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Title: Corrosion-fatigue interactions of high-temperature nickel alloys
Authors: Child, Daniel
Issue Date: 2012
Publisher: © Daniel Child
Abstract: Corrosion and fatigue aspects of high temperature turbine components made from Alloy 720Li and RR1000 have been studied with a view to understanding the potential failure mechanisms occurring in these materials. Understanding of such failure mechanisms is important in order to make safety improvements and significant cost savings by reducing engine downtime. Some ex-service discs currently exhibit pit-like features at a specific location on the firtree lobes, which potentially may lead to more serious alloy fissuring. Shot peening is currently employed to improve fatigue resistance at the surface of components. This work aims to fully characterise these pits and fissure features in addition to shot peening, and the components in general, using advanced analytical techniques, in order that the failure mechanism(s) can be determined and mitigated against. Engine turbine discs experience lower temperatures than the blades they house, typically between 600°C and 800°C, although this is constantly pushed higher by a drive for improved efficiency and reduced emissions. Materials experiencing these temperatures, in addition to aggressive sulphur-containing environments are susceptible to hot corrosion. A review of existing literature identifies three hot corrosion mechanisms, defined according to the temperature range: non-layer-type (Type I) corrosion, transitional-type corrosion and layer-type (Type II) corrosion. Each type of attack is characterised by different microstructural features, which have been used for comparison throughout the study. Study of the baseline condition of Alloy 720Li established the presence of a γ'/γ phase structure with an average grain size of ~ 4 µm and grain boundary M23C6 precipitates. The alloy also exhibited local grain size reductions by approximately 50%, known as banding . Post-exposure oxide development was characterised by an outer layer of Ni-rich oxide with an underlying layer of Cr, Ti, Al-rich oxide, observed by Secondary Electron Microscopy and Scanning Transmission Electron Microscopy with Energy Dispersive X-ray analysis. Oxide morphologies from post-exposure samples were comparable to the transition-type characteristics described by Viswanathan [1]. Salt exposure was thought to be highest at the edge-of-bedding face (located on the underside of the lobe), where a Cr, Ti-rich sulphide layer within the oxide was observed, indicating more severe attack. Cr and Al appear to be initially drawn from the alloy through grain boundaries as SOX diffuses inward to react and form sulphides. The edge-of-bedding regions displayed more extensive signs of attack due to the additional operational stresses, leading to some examples of fissuring from the edge-of-bedding channels (pit-like features at the edge-of-bedding) into the alloy. Suitable environmental and stress conditions for future laboratory trials were determined by harmonising the microstructural characteristics of test and ex-service specimens. Propagating alloy fissures were found to occur only beneath the strain hardened depth induced by shot peening. The study of shot peening in Alloy 720Li firtrees, was enabled using an Electron Back Scattered Diffraction (EBSD) based technique developed in this study to measure the depth of strain hardening influence, based on the orientation deviation of individual data points within a grain compared to the grain average, which are affected by the shot peening process. The technique showed that exposure during shot peening is lower at the critical edge-of-bedding location, probably as a result of poor accessibility of the shot. Banding, i.e. the local reduction of grain size, at the surface was shown to have a significant impact on strain hardened depth, with between a 30 and 60% reduction in strain hardened depth observed, potentially lowering the strain hardened depth in the critical edge-of-bedding region to as low as 15 µm (compared to the desired depth of approximately 55 µm). RR1000 component sections showed similar shadowing problems causing a reduced strain hardened depth, although an improvement is achieved by using a coarse grain variant of the alloy. An investigation into the relative effects of shot peening intensity and shot size revealed a large variation in strain hardened depth between similar samples. Shot peening intensity was observed to increase strain hardened depth in addition to increasing surface roughness. Shot size was found to have little to no effect on strain hardened depth but did appear to reduce surface roughness, desirable for limiting the ingress of attacking species and fatigue crack initiation sites. A duplex peening process achieved a comparatively high strain hardened depth with low surface roughness. Study of differently shot peened and corroded samples showed an increase in corrosion levels compared to a non-peened sample, thought to be due to the increase in elemental diffusivity induced by the surface treatment. The current process specification 6-8A intensity appears suitable to lose the least proportion of strain hardened depth due to corrosion, but surface roughness could be reduced by increasing shot size. The shot peening process was shown to be an area with much scope for future investigation and optimisation in order to improve the current situation, for a potential increase in component lifetime. A serial milling methodology for three-dimensional data collection of both Energy Dispersive X-ray (EDX) and EBSD data was also improved to enable complex reconstruction of regions of interest. Modified sample preparation enabled the elimination of data shadowing (the blocking of signal by geometrical sample constraints) and the optimisation of slice alignment. Further work focussed on the improvement of reconstruction methods based on a consideration of electron interaction volume, simulated by a computer-based model. Reconstructed data showed alloy fissuring in three-dimensions to be intergranular and complex in nature. Banding was also observed in three dimensions, with the volume proportion of γ' quantified for both coarse and fine grain regions. A further three-dimensional reconstruction showed a laboratory-induced fatigue crack to have no bias towards the γ' or γ phase, but it did show transgranular cracking through a particularly large γ phase grain. Grain reference orientation deviation also allowed the observation of regions of highest alloy deformation around the crack path. The technique developed provided justification for the collection of combined EBSD and EDX data. Combined data allowed the distinction between γ' and γ phase, identification of coherent γ'/γ interfaces, confirmation of γ' connectivity and accurate crack path reconstruction, making the technique useful for a number of potential future applications.
Description: This thesis is confidential until 30th April 2017. A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.
URI: https://dspace.lboro.ac.uk/2134/9732
Appears in Collections:Closed Access (Aeronautical and Automotive Engineering)

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