Thermal stability and elastic properties of Mg3Sb2 and Mg3Bi2 phases from first-principles calculations

Structural stabilities, elastic properties and electronic structures have been determined from first-principles calculation by using Castep and Dmol program based on the density functional theory for Mg17Al12, Mg3Bi2 and Mg3Sb2 phases. The calculation results of heat of formation and cohesive energy indicate that Mg3Sb2 has the strongest alloying ability as well as the highest structural stability. The calculations of thermodynamic properties show that the Gibbs free energy of Mg17Al12, Mg3Bi2 and Mg3Sb2 change with the elevated temperature, when the temperature is below 423 K, Mg3Sb2 and Mg3Bi2 are more stable than Mg17Al12 phases and Sb or Bi addition to the Mg–Al-based alloys can improve the heat resistance. The densities of states (DOS) of these compounds are given. The elastic parameters are calculated, then the bulk modulus, shear modulus, Young’s modulus and Poisson ratio value are derived. All the results of the analysis indicate that adding Sb or Bi to Mg–Al alloy could improve the ductility and plasticity by forming Mg3Sb2 and Mg3Bi2 phases.