The mechanical properties of polycrystalline materials are governed by the underlying microstructure. In this context, in this dissertation, the role of grain boundaries on the mechanical response of two technologically important materials namely silicon and silicon carbide are examined. In particular, the dynamics of silicon carbide and silicon symmetric tilt bicrystals under shear load are characterized via molecular dynamics simulations. Cubic silicon carbide bicrystals with low-angle grain boundaries exhibit stick-slip behavior due to athermal climb of edge dislocations along the grain boundary at low temperatures. With increasing temperature, stick-slip becomes less pronounced due to competing dislocation glide, and at high-temperatures, structural disordering of the low-angle grain boundary inhibits stick-slip. In contrast, structural disordering of the high-angle grain boundary is induced under shear even at low temperatures, resulting in a significantly dampened stick-slip behavior. When a single layer graphene sheet is introduced at the grain boundary of the symmetric tilt silicon-carbide bicrystals, the resultant shear response is dictated by the orientation of the graphene sheet. Specifically, when the graphene layer is oriented perpendicular to the gain boundary, stick-slip behavior displayed by the low-angle grain boundaries is inhibited, though both low-angle and high-angle grain boundaries exhibit displacement along crystallographic planes parallel with the applied shear direction. On the other hand, when the graphene sheet is parallel to the grain boundary, shear deformation at the grain boundary for both low-angle and high-angle bicrystals is diminished. In silicon bicrystals, high-angle grain boundaries demonstrate coupled motion, characterized by an additional normal motion of the grain boundary. Interestingly, this phenomenon was observed previously in metallic materials. Further, the grain boundary coupling factor, which is ratio of the grain boundary normal velocity to the grain translation velocity, matches the predicted geometric value. The underlying atomic scale mechanisms that govern the grain boundary coupled motion consists of concerted rotations of silicon tetrahedra within the grain boundary. For low-angle grain boundaries in silicon, the activation of dislocation glide along the predicted slip-plane takes precedence and no grain boundary coupling is observed. This behavior is similar to that of silicon carbide seen at high-temperatures but for silicon it occurs for a large temperature window.

This research work presents the first comprehensive study on nanocrystalline materials produced in bulk quantities using a novel continuous electrodeposition process. A series of nanocrystalline Ni and Ni-Fe alloy continuous foils were produced and an intensive investigation into their structure and various properties was carried out. High-resolution transmission electron microscopy (HR-TEM) revealed the presence of local strain at high and low angle, and twin boundaries. The cause for these local strains was explained based on the interpretation of non-equilibrium grain boundary structures that result when conditions of compatibility are not satisfied. HR-TEM also revealed the presence of twin faults of the growth type, or “growth faults”, which increased in density with the addition of Fe. This observation was found to be consistent with a corresponding increase in the growth fault probabilities determined quantitatively using X-ray diffraction (XRD) pattern analysis. Hardness and Young’s modulus were measured by nanoindentation. Hardness followed the regular Hall-Petch behaviour down to a grain size of 20 nm after which an inverse trend was observed. Young’s modulus was slightly reduced at grain sizes less than 20 nm and found to be affected by texture. Microstrain based on XRD line broadening was measured for these materials and found to increase primarily with a decrease in grain size or an increase in intercrystal defect density (i.e., grain boundaries and triple junctions). This microstrain is associated with the local strains observed at grain boundaries in the HR-TEM image analysis. A contribution to microstrain from the presence of growth faults in the nanocrystalline Ni-Fe alloys was also noted. The macrostresses for these materials were determined from strain measurements using a two-dimensional XRD technique. At grain sizes less than 20 nm, there was a sharp increase in compressive macrostresses which was also owed to the corresponding increase in intercrystal defects or interfaces in the solid.

