Microstructure controls many physical properties of a material such as strength, ductility, 1density, conductivity, which, in turn, determine the application of these materials. This thesis work focuses on studying microstructural features (such as grain size, shape, defects, orientation gradients) and mechanical properties (such as hardness and yield strength) of multilayered materials that have undergone different loading and/or operating conditions. Two materials that are studied in detail are 18 nm Cu-Nb nanolaminates and 3D printed Inconel 718. Copper-Niobium (Cu-Nb) nanolaminate is a highly stable, high strength, nuclear irradiation resistant composite, which is destabilized with application of high pressure torsion (HPT). This work focuses on understanding the deformation and failure behavior of Cu-Nb using a novel orientation mapping technique in transmission electron microscopy in (TEM) called Automated Crystal Orientation Mapping (ACOM) and Digistar (ASTARTM) or Precession Electron Diffraction (PED). A new theory is postulated to explain strengthening mechanisms at the nanoscale using a data analytics approach. In-situ TEM compression and tensile testing is performed to image dislocation movement with the application of strain. This experiment was performed by Dr. Lakshmi Narayan Ramasubramanian at Xi’an Jiaotong University in China. Another major aspect of this research focuses on the design, fabrication, and microstructural characterization of 3D printed Inconel 718 heat exchangers. Various heat exchanger designs, machine resolution, printing techniques such as build orientation, power, and velocity of the laser beam are explored. Microstructural and mechanical properties of printed parts (before and after heat treatment) are then analyzed to check consistency in grain size, shape, porosity, hardness in relation to build height, scan parameters, and design. Various tools have been utilized such as scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), x-ray computed microtomography (at Advanced Photon Source at Argonne National Lab), hardness and micro-pillar compression testing for this study.

Superligas à base de níquel vêm sendo extensivamente utilizadas em diversas aplicações nas indústrias devido ao excelente comportamento mecânico e anticorrosivo. No entanto, essas ligas possuem certas particularidades que levam à necessidade de desenvolvimento de técnicas de inspeção e caracterização metalúrgica, como forma de garantir a integridade estrutural dos componentes fabricados com essas ligas. Neste trabalho, a técnica de correntes parasitas foi aplicada à superligas à base de níquel com duas propostas distintas: 1) Em um primeiro momento foi realizado o estudo da evolução microestrutural da liga Inconel 718 durante o processo de envelhecimento através da combinação do ensaio por correntes parasitas, análise de difração de raios-X, análise metalográfica, medidas de dureza e tamanho de grão. As medidas foram realizadas em amostras submetidas a diferentes ciclos de tratamentos térmicos variando entre 620-1035°C. Os resultados mostraram que as diferentes microestruturas do Inconel 718 têm efeitos distintos na condutividade elétrica quando medidos através da técnica de correntes parasitas. A influência da microestrutura na condutividade pode ser mostrada sendo devido à competição de dois efeitos sobre o espalhamento de elétrons: a purificação da matriz e a morfologia, distribuição e tamanho dos precipitados. A combinação dos valores de dureza e condutividade elétrica provou ser uma forma rápida e prática de determinar o nível de envelhecimento da liga; 2) Em um segundo momento foi desenvolvido um processo de otimização de sensores através de modelagem por elementos finitos (MEF). Através de uma metodologia de otimização, os parâmetros de construção e operação de um sensor foram otimizados para inspeção de defeitos superficiais e subsuperficiais esperados em materiais cladeados com Inconel 625. O sensor com a geometria ótima foi construído e testado a fim de verificar a eficiência do processo de otimização. Uma ótima correlação entre os resultados numéricos e experimentais foi encontrada e o sensor ótimo se mostrou eficiente na inspeção de pequenos defeitos superficiais e subsuperficiais na liga Inconel 625 quando operado nas frequências apropriadas. / Nickel-based superalloys have been extensively used in various industries due to its unique mechanical and corrosion behavior. However, these alloys show particular characteristics which lead to the need for specific inspection and metallurgical characterization techniques in order to ensure the structural integrity of components manufactured from these alloys. In this work , the eddy current technique was applied to nickel-base superalloys with two aims: 1 ) Firstly, the microstructural evolution of Inconel 718 during aging processes has been studied through a combination of eddy current testing, X-ray diffraction analysis, metallography, hardness and grain size measurements. Measurements were carried out in samples subjected to different heat treatment cycles between 620-1035°C. Results show that different microstructures of Inconel 718 have a distinguishable effect on electrical conductivity when this is measured through an appropriately sensitive technique (i.e. eddy current testing). The influence of microstructure on conductivity could be shown to be due to the competition between two effects on the scattering of electrons: matrix purification and precipitate size, distribution and morphology. A combination of hardness values and electrical properties proved to be a fast and practical way of determining the stage of aging of the alloy; 2) An optimization method of eddy current sensor design was developed through finite element modeling (FEM). Through a methodology of optimization, the construction and operation parameters of the sensor were optimized for inspection of superficial and subsuperficial defect, commonly found in weld overlay Inconel 625 claddings. A prototype of this sensor with the optimum geometry was built and tested on blocks identical to those considered in the models in order to verify the efficiency of the optimization process. A very good agreement between numerical and experimental results was found. Moreover, the optimal sensor was efficient to detect small surface and subsurface defects in Inconel 625 when operated at appropriate frequencies.

