Fe-5 mass%Al合金の1073 Kにおける内部酸化・窒化挙動

抄録

The internal oxidation and nitridation behavior of an Fe-5 mass%Al alloy was investigated at 1073 K in oxygen, nitrogen-20 vol%oxygen(mix), and air atmospheres for up to 57.6 ks using thermo-gravimetry, scanning electron microscope, electron probe micro analyzer, and X-ray diffraction analysis. The time dependence of the amount of oxidation showed very high oxidation rate at the initial stage, a transient stage with a rapid rate decrease, and then steady state oxidation. The critical times between the transient and steady states were 900 s in air, and 2 ks in both the mix and in O₂. In the “steady state” condition, the oxidation rate in O₂ was very slow, while it was relatively faster in the mix and in air. In O₂, both a thin duplex (Fe₂O₃ and Al₂O₃) and a thick triplex (Fe₂O₃, Fe₃O₄, and FeAl₂O₄) scales formed and the film-like internal oxides grew beneath the thick triplex scale. In the mix and in air a thick scale formed with a triplex structure of Fe₂O₃, Fe₃O₄, and FeAl₂O₄ accompanied by an internal oxidation of duplex FeAl₂O₄ and Al₂O₃ zones, which grew perpendicularly to an alloy/scale interface, and spike-like AlN precipitates formed beneath the internal oxidation zone. Further, film-like Al₂O₃ formed locally at the top of internal AlN precipitates. A precipitate free zone (PFZ) formed between the internal oxidation and nitridation zone, and a formation mechanism for the PFZ was proposed.