In this research, controlled Waspaloy microstructures were produced with the objective of studying microstructural evolution in this alloy via electrically-based ac/dc non-destructive techniques. Correlations were developed between electrical measurements and alternate characterization techniques such as Ultra Small Angle X-ray Scattering (USAXS), Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) to gain a complete understanding of the microstructural transformations and the associated mechanisms. Three different sets of controlled microstructures were produced in this research. In Set I microstructures, matrix (gamma) grain sizes of 13, 52 and 89 micrometers were obtained after solution-treatments at 1045 and 176;C, 1090 and 176;C and 1145 and 176;C respectively. A vacancy stabilization treatment at 1045 and 176;C followed after which, the specimens were aged at 800 and 176;C for times ranging from 0.1 hrs to 100 hrs to vary the gamma prime precipitate size distribution. In Sets II and III, the solution-treatment was only conducted at 1145 and 176;C, with the stabilization treatment conducted only in Set II. Subsequently, aging experiments were conducted at 725 and 176;C (or 700 and 176;C in Set II), 800 and 176;C and 875 and 176;C for times up to 100 hrs. DC four-point probe resistivity of specimens increased to a maximum upon initial aging from the solution-treated condition and showed a decreasing trend thereafter with successive aging. This, in addition to complementary evidence from SEM and USAXS, led to the conclusion that gamma prime nucleation-growth was complete by the time the resistivity maximum was observed. Resistivity variations that ensued upon successive aging after the maximum were attributed to microstructural/compositional changes due to gamma prime coarsening. The height of the maximum decreased drastically with increase in aging temperature from 725 and 176;C to 800 and 176;C, while the resistivity did not increase from the solution-treated condition upon aging at 875 and 176; C. Coarsening studies based on USAXS analysis indicated an LSW type volume diffusion mechanism of coarsening in Waspaloy, with an average coarsening rate constant of 3.25x10-29 [m3/sec] for Set I specimens aged at 800 and 176;C. Analytical and Finite Element (FE) models of two-probe impedance and dc four-point probe resistivity methods were developed to gain insight into the measured response and the accurate determination of material properties. AFM-based localized electrical examination of sub-grain Waspaloy microstructures was successfully conducted using electrostatic force microscopy (EFM), scanning Kelvin probe microscopy (SKPM) and current-AFM (I-AFM) electrical modes. I-AFM experiments revealed that the conductivity of the gamma prime phase was lower than that of the gamma phase.

