Study On Thermal Resistance Of Binary Mg-x(Gd?Tb?Dy?Yb) Alloys

In the fields of aerospace,national defense,and military,some devices need to be lightweight,have high strength,and have good heat/thermal resistance.Current magnesium alloys fulfill the first two requirements,but the thermal resistance performance is mediocre.They often need to be coated with a thermal barrier coating to improve their heat resistance.However,thermal barrier coatings not only fail to meet the requirement of light weight but also has problems related to cracking,falling off,and high cost.Therefore,magnesium alloys with excellent intrinsic heat resistance should be developed urgently.In this paper,four rare-earth elements,namely,Gd,Tb,Dy,and Yb,were selected as additive elements to the alloy,and four groups of binary alloys were designed,namely,Mg-x(Gd,Tb,Dy)(x=0.5,1.4,2.3 at.%)and Mg-xYb(x=0.1,0.5,0.9 at.%).With the use of a metallographic microscope,X-ray diffractometer,scanning electron microscope,transmission electron microscope,and laser flasher,the effects of alloy composition,solution treatment,and aging treatment on the microstructure and thermal resistance of the alloy were studied systematically.Moreover,the heat resistance mechanism of the alloy was explored,thus providing data support and theoretical reference for the design and development of high-thermal-resistance magnesium alloys.The Mg-xRE alloy is composed of a-Mg and the corresponding second phases(Mg5Gd,Mg3Tb,Mg3Dy,and Mg2Yb).With an increase in rare-earth-element content,the volume fraction of the second phase in the four groups of alloys increased,the second phase changed from being point-like to being a semi-continuous network or block,and the continuity gradually increased.With the increase in the volume fraction of the second phase and the enhancement of the continuity,the lattice distortion of the matrix increased,and the thermal resistance of the alloy improved.The thermal resistivity of the as-cast alloy with the same composition was as follows:Mg-Dy>Mg-Tb>Mg-Gd>Mg-Yb.After the solution treatment,the second phase in the alloy decreased noticeably,most of the rare-earth atoms dissolved into the magnesium matrix,and the degree of lattice distortion of the matrix increased greatly.With the increase in rare-earth element content,the number of rare-earth atoms entering the matrix increased,and the degree of lattice distortion of the matrix increased gradually.For alloys with the same content,the thermal resistance of the solid-solution alloy was greater than that of the as-cast alloy.The different rare-earth elements had varied effects on the thermal resistance of the alloy.The thermal resistance efficiency of the four rare-earth elements was as follows:Gd?Tb>Dy>Yb.After the aging treatment,an ellipsoidal phase was precipitated in the Mg-2.3Gd and Mg-2.3Tb alloys,a small amount of ?' phase was precipitated in the Mg-2.3Dy alloy,and a rod-shaped Mg2Yb phase was precipitated in the Mg-0.9Yb alloy.The results showed that the thermal resistance of the alloy decreased evidently after the aging treatment compared with that of the solid-solution state.Hence,the promotion effect of phase precipitation on the thermal resistance of the alloy was less than the inhibition effect of the lattice distortion reduction.Compared with the thermal resistivity of the as-cast and solid-solution alloys,the thermal resistivities of the Mg-2.3Dy alloy were as follows:solid-solution state>aged state>as-cast state;the thermal resistivities of the three other groups of alloys were as follows:solid-solution state>as-cast state>aged state.The thermal resistivities of the four groups of aged alloys were as follows:Mg-Dy>Mg-Tb>Mg-Gd>Mg-Yb.