Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


Smith_Dissertation_OSU_final.pdf (12.72 MB)

ETD Abstract Container

Abstract Header

Orientation and Alloying Effects on Creep Strength in Ni-Based Superalloys

Smith, Timothy M, Jr.
http://rave.ohiolink.edu/etdc/view?acc_num=osu1480599553519057

Abstract Details

2016, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
The creep deformation mechanisms present during creep at intermediate stress and temperatures in ME3 were further investigated using diffraction contrast imaging. Both conventional transmission electron microscopy and scanning transmission electron microscopy were utilized. Distinctly different deformation mechanisms become operative during creep at temperatures between 677-815ºC, and at stresses ranging from 274-724MPa. Both polycrystalline and single crystal creep tests were conducted. The single crystal tests provide new insight into grain orientation effects on creep response and deformation mechanisms. Creep at lower temperatures (760C) resulted in the thermally activated shearing modes such as microtwinning, stacking fault ribbons and isolated superlattice extrinsic stacking faults (SESFs). In contrast, these faulting modes occurred much less frequently during creep at 815ºC under lower applied stresses. Instead, the principal deformation mode was dislocation climb bypass. In addition to the difference in creep behavior and creep deformation mechanisms as a function of stress and temperature, it was also observed that microstructural evolution occurs during creep at 760C and above, where the secondary coarsened and the tertiary precipitates dissolved. Based on this work, a creep deformation mechanism map is proposed, emphasizing the influence of stress and temperature on the underlying creep mechanisms. Next, the effects of varying crystal orientation and composition on active deformation modes are explored for two different, commercially used Ni-base disk alloys, ME3 and ME501. Understanding these effects will allow for improved predictive deformation modeling and consequently faster advancements in Ni-base alloy development. In order to investigate these effects, compression creep tests were conducted on [001] and [110] oriented single crystal specimens of the disk alloys ME3 and ME501, at different stress/temperature regimes. At 760 C and below, a prominent creep anisotropy exists between the two orientations, with the [110] oriented samples exhibiting superior creep strength. At 815 C, the creep anisotropy disappeared between the two orientations. Through bright field scanning transmission electron microscopy, it was determined that the existence of creep anisotropy is a result of differences in deformation modes between the different orientations and alloy compositions. Results of phase field modeling in which the interaction of dislocations with realistic precipitate structures is also conducted to further advance predictive creep deformation models. Furthermore, the local compositional and structural changes occurring in association with stacking faults in ME501 are characterized and related to the possible rate-controlling processes during creep deformation at intermediate temperatures. These rate-controlling processes are not presently understood. In order to promote stacking fault shearing, compression creep tests on specially prepared single crystals of ME501 were conducted at 760°C in the [001] orientation. Scanning transmission electron microscopy (STEM) imaging was coupled with state-of-the-art energy dispersive X-ray (EDX) spectroscopy to reveal for the first time an ordered compositional variation along the extrinsic faults inside the precipitates, and a distinct solute atmosphere surrounding the leading partial dislocations. The local structure and chemistry at the extrinsic fault is consistent with the phase, a D024 hexagonal structure. Density Functional Theory (DFT) and high angle annular dark field (HAADF)-STEM image simulations are consistent with local phase formation and indicate that a displace-diffusive transformation occurs dynamically during deformation. Additional investigation into the chemical segregation changes associated with faults in ME3 and ME501 is analyzed. Compression creep tests were conducted on [001] oriented samples at 760C in stress regimes where microtwin and stacking fault formations prominently occurred. High resolution EDX was performed in regions where stacking faults had terminated inside of a precipitate, capturing the process as it was transpiring when the creep test had ended. Again, the presence of elemental segregation was observed along superlattice stacking faults as well as multiple examples of a Co and Cr rich Cottrell atmosphere around the leading Shockley partials. The presence and interaction of newly discovered tertiary particles with the formation of these faults is explored. These combined observations lead to the creation of a new microtwin formation model incorporating the diffusion processes now known to ensue during twin development. Finally, a new “phase-transformation strengthening” mechanism that resists high temperature creep deformation in Nickel-based superalloys, where specific alloying elements inhibit the deleterious deformation mode of microtwinning at temperatures above 700 C is introduced. Ultra-high-resolution structure and composition analysis via scanning transmission electron microscopy, combined with density functional theory calculations, reveals that a superalloy with higher concentrations of the elements Titanium, Tantalum, and Niobium encourage a shear-induced solid-state transformation from the to phase along stacking faults in ¿' precipitates, which would normally be the precursors of deformation twins. This nanoscale phase creates a low energy structure that inhibits thickening of stacking faults into twins, leading to significant improvement in creep properties.
Michael Mills (Advisor)
251 p.
Materials Science
Ni-based Superalloys, Creep, Phase Transformation, Modeling, Deformation, Scanning Transmission Electron Microscopy, Characterization

Recommended Citations

Citations

  • Smith, Jr., T. M. (2016). Orientation and Alloying Effects on Creep Strength in Ni-Based Superalloys [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1480599553519057

    APA Style (7th edition)

  • Smith, Jr., Timothy. Orientation and Alloying Effects on Creep Strength in Ni-Based Superalloys . 2016. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1480599553519057.

    MLA Style (8th edition)

  • Smith, Jr., Timothy. "Orientation and Alloying Effects on Creep Strength in Ni-Based Superalloys ." Doctoral dissertation, Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1480599553519057

    Chicago Manual of Style (17th edition)

osu1480599553519057
2,599
© 2016, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.