[pt] A superliga a base de níquel 59-23Cr-16Mo foi desenvolvida na década de 90 e tem sido aplicada em indústrias químicas e petroquímicas, bem como em equipamentos de controle ambiental. Esta liga tem apresentado, ocasionalmente, problemas de corrosão por pits em regiões específicas da zona termicamente afetada (ZTA) de juntas soldadas. Com o propósito de estudar os mecanismos atuantes nos processos de precipitação da liga, amostras da mesma foram submetidas a tratamentos de envelhecimento isotérmicos, às temperaturas de 700oC e 900oC, durante intervalos de tempo de 1 a 100 horas. Um estudo da evolução microestrutural do material, a partir da condição de como recebido, foi conduzido através de microscopia ótica, microscopia eletrônica de varredura (MEV), espectroscopia por dispersão de energia de (EDS), difração de elétrons retroespalhados (EBSD), difração de raios-X, microscopia eletrônica de transmissão convencional e de alta resolução. O desempenho mecânico do material, em suas diferentes condições microestruturais, foi avaliado por meio de ensaios de tração uniaxial bem como de micro-dureza. Variações da resistividade elétrica da liga devido a evolução microestrutural foi investigada e o perfil de resistividade elétrica ao longo de uma junta soldada foi levantado. Os resultados obtidos indicaram a presença de reflexões {1, 1/2, 0} nos padrões de difração da maioria das microestruturas em questão. Tais reflexões são características de um estado de ordenação de curto alcance (SRO), e se constituem em evidência da ocorrência de reações de ordem através do mecanismo de empilhamento de planos cristalinos {4,2,0}. Uma vasta população de nanoprecipitados referentes à fase Ni2(Mo,Cr), foi detectada em todas as condições analisadas no presente trabalho. Também foram observados, nas amostras envelhecidas, microprecipitados Ni(Mo,Cr)2, originados a partir de reações de ordem, ocorrendo preferencialmente na área de contornos de grão. O comportamento mecânico do material é associado à precipitação desta fase, a qual governa tanto a transição entre modos de ruptura intragranular para intergranular quanto a degradação da ductilidade da liga. Significativas variações da resistividade elétrica foram detectadas ao longo da ZTA, não associadas à quaisquer alterações microestruturais tendo sido, portanto, atribuídas à variações na quantidade de SRO. / [en] The nickel base superalloy, 59-23Cr-16Mo, was developed in the nineties and has since been used in chemical and petrochemical industries as well as in equipment for environmental control. The alloy has, occasionally, been presenting problems of pit corrosion in the heat affected zones (HAZ) of welded joints. With the purpose of elucidating the mechanisms associated with precipitation processes, specimens were taken from the as-received alloy and submitted to isothermal aging at two different temperatures of 700 and 900oC, for time intervals varying from 1 to 100 hours. A study of the microstructural evolution accompanying the aging treatment was then conducted by means of X-ray diffraction, optical and scanning electron microscopies, making use of the energy dispersion spectroscopy (EDS) and electron back scattering diffraction (EBSD) techniques. Conventional and high resolution electron microscopies were also used as additional tools in order to follow the nanoescale precipitation processes. The mechanical behavior of the alloy in its different microstructural conditions was evaluated by means of uniaxial tensile tests as well as by microhardness measurements. Variations in electrical resistivity accompanying microstructure evolution were followed and electrical resistivity of a TIG welded joint was mapped. The results obtained have indicated the presence of {1, 1/2, 0} reflections in the diffraction patterns of the majority of the microstructures in question. Such reflections, which are typical of short range ordered (SRO) state, indicate that the alloy is prone to precipitation mechanisms via ordering reactions, involving atomic stacking on {4,2,0} planes. A large population of nanoprecipitates, referring to the Ni2(Mo,Cr) phase, was detected in all of the microstructural conditions considered in this work. In addition, the aged specimens were also found to contain microscale Ni2(Mo,Cr)2 particles formed at the grain boundaries via ordering reactions occurring preferentially in the grain boundary area. The mechanical behavior of the aged alloy is strongly related to the precipitation of this microscale phase, which governs the ductility level as well as the transition in the fracture mode from intragranular to intergranular. Finally, the electrical resistivity variation within the HAZ was not associated with microstrutural alterations and therefore was attributed to variations in the quantity of the short range ordered structures.

