Evaluation of oxidation related damage caused to a gas turbine disc alloy between 700 and 800°C

Abstract

This paper presents the results of a study targeted at characterising the oxidation behaviour of a new nickel based disc alloy (RR1000) at intermediate temperatures. Isothermal exposures were carried out using a thermo-microbalance at temperatures in the range 700 – 800°C for exposures up to 200 hours. Cyclic oxidation exposures were carried out at, 700 and 750°C for up to 1000 hours with 100 hour cycles, using mass change to monitor the materials performance. The mass gain data obtained have been used to derive oxidation reaction rate parameters, using established methodologies, with parabolic rate constants varying between 1.4×10–5 mg2/cm4/h at 700°C and 8.4×10–4 mg2/cm4/h at 800°C.Surface oxides were initially analysed using scanning electron microscope/energy dispersive X-ray analysis (SEM/EDX) and X-ray diffraction (XRD) techniques. The results showed that slowly-growing chromium-rich oxide scales had formed on the surface of the samples during these exposures. A more detailed study of cross-sections through the oxide layers and underlying alloy was undertaken using a focused ion beam (FIB) system. Measurements of the thin oxides observed showed ranges of thicknesses from 0.08μm up to 1.9μm, that were consistent with the mass change data gathered. However, the FIB examinations also revealed significant sub-surface damage that contained a mixture of grain boundary pores, a second phase depletion zone, and grain recrystallisation. The depth of these sub-surface damage zones was greater than the thickness of the oxide layers and is believed to have a major impact on the fatigue performance of this disc alloy. The use of the FIB system has enabled characterisation of the development of both the oxide layers and sub-surface damage zones as a function of exposure time and temperature.