TEM observation of phase transformations of alumina scales formed on Al-deposited Fe-Cr-Al foils

The phase transformations and structural changes of Al2O3 scales formed on an Al-deposited Fe-19Cr-3.2Al foil (totally 4.8 %Al, Al-deposited foil) during oxidation in air were studied mainly at 1373 K. A conventional Fe-20Cr-5Al foil (alloy foil) was similarly studied for comparison. The two kinds of foils had a thickness of 50 mum and similar amounts of rare earth elements. Extensive study using X-ray diffractometry, SEM and TEM revealed that scales formed on the Al-deposited foils transform from gamma- to theta- to alpha -Al2O3 with definite stages in a TTT type diagram. As the oxidation temperature rises, each transformation completes in a shorter period. Conversely, even 10 s oxidation resulted in scales consisting mainly of alpha -Al2O3 on the alloy foil. The Al-deposited foil oxidises faster than the alloy foil at 1373 K during the initial period of about 9 ks, after which this relationship is inverted. These features are attributable to the fact that during the initial stage scales on the Al-deposited foils consist mainly of theta -Al2O3, while those on the alloy foils are alpha -Al2O3, and during the later stage equiaxial and columnar grains composing scales are larger for the Al-deposited foils.