High-temperature oxidation of Fe3Al-based iron aluminides in oxygen

The isothermal high-temperature oxidation behavior of Fe3Al-based iron aluminides in oxygen has been studied. Fe-25Al was oxidized at 1225, 1330, 1425 and 1530 K, while Fe-28Al, Fe-24Al-5Cr, Fe-24Al-5Ti, Fe-28Al-2Cr and Fe-30Al-4Cr (all compositions in atom percent) were oxidized at 1330 K. The weight gain data were analyzed and rate constants (k(p)) determined by assuming a parabolic rate law. The variations of instantaneous parabolic rate constant with time reflected the complexity of the oxidation behavior. These have been attributed to the changes taking place in the nature and properties of the scale as a function of time. The values of k(p) for oxidation of Fe3Al were one to two orders of magnitude lower than those for Ti3Al-based intermetallics. As revealed by X-ray diffraction, the scale formed on Fe-25Al was predominantly alpha -Al2O3 at higher temperatures, while theta -Al2O3 was observed after oxidation at lower temperatures. The observed kinetics matched with alpha -Al2O3-formation kinetics at higher temperatures and theta -Al2O3-formation kinetics at lower temperatures. For all the other intermetallics, only alpha -Al2O3 was identified at 1330 K. The whisker morphology of theta -Al2O3 and the ridged morphology of alpha -Al2O3 were confirmed by scanning electron microscopy. Alloying with Cr or Ti increased the oxidation rate of iron aluminides, especially during the initial stages. Addition of Ti changed the nature, color, and morphology of the scale, leading to improved adherence. (C) 2001 Elsevier Science Ltd. All rights reserved.