FIRST-PRINCIPLES CALCULATIONS OF STRUCTURAL STABILITIES AND ELASTIC PROPERTIES OF AB2 TYPE INTERMETALLICS IN ZA62 MAGNESIUM ALLOY

Structural stabilities, elastic properties and electronic structures of Mg2Sn, MgZn2 and MgCu2 in ZA62 magnesium alloy have been determined from first-principles calculations by using Castep and Dmol program based on the density functional theory. The calculated heats of formation and cohesive energies showed that Mg2 Sn has the strongest alloying ability and MgCu2 the highest structural stability. The calculated bulk moduli (B), anisotropy values (A), Young's moduli (E), shear moduli (G) and Poisson ratio (ν) showed that MgZn2 and MgCu2 both are ductile, on the contrary, Mg2 Sn is brittle, and among the three phases MgZn2 is a phase with the best plasticity. Their tested melting temperatures are within the ranges calculated from elastic constants (±300 K) and bulk moduli (±500 K), the estimated values from elastic constant have the smallest average relative error, the calculated melting temperature of Mg2Sn phase is in well agreement with the experimental one and the error relative to the experiment result is about 0.31%. MgCu2 has higher melting temperature, i.e. better structural stability among the three compounds. The calculations of thermodynamic properties show that the Gibbs free energy of MgCu2 is also the smallest within 298-573 K range, indicating the structural stability of MgCu2 does not change with the elevated temperature. The calculations of the density of states (DOS) and Mulliken electronic populations showed that the reason of MgCu2 having highest structural stability in ZA62 magnesium alloy attributes to MgCu2 phase having more ionic bonds below Fermi level compared with those of Mg2 Sn and MgZn2 phases.