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Made available in DSpace on 2014-10-09T12:51:41Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:10:25Z (GMT). No. of bitstreams: 0 / Um dos principais mecanismos de falha que causam riscos de fratura a reatores de água pressurizada é a corrosão sob tensão de ligas metálicas em água do circuito primário (CSTAP). É causada por uma combinação das tensões de tração, meio ambiente em temperatura e microestruturas metalúrgicas susceptíveis. Ela pode ocorrer, dentre outros locais, nos bocais do mecanismo de acionamento das barras de controle. Essa fratura pode causar acidentes que comprometem a segurança nuclear através do bloqueio das barras de controle e vazamentos de água do circuito primário reduzindo a confiabilidade e a vida útil do reator. O objetivo desta Tese de Doutorado é o estudo de modelos e uma proposta de modelagem para fraturas por corrosão sob tensão em liga 75Ni15Cr9Fe (liga 600), em água de circuito primário de reator de água pressurizada nesses bocais. São superpostos modelos eletroquímicos e de mecânica da fratura e validados com dados obtidos em experimentos e na literatura. Na parte experimental foram utilizados resultados obtidos pelo CDTN no equipamento recém-instalado de ensaio por taxa de deformação lenta. Na literatura está proposto um diagrama que exprime a condição termodinâmica de ocorrerem diversos modos de CSTAP na liga 600: partiu-se de diagramas de potencial x pH (diagramas de Pourbaix), para a liga 600 imersa em água primária à alta temperatura (3000C a 3500C). Sobre ele, determinaram-se os submodos de corrosão, a partir de dados experimentais. Em seguida acrescentou-se uma dimensão adicional ao diagrama, correlacionando uma variável a que se denominou fração de resistência à corrosão sob tensão. No entanto, é possível acrescentar-se outras variáveis que exprimem a cinética de iniciação e/ou crescimento de trinca, provenientes de outras modelagens de CSTAP. A contribuição original deste trabalho se insere nessa fase: partindo-se de uma condição de ensaio de potencial versus pH, foram iniciadas as modelagens de um modelo empírico-comparativo, um semi-empírico-probabilístico, um de tempo de iniciação e um de taxa de deformação, a partir dos ensaios experimentais e superpostas a essa condição. Esses exprimem respectivamente a susceptibilidade à CSTAP, o tempo de falha, e nos dois últimos o tempo de iniciação de falha por corrosão sob tensão. Os resultados foram comparados com os da literatura e se mostraram coerentes. Através desse trabalho, obteve-se uma metodologia de modelagem a partir de dados experimentais. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Nickel-base superalloys have been emerged as materials for gas turbines used for jet propulsion and electricity generation. The strength of the superalloys depends mainly from an ordered precipitates of L12 structure, so called gamma prime (γ’) dispersed within the disorder γ matrix. The Ni-base alloys investigated in this dissertation comprise both model alloy systems based on Ni-Al-Cr and Ni-Al-Co as well as the commercial alloy Rene N5. Classical nucleation and growth mechanism dominates the γ’ precipitation process in slowed-cooled Ni-Al-Cr alloys. The effect of Al and Cr additions on γ’ precipitate size distribution as well as morphological and compositional development of γ’ precipitates were characterized by coupling transmission electron microscopy (TEM) and 3D atom probe (3DAP) techniques. Rapid quenching Ni-Al-Cr alloy experiences a non-classical precipitation mechanism. Structural evolution of the γ’ precipitates formed and subsequent isothermal annealing at 600 °C were investigated by coupling TEM and synchrotron-based high-energy x-ray diffraction (XRD). Compositional evolution of the non-classically formed γ’ precipitates was determined by 3DAP and Langer, Bar-on and Miller (LBM) method. Besides homogeneous nucleation, the mechanism of heterogeneous γ’ precipitation involving a discontinuous precipitation mechanism, as a function of temperature, was the primary focus of study in case of the Ni-Al-Co alloy. This investigation coupled SEM, SEM-EBSD, TEM and 3DAP techniques. Lastly, solute partitioning and enrichment of minor refractory elements across/at the γ/ γ’ interfaces in the commercially used single crystal Rene N5 superalloy was investigated by using an advantage of nano-scale composition investigation of 3DAP technique.

