| Magnesium alloy has the advantages of low density,high specific strength,good damping and excellent processing performance.It is the most important lightweight structural material in the 21st century,and has been widely used in light weight and other aspects of automobiles.However,the low strength of pure magnesium,poor plasticity,limiting its application,and alloying can effectively improve the performance of magnesium-based alloys.There are many kinds of ternary compounds in Mg-Zn-Y system,which provide more possibilities for the design of Mg-based alloys,now the ternary compounds and their relationship with the magnesium-based solid solution has not been systematically studied.There are few researches on the dynamic mechanical properties of magnesium alloys at high strain rates,but the actual service life of magnesium alloys under dynamic loading is very important.Therefore,we designed the alloys with different molar ratios of Y/Zn.The phase equilibrium relationship between Mg-based solid solutions and ternary compounds was studied by means of optical microscope,scanning electron microscopy,X-ray diffraction and transmission electron microscopy.The isothermal cross section of Mg-Zn-Y ternary system at 335? was established.At the same time,based on the research of phase equilibrium,three alloys of Zn1.4Y0.6,Zn2.8Y0.8 and Zn2.8Gd0.8 were designed,and the effects of different compound types and different rare earth elements on the microstructure and the mechanical properties at high strain rates(100,267,417,667/s)were studied,providing a theoretical basis for the design of Mg-Zn-RE alloy composition and the application under high strain rates.(1)When the Mg-Zn-Y ternary system equilibrates at 335?,the ternary compounds coexisting in the Mg-rich and Mg-based solid solutions are X phase,W phase and H phase and ? phase with the decrease of Y/Zn ratio.When Y/Zn is greater than 1.191,long-period X phase begins to appear;0.543<Y/Zn<1.191,the alloy is in ?-Mg+W+X three-phase equilibrium region;0.477<Y/Zn<0.543,a single strengthening phase W phase;0.239<Y/Zn<0.477,in the three-phase equilibrium region of ?-Mg+H+W;0.16<Y/Zn<0.236,there is an a-Mg+I+H three-phase equilibrium zone.(2)In the extrusion process with extrusion temperature of 300? and extrusion ratio of 16,the strengthening phases of Zn1.4Y0.6 alloy are W and H phases distributed along the extruded direction;the strengthening phase of Zn2.8Y0.8 alloy are the I,Z phase;the strengthening phase of Zn2.8Gd0.8 alloy are the single W phase.(3)After extrusion,Zn1.4Y0.6 and Zn2.8Y0.8 alloys occurred incomplete dynamic recrystallization,the recrystallization degree of Zn1.4Y0.6 alloy was obviously higher than that of Zn2.8Y0.8 alloy,and the grain size is smaller,the average grain size is 3.1?m,4.21?m.Zn2.8Gd0.8 alloy occurred complete dynamic recrystallization with an average grain size of 3.33?m.(4)As the strain rate increases from 10-3/s to 667/s,the yield strength and tensile strength of the alloy increase,while the elongation of the alloy first decreases and then increases.The yield strength of Zn1.4Y0.6 alloy increased from 221 MPa at 10-3/s to 426MPa at 667/s,the tensile strength increased from 295Mpa to 446Mpa,the elongation at quasi-static state was 14.5%,and then decreased to 417/s of the 5%,elongation rate of 667/s strain rate increased to 9.4%.The yield strength of Zn2.8Y0.8 alloy increased from 132 MPa at 10-3/s to 335 MPa at 667/s,the tensile strength increased from 279 MPa to 389 MPa,the elongation at quasi-static state was 14.1%,and then decreased to 5.8%at 417/s,elongation increased to 16.3%at 667/s.The elongation rate of Zn2.8Gd0.8 alloy increased from 147MPa at 10-3/s to 417MPa at 667/s,tensile strength increased from 267Mpa to 451Mpa,The elongation at static state was 14.5%at the static state,and then decreased to 6.8%at the lowest point of 267/s.The elongation at the strain rate of 667/s was increased to 12%.(5)As the strain rate increased from 10-3/s to 667/s,the yield strength of Zn1.4Y0.6 alloy increased by 92.8%and the tensile strength increased by 51.2%,and the yield strength of Zn2.8Y0.8 alloy increased by 153.8%%,tensile strength increased by 39.4%;Zn2.8Gd0.8 alloy yield strength increased by 183.7%,tensile strength increased by 68.9%.There is a largest increase on alloy yield and tensile strength of Zn2.8Gd0.8. |
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