In this master thesis, work done in the TURBO POWER project High temperature fatigue crack propagation in nickel-based superalloys during spring 2012 will be presented. The overall objective of this project is to develop and evaluate tools for designing against fatigue in gas turbine applications, with special focus on the crack propagation in the nickel-based superalloy Inconel 718. Experiments have been performed to study the effect of initial overloads, and it has been shown that even for small initial overloads a significant reduction of the crack growth rate is received. Furthermore, FE simulations have been carried out in order to describe the local stress state in front of the crack tip since it is believed to control, at least partly the diffusion of oxygen into the crack tip and thus also the hold time crack growth behaviour of the material. Finally, an evaluation method for the stresses is presented, where the results are averaged over an identifiable process/damaged zone in front of the crack tip.

Cette étude traite de la prise en compte du gradient de contrainte et du frottement pour calculer l'amorçage de fissures en fatigue. On s'intéresse notamment au dimensionnement d'une manille en Inconel 718 munie de bagues en Inconel 718montées avec interférence. Pour cela, on réalise des essais de fretting et de fatigue pour étudier les conditions d'amorçage de fissures en présence de gradient de contrainte et de frottement. Les résultats expérimentaux mettent en évidence l'effet du gradient des contraintes sur l'amorçage de fissures et justifient la prise en compte du gradient pour le dimensionnement en fatigue des structures. A travers ces essais balayant une gamme de gradient de contrainte étendue, on confirme que la présence d'un gradient de contrainte retarde l'amorçage de fissures. Les avantages et les inconvénients de plusieurs approches de fatigue multiaxiale intégrant le gradient de contrainte sont étudiés. De cette analyse, une proposition est faite permettant de décrire au mieux les résultats expérimentaux de fatigue et de fretting. Cette approche s'appuie sur l'utilisation d'une fonction de poids dépendant du gradient de la pression hydrostatique. Cette approche est similaire à des approches mises en oeuvre sur d'autres problématiques de fatigue ce qui permet de dégager quelques points de convergence pour traiter le problème du gradient de contrainte en fatigue. Un essai de fatigue sur une manille munie d'une bague montée avec interférence est réalisé. L'effet du gradient de contrainte sur l'amorçage de fissures dans ce système est trop faible pour que l'utilisation d'une approche intégrant l'effet de gradient soit pertinente. Cette observation est cohérente avec les résultats expérimentaux précédents obtenus dans cette étude. L'inconel 718 apparaît donc comme un matériau peu sensible à l'effet de gradient de contrainte. Une approche de fatigue multiaxiale de type Crossland permet une description raisonnable de la durée de vie du système. Une analyse numérique montre que la tenue en fatigue du système est peu sensible au coefficient de frottement alors que l'interférence a un impact beaucoup plus important.

