Environmentally Assisted cracking (EAC) is a very critical materials science problem that concerns many technological areas such as petrochemical engineering, aerospace operations and nuclear power generation, in which cracking or sudden failure of materials may happen at stress far below the tensile strength. This type of corrosion is initiated at the microscopic level and is complicated due to the combination of chemistry (reaction caused by corrosive agents) and mechanics (varying load). As EAC is generally related to the segregation of impurity elements to defects (mainly grain boundaries), the symptoms of risk may not be apparent from the exterior of the metal components: hence EAC remains latent and gives no sign of warning until the failure occurs. Due to its intricate nature, conducting experiments on this phenomenon involves difficulties and requires much effort. In this work, we employed advanced molecular simulation techniques to study EAC in order to give insight into its atomistic behavior. First, Density-Functional Theory (DFT) method was used to investigate the fundamental processes and mechanism of EAC-related issues at the nanoscale level, with two case studies concerning the stress corrosion in iron and hydrogen embrittlement in palladium. When segregating to the grain boundary (GB) of iron, different impurity elements such as sulfur, phosphorus and nitrogen raise corrosion failures in a variety of ways. Hydrogen atoms, due to their mobility and small atomic size, are able to form high occupation at crystal defects, but show different interactions to vacancy and GB. Then, we used the classical Molecular Dynamics (MD) method to gain an understanding of the dynamic response of materials to mechanical load and the effects of temperature, strain and extreme conditions (high pressure shock compression) on structural properties. The MD simulations show that hydrogen maintains the highest localization at grain boundaries in the vicinity of ambient temperatures, and grain boundaries are the preferred nucleation sites for dislocations and voids. This computational work, using DFT and MD techniques, is expected to contribute to the better understanding on chemistry and mechanisms of complex environment-assisted cracking phenomenon at a fundamental level in order to beneficially complement conventional laboratory approaches.

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.

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.

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.

In this thesis, we have performed systematical study of the complex impedance spectra(CIS) with the manganeses oxide thin films by the equivalent circuit model(ECM) composed of resistance and capacitance. The ECM has been utilized in analog of the electrical and dielectric properties of the granular films. The purpose of this research is to understand how the electrical- and magneto-transport properties in La0.67Ca0.33MnO3(LCMO),La0.8Ba0.2MnO3(LBMO),La0.67Sr0.33MnO3(LSMO(113)) and La0.67Sr1.33MnO4 (LSMO(214)) thin films, at various magnetic fields and temperatures. First of all, we demonstrate that the LSMO(214) and LSMO(113) can be sensitively affected by magnetic states on the manganite films. Our result provides further understanding of the dielectrics variation during the phase transition from an AFM insulating phase and/or a ferromagnetic metallic phase to a paramagnetic PM metallic phase. It is known that the strong correlation between the itinerant carriers and the local magnetic moments is the mechanism for FM/PM phase transition for LSMO(113), while the direct magnetic exchange coupling governed the AFM/PM phase transition and an indirect coupling to the status of intrinsic carriers for LSMO(214) films. These transitions can not be concludes directly by using a dc resistance measurement but can be clearly distinguished by the CIS measurement. On the other hand, the dc resistance (Rdc) and the relaxation time(£n) have the same tendency that this indicates the changes of £n matches to the electric transport properties for LCMO_90min and LSMO(214) thin films. We focus on the the dielectric properties of both samples are insensitive to temperature, revealing that the dielectric behavior is independent of magnetic phase transition but strongly associated with the transport properties. Therefore, the magnetic transitions can be most thoroughly investigated by combining CIS measurements and RC ECM, as well as by making dc resistance measurements. Moreover, the relative change of M£q(ac) is nearly larger than the dc resistive variation. This phenomenon, called giant magneto-impedance effect (GMI), implies that thehigh-frequency magnetotransport effect may enhance the performance of these manganese oxides for sensing the magnetic field. The CMI, have been analyzed by ECM, including two sets of parallel R and capacitance (C) components in series. The analyzing results the specific feature of grain boundaries(GBs) can be attributed to the interplay of magnetic moment spin disorder to ordering. The grain boundary (GB) effect can enhance low field magnetoresitance (LFMR) for artificial GBs, but shows very limited enhancement for those GBs in epitaxial films. This study finds that artificial GBs, which exhibit large LFMR, can be modeled as a non-conductive layer which disconnects the lattice periodicity of adjacent grains and contains no magnetic ions. The GBs in the present fully strained epitaxial film, which shows a relatively smaller LFMR, are more similar to a semi-continuous grain with continuous distribution of magnetic ions that align loosely parallel to the grain magnetic moment. In addition, we report in this study the high frequency magneto-transport properties, based on the classical model, of La0.8Ba0.2MnO3 and La0.67Ca0.33MnO3 thin films around their ferromagnetic transitions and under an external magnetic field. It is found that the specific features of magneto-impedance can be correlated with the complex magnetization response and the dielectric relaxation in corresponding phase states. The fast dielectric relaxation time, £nE, and the slow magnetic response, £nH, reflect the interplay of itinerant carriers and the magnetic coupling to the ac electromagnetic wave, indicating that the double exchange, or hopping, of carriers between O 2P and Mn 3d-eg states occur prior to the indirect magnetic coupling of adjacent Mn ions via strong Hunt¡¦s rules. Applied magnetic field enhances both electric and magnetic dipoles are now responding faster to the electromagnetic wave. The results of our work may provide a fundamental understanding of high frequency magnetic and electrical properties of the manganite films, and imply tips for device application of the films.

