Influence of Oxidizing Atmosphere on the Oxidation of Ni-based Superalloy Rene 65

Abstract

The influence of oxidizing atmosphere on the degradation mechanisms of the Ni-based superalloy Rene 65 was studied in this work. Oxidation was carried out in synthetic air, argon and water vapor (Ar + 18 vol.%H₂O) between 700 and 900 °C, with samples built following additive manufacturing and forging processing routes. The results showed that the processing route and hence, the derived microstructure did not significantly affect the oxidation behavior. In contrast, the oxidizing atmosphere markedly modified the oxidation kinetics, the growth of the oxide layers and the overall oxidation mechanisms. The resulting thin oxide scales were made of NiCr₂O₄ and Cr₂O₃ at 700 °C, while at 800 and 900 °C the oxide layer was composed of an external Cr₂O₃ layer on top of an internal α-Al₂O₃ layer resulting from the lower partial pressure of oxygen underneath the chromia layer. The presence of nitrogen in the synthetic air favored the internal formation of TiN, while the absence of nitrogen in argon revealed the doping effect of Ti on the formation of the Cr₂O₃ layer. The effect of water vapor in Ar was not significant as the oxidation behavior was close to that observed under argon.