Ni-based superalloys are used for high-temperature components of gas turbines in both industrial and aerospace applications due to their ability to maintain dimensional stability under conditions of high stress and strain. The oxidation resistance of these alloys often dictates their service lifetime. This study focuses on the isothermal oxidation behaviour of three nickel-based superalloys; namely, polycrystalline cast IN738LC, single-crystal N5 and a ternary Ni-Fe-Cr (TAS) powder metallurgy alloy. The isothermal oxidation tests were conducted at 900°C in the static air up to 1000h and the specific aspects studied were the oxidation behaviour of these chromia-forming and alumina-forming alloys that are used extensively in industry. In particular, the behaviour of oxide scale growth and subsurface changes were analysed in detail using various techniques such as SEM, EDS and AFM. From the isothermal oxidation kinetics, the oxidation rate constant, kp was calculated for each alloy and found to be; kp = 2.79 x 10-6 mg2.cm-4.s-1 for IN738LC, kp = 1.42 x 10-7 mg2.cm-4.s-1 for N5 and kp = 1.64 x 10-7 mg2.cm-4.s-1 for TAS. Based on a microstructural analysis, IN738LC exhibited a continuous dense outer scale of Cr2O3 and discontinuous inner scale of Al2O3, whereas N5 and TAS showed a dense outer scale of Al2O3 alone. The results suggested that the N5 and PM-TAS alloys are more oxidation resistant than the IN738LC under these conditions. / October 2016

An experimental investigation to study the influence of base metal composition on the time required to achieve complete isothermal solidification (tf) during TLP bonding of three Ni-base superalloys was performed. Alloys IN 738, DS Rene80 and DS IC 6 show similar behaviour when bonded at 1100 oC, with comparable tf. However, at higher temperatures, IN 738 requires extended period of time (as compared to DS Rene80 and DS IC 6) to achieve complete isothermal solidification. The prolonged tf in IN 738 appears to be caused by a more pronounced reduction in concentration gradient of the diffusing solute within the material during bonding. In contrast, the shorter complete isothermal solidification time experienced by alloy DS IC6 is attributable to its capability to better accommodate the diffusing solute, through the formation of densely packed second-phase precipitates in the diffusion affected zone (DAZ).

An experimental investigation to study the influence of base metal composition on the time required to achieve complete isothermal solidification (tf) during TLP bonding of three Ni-base superalloys was performed. Alloys IN 738, DS Rene80 and DS IC 6 show similar behaviour when bonded at 1100 oC, with comparable tf. However, at higher temperatures, IN 738 requires extended period of time (as compared to DS Rene80 and DS IC 6) to achieve complete isothermal solidification. The prolonged tf in IN 738 appears to be caused by a more pronounced reduction in concentration gradient of the diffusing solute within the material during bonding. In contrast, the shorter complete isothermal solidification time experienced by alloy DS IC6 is attributable to its capability to better accommodate the diffusing solute, through the formation of densely packed second-phase precipitates in the diffusion affected zone (DAZ).

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.

No description available.

Superalloys have performed well as blade and disc materials in turbine engines due to their exceptional elevated temperature strength, high resistance to creep, oxidation and corrosion as well as good fracture toughness. This study explores the use of a relatively new technique of strain measurement, high resolution electron backscatter diffraction (HR-EBSD) to measure local deformation fields. The heart of the HR-EBSD technique lies in comparing regions in EBSD patterns from a strained region of a sample to those in a pattern from an unstrained region. This method was applied to study the elastic strain fields and geometrically necessary dislocation density (GND density) distribution near hard carbide particles in a nickel-based superalloy MAR-M-002. Significant thermal strains were initially induced by thermal treatment, which included a final cooling from the ageing temperature of 870°C. Elastic strains were consistent with a compressive radial strain and tensile hoop strain that was expected as the matrix contracts around the carbide. The mismatch in thermal expansion coefficient of the carbide particles compared to that of the matrix was sufficient to have induced localized plastic deformation in the matrix leading to a GND density of 3 x 10¹³ m^–2 in regions around the carbide. These measured elastic strain and GND densities have been used to help develop a crystal plasticity finite element model in another research group and some comparisons under thermal loading have also been examined. Three-point bending was then used to impose strain levels within the range ±12% across the height of a bend bar sample. GND measurements were then made at both carbide-containing and carbide-free regions at different heights across the bar. The average GND density increases with the magnitude of the imposed strain (both in tension and compression), and is markedly higher near the carbide particles. The higher GND densities near the carbides (order of 10¹⁴ per m²) are generated by the large strain gradients produced around the plastically rigid inclusion during monotonic mechanical deformation with some minor contribution from the pre-existing residual deformation from thermal loading. A method was developed of combining the local EBSD measurements with FE modelling to set the average residual strains within the mapped region even when a good strain free reference point was unavailable. Cyclic loading was then performed under four point loading to impose strain levels of about ±8% across the height of bend bar samples. Similar measurements as in the case of monotonic deformation were made at several interruptions to fatigue loading. Observations from the cyclic loading such as slip features, carbide cracking, GND density accumulation have been explored around carbide particles, at regions away from them and near a grain boundary.

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.

No description available.

During operational conditions of internal combustion turbines of turbojet engines, the impeller blades 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 clasifikation 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 950C. 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 doctoral thesis is focused on the effect of dwells on the low cycle fatigue behaviour of advanced high temperature materials. 10 minutes strain holds are introduced into the cyclic straining of cast Ni-based superalloy IN792-5A and cast intermetallic alloy TiAl-7Nb which were chosen as experimental material. Fatigue experiments were conducted in strain control mode with constant total strain amplitude and strain rate. IN792-5A was subjected to continuous cyclic loading test as well as fatigue tests either with tensile peak strain holds or compressive peak strain holds at temperature 800 °C. TiAl-7Nb was experimentally examined by continuous cyclic loading tests and also by fatigue test with tensile peak strain hold times at temperature 750 °C. Cyclic hardening/softening curves, cyclic stress-strain curves, Manson-Coffin curves and Basquin curves were obtained. Stress relaxation data were measured during hold times. Mean stress evolution was observed and documented. Microstructure of both alloys was observed in as-received state and also after cyclic loading by means of SEM and TEM. Surface relief investigation revealed cyclic plastic strain localisation as well as fatigue crack initiation sites. Fatigue crack propagation was described using fracture surface and longitudinal gauge section observations. Dislocation structures developed in the course of fatigue tests were studied and documented.