No description available.

Aluminum-magnesium alloy (AA5083-H116) is primarily designed for marine applications such as in ship hulls and deckhouses. Its excellent combination of physical and mechanical properties makes it useful for other applications such as aircraft construction, military equipment and vehicles and automobiles.<p>This study investigated the effect of time and temperature of sensitization on the mechanical and chemical properties of AA5083-H116 such as tensile strength, yield strength and susceptibility to intergranular corrosion (IGC). Test specimens were sensitized at various temperatures (80oC, 100oC, 175oC and 200oC) for up to 672 h (4 weeks). Microhardness measurements, tensile testing, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), inductively coupled plasma/mass spectrometry (ICP/MS) and nitric acid mass loss tests (NAMLT) were used to evaluate these effects. <p>The results obtained show that the mechanical properties of AA5083-H116 deteriorated with increasing sensitization temperature and time. The adverse effect on these properties was attributed to reduction in dislocation density and recrystallization at higher temperatures. The as-received specimens and those sensitized at 80oC showed no susceptibility to IGC. However, at higher sensitization temperatures and longer resident times, resistance to IGC decreased dramatically. The reduction in IGC resistance was attributed to precipitation of secondary phases along the grain boundaries.

Aluminum-magnesium alloy (AA5083-H116) is primarily designed for marine applications such as in ship hulls and deckhouses. Its excellent combination of physical and mechanical properties makes it useful for other applications such as aircraft construction, military equipment and vehicles and automobiles.<p>This study investigated the effect of time and temperature of sensitization on the mechanical and chemical properties of AA5083-H116 such as tensile strength, yield strength and susceptibility to intergranular corrosion (IGC). Test specimens were sensitized at various temperatures (80oC, 100oC, 175oC and 200oC) for up to 672 h (4 weeks). Microhardness measurements, tensile testing, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), inductively coupled plasma/mass spectrometry (ICP/MS) and nitric acid mass loss tests (NAMLT) were used to evaluate these effects. <p>The results obtained show that the mechanical properties of AA5083-H116 deteriorated with increasing sensitization temperature and time. The adverse effect on these properties was attributed to reduction in dislocation density and recrystallization at higher temperatures. The as-received specimens and those sensitized at 80oC showed no susceptibility to IGC. However, at higher sensitization temperatures and longer resident times, resistance to IGC decreased dramatically. The reduction in IGC resistance was attributed to precipitation of secondary phases along the grain boundaries.

No description available.

Austenitic stainless steels are frequently used for engineering applications in aggressive environments. Typical sources of component failures are associated with localized attack at grain boundaries, such as intergranular corrosion and stress corrosion cracking. To prevent premature failures, structural integrity assessments are carried out, with the aim of predicting the maximum likelihood of cracking that may develop. For accurate predictions it is of great importance to know the interaction of parameters involved in life-determining processes. This PhD thesis investigates the effect of microstructure and stress on intergranular stress corrosion cracking in Type 302 / Type 304 austenitic stainless steels. High-resolution X-ray tomography has been successfully applied to examine, for the first time in 3-dimensions, in-situ, the interaction between microstructure and crack propagation. The development and subsequent failure of crack bridging ligaments has been observed and correlated with regions of ductile tearing persistent on the fracture surface. These ductile regions were consistent with the morphology of low-energy, twin-type grain boundaries, and are believed to possess the capability of shielding the crack tip. Following this observation, a new grain bridging model has been developed, in order to quantify the effect of static stress and crack bridging on the maximum likely crack length. The model was compared and evaluated with in the literature available percolation-like models. Intergranular stress corrosion tests in tetrathionate solutions have been designed and carried out to validate the new model. The assessment comprised,(i) a thorough examination of the microstructure and analysis parameters employed,(ii) the determination of the degree of sensitisation with subsequent crack path investigations,(iii) the identification of a suitable test system with associated grain boundary susceptibility criteria,(iv) the application of Grain Boundary Engineering (GBE) for microstructure control,(v) statistical crack length assessments of calibrated IGSCC test specimens. The results of these tests showed that the new model successfully predicts the magnitude of stress and the effect of grain boundary engineering on the maximum crack lengths.

