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Exploration of Phase Stability and Hot Workability of Polycrystalline Co-Al-W-Base Superalloys

Wertz, Katelun N
http://orcid.org/0000-0001-7121-1444
http://rave.ohiolink.edu/etdc/view?acc_num=osu1555455335988532

Abstract Details

2019, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
Nickel-base superalloys are routinely employed for structural components within the late-stage compressor and turbine sections of gas turbine propulsion engines due to their unique combination of ductility and strength at elevated temperatures. The desirable performance of this material class is a direct consequence of an aggregate microstructure containing a disordered γ-FCC phase strengthened by ordered γ'-L12 precipitates. In recent years, a novel alloying system with microstructural characteristics analogous to nickel-base superalloys has garnered significant interest within the aerospace community. At specific compositions, the ternary Co-Al-W system exhibits similar L12 precipitates within a FCC matrix, but with the added advantage of a solidus temperature approximately 100 - 150 °C higher than observed in nickel-based systems. This effort adds to other alloy development investigations assessing the potential of this new alloy class for commercial transition into aerospace propulsion applications. Two aspects of Co-Al-W-base alloys were probed in detail: (i) microstructural stability after exposure to an elevated temperature for extended times and (ii) the hot deformation behavior of polycrystalline alloys under conditions relevant to the industrial thermo-mechanical processes necessary for component fabrication. In many previous publications, the Co3 (Al,W)- γ' strengthening phase in the Co-Al-W ternary system has been proposed as thermodynamically metastable at desired application temperatures. Bulk specimens of five Co-rich compositions of the Co-Al-W ternary and Co-Al-W-Ni quaternary systems were characterized after isothermal aging near 850 °C to assess previously unevaluated γ' W:Al ratios and confirm the effect of Ni alloying at exposure times up to 5000 hours. The aged microstructures, phase fractions, and phase compositions were evaluated with the intent of informing computational thermodynamic simulations for the Co-rich end of Co-Al-W and Co-Al-W-Ni phase diagrams. Contrary to prior findings, the L12 phase in the model Co-9Al-9W (at%) system appeared coherent and stable near 850 °C, even after extended aging. Ni additions were confirmed to suppress the formation of undesired phases. Further, conventional thermo-mechanical processing of a highly-alloyed model Co-Al-W alloy was explored via hot compression testing at temperatures and deformation rates pertinent to industrial wrought processes. Flow behavior and microstructural evolution of a Co-10Al-4W-25Ni-2Ta-2Ti (at%) alloy were evaluated as a function of temperature, strain, and applied strain rate. To evaluate the potential hot workability of the novel alloy class, the mechanisms and kinetics controlling dynamic recrystallization at supersolvus and subsolvus temperatures were quantitatively established and compared to published literature on nickel-base superalloys. The investigated alloy was found to exhibit similar hot deformation behavior to commercial superalloys, suggesting industrial fabrication knowledge pertinent to Ni-base superalloys could be leveraged to enable rapid transition of the novel Co-Al-W-base alloys.
Michael Mills (Advisor)
Stephen Niezgoda (Advisor)
Yunzhi Wang (Committee Member)
Kiran D'Souza (Committee Member)
159 p.
Materials Science; Metallurgy
Superalloy; Cobalt; Co-Al-W; Phase Stability; Hot Workability; Dynamic Recrystallization; Long Term Aging; Gamma Prime; Supersolvus;

Recommended Citations

Citations

  • Wertz, K. N. (2019). Exploration of Phase Stability and Hot Workability of Polycrystalline Co-Al-W-Base Superalloys [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1555455335988532

    APA Style (7th edition)

  • Wertz, Katelun. Exploration of Phase Stability and Hot Workability of Polycrystalline Co-Al-W-Base Superalloys. 2019. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1555455335988532.

    MLA Style (8th edition)

  • Wertz, Katelun. "Exploration of Phase Stability and Hot Workability of Polycrystalline Co-Al-W-Base Superalloys." Doctoral dissertation, Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1555455335988532

    Chicago Manual of Style (17th edition)

osu1555455335988532
254
© 2019, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.