The need for industrial gas turbines to operate at higher temperatures and/or speeds has resulted in the continual modification of nickel-based superalloys to provide better high-temperature strength and corrosion resistance for components such as hot-section turbine blades. Thermal-Mechanical Fatigue (TMF) cracking, accelerated by the oxidation that forms as a result of the exposure of bare metal during the crack-opening stages, is one of the most common forms of damage experienced by service-run turbine blades. Due to the high costs associated with manufacturing nickel-based superalloy components, damaged turbine blades must be repaired to restore their original mechanical properties. One such method, Activated Diffusion Brazing (ADB), is under development for this purpose, and involves melting a two-part powder mixture into a damaged region. However, the tenacious oxides formed on nickel-based superalloy components provide an obstacle for the repair process, and must be removed. Fluoride-Ion Cleaning (FIC) uses flowing hydrogen and HF gas to remove tenacious oxide scales through a set of chemical reactions, leaving cleaned components free of oxide compounds and depleted of the strong oxide-formers of Al and Ti. GTD111 is a nickel-based superalloy containing the strong oxidizing elements of Al, Ti, and Cr, and is similar in composition to other nickel-based superalloys such as DD8 and Rene95. Literature concerning the oxidation, cleaning, and brazing of this particular alloy is limited, and as such this thesis serves as a comprehensive overview of the chemical effects of each above process on GTD111. The objectives of this project are to determine, through SEM-EDX and element mapping analysis, the oxidation behavior of nickel-based superalloy GTD111, the effects of oxidation and FIC on the chemistry near the surface of this particular alloy, and the effects of mixing ratio and paste viscosity on the quality of repairs made by ADB. Notches of 8 mm depth and 0.25 mm width were made in coupons of GTD111 via wire-EDM and samples were oxidized between 1 and 452 hours at 900°C. Samples oxidized between 96 and 452 hours were sectioned in half and one half of each sample was cleaned via the standard FIC process at Ti-Coating Inc. Notches of 8 mm depth and 1 mm width, also made via wire-EDM, were repaired by the ADB process with a bonding temperature of 1220°C and a holding time of 65 minutes. Time-dependent multi-layer oxide growth was observed on all samples, consisting of an innermost discontinuous aluminum oxide region, followed by a thin continuous band of Ni-W-Ta oxide and a thicker, very dense chromium oxide layer. Some oxidation times exhibited the presence of weak, inconsistent oxide regions rich in Ni and/or Ti. Since the GTD111 alloy does not contain sufficient amount of Al to form a continuous layer – as 5-7% Al is required – oxidation resistance was provided mainly by the formation of the dense chromium oxide layer. A region heavily depleted of Al and Ti and therefore the strengthening gamma prime phase was observed below and surrounding the Al-rich oxide regions. Chemical analysis of cleaned samples showed that the standard FIC process at Ti-Coating Inc. was able to remove all oxide compounds formed during oxidation at 900°C, and that the prior oxidation time had no effect on the chemistry within the surface of the cleaned samples; however, the depths of elemental and gamma prime phase depletion were affected. The elemental depletions of Al and Ti have been observed in past studies, but depletions of Ni and concentrations of Cr near the surfaces of cleaned components have not been previously observed. Preliminary brazing trials made with varying paste viscosities demonstrated the importance of paste pre-placement and maintaining the molten filler metal within the notch, as better pre-placement resulted in higher densities in the braze-repaired region of the brazing trial samples. Although porosity was observed on all samples, the paste pre-placement was found to be more important in reducing porosity than the mixing ratio and paste viscosity, although using an appropriate paste viscosity allowed for better pre-placement.

