As superligas a base de níquel apresentam limitações para uso em temperaturas acima de 600 °C. Por este motivo, os revestimentos protetivos podem ser usados como barreiras para evitar a propagação de nucleação e fissuras. O presente trabalho tem por objetivo avaliar as propriedades mecânicas em altas temperaturas de amostras nitretadas por plasma da superliga INCONEL 625 foram realizados ensaios de tração e fluência em amostras com e sem tratamento. Nos ensaios de tração, nas temperaturas de 200 a 700 °C, o material apresentou uma oscilação nos valores de tensão a partir do início da deformação plástica. Este comportamento da curva tensão-deformação serrilhada pode estar associado com a difusão de um soluto intersticial ou pela difusão de átomos substitucionais ou pelo envelhecimento da deformação dinâmica (DSA). Em 600 °C, o aumento da taxa de deformação promoveu um aumento da amplitude e da frequência de oscilação da tensão. Sob níveis de tensões intermediários ou elevados o mecanismo de fluência dominante da liga Inconel 625 foi por escorregamento de discordâncias mostrando-se semelhante ao de muitos metais puros e ligas de soluções sólidas. / Nickel-based superalloys have limitations for use at temperatures above 600 °C. For this reason, protective coatings can be used as barriers to avoid both nucleation and crack propagation. Thus, to evaluate the mechanical properties at high temperatures of plasma nitrided samples of the INCONEL 625 superalloy, tensile and creep tests were performed on samples with and without treatment. In the tensile tests at temperatures of 200 to 700 °C the material showed an oscillation in the tension values from the beginning of the plastic deformation. This behavior of the serrated strain-strain curve may be associated with the diffusion of an interstitial solute or by the diffusion of substitutional atoms or by dynamic strain aging effect. At 600 °C, the increase in strain rate promoted an increase of the amplitude and oscillation frequency of the stress. Under levels of intermediate of elevated stresses, the dominant creep mechanism of the Inconel 625 alloy was like that of many pure metals and alloys of solid solutions which is by slippage of dislocations.

Introduction Annealed Inconel 718 alloy was chosen for the beam window at the Los Alamos Neutron Science Center (LANSCE) Isotope Production Facility (IPF) [1]. The window was replaced after 5 years of operation. Mechanical properties and microstructure changes were measured to assess its expected lifetime. Material and Methods A cutting plan was developed based on the IPF rasterred beam profile (FIG. 1). 3-mm OD samples were cut out from the window and thinned to 0.25-mm thick. Shear punch tests were per-formed at 25 °C on 21 samples to quantify shear yield, ultimate shear stress, and ductility. From 1-mm OD, 0.25-mm thick shear punched out disks, 4 TEM specimens of ~30×10×2 μm were obtained using standard FIB lift-out techniques. TEM was performed on an FEI Tecnai TF30-FEG operating at 300 kV. Results and Conclusions TABLE 1 shows MCNPX tally results of accumulated dpa, He and H content from both protons and neutrons fluences and ANSYS steady-state irradiation temperature for the 3-mm OD samples [2]. These peak values are at the peak density of Typically increases in shear yield and shear maximum stress occur with increasing dose. In this case, highest shear yield and ultimate stress was on the lowest dose samples at the outer edge (FIG. 2). Optical microscopy images of the fracture surfaces on the shear punched out disks show no significant change in the fracture mode or reduction in ductility in the un-irradiated, high and low dose irradiated samples. One un-irradiated and 4 irradiated samples (5, E, 16 and 19) were selected for TEM analysis. Figure 3 shows bright field TEM images of an un-irradiated, high and low dose irradiated samples. Un-irradiated sample shows some dislocations and some large precipitates. The high dose sample #5 (~11 dpa, 122 oC) shows small loops and dislocations (left and center images) and no γ\' or γ\'\' precipitates in SAD from z = [011] (right image). Low dose sample #19 (~0.7 dpa, 40 oC) shows a high density of dislocation loops (left image), high density of H/He bubbles (center image) and presence of γ\'\' precipitates in SAD from z = [011] (right image). Radiation induced-hardening is highest at the low dose region in the outer most edge. The hardening from γ\'\' precipitates is determined to be more pronounced than that from trapped bubbles. The lack of significant hardening in the highest dose region is attributed to a lower dis-location density and no γ” precipitates or bubbles [3]. Identification of H or He bubbles and the higher accumulation of these bubbles in the low dose region (no direct beam hitting) warrant further studies. Despite the evidence of irradiation-induced hardening, this spent beam window appears to retain useful ductility after 5 years in service. At the conclusion of 2013 run cycle, the current in-service beam window had reached the same dpa as of the spent window. We plan to extend the service of the current in-service window until it reaches its intended design threshold limit of ~20 dpa (in the highest dose region). Additional measurements at higher dpa values will enable better decision-making in managing risks of the window failure.

