Hot section components of land-based gas turbines are subject to extremely harsh, high temperature environments and require the use of advanced materials. Directionally solidified Ni-base superalloys are often chosen as materials for these hot section components due to their excellent creep resistance and fatigue properties at high temperatures. These blades undergo complex thermomechanical loading conditions throughout their service life, and the influences of blade geometry and variable operation can make life prediction difficult. Accurate predictions of material response under thermomechanical loading conditions is essential for life prediction of these components. Complex crystal viscoplasticity models are often used to capture the behavior of Ni-base superalloys. While accurate, these models are computationally expensive and are not suitable for all phases of design. This work involves the calibration of a previously developed reduced-order, macroscale transversely isotropic viscoplasticity model to a directionally solidified Ni-base superalloy. The unified model is capable of capturing isothermal and thermomechanical responses in addition to secondary creep behavior. An extreme reduced order microstructure-sensitive constitutive model is also developed using an artificial neural network to provide a rapid first-order approximation of material response under various temperatures, rates of loading, and material orientation from the axis of solidification.

Cast nickel-aluminum bronze (NAB) alloy is specified for many marine applications, including ship propellers, due to its excellent corrosion-resistance combined with acceptable mechanical properties. Friction stir processing (FSP) can be used to improve the alloyâ s mechanical properties by localized microstructure modification in the cast material. FSP converts an as-cast microstructure to a wrought condition in the absence of macroscopic shape change, closes porosity, and provides a means to surface harden the castings. The closure of porosity near the surface of the material may shorten the manufacturing and processing time for ship propellers. The surface hardening of cast NAB alloy can be used to increase the wear life of ship propellers. Rockwell Scientific Corporation (now Teledyne Scientific Corporation) supplied three Nickel Aluminum Bronze alloy plates which have been friction stir processed in a raster pattern under a Defense Advanced Research Project Agency (DARPA) project. Each plate had been processed using a different tool RPM and IPM (inches per minute of transverse) combination. Miniature tensile samples were sectioned from the FSP zone and surrounding base metal and mechanical property distributions were determined in these regions. The material within the FSP zone exhibited consistently higher yield strengths, ultimate tensile strengths, and ductilities than the as-cast base metal.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, 2016. / Metal dusting (MD) is a severe type of corrosion that occurs mainly in petrochemical industries. The occurrence of MD is mainly due to syngas attacking Fe-, Ni- and Co-based alloys at elevated temperatures. More recently, literature has shown that apart from syngas, liquid hydrocarbon sources have been causing problems on platformer units in refineries. In the first part of this study a highly corrosion resistant Ni-based alloy (Hastelloy C276), in its polished form, was subjected to MD conditions at 800 °C using a liquid hydrocarbon (toluene) and helium (carrier gas) for 1 h. Exposure to these conditions revealed the formation of carbon nanofilaments and graphite layers which were confirmed by laser Raman spectroscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Burning off the carbon nanofilaments and the graphite layers in laboratory air for 1 h at 800 °C revealed that pits were formed on the Hastelloy C276. These same pits were not evident when Hastelloy C276 was exposed to either the carrier gas (helium) or laboratory air alone. Besides MD being a continuous problem in industry, this mechanism has been shown to be beneficial in the synthesis of carbon nanofilaments viz., carbon nanofibers (CNTs) and nanotubes (CNFs). In the second part of this study, unpolished Hastelloy C276 blocks (as opposed to polished blocks) were used to synthesize carbon nanofilaments. This was done as prior studies had shown that carbon nanofilaments were produced with better quality and greater yields this way. Here the flow rate (80, 160 and 240 mL/min) and reaction duration (10, 15, 30, 45, 60, 120 and 240 min) were studied using toluene (a liquid hydrocarbon). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to assess the quality and quantity of the carbon nanofilaments synthesized. Besides the formation of carbon nanofilaments, a less important material known as graphite particle structures (GPSs) were also synthesized. These studies collectively showed that MD had taken place on the surface of Hastelloy C276 when exposed to toluene at 800 °C. / TG2016

