Advancements in the understanding of the oxidation properties of a nickel-based superalloy

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Reynolds, Thomas David (2020). Advancements in the understanding of the oxidation properties of a nickel-based superalloy. University of Birmingham. Ph.D.

Reynolds2020PhD.pdf
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Abstract

The high-temperature strength of nickel-based superalloys leads their extensive use in the hot sections of gas turbine engines, where material degradation by high-temperature oxidation can occur. High-temperature oxidation is a life-limiting and safety-critical application that can cause component failure, and therefore understanding the oxidation process is essential.

Oxidation testing is generally performed in static laboratory air under isothermal conditions, which is not representative of the actual service environment. In addition, testing is often performed on polished or ground specimens, while service components may be subject to additional surface modifications to improve mechanical performance. RR1000 is a polycrystalline nickel-based superalloy which is utilised as high-pressure turbine disc material in aero-engines which has been studied extensively in previous work however limitations have been identified in the previous testing.

In this work the effect of several surface modifications have been examined: shot-peening, turning and swaging; initially under static lab-air and under more realistic environments such as elevated pressure air and flowing wetted air. Surface modifications were shown to disrupt the normal oxide growth behaviour and mechanisms have been presented in this thesis to explain the process by which this occurred.
Elevated pressure air was shown not to impact the oxidation performance of RR1000 significantly. However, flowing wetted air was shown to be detrimental to the oxidation performance under certain surface conditions.

The resistance of RR1000 to cyclic oxidation was examined with direct comparison to data from isothermal testing, and no differences were observed, confirming the validity of isothermal testing for this material.

The key contribution of this work is the discovery of a significant effect of heating rate. It has been shown that modifying the initial heating rate of the alloy significantly impacts its long-term oxidation behaviour. A particular heating rate was identified that offered a marked improvement in the oxidation properties of RR1000 at 650°C, and this was validated using synchrotron diffraction, high-resolution SEM imaging, and detailed thermodynamic calculations to propose a mechanism as to how this beneficial behaviour manifests.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):