Study On The Microstructure And Mechanical Properties Of Mg-Gd(-Y-Sm-Zr)heat Resistant Magnesium Alloys

Mg-Gd series alloys have a broad application prospect in the military industry,aerospace and automotive and other fields owing to its excellent ageing strengthening effect and good heat resistance.Heat resistant magnesium rare earth(Mg-RE)alloys of Mg-Gd-Y-Sm-Zr were designed and developed based on high tensile strength at room temperature.Especially,the tensile strength was improved obviously with increasing tensile temperature from room temperature to 250 ?,which existed abnormal temperature effect of tensile strength.The microstructure and mechanical properties of Mg-Gd,Mg-Gd-Sm-Zr and Mg-Gd-Y-Sm-Zr series alloys were systematically investigated by optical microscope,X-ray diffraction,scanning electron microscope with energy spectrum,transmission electron microscope,tensile and creep tests.On this basis,the law of the abnormal temperature effect of tensile strength of different series alloys was studied,and the micro-mechanism of the abnormal temperature effect was deeply discussed.The results showed that:The mechanical properties of aged Mg-12 Gd alloy was best and began to appear abnormal temperature effect of tensile strength by research of Mg-(5-20)Gd binary alloys,which was composed by the matrix ?-Mg and base-centred orthorhombic ?' phase.The abnormal temperature effect of tensile strength of the alloys was more obviously appeared when Gd content was higher.The aged Mg-(6-15)Gd-1Sm-0.5Zr alloys mainly consists of matrix ?-Mg and granular ?' phase through the study of effect of Gd addition on the microstructures and mechanical properties of the alloys.The alloy grain size decreased first and then increased as ?' phase gradually increased,and the alloy grain size will be smaller when Gd is 9 %,tensile strength and yield strength improved to some extent from room temperature to 300 ? as the Gd content increased,and elongation were reduced under 200 ?,increased higher than 200 ? with the increase of Gd content.The tensile strength increased with the tensile temperature rising,and had obvious abnormal temperature effect when Gd content was more than 9 wt.%.The research results of effect of Y addition on the microstructures and mechanical properties of Mg-12Gd-(0-5)Y-1Sm-0.5Zr alloys showed that the microstructure of aged alloys were mainly composed of the matrix ?-Mg and elliptic spherical ?' phase.The tensile strength of the alloys were higher rised with the increase of stretching temperature from room temperature to 300 ?,and reached their maximum respectively at 250 ?(The tensile strength of Mg-12Gd-3Y-1Sm-0.5Zr alloy is 323 MPa),and the anomalous temperature dependence phenomenon of tensile strength was obvious.The microstructures of Mg-12Gd-(0-5)Y-1Sm-0.5Zr alloys mainly composed of the matrix ?-Mg and elliptic spherical ?' phase.The tensile strength of the alloys gone up with the increase of stretching temperature from room temperature to 300 ?,and reached their maximum respectively at 250 ?(The tensile strength of Mg-12Gd-3Y-1Sm-0.5Zr alloy is 322 MPa),and the abnormal temperature effect of tensile strength was obvious.High temperature XRD experiment analysis results showed that the aged Mg-12Gd-3Y-1Sm-0.5Zr alloy diffraction spectrum and phase structure change insignificantly from room temperature to 300 ?.After further analysis and calculation,the axis ratio(c/a)value of ?-Mg matrix decreases,especially the axis ratio(c/a)of ?-Mg matrix reduced obviously,which was better for the alloy to start the slip plane and coordination in the process of tensile deformation,and it was helpful to improve the strength of high temperature at 200 ? and 250 ?.The in situ transmission electron microscopy(in-situTEM)analysis from room temperature to 300 ? results showed that the partial granular,semi-coherent with matrix and base-centred orthorhombic ?' phase in the aged Mg-Gd-Y-Sm-Zr series alloy changed dispersively distributed,size of a few nanometers,coherent with matrix and face centered cubic ?1 phase in the process of in-situ heated,?1 phase was coherent with matrix,and the heat resistance was more stronger when the phase form more round and the size more smaller,which can effectively improve alloy high temperature mechanical properties at 200 ? and 250 ?.The ?1 phase grew up,and then evolution to be face centered cubic ? phase with 200~300 nm and that were larger in size and didn't share the lattice with the matrix with the further increase of heating temperature,which influence performance of the alloy,and the tensile strength decreased.This was iconsidered as one of the important reasons to generate the abnormal temperature effect of tensile strength.TEM analysis showed that 18R-type long-period stacking order structure(LPSO)and ?1 phase appeared of aged Mg-12Gd-2Y-1Sm-0.5Zr alloy after high temperature tensile of 200 ? and 250 ? based on the high temperature tensile test specimen restored to room temperature.The LPSO structure had excellent thermal stability and plastic deformation ability,and the high temperature tensile strength increased.If the temperature were further improved(300 ?),the alloy tensile strength decreased due to the matrix to soften and the strengthening phase decreased.This was iconsidered as another important reason to generate the abnormal temperature effect of tensile strength.Aged Mg-12Gd-3Y-1Sm-0.5Zr alloy has excellent creep resistance at(200 ?,250 ?,300 ?)/(50 MPa,70 MPa,90 MPa).Especially,its creep performance was better than that of WE43 magnesium alloy.The creep mechanism of Mg-12Gd-3Y-1Sm-0.5Zr alloy was mainly dislocation glide and grain boundary sliding mechanism.