Cette étude porte sur la construction d’un modèle de prévision de la durée de vie à écaillage des barrièresthermiques protégeant les aubes de turbines aéronautiques, par le développement et l’exploitation d’essaisd’adhérence. La chaîne de modélisation de la durée de vie proposée comporte trois étapes. Tout d’abord, leschamps mécaniques dans les différentes couches du système sont évalués par un modèle semi-analytique decomportement de la structure multicouche, qui a été modifié pour favoriser son adaptabilité industrielle. Àpartir de l’histoire thermo-mécanique du substrat comme donnée d’entrée, qui peut être extraite d’un calculd’aube par EF, ce modèle prévoit la déformation de l’interface entre la pièce métallique et sa protectioncéramique lorsqu’un champ de contraintes lié à l’oxydation du métal lui est appliqué. Des mesures de rumpling,provoqué par le vieillissement du système à différentes températures, ont permis d’identifier et de valider lemodèle.Dans un deuxième temps, l’énergie d’adhérence est estimée au travers d’un modèle d’endommagements’appuyant sur la réponse mécanique du modèle de comportement précédent. L’endommagement, écrit àl’échelle de l’interface et découplé du comportement mécanique, a été identifié sur l’énergie d’adhérencequantifiée expérimentalement. Afin de caractériser au mieux l’énergie d’adhérence de la barrière thermiquesur son substrat, plusieurs essais ont été mis en oeuvre, permettant de solliciter l’interface dans une largegamme de mixité modale. Pour ce faire, des essais spécifiques ont été développés pour se rapprocher d’unepropagation de la fissure interfaciale en mode de cisaillement. Finalement, un critère énergétique permet dedéterminer la durée de vie du système, par comparaison de l’énergie d’adhérence et de l’énergie disponibledans le système pour la propagation d’une fissure interfaciale. Cette chaîne de prévision de la durée de vie estapplicable en post-traitement d’un calcul d’aube. Il a été montré que les tendances expérimentales sontcorrectement reproduites par la chaîne de durée de vie mise en place. / The aim of this study is to build a lifetime assessment model for thermal barrier coatings protecting aircraftturbine blades, by setting up and using adhesion tests. The model involves three steps: first, the mechanicalfields inside the layers are computed by a semi-analytical model of the multi-layered system behaviour, whichwas improved to fit the industrial demands. Given the thermo-mechanical history of the substrate (which canbe derived from FE computations), the model computes the interface strains between the metallic substrateand the ceramic protection under a stress field induced by oxidation. The model has been identified andvalidated with respect to rumpling measurements for different ageing temperatures of the system.During a second step, the interface toughness is estimated through a damage model depending on themechanical response of the multi-layered system. The damage parameters have been identified on toughnessmeasurements, and are not coupled to the multi-layer behaviour. In order to characterize the TBC toughness,several shear mode interface crack propagation tests have been developed and carried out.Finally, an energetic approach allows computing the system lifetime by comparing the decreasing interfacetoughness to the elastic stored energy. This lifetime assessment model can be applied as a post-processing of afinite element computation on a turbine blade and it has been shown that the experimental trends areconsistent with the lifetime given by the model.

Cette étude traite de l'impact du vieillissement thermomécanique sur la microstructure et sur les propriétés mécaniques du nouveau superalliage base Nickel pour disque de turbine René 65.Le vieillissement thermique conduit à trois évolutions microstructurales majeures, à savoir la croissance des précipités y' intragranulaires et à la nucléation de particules TCP aux joints de grains accompagnés d'une ségrégation de molybdène. Une méthode innovante basée sur des traitements thermiques adaptés a permis de dissocier les effets de ces deux évolutions microstructurales sur les propriétés en fluage et fatigue-temps de maintien à 700° Cdu René 65. La croissance des précipités y' intragranulaires est majoritairement responsable de l'abattement des propriétés mécaniques. Il s'avère néanmoins que la présence des particules TCP aux joints de grains ainsi que la ségrégation de molybdène affectent également le comportement viscoplastique et la durabilité de l'alliage, contribuant à un abattement supplémentaire des propriétés mécaniques. Ce phénomène est attribué à l'adoucissement localisé de la matrice au voisinage des particules TCP et des joints de grains par la perte d'éléments durcissants de la solution solide y. / This study focused on the impact of thermo-mechanical aging on the microstructure and on the mechanical properties of the new nickel-based superalloy René 65 for turbine disk applications.Thermal aging causes three main microstructural evolutions, namely the intragranular y'-growth, the nucleation of TCP particles at grain boundaries along with a segregation of molybdenurn. An innovative method based on appropriated thermal treatments enabled to dissociate these microstructural evolutions' impacts on the René 65 creep and dwell-fatigue properties at 700°C.The y'-growth is mainly responsible of the overall mechanical proprerties degradation. However, it turns out TCP particles and the molybdenum segregation at grain boundaries also affect negatively the alloy viscoplastic behavior and its durability, contributing to an additional decrease in its mechanical properties. This phenomenon is attributed to the softening of the matrix locally at grain boundaries by solid solution elements depletion in favor of TCP precipitation.

