Elastic Properties And Electronic Structure Of (Ti_5-xMg_X)Si₃and Ti₅X₃（X=Si, Ge, Sn, Pb）

Low density, high strength, and excellent corrosion resistance are the main properties that make lightweight structural materials of titanium attractive for applications in aerospace industries, automobile and biological engineering. The present study shows that addition of magnesium and IVA-group element to Ti-based alloys can result in make to density of reduction and further improvement mechanical properties, so the purpose of this thesis is to study the structural stability, elastic properties, ideal strength and electronic properties of isostructural of (Ti5-xMgx)Si3and Ti5X3(X=Si, Ge, Sn, Pb) by first-principles calculations within the frame work of density functional theory. It will provide helpful theoretical basis to design new materials and predict the properties of materials. The main contents are as follows:1. The structural stability, elastic and electronic properties of (Ti5-xMgx)Si3with D88structure have been investigated by first-principles calculations. The obtained formation enthalpies demonstrated that the structure stability of (Ti5-xMgx)Si3was eventually lowered with increasing x. Then the independent elastic constants of (Ti5-xMgx)Si3were calculated, and the obtained results indicated that (Ti2Mg3)Si3and Mg5Si3were mechanically instable. With VRH approximation, the calculated elastic modulus B, G and E of (Ti5-xMgx)Si3were reduced with increasing x, whereas the ductility was somewhat improved. The elastic anisotropy of (Ti5-xMgx)Si3was also studied in details. Furthermore, the electronic density of states and charge density distribution were investigated to reveal the inherent mechanism for stability and elastic properties of (Ti5-xMgx)Si3phases..2. The structural stabilities, elastic and electronic properties of Ti5X3(X=Si, Ge, Sn, Pb) compounds with D88structure have been systematically investigated within the frame work of density functional theory. With increase of atomic number of the element X, the calculated lattice constants of Ti5X3compounds are found to increase, while stabilities of these compounds decrease because the formation enthalpies increase. Moreover, with exception of C12, the calculated elastic constants are reduced with increase of atomic number of element X, the bulk, shear and Young’s modulus are also lowered, while Poisson’s ratio is larger, and the ductility is slightly improved. The elastic anisotropies and ideal strength of Ti5X3were further studied shows that the maximum stress in each direction of strain are decreased with increase of atomic number of element X. Ti5X3(X=Si, Ge) shows that the ideal tensile strength occurs in the[0001], while the shear strength was found in(0001)[1010]. Ti5X3(X=Sn, Pb) shows that the ideal tensile strength occurs in the [1120] and [1010], respectively. While the shear strength of Ti5X3(X=Sn, Pb) was found in(1010)[1120]. The density of states and charge density distribution reveal bonding feature of Ti5X3. Especially, the X-X and Ti-X covalent bonding strength is found to decrease with increase of atomic number of element X.