Browsing by Author "Janisch, Rebecca"

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    Segregation of interstitial impurities at grain boundaries in transition metals : electronic structure calculations
    (2003) Janisch, Rebecca; Elsaesser, Christian (Prof. Dr.)
    In general the macroscopic properties of polycrystalline materials and hence their potential technological applications are crucially influenced by the behaviour of their microstructural interfaces. The cohesion at these grain boundaries affects the hardness, deformability and toughness of the material and it can be enhanced or decreased by segregated impurities. In this context, it is essential to understand interfacial cohesion and impurity segregation in detail, and it is vital to investigate the relationship between these microscopic features and the macroscopic properties in order to find general rules describing the latter. Knowing these rules will enable us not only to understand but also to some extent manipulate the material properties. In this work the influence of the interstitial impurities B, C, N, O, and H on the interfacial cohesion and structure of a special grain boundary in the body-centered cubic transition metals Nb and Mo is investigated by means of ab-initio calculations. The method is based on the local-density-functional theory (LDFT) and uses the mixed-basis pseudopotential approach. The analysis of interface energies and geometric translation states, site-projected densities of states and bonding electron densities yields systematic trends according to the valence configuration of the involved elements. In a continuated study the influence of a MoC precipitate at the Mo grain boundary on the atomic structure and the material's properties is investigated. This investigation facilitates the interpretation of high-resolution transmission-electron-microscopy (HRTEM) images of the investigated grain boundary. Within this part of the work the limits of a sensible use of the ab-initio method are reached. Thus, in the last part of the thesis, the possibilities of describing the Mo-C system by a semi-empirical tight-binding model and of fitting the models free parameters to ab-initio data of different bulk phases of molybdenum carbides and semicarbides are discussed.