The influence of CrTaO4 layer on the oxidation behavior of a directionally-solidified nickel-based superalloy at 850-900 °C

The influence of CrTaO4 on the oxidation behavior of a directionally solidified nickel-based alloy has been investigated at the temperatures between 850 degrees C and 900 degrees C for the different time intervals ( the longest time is 300 h). The oxide scale consists of three layers. The outer layer is mainly composed of Cr2O3, TiO2, and NiCr2O4, the middle layer is CrTaO4, and the inner oxide layer mainly consists of Al2O3. The continuous CrTaO4 layer is formed after oxidation for certain times. The formation time of continuous CrTaO4 layer shortens with the temperature increase. Furthermore, the layer causes the transformation of the oxidation kinetics from the parabolic law to the cubic law. The control of oxidation process converts from the outward diffusion of Cr, Ti, and Ni to the inward diffusion of O when the continuous CrTaO4 layer is established. The mechanism clarifies the influence of Ta on the oxidation behavior of Ni-based superalloys from the oxidation kinetics. (C) 2017 Elsevier B.V. All rights reserved.