Abnormal high growth rates of metastable aluminas on FeCrAl alloys

Experimental evidence in high temperature oxidation of alumina-forming alloys has accumulated that the overall growth kinetics of the oxide scale are slower for 1000 degrees C, where the stable alpha-Al2O3 phase predominates, than for 900 degrees C where metastable gamma-Al2O3 and/or theta-Al2O3 polymorphs predominate. This intriguing behaviour has been unanimously related to the substantial presence of twin boundaries and the cation vacancy network intrinsic to the metastable aluminas allowing faster diffusion than in the nearly close packed corundum structure. This paper shows that this abnormal growth rate accompanying the presence of stable alumina polymorphs in platelets or needle-like morphology is rather due to the formation of a corundum-alumina-rich compact layer from an outer metastable layer by the concomitant sintering at the intersection vertices of the platelets and secondary recrystallization in these platelets. These phenomena are illustrated from oxidation tests performed on thin FeCrAl foils in both a conventional muffle furnace (designed by AET) and thermogravimetric analysis furnace (TGA) over the temperature range of 800-1300 degrees C using field emission scanning electron microscope (FEG-SEM), transmission electron microscope (TEM), electron probe microanalysis (EPMA), atomic force microscope (AFM), grazing incidence X-ray diffraction (GIXRD) and image analysis (IA) techniques.