Abstract:

A β-NiAl alloy with normal purity, a S-doped and a Dy and S co-doped β-NiAl alloys were prepared by arc-melting and their corresponding S contents were less than 20 × 10⁻⁶, 33 × 10⁻⁶ and 22 × 10⁻⁶, respectively. The isothermal oxidation behavior of the alloys at 1200 °C was investigated and the extent of S segregation at the scale–alloy interface was determined by scanning Auger microscopy. S-doping had no significant effect on the phase transformation rate from θ- to α-Al₂O₃, while the addition of Dy retarded this process. For the S-doped alloy, scale rumpling occurred only after 2 h thermal exposure and numerous large voids were observed at the scale–alloy interface where S segregated. In contrast to this, the oxide scale formed on the Dy and S co-doped alloy still remained flat even after 50 h isothermal oxidation and only small voids existed at the interface where S segregation was not detected.

Key words: NiAl, Dy, S, Isothermal oxidation, Phase transformation, Segregation