TEM ANALYSIS OF THE GROWTH OF OXIDE SCALES AT DIFFERENT TEMPERATURES IN FeAl GRADE 3 INTERMETALLIC ALLOY

Upon the isothermal oxidation of an ODS FeAl Grade 3 intermetallic alloy, a structured oxide scale is developed between 800 and 950o C. TEM studies have revealed a three-layered structure with a top nanoequiaxed alumina, a central alumina and a bottom Fe-Al spinel. At these temperatures, aluminium outward diffusion does not seem to be suppressed neither by the spinel sub-layer nor by the yttria present in the superalloy. However, the formation of metastable θ-Al2O3 seems to be significantly hindered. On the contrary, at 1000o C, the spinel phase no longer forms but a columnar alumina layer topped up with a nanoequiaxed structure. Yttria seems then to segregate at the scale/alloy interface and to randomly coarsen to produce an Y-Al oxide phase.