TEM OBSERVATION OF AL2O3 SCALES FORMED ON AL-DEPOSITED FE-CR-AL ALLOYFOIL

The development and phase transformations of Al2O3 scales formed on Al -deposited Fe-Cr-Al foil and Fe-20Cr-5Al foil have been studied during the oxidation at 1173K in air using TEM, SEM and XRD. The both kinds of foils bad very similar chemical compositions, however the oxidation started on surface of high Al content for the Al-deposited foil. The scale formed on it has a two-layer structure during the initial oxidat ion period : the outer layer consists of gamma-Al2O3 and the inner lay er of mainly line-grain theta-Al2O3. The phase transformation from gam ma to theta takes place at the scale / substrate interface. As oxidati on proceeds needle-like crystals start to grow at the interface of the two layers, pass through the outer layer, and extended out from the s cale surface. They consist oi twin crystals of gamma-Al2O3, some of wh ich transform to theta-Al2O3, with twin boundaries parallel to their g rowth directions. It is proposed that the twin boundaries provide fast diffusion paths for Al cations. During the growth of needle-like crys tals, the inner layer consists of fine-grain theta-Al2O3. After about 180ks oxidation alpha-Al2O3 grains nucleate at the scale / substrate i nterface and after about 360ks the major part of the scale transform t o alpha-Al2O3. Its grain size is 1 to 2 mu m and thus one or two grain s occupy the scale thickness. Small voids are formed within alpha-Al2O 3 grains. This is attributable to volume decrease by about 12% accompa nying the theta to alpha transformation. Contrary to the Al-deposited foil, scales formed on the Fe-20Cr-5Al foil transform to alpha-Al2O3 i n short periods.