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As curvas de polarização potenciodinâmicas cíclicas da liga de níquel Inconel 625, em solução de NaCl 3,56 %, pH 7,5, desaerada e a temperatura ambiente, obtidas com base na metodologia descrita na norma ASTM G61, foram estudadas com o objetivo de interpretá-las, especialmente no trecho após a reversão da polarização. Para esse estudo, foram realizadas análises complementares por MEV/EDS e por espctroscopia Raman da superfície do eletrodo de trabalho e determinação qualitativa por EDS da composição do eletrólito coletado nas vizinhas do eletrodo de trabalho. Medidas de pH e determinação do teor de O2 do eletrólito junto ao eletrodo de trabalho também foram feitas. Os resultados obtidos mostraram que, entre o potencial de circuito aberto até potencial de quebra (Eb), da ordem de 0,600 V (ECS), a curva de polarização apresenta um comportamento passivo com tendência a atingir a densidade de corrente da ordem de 10-6 A/cm². Nessa região, ocorre o espessamento da camada devido à formação principalmente de Cr2O3. Após Eb, ocorre a mudança na inclinação da curva, devido à ocorrência da reação de oxidação da água, comprovada pela acidificação do eletrólito. No potencial de 0,750 V (ECS) ocorre uma inflexão na curva, causando a diminuição discreta da corrente, atribuída à formação de MoO42- (gel) que evita a incorporação de ânions como Cl- ou OH-, dificultando, porém não evitando, a oxidação/dissolução da camada passiva, comprovada pela presença de íons de Cr e de Ni no eletrólito, caracterizando uma região transpassiva. Após a reversão da polarização no potencial de 1,130 V (ECS), a curva apresenta uma histerese positiva, com valores de densidades de correntes menores em relação ao mesmo potencial aplicado durante a polarização direta, indicando a ocorrência de repassivação devido principalmente à formação de uma camada rica em Mo sobre a liga, cuja formação pode ser atribuída à transformação do molibdato para o óxido MoO3 decorrente da forte acidificação do eletrólito verificado nesse trecho da curva. Ainda, após a inversão da polarização, ocorre a transição da corrente anódica para catódica em um potencial próximo de 0,720 V (ECS). Do potencial de 0,600 V (ECS) até 0,200 V (ECS), observa-se uma região de passivação secundária, atribuída à formação do MoO3 de maneira mais significativa. Ao final a densidade de corrente volta a aumentar, indicando o início da oxidação/dissolução do MoO3 para Mo3+. / Cyclic potentiodynamic polarization curves of the nickel alloy Inconel 625 in 3.56 % NaCl solution, pH 7.5, deaerated and at ambient temperature, obtained according to the ASTM G61, were studied in order to interpret them, especially after the reversion of the scanning direction. For this study, complementary analyzes by MEV/EDS and by Raman spectroscopy of the working electrode surfaces as well as EDS analyses of the electrolyte collected near the working electrode were carried out. The pH values and O2 content of the electrolyte near the working electrode were also obtained. From the open circuit potential (OPC) up to breakdown potential (Eb), at about 0,600 V (ECS), the polarization curve presents a passive behavior with a tendency to reach the current density of 10-6 A/cm². In this region, the thickening of the passive film takes place mainly due to the formation of Cr2O3. After Eb, a change of the curve slope occurs due to the oxygen evolution reaction which is confirmed by the acidification of the electrolyte. At the 0,750 V (ECS) potential, an inflection of the curve is observed causing a discrete decrease in the current which was attributed to the formation of MoO42- (gel) which, in turn, avoids the incorporation of anions as Cl- or OH- in the passive film. This fact makes the oxidation/dissolution of the passive film difficult but does not avoid it since Cr and Ni ions were detected in the electrolyte after the inflection, characterizing a transpassive region. After the reversion of the scanning direction at the potential 1,130 V (ECS), the curve shows a positive hysteresis with lower current densities than the values obtained at the same potential applied during the direct polarization. This indicates the occurrence of repassivation mainly due to the formation of a Mo rich layer on the alloy whose formation can be attributed to the transformation of the molybdate to the MoO3 because of the strong acidification of the electrolyte. After the reversion of the polarization, a transition from the anodic current to cathodic occurs at a potential close to 0,720 V (ECS). From the potential of 0,600 V (ECS) up to 0,200 V (ECS), a secondary passivation region is observed which was attributed to the significant formation of MoO3. At the end of the reverse polarization, the current increases again, indicating the initiation of the oxidation/dissolution of MoO3 to Mo3+.

