First-principles study of defects instructural materials

Sammanfattning: In this thesis, first we focus on the Helium (He) and He bubbles behavior in three kinds of the most promising candidate structural materials for future fusion reactor. These materials are vanadium, silicon carbide (SiC) composites, and reduced activation ferritic-martensitic (RAFM) steels. Second we investigate the intrinsic stacking fault of face-centered cubic (fcc) metals and alloys, with special emphasis on the interfacial energy between fcc and hexagonal close packed (hcp) phases. The present research has been carried out using modern ab initio quantum mechanical tools based on Density Functional Theory.The microscopic mechanism of He trapping in vacancies and voids in structural materials has been examined using first-principles calculations based on pseudopotential method as implemented in the Vienna ab initio Simulation Package (VASP). For body-centered cubic (bcc) vanadium (paper I), the trapping energies for multiple He atoms in monovacancy and 9-atom small void (about 0.6 nm in diameter) have been investigated. It is found that monovacancy and 9-atom void capture at least 18 and 66 He atoms, respectively. The corresponding internal pressure caused by He cluster is as large as 7.5 and 19.3 GPa. The He-He distance constrained in small void is shorter than in gas-phase Hen clusters. This finding is consistent with the results obtained for the radial distribution function. For hexagonal 6H–SiC (paper II), the interactions between a He (in one vacancy, Va) and HenVam clusters (n, m = 1 – 4) have been investigated. For a specified vacancy number (i.e. m fixed) in HenVam, the bind energy decreases with increasing He atoms, meaning that it becomes increasingly difficult for trapping more He atoms due to the He-He repulsion. This phenomenon is further confirmed by the attractive interaction between a vacancy and HenVam that expands the void space to release He-He repulsive interaction. However, bulk 6H–SiC has a weak capacity to capture He atoms (14 He atoms) due to its brittle property. The estimated internal pressure (2.5 GPa) has the same order of magnitude as the experimental value (0.8 GPa). For ferromagnetic bcc iron (Fe) (paper III), we concentrate on the effect of chromium (Cr) and tungsten (W) alloying elements on the He stable interstitial position, migration energy and trapping energy. The formation energies of He in tetrahedral interstitial site (T-site) and octahedral interstitial site (O-site) with different number of Cr and W atoms have been studied. The He formation energy trends with increasing Cr and W content are non-linear, respectively. It is found that the antiferromagnetic Cr-Cr coupling in bcc Fe transforms to ferromagnetic coupling, and the repulsion between He and W is larger than in pure W host lattice. The He migration energy and the number of He atoms trapped by monovacancy become lower compared to pure Fe due to the additional Cr and W. It is found that Cr and W lead to higher trapping energies for multiple He and slightly hamper He trapping in vacancy compared to pure bcc Fe.In the second part of the thesis (paper IV) the stacking fault energy (SFE) and interfacial energy of six fcc metals and Fe-Cr-Ni alloys have been studied. SFE γ plays an important role in determining the plastic deformation mechanism of fcc metals and thus is a fundamental parameter describing and understanding the mechanical properties of high-technology alloys. Small SFE favors twinning, and high SFE favors dislocation slip. The formation energy of the interface between fcc(111)/hcp(0001) is a key parameter in determining the SFE when using standard thermodynamic approaches. In this thesis, two other models that are commonly used in the ab initio calculation of the SFE are considered. One is based on the supercell technique with one intrinsic stacking fault pure unit cell, and the other on the axial interaction model. Due to the different conditions for hcp structures in entering the thermodynamic model and the above ab initio models, we differentiate between the actual interfacial energy σ for the coherent fcc(111)/hcp(0001) interface and the "pseudo-interfacial energy (σ∗)", the latter appearing in the thermodynamic expression for the SFE. Using the first-principles exact muffin-tin orbitals method (EMTO) in combination with the coherent potential approximation (CPA), we investigated the coherent and pesudo-interfacial energy for six fcc metal (Al, Ni, Cu, Ag, Pt, and Au) and three Fe-Cr-Ni alloys. It is found the two interfacial energies remarkable differ from each other. Our results form the first systematic first-principles data for the interfacial energies of monoatomic fcc metals and austenitic stainless steels and are expected to be used in future thermodynamic predictions.