Cette étude financée par SNECMA et le CNES a pour objectif de mieux comprendre et caractériser le résultats de la solidification de l'inconel 718. Cet objectif est motivé par le besoin grandissant de fiabilité sur les pièces brutes de fonderies utilisées dans l'assemblage des moteurs de fusée. En effet, lors de la solidification, la ségrégation chimique provoque la formation de phase fragile. C'est notamment le cas des phases de Laves. Ainsi, un four de fonderie sous vide et un moule instrumenté ont été mise au point afin de couler au laboratoire des plaques de géométrie variable. Le but étant d'obtenir plusieurs échantillons obtenu sous différente conditions de solidifications. Une méthode d'analyse quantitative au MEB a par la suite été mise au point pour analyser les 4 coulées exploitables. Elle a permis de quantifier la quantité d'intermétalliques en fonction de la vitesse de refroidissement. Mais aussi leur répartition vis à vis des joints de grains d'une part et l'impact de la désorientation entre chaque grain d'autre part. La ségrégation chimique a aussi pu être quantifiée selon ces mêmes paramètres.De travaux numériques ont été entrepris en parallèles avec tout d'abord la simulation macroscopique des coulées avec le logiciel Thercast. Ces simulations ont permis de connaître les conditions de solidification de l'ensemble des échantillons coulés et ont fourni des données d'entré pour les autres aspects numériques. Par exemple, avec un modèle élément fini et automate cellulaire, la formation de la structure granulaire a pu être modélisée. Enfin, la simulation de la microségrégation a permis de valider notre compréhension des phénomènes prédominant lors de la solidification de l'inconel 718. Ces simulations numériques apportent à la fois un éclairage sur les résultats expérimentaux mais aussi démontrent leurs possibilités dans l'optique de mettre en place un modèle global de solidification recouvrant ces trois échelles pour des pièces industrielles

The objective of this work is to study crack path deflection under proportional and non-proportional mixed-mode fatigue and predict crack branching direction based on linear elastic fracture mechanics (LEFM) driving forces. Under proportional in-phase mixed Mode I / Mode II loading conditions, crack growth direction has previously been observed in some materials to shift from tensile-dominated Mode I to shear-dominated Mode II or mixed-mode crack growth at higher proportions of initial Mode II loading, but non-proportional loads are not well-characterized. An LEFM approach is desired in order to implement the model in crack growth software such as the boundary element-based fracture analysis package FRANC3D. A novel specimen configuration has been designed and analyzed for generation of wide ranges of mixed-mode loading conditions in a single test. This specimen and a more conventional thin-walled tubular specimen have been used to test polycrystalline nickel-base superalloy Inconel 718 under proportional in-phase and 3 kinds of non-proportional fatigue loading. Stress intensity factors for the various configurations have been analyzed with FRANC3D. Modal transition from Mode I (tensile) to Mode II (shear) crack branching has been observed in several load cases. Qualitative microscopy of fracture surfaces was used to characterize the difference between crack branch modes. An LEFM approach based on an effective stress intensity factor range, which incorporates the maximum value and range of each appropriate stress intensity (Mode I or Mode II), has been used to successfully predict the crack deflection angles, and in some cases to quantify modal transition, within each load case considered. Variability between load cases and specimen configurations points to the limitations of LEFM in providing a general predictor of crack path behavior across all types of non-proportional mixed mode loading.

