| Magnesium alloys have been extensively applied in the automobile and aerospace industries over the past years because of the excellent advantages of low densities, high specific strengths and stiffnesses, superb electromagnetic shielding capacities, low toxicities, excellent machinability, castability and recyclability. However, the application of magnesium alloys in modern industry is still limited due to the restrained mechanical properties, especially the poor creep, corrosion and ignition resistance at high temperature. Therefore, improvements of Mg alloys are required and knowledge of their mechanical properties is important for material performance. Addition of rare earth (RE) elements is known to be one the most effective ways to improve the mechanical properties of the magnesium alloys. In this paper, the first-principles calculations based on density functional theory are carried out to investigate the elastic and electronic properties of the Mg2Y(Ca) and tI26-type Mg12RE alloys. The obtained main conclusions are as follows:1. The optimized lattice parameters of Mg2Ca and Mg2Y phases have been found to be in excellent agreement with the available experimental value. The negative formation enthalpy shows structural stability of the two phases and the Mg2Ca phase is more stable. The five independent elastic constants are calculated, showing that both phases are mechanically stable. Then the bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratio v of polycrystalline aggregates are derived within the Voigt-Reuss-Hill (VRH) approximation, and the relevant mechanical properties Mg2Ca and Mg2Y phases are also further discussed. The elastic anisotropy of the two phases is also discussed in details, indicating that the Mg2Ca and Mg2Y phases exhibit small elastic anisotropy. Finally, according to the analysis of density of states and charge distribution, the higher stability of Mg2Ca may be attributed to the stronger directional covalent bonding.2. A detailed theoretical study on structural, elastic and electronic properties of tI26-type Mg12RE (RE=Ce, Pr and Nd) phases has been carried out by means of first-principles calculations. The optimized lattice parameters are in good agreement with the available experimental value, and the obtained formation enthalpies show that with increasing atomic number of RE, the stability of Mg12RE alloys is lower. The elastic constants and polycrystalline elastic parameters B, G, E and v within the Voigt-Reuss-Hill (VRH) approximation of tI26Mg12RE are further calculated, the obtained results demonstrate that with increasing atomic number of RE, B, G, E and v are larger, and all Mg12RE alloys behave in a brittle manner. A three-dimensional directional representation together with the projection on ab and bc(ac) planes is employed to reveal visually the variation of the elastic modulus with crystallographic direction in more detail. It is found that the elastic anisotropy on ab is different from that on bc(ac) planes and all tI26Mg12RE phases display similar feature of elastic anisotropy. The density of states, charge distribution, charge density difference and the Bader charge are also further investigated, the obtained results show that obvious RE-RE and RE-Mg covalent bonding are formed for the tI26Mg12RE alloys. |
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