Optimizing the imperfect oxidation performance of iron aluminides

Iron aluminides (Fe₃Al-based) are attractive corrosion-resistant, high temperature materials due to their excellent sulfidation resistance and large Al reservoir. However, at high temperatures (>1000°C) iron aluminides show excessive scale spallation compared to doped FeCrAl. This behavior is attributed to the inherently high coefficient of thermal expansion of these materials which results in higher cooling strains in the alumina scale. Although this problem cannot be corrected by minor alloying additions or impurity control, composition optimization can still be used to maximize performance. A multi-year effort has examined various reactive element additions and shown that optimized Hf doping (0.05 at.% Hf) provides the best observed oxidation performance at 1100-1200°C. Hafnium lowers the scale growth rate by an order of magnitude compared to undoped Fe₃Al, but does not improve the resistance to scale spallation. Possible mechanisms for the role of Hf on the scale growth rate are discussed.