<p>Thermal barrier coatings (TBCs) made of low-thermal-conductivity ceramic topcoat, metallic bond coat and metallic substrate, have been extensively used in gas turbine engines for thermal protection. Recently, additive manufacturing (AM) or 3D printing techniques have emerged as promising manufacturing techniques to fabricate engine components. The motivation of the thesis is that currently, application of TBCs on AM’ed metallic substrate is still in its infancy, which hinders the realization of its full potential.</p> <p>The goal of this thesis is to understand the processing-structure-property relationship in thermal barrier coating deposited on AM’ed superalloys.</p> <p>The APS method is used to deposit 7YSZ as the topcoat and NiCrAlY as the bond coat on TruForm 718 substrates fabricated using the direct metal laser sintering (DMLS) method. For comparison, another TBC system with the same topcoat and bond coat is deposited using APS on wrought 718 substrates. For thermomechanical property characterizations, thermal cycling, thermal shock (TS) and jet engine thermal shock (JETS) tests are performed for both TBC systems to evaluate thermal durability. Microhardness and elastic modulus at each layer and respective interfaces are also evaluated for both systems. Additionally, the microstructure and elemental composition are thoroughly studied to understand the cause for better performance of one system over the other.</p> <p>Both TBC systems showed similar performance during the thermal cycling and JETS test but TBC systems with AM substrates showed enhanced thermal durability especially in the case of the more aggressive thermal shock test. The TBC sample with AM substrate failed after 105 thermal shock cycles whereas the one with wrought substrate endured a maximum of 85 cycles after which it suffered topcoat delamination. The AM substrates also demonstrated an overall higher microhardness and elastic modulus except for post thermal cycling condition where it slightly underperformed. This study successfully demonstrated the use of AM built substrates for an improved TBC system and validated the enhanced thermal durability and mechanical properties of such a system.</p> <p>A modified YSZ TBC architecture with an intermediate Ti3C2 MXene layer is proposed to improve the interfacial adhesion at the topcoat/bond coat interface to improve the thermal durability of YSZ</p> <p>12</p> <p>TBC systems. First principles calculations are conducted to study the interfacial adhesion energy in the modified and conventional YSZ TBC systems. The results show enhanced adhesion at the bond coat/MXene interface. At the topcoat/MXene interface, the adhesion energy is similar to the adhesion energy between the topcoat and bond coat in a conventional YSZ TBC system.</p> <p>An alternative route is proposed for the fabrication of YSZ TBC on nickel base superalloy substrates by using the SPS technology. SPS offers a one-step fabrication process with faster production time and reduced production cost since all the layers of the TBC system are fabricated simultaneously. Two different TBC systems are processed using the same heating protocol. The first system is a conventional TBC system with 8YSZ topcoat, NiCoCrAlY bond coat and nickel base superalloy substrate. The second system is similar to the first but with an addition of Ti3C2 MXene layer between the topcoat and the bond coat. Based on the first principles study, addition of Ti3C2 layer enhances the adhesion strength of the topcoat/bond coat interface, an area which is highly susceptible to spallation. Further tests such as thermal cycling and thermal shock along with the evaluation of mechanical properties would be carried out for these samples in future studies to support our hypothesis.</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. 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. 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. 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.

No description available.

