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GRAIN BOUNDARY ENGINEERING OF POWDER-PROCESSED NI-BASE SUPERALLOY RR1000
Title
GRAIN BOUNDARY ENGINEERING OF POWDER-PROCESSED NI-BASE SUPERALLOY RR1000
Creator
Detrois, Martin
Date
2016, 2016-05
Description
Grain boundary engineering (GBE) has been used to improve the properties of various polycrystalline materials by optimization of their grain...
Show moreGrain boundary engineering (GBE) has been used to improve the properties of various polycrystalline materials by optimization of their grain boundary network. Traditional processing routes for GBE often require multiple iterations of cold work followed by short annealing cycles where each iteration imparts a modest increase in the fraction of special grain boundaries. Multiple iterations are then required to achieve sufficiently high fractions (>50%) that result in the improved properties. Thus, this GBE approach is not suitable for the fabrication of large, complex-shaped structures and leads to added manufacturing lead time and cost. In this investigation, the Ni-base superalloy RR1000 used as turbine discs in gas turbine engines manufactured by Rolls-Royce, was considered for GBE using alternative processing routes more suitable to the forging of Ni-base superalloy components. A preliminary study of the e↵ects of hot deformation parameters closer to typical industrial processing revealed that the length fraction of ⌃3 boundaries increased from 35% to 52% following a single deformation/anneal cycle. Deformation parameters that resulted in strain accommodation via superplastic flow did not enhance the formation of ⌃3 boundaries upon annealing. Whereas deformation parameters that resulted in a dominant dislocation-based plasticity flow mechanism promoted the formation of annealing twins. Using misorientation maps and by estimating the stored strain energy from deformation, equations for the length fraction and density of ⌃3 boundaries were generated for high-temperature GBE of RR1000. The grain boundary characters obtained via high-temperature deformation, however, are less ideal than those resulting from traditional cold rolling. The underlying mechanisms responsible for the formation of ⌃3n boundaries during high-temperature GBE were further investigated. A larger starting grain size prior to deformation was found to be unfavorable to the formation of twin boundaries from twin-reorientation and annihilation of preexisting twins. While recrystallization was found to populate the microstructure with grains that contained very few twin boundaries, post-deformation texture was found to promote the formation of ⌃3 boundaries and triple junctions when Goss texture was present. A final consideration of larger scale forgings was used to raise an outlook on the current issues and the potential of high-temperature GBE for turbine engines.
Ph.D. in Materials Science and Engineering, May 2016
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RECRYSTALLIZATION MECHANISM FOR TWO NICKEL BASED SUPERALLOYS
Title
RECRYSTALLIZATION MECHANISM FOR TWO NICKEL BASED SUPERALLOYS
Creator
Balandra, Ombeline
Date
2016, 2016-05
Description
The demand for enhanced structural performances materials is growing every year. A lot of technological advancements in the sector of...
Show moreThe demand for enhanced structural performances materials is growing every year. A lot of technological advancements in the sector of aerospace or nuclear are in constant need for materials with good mechanical properties and high temperature resistance. The alloys commonly used for these features are Nickel-based superalloys as they exhibit high strength and good resistance to corrosion and oxidation. To improve their mechanical behavior, recent studies have focused on grain refinement methods. Among these methods to obtain the finest grain size distribution, one is particularly advantageous for it low cost and feasibility: severe plastic deformation.In this study, the deformation mechanism of two high performance, low stacking fault energy nickel-based alloys are investigated. The first alloy, Monel 400, it is a single FCC phase material. The second one is Inconel 625 wich has a two-phase (γ and γ’) microstructure. During hot deformation, the γ’ precipitate may be present in the γ phase and above a certain solvus temperature, the γ phase exists in the material. The restoration mechanism for FCC crystals is well known, and particular attention was given in this report to the recrystallization response and flow behavior of Inconel 625 for sub-solvus temperatures. In the introduction a brief review of the current state of literature on the deformation response of Nickel-based superalloys is provided. Samples were compressed under various temperatures and strain rate conditions using a Gleeble-3500 thermo-mechanical simulator and flow stress curves were extracted. To characterize both qualitatively and quantitatively the deformation, samples were then analyzed using standard microscopy, scanning electron microscopy and electron backscatter diffraction analysis. The resulting images and maps combined with flow stress curves have lead to the formulation of constitutive models of the recrystallization process using three parameters, the stress, grain size and recrystallized volume fraction.The data shows that deformation is first accommodated through dynamic recovery with the formation of sub-grains structures. Then, after the experimental strain reaches a critical value, recrystallized grains form within the microstructure. EBSD analysis show a trend for new recrystallized grain to grow under certain conditions. Results show a trend of increasing the grain size with increasing the strain and decreasing the Zener-Hollomon parameter and an increasing recrystallized volume fraction with increasing the strain and Zener-Hollomon parameter.
M.S. in Material, Mechanical and Aerospace Engineering, May 2016
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