FIRST-PRINCIPLES CALCULATION OF ELECTRONIC STRUCTURE AND ELASTIC PROPERTY OF AB(2) TYPE INTERMETALLICS IN Mg-Zn-Ca ALLOY

Electronic structure and elastic properties of Mg_2Sn,Mg_2Ca and MgZn_2 phases were investigated by means of first-principles calculations from CASTEP program based on density functional theory(DFT).The calculated lattice parameters were in good agreement with the experimental and literature values.The calculated heats of formation and cohesive energies show that Mg_2Sn has the strongest alloying ability and structural stability.The density of states(DOS) of Mg_2Sn,Mg_2Ca and MgZn_2 phases were calculated to analyze the mechanism of structural stability and mechanical properties.The calculated band structures show that Mg_2Sn phase has the widest bandgap.The electron density difference indicate that bonding characteristics of Mg_2Sn,Mg_2Ca and MgZn_2 phases were all covalent bond,ionic bond and metallic bond.The elastic constants of Mg_2Sn,Mg_2Ca and MgZn_2 phases are calculated,the bulk moduli,shear moduli,Young's moduli and Poisson's ratio are then derived.Bulk moduli is assumed to be the ability of material resistance to volume change by applied stress,the larger bulk modulus of MgZn_2 phase shows that it has stronger ability to resist deformation.Shear moduli is a measure of resistance to shear strain deformation under the deformation condition of shear stress,the larger shear moduli value of Mg_2Sn phase indicates that it has the stronger ability to resist shear strain deformation.The calculated Poisson's ration shows that MgZn_2has the largest value,and then followed by Mg_2Ca and Mg_2Sn.Hence,the plasticity of MgZn_2 phase is the best.The calculated Young's moduli of Mg_2Sn phase has the largest value and MgZn_2 phasehas the smallest value.Hence,among the three phases Mg_2Sn phase has the strongest stiffness.The ratio of the shear moduli to bulk moduli of phase can be used to demonstrate the brittle and ductile of materials.The critical value,which separates ductility from brittleness.is about 0.57.A higher G/B_u value is associated with brittleness,otherwise is ductility.The calculated values of Mg_2Sn,Mg_2Ca and MgZn_2 phases are 0.66.0.53 and 0.18.respectively.The results show that Mg_2Sn is brittle.Mg_2Ca and MgZn_2 are both ductile.