Additive manufacturing (AM) is a growing process interesting many companies in many industries. Thereare multiple processes within the familty of AM, but this study focuses on laser blown powder (LBP). LBP involves a laser beam focused on the substrate with powder being blown into the laser beam. The laser beam melts both the powder and the surface of the substrate and as the laser beam moves and the melt pool solidies it leaves a bead of solid material behind. These beads are placed next to each other creating a layer which are then stacked, building the wanted geometry. As the method develops new materials are tested and this study analyses Haynes 282 powder onto Inconel 718 substrate. Multiple process parameters are involved in the LBP method and this study focuses on the impact of laser effect and scanning speed. Each value on the process parameters was inspired by previous reports with similar equipment and process. The laser effect ranges from 1600 W to 700 W, scanning speed ranges between 900 mm/min to 300 mm/min and the powder feeding rate was also varied from 4 g/min to 3 g/min. Each sample was built as a single bead and a multilayer specimen, which is ve layers and 16 beadswide at the bottom and 12 beads wide at the top. When analyzing the samples images from microscopes were mostly used for obtaining results. An image software called ImageJ allowed measurements in an image to obtain penetration depth or primary dendrite arm spacing. ImageJ also allowed measurements of porosity by turning the image binary and calculate the fraction of white and black. The results consists of numerical values and visual analysis of the bead geometry, minimum and maximum penetration, microstructure, porosity, hardness and cracks. The results show an increased bead width around 2 mm to 4 mm and decreased bead height around 0,2 mm to 0,7 mm of single beads with increased laser effect. Increased maximum penetration depth around, 200 μm to 500 μm, withincreased laser effect. More remelt between each deposited layer causing longer dendrites with increasinglaser effect. Porosity is decreased with an increased laser power, going from 0,04 % to 0,15 %. No distinct difference in hardness is observed between the samples, ranging between 255 HV to 310 HV. It is believed that aging causes the increased hardness right above the fusion zone. Cracks were found between dendrites and is believed to be caused by Laves-phases. Most results are comparable to previous similar studies, both as trends and numerical values. The statistics of the study is limited, meaning that all results should not be taken as granted but as a general guide line for more studies. The purpose and goals of the study has been met and completed.

abstract: The use of solar energy to produce power has increased substantially in the past few decades. In an attempt to provide uninterrupted solar power, production plants may find themselves having to operate the systems at temperatures higher than the operational capacity of the materials used in many of their components, which affects the microstructural and mechanical properties of those materials. Failures in components that have been exposed to these excessive temperatures have been observed during operations in the turbine used by AORA Solar Ltd. A particular component of interest was made of a material similar to the Ni-based superalloy Inconel 718 (IN 718), which was observed to have damage that is believed to have been initiated by Foreign Object Damage (FOD) and worsened by the high temperatures in the turbine. The potential links among the observed failure, FOD and the high temperatures of operation are investigated in this study. IN718 is a precipitation hardened nickel superalloy with resistance to oxidation and ability to withstand high stresses over a wide range of temperatures. Several studies have been conducted to understand IN 718 tensile and fatigue properties at elevated temperatures (600- 950°C). However, this study focuses on understanding the behavior of IN718 with FOD induced by a stream of 50 μm Alumina particles at a velocity of 200 m/s. under high cycle fatigue at an elevated temperature of 1050 °C. Tensile tests were conducted for both as-received and heat treated (1050 °C in air for 8hrs) samples at room and high temperature. Fatigue tests were performed at heat treated samples at 1050 °C for samples with and without ablation. The test conditions were as similar as possible to the conditions in the AORA turbine. The results of the study provide an insight into tensile properties, fatigue properties and FOD. The results indicated a reduction in fatigue life for the samples with ablation damage, where crack nucleation occurred either at the edge or inside the ablation region and multisite cracking was observed under far field stresses that were the same than for pristine samples, which showed single cracks. Fracture surfaces indicate intergranular fracture, with the presence of secondary cracks and a lack of typical fatigue features, e.g., beach marks which was attributed to environmental effects and creep. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2017