This research work presents the first comprehensive study on nanocrystalline materials produced in bulk quantities using a novel continuous electrodeposition process. A series of nanocrystalline Ni and Ni-Fe alloy continuous foils were produced and an intensive investigation into their structure and various properties was carried out. High-resolution transmission electron microscopy (HR-TEM) revealed the presence of local strain at high and low angle, and twin boundaries. The cause for these local strains was explained based on the interpretation of non-equilibrium grain boundary structures that result when conditions of compatibility are not satisfied. HR-TEM also revealed the presence of twin faults of the growth type, or “growth faults”, which increased in density with the addition of Fe. This observation was found to be consistent with a corresponding increase in the growth fault probabilities determined quantitatively using X-ray diffraction (XRD) pattern analysis. Hardness and Young’s modulus were measured by nanoindentation. Hardness followed the regular Hall-Petch behaviour down to a grain size of 20 nm after which an inverse trend was observed. Young’s modulus was slightly reduced at grain sizes less than 20 nm and found to be affected by texture. Microstrain based on XRD line broadening was measured for these materials and found to increase primarily with a decrease in grain size or an increase in intercrystal defect density (i.e., grain boundaries and triple junctions). This microstrain is associated with the local strains observed at grain boundaries in the HR-TEM image analysis. A contribution to microstrain from the presence of growth faults in the nanocrystalline Ni-Fe alloys was also noted. The macrostresses for these materials were determined from strain measurements using a two-dimensional XRD technique. At grain sizes less than 20 nm, there was a sharp increase in compressive macrostresses which was also owed to the corresponding increase in intercrystal defects or interfaces in the solid.

The aim of this thesis is to investigate the effects of additives, reactive elements and impurities, on the lifetime of thermal barrier coatings. The thesis consists of a number of studies on interface adhesion, impurity diffusion, grain boundary sliding and cleavage processes and their impact on the mechanical behaviour of grain boundaries. The effects of additives and impurity on interface adhesion were elaborated by using total energy calculations, electron localization and density of states, and by looking into the atomic separations. The results of these calculations allow the assessment of atomic level contributions to changes in the adhesive trend. Formation of new bonds across the interface is determined to improve the adhesion in reactive element(RE)-doped structures. Breaking of the cross interface bonds and sulfur(S)-oxygen(O) repulsion is found responsible for the decreased adhesion after S segregation. Interstitial and vacancy mediated S diffusion and the effects of Hf and Pt on the diffusion rate of S in bulk NiAl are studied. Hf is shown to reduce the diffusion rate, and the preferred diffusion mechanism of S and the influence of Pt are revealed to be temperature dependent. Finally, the effects of reactive elements on alumina grain boundary strength are studied. Reactive elements are shown to improve both the sliding and cleavage resistance, and the analysis of atomic separations suggest an increased ductility after the addition of quadrivalent Hf and Zr to the alumina grain boundaries.

Turbine components operate under severe loading conditions and at high and varying temperatures that result in thermal stresses in the presence of temperature gradients created by hot gases and cooling air. Moreover, static and cyclic loads as well as the motion of rotating components create mechanical stresses. The combined effect of complex thermo-mechanical stresses promote nucleation and propagation of cracks that give rise to fatigue and creep failure of the turbine components. Therefore, the relationship between thermo-mechanical stresses, chemical composition, heat treatment, resulting microstructure, operating temperature, material damage, and potential failure modes, i.e. fatigue and/or creep, needs to be well understood and studied. Artificial neural networks are promising candidate tools for such studies. They are fast, flexible, efficient, and accurate tools to model highly non-linear multi-dimensional relationships and reduce the need for experimental work and time-consuming regression analysis. Therefore, separate neural network models for γ’ precipitate strengthened Ni based superalloys have been developed for predicting the γ’ precipitate size, thermal expansion coefficient, fatigue life, and hysteresis energy. The accumulated fatigue damage is then estimated as the product of hysteresis energy and fatigue life. The models for γ’ precipitate size, thermal expansion coefficient, and hysteresis energy converge very well and match experimental data accurately. The fatigue life proved to be the most challenging aspect to predict, and fracture mechanics proved to potentially be a necessary supplement to neural networks. The model for fatigue life converges well, but relatively large errors are observed partly due to the generally large statistical variations inherent to fatigue life. The deformation mechanism map for 1.23Cr-1.2Mo-0.26V rotor steel has been constructed using dislocation glide, grain boundary sliding, and power law creep rate equations. The constructed map is verified with experimental data points and neural network results. Although the existing set of experimental data points for neural network modeling is limited, there is an excellent match with boundaries constructed using rate equations which validates the deformation mechanism map.