Austenitic stainless steels have a major problem during solution annealing or welding in the temperature range of 500-800 &deg / C due to the formation of chromium carbide, which causes chromium depleted areas along grain boundaries. This means that the structure has become sensitized to intergranular corrosion. Susceptibility to intergranular corrosion can be determined by means of destructive acid tests or by nondestructive electrochemical potentiokinetic reactivation (EPR) tests. The EPR test, which provides quantitative measurements, can be practiced as single loop or double loop. Single loop EPR method for AISI 304 and 304L type stainless steels was standardized / however double loop EPR (DLEPR) method has not been validated yet. In this study, the degree of sensitization was examined in AISI 304L and 316L type steels by DLEPR method whose experiments have been carried out on sensitive and nonsensitive steels to examine and determine the detailed parameters / solution temperature, concentration and scan rate of the DLEPR method. In order to determine the degree of sensitization, oxalic acid, Huey and Streicher tests were carried out and revealed microstructures and measurements of weight loss by the acid tests were then correlated with DLEPR method results, as a first step towards standardization of DLEPR method for 316L steels. Best agreement was provided with test parameters which are 1M H2SO4 + 0.005M KSCN at 3 V/hr scan rate with 30 &deg / C solution temperature. It was concluded that specimens can be classified as step, dual and ditch, if the Ir:Ia ratios were obtained to be between 0 to 0.15, 0.15 to 4.0 and 4.0 to higher respectively.

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CoordenaÃÃo de AperfeiÃoamento de NÃvel Superior / Os aÃos inoxidÃveis austenÃticos sÃo conhecidos pela sua maior resistÃncia à corrosÃo e boas propriedades mecÃnicas a altas temperaturas. No entanto estes aÃos sÃo suscetÃveis à corrosÃo intergranular, causada pela segregaÃÃo de carboneto de cromo (M23C6) nos contornos de grÃo. Esse tipo de corrosÃo à causado pelo empobrecimento de cromo nas regiÃes adjacentes aos contornos de grÃo, fazendo com que o aÃo se caracterize como sensitizado. Uma alternativa para reduzir esse efeito à a reduÃÃo do teor de carbono em soluÃÃo sÃlida, diminuindo assim a formaÃÃo de carbonetos a elevadas temperaturas. Neste trabalho foi avaliada a resistÃncia à sensitizaÃÃo dos aÃos AISI 317 e AISI 317L a partir do estado como recebido quando submetido na faixa de temperatura de 400ÂC a 700ÂC por diferentes perÃodos de tempo. Foi tambÃm estudado um tratamento de solubilizaÃÃo adequado para minimizar os efeitos da sensitizaÃÃo no material como recebido submetido nas condiÃÃes escolhidas de tratamento tÃrmico. Foram realizados tratamentos tÃrmicos nos tempos de 1h, 12h, 24h, 72h e 96h nas temperaturas de 400ÂC, 500ÂC, 600ÂC e 700ÂC. Foram realizados tratamentos de solubilizaÃÃo na temperatura de 1100ÂC por 20, 60 e 240 minutos em ambos os aÃos e em seguida foram repetidos os tratamentos tÃrmicos nas condiÃÃes onde o material apresentou sensitizaÃÃo na condiÃÃo como recebido. ApÃs cada tratamento foi realizada uma caracterizaÃÃo microestrutural pelas tÃcnicas de microscopia eletrÃnica de varredura e microscopia Ãtica. Foram realizados ensaios de reativaÃÃo potenciocinÃtica por Double Loop (DL-EPR) para uma avaliaÃÃo quantitativa do grau de sensitizaÃÃo das amostras. A exposiÃÃo dos aÃos na temperatura de 700ÂC acarretou a sensitizaÃÃo dos mesmos, porÃm na condiÃÃo como recebido, o aÃo AISI 317L mostrou-se sensitizado mais cedo em comparaÃÃo ao aÃo AISI 317. O tratamento tÃrmico de solubilizaÃÃo à 1100ÂC foi mais efetivo em reduzir a sensitizaÃÃo do aÃo AISI 317L, mostrando que o tempo de 20 minutos à suficiente para melhorar sua resistÃncia ao empobrecimento de cromo. / Austenitic stainless steels are known for their higher corrosion resistance and good mechanical properties at high temperatures. However, these steels are susceptible to intergranular corrosion caused by segregation of chromium carbide (M23C6) at grain boundaries. This type of corrosion is caused by the depletion of chromium in the regions adjacent to grain boundaries, making the steel as sensitized. An alternative to reduce this effect is to reduction of the carbon content in solid solution, thus decreasing the formation of carbides at high temperatures. In this work it was evaluated the resistance to sensitization of AISI 317 and AISI 317L steels from the state as received when submitted in the temperature range 400  C to 700  C for different periods of time. It was also investigated a solution annealing temperature appropriate that it could minimize the sensitization effects for the material in the as received condition under the selected conditions of heat treatment. Heat treatments were performed in the time of 1h, 12h, 24h. 72h and 96h at temperatures of 400 ÂC, 500 ÂC, 600 ÂC and 700 ÂC. Solution annealing treatments were performed at temperature of 1100ÂC for 20, 60 and 240 minutes in both steel and then heat treatments were repeated under conditions where the material presented in the sensitization condition as received. After each treatment a micro structural characterization was held using both a metallurgical and a scanning electron microscope. Tests were performed with Double Loop potenciokinetic reactivation method (DL-EPR) to quantitatively assess the sensitization degree of the samples. The exposure of the samples at a temperature of 700  C led to the sensitization, but as received AISI 317L steel was sensitized early in comparison to the AISI 317 steel. The solution anneal heat treatment at 1100ÂC was more effective in reducing the sensitization of the AISI 317L steel, showing that the time of 20 minutes is enough to improve their resistance to depletion of chromium.

