Early stages of the oxidation of a FeCrAlRE alloy (Kanthal AF) at 900 \ub0C: A detailed microstructural investigation

Early stages of the evolution of Al2O3 scales formed on a FeCrAlRE alloy (Kanthal AF) have been investigatedby analytical TEM. The samples were oxidized isothermally at 900 \ub0C in dry O2 or O2 + 40% H2O for1 h or 24 h. All oxide scales exhibited a two-layered structure, with a continuous inward growing a-Al2O3inner layer and an outward growing outer layer. After 1 h, the outer oxide layer consisted of c-Al2O3 inboth environments. After 24 h exposure in dry O2, the c-Al2O3 in the outer oxide layer was partly transformedto a-Al2O3 and spinel oxide (Mg1-xFexAl2O4). In contrast, the c-Al2O3 in the outer layer was nottransformed after 24 h in O2 + 40% H2O, showing that water vapour stabilizes c-Al2O3. All oxide scalescontained a Cr-rich band, a product of the initial oxidation. The inner a-Al2O3 layer is suggested to nucleateon Cr2O3 or Cr2-xFexO3 in the initial oxide