Short-Term Oxidation in HT-SEM of the Pt-Containing TROPEA Single Crystal Ni-Based Superalloy from 680 to 1000 °C

Abstract

Short-time oxidation exposures of the Ni-based TROPEA single crystal superalloy was implemented to determine the nature and quantities of transient oxides in the 680–1000 °C temperature range. Experiments were carried out in situ in the SEM with reduced air pressure (150 Pa, \({P}_{{O}_{2}}\) ~ 31.5 Pa) compared to atmospheric conditions (10⁵ Pa, \({P}_{{O}_{2}}\) ~ 2.1 10⁴ Pa). TEM characterization after oxidation showed the complexity of the oxidation products developed. Aluminum underwent internal oxidation between 680 and 1000 °C. During the limited duration of oxidation, the TROPEA alloy only formed a continuous alumina layer at 1000 °C. At 680 and 850 °C, the low diffusion rate and small amount of Al in the Ni-based single crystal led to the formation of a significant amount of transient oxides such as (Ni,Co)O, compared to the desired chromia or alumina protective oxides. The lower the temperature, the smaller the size of the internal Al₂O₃ precipitates and the larger the transient oxide amount, which would lower the resistance of TROPEA to Type II hot corrosion. In contrast after a transient period shorter than 22 h, during which multiple transient oxide developed, the oxidation resistance would be ensured at 1000 °C by the formation of a continuous Al₂O₃ scale.