In this work, a constitutive theory is developed, within the context of continuum mechanics, to describe the creep deformation of single crystal superalloys. The con- stitutive model that is developed here is based on the fact that as bodies deform the stress free state that corresponds to the current configuration (referred to as the "natural configuration", i.e., the configuration that the body would attain on the removal of the external stimuli) evolves. It is assumed that the material possesses an infinity of natural (or stress-free) configurations, the underlying natural configuration of the body changing during the deformation process, with the response of the body being elastic from these evolving natural configurations. It is also assumed that the evolution of the natural configurations is determined by the tendency of the body to undergo a process that maximizes the rate of dissipation. Central to the theory is the prescription of the forms for the stored energy and rate of dissipation functions. The stored energy reflects the fact that the elastic response exhibits cubic symmetry. Consistent with experiments, the elastic response from the natural configuration is assumed to be linearly elastic and the model also takes into account the fact that the symmetry of single crystals does not change with inelastic deformation. An ap- propriate form for the inelastic stored energy (the energy that is `trapped' within dislocation networks) is also utilized based on simple ideas of dislocation motion. In lieu of the absence of any experimental data to corroborate with, the form for the inelastic stored energy is assumed to be isotropic. The rate of dissipation function is chosen to be anisotropic, in that it reflects invariance to transformations that belong to the cubic symmetry group. The rate of dissipation is assumed to be proportional to the density of mobile dislocations and another term that takes into account the damage accumulation due to creep. The model developed herein is used to simulate uniaxial creep of <001>, <111> and <011> oriented single crystal nickel based su- peralloys for a range of temperatures. The predictions of the theory match well with the available experimental data for CMSX-4. The constitutive model is also imple- mented as a User Material (UMAT) in commercial finite element software ABAQUS to enable the analysis of more general problems. The UMAT is validated for simple problems and the numerical scheme based on an implicit backward difference formula works well in that the results match closely with those obtained using a semi-inverse approach.

Ni-base superalloys are widely used in hot sections of gas turbine engines due to the high resistance to fatigue and creep at elevated temperatures. Due to the demands for improved performance and efficiency in applications of the superalloys, new and improved higher temperature alloy systems are being developed. Constitutive relations for these materials need to be formulated accordingly to predict behavior of cracks at notches in components under cyclic loading with peak dwell periods representative of gas turbine engine disk materials. Since properties are affected by microstructure at various length scales ranging from 10 nm tertiary γ' precipitates to 5-30 μm grains, hierarchical multiscale modeling is essential to address behavior at the component level. The goal of this work is to develop a framework for hierarchical multiscale modeling network that features linkage of several fine scale models to incorporate relevant microstructure attributes into the framework to improve the predictability of the constitutive model. This hierarchy of models is being developed in a collaborative research program with the Ohio State University. The fine scale models include the phase field model which addresses dislocation dissociation in the γ matrix and γ' precipitate phases, and the critical stresses from the model are used as inputs to a grain scale crystal plasticity model in a bottom-up fashion. The crystal plasticity model incorporates microstructure attributes by homogenization. A major task of the present work is to link the crystal plasticity model, informed by the phase field model, to the macroscale model and calibrate models in a top-down fashion to experimental data for a range of microstructures of the improved alloy system by implementing a hierarchical optimization scheme with a parameter clustering strategy. Another key part of the strategy to be developed in this thesis is the incorporation of polycrystal plasticity simulations to model a large range of virtual microstructures that have not been experimentally realized (processed), which append the experimentally available microstructures. Simulations of cyclic responses with dwell periods for this range of virtual (and limited experimental) polycrystalline microstructures will be used to (i) provide additional data to optimize parameter fitting for a microstructure-insensitive macroscopic internal state variable (ISV) model with thermal recovery and rate dependence relevant to the temperatures of interest, and (ii) provide input to train an artificial neural network that will associate the macroscopic ISV model parameters with microstructure attributes for this material. Such microstructure sensitive macroscopic models can then be employed in component level finite element studies to model cyclic behavior with dwell times at smooth and cracked notched specimens.

Understanding the effects of aged microstructures on the thermomechanical fatigue (TMF) properties of nickel-base (Ni-base) superalloys remains unclear. Few experimental results are currently available in this area, and of the limited results available, some promote aged microstructures as beneficial, while others as detri- mental. The importance of these aged structures arises from the fact that when components used in the hot sections of gas turbine engines remain in service for ex- tended periods of time, the local temperature and stress provides the catalyst for the evolution of the microstructure. An experimental assessment of a negative misfit directionally solidified (DS) Ni- base superalloy was undertaken to characterize the aging kinetics and understand the influence of the TMF cycle temperature extremum, temperature-load phasing, mean strain, creep-fatigue, orientation effects, and microstructure on TMF fatigue crack initiation. To determine the effects of aging on the TMF response, the as-heat- treated alloy was artificially aged to three unique microstructures identified in the aging kinetics study. The experiments revealed that not all aged microstructures are detrimental to the fatigue life behavior. Specifically, when the γ′ precipitates age in a manner to align themselves parallel to the axis of the applied stress, an increase in the fatigue life over that of the as-heat-treated microstructure is observed for out-of-phase TMF with dwells. To extend the experimental understanding of the aged microstructures into ser- vice component design and life analysis, a temperature-dependent crystal viscoplas- ticity (CVP) constitutive model is developed to capture the sensitivity of the aged microstructure through embedding additional variables associated with the current state of the γ′ particles. As a result of the adaptations, the CVP model has the ability to describe the long-term aging effects of directional coarsening relevant to the analysis industrial gas turbine hot section components.

