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dc.contributor.authorMcGregor, Meganen
dc.description.abstractIn modern gas turbine engines, leakage of gas from the flow path reduces the efficiency of the engine. This leakage is currently ameliorated in the high-pressure turbine by using shrouded turbine blades, which give a high sealing efficiency. However, the shroud is manufactured from the same superalloy as the blade, which has a high density of 8.70 g cm⁻³ and hence limits the creep life of the turbine blade. Shroudless blades can be adopted to combat the issue of excessive creep. To achieve a suitable seal with a shroudless blade, the blade can be tipped with an abrasive system that cuts into an abradable liner on the static counter face. This should allow low clearance to be maintained during service. Currently, such systems for high-temperature use are composed of cubic boron nitride (cBN) abrasive particles embedded in a NiCoCrAlY anchor phase. Previous work has shown that at the operating temperatures achieved in the high-pressure turbine, the NiCoCrAlY is unable to retain the embedded cBN particles under abrasion. A new anchor phase system based on the intermetallic NiAl, strengthened with a reinforcing Laves phase, NiTaAl, has been identified as a possible replacement material with sufficient strength at high temperatures. In addition to high strength at the required temperatures, anchor phase materials must have sufficient oxidation resistance to maintain seals for the operational lifetime of the engine. Compatibility between the anchor phase and the underlying CMSX-4 turbine blade is also essential, as interdiffusion occurs over the lifetime of the blade. These two requirements, and preliminary wear behaviour, were investigated for the Ni-Al-Ta ternary system. The isothermal oxidation behaviour of a series of Ni-Al-Ta alloys with varying tantalum content was studied at 1100 °C using static oxidation testing and thermogravimetric analysis. A layer of Al₂O₃ is found to form on samples containing up to 20 at.% tantalum with parabolic oxidation behaviour observed for these alloys. At higher tantalum contents, parabolic oxidation behaviour is initially observed, which then transitions into a rapidly increasing rate of oxidation. Ni-Al-Ta/CMSX-4 diffusion couples were investigated experimentally and computationally. After heat treatment at 1100 °C, the NiTaAl Laves phase in the anchor phase had transformed to Ni₂TaAl Heusler phase, and topologically close-packed phase precipitation was observed in the CMSX-4. A CALPHAD-based model, using Thermo-Calc and DICTRA, developed for this system was able to predict the concentration profiles across the diffusion couple at 1000 °C. However, the model predictions were poor at higher temperatures due to the limited diffusion data available for the intermetallic phases. The wear behaviour of a Ni-Al-Ta alloy was studied up to 1100 °C using a pin-on-disc abrasion rig. The Laves phase showed a layer of plastic deformation at the highest test temperatures. Preliminary attempts to deposit cBN in the new anchor phase material indicated that some cBN may be retained after abrasion. Careful consideration must be given to any future manufacturing method for the coating, to maximise cBN retention and minimise reaction with the material.en
dc.rightsAll rights reserveden
dc.rightsAll rights reserveden
dc.rightsAll rights reserveden
dc.subjectaviation materialsen
dc.subjectabrasive coatingen
dc.titleNi-Al-Ta alloys for high temperature abrasive coatingsen
dc.type.qualificationnameDoctor of Philosophy (PhD)en
dc.publisher.institutionUniversity of Cambridgeen
dc.publisher.departmentDepartment of Materials Science and Metallurgyen
dc.contributor.orcidMcGregor, Megan [0000-0001-7076-8929]
dc.publisher.collegeDepartment of Materials Science and Metallurgy
dc.type.qualificationtitlePhD in High Performance Structural Metallicsen
pubs.funder-project-idEPSRC (1642234)
pubs.funder-project-idEPSRC (1642234)
cam.supervisorClegg, William

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