First-Principles Calculation Of The Structural Stability And Thermodynamic Properties Of The Aging Strengthening Phase In Mg-Gd Alloys

The Mg-Gd series alloy has good creep resistance at high temperature,and the effect of strengthening is remarkable at low temperature.Up till the present moment,this is a clear matter about the aging precipitation sequence of Mg-Gd alloy,but there are still some controversies related to the crystal structure and thermodynamic and mechanical properties.In this paper,the main precipitated phases,such as,?''-Mg₃Gd,?₁-Mg₃Gd,?'-Mg₇Gd and stable phase Mg₂Gd,and a more systematically analysis of?₁-Mg₃Gd and?'-Mg₇Gd with better thermodynamics and mechanical stability.?1?In this paper,studies on?''-Mg₃Gd?A???''-Mg₃Gd?B???''-Mg₃Gd?C?,Mg₂Gd,?₁-Mg₃Gd and?'-Mg₇Gd intermetallic compounds structural stability and thermodynamic properties show that:the formation enthalpies are all negative,Mg-Gd phase is thermodynamically stable at zero pressure,?₁-Mg₃Gd has the highest stability,followed by?'-Mg₇Gd;With the increase of Gd elements in intermetallic compounds,the volume deformation resistance of the alloy phase is improved in the Mg-Gd alloy.?₁-Mg₃Gd and?'-Mg₇Gd have better mechanical properties and play a role in strengthening the aging.?₁-Mg₃Gd has the largest shear modulus G and Young's modulus E,and the difference in sound velocity in each direction is the largest,resulting in the largest anisotropy,however,the situation of the?'-Mg₇Gd structure is the opposite;thermal conductivity is in all directions There is anisotropy.The?''-Mg₃Gd?C?with the hexagonal structure has proved to be thermodynamically unstable because the constant volume heat capacity C_v continues to increase with temperature;The higher the Debye temperature,the stronger the covalent bond and the increased brittleness of the material.The Debye temperature of?'-Mg₇Gd structure is the highest,which is a brittle material;The Debye temperature of Mg₂Gd is the lowest,which is a ductile material.The?'-Mg₇Gd structure is sensitive to temperature,and its creep resistance deteriorates at high temperature;The local density of states shows that Mg-Mg and Mg-Gd covalent bonds and Mg-Mg around the Gd atoms form covalent bonds,which makes Gd have an exceptionally high solution strengthening efficiency in magnesium alloy.The density of state analysis shows that among the three?''-Mg₃Gd phases,the bonding peak of?''-Mg₃Gd?B?is more biased towards the low energy region of the occupied state,and the peak is lower,while Mg₂Gd,?₁-Mg₃Gd,and?'-Mg₇Gd are in the valley indicates that the latter electronic state is more stable.In summary,?''-Mg₃Gd?B?is relatively stable among the three structures.?2?Finally,because?₁-Mg₃Gd and?'-Mg₇Gd have good structural stability and good mechanical and thermodynamic properties,the lattice parameters,elastic constants,elastic modulus,thermal conductivity,the local electron localization function and state density of?₁-Mg₃Gd and?'-Mg₇Gd were calculated under different pressures of 0,10,20 Gpa.The results show that the enthalpy of formation of?₁-Mg₃Gd and?'-Mg₇Gd are both negative,thermodynamically stable,and as the pressure increases,the smaller the absolute value of enthalpy of formation,the less likely the structure is to form;the elastic modulus of?₁-Mg₃Gd is higher than that of?'-Mg₇Gd under limited pressure,when the G/B ratio of?₁-Mg₃Gd phase is greater than 0.57,which are both brittle,and?'-Mg₇Gd changes from brittle to ductile at20Gpa;Under different pressures of?₁-Mg₃Gd,the difference in sound velocity in all directions weakens,resulting in reduced anisotropy.On the contrary,?'-Mg₇Gd increases anisotropy,and such calculation results can support mechanical analysis.The thermal conductivity of?'-Mg₇Gd under different pressures is less than?₁-Mg₃Gd.In the?₁-Mg₃Gd structure,the distance between the Mg atom and the Gd atom and the charge accumulation increase,and the covalent bond strength increases with pressure.In the?'-Mg₇Gd structure,because the percentage of Gd atoms is less than that of?₁-Mg₃Gd,the accumulation of electrons between Gd-Mg atoms increases in locality with pressure.At 20Gpa,the electron delocalization between Mg-Mg atoms is enhanced,resulting in an increase in the metal bond,and the material exhibits a transition from brittleness to ductility,in which covalent bonds have always played a key role.The calculation of the density of states shows that the distance between the atoms of the Mg-Gd phase decreases,the maximum bonding peak of the total density of states becomes wider and the peak intensity decreases under pressure,which shows that the pressure can improve the structural stability of the selected alloy.Among the occupied states,the total density of states mainly comes from the contributions of Mg s,p orbital and Gd d,indicating the role of covalent bonds and the increase in the strength of?'-Mg₇Gd covalent bonds with increasing pressure.