OXIDATION OF FE-CR-AL AND FE-CR-AL-Y SINGLE-CRYSTALS

Single crystal samples of the alloy Fe-20%Cr-5%Al with and without Y-d oping were used to study the ''reactive element'' (RE) effect, which c auses improved oxidation behaviour and formation of a protective Al2O3 layer on this alloy. The oxidation was followed by AES at 10(-7) mbar O-2 up to about 1000 degrees C. Most observations were peculiar for t his low pO(2) environment, but yttrium clearly favors the formation of Al-oxide and stabilizes it also under these conditions, probably by f avoring its nucleation. The oxides formed are surface compounds of abo ut monolayer thickness, not clearly related to bulk oxides. Furthermor e, the morphologies of oxide scales were investigated by SEM, after ox idation at 1000 degrees C for 100 h at 133 mbar O-2. On Fe-Cr-Al the s cale is strongly convoluted and tends to spalling, whereas the presenc e of Y leads to flat scales which are well adherent. This difference i s explained by a change in growth mechanism. The tendency for separati on of oxide and metal was highest for the samples with low energy meta l surface, i.e. (100) and (110), the scale was better adherent on the (111) oriented surface and on the polycrystalline specimen, since in t he latter cases the overall energy for scale/metal separation is highe r. All observations, from the low and from the high pO(2) experiments, are discussed in relation to the approximately ten mechanisms propose d in the literature for explanation of the RE effects.