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

Abstract

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 Cr0.12Ti0.78O1.74–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.