Basic Research About Controlling Of Microstructure And Mechanical Properties For Mg-3Y-2Zn-0.6Zr-Gd/Sm Magnesium Alloys

Mg-Y-Zn-Zr based magnesium alloys are thought as a potential alloys and have very wide applications in automotive,aerospace and other industries.However,the mechanical properties of the current Mg-Y-Zn-Zr based magnesium alloys do not meet the requirement of high-strength magnesium alloys.Therefore,it is very necessary to further research and develop Mg-Y-Zn-Zr based magnesium alloys.It is well known that alloying and/or micro-alloying which are thought as an important method to improve the mechanical properties at room and elevated temperatures for magnesium alloys,has successfully used to develop magnesium alloys with high-performance.At present,among the various alloying and/or micro-alloying elements for the modification of magnesium alloys with high-performance,rare earth elements Gd and Sm have received much attention due to their high precipitation strengthening effects,obvious improvement in the properties and so on.However,up to now,the investigations about the modification of Mg-Y-Zn-Zr based alloys by using Gd and Sm especially a small amount of Gd and Sm are very limited.Therefore,the basic research about the controlling of the microstructure and mechanical properties for Mg-Y-Zn-Zr based magnesium alloys with Gd and Sm alloying and/or micro-alloying,has very important theoretical significance and practicable value in developing Mg-Y-Zn-Zr based magnesium alloys with high properties and widening their application.Based on the Mg-3Y-2Zn-0.6Zr-Gd/Sm(wt.%)based experimental magnesium alloys designed in this paper,both the effects of Gd and Sm additions and their adding amount on the microstructure and mechanical properties of the Mg-3Y-2Zn-0.6Zr based experimental alloys and the effects of heat treatment on the microstructure and mechanical properties of the Mg-3Y-2Zn-0.6Zr-Gd/Sm experimental alloy,were investigated by using optical microscope(OM),X-Ray diffraction(XRD),scanning electron microscope(SEM),differential scanning calorimetry(DSC)and properties testing,and the following main research results were obtained:1)The as-cast microstructure of the Mg-3Y-2Zn-0.6Zr experimental magnesium alloy is mainly composed of ?-Mg,Mg12YZn(LPSO),particle-like Mg3Y2Zn3 and a small amount of Mg24Y5 phases,thereinto the LPSO phases exhibit the continuous net distribution along grain boundaries.2)After adding 1.0-6.0 wt.%Gd to the Mg-3Y-2Zn-0.6Zr experimental magnesium alloy,the change in the type of alloying phases in the as-cast microstructure of the alloys is not obvious,and since Y in the Mg12 YZn phase are partial replaced by Gd,the Mg12 YZn phase mainly exists in the form of the Mg12(Y,Gd)Zn intermetallic compound.At the same time,after 1.0-6.0 wt.%Gd additions to the Mg-3Y-2Zn-0.6Zr alloy,the microstructure of the alloys is refined,and the secondary phases in the alloys exhibit the continuous and quasi-continuous distribution along grain boundaries and within grains and their amount increases,which becomes more obvious along with Gd adding amount.In addition,the effects of Gd addition on the as-cast tensile properties at room temperature(RT)for the Mg-3Y-2Zn-0.6Zr alloy are closely related to Gd adding amount.For example,1.0wt.%Gd addition to the Mg-3Y-2Zn-0.6Zr alloy causes the tensile properties especially elongation to be obviously improved.However,after adding 2.0-6.0 wt.%Gd to the Mg-3Y-2Zn-0.6Zr alloy,the tensile properties of the alloys decrease.3)Heat treatment has a certain degree of effects on the tensile properties at RT for the Mg-3Y-2Zn-0.6Zr based magnesium alloys.After treated by T4(500?×24h+water quenching),the tensile properties for the alloys with 1-2 wt.% Gd additions and without Gd addition decrease,and a slight increase in the tensile properties for the alloys with 4-6 wt.%Gd additions is found.In adition,after treated by T6(500 ? × 24h+water quenching+225?×12h+air cooling)for the alloys with and without Gd addition,the tensile properties of the alloys with 2-4 wt.%Gd additions and without Gd addition increase but the tensile properties of the alloys with 1 wt.% and 6 wt.%Gd additions decrease.4)After 1.0-2.0 wt.%Sm additions to the Mg-3Y-2Zn-0.6Zr experimental magnesium alloy,the change in the type of alloying phases in the as-cast microstructure of the alloy is not obvious,and since Y in the Mg12 YZn phase are partial replaced by Sm,the Mg12 YZn phase mainly exists in the form of the Mg12(Y,Sm)Zn intermetallic compound.At the same time,after 1.0-2.0 wt.%Sm additions to the Mg-3Y-2Zn-0.6Zr alloy,the microstructure of the alloys is refined and the amount of the secondary phases in the alloys increases,which is very obvious for the alloy with 2 wt.%Sm addition.Although the microstructures of the Sm-containing Mg-3Y-2Zn-0.6Zr alloys are rfined,their tensile properties at RT decrease.In addition,heat treatment has great influence on the tensile properties at RT for the Mg-3Y-2Zn-0.6Zr experimental magnesium alloys with and without Sm addition.After treated by T4(500?×24h+water quenching),the tensile properties of the alloys with and without Sm addition decrease.However,after treated by T6(500?×24h+water quenching+225?×12h+air cooling),the tensile properties of the alloys with and without Sm addition increase,especially the T6-treated alloy with 1 wt.%Sm addition,whose tensile and yield strengthes and elongation at RT respectively reach 224 MPa,120 MPa and 10.9%,obtains the relativly high tensile properties.