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E-Thesis 145 views

Oxidation and its Effect on the Fatigue Properties of the Nickel Based Superalloy RR1000 / DAVID LEWIS

Swansea University Author: DAVID LEWIS

  • Redacted version - open access under embargo until: 29th July 2025

DOI (Published version): 10.23889/SUthesis.58632

Abstract

As temperatures within the aero engine increase, oxidation will begin to have a greater effect on the high temperature materials. How this damage will affect the mechanical properties of the high pressure turbine rotor material i.e. nickel superalloy RR1000, must be understood. In this study an atte...

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Published: Swansea 2021
Institution: Swansea University
Degree level: Doctoral
Degree name: EngD
Supervisor: Whittaker, Mark T. ; Mignanelli, Paul M.
URI: https://cronfa.swan.ac.uk/Record/cronfa58632
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Abstract: As temperatures within the aero engine increase, oxidation will begin to have a greater effect on the high temperature materials. How this damage will affect the mechanical properties of the high pressure turbine rotor material i.e. nickel superalloy RR1000, must be understood. In this study an attempt was made to understand how the oxides form and the effect they will have on the fatigue performance of the alloy. Initial thermal exposures were undertaken under no-load and tensile and compressive loads in order to determine the variation in oxide characteristics. Subsequent thermal exposures were undertaken to determine the effect oxidation has on the fatigue lives of the nickel-based superalloy. The oxide morphologies were the same throughout, but the application of an external load caused increased reaction rates, with compressive loads causing greater rate increase than tensile. Methodologies were determined using forms of the Arrhenius relationship to numerically compare the oxidation reaction. Tensile and compressive oxidation asymmetry was believed to be related to the mechano-chemical nature of the reaction. The pre-fatigue thermal exposures initially caused a considerable reduction in the fatigue lives with increasing oxidation. However, the longest exposure time resulted in an S-N curve that lay between the shortest and mid length exposures. A range of analyses were undertaken to determine the presence of any relevant mechanisms that caused this unexpected life improvement. It was found that few of the mechanisms investigated were likely to have an effect on the change in life. The exceptions were; reduction in dislocation movement and crack deflection as a result of a recrystallised zone at the surface, changes in the tertiary γ’ size, variation in hardness of different regions due to a γ’ depleted plastic zone and a hard ceramic oxide, and reduced initiations due to the presence of a ‘healing’ chromium oxide scale. Notch fatigue tests were performed to determine the importance of the findings to components in service and it was found that the notch acted as the cause of crack initiation, effectively mitigating against the effects of the oxidation damage.
Item Description: A selection of third party content is redacted or is partially redacted from this thesis due to copyright restrictions.
Keywords: Superalloy, oxidation, fatigue, microscopy, nano-indentation
College: Faculty of Science and Engineering