Theoretical Study Of Point Defects In Mg₂₄Y₅ And MgCu₂

As excellent light-weight structural materials, Mg alloys have advantages such as low density, high specific strength and specific stiffness. Therefore, Mg alloys have been widely applied. Point defects strongly affect the structure and properties of materials, so limit to some extent for the property and application of Mg alloy materials. The experimental identification of defect is typically difficult and indirect, therefore theoretical investigations of defects are very necessary. First-principles calculations have emerged as a powerful approach that complements experiments and can serve as a predictive tool in the identification and characterization of defects for Mg alloys. Therefore, First-principles calculations with GGA approximation are performed to study the point defects in Mg24Y5 and MgCu2. The main investigations are as following:Firstly, eight possible native point defects in Mg24Y5 are studied from the density functional theory (DFT) calculations. The energetic results show that antisite defects with lower formation energies are energetically favored over vacancies. Under both Mg-rich and Y-rich conditions, MgY1 defect is dominant due to the lowest defect formation energy, followed by YMg2, MgY2 and YMg1, which reasonably explains the Mg-rich off-stoichiometry of Mg24Y5 alloys. For vacancy defects, the formation energies on the two Mg sublattices are smaller than ones on two Y sublattices, and VMg1 has the lowest formation energy, while VY2 is most unlikely due to the highest formation energy originating from the large mismatch in atomic size and the strong interaction of Y with surrounding atoms. The defect concentration distribution as a function of temperature and stoichiometry is further obtained from grand canonical statistics. The size factor and electronic structure are further discussed, demonstrating that Mg24Y5 alloy is typical system in which the point defects are evidently affected by strong size effect due to the large atomic size radius and strong electronic factor caused by the intensive interaction of Y with the surrounding atoms.Then, the point defect in C15 Laves phase MgCu2 is also studied employing first-principles calculations. The formation enthalpies of point defects and defective C15 MgCu2 compound show that the dominating defect structure is MgCu and CuMg antisite defect on Mg-rich and Cu-rich side of off-stoichiometry, respectively. From grand canonical statistics, the concentration of various point defects as a function of composition of MgCu2 is analyzed. The calculated local geometrical variation around defect demonstrates an apparent size effect, and the electronic densities of states and charge density distribution indicate the gradually weaker bonding from MgCu to VCu and from CuMg to VMg systems. Furthermore, doping of Zn atom in defective MgCu2 has been investigated. It is found that because of the relative lower formation enthalpy, Zn has a stronger preference for occupying the Cu sublattice than for Mg sublattice. The further investigated electronic structure reveals that the stability of Zn-doped system is associated with the stronger covalent bonds between Cu-Cu and Zn-Cu bond.