Study On Phase Equilibra And Hot Extrusion Microstructures Of Mg-Zn-Y-Ca Alloys

Previous investigations indicated that Mg-Zn-Y based alloys exhibit good mechanical properties at elevated temperatures as well as room temperature.Due to the fact that Ca addition to Mg-Zn-Y ternary alloys can enhance the strength,Mg-Zn-Y-Ca alloys has great potential in the high-temperature wrought magnesium alloys.In this paper,the as-cast microstructures in the Mg-Zn-Y-Ca alloys with different Zn/Y atomic ratios have been studied by SEM-EDS,XRD,DSC.Phase equilibra and phase transformations under 450 and 500? have been investigated.Based on this,the extruded Mg-Zn-Y-Ca alloy compostions have been designed.The effects of extrusion temperatures on the microstructures and mechanical properties of the extruded Mg alloys were studied.The as-cast Mg-Zn-Y-Ca alloys mainly consist of W-Mg3Y2Zn3,X-Mg12ZnY,I-Mg3YZn6 and T1-Ca2Mg6Zn3 ternary intermetallic compounds and Mg2Ca.The precipitates change in the order of I+T1+Mg2Ca,W+T1+Mg2Ca,X+W+T1+Mg2Ca and X+T1+Mg2Ca with the increase of Y/Zn ratio.Among the five kinds of strengthening phases above,the T1 phase is stable up to a temperature lower than 410?.With the temperature increase,Mg2Ca phase melts at about 465?.X phase,the long period ordered phase,and the W phase are stable at both 450 and 500?.The isothermal diagram at 450? has been established.There are one four-phase equilibrium of ?-Mg+X+W+Mg2Ca in the Mg-Zn-Y-Ca alloys;two three-phase areas,which are ?-Mg+W+liquid,?-Mg+Mg2Ca+W.The isothermal diagram at 500 ? has also been established.There is a four-phase reaction at 495 ?:liquid+X phase??-Mg+W phase.On the basis of the research work above,Mg97.5Zn1.0Y1.0Ca0.5 and Mg98Zn1.0Y0.5Ca0.5 alloys were designed for hot extrusion experiments at 300,330,360 and 390?,respectively.It is found that the strengthening phases of the Mg97.5Zn1.0Y1.0Ca0.5 alloy are X and Mg2Ca,and the strengthening phases of the Mg98Zn1.0Y0.5Ca0.5 alloy are W and Mg2Ca.With the increase of extrusion temperature,the grain size first decreases and then increases,and the grain size is smallest at 330 ?.Mg97.5Zn1YiCa0.5 and Mg98Zn1Y0.5Ca0.5 alloys did not undergo complete dynamic recrystallization at 300?,and there were still some coarse primary grains existed in the extruded state.The complete dynamic recrystallization occurred at 330?.The extruded structure consisted of a large number of equiaxed grains with the grain size about 4.55 and 5.88 ?m,respectively.When the extrusion temperature continues to rise to 360,390?,the recrystallized grains become coarsen and grow up.The mechanical properties of the extruded alloys were investigated.The results show that the magnesium alloy rod prepared at 330? has good mechanical properties.The yield strength of MgZn1Y1Ca0.5 alloy reaches to 283 MPa and the tensile strength reaches 316 MPa.The yield strength of MgZn1Y0.5Ca0.5 alloy reaches 211 MPa and the tensile strength reaches 260 MPa.The grain size difference of the two alloys is not large at each extrusion temperature,but the mechanical properties are much different,which indicates that the strengthening effect of X phase is better than that of W phase in Mg-Zn-Y-Ca alloy system.