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.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22777号 / 工博第4776号 / 新制||工||1747(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 乾 晴行, 教授 安田 秀幸, 教授 辻 伸泰 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Indiana University-Purdue University Indianapolis (IUPUI) / Additively manufactured (AM) metals have been increasingly fabricated for structural applications. However, a major hurdle preventing their extensive application is lack of understanding of their mechanical properties. To address this issue, the objective of this research is to develop a computational model to simulate the creep behavior of nickel alloy 718 manufactured using the laser powder bed fusion (L-PBF) additive manufacturing process. A finite element (FE) model with a subroutine is created for simulating the creep mechanism for 3D printed nickel alloy 718 components. A continuum damage mechanics (CDM) approach is employed by implementing a user defined subroutine formulated to accurately capture the creep mechanisms. Using a calibration code, the material constants are determined. The secondary creep and damage constants are derived using the parameter fitting on the experimental data found in literature. The developed FE model is capable to predict the creep deformation, damage evolution, and creep-rupture life. Creep damage and rupture is simulated as defined by the CDM theory. The predicted results from the CDM model compare well with experimental data, which are collected from literature for L-PBF manufactured nickel alloy 718 of creep deformation and creep rupture, at different levels of temperature and stress. Using the multi-regime Liu-Murakami (L-M) and Kachanov-Rabotnov (K-R) isotropic creep damage formulation, creep deformation and rupture tests of both the secondary and tertiary creep behaviors are modeled. A single element FE model is used to validate the model constants. The model shows good agreement with the traditionally wrought manufactured 316 stainless steel and nickel alloy 718 experimental data collected from the literature. Moreover, a full-scale axisymmetric FE model is used to simulate the creep test and the capacity of the model to predict necking, creep damage, and creep-rupture life for L-PBF manufactured nickel alloy 718. The model predictions are then compared to the experimental creep data, with satisfactory agreement. In summary, the model developed in this work can reliably predict the creep behavior for 3D printed metals under uniaxial tensile and high temperature conditions.

Predicting new materials through virtually screening a large number of hypothetical materials using supercomputers has enabled materials discovery at an accelerated pace. However, the innumerable number of possible hypothetical materials necessitates the development of faster computational methods for speedier screening of materials reducing the time of discovery. In this thesis, I aim to understand and apply two computational methods for materials prediction. The first method deals with a computational high-throughput study of superalloys. Superalloys are materials which exhibit high-temperature strength. A combinatorial high-throughput search across 2224 ternary alloy systems revealed 102 potential superalloys of which 37 are brand new, all of which we patented. The second computational method deals with a machine-learning (ML) approach and aims at understanding the consistency among five different state-of-the-art machine-learning models in predicting the formation enthalpy of 10 different binary alloys. The study revealed that although the five different ML models approach the problem uniquely, their predictions are consistent with each other and that they are all capable of predicting multiple materials simultaneously.My contribution to both the projects included conceiving the idea, performing calculations, interpreting the results, and writing significant portions of the two journal articles published related to each project. A follow-up work of both computational approaches, their impact, and future outlook of materials prediction are also presented.

During operational conditions of internal combustion turbines of turbojet engines, the impeller are stressed by load cycles which vary in time, temperature and stress. In the course of operation, the blades are exposed to a considerable number of degradation effects, particularly high-temperature corrosion, fatigue processes and creep. The presented work is aimed at the study of the structure cast nickel-base superalloy INCONEL 713 LC, creep tests exposured. The structural changes (degradation) was clasification by the help of image analyses. Operational conditions of heat parts, used in gas turbines and turbo-compressors, were simulated by creep tests of Inconel 713 LC nickel superalloy during a constant load of temperatures ranging from 750 up to 950°C. Changes in the structure dependent upon time, temperature and stress applied were analysed by means of light and electron microscopy methods and image analyses after the fracture of test bars.

The present diploma thesis is focused on the effect of AlCr layer on the low cycle fatigue behaviour of cast polycrystalline superalloy IN713LC at 800°C. Protective layer is made by chemical vapour deposition followed by heat treatment. Fatigue tests were conducted in strain control mode with constant total strain amplitude and strain rate. The fatigue behaviour is assessed by cyclic hardening/softening curves, cyclic stress-strain curves, Manson-Coffin curves and Basquin curves. Microstructure was observed in as-received state and also after cyclic loading by means of optical microscopy and SEM.

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