Investigations on the Cyclic Oxidation Behaviour of the Ni-Based Superalloy AD730™ at Elevated Temperature

Abstract

In this study, the cyclic oxidation behaviour of the polycrystalline Ni-based superalloy AD730™ was investigated. Cyclic oxidation tests were carried out in laboratory air at temperatures of 800 °C, 900 °C, 1000 °C and 1100 °C up to 500 cycles (1 h per cycle). The oxidation behaviour of AD730™ approximately followed the parabolic rate law, and the average activation energy is about 312 kJ/mol. At lower temperatures, a two-stage oxidation kinetics were observed, and the transition time decreased as the oxidation temperature increased. SEM observations indicated the formation of continuous but relatively irregular oxide layers with a surface nodular-type structure that thickened with increasing temperature. SEM–EDS analysis revealed that the oxide layers mainly consisted of a Cr₂O₃ scale covered by TiO₂. Aluminum was internally oxidized to form Al₂O₃ as elongated precipitates, which grew along grain boundaries via branch-like growth. Furthermore, TiO₂ and TiN precipitates were found within the internal oxidation zone. The internal oxidation depth increased with rising temperature.