Nanocrystalline (NC) metals and alloys are known to possess superior mechanical properties, e.g., strength, hardness, and wear-resistance, as compared to conventional microcrystalline materials. NC metals are characterized by a mean grain size <100 nm; in this grain size regime, inelastic deformation can occur via a combination of interface-mediated mechanisms viz., grain boundary sliding/migration, and dislocation nucleation from grain boundary sources. Recent studies have suggested that these interface-mediated inelastic deformation mechanisms in fcc metals are influenced by non-glide stresses and interfacial free volume, unlike dislocation glide mechanisms that operate in microcrystalline fcc metals. Further, observations of tension-compression strength asymmetry in NC metals raise the possibility that yield and inelastic flow in these materials may not be adequately described by solely the deviatoric stress. Unfortunately, most literature concerning the mechanical testing of NC metals is limited to uniaxial deformation or nanoindentation techniques, and the multiaxial deformation behavior is often predicted assuming initially isotropic yield and subsequent flow normal to the yield surface. The primary objective of this thesis is to obtain a better understanding of the nature of inelasticity in NC metals by simulating multiaxial deformation at the atomistic resolution, and developing methods to interpret the results in ways that would be useful from a continuum constitutive modeling viewpoint. First, we have presented a novel, statistical mechanics-based approach to unambiguously resolve the elastic-plastic transition as an avalanche in the proliferation of mobile defects. This approach is applied to nanocrystalline Cu to explore the influence of pressure and multiaxial stress states on the inelastic deformation behavior. The results suggest that initial yield in nanocrystalline Cu under biaxial loading is only weakly anisotropic in the 5 nm grain size regime, and that plastic flow evolves in a direction normal to the von Mises yield surface. However, triaxial deformation simulations reveal a significant effect of the superimposed hydrostatic stress on yielding under shear. These results are analyzed in detail in order to assess the influence of pre-existing internal stresses and interfacial excess volume on the inelastic deformation behavior. Further, we have studied the effects of imposed hydrostatic pressure on the shear deformation behavior of Cu bicrystals containing symmetric tilt interfaces, as well as Cu nanocrystals of different grain sizes. Most interfaces exhibit an increase in shear strength with imposed compressive hydrostatic pressure. However, for some interfaces, this trend is reversed. Neither the sign nor the magnitude of the pressure-induced elevation in shear strength appears to correlate with interface structure or particular deformation mechanism(s). In Cu nanocrystals, we observe that imposed compressive pressure leads to strengthening under shear deformation, and the effect of imposed pressure on the shear strength becomes stronger with increase in grain size or temperature. Activation parameters for shear deformation have been computed for these nanocrystals, and computed values seem to agree with existing experimental and theoretical estimates. Finally, we have proposed some modifications to conventional isothermal molecular dynamics algorithms, in order to isolate dislocation nucleation events from interfacial sources, and thereby permit explicit computation of the activation parameters for such events.

Fuel cladding is one of the most critical components of nuclear reactors; so it is important to improve our understanding of various properties and behaviors of the cladding under different conditions approximating the nuclear reactor environment. Moreover, the efficiency of energy production, in addition to safety concerns, has resulted in progressive improvement of nuclear reactors design from Generation I to Generation IV. To complement this progressive trend, materials used for fuel cladding need to be improved or new materials should be developed. In this thesis, I address problems in the improvement of present fuel cladding and also investigate fuel cladding materials to be used in future Generation IV nuclear reactors. In the case of current Zircaloy-4 fuel claddings, a detailed evaluation of the surface roughness effects on their performance and properties of Zircaloy-4 fuel claddings was studied. A smoother surface on Zircaloy-4 cladding tubes is demanded by the customers; however no systematic study is available addressing the effect of surface roughness on the claddings’ performance. Thus the effects of surface roughness on texture, oxidation, hydriding behaviors and mechanical properties of Zircaloy-4 cladding tubes were investigated using various methods. It was found that surface roughness has some effects on the oxidation of Zircaloy-4. Increasing the surface roughness would increase the weight gain, however, this effect was more pronounced at the initial oxidation stages. Synchrotron techniques were used to characterize the electronic structure of zirconium alloys in their oxidized and hydrided states. With this approach, complex interactions between hydrogen and oxygen in the zirconium matrix could be investigated, which could not be resolved using conventional methods. As a candidate for future fuel cladding material, Incoloy 800H/HT, which is expected to be considered in super-critical water-cooled Gen IV reactors, was studied in order to optimize microstructure, texture and grain boundary characteristics. A specific Thermo-Mechanical Processing (TMP) was employed to manipulate the texture, microstructure and grain boundary character distribution. The deformation and annealing textures of thermo-mechanically processed samples were investigated by means of X-ray diffraction and orientation imaging microscopy. It was found that different rolling paths lead to different textures. The origin of different textures in differently (unidirectional and cross) rolled Incoloy 800H/HT at high deformation strains were investigated. In addition, the recrystallization kinetic of differently rolled samples was studied. It was found that the oriented nucleation plays an important role in determining the recrystallization texture. Unidirectional rolled samples exhibited a faster recrystallization kinetic compared with cross rolled ones, due to the presence of γ-fibre. The effect of the aforementioned microstructural parameters (grain size, texture and GBCD) on the oxidation resistance of Incoloy 800H/HT in super-critical water was investigated. It was found that the oxidation resistance of Incoloy 800H/HT can be improved by TMP. The optimum TMP process for enhancing the oxidation resistance was proposed. Microstructural parameters that can improve the oxidation resistance of Incoloy 800H/HT were identified. These findings will contribute to the effective selection of fuel cladding material for application in Gen IV SCW reactors.

