The microstructural evolution and mechanical properties at elevated temperatures of a recently fabricated fine-grained AA6xxx aluminium sheet were evaluated and compared to the commercially fabricated sheet of the same alloy in the T4P condition. The behaviour of the fine-grained and T4P sheets was compared at elevated temperatures between 350°C and 550°C, as well as room temperature. Static exposure to elevated temperatures revealed that the precipitate structure of the fine-grained material did not change extensively. The T4P material, however, underwent extensive growth of precipitates, including a large amount of grain boundary precipitation. At room temperature, the T4P material deformed at much higher stresses than the FG material, but achieved lower elongations. Deformation at elevated temperatures revealed that the fine-grained material achieved significantly larger elongations to failure than the T4P material in the temperature range of 350°C-450°C. Both materials behaved similarly at 500°C and 550°C. Above 500°C, the grain size was greatly reduced in the T4P material, and only a slightly increased in the fine-grained material. At temperatures above 450°C, the elongation to failure in both materials generally increased with increasing strain-rate. The poor performance of the T4P material at low temperatures was attributed to the precipitate characteristics of the sheet, which lead to elevated stresses and increased cavitation. The deformation mechanism of both materials was found to be controlled by dislocation climb, accommodated by the self diffusion of aluminium at 500°C and 550°C. The deformation mechanism in the fine-grained material transitioned to power law breakdown at lower temperatures. At 350°C to 450°C, the T4P material behaved similarly to a particle hardened material with an internal stress created by the precipitates. The reduction in grain size of the T4P material after deformation at 500°C and 550°C was suggested to be caused by dynamic recovery/recrystallization. The role of a finer grain-size in the deformation behaviour at elevated temperatures was mainly related to enhanced diffusion through grain boundaries. The differences in the behaviour of the two materials were mainly attributed to the difference in the precipitation characteristics of the materials.

The microstructural evolution and mechanical properties at elevated temperatures of a recently fabricated fine-grained AA6xxx aluminium sheet were evaluated and compared to the commercially fabricated sheet of the same alloy in the T4P condition. The behaviour of the fine-grained and T4P sheets was compared at elevated temperatures between 350°C and 550°C, as well as room temperature. Static exposure to elevated temperatures revealed that the precipitate structure of the fine-grained material did not change extensively. The T4P material, however, underwent extensive growth of precipitates, including a large amount of grain boundary precipitation. At room temperature, the T4P material deformed at much higher stresses than the FG material, but achieved lower elongations. Deformation at elevated temperatures revealed that the fine-grained material achieved significantly larger elongations to failure than the T4P material in the temperature range of 350°C-450°C. Both materials behaved similarly at 500°C and 550°C. Above 500°C, the grain size was greatly reduced in the T4P material, and only a slightly increased in the fine-grained material. At temperatures above 450°C, the elongation to failure in both materials generally increased with increasing strain-rate. The poor performance of the T4P material at low temperatures was attributed to the precipitate characteristics of the sheet, which lead to elevated stresses and increased cavitation. The deformation mechanism of both materials was found to be controlled by dislocation climb, accommodated by the self diffusion of aluminium at 500°C and 550°C. The deformation mechanism in the fine-grained material transitioned to power law breakdown at lower temperatures. At 350°C to 450°C, the T4P material behaved similarly to a particle hardened material with an internal stress created by the precipitates. The reduction in grain size of the T4P material after deformation at 500°C and 550°C was suggested to be caused by dynamic recovery/recrystallization. The role of a finer grain-size in the deformation behaviour at elevated temperatures was mainly related to enhanced diffusion through grain boundaries. The differences in the behaviour of the two materials were mainly attributed to the difference in the precipitation characteristics of the materials.

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<p>Ni-based superalloys have been widely used for high temperature applications such as turbine blades for jet propulsion and power plants due to their excellent creep, fatigue, and corrosion resistance. But as the demand for higher temperature capability and strength increases, there remains a need to better understand high temperature deformation mechanisms and improve and strengthen superalloys at these elevated temperatures. Recently, a correlation has been observed between solute segregation at planar defects (stacking faults, antiphase boundaries, etc) and enhanced high temperature creep properties – known colloquially as phase transformation strengthening. Experimentally, regardless of alloy composition, strong Co segregation at planar defects along with Cr has been observed. In addition, it has been suggested by density functional theory work that Co would promote Cr concentration at stacking faults by forming strong Cr-Co bonds. Based on these findings, it was hypothesized the presence of Co provides a significant thermodynamic driving force for segregation to planar defects. </p><p>In order to further investigate the correlation between solute segregation and deformation mechanisms the fabrication of a planar front single crystal Ni-based superalloy and its microstructure, alloy composition, and microhardness properties of the as-zone melted and solution heat treated states were investigated and compared to the directionally-solidified state to study the effect of microsegregation on these alloy characteristics. Next, new Co-containing, Cr-free alloys are designed to optimize g-g’ volume fraction, size, and morphology to mimic microstructures observed in single crystal superalloys. The general alloy design strategy and approach are outlined, and the composition, microstructure, phase transformation temperatures, and mechanical properties of new Cr-free and Co-containing alloys are reported. A new set of Cr-free alloys have thus been designed, with modifications of Nb, Ta, and Ti additions ranging from 3 to 7 at.% to investigate the role of these elements on the phase transformation strengthening mechanism at elevated temperatures.</p><p></p>

