The Properties Of Mg2_X(X=Si,Ge,Sn)phases By First-principles Calculations

Magnesium alloy is the lightest metal material at present,with the advantages of low density（1.8g/cm³）,good heat dissipation,high strength and stiffness,good electromagnetic shielding effect,good shock absorption,high recycling efficiency,etc.It is known as“21st Century green metallic material”.It is widely used in the field of transportation,3C products（communications,computers and consumer electronics）,aerospace,etc.Mg₂X（X=Si,Ge,Sn）alloy is a more promising and representative intermetallic compound.Mg₂X（X=Si,Ge,Sn）alloys are rich in storage and their crystal structure is CaF₂（face centered cubic）.Mg₂X（X=Si,Ge,Sn）intermetallic compounds have the advantages of high temperature resistance,high melting point,low thermal conductivity,high hardness,high electrical conductivity,corrosion resistance,etc.They have important application prospects in the fields of electronic components,energy components and other fields.In the work,the Cambridge Sequential Total Energy Package（CASTEP）is used to complete the computational tasks,which is a first-principles method of computation based on the pseudopotential plan-wave within density functional theory（DFT）.We studied the structural parameters,mechanical properties,thermodynamic properties,electronic properties and pressure effect on performance of Mg-Si,Mg-Ge,Mg-Sn compounds.The calculated structural parameters,mechanical properties and electronic properties of Mg₂X（X=Si,Ge,Sn）metal compounds at zero pressure are very close to experimental results and other theoretical results,indicating that the calculation method is feasible.The results show that the formation heat of the three is negative,indicating that both of the three can form stable compounds.For the Mg₂Ge compound is easier to forma and has stronger alloying ability than that of the metastable phases.The most stable structure is Mg₂Si alloy.Mg₂X（X=Si,Ge,Sn）alloys are all brittle materials and all have metallic properties.The order of resistance deformationability,shear-resistance ability andstiffness size is Mg₂Si>Mg₂Ge>Mg₂Sn.The covalent bond strength of the three alloys is arranged as:Mg₂Si<Mg₂Ge<Mg₂Sn.The effect of pressure on the Mg₂X（X=Si,Ge）alloy was studied.The results show that both Mg₂Si and Mg₂Ge are brittle at 0-10GPa,and when the pressure reaches 15GPa,both Mg₂Si and Mg₂Ge begin to exhibit ductile properties.With the increase of pressure,the plasticity of the material has been improved.Debye temperature and bulk modulus of Mg₂Si and Mg₂Ge phases decrease with increasing temperature at 0-1000K.At the same temperature,Debye temperature and bulk modulus increase with pressure.The isothermal capacity C_v and isobaric heat capacity of C_p all decrease with the increase of pressure,and the heat capacity of Mg₂Ge is higher than that of Mg₂Si.The alloying of Mg₂X（X=Si,Ge）alloy was carried out.After the alloying element Bi was added,the forming ability was reduced.The alloying element Bi doped Mg₂X（X=Si,Ge）easily replaced the X atom position.The structural stability was ranked as follows:Mg₇X₄Bi>Mg₈X₄>Mg₈X₄Bi>Mg₈X₃Bi.Although the Mg₈X₃Bi alloy is most easily formed,the structure is the most unstable.The doping of alloying element Bi makes Mg₂X（X=Si,Ge）alloy transform from brittle material to ductile material.The plasticity of Mg₂X（X=Si,Ge）alloy increases and the stiffness of Mg₂X（X=Si,Ge）alloy decreases.The plasticity of Mg₇Ge₄Bi is stronger than that of Mg₇Si₄Bi alloy.At the same time,the ability to resist deformation and resist plastic deformation are all reduced.Feimi surface shifted toward the region of high energy level.Electron density of states corresponding to Fermi level increased.Doping can enhance the conductivity of Mg₂X（X=Si,Ge）alloy,and the conductivity of Mg₇Si₄Bi is better than that of Mg₇Ge₄Bi.The performance of the Mg₁₆X₈（X=Si,Ge）unit cell containing a point defect was investigated.The lattice distortion in the cell caused the cell expansion and the lattice constant became large.At the same location,vacancy defects are easier to form than counter defects.The defect is more stable in the Mg position than in the X（X=Si,Ge）position.At the same time,the defects can reduce the resistance to deformation,shear ability and stiffness of Mg₁₆X₈（X=Si,Ge）alloys.When the reverse defect is produced in the same position,the unit cell has higher shear strain resistance and stiffness,and the plasticity and ductility are weaker.In the Mg₁₆Ge₈ unit cell,when the defect occurs in the Mg position,the crystal has higher resistance to deformation,greater resistance to shear strain and stiffness,and weaker ductility and ductility.