High-temperature oxidation behavior of ODS-Fe₃Al

The high-temperature oxidation behavior of an oxide dispersion-strengthened (ODS) Fe₃Al alloy has been studied during isothermal and cyclic exposures in oxygen and air over the temperature range 1000 to 1300°C. Compared to commercially available ODS-FeCrAl alloys, it exhibited very similar short-term rates of oxidation at 1000 and 1100°C, but at higher temperatures the oxidation rate increased because of increased scale spallation. Over the entire temperature range, the oxide scale formed was α-Al₂O₃, with the morphological features typical of reactive-element doping and was similar to those formed on the ODS-FeCrAl alloys. Although initially this scale appeared to be extremely adherent to the Fe₃Al substrate, an undulating metal-oxide interface formed with increasing time and temperature, which led to cracking of the scale in the vicinity of surface undulations accompanied by a loss of small fragments of the full-scale thickness. In some instances, the surface undulations appeared to have resulted from gross outward local extrusion of the alloy substrate. Similar features developd on the FeCrAl alloys, but they were typically much smaller after a given oxidation exposure. The ODS-Fe₃Al alloy has a significantly larger coefficient of thermal expansion (CTE) than typical FeCrAl alloys (approximately 1.5 times at 900°C) and this appears to be the major reason for the greater tendency for scale spallation. The stress generated by the CTE mismatch was apparently sufficient to lead to buckling and limited loss of scale at temperatures up to 1100°C, with an increasing amount of substrate deformation at 1200°C and above. This deformation led to increased scale spallation by producing an out-of-plane stress distribution, resulting in cracking or shearing of the oxide.