Effects of Hf, Y, and Zr on Alumina Scale Growth on NiAlCr and NiAlPt Alloys

The effects of Hf, Y, and Zr additions on the growth of a thermally grown alumina scale formed on NiAlCr and NiAlPt alloys were investigated. Isothermal and thermal cycling oxidation experiments were carried out at 1150 °C, and cross sections of the oxidized samples were characterized using scanning electron microscopy. It was observed that single doping as well as co-doping of Hf, Y, and Zr reduces the rate of alumina scale growth on NiAlCr and NiAlPt alloys, with Hf showing a more significant effect than Y or Zr. The following possible contributing factors to these observations were assessed: (1) the ionic size of the minor alloying elements and (2) the bond strength between the doping elements and oxygen in the oxide grain boundaries in terms of formation and melting enthalpies of oxides. It was concluded that the bond strength between the doping elements and oxygen within the oxide grain boundaries plays an important role in retarding the alumina scale growth.