The Research On Microstructures,Mechanical Properties,Creep Properties And Deformation Behavior Of Mg-Sm Multi-component Alloys

As new light metal structure materials,magnesium alloys have been widely used in automobile,aerospace,and electronic industries.However,the poor heat resistance of magnesium alloys has limited their application.It is an effective method to improve the heat resistance of magnesium alloys to add rare earth elements to them.The price of samarium(Sm)is relatively low among all the rare earth elements,and the maximum solubility in Mg is 5.8wt.%.Besides,the reports about Mg-Sm alloy are relatively inadequate among Mg-RE alloys.Therefore,several kinds of multicomponent Mg-Sm alloys are designed and investigated in this paper to obtain low-cost and medium high heat-resistant Mg-RE alloys.This article can be divided into three parts:Part I: The EZ33(Mg-3.5wt.%Nd-3.0wt.%Zn-1wt.%Zr)alloy developed by the US is a mature commercial Mg alloy.However,the Nd element is expensive,which limits its application.In addition,Zr element is always added to Mg-RE alloys to refine the grain.However,its high price and low utilization rate is another factor to limit the application of Mg-RE alloys.Former researches show that Al2 RE phase can form when Al is added into Mg-RE alloy.Al2 RE phase has a good coherent relationship with Mg matrix,and the atomic spacing misfit and latticemisfit are low.This phase is an effective heterogeneous nucleation core during solidification process of Mg alloy,which is a promising grain refiner for Mg alloys.Therefore,Mg-4Sm-Al-0.3Mn-x Zn(x=0,1,2,3,4 wt.%)alloys have been designed with substituting Sm,Al,Mn for Nd,Zr in EZ33 alloy.The microstructures and mechanical properties of the alloys are investigated.The as-cast Mg-4Sm-Al-0.3Mn-Zn alloy has the best mechanical properties,the tensile yield strength is 106 MPa at room temperature,which is only slightly lower than that of the EZ33 alloy at T5 state(110MPa).The as-extruded Mg-4Sm-Al-0.3Mn-2Zn alloy has the best mechanical properties,its yield strength(YS),ultimate tensile strength(UTS)and elongation(EL)are 269 MPa,298MPa and 16.7%,respectively.A novel and unreported Mg-Sm-Al ternary phase has been found in the Mg-4Sm-Al system in this paper.The precipitates are basal plates with 1~10?m long and 20~50nm wide,and have a hexagonal structure with a=0.556 nm.The orientation relationship between the new phase and matrix is [01-10]Mg-Sm-Al||[-2110]Mg,(0001)Mg-Sm-Al||(0001)Mg.More interestingly,after solution treatment at 783 K for 8h,the number density of the ternary phase increases obviously.In the Mg-Sm-Al ternary alloy,when the atomic ratio of Al and Sm ranges from 1.4 to 1.9,the plate-like phase might form in the as-cast Mg-Sm-Al alloy.The range may be larger than 1.4~1.9,but it must be less than 0.93~2.8.The crystal structure of Mg-Sm-Al ternary phase remained unchanged after 510°C/ 8 h solution treatment and 200°C/64 h isothermal aging.It is an effective method to strengthen Mg alloys to add light rare earth element and heavy rare earth element to Mg alloys simultaneously.However,the reserves of heavy rare earth elements are low and their price is high.Therefore,in this paper,1wt.%Gd elements are added to Mg-4Sm-Al-0.3Mn-Zn alloy.The effect of a small quantity of heavy rare earth elements addition on the microstructure and properties of the alloy was discussed.The results of research reveal that 1wt.%Gd element addition can significantly improve value of the peak-aged hardness of Mg-4Sm-Al-0.3Mn-Zn alloy,as well as the mechanical properties of the as-cast and as-extruded Mg-4Sm-Al-0.3Mn-Zn alloys.Part II: Our team have investigated the microstructures and mechanical properties of Mg-4Sm-0.5Zr-x Zn(x=0,1,2,3,4wt.%)alloys to replace the expensive EZ33 alloy before.The results show that as-extruded Mg-4Sm-4Zn-0.5Zr alloy has the best mechanical properties.On this basis,a small quantity of Gd element(0.5wt.