Vacuum-Arc Remelting (VAR) is an important process for manufacturing Ti- and Ni-based superalloys. Currently, the sources and mechanisms behind microstructural anomalies produced in VAR superalloy ingots are not well understood. In order to help understand formation processes, a method of characterizing specific anomalies in VAR ingots is desired. This paper presents a method of characterizing the composition and morphology of anomalies in VAR alloy ingots using a combination of serial sectioning and X-ray fluorescence (XRF) energy dispersive spectroscopy (EDS) techniques. This process is demonstrated on a dirty white spot from an Alloy 718 sample. The white spot of interest was serial polished and 2-D XRF EDS maps were acquired at each polish depth. The EDS maps were then stacked to form a 3-D representation of the white spot. In addition, SEM and optical microscopy techniques were used to further characterize the composition and morphology of the dirty white spot. The dirty white spot is composed of both Ti-enriched and Nb-depleted regions. The 2-D EDS maps acquired with the XRF equipment provided adequate contrast for creating a 3-D representation of the Ti-rich region of the dirty white spot. However, contrast was not sufficient to create a 3-D representation of the Nb-depleted region. The XRF EDS equipment combined with SEM and optical microscopy techniques provided valuable information about the morphology and composition of the Alloy 718 dirty white spot. It is concluded that this dirty white spot was produced by fall-in from either the crown or shelf regions during the VAR process. / text

The simulation and design of advanced materials for fatigue resistance requires an understanding of the response of their hierarchical microstructure attributes to imposed load, temperature, and environment over time. For Ni-base superalloy components used in aircraft jet turbine engines, different competing mechanisms (ex. surface vs. subsurface, crystallographic vs. inclusion crack formation, transgranular vs. intergranular propagation) are present depending on applied load, temperature, and environment. Typically, the life-limiting features causing failure in Ni-base superalloy components are near surface inclusions. Compressive surface residual stresses are often introduced in Ni-base superalloy components to help retard fatigue crack initiation and early growth at near surface inclusions and shift the fatigue crack initiation sites from surface to sub-surface locations, thereby increasing fatigue life. To model the effects of residual stresses, inclusions, and microstructure heterogeneity on fatigue crack driving force and fatigue scatter, a computational crystal plasticity framework is presented that imposes quasi-thermal eigenstrain to induce near surface residual stresses in polycrystalline Ni-base superalloy IN100 smooth specimens with and without nonmetallic inclusions. In addition, the effect of near surface inclusions in notched Ni-base superalloy components on MSC growth and fatigue life scatter was investigated in this work. A fatigue indicator parameter (FIP)-based microstructurally small crack (MSC) growth model incorporating crack tip/grain boundary effects was introduced and fit to experiments (in both laboratory air and vacuum) for the case of 1D crack growth and then computationally applied to 3D crack growth starting (1) from a focused ion beam (FIB) notch in a smooth specimen, (2) from a debonded inclusion located at different depths within notched components containing different notch root radii, and (3) from inclusions located at different depths relative to the surface in smooth specimens containing simulated shot peened induced residual stresses. Computational predictions in MSC growth rate scatter and distribution of fatigue life were in general accordance with experiments. The general approach presented in this Dissertation can be used to advance integrated computational materials engineering (ICME) by predicting variation of fatigue resistance and minimum life as a function of heat treatment/microstructure and surface treatments for a given alloy system and providing support for design of materials for enhanced fatigue resistance. In addition, this framework can reduce the number of experiments required to support modification of material to enhance fatigue resistance, which can lead to accelerated insertion (from design conception to production parts) of new or improved materials for specific design applications. Elements of the framework being advanced in this research can be applied to any engineering alloy.