Superligas à base de níquel vêm sendo extensivamente utilizadas em diversas aplicações nas indústrias devido ao excelente comportamento mecânico e anticorrosivo. No entanto, essas ligas possuem certas particularidades que levam à necessidade de desenvolvimento de técnicas de inspeção e caracterização metalúrgica, como forma de garantir a integridade estrutural dos componentes fabricados com essas ligas. Neste trabalho, a técnica de correntes parasitas foi aplicada à superligas à base de níquel com duas propostas distintas: 1) Em um primeiro momento foi realizado o estudo da evolução microestrutural da liga Inconel 718 durante o processo de envelhecimento através da combinação do ensaio por correntes parasitas, análise de difração de raios-X, análise metalográfica, medidas de dureza e tamanho de grão. As medidas foram realizadas em amostras submetidas a diferentes ciclos de tratamentos térmicos variando entre 620-1035°C. Os resultados mostraram que as diferentes microestruturas do Inconel 718 têm efeitos distintos na condutividade elétrica quando medidos através da técnica de correntes parasitas. A influência da microestrutura na condutividade pode ser mostrada sendo devido à competição de dois efeitos sobre o espalhamento de elétrons: a purificação da matriz e a morfologia, distribuição e tamanho dos precipitados. A combinação dos valores de dureza e condutividade elétrica provou ser uma forma rápida e prática de determinar o nível de envelhecimento da liga; 2) Em um segundo momento foi desenvolvido um processo de otimização de sensores através de modelagem por elementos finitos (MEF). Através de uma metodologia de otimização, os parâmetros de construção e operação de um sensor foram otimizados para inspeção de defeitos superficiais e subsuperficiais esperados em materiais cladeados com Inconel 625. O sensor com a geometria ótima foi construído e testado a fim de verificar a eficiência do processo de otimização. Uma ótima correlação entre os resultados numéricos e experimentais foi encontrada e o sensor ótimo se mostrou eficiente na inspeção de pequenos defeitos superficiais e subsuperficiais na liga Inconel 625 quando operado nas frequências apropriadas. / Nickel-based superalloys have been extensively used in various industries due to its unique mechanical and corrosion behavior. However, these alloys show particular characteristics which lead to the need for specific inspection and metallurgical characterization techniques in order to ensure the structural integrity of components manufactured from these alloys. In this work , the eddy current technique was applied to nickel-base superalloys with two aims: 1 ) Firstly, the microstructural evolution of Inconel 718 during aging processes has been studied through a combination of eddy current testing, X-ray diffraction analysis, metallography, hardness and grain size measurements. Measurements were carried out in samples subjected to different heat treatment cycles between 620-1035°C. Results show that different microstructures of Inconel 718 have a distinguishable effect on electrical conductivity when this is measured through an appropriately sensitive technique (i.e. eddy current testing). The influence of microstructure on conductivity could be shown to be due to the competition between two effects on the scattering of electrons: matrix purification and precipitate size, distribution and morphology. A combination of hardness values and electrical properties proved to be a fast and practical way of determining the stage of aging of the alloy; 2) An optimization method of eddy current sensor design was developed through finite element modeling (FEM). Through a methodology of optimization, the construction and operation parameters of the sensor were optimized for inspection of superficial and subsuperficial defect, commonly found in weld overlay Inconel 625 claddings. A prototype of this sensor with the optimum geometry was built and tested on blocks identical to those considered in the models in order to verify the efficiency of the optimization process. A very good agreement between numerical and experimental results was found. Moreover, the optimal sensor was efficient to detect small surface and subsurface defects in Inconel 625 when operated at appropriate frequencies.

abstract: Additive manufacturing (AM) describes an array of methods used to create a 3D object layer by layer. The increasing popularity of AM in the past decade has been due to its demonstrated potential to increase design flexibility, produce rapid prototypes, and decrease material waste. Temporary supports are an inconvenient necessity in many metal AM parts. These sacrificial structures are used to fabricate large overhangs, anchor the part to the build substrate, and provide a heat pathway to avoid warping. Polymers AM has addressed this issue by using support material that is soluble in an electrolyte that the base material is not. In contrast, metals AM has traditionally approached support removal using time consuming, costly methods such as electrical discharge machining or a dremel. This work introduces dissolvable supports to single- and multi-material metals AM. The multi-material approach uses material choice to design a functionally graded material where corrosion is the functionality being varied. The single-material approach is the primary focus of this thesis, leveraging already common post-print heat treatments to locally alter the microstructure near the surface. By including a sensitizing agent in the ageing heat treatment, carbon is diffused into the part decreasing the corrosion resistance to a depth equal to at least half the support thickness. In a properly chosen electrolyte, this layer is easily chemically, or electrochemically removed. Stainless steel 316 (SS316) and Inconel 718 are both investigated to study this process using two popular alloys. The microstructure evolution and corrosion properties are investigated for both. For SS316, the effect of applied electrochemical potential is investigated to describe the varying corrosion phenomena induced, and the effect of potential choice on resultant roughness. In summary, a new approach to remove supports from metal AM parts is introduced to decrease costs and further the field of metals AM by expanding the design space. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2018