Ligas de níquel têm atualmente uma vasta gama de aplicações, sendo a agressividade do meio e as elevadas temperaturas que estas ligas suportam, um diferencial excepcional. A liga UNS N07090, objeto deste trabalho, encontra aplicações muito diversas, entre elas turbocompressores, sistemas de exaustão e de pós-tratamento de motores a diesel. Nestas aplicações a liga está exposta a gases, que ao condensarem formam ácido sulfúrico (H2SO4). Torna-se, assim, importante, o conhecimento da resistência à corrosão da liga nesse meio. O presente trabalho tem como objetivo caracterizar o comportamento eletroquímico da liga através de curvas de polarização potenciodinâmica em diferentes concentrações de ácido sulfúrico. Observou-se que quanto maior a concentração de ácido, maior é a corrosão da liga. Este fato pode ser discutido pela adsorção do íon sulfato (SO42-), que prejudica tanto a formação quanto a cinética de crescimento da película passiva do níquel. Em contrapartida, a presença de Cr na liga UNS N07090 apresentou um benefício sobre a resistência à corrosão desta muito acima do esperado, influenciando de forma direta as curvas de polarização da liga e mantendo-a em condição de elevada resistência à corrosão em detrimento do pior desempenho observado em outros elementos pertecentes à liga, como Co, Al e Ti. Tempos de imersão de 24h em ácido sulfúrico elevam discretamente os parâmetros de corrosão da liga, mantendo-os, no entanto em patamares bastante baixos, permitindo finalmente concluir que a liga UNS N07090, quando exposta a concentrações que variam de 1M a 4M H2SO4, a 25°C, apresenta boa resistência à corrosão e que esta não se altera com o tempo de exposição. A análise dos resultados mostrou que o processo corrosivo é controlado por reações catódicas de sulfato e hidrogênio, as quais podem ter diferentes contribuições dependendo da concentração do ácido e do tempo de imersão da liga UNS N07090 no meio corrosivo. / Nickel alloys have a wide range of application nowadays due to their excellent resistance to aggressive media and high temperatures. UNS N07090, the nickel alloy studied herein, is used for turbochargers, exhaust and after treatment systems of diesel engines. These applications expose the alloy to hot gases, which condensate forming sulfuric acid (H2SO4). It is thus important to know the corrosion resistance of the alloy in this media. The goal of this work is the characterization of the electrochemical behaviour of UNS N07090 alloy through potentiodynamic polarization curves in different sulfuric acid concentrations. On the one hand, higher acid concentrations were noticed to lead to higher corrosion of the alloy. This phenomenon can be discussed by sulfate (SO42-) adsorption, which negatively influences not only the formation but also the growth of the passive film on nickel. On the other hand, the presence of Cr in UNS N07090 composition, improves the corrosion resistance of the alloy far more than expected, directly affecting the shape of the polarization curves of the alloy, placing it in a condition of high corrosion resistance despite the worst performance shown by the other elements present in the chemical composition of the alloy, such as Co, Al and Ti. 24h immersion periods in sulfuric acid slightly raise corrosion parameters of UNS N07090 alloy, but still ranking the alloy in very good corrosion resistance. Finally, it can be concluded that UNS N07090 alloy exposed to 1M to 4M H2SO4 at 25°C presents good corrosion resistance and that this is not affected by longer immersion periods. The results showed that the corrosion process of alloy UNS N07090 is controlled by sulfate and hidrogen cathodic reactions, which have stronger or weaker contribution depending on acid concentration and immersion time on the corrosive media.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Carl J. White. / M.S.