Grinding is an abrasive machining process used for the final shaping of components that require very smooth surfaces and a high dimensional accuracy. In recent years, the costs of industrial grinding operations have increased with a greater demand for high-strength, low-weight superalloy components. Titanium and nickel-based alloys are desirable for their high creep-rupture strength and corrosion and oxidation resistance in high-temperature environments. However, they are very difficult to grinding due to a combination of poor thermal properties, rapid work-hardening, and a high level of chemical reactivity. In this thesis, two methods are investigated to improve process performance in plunge centerless grinding of Inconel 718 and Ti-6Al-4V superalloy fasteners: (i) economic optimization of grinding process parameters and (ii) reduced quantity lubrication using a graphite nanoplatelet-enhanced grinding fluid. In the first part, a systematic methodology is presented for finding the optimum parameters in two stages: (i) modeling of process and part quality constraints, and (ii) determination of optimum grinding conditions in the feasible operating region. In the second part, the performance of a graphite nanoplatelet-enhanced grinding fluid in reduced quantity lubrication centerless grinding is evaluated to assess its potential as a cost-effective alternative to the traditional flood cooling method. The results of the study indicate that an appreciable reduction in the cost of the superalloy grinding operation can be achieved by operating at the optimum parameters. In addition, it is shown that the application of a graphite-enhanced fluid at a reduced flow rate is more effective than high-volume flood cooling in reducing specific grinding energy levels and wheel wear rates, thus offering the potential to increase process productivity.

Vacuum Arc Remelting (VAR) is a secondary process used for homogenization of high-melting-point and oxygen-sensitive materials such as superalloys and titanium alloys. The VAR process is carried out with the aim of melting a large consumable electrode in such a way that the resulting ingot has improved homogeneity. The Specialty Metals Processing Consortium (SMPC) has spent the past 20 years developing technology to improve control over the final ingot remelting and solidification processes to alleviate conditions that lead to the formation of inclusions and segregation. Channel segregates are concentration defects arising during the solidification of large-diameter solute-rich alloys. As manufacturers for turbine engines and generators call for larger ingots, it becomes more difficult to produce them without these defects. If, however, liquid pool depth can be controlled precisely to stabilize the solidification zone in the ingot, we could, in principle, produce larger ingots that are defect free. A problem arises because measurements obtained from the VAR furnace do not give enough information to accurately estimate the liquid pool shape in dynamic melting situations. Also, the solidification process in VAR is extremely complex due to the multiple physical domains present and a high-fidelity model is required to give an accurate description of the dynamic process. The Basic Axisymmetric Remelting (BAR) code was initially developed by Lee Bertram at Sandia National Laboratories as a high-fidelity multi-energy model to describe ingot casting in this system. In this work we present a new strategy to improve the accuracy of the estimates used in the control system. This strategy consists of implementing BAR as a new set of measurements to be used by the estimator. This new strategy was used in tests jointly sponsored by SMPC and Los Alamos National Laboratory (LANL) in February 2011 using a laboratory-scale furnace and alloy 718 electrodes. / text

The low cycle thermo-mechanical fatigue of laser beam welded conventionally cast Inconel 738, directionally solidified René 80 and single crystal René N5 has been evaluated. Results have been compared to gas tungsten arc and baseline alloy conditions. Metallographic examination of laser beam welds and the associated heat affected zone were conducted by scanning electron microscopy and energy dispersive spectroscopy. The impact of laser beam welding on thermo-mechanical fatigue properties of Inconel 738, René 80 and René N5 has been evaluated and recommendations for improvements and areas of further research have been presented.

The low cycle thermo-mechanical fatigue of laser beam welded conventionally cast Inconel 738, directionally solidified René 80 and single crystal René N5 has been evaluated. Results have been compared to gas tungsten arc and baseline alloy conditions. Metallographic examination of laser beam welds and the associated heat affected zone were conducted by scanning electron microscopy and energy dispersive spectroscopy. The impact of laser beam welding on thermo-mechanical fatigue properties of Inconel 738, René 80 and René N5 has been evaluated and recommendations for improvements and areas of further research have been presented.

Slurry coating technique, as one of the most popular deposition methods, is widely used to produce various material coatings. This method includes two processes: spraying, brushing or dipping of slurry, and sintering heat treatment of the coated specimen. Superalloys and stainless steels are the most common materials used as either coating materials or substrate materials because of their excellent corrosion, wear, high-temperature and mechanical properties. This research is aimed at investigating the influence of the sintering heat treatment in the slurry coating process developed at Kennametal Stellite Inc. on the microstructure, hardness and wear behavior of superalloys and stainless steels. Low-carbon Stellite 22, cobalt-based Tribaloy T-400C, martensitic AISI 420 and AISI 440C stainless steels are studied in this research. The microstructure, hardness and wear resistance of these alloys before and after the heat treatment are investigated, stressing the influence of the heat treatment on these material characteristics. The hardness and wear tested are conducted on these alloys at both room temperature and at elevated temperatures. The worn surfaces of each specimen are analyzed using a Scanning Electron Microscope (SEM) with backscatter electron imaging (BEI) and energy dispersive X ray (EDX) spectrum. It is demonstrated that the heat treatment alters the microstructures of these alloys differently; it increases the hardness but affects the wear resistance more complexly than hardness. At room temperature, the wear resistance of these alloys is governed by their microstructures. However, at high temperatures, oxidation, resulting in formation of oxide films on the specimen surface, influences the wear resistance significantly.

