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DIFFUSION REACTIONS AT METAL–OXIDE INTERFACES AND THE EFFECT OF AN APPLIED ELECTRIC FIELD by Yeonseop Yu Department ofMaterials Sciences and Engineering CaseWestern Reserve University，Cleveland, Ohio August 2005 Abstract Metal-oxide interfaces play a major role in a variety of applications, such as packaging for electronic devices, oxide-dispersion-strengthening of alloys, and thermal barrier coatings. For many of these applications, it would be a great advantage if it was possible to engineer the mechanical properties of metals–oxide interfaces. The present thesis describes experiments I have carried out on amodelmetal–oxide interface to understand diffusion reactions and the effect of an external electric field across the interface. I have focused on investigating the formation of a reaction phase, morphological changes, and redistribution of ions at the interface. These features are believed to impact themechanical properties of ametal–oxide interface.A planar Al–MgAl2O4 interface was considered as a suitable model system due to the small lattice mismatch between aluminum and MgAl2O4 and its coherent interface. Thin film of aluminum on a planar MgAl2O4(001) substrate was prepared bymolecular beamepitaxy (MBE), electron beamevaporation, and thermal evaporation. After annealing of different polarity at 620-630 C for 5- 40 h with and without an applied voltage, the interface was investigated using conventional transmission electron microscopy (CTEM), high-resolution TEM(HRTEM), X-ray energy-dispersive spectroscopy (XEDS), electron spectroscopic imaging (ESI), and electron energy-loss spectroscopy (EELS). When Al–MgAl2O4 interfaces were annealed without an applied electric field, a {111}- and {100}-faceted phase formed at the interface toward the aluminum film. The reaction phase is believed to be -Al2O3, which is formed by oxidation of aluminum at the interface and diffusion of oxygen into the aluminum film. It was also observed that magnesium ions at the interface were replaced by aluminum ions which diffused fromthe aluminum film. When Al–MgAl2O4 interfaces were annealed with an applied electric field, the results were different depending on the polarity of the applied voltage. When a positive voltage (Ua > 0V) was applied to the aluminum film, magnesium ions migrate into the MgAl2O4 substrate from the interface, which results in a Mgdeficient layer close to the interface. In the case of (Ua < 0V), pyramidal islands of {111}-faceted MgAl2O4 formin the presence of trapped or dissolved oxygen in the aluminum film. The rate-limiting step of reaction is considered to be the lateral growth of pyramidal MgAl2O4 islands, in particular the interfacial diffusion of oxygen to the MgAl2O4 islands. The results of the present study demonstrate that themorphology, the spatial distribution of ions, and, consequently, themetal–oxide bonding can be substantially modified by annealing at elevated temperatures, and that the effect of annealing can be strongly enhanced or hindered by applying an electric field. Since metal–oxide adhesion and the fracture energy sensitively depend on the spatial distribution of atomic species and the interface morphology, the results of the present study further suggest that annealing - in particular under applied electric fields - can have profound effects on the technologically important properties of metal–oxide interfaces. 链接：http://www.namipan.com/d/1.pdf/4 ... ef894d2436f60b5a600 THEORY AND MODELING OF MICROSTRUCTURAL EVOLUTION IN POLYCRYSTALLINE MATERIALS: SOLUTE SEGREGATION, GRAIN GROWTH AND PHASE TRANSFORMATIONS by Ning Ma, M.S. Ohio State University,2005 ABSTRACT To accurately predict microstructure evolution and, hence, to synthesis metal and ceramic alloys with desirable properties involves many fundamental as well as practical issues. In the present study, novel theoretical and phase field approaches have been developed to address some of these issues including solute drag and segregation transition at grain boundaries and dislocations, grain growth in systems of anisotropic boundary properties, and precipitate microstructure development in polycrystalline materials. The segregation model has allowed for the prediction of a first-order segregation transition, which could be related to the sharp transition of solute concentration of grain boundary as a function of temperature. The incorporating of interfacial energy and mobility as functions of misorientation and inclination in the phase field model has allowed for the study of concurrent grain growth and texture evolution. The simulation results were analyzed using the concept of local grain boundary energy density, which simplified significantly the development of governing equations for texture controlled grain growth in Ti-6Al-4V. Quantitative phase field modeling techniques have been developed by incorporating thermodynamic and diffusivity databases. The models have been validated against DICTRA simulations in simple 1D problems and applied to simulate realistic microstructural evolutions in Ti-6Al-4V, including grain boundary α and globular α growth and sideplate development under both isothermal aging and continuous cooling conditions. The simulation predictions agree well with experimental observations. 链接：http://www.namipan.com/d/0.pdf/f ... e3333be6a405d9e2800 |
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