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Title

HIGH TEMPERATURE GRAIN BOUNDARY ENGINEERING OF POWDER PROCESSED NICKEL BASED SUPERALLOYS

Creator

Mccarley, Joshua B.

Date

2018, 2018-05

Description

The concept of grain boundary engineering (GBE) has been effectively utilized to improve the properties of polycrystalline Nickel based...
Show moreThe concept of grain boundary engineering (GBE) has been effectively utilized to improve the properties of polycrystalline Nickel based superalloys via triggering the formation of special High-angle grain boundaries which are highly coherent. Current processing routes which implement GBE often require multiple iterations of room temperature deformation also characterized as cold work, followed by short annealing cycles whereby each applied iteration results in limited to modest enhancements in the fraction of the aforementioned special grain boundaries. As such, a substantial number of iterations are required to obtain fractions large enough (>50%) to effectively yield improved resistance to corrosion, creep, and fatigue. The current application of GBE on high-strength materials is not suitable for the production of large, complex-shaped structures, and has also been noted to increase manufacturing lead time and cost. In this investigation, alternative processing routes which utilize GBE at elevated temperatures for powder processed Nickel based superalloys used as turbine discs in gas turbine engines manufactured by Rolls-Royce. The material which serves as the central focus of the forthcoming investigation is an experimental low stacking fault energy Nickel based superalloy containing a cobalt concentration of 24 wt.%. A preliminary study which focused on the effects of hot deformation parameters similar to typical industrial applications revealed the materials ability to enhance its Σ3 twin boundary length fraction from 21% to 53% following a single deformation/anneal cycle. Subsequently, the investigated deformation parameters displayed a near consistent triggering of dislocation based plasticity mechanisms which promoted the formation of annealing twin boundaries. Although the experimental Nickel based superalloy exhibited the ability to promote extensive fractions of annealing twins, the resultant grain boundary characters produced via hot deformation displayed varying magnitudes of dynamically recrystallized grains which are generally considered to be detrimental in GBE practices. Mechanisms which assist in optimizing the formation of Σ3 twin boundaries during high-temperature GBE were further investigated. The enhanced cobalt concentration possessed by the experimental alloy was observed to have effectively enhanced the materials ability to rapidly store high magnitudes of strain energy upon hot deformation when compared to a commercially available Nickel based superalloy possessed a lower Co. concentration. Reducing the volume fraction of primary gamma prime precipitates present in the experimental alloys microstructure was noted to have effectively slowed the onset of dynamic recrystallization, while still promoting favorable fractions of annealing twins. A critical strain was later identified at which the onset of dynamic recrystallization was predominately avoided in the experimental alloy during hot deformation, and the strain induced boundary migration mechanism was effectively triggered which promoted the largest observed length fractions of Σ3 twin boundaries. A final investigation which considered what effect distinct microstructural features have on the kinetics which drive twin formation was also performed to assist in providing insight on optimizing applications related to GBE for advanced turbine engines.
Ph.D. in Materials Science and Engineering, May 2018
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