CoordenaÃÃo de AperfeiÃoamento de NÃvel Superior / A soldagem de revestimentos de ligas de nÃquel à utilizada pelas indÃstrias de petrÃleo e gÃs como opÃÃo de construÃÃo e reparo de estruturas e equipamentos submetidos em ambiente de condiÃÃes severas com o intuito de apresentar desempenhos necessÃrios de resistÃncia à corrosÃo. Muitos trabalhos foram desenvolvidos com superligas de nÃquel AWS ERNiCrMo-3, AWS ERNiCrMo-4 e AWS ERNiCr-Mo14, para avaliar resistÃncias à corrosÃo e a Ãltima se mostrou bem superior, porÃm o seu custo tambÃm à bem mais elevado que as outras duas. Assim, o trabalho vem aliar a alta produtividade do processo de soldagem MIG/MAG â DA atravÃs da soldagem de revestimentos com as ligas AWS ERNiCrMo-3 e AWS ERNiCrMo-4 na intenÃÃo de se obter uma mistura que apresente tanto a composiÃÃo quÃmica prÃxima a da liga AWS ER NiCr-Mo-14 como desempenho de resistÃncia a corrosÃo, porÃm com menor custo. Estudos exploratÃrios foram realizados por meio de soldagem com simples deposiÃÃo para determinaÃÃo da faixa de velocidade de soldagem entre 50 e 100 cm/min, modo de operaÃÃo da fonte em corrente constante pulsada e gÃs de proteÃÃo ArgÃnio puro. Os revestimentos foram soldados com menor energia (7,5 kJ/cm), tecimento triangular, eletrodos posicionados um atrÃs do outro em relaÃÃo a velocidade de soldagem e sem defasagem entre as correntes. Nessa etapa variou-se Ãngulo de ataque e o percentual de sobreposiÃÃo dos cordÃes. A melhor condiÃÃo apresentada foi menor diluiÃÃo, 9,75%, e elevado reforÃo, 3,92 mm, sendo essa obtida utilizando-se a tocha inclinada de 15Â, e sobreposiÃÃes de 50%. A relativa baixa diluiÃÃo encontrada e o elevado reforÃo reduziram a necessidade de uma segunda camada, o que torna o processo uma boa opÃÃo para operaÃÃes de revestimento para indÃstria de petrÃleo e gÃs. / Weld overlayâs Nickel based alloy are used by the oil and gas industry as an option for construction and repair of structures and equipment subjected to severe environment conditions with the purpose to provide performance required like for corrosion resistance. Many works have been developed with Nickel based superalloys AWS ERNiCrMo-3, AWS ERNiCrMo-4 e AWS ERNiCrMo-14, evaluating among several characteristics the last alloy proved to be upper, but their cost is also higher than the other two. Therefore the present work intends to combine the high productivity of GMAW double wire for welding overlay applications using alloy with AWS ERNiCrMo-3, AWS ERNiCrMo-4 with the intention of obtain a mixture presenting both chemical composition similar to the AWS ER NiCrMo-14 such as corrosion resistance performance, but with less cost. Exploratory studies were performed by means of welding with simple deposition to determine the range of welding speed between 50 and 100 cm / min, the operating mode of the source constant current and pulsed pure argon shielding gas. The coatings were welded with low energy (7,56 kJ/cm), triangle weaving, electrodes positioned one behind the other in relation to the welding speed, without out of phase the currents. This step was varied angle of attack and the percentage of overlap the weld beads. The best condition was presented lower dilution, 9.75%, and a high thickness 3.92 mm and that obtained using the torch inclined at 15  and overlap of 50%. The relatively low dilution and high thickness found reduced the need for a second coating, which makes the process a good option for coating operations.

