Oxidation behavior of a high Hf nickel-based superalloy in air at 900, 1000 and 1100°C

In order to investigate the oxidation behavior of a nickel-based superalloy containing high hafnium content (1.34 wt%), isothermal oxidation tests were performed at 900, 1000 and 1100°C for up to 200 h. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) were applied to study the oxidation behavior. Weight gain of the experimental superalloy exhibits a parabola-like curve and no spallation of the oxide scale was observed in the oxidation tests. The alloy presents excellent oxidation resistance and no HfO₂ is observed in the oxide scale at 900°C. Elevating the oxidation temperature up to 1000°C, HfO₂ particles form in the spinel phases of the scale, and “pegs” HfO₂ is observed within and beneath the inner layer of Al₂O₃ after 200 hours. As the oxidation temperature elevated to 1100°C, “pegs” HfO₂ is observed at the early stage of oxidation test (within 25 hours). Formation mechanism of HfO₂ and the impact on oxidation resistance are investigated based on the analysis of oxidation tests results at different temperatures.