Composition Design And Optimization Of Heat Treatment Process For Ultra High Strength Mg-Gd-Y-Zn-Zr Alloys

As the lightest metal structural materials,Magnesium alloys exhibit many merits including pollution-free,non-toxic,recycling and good damping,are more and more extensively applied in automotive and aerospace industries.However,applications of magnesium alloys are still limited due to the poor strength.To overcome this shortcoming,Mg-Gd-Y-Zn-Zr alloys with high rare earth content were prepared by water cooling casting,and were strengthed by homogenization treatment,extrusion and aging treatment.The effects of microstructures and mechanical properties by different solution processes,different extrusion temperature,differernt Gd,Y,Zn content were analyzed to obtain better comprehensive mechanical properties.LPSO structure can effectively improve the strength and the elongation of the rare earth magnesium alloys.At the same time,the LPSO structure was the main residual second phase after the homogenization treated of Mg-Gd-Y-Zn-Zr alloy.The rare earth magnesium alloy will precipitate a large number of nanoscale ?' phases during aging hardening.The ?' phase has a better strengthening effect than the LPSO structure but will lead to the poor plasticity.The precipitation of ?' phase can be controlled by controlling the LPSO structure content The LPSO structure will occupy rare earth elements,which will reduce the aging precipitation of ? ' phase.The effects of Gd/Y ratios in the Mg-Gd-Y-Zn-Zr(Gd+Y=15wt.%)alloys were studied,the increase of Y/Gd ratios caused the incease the total amount of the second phase while the 14H-LPSO structure after the homogenization treated also increased.Therefore,when the content of Y element is small,the increase of the second phase increases the strength of the alloy.When the content of Y element is too high,too much LPSO structure will greatly reduce the precipitation of the ?' phase.Even if the total amount of the second phase increases,the strength of the alloy is still decreasing.In the Mg-Gd-Y-Zn-Zr(Gd+Y=15wt.%)alloys,Gd/Y ratio has the best performance at about 2.Once Y/Gd is greater than 1,the comprehensive performance will be poor.The peak aged Mg-9.7Gd-5.8Y-1.6Zn-0.3Zr alloy has the excellent UTS,YTS and elongation,i.e.549 MPa,485MPa and 8.1%,respectively.The effects of the Zn content in the Mg-10Gd-5Y-x Zn-0.4Zr alloys were studied,the increase of Zn content caused the incease the total amount of the second phase while the 14H-LPSO structure also increased rapidly.when the Zn content increases to 4.8 wt.At %,almost no ?' phase was precipitated.Adding Gd or Y to the Mg-Gd-Y-Zn-Zr(Gd+Y=15wt.%)alloy alone,it is found that increase of Gd is more effective than increase of Y,because Y element contributes to the formation of LPSO structure,which decreases the precipitation of ?' phase.Compared with the first stage homogenization,the residual 14H-LPSO structure is less after the secondary homogenization treatment,and more ?' phases can be precipitated during the aging at 200°C.The total amount of the second phase did not change,the aging strengthening effect of the alloy was strengthened,and the yield strength of the peak aged alloy increased and the elongation decreased.The peak aged Mg-9.7Gd-5.8Y-1.6Zn-0.3Zr alloy has a tensile strength of 574 MPa,a yield of 516 MPa and an elongation of 4.4%,which is higher than that of the first stage homogenization.