Connolley, T., Reed, P.A.S. and Starink, M.J.,
A study of the role of (Nb,Ti)C carbides in fatigue crack initiation in IN718 at 20oC and 600oC
Strang, A, Banks, W.M., Conroy, R.D., McColvin, G.M., Neal, J.C. and Simpson, S. (eds.)
In Parsons 2000 Advanced Materials for 21st Century Turbines and Power Plant: Proceedings of the Fifth International Charles Parsons Turbine Conference, Churchill Collge, UK, July 2000.
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It is generally accepted that primary carbides can act as fatigue crack initiation sites in nickel-based superalloys. Oxidation of primary carbides is suggested as one cause of fatigue crack initiation at elevated temperatures, though little has been published about the precise mechanism by which this occurs. This paper presents the results of preliminary studies on high temperature fatigue crack initiation in Inconel 718, a material used extensively for components in gas turbines. Comparisons are made between material taken from an extruded bar and a turbine disc forging.
Fatigue tests were performed in air at 600oC on U-notch specimens, designed to simulate the stress concentration factor of a blade root fixing. Fatigue lives using 1-1-1-1 and 1-30-1-1 trapezoidal waveforms are compared. Differences in lifetimes between the extruded bar and disc material are discussed with reference to their different microstructures. Post-fracture analysis using optical and scanning electron microscopy indicates that swelling and eruption of oxidising (Nb,Ti)C carbides is a significant cause of fatigue crack initiation.
To further investigate the role of erupted carbides in crack initiation, some tests were performed at room temperature, using a 10 Hz sinusoidal waveform. Characterisation of surface crack initiation behaviour in plain, polished bend bars is presented, comparing the as-polished condition with specimens thermally exposed for 4 hours at 600oC to induce carbide oxidation.
The results of the high temperature and room temperature studies are discussed, concentrating on possible mechanisms of fatigue crack initiation caused by the oxidation of primary carbides.
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