This thesis describes the effect of dislocation slip on superplastic deformation of AZ31 magnesium alloy. Through two different routes of ECAE (equal channel angular extrusion), two types of specimens having the same grain size but different texture were obtained. One is favorable for basal slip and the other is not. Under the same condition of deformation, the strain rate sensitivity and contribution of grain boundary sliding to total elongation in these two different specimens are almost the same. As for elongation, not much difference was found. The present results demonstrate that the relationship between dislocation slip and grain boundary sliding in superplastic AZ31 magnesium alloy is non-obvious.

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Following the first report on electronic transport measurements of graphene, an atom-thick carbon material, many scientists have devoted effort to understand its fundamental properties. In this work, the mechanical properties of graphene-based materials, including monolayer graphene oxide and chemical vapor deposition (CVD) grown graphene, were determined using membrane structures. Furthermore, a membrane structure was used to demonstrate thermoacoustic sound generation from monolayer graphene. In order to realize the mechanical characterization, reproducible methods to fabricate graphene membranes were developed using dry and wet transfer techniques. A novel dry transfer technique produced graphene-sealed microchambers without trapping liquid inside. An improved wet transfer technique enabled the transfer of graphene onto perforated substrates. Monolayer graphene oxide was mechanically tested using scanning atomic force microscopy (AFM) combined with finite element analysis of the data. The mechanical deformation was measured by scanning AFM tips over the suspended graphene oxide membranes. The Young’s modulus of the membranes was obtained by analyzing the deformation using finite element analysis together with a mapping technique. In addition, membranes with 2 and 3 layers of graphene oxide were identified using transmission electron microscopy and mechanically characterized. Moreover, these same methods were used for measuring mechanical properties of ultra-thin amorphous carbon membranes. Bulge tests, which apply uniform pressure on the suspended membrane, revealed the mechanical behavior of polycrystalline graphene grown on copper foils by chemical vapor deposition. In particular, the effect of grain boundaries on the elastic properties of polycrystalline graphene was studied by correlating its Young’s modulus with the density of grain boundaries within the membranes. It was observed that a large number of grain boundaries softened the graphene membranes. Graphene, along with monolayer hexagonal boron nitride, is the ultimate limit of thin materials. Thus, it is an ideal candidate as a thermoacoustic sound source because of its low heat capacity per unit area. The work presented here provides the first demonstration of thermoacoustic sound generation from large-area monolayer graphene. A fundamental understanding of the influence of the underlying substrates was achieved by comparing the acoustic performance of graphene membranes on various patterned substrates with different porosities. / text

Nanocrystalline (NC) metals with averaged grain size smaller than 100 nm have shown promising mechanical properties such as higher hardness and toughness than conventional coarse-grained metals. Unlike conventional metals in which the deformation is controlled by dislocation activities, the microstructural evolution in NC metals is mainly dominated by grain rotation and stress-driven grain boundary motion (SDGBM) due to the high density of grain boundaries (GBs). SDGBM is thus among the most studied modes of microstructural evolution in NC materials with particular interests on their fundamental atomistic mechanisms. In the first part of this thesis, molecular dynamics simulations were used to investigate the influences of Triple Junctions (TJs) on SDGBM of symmetric tilt GBs in copper by considering a honeycomb NC model. TJs exhibited asymmetric pinning effects to the GB migration and the constraints by the TJs and neighboring grains led to remarkable non-linear GB motion in directions both parallel and normal to the applied shear. Based on these findings, a generalized model for SDGBM in NC Cu was proposed. In the second part, the interaction of SDGBM with crack, voids and precipitates was investigated. It was found that depending on the GB structure, material type and temperature, there is a competition between different atomistic mechanisms such as crack healing, recrystallization and GB decohesion. It is hoped that the findings of this work could clarify the micro-mechanisms of various experimental phenomena such as grain refinement in metals during severe plastic deformation, which can be used to design optimized route of making stabilized bulk NC metals. / February 2016

Three sheet materials, including high purity aluminum, commercial purity aluminum, and an aluminum-magnesium alloy with 3 wt% magnesium, were produced into single-crystals in the solid-state. The method, developed in 1939 by T. Fujiwara at Hiroshima University, involves straining a fully recrystallized material then passing it into a furnace with a high temperature gradient at a specific rate. This method preserves composition and particulate distributions that melt-solidification methods do not. Large single crystals were measured for their orientation preferences and growth rates. The single-crystals were found to preferably orient their growth direction to the <120> to <110> directions, and <100> to <111> directions normal to the specimen surface. The grain boundary mobility of each material was found to be a function of impurity content. The mobility constants observed were similar to those reported in the literature, indicating that this method of crystal growth provides an estimate of grain boundary mobility. This is the first study the effect of impurities and alloying to this single-crystal production process, and to show this method’s applicability in determining grain boundary mobility information. / text