The increasing technological need to push service conditions of structural materials to higher temperatures has motivated the development of several alloy systems. Among them, superalloys are an excellent candidate for high temperature applications because of their ability to form coherent ordered precipitates, which enable the retention of high strength close to their melting temperature. The accelerated kinetics of solute diffusion, with or without an added component of mechanical stress, leads to coarsening of the precipitates, and results in microstructural degradation, limiting the durability of the materials. Hence, the coarsening of precipitates has been a classical research problem for these alloys in service. The prolonged hunt for an alternative of nickel base superalloys with superior traits has gained hope after the recent discovery of Co-Al-W based alloys, which readily form high temperature g precipitates, similar to Ni base superalloys. In the present study, coarsening behavior of g precipitates in Co-10Al-10W (at. %) has been carried out at 800°C and 900°C. This study has, for the first time, obtained critical coarsening parameters in cobalt-base alloys. Apart from this, it has incorporated atomic scale compositional information across the g/g interfaces into classical Cahn-Hilliard model for a better model of coarsening kinetics. The coarsening study of g precipitates in Ni-14Al-7 Cr (at. %) has shown the importance of temporal evolution of the compositional width of the g/g interfaces to the coarsening kinetics of g precipitates. This study has introduced a novel, reproducible characterization method of crystallographic study of ordered phase by coupling of orientation microscopy with atom probe tomography (APT). Along with the detailed analysis of field evaporation behaviors of Ni and Co superalloys in APT, the present study determines the site occupancy of various solutes within ordered g precipitates in both Ni and Co superalloys. This study has explained the role of structural and compositional gradients across the precipitates (g)/matrix (g) interfaces on the coarsening behavior of coherent precipitates in both Ni and Co-base superalloys. The observation of two interfacial widths, one corresponding to a structural order-disorder transition, and the other to the compositional transition across the interface, raises fundamental questions regarding the definition of the interfacial width in such systems. The comparative interface analysis in Co and Ni superalloy shows significant differences, which gives insights to the coarsening behaviors of g precipitates in these alloys. Hence, the principal goal of this work is to compare and contrast the Co and Ni superalloys and also, to accommodate atomic scale information related to transitions across interfaces to coarsening models for a better practical applicability of coarsening laws to various alloys.

Orientador: Fernando Eidi Takahashi / Banca: Sebastião Simões Gomes / Banca: Sigmar de Mello Rode / Resumo: Este trabalho avaliou a alteração linear de fundições com liga de níquel-cromo (Verabond II) obtidas com dois revestimentos fosfatados, sendo um com carga refratária à base de cerâmica granulada (Microfine 1700) que permite a introdução do anel após a presa diretamente em um forno pré-aquecido (750°C) e outro à base de cristobalita (Bellavest-T) de procedimento convencional. Quarenta padrões de cera (Bredent life-color-wachs) foram obtidos por meio de moldagem direta de uma matriz de aço inoxidável. Vinte padrões de cera foram incluídos com Microfine 1700 e vinte com Bellavest-T. Com o auxílio de um projetor de perfil com precisão de 0,001mm, mediu-se a distância entre dois pontos pré-estabelecidos na matriz. A mesma distância foi medida posteriormente na peça metálica reproduzida em liga de níquel-cromo, obtendo-se assim, as medidas das possíveis alterações dimensionais. Os resultados evidenciaram que a média da alteração linear das fundições com os dois revestimentos em relação à matriz, através do Test T de Student foi significante estatisticamente em nível de 5% para o Bellavest-T e em nível de 1% para o Microfine 1700. As conclusões foram as seguintes: a) os dois revestimentos não conseguiram compensar a contração da liga metálica e da cera no processo de fundição; b) a média das distâncias das fundições obtidas com revestimento Microfine 1700, apesar de apresentarem-se menores que a matriz e as fundições com o revestimento Bellavest-T, revelaram medidas mais constantes / Abstract: This study evatuated the linear alteration of castings with NiCr alloy (Verabond II) obtained with two phosphate-bonded investments, one with granuled ceramic refractory load (Microfine 1700), which allows the introduction of the ring after setting directly in a pre-heated oven (750ºC); and another one, cristobalite-based (Bellavest-T) for conventional procedure. Forty wax patterns (Bredent life - color wachs, regular type II) were obtained through the direct molding of a stainless steel plate. Twenty wax patterns were invested in Microfine 1700 and twenty in Bellavest-T. A profile projector (0.001mm precision) measured the distance between 2 pre-established points in the matrix. The same distance was measured on the wax patterns and on the NiCr specimens produced, hereby obtaining the measure of the possible dimensional alterations of the materials. The results showed significance between the 2 investments (Bellavest-T, 5% and Microfine 1700, 1%) as compared to the matrix, through the Student's t test. The conclusions were: a) the two investments could not compensate the contraction of the alloy and wax in the casting process; b) the mean distance of the castings obtained with Microfine 1700, despite the fact of being smaller than the matrix and than the castings of Bellavest-T, showed more constant dimensions / Mestre

