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Effects of Advanced Surface Treatments on the Fatigue Behavior of ATI 718Plus at Room and Elevated Temperatures

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2017, PhD, University of Cincinnati, Engineering and Applied Science: Mechanical Engineering.
Fatigue failure is a major reason behind the failure of mechanical components and machine parts. In turbine engines and related applications, the components are subjected to cyclic loading at elevated temperatures. Superalloys have high strength and environmental resistance to perform under extreme high temperatures and stress conditions. Improvement in the strength, fatigue life, and/or temperature capabilities of these superalloys will yield huge economic benefits. To address these challenges, surface treatment techniques are implemented to improve the fatigue behavior of currently used superalloys at elevated temperatures. This study investigates Ultrasonic Nano-crystal Surface Modification (UNSM) and Laser Shock Peening (LSP) as techniques to improve strength and fatigue behavior of ATI 718 Plus (718Plus) at room and elevated temperatures. The effect of temperature and strain rate on the strength, ductility, and failure behavior of 718Plus was investigated. The results showed that with the increase of temperature at slow strain rate, there is a small reduction in the yield strength, a large drop in ductility, and a change in fracture mode from ductile transgranular to brittle intergranular cracking. Analysis of the microstructure showed that the driving mechanism at higher temperatures and slower strain rates is oxygen-induced intergranular cracking, a dynamic embrittlement mechanism and that the d precipitates on the grain boundaries are facilitators. Increase of strain rate at 704 °C caused a small increase in the yield strength, a huge increase in the ductility, and a change in fracture mode from brittle to ductile failure. This showed that the driving mechanism at higher strain rates was Portevin–Le Chatelier effect. Finally, 718Plus has superior fatigue behavior at its operation temperature (650 °C) compared to room temperature due to the strengthening of the ?' precipitates which increased its endurance limit by ~20% (~145 MPa). The repetitive strikes from UNSM induced plastic deformation that led to nano-sized crystallites, high density of twins, and high dislocations density in the near surface, coupled with extremely high magnitude surface compressive stresses (-1200 MPa) and increase in the surface hardness by ~2.3 GPa. This led to increase in the room temperature yield strength by ~ 13% (~ 145 MPa) and endurance limit by ~ 13% (~ 100 MPa). At 650 °C, UNSM retained 56% of its residual stresses after 140 hours. In addition, the microstructure created by UNSM remained stable at 650 °C. The high retained residual stress and stable microstructure led to an increase in the yield strength by ~ 11% (~105 MPa) and endurance limit by ~ 8% (~ 67 MPa) of 718Plus at its operation temperature. The shockwave by LSP induced plastic deformation that greatly increased the dislocation density in the form of dislocation entanglements and slip bands. LSP created high residual stresses (-750 MPa) and increased the surface hardness by ~ 1.62 GPa. This led to increase in the room temperature yield strength by ~ 16% (~ 175 MPa) and endurance limit by ~ 15% (~ 110 MPa). At 650 °C, LSP retained 68% of its residual stresses after 140 hours. In addition, the microstructure created by LSP remained stable at 650 °C. The high retained residual stress and stable microstructure led to an increase of in the yield strength by ~14% (~ 140 MPa) and endurance limit by ~10% (~ 90 MPa) of 718Plus at its operation temperature. The improvement in fatigue behavior 718Plus was due to the shielding provided by UNSM & LSP. The compressive stress shielding hindered the crack initiation and lowered the crack propagation rates. In addition, the near surface microstructure created a barrier that restricted the movement of dislocations to the surface. Thus, the required cycles to create extrusions and intrusions, which lead to crack initiation, was much higher and the crack propagation rates especially near the surface were much lower.
Vijay Vasudevan, Ph.D. (Committee Chair)
Woo Kyun Kim, Ph.D. (Committee Member)
Yijun Liu, Ph.D. (Committee Member)
Dong Qian, Ph.D. (Committee Member)
Jing Shi, Ph.D. (Committee Member)
143 p.

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Citations

  • Kattoura, M. (2017). Effects of Advanced Surface Treatments on the Fatigue Behavior of ATI 718Plus at Room and Elevated Temperatures [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1504873748777613

    APA Style (7th edition)

  • Kattoura, Micheal. Effects of Advanced Surface Treatments on the Fatigue Behavior of ATI 718Plus at Room and Elevated Temperatures. 2017. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1504873748777613.

    MLA Style (8th edition)

  • Kattoura, Micheal. "Effects of Advanced Surface Treatments on the Fatigue Behavior of ATI 718Plus at Room and Elevated Temperatures." Doctoral dissertation, University of Cincinnati, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1504873748777613

    Chicago Manual of Style (17th edition)