This research is concerned with a single crystal nickel-base superalloy which has been developed for application as a high pressure turbine blade material in jet aircraft engines. The microstructures and mechanical properties of superalloys, including the effects of heat-treatments, have been reviewed. The effects of heat-treatments on the γ' precipitate distributions have been investigated. During ageing at 900°C or 800°C, the precipitates adopt an irregular, rounded and highly interconnected microstructure, indicative of precipitate coalescence, whilst at higher ageing temperatures a regular cuboidal precipitate morphology is formed. The kinetics of precipitate coarsening have been investigated, and slight deviations from the power-law predicted by a number of theoretical models were observed. These deviations have been discussed in terms of a progressive transition in the dominant coarsening mechanism. Constant load creep tests were carried out, and although the tensile axis was nominally parallel to [001], the degree and direction of misorientation were found to be critical to the extent of the primary creep strain. Primary creep was shown to proceed by slip on a single (111)[112] system, until the activation of intersecting slip systems brings about the onset of the secondary creep stage. The extent of primary creep has been shown to be reduced by application of a final ageing treatment at 870°C. Precipitate shear by paired dislocations in intense slip bands occurs during high strain-rate deformation at both ambient temperature and at 750°C. Application of a final ageing treatment at 870°C was found to increase the 0.2% proof stress and to bring about the activation of an alternative mode of precipitate shear by dissociated dislocations. The 870°C ageing treatment was shown to cause slight chemical changes at the γ/γ' interfaces, and these are believed to have caused the observed changes in mechanical properties.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:351442 |
| Date | January 1984 |
| Creators | Hopgood, Adrian A. |
| Contributors | Martin, John Wilson |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:71780c17-f658-40b6-9400-4ec40f5a7381 |
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