MICROSTRUCTURAL CHARACTERIZATION AND ADHERENCE OF ALPHA-AL2O3 OXIDE SCALES ON FE-CR-AL AND FE-CR-AL-Y ALLOYS

Several features of the microstructure and the adherence of alumina scales formed on Fe-Cr-Al and Fe-Cr-Al-Y single- and polycrystalline alloys after oxidation at 1000-degrees-C were examined. The convolutions of the scale and especially of the scale/alloy interface are thought to be the major reason of poor spallation resistance of scales on the yttrium-free alloy. The flat oxide scale on the even interface of the yttrium-doped alloy, on the contrary, exhibits excellent adherence upon cooling. Interfacial cavities observed on the Fe-Cr-Al alloy result from the scale undulation under compressive growth stresses. The shape and the number of cavities depend on the initial surface orientation and most probably reflect a balance between the interfacial energy of convoluted substrate in contact with the oxide layer and the energy of separated surfaces.