First-principles Study Of β',β" And 6H LPS Phases In Mg-rare Earth Alloy

Magnesium alloys have important application value and prospect in the aerospace, transportation, electronics fields due to their low density, excellent mechanical properties, and electromagnetic shielding ability. However, the applications are still limited because of the lower strength, inferior fatigue and creep resistance at elevated temperature. Mg-rare earth alloys have received material researchers'great interest because of their high strength, good creep resistance and thermal stability. In this paper, we carried out first-principles calculations based on density functional theory to study on the structural, mechanical and electronic properties of the intermediate phase in Mg alloys. The obtained main results are as follows:1. We determined the lattice parameters ofβ' phase in Mg–Gd alloy theoretically and calculated the nine independent elastic constants of the orthogonal crystal. The nine independent elastic constants did not meet the mechanical stability criteria, which suggest the reported Mg15Gd is a mechanically unstable structure. The polycrystalline bulk modulus B, shear modulus G, Young's modulus E, and Poisson's ratioνfor Mg7Gd are calculated within the scheme of Voigt–Reuss–Hill(VRH) approximation. We found the covalent bond was formed between the Mg and Gd atoms based on the analysis of the density of states and charge density. Finally, the density, wave velocity and Debye temperature of Mg7Gd are calculated.2. The structural parameters ofβ" phase in Mg–Gd alloy are determined. The obtained formation enthalpy indicates that theβ" phase Mg3Gd could be formed energetically. The nine independent elastic constants are calculated, which suggest the Mg3Gd is mechanically stable. The polycrystalline bulk modulus B, shear modulus G, Young's modulus E and Poisson's ratioνforβ" phase Mg3Gd are calculated from the elastic constants. It seems that the improvement of mechanical properties is limited after alloying from these parameters. Theβ" phase Mg3Gd is a ductile material and exhibits the elastic anisotropy apparently. Just as theβ' phase, the directional covalent bonding was formed in the system.3. The microstructure, stability and electronic properties of the 6H-type ABACAB LPS phase in Mg₉₇Zn₁Y₂ are investigated theoretically. The enrichment of Y and Zn atoms occurs in the stacking fault defect layers, which is in accordance with the experimental observations. The present calculated results still show clearly that the Y and Zn atoms are preferably arranged closely. It is reasonable that the two atoms arrange closely to cancel strain due to the different atomic radii, and then lead the structure energetically more stable. The Y element enhances the structural stability from the calculated cohesive energy. It is clearly that the Y element would improve the creep property of alloy. The Zn atoms located in two different misfit fault layers would lead to lattice distortion. Further analysis shows that the intimate arrangement of Y and Zn atoms would weaken the distortion. Based on the analysis of the density of states, we found the covalent bonding exists between the Mg and Gd atoms which would enhance the strength of alloy. The hybridization between Mg and Zn is clear in entire region and the variation of hybridization is small. So this is helpful for improving the ductility of the 6H-type LPS structure.