Study On The Formation Of Ternary Compound Phase In Mg-Zn-Gd System And Its Alloy Properties

Magnesium alloy is widely used because of its high specific strength,specific stiffness,excellent damping performance and electromagnetic shielding performance,good processing performance and easy recovery.At the same time,the disadvantages of magnesium alloy,such as low plasticity and poor corrosion resistance,greatly limit its more extensive development space and application potential.It is found that the properties of magnesium alloy can be improved by adding rare earth elements into the alloy,so in recent years,rare earth magnesium alloy has become a hot spot in the field of magnesium alloy research.At present,the research of Mg-Zn-Gd alloy is still in the initial stage,and the composition and evolution of phases in the alloy are not clear.Therefore,in this paper,Mg-Zn-Gd system alloy as the main research object,to explore the evolution of phases and alloy properties.In this paper,a series of Mg-Zn-Gd alloys with different experimental conditions and process parameters were prepared by conventional casting method.The effects of alloy composition,melt holding time and solidification speed on the microstructure and composition of Mg-Zn-Gd alloy were studied by various analysis and test methods.The evolution rule of ternary rod-like phase and the rod-like phase in the alloy were mainly studied and analyzed The thermal stability of Mg-Zn-Gd alloy.The results show that:The microstructure of Mg-Zn-Gd alloy with different content of rare earth Gd is different.There are?-Mg phase and Mg₇Zn₃ phase in all three alloys.In addition,there are petal quasicrystal phase in Mg₆₇Zn₃₀Gd₃ alloy,dispersed point gdzn phase and rod-shaped phase in Mg66Zn30Gd4 alloy,and point gdzn phase and irregular shape phase in Mg₆₅Zn₃₀Gd₅ alloy.It is found that the rod-shaped phase of Mg₆₆Zn₃₀Gd₄alloy contains Mg,Zn and Gd,and the content of each element is similar to that of quasicrystal phase.The diffraction spots of rod-shaped phase are complex and do not have quasicrystal diffraction characteristics.Based on the study of the evolution law of the symmetrical rod-shaped phase in Mg₆₆Zn₃₀Gd₄ alloy,it is found that there are two ways of the evolution of the rod-shaped phase:the first is that the lamellar eutectic structure is formed in the melt of the alloy,and along the middle linear?-Mg phase,the two parts are divided into two symmetrical parts,and then the two parts of the dendrite phase are respectively further evolved from the dendrite phase to the rod-shaped phase,and finally the symmetrical distribution is formed The second is to form lamellar eutectic structure in the alloy melt.The eutectic structure is also divided into two symmetrical parts with the intermediate linear?-Mg phase as the boundary line.Then the eutectic structure as a whole evolves into a thick rod-shaped phase structure,and then from both ends of the rod-shaped phase to the internal continuous depression,and finally split into two parts to form the rod-shaped phase with symmetrical distribution.The holding time,composition and solidification speed of the melt all have an important influence on the microstructure of Mg-Zn-Gd alloy,especially the formation of the rod-shaped phase.In the study of the properties of Mg_70-xZn30Gdx_?x=3,4,5?,it is found that the thermal stability of the rod-like phase is higher than that of the quasicrystal phase,and the temperature coefficient of the negative resistivity appears during the heating process of the three-dimensional rod-like phase,so it is speculated that the atomic spatial arrangement structure of the three-dimensional rod-like phase is similar to the quasiperiodic structure or metastable structure.The corrosion resistance of the ternary rod-shaped phase is the best,so the higher the volume fraction of the ternary rod-shaped phase is,the better the corrosion resistance of the alloy is.The hardness and wear resistance of Mg₆₇Zn₃₀Gd₃ alloy are the best because the quasicrystal phase plays the role of particle reinforced phase.The hardness and wear resistance of Mg₆₆Zn₃₀Gd₄ alloy are slightly worse than that of quasicrystal alloy.There is no regular particle reinforced phase in Mg₆₅Zn₃₀Gd₅ alloy,so the mechanical properties of the alloy are the worst among them.The analysis of the alloy composition and thermal stability shows that the rod-shaped phase is close to the quasicrystal phase.The negative temperature coefficient of the resistivity temperature curve of the rod-shaped phase indicates that the crystal has a quasi periodic metastable structure,but the diffraction spot image of the rod-shaped phase does not have the quasicrystal characteristics.Therefore,it is speculated that the rod-shaped phase is a"ternary quasi periodic structure"phase.