Hot section components in gas turbines can be NiCoCrAlY-coated to provide the component with an Al reservoir that maintains a protective oxide layer on its surface. Over the service life of the component, the coatings degrade by composition and phase changes due to oxidation/hot-corrosion, and multicomponent interdiffusion from and into the superalloy substrate. In this study, the rate of Al interdiffusion into selected Ni-base superalloys using various diffusion couples of two-phase NiCoCrAlY (beta + gamma) and single beta-phase NiAl with the selected alloys is measured. The diffusion couples were examined with an emphasis on the composition-dependence of Al interdiffusion. Microstructural analysis of the NiCoCrAlY vs. superalloys couples is performed to examine the dependence of coatings lifetime on the superalloy composition. The beta-NiAl diffusion couples were analyzed to determine the integrated, apparent and average effective interdiffusion coefficient as a function of superalloy's composition. Concentration profiles were obtained by EPMA of the NiAl vs. superalloy diffusion couples. Findings of this study show that the lifetimes of NiCoCrAlY are heavily dependent on superalloy compositions. The rate of interdiffusion in the diffusion couples is affected by the refractory precipitate phase microstructure structures in the interdiffusion zones as well as by component interactions. The results of the beta-NiAl diffusion couples show that increasing concentrations of Cr, Mo and Ti in the superalloy increase the Al effective interdiffusion coefficient into the superalloy, while increasing concentrations of Al, Ta and W reduce it. Thus NiCoCrAlY-superalloy systems may be designed to produce optimal microstructures in the interdiffusion zone and minimize Al interdiffusion by consideration of these diffusional interactions.

The development of a process chain for Inconel 718 production utilizing Binder Jetting has been investigated. Different powder sources were compared by the effect they had on machine compatibility, powder bed packing, recyclability, green density, sintering parameters, final density, porosity, and mechanical properties. The three powder lots investigated originated from two different production sites. One of the three powder lots has a finer powder size distribution, due it being produced simultaneously with another powder lot with a coarser powder size distribution fraction. This synergy production results in a higher yield of the atomization process and thus is economically and environmentally beneficial. The compatibility between powder lots and Binder Jetting machine was investigated using new powder and recycled powder. By using recycled powder in the process an increase in green density by 5% could be achieved. Several temperature and hold time relations were tested to develop a sintering program with an acceptable final density above 94% of theoretical density. 1270◦C with a hold time of 4h generated the best results. Sintered samples did not reach acceptable strength properties. The elongation value was twice as high as required for one of the powder lots using recycled powder. Post heat treatment generated samples with an acceptable yield strength but highly reduced elongation properties.

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In sub-millimeter sheets of wrought H-X750 Nickel-base superalloy, aluminum-rich coatings are bonded to matrix with a vapor phase aluminization process. If an appropriate amount of aluminum is bonded to matrix with homogenization treatment, the resulting diffusion couple will diffuse into coherent (g/g’) heterogeneous phases creating matrix that is both precipitation and solid solution strengthened. The diffusional mechanisms for solid solution mass transport involved with the growth and dispersion of bonded aluminum-rich coatings in the aluminization process only differ from the no external mass flow homogenization process with annealing treatment in that the boundary conditions are different. In each case these forces that activate diffusion at the macroscopic level are connected to the activation energies of random walks of atoms on a wide scale at the angstrom level. An overview of wrought Nickel-base superalloy is presented. Starting with thin sheets the alloy will be aluminized and homogenized. The research from this study will determine the parameters for the movement of the phase boundaries, mass transport, and the time variant concentration fields for both the aluminization and homogenization processes. This is predictable for both single dimension fluxes assuming the interdiffusivities and fluxes at the phase boundaries are known. Because mass-transport is related to the movement of the phase boundaries through density, an investigation into the less dense aluminum-rich coatings and resultant matrix is also included.