%Gd,1.0 wt.%Gd and 1.5wt.%Gd)was added to Mg-4Sm-4Zn-0.5Zr alloy.And it has been investigated that the effect of a small quantity of Gd element addition on the microstructure and mechanical properties of as-cast and as-extruded Mg-4Sm-4Zn-0.5Zr alloys.The experimental results reveal that:(i)the addition of Gd can effectively refine the grains of as-cast alloy.With the increase of Gd content,the grain sizes of the alloys are 39?m,33?m and 29?m respectively.(ii)Both as-cast and as-extruded Mg-4Sm-4Zn-0.5Zr-1.5Gd alloy exhibit the best mechanical properties.The yield strength of as-cast Mg-4Sm-4Zn-0.5Zr-1.5Gd alloy is 176 MPa at room temperature,which is obviously higher than that of EZ33-T5 alloy(110MPa).The yield strength,ultimate tensile strength and elongation of as-extruded Mg-4Sm-4Zn-0.5Zr-1.5Gd alloy at room temperature are 255 MPa,334MPa and 23.2% respectively.The thermal isothermal compression experiment of Mg-4Sm-4Zn-0.5Zr-1.5Gd alloy was carried out with condition of 350°C~450°C/0.001s-1~1s-1?The rheological stress-strain curve of the alloy was obtained.The constitutive equation was obtained,as shown below:(?)where,(?)?Part III: Mg-4.3Sm-3.6Nd-Zr and Mg-3.9Sm-3Nd-1.2Gd-Zn-Zr alloys have been investigated by our team before whose total rare earth content is close to each other.The latter exhibits good heat resistance after solution treatment at 525 oC for 8h and aging at 200 oC for 16 h.The alloy has an ultimate tensile strength of 213 MPa and elongation of 8.6% at 300 oC.In the previous work,the heat resistance mechanism of the alloy was not been deeply studied.Therefore,the heat resistance mechanism of Mg-3.9Sm-3Nd-1.2Gd-Zn-Zr alloy was systematically studied and discussed in this paper.The creep property of peak-aged Mg-3.9Sm-3Nd-1.2Gd-Zn-Zr alloy was tested and analyzed.On the basis of this heat resistance mechanism,the Mg-4Sm-4Gd-Zn-Zr alloy with equal rare earth content was designed,and the microstructure and mechanical properties of Mg-4Sm-4Gd-Zn-Zr alloy were studied to verify the universal applicability of the heat resistance mechanism.The results show that there are three types of “precipitates” in peak-aged Mg-3.9Sm-3Nd-1.2Gd-Zn-Zr alloy:(i)basal precipitate,? phase(FCC,a=0.72nm);(ii)prismatic precipitates,?? phase(orthorhombic structure,a=0.64 nm,b=3.334 nm,c=0.52nm);(iii)Zn Zr phase,some of them neither parallel to nor perpendicular to c axis of Mg matrix,is special “pyramidal precipitate”.The good heat resistance of peak-aged Mg-3.9Sm-3Nd-1.2Gd-Zn-Zr alloy is attributed to ? series phase,? phase and Zr Zn phase hindering basal and non-basal slip at elevated temperature simultaneously.The tensile creep behavior of peak-aged Mg-3.9Sm-3Nd-1.2Gd-Zn-Zr alloy was studied.The steady creep rate of the alloy at 200 °C/60 MPa is 1.134E-10s-1,which is slightly lower than that of the WE43(T6)alloy.The result indicates that the creep resistance of the alloy is comparable to that of commercial WE43 alloy.When the creep tests of peak-aged Mg-3.9Sm-3Nd-1.2Gd-Zn-Zr alloy were carried out under condition of 175oC/60~100MPa,the creep stress exponent was 2.7,and the dominant creep mechanism should be grain boundary slip.When the creep tests were carried out under condition of 200-225oC/60-100 MPa,the creep stress exponent was between 3~7,and the dominant creep mechanism should be dislocation slip.Peak-aged Mg-4Sm-4Gd-Zn-Zr alloy exhibits good mechanical properties at 300 oC,The yield strength of peak-aged Mg-4Sm-4Gd-Zn-Zr alloy is higher than that of peak-aged Mg-3.9Sm-3Nd-1.2Gd-Zn-Zr alloy.There are also three types of precipitates in peak-aged Mg-3.9Sm-3Nd-1.2Gd-Zn-Zr alloy: basal precipitates,prismatic precipitates and Zn Zr phase.Therefore,the heat resistance mechanism proposed in this paper is proved to have applicability to some extent,and it is worth further study and popularization.