Study On Properties Of Rare Earth Reinforced Phase And Mg-Zn-RE Alloy

Lightweight and environmental protection are became the primary requirements of the automotive industry,especially the development of new energy vehicles,which makes the automotive industry pay more attention to automotive structural materials.However,due to the poor corrosion resistance of magnesium alloys,that greatly limits its large-scale application in the industrial field.In recent years,theMg-Zn-RE alloy obtained by adding rare earth elements to theMg-Zn alloy has attracted great interest from researchers.Among them,the light rare earth elements La and Ce are added to theMg-Zn alloy will form a variety of intermetallic compounds.In this paper,by adding light rare earth La and Ce to Mg-4Zn alloy,the mechanical properties and thermodynamic properties of the binary and ternary rare earth phases are explored through microstructure characterization and performance experiments.So as to provide a theoretical basis for the development of a new type of magnesium alloy.In the experiment,Mg-4Zn-2.4RE(RE=Ce,La)-0.5Zr as-cast alloy was prepared,and the as-cast alloy was treated with solution treatment and aging treatment.In the as-cast alloy without adding rare earth elements,there are mainly?-Mg andMg-Zn phases,and the grain size is very coarse,while the grain size after adding La or Ce elements become fine.The optical microstructure shows that the grain refinement effect of Ce element is better than that of La.In addition,the best condition for solution treatment of the as-cast alloy is 460?for 10h,and the best process condition for aging is 200? for 16h.The microstructure of the as-cast alloy under the scanning electron microscope is mainly the non-equilibrium eutectic structure at the grain boundary andMg matrix.After solution treatment,the non-equilibrium eutectic structure at the grain boundary dissolves.There are many bright white and fineMg₁₂Ce andMg₁₂La precipitated phases in the alloy after solution+aging treatment,and there are CeMgZn₂and LaMgZn₂phases at the grain boundaries.In XRD diffraction analysis,the main diffraction peaks of the as-cast alloy are contributed by?-Mg,H-MgZn₂and M-MgZn₂phases.After the solution treatment,the diffraction peak of the binary rare earth phase appears,and the diffraction peak of theMg-Zn phase is weakened.Then after solution+aging treatment,the peak of H-MgZn₂phase reappeared,indicating that H-MgZn₂phase was precipitated after aging treatment.The results of the micro Vickers hardness test show that the alloy reaches its peak hardness after aging for 16h.The hardness of the alloy with Ce element is 63.10H_v,while the La element is 60.78H_v.In first-principles calculations,except that the formation of M-MgZn₂phase is an endothermic reaction,the formation of the remaining five phases are all exothermic reactions,and the formation ability from strong to weak is:CeMgZn₂>LaMgZn₂>H-MgZn₂>Mg₁₂La>Mg₁₂Ce>M-MgZn₂.In the analysis of the density of states,there are covalent bonds in the M-MgZn₂,H-MgZn₂,Mg₁₂Ce andMg₁₂La phases.In addition,ionic bonds are formed in the binary Mg₁₂RE,but no obvious covalent bonds exist in the ternary phase.The single crystal elastic constant shows that CeMgZn₂phase is difficult to be compressed along the a-axis direction;along the c-axis direction,H-MgZn₂ has the lowest compressibility.The polycrystalline elastic constants show that the hardness of the rare earth phase is higher than that of theMg-Zn phase,which indicates that the addition of rare earth elements can enhance the hardness of the material.The shear modulus anisotropy factor indicates that LaMgZn₂has the highest shear anisotropy on the(100)and(001)planes,while M-MgZn₂has the largest anisotropy on the(010)plane.Debye temperature is positively correlated with thermal conductivity and thermal stability.The calculated Debye temperature in descending order is:Mg₁₂La>Mg₁₂Ce>H-MgZn₂>LaMgZn₂>M-MgZn₂>CeMgZn₂.That meansMg₁₂La has the highest thermal conductivity and best thermal stability among these six phases.Both the experimental and calculation results show that the structure and performance of the alloy with Ce element are better than the La element.In the XRD test analysis result,the peak of the H-MgZn₂phase reappeared after the aging,but the M-MgZn₂phase's peak did not appear.In the formation enthalpy calculations,the M-MgZn₂phase is the metastable phase,while the H-MgZn₂phase is stably.The experimental and calculated results are consistent.Besides,in the hardness test,the hardness of the alloy with Ce element added is higher than that of the La element.Then in the calculation of elastic constants,the CeMgZn₂ phase has the largest hardness among the six phases,regardless of whether it is a binary or ternary rare earth phase,their hardness is greater than that of theMg-Zn phase,which shows that the addition of rare earth elements can effectively enhance the hardness of the alloy.The calculated results are completely consistent with the experimental data.