Orientadores: Antonio Jose Ramirez, Rubens Caram Junior / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-11T13:06:54Z (GMT). No. of bitstreams: 1 TorresLopez_EdwarAndres_M.pdf: 39783627 bytes, checksum: 03310e12f2969671a1edcad6da907e3b (MD5) Previous issue date: 2008 / Resumo: Foi desenvolvido um experimento para estudo in-situ dos processos de deformação a alta temperatura no interior do microscópio eletrônico de varredura, permitindo assim o estudo do fenômeno de trincamento a alta temperatura, denominado fratura por queda de ductilidade. Este experimento teve como finalidade o estudo específico das ligas de níquel AWS A5.14 ERNiCrFe-7 e ERNiCr-3AWS empregadas como metal de aporte para a soldagem de ligas de Ni. Instrumentação cientifica dedicada foi desenvolvida e modificada de modo a superar os desafios impostos pelas condições experimentais adversas associadas as elevadas temperaturas necessárias, à compatibilização do experimento com o nível de vácuo necessário na câmera do microscópio e finalmente, à estabilidade mecânica necessária para o acompanhamento do processo de deformação em escala micrométrica juntamente com os processos de aquecimento e de aplicação de forças elevadas. Utilizando esta instrumentação foram definidas as condições e procedimentos adequados para o acompanhamento do processo de deformação das ligas de Ni AWS A5.14 ERNiCrFe-7 e ERNiCr-3 em temperaturas entre 700 e 1000 °C, de forma a estudar as condições de inicio da fratura por queda de ductilidade nestes materiais. Porém, a instrumentação desenvolvida permite não apenas estudar o fenômeno de fratura por queda de ductilidade e avaliar o desempenho de ligas experimentais, mas também o estudo tanto qualitativo como quantitativo de diversos outros fenômenos de fratura e transformação de fase / Abstract: An in-situ high temperature deformation experiment was developed and adapted be performed within the vacuum chamber of a scanning electron microscope in order to study the high temperature cracking phenomenon known as ductility-dip cracking. This experiment was specifically applied to the study of Ni-base alloys AWS A5.14 ERNiCrFe-7 e ERNiCr-3, which are commonly used as filler metal to weld Ni- and Fe- based alloys. Dedicated scientific instrumentation was developed and modified to overcome the challenges imposed by the severe experimental conditions as elevated temperatures and forces, the compatibleness with the microscope vacuum chamber, and the required mechanical stability to track deformation processes at the micro scale. Using this instrumentation were defined and optimized the conditions to study the deformation of Ni-base alloys AWS A5.14 ERNiCrFe-7 e ERNiCr-3 alloys between 700 and 1000 °C and therefore, helps to elucidate the causes of ductility-dip cracking phenomenon . However, the developed instrumentation is a powerful tool to perform several other qualitative and quantitative studies of deformation, cracking phenomena and phase transformations in different materials / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Thermomechanical processes such as hot rolling have been used in the industry for a long time to process and shape metals to a desired form with specific properties. However it can be difficult to make changes to the different process parameters. That's where it is beneficial to use a hot deformation simulator such as the Gleeble 3800-GTC. It can be used to test metals in a controlled environment where the deformation, temperature and other parameters are easily changed. When the machine uses a Hydrawedge module, it is able to simulate hot rolling using uniaxial compression at high temperatures. Swerim AB has one such machine and has requested to investigate what occurs inside a specimen during testing in the Gleeble, specifically inside two low-alloyed steels with a hardness between 400 and 500 HV. Such tests were replicated using LS-DYNA, an FE software. The goal was to acquire true stress-strain graphs that showed similar behaviour to the data from the Gleeble and plots of the effective plastic strain which could be correlated to the grain structure pattern inside the deformed cylinders. An FE-model was created which replicates the procedure. The model was verified through numerous steps. An initial mesh verification was done where the simulation time took at least 5 hours and at most 86 hours. Using a technique called mass scaling, the elements inside the model were manipulated using additional mass to increase their time step and reduce the computational time. A verification of the mass scaling was done where the computational time was weighed off against accuracy. Afterwards the friction had to be verified where it was found that the Gleeble test specimens were deformed more than necessary which was taken into account and the models were adjusted for friction verification. After all was said and done, the model had a reasonable friction coefficient with an optimal mesh and mass scaling configuration. The resulting model simulated a test of 0.5 seconds in 15 minutes and only costing at most 10 MPa in accuracy when experimental results have maximum values between 110 to 220 MPa depending on the scenario. This equals an approximate error of around 5-10%. When investigating the grain structure after 100 seconds of relaxation, the computational time amounted to 52 hours but could be reduced to 12 hours when simulating 30 seconds as there was no change in the effective plastic strain after that time. The final model has a high enough accuracy which, when combined with the Gleeble, is able to confirm material models and describe what occurs in the material during conditions akin to hot rolling.