The analysis of microstructural shapes is an underutilized tool in the field of materials science. Typical observations of morphology are qualitative, rather than quantitative, which prevents the identification of relationships between shape and the mechanical properties of a material. Recent advances in the fields of computer vision and high-dimensional analysis have made computer-based shape characterization feasible on a variety of materials. In this work, the relationship between microstructural shapes, and the properties and function of the material as a whole, is explored using moment invariants as global shape descriptors. A diifferent relationship is examined in each of three material systems: how the three-dimensional shapes of cells in the cotyledons of the plant Arabidopsis Thaliana can be used to identify cell function; the two-dimensional shapes of additive manufacturing feedstock powder and the ability to distinguish between images of powders from different samples; and the two-dimensional shapes of ' precipitates and their influence on the creep resistance of single crystal nickel-base superalloys. In the case of Arabidopsis Thaliana cotyledon cells, three-dimensional Zernike and Cartesian moment invariants were used to quantify morphology, and combined with size and orientation information. These feature sets were then analyzed using unsupervised and supervised machine learning methods. Moderate success was found using unsupervised methods, indicating that natural delineations in the data correlate to cell roles to some degree. Using supervised methods, a success rate of 90% was possible, indicating that these features can be used to identify cell function. The ability of two-dimensional Cartesian moment invariants to distinguish meaningful features in particles of additive manufacturing feedstock was tested by using these features to classify images of feedstock. Ultimately, simple histogram matching methods were unsuccessful, likely because they rely on the most common particles to draw conclusions. A bag-of-words method was used, which uses high-dimensional visualization and clustering techniques to classify individual particles by common features. Histograms of particle clusters are then used to represent each image. This method was far more successful, and a correct classification rate of up to 90% was found, and comparable rates were discovered using invariants which describe the shapes only broadly. This indicates that moment invariants are an effective measure of the morphologies of these types of particles, and can be used to classify powder shapes, which control many properties which are relevant to the additive manufacturing process. In the case of the superalloys, it has been shown that the shape distribution of ' precipitates can be tracked using second order moment invariants. In addition, several loworder moment invariants are shown to correlate to creep resistance in four alloys examined, which supports the idea that the shape of precipitates plays role in determining creep resistance in these alloys.

Cette étude s’est focalisée sur la compréhension des mécanismes de déformation et d’endommagementd’un superalliage à base de Nickel (Udimet 720Li) utilisé pour la confection de disques de turbine hautepression soumis à des sollicitations monotones et cycliques à des températures inférieures à 500 °C.L’amorçage de fissures de fatigue à 20 °C et 465 °C a été systématiquement observé suivant des facettescristallographiques pour différents états métallurgiques. Une étude quantitative des traces de glissementplastique en surface a permis de montrer que cet endommagement est la conséquence d’une nontransmission de la déformation plastique entre grains voisins. Plus précisément, un mécanisme de rotationcristalline localisée au sein de micro-volumes à l’extrémité de bandes de glissement dans le grain voisin aété identifié à l’origine des fissures.Les configurations cristallographiques favorisant ce processus impliquent notamment les relations entresystèmes de glissement de part et d’autre du joint de grains (angle de TWIST), ou encore la présence d’uneprécipitation fine participant au confinement des bandes de glissement. / This study contributes to the understanding of the deformation and damage mechanisms in a Nickelbasedsuperalloy (the Udimet 720Li) used for the manufacturing of aeroengines high pressure turbinedisks submitted to monotonic and cyclic loadings at temperatures below 500 °C under service conditions.Fatigue crack initiation at 20 °C and 465 °C was systematically observed at large crystallographic facetswhatever the microstructure. An extensive quantitative study of the slip activity at the surface showed thatcrack initiation results from a difficult slip transmission between neighbouring grains. In more details, alocalized crystallographic rotation confined in micro-volumes detected at the tip of slip bands interactingwith the grain boundary were identified to trigger crack initiation.The crystallographic configurations favouring such a process are closely related to the TWIST anglebetween active slip systems on each side of the grain boundary and to a dense and fine precipitationconfining the plasticity.Mots-clés

Slurry coating technique, as one of the most popular deposition methods, is widely used to produce various material coatings. This method includes two processes: spraying, brushing or dipping of slurry, and sintering heat treatment of the coated specimen. Superalloys and stainless steels are the most common materials used as either coating materials or substrate materials because of their excellent corrosion, wear, high-temperature and mechanical properties. This research is aimed at investigating the influence of the sintering heat treatment in the slurry coating process developed at Kennametal Stellite Inc. on the microstructure, hardness and wear behavior of superalloys and stainless steels. Low-carbon Stellite 22, cobalt-based Tribaloy T-400C, martensitic AISI 420 and AISI 440C stainless steels are studied in this research. The microstructure, hardness and wear resistance of these alloys before and after the heat treatment are investigated, stressing the influence of the heat treatment on these material characteristics. The hardness and wear tested are conducted on these alloys at both room temperature and at elevated temperatures. The worn surfaces of each specimen are analyzed using a Scanning Electron Microscope (SEM) with backscatter electron imaging (BEI) and energy dispersive X ray (EDX) spectrum. It is demonstrated that the heat treatment alters the microstructures of these alloys differently; it increases the hardness but affects the wear resistance more complexly than hardness. At room temperature, the wear resistance of these alloys is governed by their microstructures. However, at high temperatures, oxidation, resulting in formation of oxide films on the specimen surface, influences the wear resistance significantly.