Effect of Mo dispersion size and water vapor on oxidation of two-phase directionally solidified NiAl–9Mo in-situ composites

Oxidation of NiAl–9Mo eutectics with three different second-phase Mo dispersion sizes was investigated at 900 °C in dry and wet air. Good oxidation resistance via alumina formation was observed in dry air, with MoO3 volatilization minimized by submicron Mo dispersions. However, although mass change measurements were similar in dry/wet air, extensive volatilization and in-place internal oxidation of prior Mo phase regions was observed in wet air. The ramifications of this phenomenon for the development of multiphase high-temperature alloys are discussed.