<p>This master’s thesis work is a study of oxidation and degradation of nickel-base alloys at high temperatures. The materials studied are designed for use in critical gas turbine components such as turbine blades and vanes. Some of the alloys are used today, whereas others have not yet entered commercial application. In order to maximize the efficiency of gas turbines, there is an ambition to maximize the operating temperatures. There is therefore a demand for materials which can withstand the damage mechanisms active at high temperatures. Among these damage mechanisms are oxidation and microstructural degradation.</p><p>To investigate the oxidation resistance of 7 different monocrystalline and polycrystalline alloys, samples have been exposed isothermally in still air at temperatures between 850 and 1000°C, for exposure times of up to 20000h. Two of the alloys were also exposed cyclically at 950°C. Oxidation during the heat treatment resulted in significant weight changes, which were measured after each cycle for cyclically exposed samples and after completed heat treatment for isothermally exposed samples. The weight change data was used to evaluate the relative oxidation resistance of the alloys. The ranking of the alloys with respect to oxidation resistance was generally in agreement with the oxidation resistance predicted by a simple consideration of the Cr and Al contents of the alloys. However, the single-crystal alloy PWA1483 displayed better oxidation resistance than predicted from its chemical composition.</p><p>Metallographic analysis of the samples indicated that the oxide scales formed consisted of several different types of oxides. The oxide scales were mainly composed of Cr2O3 and Al2O3. Fragments of the oxide scales spalled off, primarily during cooling but also in some cases during the long-term heat treatments. Spalling of the oxide scale accelerated the oxidation process, since the ability of the oxide scale to impede diffusion decreased with its decrease in thickness. Oxidation caused depletion of Al and thereby local dissolution of the aluminum-rich γ′ particles, which are of vital importance to the mechanical properties of the material. A γ′ depleted zone thereby formed underneath the oxide scale. In this zone nitrides and needle-like particles, believed to be topologically close packed μ phase, precipitated during heat treatment. Recrystallization in the depletion zone was observed in some of the monocrystalline materials. MC carbides (M=metal) present in the virgin material decomposed during heat treatment and M23C6 carbides were formed. The γ′ particles coarsened during heat treatment, which resulted in decreased hardness. The hardness decreased with exposure temperature up to 950°C, as expected due to the increased coarsening rate. At 1000°C an unexpected increase in hardness was observed for all sample materials except one. A possible explanation for this hardness increase is redistribution of γ′, by dissolution of γ′ during heat treatment and reprecipitation during cooling as much finer particles. A fine dispersion of γ′ is expected to contribute more to the hardness than a corresponding volume of γ′ in the form of larger particles. For some of the sample series, clear correlations between hardness and γ′ particle size or exposition time were found. These relationships could potentially be used to estimate the exposure temperature of service-exposed material.</p><p>A numerical model was implemented in Matlab to describe the process of oxide growth and spalling, cycle by cycle. The model was successfully adapted to experimental data from the cyclic oxidation measurements. The general applicability of the model to cyclic oxidation data at different temperatures and cycle frequencies was not investigated. At long times of cyclic exposure, the net weight loss of the samples could be well approximated as a linear function of the number of cycles. However, during the last few cycles the amount of oxide spalled in each cycle suddenly decreased. This change in spallation behavior was mainly observed for the samples cooled in air between every cycle and to a much smaller extent for the samples cooled in water. The proposed explanation is that spalling occurred preferentially at a weak subscale interface and that the spalling propensity decreased with decreasing area of this weak interface. The deviating results of the last few cycles were not included in the modeling of the cyclic oxidation process.</p>

The goal of this work is to establish a life prediction methodology for thermomechanical loading of the Ni-base superalloy Mar-M247 over a larger temperature range than previous work. The work presented in this thesis extends the predictive capability of the Sehitoglu-Boismier unified thermo-viscoplasticity constitutive model and thermomechanical life model from a maximum temperature of 871C to a maximum temperature of 1038C. The constitutive model, which is suitable for predicting stress-strain history under thermomechanical loading, is adapted and calibrated using the response from isothermal cyclic experiments conducted at temperatures from 500C to 1038C at different strain rates with and without dwells. In the constitutive model, the flow rule function and parameters as well as the temperature dependence of the evolution equation for kinematic hardening are established. In the elevated temperature regime, creep and stress relaxation are critical behaviors captured by the constitutive model. The life model accounts for fatigue, creep, and environmental-fatigue damage under both isothermal and thermomechanical fatigue. At elevated temperatures, the damage terms must be calibrated to account for thermally activated damage mechanisms which change with increasing temperature. At lower temperatures and higher strain rates, fatigue damage dominates life prediction, while at higher temperatures and slower strain rates, environmental-fatigue and creep damage dominate life prediction. Under thermomechanical loading, both environmental-fatigue and creep damage depend strongly on the relative phasing of the thermal and mechanical strain rates, with environmental-fatigue damage dominating during out-of-phase thermomechanical loading and creep damage dominating in-phase thermomechanical loading. The coarse-grained polycrystalline microstructure of the alloy studied causes a significant variation in the elastic response, which can be linked to the crystallographic orientation of the large grains. This variation in the elastic response presents difficulties for both the constitutive and life models, which depend upon the assumption of an isotropic material. The extreme effects of a large grained microstructure on the life predictions is demonstrated, and a suitable modeling framework is proposed to account for these effects in future work.

