Elastic Properties Of Mg-y(Zn) Solid Solution And Mg-Ce Intermetallics

Lightweight Mg alloys have attracted increasing interest in recent years for potential applications in the aerospace, aircraft and automotive industries due to their high strength/weigh ratio. However, the application of magnesium alloys in modern industry is limited because of the restrained mechanical properties, especially at high temperature. Hence, much work has been devoted to improve the mechanical properties of Mg-based alloys. In general, solid-solution strengthening and precipitation strengthening are the two major methods to increase the strength of magnesium alloys. Addition of alloying elements plays very important role in optimizing the microstructure and mechanical properties. However, the research has been focused only on experiments, the atomic scale computer simulation for the elastic properties of Mg alloys has rarely been reported and the effect of solute atoms on the elastic properties has not been well understood.First-principles calculations were used to study structural, elastic and electronic properties of Mg-R(R=Y, Zn) solid solution and Mg-Ce intermetallics. The atomic scale computer simulation was used to explain strengthening mechanism. The main contents of this dissertation are as following:1. The effects of Y and Zn atoms on structural stability and elastic properties of Mg solid solution are investigated by first-principles calculations based on density function theory. Five supercell models (Mg₉₆, Mg₉₅Y₁, Mg₉₄Y₂, Mg₉₅Zn₁ and Mg₉₄Zn₂) are adopted to simulate the variation of solute atoms content. Structural optimization is performed firstly, and lattice parameters of Mg–R (R=Y, Zn) systems are found to be dependent on the atomic radius and the number of the solute atoms. The calculation of cohesive energy shows that the Y element makes the cohesive energy decrease significantly whereas Zn element increases slightly the cohesive energy.Then the five independent elastic constants of Mg solid solution are calculated. The shear modulus G, bulk modulus B, Young's modulus E and Poisson ratioνare also derived by Voigt-Reuss-Hill (VRH) approximation, indicating a strong dependence of the elastic modulus on concentration of the solute atoms. The investigation of the B/G ratio shows that the ductility is improved, and shows the influence of Y is much stronger than Zn. In the end, The calculated electronic density of state (DOS) shows the bonding contribution from the interaction between Y (Zn) and Mg atoms is increasingly strengthened with increasing solute atoms.2. First-principles calculations have been carried out to investigate the elastic properties and electronic structures of the main binary Mg-Ce intermetallics MgCe, Mg₂Ce and Mg₃Ce. The optimized equilibrium lattice constants are in agreement with the available experimental values, and the structural stability of the MgCe, Mg₂Ce and Mg₃Ce are studied from the calculated heat of formation and cohesive energy. The elastic constants C_ij of the three intermetallics are calculated, then the shear modulus, bulk modulus, Young's modulus, Cauchy pressure, and the ratio of bulk to shear modulus B/G are further investigated. The obtained results indicate that Mg₂Ce has the best ductility and plasticity, while Mg₃Ce is the hardest and the most brittle among the three Mg-Ce intermetallics. Then the elastic anisotropic constant showed that Mg₂Ce is less anisotropic than the others. The density of states and charge densities distribution of the Mg-Ce intermetallics are also calculated to reveal the underlying mechanism for the structural stability and elastic properties of these Mg-Ce intermetallics.