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Microstructure and thermo-mechanical properties of gradient nickel alloysJie Ding (8771438) 28 April 2020 (has links)
<p>Gradient structured (GS) metallic
materials have shown unique properties including the synergy of high strength
and good ductility, improved fatigue and wear/friction resistance etc. One of
the severe surface modification technique, surface mechanical grinding
treatment (SMGT), has been proven an effective method for the generation of
gradient structures in metallic materials. Most of Ni-based superalloys are
precipitation strengthened and with an extraordinary combination of high
strength, ductility and resistance to oxidation at high temperatures. The
precipitation behaviors of these materials are sensitive to their initial
microstructures. This thesis focuses on the microstructure evolution and
mechanical behaviors of two types of gradient Ni alloys. </p>
<p>GS Hastelloy C-22HS and Inconel
718 (IN718) Ni-based superalloys were fabricated through the SMGT technique.
The gradient structures consist of nanograined (NG) or nanolaminate (NL)
surface layer and the subsurface layers with deformation twins. <i>In situ </i>compression test results reveal
that intergranular back stress may contributes to the high work hardening capability
of the GS C-22HS alloy. Mo-rich thick grain boundaries (GBs) formed in the
gradient C-22HS samples after heat treatment. <i>In situ</i> micropillar compression studies coupled with molecular
dynamics (MD) simulations reveal that the Mo-rich thick GBs are stronger
barriers than conventional thin GBs to the transmission of dislocations,
leading to significant strengthening. Furthermore, the formation of thick GBs
also contributes to the improvement of thermal stability of nanograins in the
C-22HS alloys. The gradient microstructures in the IN718 alloy changed the
precipitation behavior and thermal stability of nanograins in the alloy. The studies
on precipitation behaviors of GS IN718 alloy reveal that η phase formed in the
severely deformed surface NG layer after annealing at 700 <sup>o</sup>C. Thermal
stability studies show that NG IN718 alloy with grain sizes smaller than the
critical value of ~ 40 nm is thermally more stable than their coarse-grained
counterpart. The underlying mechanisms of strengthening and improved thermal
stability of the gradient Ni-based superalloys are discussed based on
transmission electron microscopy studies and MD simulations. This work suggests
that tailoring the gradient microstructures may lead to the discovery of
metallic materials with novel mechanical and thermodynamic properties. </p>
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Influência do teor de carbono na formação da camada de nitretos e nas propriedades mecânicas em aços da família 43XX submetidos à nitretação a plasma / Carbon content influence on the formation of the nitride layer and on the mechanical properties of steels from 43XX subjected to plasma nitriding treatmentSakamoto, Felipe Sannomiya 16 February 2018 (has links)
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Previous issue date: 2018-02-16 / Os aços de alta resistência da série AISI 43XX apresentam importantes aplicações na indústria aeroespacial e automobilística. Devido as suas características, podem atingir altos limites de escoamento e resistência. A presença de elementos de liga como cromo, molibdênio e níquel juntamente com o teor de carbono, variando neste estudo de 0,20 a 0,70 %, podem promover melhorias nas propriedades mecânicas. O tratamento de superfície de nitretação utilizado pode promover melhorias nas propriedades de corrosão, desgaste e aumento da dureza superficial. Neste estudo foi aplicado o tratamento de nitretação a plasma em amostras dos aços da família AISI 43XX e foi avaliada, de forma comparativa, as alterações na superfície e na microestrutura dos aços nitretados em relação aos não nitretados. A elevação do teor de carbono pode dificultar ou reduzir a formação da camada de nitretos, portanto, é importante avaliar a influência do percentual de carbono nestas estruturas. As características microestruturais foram analisadas por microscopia óptica, microdureza e difração de raios-X. Foram também estudadas a influência do teor de carbono no aço apenas temperado e temperado e revenido. Com o aumento do teor de carbono do aço foi observada a elevação na dureza do aço temperado ou apenas recozido. O tratamento termoquímico de nitretação a plasma elevou a dureza da superfície para valores em torno de 800 HV, dureza superior à encontrada no substrato. A profundidade da zona de difusão de nitrogênio foi menor para os aços com teores de carbono mais elevados. A autenita retida aumentou juntamente com o teor de carbono, mas só foi possível quantificar a fração volumétrica desta fase pela microscopia óptica, uma