Oxidation-fatigue mechanisms at moderate service temperatures in single crystal turbine blade materials

• Main Author: Evangelou, Angelos
• Other Authors: Reed, Philippa
• Published: University of Southampton 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.741673

The shortage of fossil fuels and the emergence of renewable energy technologies have increased the demand for a more variable and efficient power output from conventional gas turbine units. Single crystal Ni-based superalloys have long been the materials of choice for high temperature, gas turbine blade, applications due to their excellent fatigue, creep and oxidation resistance. However, the increased number of start-ups and shut downs, produce complex, unpredictable, cyclic loadings even at moderate temperatures, where the fatigue - oxidation behaviour of such materials is less well understood. The gas turbine industry has significant economic incentives to optimise maintenance scheduling and determine the useful lifetime of such components and thus a substantial effort is put in the development of damage tolerant, life assessment methods. This thesis aims to elucidate the mechanisms of oxidation - fatigue damage at moderate service temperatures, in single crystal, Ni-based superalloy turbine blade materials, in order to provide the basis for a physics based lifing model accounting for fatigue oxidation interactions. The oxidation behaviour of two commercially available single crystal nickel based superalloys (in CMSX-4 and MD-2) has been investigated at the lower operating temperature range (450-550°C) of an industrial gas turbine blade. Isothermal oxidation was carried out for varying times up to 640h and it was found that exposure resulted in a sub-micron thick oxide. The external and internal oxide kinetics were studied via high resolution image analysis and both showed sub-parabolic growth rates. Thermogravimetric tests indicated that the overall oxidation growth obeys a near quartic power law while parabolic kinetics can describe the transient oxidation period. Characterisation of the resulting oxides was carried out using field emission gun scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Results from thermodynamic modelling (Thermo-Calc) of the oxide formation are also presented. Low temperature oxidation in these Ni-based superalloys begins with the formation of porous NiO protrusions over the γ matrix. As the oxygen partial pressure drops in the substrate, a transition alumina forms near the surface within the γ’. The oxygen anions that diffuse into the alloy, preferentially through the γ/γ’ interface and come in contact with the internal Al-rich γ’ phase. This leads to the preferential oxidation of the γ’ particles internally while the γ matrix remains relatively unaffected. The notch fatigue initiation process has been studied at 450-550oC in CMSX-4 in both air and vacuum (low oxygen partial pressure) environments, to assess the effect of oxidation. Detailed, electron microscopy, fractography indicated that crack initiation in an oxidising environment at 450°C and 550°C, is dominated by subsurface porosity while initiation at lower temperatures and low oxygen partial pressure environments result in crystallographic cracking promoted by surface defects. At these temperatures, oxidation acted as a retardation mechanism to surface initiation processes by plugging porosity and interfering with the resulting strain levels. Porosity not only controlled initiation but also produced significant scatter in the fatigue lives obtained thus constituting an inherent feature of service conditions and should be considered in lifing approaches. Dwell-fatigue testing was conducted on CMSX-4 samples at 450°C and 550°C in air and vacuum (low oxygen partial pressure). The effects of frequency (dwell) on the fatigue crack growth behaviour were studied using a constant stress intensity range test and blocks of alternating frequencies. It was found that at 550oC, a dwell time of 20s or higher (< 0.04Hz) promotes a mixed time/cycle dependent crack growth rate. In order to further investigate the effect of dwell on the crack tip damage, pre-cracked samples were held under sustained loads for 12h. The resulting crack tips were examined using transmission electron microscopy and the resulting oxides using high resolution energy dispersive spectroscopy. Cracks forming under long dwell fatigue conditions had a complex morphology and formed several sub-branches that resulted in rougher fracture surfaces. During dwell fatigue crack propagation at intermediate temperatures, several competing mechanisms contribute synergistically to damage. In addition, the effects of oxidation were found to be two-fold. Strain assisted oxygen diffusion at small distances ahead of the crack tip can promote fracture at the γ/γ’ interface while oxide formation on the crack tip surfaces can bridge the crack tip opening and reduce the effective driving force.