Investigations Into The Structural Character And Properties Of Multiple Reinforced Phases In Mg-Si-RE Alloys

As a new structural material with low density and high strength,magnesium alloy is widely used in the automotive and aerospace fields.The main drawback of magnesium alloys is poor mechanical properties at high temperatures.The research is focused on adding high content of Si elements and low content rare earth elements Ce,La in the past.The reaction between Si and Mg to generate Mg2Si high temperature phase is distributed in the magnesium matrix,and the rare earth element has an adsorption poisoning effect to reduce the Mg2Si phase size.However,the modification effect of rare earth elements on Mg2Si reinforced phase is limited.Therefore,the content of Si is reduced and the content of rare earth is increased to prepare Mg-1 Si-3 RE(RE=Ce,La)alloy in this work.Based on experimental characterization,the formation mechanism,morphological characteristics,and orientation relationship of the phase interface were deeply investigated.The mechanical and thermodynamic properties of the mesophase were calculated based on the first-principles to provide a reliable theoretical basis for the development of new magnesium alloy materials.Mg-Si-RE(RE=Ce,La)alloys were prepared by gravity casting and rapid solidification processes.Based on the previous analysis,it was found that the quadrilateral plate-like white rare earth phase CeMg2Si2 with a size of about 10 μm formed in Mg-1Si-3Ce alloys prepared by the gravity casting process.Bright white multi-rare earth phases LaMg2Si2 formed in the Mg-1Si-3La prepared by gravity casting process.The rectangular plate-like rare earth phase CeLaMg4Si4 and long rod-like rare earth phases(CeLa)MgSi formed in Mg-1Si-1.5Ce-1.5La alloys.The micro-analysis results of Mg-1Si-3RE(RE=Ce,La)prepared by the rapid solidification process were found that the quadrilateral sheet CeMg2Si2 and octahedral LaMg2Si2 with a size of about 5 μm were formed after the addition of Ce and La,respectively.While only quadrilateral lamellar CeLaMg4Si4 phases formed in Mg-1Si-1.5Ce-1.5La,it was found by TEM that the doped rare earth phase was embedded in Mg2Si with the(002)plane of rare earth and Mg2Si(020)plane at 51.96°.The Vickers hardness test results show that CeMg2Si2 has the largest Vickers hardness and Young’s modulus,followed by Mg2Si and CeLaMg4Si4,and LaMg2Si2 has the smallest Vickers hardness and Young’s modulus.Thus,CeMg2Si2 can be used as an excellent Mg-based reinforcing phase.Meanwhile,the higher the hardness of the alloy phase,the greater the corresponding Young’s modulus.The room temperature compressive performance test results show that the maximum compressive strength of Mg-1Si-3Ce alloy is 260 MPa,followed by Mg-1Si alloy and Mg-1Si-1.5Ce-1.5La,and the compressive strength of Mg-1Si-3La is the smallest.The structural stability,mechanical properties,elastic anisotropy and Debye temperature of the precipitated and doped phases in the Mg-1 Si-3 RE(RE=Ce,La)alloy were calculated.According to the calculated cohesive energy and enthalpy of formation of Mg-1 Si-3 RE(RE=Ce,La),the cohesive energy of the six phases,CeSi2,LaSi2,Mg2Si,LaMg2Si2,CeLaMg4Si4,and CeMg2Si2,are all negative,indicating that these six phases are stable molecular structures.The phases are exothermic reactions except for the formation of CeMg2Si2 and CeLaMg4Si4.The structural stability and ease of formation of these six phases are from weak to strong is:CeSi2>LaSi2>Mg2Si>LaMg2Si2>CeLaMg4Si4>CeMg2Si2O According to the density-of-state curve,the Mg2Si,CeMg2Si2,LaMg2Si2,CeSi2,LaSi2,and CeLaMg4Si4 phases have undergone hybridization and formed a large number of covalent bonds.The population analysis results show that all six phases form ionic bonds and covalent bond,and the order of the strength of the covalent bond is:CeSi2>LaSi2>CeLaMg4Si4>LaMg2Si2>CeMg2Si2>Mg2Si;the metal strength is:CeSi2>LaSi2>CeLaMg4Si4>Mg2Si>LaMg2Si2>CeMg2Si2.The single crystal elastic constant shows that the value of C33 as a whole is larger than the value of C11,indicating that the resistance to pressure deformation of CeMg2Si2,LaMg2Si2,CeSi2,LaSi2,and CeLaMg4Si4 on the z-axis is greater than its resistance to deformation on the x-axis.The calculation results of elastic properties show that the resistance to volume change and the resistance to shear stress in order from strong to weak are:LaSi2>CeSi2>LaMg2Si2>CeLaMg4Si4>CeMg2Si2>Mg2Si and CeLaMg4Si4>LaMg2Si2>LaSi2>CeMg2Si2>Mg2Si>CeSi2,respectively.The order of stiffness from large to small is:LaSi2>LaMg2Si2>CeLaMg4Si4>CeMg2Si2>CeSi2>Mg2Si,and the plasticity from large to small is:CeSi2>LaSi2>LaMg2Si2>CeLaMg4Si4>CeMg2Si2>Mg2Si,and these six phases is brittle,the hardness is:CeMg2Si2>Mg2Si=CeLaMg4Si4>LaMg2Si2>LaSi2>CeSi2.The elastic anisotropy results show that the general anisotropy index is:CeSi2>CeLaMg4Si4>LaMg2Si2>CeMg2Si2>LaSi2>Mg2Si,CeLaMg4Si4 has the largest Young’s modulus anisotropy on the(010)and(001)planes.The bulk modulus anisotropy of CeLaMg4Si4 is largest in(100),(010),and(001)planes,and the shear modulus anisotropy of the(100)and(010)planes is largest.Debye temperature calculation results show that;Mg2Si>LaMg2Si2>CeLaMg4Si4>CeMg2Si2>LaSi2>CeSi2,implying that the bonding between Mg2Si atoms is strong,which corresponds to the strongest thermal conductivity and thermal conductivity.The comprehensive analysis indicates that the solid solution strengthening effect can form by addition of rare earth elelment Ce.The yield strength of magnesium alloy can be enhanced due to CeMg2Si2、LaMg2Si2 and CeLaMg4Si4 pahses can form semi-coherent interfaces with Mg2Si phase.Meanwhile,the formed CeMg2Si、LaMg2Si2 and CeLaMg4Si4 phases distributed in the magnesium matrix result in in the dispersion strengthening effect The results of calculation and experiment show that CeMg2Si2 phase has relatively excellent strengthening effect,which can be applied in industry as an excellent strengthening phase.