vez que, pela técnica de difração de raio X, é de difícil detecção concentrações abaixo de 5% ou distribuídas de forma fina e homogênea. / The high strenght steel from series AISI 43XX are heavely applied on the aerospacial and automotive industry. Due to their chacteristics, they reach high yield values and resistence levels. The presence of chrome, nickel and molybdenum allied with the carbon content, from 0,20 to 0,70% in this study, can provide improvements on the mechanical properties. The nitriding surface threatment used may provide improvement on corrosion, wear properties and increase the surface hardness. In this study was applied the nitriding plasma threatment on samples of family 43XX steel and was evaluated, on comparative basis, the alterations on the surface and in the microstructure of nitrided steels in comparison to not nitrided steels. Higher carbon content may hamper or reduce the nitrides deposition, therefore, it is important to evaluate the influence of the carbon concentration in theses structures. The microstructure characteristics were analysed by optical microscopy, microhardness and X-ray difraction. The nitriding plasma thermochemical threatment raised the surface hardness to values of about 800 HV, higher than the values measured in the substract. The nitrogen diffused zone of higher carbon content steels was thicker than the lower. The content of arrested austenite increased as the carbon contend increased, but was just possible to measure it by optical microscopy, because it is difficult to measure lower contents than 5% or homogen and thick distributed by X-ray difraction.
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Performance characterisation of duplex stainless steel in nuclear waste storage environmentOrnek, Cem January 2016 (has links)
The majority of UK’s intermediate level radioactive waste is currently stored in 316L and 304L austenitic stainless steel containers in interim storage facilities for permanent disposal until a geological disposal facility has become available. The structural integrity of stainless steel canisters is required to persevere against environmental degradation for up to 500 years to assure a safe storage and disposal scheme. Hitherto existing severe localised corrosion observances on real waste storage containers after 10 years of exposure to an ambient atmosphere in an in-land warehouse in Culham at Oxfordshire, however, questioned the likelihood occurrence of stress corrosion cracking that may harm the canister’s functionality during long-term storage. The more corrosion resistant duplex stainless steel grade 2205, therefore, has been started to be manufactured as a replacement for the austenitic grades. Over decades, the threshold stress corrosion cracking temperature of austenitic stainless steels has been believed to be 50-60°C, but lab- and field-based research has shown that 304L and 316L may suffer from atmospheric stress corrosion cracking at ambient temperatures. Such an issue has not been reported to occur for the 2205 duplex steel, and its atmospheric stress corrosion cracking behaviour at low temperatures (40-50°C) has been sparsely studied which requires detailed investigations in this respect. Low temperature atmospheric stress corrosion cracking investigations on 2205 duplex stainless steel formed the framework of this PhD thesis with respect to the waste storage context. Long-term surface magnesium chloride deposition exposures at 50°C and 30% relative humidity for up to 15 months exhibited the occurrence of stress corrosion cracks, showing stress corrosion susceptibility of 2205 duplex stainless steel at 50°C.The amount of cold work increased the cracking susceptibility, with bending deformation being the most critical type of deformation mode among tensile and rolling type of cold work. The orientation of the microstructure deformation direction, i.e. whether the deformation occurred in transverse or rolling direction, played vital role in corrosion and cracking behaviour, as such that bending in transverse direction showed almost 3-times larger corrosion and stress corrosion cracking propensity. Welding simulation treatments by ageing processes at 750°C and 475°C exhibited substantial influences on the corrosion properties. It was shown that sensitisation ageing at 750°C can render the material enhanced susceptible to stress corrosion cracking at even low chloride deposition densities of ≤145 µm/cm². However, it could be shown that short-term heat treatments at 475°C can decrease corrosion and stress corrosion cracking susceptibility which may be used to improve the materials performance. Mechanistic understanding of stress corrosion cracking phenomena in light of a comprehensive microstructure characterisation was the main focus of this thesis.
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