The need for industrial gas turbines to operate at higher temperatures and/or speeds has resulted in the continual modification of nickel-based superalloys to provide better high-temperature strength and corrosion resistance for components such as hot-section turbine blades. Thermal-Mechanical Fatigue (TMF) cracking, accelerated by the oxidation that forms as a result of the exposure of bare metal during the crack-opening stages, is one of the most common forms of damage experienced by service-run turbine blades. Due to the high costs associated with manufacturing nickel-based superalloy components, damaged turbine blades must be repaired to restore their original mechanical properties. One such method, Activated Diffusion Brazing (ADB), is under development for this purpose, and involves melting a two-part powder mixture into a damaged region. However, the tenacious oxides formed on nickel-based superalloy components provide an obstacle for the repair process, and must be removed. Fluoride-Ion Cleaning (FIC) uses flowing hydrogen and HF gas to remove tenacious oxide scales through a set of chemical reactions, leaving cleaned components free of oxide compounds and depleted of the strong oxide-formers of Al and Ti. GTD111 is a nickel-based superalloy containing the strong oxidizing elements of Al, Ti, and Cr, and is similar in composition to other nickel-based superalloys such as DD8 and Rene95. Literature concerning the oxidation, cleaning, and brazing of this particular alloy is limited, and as such this thesis serves as a comprehensive overview of the chemical effects of each above process on GTD111. The objectives of this project are to determine, through SEM-EDX and element mapping analysis, the oxidation behavior of nickel-based superalloy GTD111, the effects of oxidation and FIC on the chemistry near the surface of this particular alloy, and the effects of mixing ratio and paste viscosity on the quality of repairs made by ADB. Notches of 8 mm depth and 0.25 mm width were made in coupons of GTD111 via wire-EDM and samples were oxidized between 1 and 452 hours at 900°C. Samples oxidized between 96 and 452 hours were sectioned in half and one half of each sample was cleaned via the standard FIC process at Ti-Coating Inc. Notches of 8 mm depth and 1 mm width, also made via wire-EDM, were repaired by the ADB process with a bonding temperature of 1220°C and a holding time of 65 minutes. Time-dependent multi-layer oxide growth was observed on all samples, consisting of an innermost discontinuous aluminum oxide region, followed by a thin continuous band of Ni-W-Ta oxide and a thicker, very dense chromium oxide layer. Some oxidation times exhibited the presence of weak, inconsistent oxide regions rich in Ni and/or Ti. Since the GTD111 alloy does not contain sufficient amount of Al to form a continuous layer – as 5-7% Al is required – oxidation resistance was provided mainly by the formation of the dense chromium oxide layer. A region heavily depleted of Al and Ti and therefore the strengthening gamma prime phase was observed below and surrounding the Al-rich oxide regions. Chemical analysis of cleaned samples showed that the standard FIC process at Ti-Coating Inc. was able to remove all oxide compounds formed during oxidation at 900°C, and that the prior oxidation time had no effect on the chemistry within the surface of the cleaned samples; however, the depths of elemental and gamma prime phase depletion were affected. The elemental depletions of Al and Ti have been observed in past studies, but depletions of Ni and concentrations of Cr near the surfaces of cleaned components have not been previously observed. Preliminary brazing trials made with varying paste viscosities demonstrated the importance of paste pre-placement and maintaining the molten filler metal within the notch, as better pre-placement resulted in higher densities in the braze-repaired region of the brazing trial samples. Although porosity was observed on all samples, the paste pre-placement was found to be more important in reducing porosity than the mixing ratio and paste viscosity, although using an appropriate paste viscosity allowed for better pre-placement.

Metal sealing rings have been used widely in compressors, turbines and hydraulic devices. Such rings can extend out due to elasticity, and keep close contact with the valve wall, resulting in the formation of a functional seal under pressure. In this project, the failure of metal sealing rings is considered. Sealing component failure due to stress relaxation can threaten the safety of the whole steam turbine. The object of this study was to examine the stress relaxation response and corresponding changes in microstructure of metal sealing rings used in nuclear steam turbine under high temperature and applied stress. The two kinds of sealing ring samples were selected for GH4145 and GH2132. In this paper, all samples were tested by accelerated simulation experiment. The test temperature was controlled at 400℃, 600℃, and 800℃. The 400℃ experiments lasted for 10, 20, 30 and 40 hours, while the 600℃ and 800℃ experiments lasted for 5, 10, 15 and 20 hours. The surface morphology was observed by metallographic analysis. It was found that the two kinds of sealing ring samples presented with a continuous development of grain coarsening and a decrease of the twins when time and test temperature were increased. The prolongation of time and increase of test temperature will drive the grain coarsening and reduce the twins faster. Many precipitates and inclusions were observed on the surface. The composition of precipitation was examined by scanning electron microscopy (SEM). It was further studied by testing samples with applied stress. The differences between the two tests and their influence on mechanical properties are discussed. The grain coarsening and twinning in the alloy will reduce the stress relaxation resistance of the material. Additionally, the precipitates and inclusions in the alloy may adversely affect the stress relaxation performance. Sealing rings using the nickel-based superalloys have stronger anti-stress relaxation performance than sealing rings made of iron-based superalloys.