De nombreuses études ont permis de développer des modèles de plasticité cristalline rendant compte de l’anisotropie d’un monocristal. Les matériaux à solidification dirigée (DS) peuvent être simulés avec des modèles semblables, sous réserve de connaître la morphologie et l'orientation cristallographique des grains contenus dans la pièce. Pour éviter ces analyses microstructurales coûteuses, il est possible de développer des approches, déterministes ou analysant les dispersions de la réponse sur un lot de << pièces synthétiques >> résolues par la méthode des éléments finis. Dans cette étude, nous avons tenté d'apporter les outils nécessaires aux deux types de modélisation. Avant tout, un modèle du monograin de DS200+Hf a été identifié pour une gamme de température allant de l’ambiante à 1200°C. Ensuite, plusieurs montées d'échelle ont été envisagées, d'abord sur un volume élémentaire représentatif (VER) puis sur une structure tridimensionnelle (éprouvette cruciforme). Sur le VER, la réponse de plusieurs modèles micromécaniques a été confrontée à des calculs de référence utilisant la méthode des éléments finis. Puis, le comportement mécanique d'une éprouvette cruciforme en DS200+Hf a été étudié, en réalisant des essais expérimentaux biaxiaux qui, ensuite, ont permis d'évaluer les prévisions du modèle. Ces résultats amènent à s'interroger sur la modélisation adaptée aux structures oligogranulaires (i.e. constituées d’un faible nombre de grains) : faut-il mailler explicitement l'échelle locale (les grains) dans la structure ou malgré la non-séparabilité des échelles, le modèle homogénéisé continue-t-il de fournir des résultats satisfaisants ? / Various studies were aimed at developing crystal plasticity models to account for the anisotropic mechanical behaviour of single crystals. Directionally solidified (DS) materials can be modeled using such approaches, taking into account the underlying crystallographic structure. It requires the knowledge of the position, shape and crystallographic orientations of grains. To prevent heavy microstructure analyses, other models have to be developed for industrial calculations, using homogenization theory or considering a batch of synthetic pieces calculated using Crystal Plasticity Finite Elements Method (CPFEM). The aim of this thesis is to bring computational tools to carry out the two types of modeling for industrial applications. First of all, a crystal plasticity model for one grain of DS200+Hf is defined ranging from room temperature to 1200°C. Some scale transition rules, using full-field or mean-field approaches, are studied first in the theoretical case of a representative volume element (RVE) and then on tri-dimensional structures in order to access overall and local responses. For the RVE responses, micromechanical models are compared with a reference produced by CPFEM for various loadings. Moreover, the mechanical behaviour of a DS200+Hf cruciform specimen is studied. Biaxial tests with digital image correlation allow us to check the model predictions. These results raise questions about the modeling of oligogranular structures (i.e. with a small number of grains): should it be accepted that the local scale must be explicitly meshed, or, despite the fact that scale separability is not ensured, can we consider that the homogenized model still produces reliable results?

High strength nickel-base superalloys have been used in turbine blades for many years because of their superior performance at high temperatures. In such environments superalloys have limited oxidation and corrosion resistance and to solve this problem, protective coatings are deposited on the surface. The positive effect of coatings is based on protecting the surface zone in contact with hot gas atmosphere with a thermodynamically stable oxide layer that acts as a diffusion barrier. During service life, mechanical properties of metallic coatings can be changed due to the significant interdiffusion between substrate and coating. There are also other degradation mechanisms that affect nickel-base superalloys such as low cycle fatigue, thermo-mechanical fatigue and creep. The focus of this work is on a study of low cycle fatigue and out-of-phase thermo-mechanical fatigue behaviour of three uncoated and coated nickel-base superalloys. Polycrystalline IN792 and two single crystals CMSX-4 and SCB were coated with four different coatings; an overlay coating AMDRY997 (NiCoCrAlYTa), a platinum aluminide modified diffusion coating RT22 and two innovative coatings with a NiW interdiffusion barrier in the interface called IC1 and IC3. A low cycle fatigue and thermo-mechanical fatigue device was designed and set-up to simulate service loading of turbine blades and vanes. The low cycle fatigue tests were run at 500oC and 900oC while the thermo-mechanical fatigue tests were run between 250oC and 900oC.To simulate long service life, some coated specimens were exposed at 1050oC for 2000 h before the tests. The main conclusions are that the presence of the coatings is, in most cases, detrimental to LCF lives of the superalloys at 500oC while the coatings do improve the LCF lives of the superalloys at 900oC. Under TMF loading conditions, the coatings have negative effect on the lifetime of IN792. On single crystals, they are found to improve TMF life of the superalloys, especially at lower strains. The tests also indicate that long-term aging influences the fatigue and fracture behaviour of coated superalloys by oxidation and diffusion mechanisms when compared to non-aged specimens. The aged specimens exhibit longer life in some cases and shorter life during other test conditions. Fatigue cracks were in most cases initiated at the surface of the coatings, growing transgranularly perpendicular to the load axis.