La prédiction des durées de vie des pièces de structures aéronautiques a toujours été une problématique à la fois complexe et capitale dans ce domaine de l’industrie. Néanmoins, la majorité des tests existant à l’heure actuelle cherche à évaluer la capacité des matériaux à résister aux sollicitations mécaniques notamment en fatigue. Les problématiques liées à l’endommagement causé par l’environnement comme la corrosion sont encore mal comprises. En effet, bien que certains tests permettent de détecter et de caractériser cet endommagement, aucun outil fiable de prédiction des vitesses de propagation des défauts de corrosion intergranulaire n’existe. Ainsi, actuellement, un défaut de corrosion détecté induit systématiquement un changement de la pièce. Les travaux de cette thèse s’inscrivent dans cette problématique ; ils ont pour but de comprendre les phénomènes de corrosion intergranulaire sur l’alliage d’aluminium 2024, le plus utilisé dans le secteur aéronautique, et d’étudier les cinétiques de propagation des défauts de corrosion. L’étude s’appuie sur une approche multi-échelle des processus de corrosion, des états microstructuraux et de l’influence de l’hydrogène. Ce projet s’inscrit également dans une dynamique de collaboration avec Airbus Group et l’Université de Bourgogne dans le cadre du projet ANR M-SCOT (Multi-Scale Corrosion Testing ANR-14-CE07-0027-01). / Nowadays, cracks kinetics is a key point in aircraft risk and reliability analysis. In particular, the propagation of corrosion defects is of special interest and could promote an early mechanical crack initiation. However, today most of the tests are calibrated to control mechanical damage and do not take into account the propagation of the corrosion defects. Indeed, when a corrosion defect is observed, the airplane part is automatically changed which leads to high manufacturing costs. The aim of this work is to understand the intergranular corrosion mechanisms and to study the propagation kinetics of the corrosion defects in an aeronautical reference alloy, i.e. the 2024-T351 aluminium alloy. A multiscale approach of the corrosion processes, the microstructural states as well as hydrogen influence was performed. This work is supported by ANR-14-CE07-0027-01 – M-SCOT: Multi Scale COrrosion Testing.