Nickel aluminide containing refractory-metal dispersoids 2: Microstructure and properties

NiAl containing 10 vol.% or 30 vol.% of molybdenum or tungsten was fabricated by hot isostatic pressing at 1000 °C or 1200 °C of powders synthesized by reactive mechanical alloying. The NiAl matrix exhibits sub-micron grains pinned by discrete refractory dispersoids with an average size between 55 nm and 110 nm. The two phases have negligible mutual solubility at room temperature, and the dispersoids exhibit only slight coarsening at the highest consolidation temperature. Dispersion-strengthening by the refractory second phase, measured by hardness indentation, is observed both at room temperature and at elevated temperature up to 700 °C, and increases with increasing volume fraction and decreasing size of the dispersoids. The oxidation resistance at 900 °C of NiAl containing 10 vol.% refractory-metal dispersoids is similar to that of unalloyed NiAl, indicating that the alumina scale protects the discontinuous refractory-metal dispersion within the NiAl matrix.