobile and aerospace application where exist the pressing demand of lightweight structures to improve fuel efficiency.It is well known that Mg-RE(Rare Earth)alloys show excellent mechanical properties,which have been attracted much attention from researchers.The effect of Zn additions on microstructures evolution and mechanical properties of Four Mg-10.5Gd-5Y-x Zn-0.5Zr(wt.%)(x=0,0.65,1.2,1.8)alloys remains obscure and need further investigation,especially for the effect of LPSO phases and ? phases on microstructure regulation and mechanical properties improvement.The microstructure of as-cast Mg-10.5Gd-5Y-xZn-0.5Zr alloys show that metastable lamellar LPSO phases increase with an increase of Zn addition.The study of microstructures of Mg-10.5Gd-5Y-xZn-0.5Zr alloy heated at 500-520 degrees for 6-24 hours shows that the optimum homogenization condition is 520 ? and 12 h.The study shows that Zn content has an important influence on microstructure evolution of as-extruded alloys.When Zn addition is 0.65 wt.%,a large number of fine lamellar 14H-LPSO phase appear in the equiaxed grains.When Zn addition is more than 1.2 wt.%,the volume fraction of deformed grains and block-shaped 18R-LPSO phases increase with the increase of Zn content.Meanwhile,the influence of precipitates on microstructure evolution change into the LPSO phase inducing by dynamic recrystallization grain nucleation from ? phases inducing by dynamic recrystallization grain nucleation.The study revealed the microstructure characteristics of the optimum strength.When extruded at 400 degrees,for Zn=1.8 wt.%,the highest volume fraction of block-shaped LPSO phases and deformed grains could improve the strength but also reduce the elongation;when Zn=1.2 wt.%,it has the highest tensile strength,because it has many LPSO phases that can contribute to dynamic recrystallization nucleation and then reduce grain size.We make a comparison among the mechanical properties of as-extruded alloys with different extrusion temperature and the results showed that: when Zn content is less than or equal to 1.2 wt.%,the yield strength,tensile strength and elongation of as-extruded alloys with the extrusion temperature of 400 degrees are higher than that of 470 degrees.But when Zn=1.8 wt.%,the results are reversed.While the extrusion temperature is 470 degrees,the contribution effect of the ? phase inducing dynamic recrystallization nucleation decreases,due to the block-shaped LPSO phases with better thermal stability and still play an important role in the dynamic recrystallization nucleation.When Zn=1.8 wt.%,there appear obvious yield platform in the tensile curves of as-extruded alloys with the extrusion temperatures of 400 degree and 470 degree.This paper believes that the presence of LPSO phases increase the critical shear stress,activating non basal slip and grain refinement,which contribute to the formation of the yield platform.By studying the thermodynamic simulation compression behavior of the alloys with Zn=0 and Zn=1.8 wt.% under different conditions,it is found that the peak stress increases with the decrease of deformation temperatures and the increase of strain rates.Besides,the alloy tends to appear the flow instability at a lower deformation temperature and a higher strain rate.By comparing the microstructure of the alloy after compression deformation,it is found that the higher deformation temperature and the lower the deformation rate contribute to dynamic recrystallization.