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.