Fabrication of nanoscale Josephson junctions and Superconducting Quantum Interference Devices (SQUID) is very promising but challenging topic in the superconducting electronics and device technology. In order to achieve best sensitivity of SQUIDs and to reproduce them easily with a straightforward method, new fabrication techniques for realization of nanoSQUIDs needs to be investigated. This study concentrates on investigation of new fabrication methodology for manufacturing nanoSQUIDs with High Temperature Bi-Crystal Grain Boundary Josephson Junctions fabricated onto SrTiO3 bi-crystal substrates using YBa2Cu3O7-δ (YBCO) thin-films. In this process nanoscale patterning of YBCO was realized by using electron beam patterning and physical dry etching of YBCO thin films on STO substrates. YBCO thin films were deposited using RF magnetron sputtering technique in the mixture of Ar and O2 gases and followed by annealing at high temperatures in O2 atmosphere. Structural characterization of YBCO thin films was done by Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDX). Superconducting properties of thin films was characterized by AC magnetic susceptibility measurements. Nanoscale structures on YBCO thin films were fabricated by one E-Beam Lithography (EBL) step followed by Reactive Ion Etching (RIE) and physical dry etching. First SiO2 thin film were deposited on YBCO by RF magnetron sputtering and it was patterned by EBL using Polystyrene (PS) as resist material and RIE. Then SiO2 was used as an etch mask for physical dry etching of YBCO and nanoscale structures on YBCO were formed.

abstract: ABSTRACT The behavior of the fission products, as they are released from fission events during nuclear reaction, plays an important role in nuclear fuel performance. Fission product release can occur through grain boundary (GB) at low burnups; therefore, this study simulates the mass transport of fission gases in a 2-D GB network to look into the effects of GB characteristics on this phenomenon, with emphasis on conditions that can lead to percolation. A finite element model was created based on the microstructure of a depleted UO2 sample characterized by Electron Backscattering Diffraction (EBSD). The GBs were categorized into high (D2), low (D1) and bulk diffusivity (Dbulk) based on their misorientation angles and Coincident Site Lattice (CSL) types. The simulation was run using different diffusivity ratios (D2/Dbulk) ranging from 1 to 10^8. The model was set up in three ways: constant temperature case, temperature gradient effects and window methods that mimic the environments in a Light Water Reactor (LWR). In general, the formation of percolation paths was observed at a ratio higher than 10^4 in the measured GB network, which had a 68% fraction of high diffusivity GBs. The presence of temperature gradient created an uneven concentration distribution and decreased the overall mass flux. Finally, radial temperature and fission gas concentration profiles were obtained for a fuel pellet in operation using an approximate 1-D model. The 100 µm long microstructurally explicit model was used to simulate, to the scale of a real UO2 pellet, the mass transport at different radial positions, with boundary conditions obtained from the profiles. Stronger percolation effects were observed at the intermediate and periphery position of the pellet. The results also showed that highest mass flux happens at the edge of a pellet at steady state to accommodate for the sharp concentration drop. / Dissertation/Thesis / M.S. Materials Science and Engineering 2011