Thesis (Ph.D. (Physics)) --University of Limpopo, 2013 / The intention of the current study is to investigate structure, ion transport and reactivity of various forms of the cobalt pentlandite, Co9S8, at different temperatures using atomistic simulation methods with the support of electronic structure calculations. The first interatomic potentials of Co9S8 were derived with input data as structure and elastic properties from experiment and electronic structure calculations respectively. The potentials were validated by running energy minimization and molecular dynamics calculations. Structure, elastic properties and phonon spectra were well reproduced, together with the complex high temperature transformations and melting of Co9S8 as deduced from crystal structure, radial distribution functions, density profiles and diffusion coefficients. Amongst the high symmetry surfaces {111}, {101} and {101} atomistic surface energy calculations proposed the {111} surface of Co9S8 as the most stable in agreement with experimental morphologies, and water adsorption energies on the such surfaces which mostly agreed with those from electronic structure calculations. The structural and ion transport variations with temperature were investigated and predicted surface melting at lower temperatures than the bulk. The effects of hydration on the surfaces at low and high temperatures were also studied. The structural and ion transport properties of Co9S8 nanoparticles of varying sizes, covered by high symmetry surfaces {111}, {101} and {100} were predicted using molecular dynamics method based on our derived interatomic potentials. The structural and ion transport properties of Co9S8 nanoparticles of varying sizes, covered by high symmetry surfaces {111}, {101} and {100} were predicted using molecular dynamics method based on our derived interatomic potentials. Generally for {111}, {101} nanoparticles, high temperature transitions were abrupt for smaller nanoparticles and these tended to disintegrate and form voids. However, for larger nanoparticles the transitions were more gradual. Transitions in the {100} bound nanoparticles were less dramatic for all sizes and the formation of voids was reduced at high temperatures. Generally, the melting temperatures of different sizes of nanoparticles increases with the particle size hence approach the bulk limit. The interaction of nanoparticles with water was investigated. / Anglo Platinum, National Research Foundation (South Africa), and The Royal Society (UK)

Thermally cycling a thin plate of nickel-based superalloy with an intense in-plane thermal gradient, or hot spot, produces thermally induced crack growth not represented by classic thermo-mechanical fatigue (TMF). With the max hot spot temperature at 1093 C (2000 F) of a 1.5 mm thick, 82.55 mm diameter circular plate of B-1900+Hf, annular buckling and bending stresses result during each thermal cycle which drive the crack initiation and propagation. A finite element analysis (FEA) model, using ANSYS 7.1, has been developed which models the buckling and as well as represents the stress intensity at simulated crack lengths upon cool down of each thermal cycle. The model approximates the out-of-plane response at heat-up within 5% error and a difference in the final displacement of 0.185 mm after twelve thermal cycles. Using published da/dN vs. Keff data, the number of cycles needed to grow the crack to the experimental arrest distance is modeled within 1 mm. The number of cycles to this point is within 5 out of 462 in comparison to the experimental test.

Engineers constantly seek advancements in the performance of aircraft and power generation engines, including, lower costs and emissions, and improved fuel efficiency. Nickel-base superalloys are the material of choice for turbine discs, which experience some of the highest temperatures and stresses in the engine. Engine performance is proportional to operating temperatures. Consequently, the high-temperature capabilities of disc materials limit the performance of gas-turbine engines. Therefore, any improvements to engine performance necessitate improved alloy performance. In order to take advantage of improvements in high-temperature capabilities through tailoring of alloy microstructure, the overall objectives of this work were to establish relationships between alloy processing and microstructure, and between microstructure and mechanical properties. In addition, the project aimed to demonstrate the applicability of neural network modeling to the field of Ni-base disc alloy development and behavior. A full program of heat-treatment, microstructural quantification, mechanical testing, and neural network modeling was successfully applied to next generation Ni-base disc alloys. Mechanical testing included hot tensile, hot hardness, creep deformation, creep crack growth, and fatigue crack growth. From this work the mechanisms of processing-structure and structure-property relationships were studied. Further, testing results were used to demonstrate the applicability of machine-learning techniques to the development and optimization of this family of superalloys.