Les motoristes aéronautiques doivent désormais montrer que la présence de petites anomalies de surface, pouvant être introduites lors d’opérations de maintenance, ne mènent pas à la rupture des pièces, et ce sur toute la durée de vie du moteur. Cette étude concerne la caractérisation de la nocivité d’anomalies de surface de type rayure et choc sur la tenue en fatigue du superalliage à base nickel René 65.Afin de découpler les effets de géométrie des effets de contraintes résiduelles, les rayures et les chocs possèdent un profil géométrique identique en V. Une technique de suivi de potentiel 3 points a été mise en place dans le but d’améliorer la détection de l’amorçage et d’avoir une information sur la morphologie du front de fissure. Les résultats expérimentaux montrent un amorçage rapide et un fort ralentissement de la vitesse de propagation dans les premiers stades. Nous avons également observé, par le biais de marquages thermiques, une évolution particulière de la forme du front de fissure s’amorçant au fond des rayures.L’utilisation de traitement thermique de relaxation a alors montré que c’est le champ mécanique hétérogène créé lors de la fabrication de ces anomalies qui contrôle la durée de vie et que c’est le paramètre physique d’ordre un à modéliser.Une stratégie de modélisation de la propagation de fissures à partir d’anomalies de type choc a été proposée. Celle-ci est basée sur la connaissance du champ de contraintes résiduelles par des simulations numériques, et sur l’application de ce champ dans un modèle numérique de propagation. Les résultats ont permis de confirmer que les contraintes résiduelles étaient bien la source du ralentissement de la propagation et également responsables de l’évolution de la forme du front de fissure. Ils ont également permis d’identifier les paramètres qui doivent être mesurés lors des contrôles non destructifs. / Anomalies, introduced during maintenance operations, are not critical for in-service life of a component. This study was undertaken to characterise the harmfulness of scratch and dent anomalies on the fatigue behaviour of the nickel based superalloy René 65.In order to separate the effects of the geometry and the residual stresses, scratches and dents have the same V-type profile. A 3 points DCPD method has been used to improve the detection of the initiation and also to have information about the crack front morphology. Experimental results showed that the initiation fatigue life is short and a slowdown of the fatigue crack growth in the first stages. We also observed, thanks to heat tints marking, aparticular crack front morphology for cracks initiating from scratches. Heat treatment has been used and showed that the heterogeneous mechanical field induced by the fabrication of the anomalies controls the fatigue life and that it constitutes one of the parameters to be taken into account in a future modelling. A modelling strategy of the crack propagation from dent anomalies has been developed. This model is based on the knowledge of the residual stress field by finite elements simulations, and the application of the calculated stress field in a numerical crackgrowth model. The results confirmed that the residual stresses were the physical source of the fatigue crack growth slow-down and also responsible for the evolution of the crack front morphology. They also allowed us to identify the parameters which have to be measured during non-destructive testing.

Experimental micromechanics of materials is a branch of science that seeks to build tight connections between composition, structure, processing and performance of materials under specific operating conditions required for particular technology applications. The present project is focused on the development of techniques that use the combination of electron, ion and X-ray microscopies to study the deformation behaviour of a particularly important class of metallic alloys used in the manufacture of aeroengines, namely, the so-called Ni-base superalloys. The complex hierarchical structure of these materials means that their macroscopic response is controlled to a great extent by the phenomena that play out on very fine scales, from angstroms (lattice spacing dimension) to nanometres (precipitates, phase boundaries, dislocations, chemical inhomogeneities) to microns (grains and their boundaries, defects and their clusters, dislocation pileups) to millimetres (component scale). Understanding the fine structure and deformation behaviour requires the development of specially configured experimental setup that allow the observation and quantification of deformation to external loading. In this study, FIB-SEM methods for sample characterization and fabrication were combined with synchrotron-based X-ray diffraction and imaging techniques, and backed up by theoretical analysis and numerical simulation, to elucidate the origins of the strength of these alloys. Micropillar compression tests using in-SEM nanoindentation were used to reveal the size dependence of the apparent strength, and connection was made with the dislocation-mediated crystal slip to provide an explanation of the observed Hall-Petch type dependence with a modified Hall-Petch equation considering both intrinsic and extrinsic characteristic lengths introduced. X-ray scattering was used in the polychromatic micro-Laue mode and using Bragg coherent diffractive imaging to reveal the crystal distortion arising due to plastic deformation. A Discrete dislocation dynamics in the 2.5D formulation was used to obtain a model description of the observed phenomena. The key outcome of the work presented in this thesis lies in the successful development of advanced observational tools and relevant theoretical or computational models for mesoscale plasticity problems for crystal with complex microstructure.