The formation of discontinuous Al₂O₃ layers during high temperature oxidation of IrAl alloys

Oxidation of IrAl alloys (50-50 at.%) was conducted at 1300 and 1600°C in air for different times. Due to the outward diffusion of Al atoms and subsequent oxidation, the formation of Al₂O₃ is accompanied with the generation of an Ir-rich phase in the vicinity areas. The oxidation zone is separated from the bulk IrAl alloy by a phase boundary. In the oxidation zones, alternating layers of Al₂O₃ and Ir-rich phase are produced at 1300°C, while irregularly intertwined; Al₂O₃ and Ir-rich phase are produced at 1600 °C. The discontinuous formation of Al₂O₃ gives rise to internal oxidation of the IrAl alloys. The thickness of the Al₂O₃ layer is in the range of tens of micrometer, and it grows thicker as its location gets farther away from the free surface. The formation of an Al₂O₃/Ir-rich phase layered structure is discussed based on the diffusion kinetics of O, Al, and Ir, the chemical reactivities of Al and Ir with respect to O, and the nucleation and growth kinetics of Al₂O₃.