Oxidation Behavior of Ni3Al and Fe3Al: II. Early Stage of Oxide Growth

Oxidation treatments of Ni3Al and Fe3Al were performed at room temperature in 0.2 atm O2 for 5 min and at 300 and 500°C in air for 5 min, and 6, 50, 100, and 200 hr. The oxides were analyzed by XPS, AES, and SEM. A model explaining the initial stages of oxide formation is suggested. At room temperature and 300°C, islands of Al2O3 and NiO combined with NiAl2O4 formed on Ni3Al. At 500°C, the Ni oxides grow laterally and cover the Al2O3 islands. Islands of Al2O3 and Fe2O3 mixed with Fe(Fe, Al)2O4 formed on Fe3Al at room temperature. At 300 and 500°C the scale is composed of an outer layer rich in Fe oxides and an inner layer rich in Al oxides. During long time exposure, islands of Fe2O3 and Fe(Fe, Al)2O4 formed at the surface by diffusion of Fe cations through the alumina layer. The oxide growth on Fe3Al reaches a steady-state regime after formation of the continuous alumina layer. At 300°C, the oxide formed on Fe3Al is thicker than on Ni3Al, whereas it is reverse at 500°C.