Les gaz rares sont d’excellents traceurs des hétérogénéités géochimiques et isotopiques présentes dans le manteau terrestre. Cependant, le stockage et le transport de ces éléments dans les minéraux mantelliques restent mal compris. Cette thèse est centrée sur les sites de stockage de l’hélium et l’argon, et leurs mécanismes de diffusion dans les roches mantelliques. Des échantillons d’olivine polycristalline ont été dopés en hélium et argon à haute température (1150 ± 25 and 1050 ± 25 °C) et haute pression (0.30 ± 0.01 GPa), et analysés par chauffage par paliers de température avec un spectromètre de masse. L’influence d’une concentration initiale hétérogène dans l’échantillon sur les diffusivités calculées a également été testée, démontrant la robustesse des paramètres de diffusion obtenus dans cette étude. Les résultats montrent que deux domaines de diffusion sont présents dans l’olivine polycristalline : (i) un domaine à haute température avec une énergie d’activation (Ea) élevée où la diffusion est contrôlée par la diffusion dans la maille cristalline, et (ii) un domaine à plus basse température avec une Ea plus faible où la diffusion est contrôlée par à la fois la diffusion dans la maille cristalline et celle dans les joints de grains. Ces deux domaines sont séparés par une température de transition qui a lieu au moment où les joints de grains ont été vidés. Mes résultats confirment que les joints de grains peuvent représenter un site de stockage significatif pour l’hélium et l’argon. Pour l’hélium, deux populations d’Ea ont été observées dans le domaine de la maille cristalline de l’olivine, interprétées comme correspondant à la diffusion de l’hélium dans les sites interstitiels (Ea = 95 ± 15 kJ.mol-1) et dans les sites vacants du Mg (Ea = 168 ± 19 kJ.mol-1). Pour l’argon, une valeur moyenne des paramètre de diffusion dans la maille cristalline (Ea = 166 ± 44 kJ.mol-1 et logD0 = −7.04 ± 1.13 avec D0 en m2.s-1) a été obtenue à partir des données de la littérature et de notre étude. De plus, les paramètres de diffusion dans les joints de grains ont été déterminés : Ea = 45 ± 12 kJ.mol-1 et D0 = 5.30 ± 1.53 * 10-13 m2.s-1 pour l’hélium, et Ea = 22 ± 5 kJ.mol-1 et log(D0) = -12.33 ± 0.3 pour l’argon avec D0 en m2.s-1. En appliquant ces résultats au manteau supérieur, il s’avère qu’une quantité conséquente d’hélium et d’argon peut être stockée aux joints de grains (~ 22% pour une taille de grain de 1 mm). En conséquence, les diffusivités globales peuvent être significativement plus élevées que celles de la maille cristalline, induisant des implications importantes pour la géochimie et la géodynamique du manteau terrestre / Noble gases are key tracers of mantle geochemical and isotopic heterogeneities and can constrain our understanding of mantle geodynamics. Nevertheless, the basic mechanisms of noble gas storage and transport in mantle minerals remain poorly understood. In this PhD thesis, I focused on helium and argon to constrain their storage sites and the diffusive mechanisms, which occur in mantle rocks. Polycrystalline olivine was doped with helium and argon at high temperature (1150 ± 25 and 1050 ± 25 °C) and high pressure (0.30 ± 0.01 GPa), followed by step heating extraction experiments. I also tested the effect of heterogeneous initial concentrations on the extracted diffusivities, and demonstrate the robustness of diffusion parameters obtained in this study. My results show that two diffusion domains are present in polycrystalline olivine: (i) a high temperature domain with high activation energy (Ea) where diffusion is only controlled by lattice diffusion, and (ii) a lower temperature domain with lower Ea where diffusion is controlled by both grain boundary and lattice diffusion. These two domains are separated by a transition temperature that depends on the depletion of helium or argon hosted in grain boundaries, i.e., the amount of helium or argon stored at grain boundaries and the temperature and duration of the step heating sequence. The results confirm that grain boundaries can represent a significant storage site for helium and argon. Moreover, I constrained argon and helium diffusion in olivine lattice. For helium, I report two different populations of Ea in the lattice diffusion domain, which are interpreted as diffusion in interstitials (Ea = 95 ± 15 kJ.mol-1) and Mg vacancies (Ea = 168 ± 19 kJ.mol-1). For argon, a mean value of diffusion parameters in olivine lattice (Ea = 166 ± 44 kJ.mol-1 and logD0 = −7.04 ± 1.13 with D0 in m2.s-1) is obtained for data from literature and this study. Furthermore, I determine grain boundary diffusion parameters: Ea = 45 ± 12 kJ.mol-1 and D0 = 5.30 ± 1.53 * 10-13 m2.s-1 for helium, and Ea = 22 ± 5 kJ.mol-1 and log(D0) = -12.33 ± 0.3 for argon with D0 in m2.s-1. Applying these results to the upper mantle reveals that high content of helium and argon can be stored at grain boundaries. As a consequence, bulk diffusivities can be significantly higher than lattice diffusivities, inducing important implications for mantle geochemistry and geodynamics