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Carbon nanotubes (CNTs) have great potential in many applications due to their unique structure and properties. However, there are still many unsolved problems hampering their real applications. This thesis focuses on three important issues limiting their applications, namely: (1) direct growth of CNTs without additional catalyst, (2) secondary growth of carbon nanotubes on primary CNT bed without using extra catalyst, (3) and CNT alignment mechanisms during the growth.<p> The CNTs used in this thesis were prepared by hot filament chemical vapor deposition (CVD) reactor and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), and Raman spectroscopy. Field electron emission (FEE) properties of the CNTs were also tested.<p> Oxidation-reduction method was adopted in direct growth of CNTs on Inconel 600 plates and proved effective. The effect of oxidation temperature on the growth of CNTs was studied. It was found that the oxidation temperature had an influence on CNT height uniformity and FEE properties: the higher the treatment temperature, the more uniform the resultant CNTs, and the better the FEE properties of the resultant CNTs. The contribution of different oxides formed at different temperatures were investigated to explain the effect of oxidation temperature on the CNT height uniformity.<p> Secondary CNTs were grown on primary ones by simply changing the carbon concentration. No additional catalyst was used during the whole deposition process. It was found that synthesizing primary CNTs at extremely low carbon concentration is key factor for the secondary growth without additional catalyst. The CNT sample grown with secondary nanotubes exhibited improved field emission properties.<p> The effect of bias voltage on growth of vertically aligned carbon nanotubes was investigated. The CNTs grown at -500V shows the best alignment. At the early growth stage, simultaneous growth of randomly oriented and aligned carbon nanotubes was observed. This was consistent with the alignment mechanism involving stress that imposed on catalyst particles on tube tips. Through the observation of CNT growth on the scratched substrates, catalyst particle size was found as another determining factor in the alignment of CNTs. Big catalyst particles promoted aligned growth of CNTs.

Carbon nanotubes (CNTs) have great potential in many applications due to their unique structure and properties. However, there are still many unsolved problems hampering their real applications. This thesis focuses on three important issues limiting their applications, namely: (1) direct growth of CNTs without additional catalyst, (2) secondary growth of carbon nanotubes on primary CNT bed without using extra catalyst, (3) and CNT alignment mechanisms during the growth.<p> The CNTs used in this thesis were prepared by hot filament chemical vapor deposition (CVD) reactor and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), and Raman spectroscopy. Field electron emission (FEE) properties of the CNTs were also tested.<p> Oxidation-reduction method was adopted in direct growth of CNTs on Inconel 600 plates and proved effective. The effect of oxidation temperature on the growth of CNTs was studied. It was found that the oxidation temperature had an influence on CNT height uniformity and FEE properties: the higher the treatment temperature, the more uniform the resultant CNTs, and the better the FEE properties of the resultant CNTs. The contribution of different oxides formed at different temperatures were investigated to explain the effect of oxidation temperature on the CNT height uniformity.<p> Secondary CNTs were grown on primary ones by simply changing the carbon concentration. No additional catalyst was used during the whole deposition process. It was found that synthesizing primary CNTs at extremely low carbon concentration is key factor for the secondary growth without additional catalyst. The CNT sample grown with secondary nanotubes exhibited improved field emission properties.<p> The effect of bias voltage on growth of vertically aligned carbon nanotubes was investigated. The CNTs grown at -500V shows the best alignment. At the early growth stage, simultaneous growth of randomly oriented and aligned carbon nanotubes was observed. This was consistent with the alignment mechanism involving stress that imposed on catalyst particles on tube tips. Through the observation of CNT growth on the scratched substrates, catalyst particle size was found as another determining factor in the alignment of CNTs. Big catalyst particles promoted aligned growth of CNTs.

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Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Melkote, Shreyes; Committee Member: Danyluk, Steven; Committee Member: Steven Liang.