First-Principles Study Of Phase Stability And Mechanical Property Several Novel Materials

With the progress in computer technology and numerical methods, first-principles calculations based on density functional theory have become a powerful tool for the investigation of material property. The first-principles calculation of the crystal defect, phase stability and mechanical property of materials has matured considerably. In this dissertation, the crystal defect, phase stability and mechanical property of three kinds of new structural materials, i.e. Laves phase, low modulusβ-Ti alloys and two-phaseγ/γ' Co alloys, are investigated theoretically by first-principles method. The concerned properties include site occupancy, phase stability, elastic property, ideal strength and magnetic property. The above researches give an important theoretical basis for the optimum design of three kinds of new structural materials.In the first chapter, we introduce the basic concept of computational materials science and development of first-principles method based on density functional theory. The applications of first-principles method in materials science are introduced at the end of this chapter.In chapter 2, Site occupancies of ternary additions in the C15 NbCr₂ and ZrCr₂ Laves phases and the ideal strength of C15 XCr₂ (X=Nb, Zr and Ti) and TiCo₂ Laves phase are investigated by first-principles calculations. The results show that Ti and Zr preferentially substitute the Nb sites in NbCr₂. V, Mo and W preferentially occupy the Cr sites in NbCr₂. For ZrCr₂, Nb and Ti preferentially occupy the Zr sites, and V, W and Mo the Cr sites. The site occupancy behaviors of the ternary additions in Laves phase can not simply be attributed to atomic size and valence electron concentration. The physical nature of the site occupancy behaviors can be explained by the hybridization between ternary and host atoms and the change in phase stability of Laves phase. The calculated results of the ideal strength of Cr-based Laves phase show that the ideal shear strength of NbCr₂ along {1 11} < 112> direction is higher than the ideal tensile strength along <001> direction, which could explain why the NbCr₂ is liable to brittle fracture. The {001} planes are the principle cleavage planes of Cr-based Laves phase. The ideal tensile strength of Cr-based Laves phase is mainly determined by interatomic bonding strength. The ideal tensile strength of TiCr₂, NbCr₂ and ZrCr₂ along <001> direction are 28.51, 28.17 and 23.32 GPa,respectively. The failure strain of TiCr₂, NbCr₂ and ZrCr₂ along <001> direction is about 0.32. The ideal tensile strength of TiCo₂ along <001> direction is found to be 26.35 GPa, and the failure strain is 0.26.In chapter 3, the phase stability, electronic structure and elastic property ofβ-type Ti-X binary alloys are studied by first-principles calculations. The results show that the lattice constants of Ti-Mo alloy decrease monotonously with an increase of Mo content, while the lattice constants of Ti-Nb and Ti-Ta alloys increase monotonously with increasing the content of Nb and Ta, respectively. The cohesive energies ofβ-type Ti-X alloys increase monotonously with increasing the contents of X element. The phase stability ofβ-type Ti-X alloys can be enhanced by increasing the valence electron concentration of Ti-X alloy. According to the analysis of elastic stability criteria, when the valence electron concentration per atom of Ti-X alloys is 4.10 or more, the Ti-X alloys withβstructure is stable. The bulk modulus, shear modulus and Young's modulus ofβ-type Ti-X alloys increase linearly with an increase of the valence electron concentration of Ti-X alloy, that is an increase of the phase stability ofβ-type Ti-X alloys. When the valence electron concentration of Ti-X alloys is 4.10, the Ti-X alloys achieve lowest elastic modulus. There is no definite correlation between elastic moduli and lattice constants in Ti-X alloys, so the interatomic bonding and elastic property of Ti-X alloys can not be predicted using the variation of lattice constants. Theβ,α'' andωphases all satisfy the criteria of elastic stability in Ti-25 at.% Nb alloy, which implies all of theβ,α'' andωmetastable phases are elastically stable. The phase stability ofβ,α'' andωphases follows the order ofα″>ω>β. The shear and Young's moduli increase with theβ,α″andωphase in order in the Ti-25 at.% Nb alloy.In chapter 4, the method of first-principles calculations has been employed to investigated the structural stability, magnetic property and elastic properties of the ternary Co₃(Al,W) and Co₃(Ge,W) precipitate in Co-Al(Ge)-W ternary alloys. The results show that both Co₃(Al,W) and Co₃(Ge,W) precipitate have L12 structure. Both L12 Co₃(Al,W) and Co₃(Ge,W) precipitate have a strengthening effect in the disordered fcc cobalt matrix, owing to the larger modulus differences between precipitate and cobalt matrix. The L12 Co₃(Al,W) precipitate is a magnetic compound, and its magnetic moments per atom is about 0.5μB. The effect of spin polarization can not be neglected for the elastic property calculation of L12 Co₃(Al,W). The calculated results of the free energy of formation show that when the content of W is higher than 23 at.%, Co₂4(Al_8-xW_x) compound has hexagonal structure at 1173 K. The L12 and hexagonal structure are coexistent in Co₂4(Al_8-xW_x) compounds at 1173 K, when the content of W is between 18.5 at.% and 23 at.% in the Co-Al-W alloy. When the content of W is lower than 18.5 at.%, Co₂4(Al_8-xW_x) compound has L12 structure at 1173 K.