Ce travail de thèse est consacré principalement à l’étude des relations entre la microstructure, le procédé de grenaillage et les champs de contraintes résiduelles dans le superalliage à base de nickel N18. Pour mettre en exergue le rôle de la microstructure, nous avons tout d’abord fabriqué quatre microstructures modèles de tailles de grains gamma et de précipités gamma' significativement différentes par différents chemins thermiques. Les échantillons ont été ensuite grenaillés par ultrason et enfin soit traités thermiquement ou sollicités en fatigue à chaud. Nous avons étudié les changements microstructuraux et mécaniques induits par chaque étape en s'appuyant sur différentes techniques de caractérisation (MEB, dureté, essais de traction et de fatigue). Nous montrons que la dureté et les propriétés en traction avant grenaillage ainsi que les modifications microstructurales et de dureté après grenaillage sont principalement dépendantes de la taille de précipités gamma'. Des mesures in situ de résistivité électrique ont permis de suivre les cinétiques de dissolution et de précipitation de la phase gamma' au cours de traitements thermiques. Les cinétiques ont été comparées à un modèle de précipitation développé pour l’alliage N18. Dans la suite, nous avons déterminé finement les contraintes résiduelles par diffraction des rayons X en laboratoire avec la méthode des « sin² psi » et au synchrotron avec la microdiffraction Laue couplée à des mesures d’énergies. La sensibilité de la microdiffraction a permis d’appréhender le rôle de la microstructure sur les champs de déformations et de contraintes à l’échelle du micromètre et de différencier la contribution des phases gamma et gamma', qui constitue l’une des principales difficultés de ce travail d’exploitation. Avant grenaillage, la déformation déviatorique est inférieure à 2 x 10-4 quelle que soit la taille de précipités gamma'. À l’issue du grenaillage, un décalage des profils de déformations et de contraintes de 100 µm est observé lorsque l'on compare la microstructure contenant de fins précipités gamma' (200 nm) à celle contenant des précipités gamma' grossiers (2000 nm). Les profils de contraintes obtenus avec la microdiffraction Laue montrent des différences significatives en comparaison à l’état de contraintes planes attendu à cœur de l’échantillon. Enfin, nous montrons qu’à l’issue d’un maintien isotherme ou d’un essai de fatigue interrompu, les déformations déviatoriques introduites par de grenaillage sont relaxées ou redistribuées / This thesis is mainly devoted to the study of the relation between microstructure, shot-peening process and residual stress fields in a N18 nickel-based superalloy. To highlight the effect of the microstructure, four simplified microstructures with significantly different gamma grain and gamma' precipitate sizes were designed using different heat treatments. Samples were then subjected to ultrasonic shot-peening and finally either to isothermal holding or to low-cycle fatigue test. Microstructural and mechanical modifications induced by each step were studied using different characterization techniques (SEM, hardness, tensile and fatigue tests). Hardness and tensile properties prior to shot-peening as well as microstructural and hardness modifications after shot-peening mainly depend on the gamma' precipitate size. In situ electrical resistivity measurements were used to follow gamma' dissolution and precipitation kinetics during heat treatments. The kinetics was compared to a model developed for the N18 alloy. Afterward, residual stresses were determined by conventional X-ray diffraction with the « sin² psi » method and synchrotron Laue microdiffraction coupled with energy measurements. The sensitivity of the microdiffraction technique allowed to understand the effect of the microstructure on strain and stress fields at the micrometer scale and to separate the contribution of gamma phase from that of gamma' phase, which is one of the major difficulties of this analysis. Prior to shot-peening, the deviatoric strain is less than 2 x 10-4 regardless of the gamma' precipitate size. After shot-peening, a shift of 100 µm on strain and stress profiles was observed between microstructures with fine gamma' precipitates (200 nm) and coarse gamma' precipitates (2000 nm). Stress profiles obtained with Laue microdiffraction method showed significant differences compared to the plane stress state expected in the sample. Finally, the deviatoric strains introduced by shot-peening are relaxed or redistributed after an isothermal holding or an interrupted fatigue test

In this paper the mechanical behaviour, both elastic and plastic, of single-crystal nickel-based superalloys has been investigated. A theoretic base has been established in crystal plasticity, with concern taken to the shearing rate on the slip systems. A model of the mechanical behaviour has been implemented, by using FORTRAN, as a user defined material model in three major FEM-programmes. To evaluate the model a simulated pole figure has been compared to an experimental one. These pole figures match each other very well. Yielding a realistic behaviour of the model.