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Weaver - TMP of Gamma-Prime Cobalt.pdf (12.81 MB)

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Thermomechanical Processing of a Gamma-Prime Strengthened Cobalt-Base Superalloy

Weaver, Donald S
http://rave.ohiolink.edu/etdc/view?acc_num=osu1543508199900005

Abstract Details

2018, Doctor of Philosophy, Ohio State University, Industrial and Systems Engineering.
A novel class of gamma-prime strengthened cobalt-based superalloys may enable a significant temperature and efficiency capability improvement relative to nickel-base superalloys for future generation turbine engine hardware. However, little information exists regarding deformation processing of these novel Co-Al-W alloys into useable product forms with the necessary microstructure refinement at an industrially relevant scale with industrially relevant processes. To address this need, an ingot metallurgy thermomechanical processing sequence was demonstrated for a novel class of cobalt-base gamma-prime containing superalloys. From an as-cast ingot, the material was characterized and a homogenization heat treatment was developed and executed to reduce residual segregation from casting. Representative ingot conversion steps using extrusion were evaluated and performed followed by a recrystallization heat treatment to produce the desired fine-grain, wrought microstructure. Deformation processing of wrought material was completed at supersolvus hot-working temperatures using both cylindrical upset specimens to establish flow-stress behavior and custom-designed double-cone upset specimens to experimentally quantify the effect of strain, strain-rate, and temperature in microstructure evolution during hot-working, including the dynamic recrystallization and grain growth. All upset testing was completed at two supersolvus temperatures (1149 °C or 1204 °C) and one of three strain-rates (0.01/s, 0.1/s, or 1.0/s) depending on the type of testing completed. Required thermophysical and thermomechanical data was determined for material property inputs to a finite element model which was used to correlate observed microstructures to location-specific thermomechanical processing history. As part of this development, a significant effort was undertaken at each stage of processing to sufficiently characterize the microstructure through optical microscopy, electron microscopy, or other specialized testing. Large-area electron backscatter diffraction was completed to document the microstructure evolution during subsequent thermomechanical processing sequences. The microstructure data from the characterization was used to fit Johnson-Mehl-Avrami-Kolmogorov equations for dynamic recrystallization and a traditional grain growth equation. The results were compared to similar equations generated for a nickel-base superalloy, Waspaloy. The microstructure fits were incorporated into a commercial finite element software package and used to predict location-specific microstructure information and compared to experimental results. This work established ingot metallurgy was a feasible processing route for this novel class of gamma-prime strengthened cobalt superalloy and determined and demonstrated thermomechanical processing sequences for successful homogenization, ingot conversion, and wrought processing at an industrially-relevant scale. The microstructure model fits, incorporated in finite element software, successfully reflected the microstructure evolution and showed similarity between a legacy nickel superalloy, Waspaloy, and confirmed the suitability of traditional JMAK microstructure evolution models for novel gamma-prime strengthened cobalt superalloys. The strong similarity between nickel- and cobalt-based gamma-prime strengthened alloys was also promising from an industrial perspective. Similarity of thermomechanical processing sequences meant conventional machines, processes, and tooling could be used for manufacturing. Utilizing updated, well-accepted existing computational modeling tools enabled process and location-specific microstructure and mechanical property simulation and optimization at less effort than a full development. The result of the demonstrated similarity was reduced barriers to transition and eventual implementation of this novel alloy class.
Rajiv Shivpuri (Advisor)
Jerald Brevick (Committee Member)
Hamish Fraser (Committee Member)
178 p.
Aerospace Materials; Engineering; Industrial Engineering; Materials Science; Metallurgy
Cobalt Superalloy; Gamma-Prime Strengthened Cobalt; Thermomechanical Processing; Ingot Metallurgy; Cobalt Homogenization; Ingot Conversion; Wrought Processing; Microstructure Evolution; Microstructure Evolution Modeling; Dynamic Recrystallization;

Recommended Citations

Citations

  • Weaver, D. S. (2018). Thermomechanical Processing of a Gamma-Prime Strengthened Cobalt-Base Superalloy [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1543508199900005

    APA Style (7th edition)

  • Weaver, Donald. Thermomechanical Processing of a Gamma-Prime Strengthened Cobalt-Base Superalloy. 2018. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1543508199900005.

    MLA Style (8th edition)

  • Weaver, Donald. "Thermomechanical Processing of a Gamma-Prime Strengthened Cobalt-Base Superalloy." Doctoral dissertation, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1543508199900005

    Chicago Manual of Style (17th edition)

osu1543508199900005
714
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