First Principles Calculations On Structural, Mechanical And Thermodynamic Properties Of Mg-Ca Alloys

Magnesium alloy is widely used in automobile,3C, aerospace and defence industriesbecause it has a series of excellent performance. However, the application of magnesiumalloys is still limited due to the restrained properties. Alloying is regarded as an important wayto improve the performance of magnesium alloy.The structural, mechanical properties, electronic structures and Debye tempertatures ofMg2Sn, MgCu2, MgZn2, Mg2Sr and Mg2Ca phases were investigated by means of first-principles calculations from CASTEP program based on density functional theory (DFT) withthe generalized gradient approximation. The calculated lattice parameters were in goodagreement with the experimental and literature values. The calculated heats of formation andcohesive energies showed that Mg2Sn had the strongest alloying ability and MgCu2had thestrongest structural stability. The elastic constants of the five kinds intermetallic compoundswere calculated and the mechanical parameters, such as bulk moduli B, shear moduli G,Young’s moduli E, Poisson’s ratio and ansiotropy A were determined by using the Voigt-Reuss-Hill (VRH) averaging scheme. The calculated results showed that Mg2Sn was brittlephase, while others were all ductile phases. Among the five phases, MgCu2had the strongeststiffness, the plasticity of MgZn2phase was the best. Melting points and hardness of the fivephases were predicted by using cohesive energy and elastic constants. Density of states (DOS),Mulliken population and charge density difference were discussed. Finally, Debyetemperatures were calculated and discussed. It could be concluded that the mechanicalproperty of Mg-Ca alloy were improved by adding Sn, Cu, Zn and Sr elements.The mechanical, electronic and thermodynamic properties of Mg2Sr and Mg2Ca withC14-type structure under pressures had been firstly studied by using the first-principlescalculations. The five independent elastic constants of Mg2Sr and Mg2Ca structures werecalculated and the mechanical parameters and Poisson’s ratio were determined by usingthe Voigt-Reuss-Hill (VRH) averaging scheme. The Cauchy pressure and elastic anisotropy ofMg2Sr and Mg2Ca were also investigated. The calculated results showed that Mg2Sr andMg2Ca were ductile phases according to the calculation results of Cauchy pressure and G/Bvalues under different pressures. In addition, the electronic densities of states (DOS) as a function of pressures were analyzed. The Debye temperatures obtained by elastic constantswere investigated. The dependences of thermodynamic properties on temperature and pressurewere also discussed by using the quasi-harmonic approximation.