Isothermal Oxidation Behavior of Dysprosium/S-Doped Β-Nial Alloys at 1200 °C

A beta-NiAl alloy with normal purity, a S-doped and a Dy and S co-doped b-NiAl alloys were prepared by arc-melting and their corresponding S contents were less than 20 x 10(-6), 33 x 10(-6) and 22 x 10(-6), respectively. The isothermal oxidation behavior of the alloys at 1200 degrees 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 theta- to alpha-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.