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

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Orientation and Alloying Effects on Creep Strength in Ni-Based Superalloys Orientation and Alloying Effects on Creep Strength in Ni-Based Superalloys DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Timothy Michael Smith Jr. Graduate Program in Materials Science and Engineering The Ohio State University 2016 Dissertation Committee: Dr. Michael Mills, Advisor Dr. Hamish Fraser Dr. Yunzhi Wang Dr. Andrew Wessman Dr. Bern Kohler © Copyright by Timothy Michael Smith Jr. 2016 Abstract 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, ii 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) iii 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 γ′ precipitates, which would normally be the precursors of deformation twins. This nanoscale phase creates a low energy structure that inhibits iv thickening of stacking faults into twins, leading to significant improvement in creep properties. v To my wife, And the rest of my family vi Acknowledgements I would be remiss if I didn’t acknowledge the many people who helped me reach this milestone in my life. First and foremost, I would like to thank my wife. I understand and am fully aware of the many sacrifices one must make when married to a graduate student. Whether it being the uncertainty of the future or the unpredictable hours spent in the lab you were always supportive and understanding throughout my time at OSU. I am truly blessed to be married to such a loving and wonderful person. Likewise, I would also like to acknowledge my family who emphasized the importance of an education and hard work. I always knew that if anything were to happen I had the support of my family to fall back on and I truly cannot over-emphasize how valuable that backing is. I respectfully dedicate this document to my wife and family with much gratitude and thanks When I was in undergraduate studies at Wright State University I was fortunate enough to obtain an internship at the Wright Patterson Air Force Base in Dayton Oh. While there, I found myself working under the guidance of Dr. Muratore. I wouldn’t know at the time but this encounter would change the direction of my life. While there, I quickly discovered that the pursuit of a PhD in Materials Science and Engineering was what I wanted to do with my life. This may not have turned out to be true if I also hadn’t been lucky enough to end up working under Dr. Mills at The Ohio State University for my PhD. I can honestly say that it has been my privilege to work in the Mills group and if I could go back and do this all over again I wouldn’t change a thing. The past and present Mills group members have also had a huge role in teaching and guiding me through my graduate studies. Ray Unocic, Patrick Phillips, Dan Coughlin, Matt Bowers, Hallee Deutchman, Don McAllister, Lee Casalena, Collin Whitt, and Connor Slone; Thank you. This thanks vii also extends to all of the great Professors and researchers in the MSE department who I have had the opportunity to collaborate with. In particular, I’d like to thank Dr. McComb, Dr. Wang, Dr. Fraser, Dr. Ghazisaeidi, and Dr. Windl (all from Ohio State) who have all made contributions towards this document. I also received help and contributions from some great colleagues that have attended school at the same time. Bryan Esser, Nik Antolin, Shahriar Hooshmand, and Duchao Lv; without your efforts and fruitful discussions very little of this document would be possible. Indeed, the collaboration and support from all of these scientists at Ohio State helped me throughout my graduate studies and I am grateful to have been given the opportunity to work with them. I need to also thank all of the individuals who helped me in my characterization efforts. Specifically, Robert Williams, Babu Viswanathan, Dan Huber, and Henk Colijn. I appreciate the time and passion each of you showed to help teach me the important characterization techniques that I would subsequently use in my research. I need to also acknowledge the GE University Strategic Alliance Program and the NSF DMREF program for the financial support. Ultimately, this research was only possible through the collaborative efforts of GE. I would like to thank everyone at GE for their support and knowledge, I am particularly grateful to Andrew Wessman, Dave Mourer, Tim Hanlon and Ken Bain. Lastly, I would like to thank my dissertation committee members for taking
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