Doyle, Joseph Constantine (2026) Study of fatigue crack propagation in single crystal nickel-based superalloys at an intermediate service temperature: effects of crystallographic orientation, oxidation and dwell times. University of Southampton, Doctoral Thesis, 236pp.
Abstract
Single crystal Ni-based superalloys have long been used as the material for turbine blades in jet engines due to their excellent corrosion and fatigue resistance, and superior mechanical properties at elevated temperatures. Complex cyclic loading conditions are experienced in turbine blades due to repeated start-ups and run downs and it has been found that the fatigue damage caused is particularly prominent in the cooler parts of the blade near the root which can experience intermediate temperatures between 450 °C and 650 °C. There are gaps in the understanding of fatigue crack propagation in single crystal superalloys at intermediate temperatures, particularly the contributing effects from crystallographic orientation, oxidation and dwell times. This thesis aims to investigate the fatigue behaviour of three different generations of single crystal superalloys, considering how the change of chemistry and microstructural features influences fatigue and oxidation performance.
The oxidation behaviours of three single crystal Ni-based superalloys (SRR-99 – 1st generation, CMSX-4 – 2nd generation, and CMSX-10 – 3rd generation) have been investigated and compared at intermediate temperature 550 °C. Isothermal oxidation tests were carried out at various exposure times ranging from 2 hours up to 1000 hours. The external and internal oxides were studied through high-resolution imaging, measurements from which revealed the oxidation kinetics of CMSX-4 and CMSX-10 followed a near-parabolic law whilst a logarithmic law better described the kinetics behaviour of SRR-99. Thermodynamic modelling, Thermo-Calc, was utilised to predict the species and composition of oxides formed in each alloy and compared to measured energy-dispersive X-ray spectroscopy and X-ray diffraction results. From the experimental results and modelling, it was found that the mechanism and oxide products formed for CMSX-4 and CMSX-10 are very similar, as NiO initially formed externally over the γ channels and the γ’ precipitates were preferentially oxidised internally at the γ/γ’ interface.
The effect of secondary orientation on the fatigue crack propagation behaviour at 550 °C for CMSX-4 and CMSX-10 has been investigated. Testing was performed on single edge-notched bend bars under constant load with the tensile axis aligned with the <001> growth direction. Two orientations were tested, orientation A, with crack growth in the <110> direction, and orientation B, with crack growth in the <010> direction. It was found that orientation B samples promoted stage I crystallographic crack along extended {111} slip bands, making the fracture surface highly faceted and deflected. Whereas, for orientation A the fracture surface showed stage II crack growth. The effect of the extended slip band cracking was larger for CMSX-4, as the faceted and deflected crack paths reduced the fatigue crack propagation rate compared to orientation A. Due to the lower oxidation resistance of CMSX-10, even in the orientation B test stage II cracking was found as oxidation homogenised slip delaying stage I crack growth. This meant there was little difference between the crack propagation rates between the two orientations for CMSX-10.
Dwell tests were performed on CMSX-10 and SRR-99 at 550 °C in both air and inert gas (low oxygen environment). It was found that in air, a dwell of 90s was long enough to induce time dependent crack growth in CMSX-10 but not in SRR-99. In addition, dwell fatigue promoted the formation of secondary cracks on the fracture surface. After repeating the dwell test in an inert gas, the crack propagation rate was significantly reduced in both alloys. Through X-ray computed tomography and examination of the sectioned fracture surface, secondary cracks were found to grow much deeper in the inert gas even to the point of becoming the dominant crack. This behaviour contributed to the slower crack propagation rate as the effected stress intensity factor is reduced by the forking of a crack. Based on these observations, it was determined that a combination of mechanisms were contributing to the crack growth under dwell. Embrittlement of the volume ahead of the crack tip driving crack growth rate, strain assisted diffusion along {111} slip bands promoting the formation of secondary cracks as well as oxidation-induced crack closure arresting cracks. Depending on the composition and oxidation resistance of the alloy, the synergy of these mechanisms will change affecting the overall crack growth rate and behaviour at these moderate service temperatures.
More information
Identifiers
Catalogue record
Export record
Altmetrics
Contributors
Download statistics
Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.