A study on the Effect of Hf Addition on the Oxidation Behavior of Fe-Cr-Al Alloys

Fe-Cr-Al and Fe-Cr-Al-Hf alloys prepared by arc melting were isothermally and cyclically exposed to air at 900 ∼ 1100 .deg. C. On the surface of the Hf-free and the Hf-added specimens exposed at 900 .deg. C, only an external scale mainly composed of Al2O3 was observed to form, while on the surface of Hf-added specimens exposed at 1000 .deg. C and 1100 .deg. C, Hf-rich internal oxides below the thin external Al2O3 scale formed. As a result of cyclic oxidation, Hf-added specimens showed much more protective behavior than Hf-free ones oxidized at all temperatures. Hf-free specimens failed after cyclic oxidation in about 150 ∼ 200 hours, but Hf-added ones didn't up to 700 hours. All the specimens failed at the cyclic tests showed numbers of polygonal-shaped aluminum nitrides formed below the external aluminum oxide layer, resulting in the consumption of protective oxide-forming component, Al