Oxidation behavior of Ni-based superalloy GH738 in static air between 800 and 1000 °C

The oxidation behavior of a nickel-based superalloy GH738 was studied by isothermal oxidation tests in still air at different temperatures, with exposure time up to 100 h. Oxidation-kinetic curves were plotted using the mass gain method. The surface and cross-sectional morphologies were observed by scanning electron microscopy (SEM). A composition analysis and an oxidation-product identification were conducted using energy-dispersive X-ray spectroscopy (EDS) and an X-ray diffraction (XRD), respectively. The results showed that GH738 exhibited parabolic oxidation-kinetic curves, with stable parabolic-rate constants at each temperatures. The activation energy of oxide growth was calculated to be 329.6 kJ·mol⁻¹. Cr₂O₃ (chromia) was the external oxidation product at 800 °C. A TiO₂–Cr₂O₃ double-layer structure was formed at 900 °C. The position of TiO₂ changed from the oxide–metal interface to the air–oxide interface by the diffusion of Ti atoms in chromia during the oxidation time at this temperature. Spallation was observed in the Cr_0.12Ti_0.78O_1.74–Cr₂O₃ multi-layer oxide of at 1000 °C, which increased the oxidation rate. For all the tests, the main internal oxide was always Al₂O₃. The entire GH738 oxidation process was interpreted by the competitive diffusion of elements (Cr, Ti, Ni, etc.) in metal matrix and chromia, while a schematic diagram of oxidation process was proposed.