Computational Study On Mechanical Stability At High Pressure And Therdynamic Properties Of Compound Phases In Mg-Zn-Y Alloys

The latest research shows that the high strength and heat resistant rare earth magnesium alloys have increased the commercial use and enhanced the practicability of magnesium alloys.In this paper,starting from the study of the stability of rare earth magnesium alloys,the binary strengthening phases MgZn₂ and Mg₂₄Y₅ and the main ternary phase W(Mg₃Y₂Zn₃)in Mg-Zn-Y alloy were selected as the research objects.The mechanical stability and thermodynamic properties of the three phases were studied by using first-principles calculation method.The enthalpy of formation,binding energy,mechanical stability,temperature,energy band structure and density of states of MgZn₂,Mg₂₄Y₅ and Mg₃Y₂Zn₃ in Mg-Zn-Y alloy were investigated under atmospheric pressure.The results show that Mg₃Y₂Zn₃ has the strongest alloying ability and is the easiest to form under atmospheric pressure.MgZn₂times;Mg₂₄Y₅ weakest;Mg₃Y₂Zn₃ has the best stability and Mg₂₄Y₅ has the worst stability.By calculation,MgZn₂,Mg₂₄Y₅ and Mg₃Y₂Zn₃ are all mechanically stable phases at atmospheric pressure.The deformation resistance and hardness of Mg₃Y₂Zn₃ phase are the best under normal pressure.MgZn₂ has the best plasticity.Mg₂₄Y₅ has the highest interatomic binding force and the fastest vibration frequency.The metallic properties of the three phases are verified by the band structure diagram and the orbital hybridization of the three phases is analyzed by the density of states diagram.No virtual frequency is observed in the phonon dispersion curve of Mg₂₄Y₅,which proves the stability of Mg₂₄Y₅under normal pressure.The phonon calculation is used to calculate the thermodynamic properties of Mg₂₄Y₅ phase from 0 to 1000 K.It's found that the enthalpy,entropy and volume heat capacity of Mg₂₄Y₅ increase as the temperature increasing.The Helmholtz free energy decreases with the increase of temperature,and Mg₂₄Y₅ shows good stability at high temperature.The mechanical stability and debay temperature of MgZn₂,Mg₂₄Y₅ and Mg₃Y₂Zn₃ at10?100 GPa were calculated.As the pressure increases,the bulk modulus B,shear modulus G and Young's modulus E have an upward trend with the increase of pressure.The resistance,deformation resistance and hardness of MgZn₂ material are the highest when the pressure increases.The deformation resistance and hardness of Mg₂₄Y₅ are the lowest.The Poisson's ratio?of the three phases indicates that Mg₂₄Y₅ has the best plasticity at 30?100 GPa.MgZn₂ has the worst plasticity.In addition,the calculation of material G/B shows that the three structures are ductile phase.The results show that the interatomic force and vibration frequency of Mg₂₄Y₅ phase are the fastest when the pressure is less than 20 GPa.The maximum interatomic force and the fastest vibration frequency of MgZn₂ are obtained at a high pressure of 20?100 GPa.Helmholtz free energy calculation at high temperature shows that the free energy relationship is Mg₂₄Y₅<Mg₃Y₂Zn₃<MgZn₂ in the range of 300?1000 K.From the perspective of free energy,Mg₂₄Y₅ shows superior stability at high temperature.