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

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

Bibliographic Details
Main Author: Kattoura, Micheal
Language:English
Published: University of Cincinnati / OhioLINK 2017
Subjects:
Mechanical Engineering
Mechanics
Advanced Surface treatments
Nickel-based superalloys
Fatigue behavior
Residual stresses
Electron microscopy
Elevated temperature testing
Online Access:http://rave.ohiolink.edu/etdc/view?acc_num=ucin1504873748777613
id ndltd-OhioLink-oai-etd.ohiolink.edu-ucin1504873748777613
record_format oai_dc
collection NDLTD
language English
sources NDLTD
topic Mechanical Engineering
Mechanics
Advanced Surface treatments
Nickel-based superalloys
Fatigue behavior
Residual stresses
Electron microscopy
Elevated temperature testing
spellingShingle Mechanical Engineering
Mechanics
Advanced Surface treatments
Nickel-based superalloys
Fatigue behavior
Residual stresses
Electron microscopy
Elevated temperature testing
Kattoura, Micheal
Effects of Advanced Surface Treatments on the Fatigue Behavior of ATI 718Plus at Room and Elevated Temperatures
author Kattoura, Micheal
author_facet Kattoura, Micheal
author_sort Kattoura, Micheal
title Effects of Advanced Surface Treatments on the Fatigue Behavior of ATI 718Plus at Room and Elevated Temperatures
title_short Effects of Advanced Surface Treatments on the Fatigue Behavior of ATI 718Plus at Room and Elevated Temperatures
title_full Effects of Advanced Surface Treatments on the Fatigue Behavior of ATI 718Plus at Room and Elevated Temperatures
title_fullStr Effects of Advanced Surface Treatments on the Fatigue Behavior of ATI 718Plus at Room and Elevated Temperatures
title_full_unstemmed Effects of Advanced Surface Treatments on the Fatigue Behavior of ATI 718Plus at Room and Elevated Temperatures
title_sort effects of advanced surface treatments on the fatigue behavior of ati 718plus at room and elevated temperatures
publisher University of Cincinnati / OhioLINK
publishDate 2017
url http://rave.ohiolink.edu/etdc/view?acc_num=ucin1504873748777613
work_keys_str_mv AT kattouramicheal effectsofadvancedsurfacetreatmentsonthefatiguebehaviorofati718plusatroomandelevatedtemperatures
_version_ 1719452942831976448
spelling ndltd-OhioLink-oai-etd.ohiolink.edu-ucin15048737487776132021-08-03T07:04:08Z Effects of Advanced Surface Treatments on the Fatigue Behavior of ATI 718Plus at Room and Elevated Temperatures Kattoura, Micheal Mechanical Engineering Mechanics Advanced Surface treatments Nickel-based superalloys Fatigue behavior Residual stresses Electron microscopy Elevated temperature testing 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. 2017-10-30 English text University of Cincinnati / OhioLINK http://rave.ohiolink.edu/etdc/view?acc_num=ucin1504873748777613 http://rave.ohiolink.edu/etdc/view?acc_num=ucin1504873748777613 unrestricted This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.

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