Nickel aluminide containing refractory-metal dispersoids: 1: Synthesis by reactive mechanical alloying

Nickel and aluminum powders were mechanically alloyed with 10 or 30 vol.% refractory metal powders (molybdenum or tungsten) for times up to 50 h. Intermetallic synthesis takes place after 10–20 h alloying, with concomitant dispersion of the inert refractory metal phase, resulting in an NiAl matrix containing fine refractory dispersoids. The reactive synthesis step is direct, without intermediate intermetallic products. In a separate processing route, the compound NiMo was first synthesized by hydrogen reduction of nickel oxide and molybdenum oxide. Subsequent reactive mechanical alloying of NiMo powders with metallic nickel and aluminum powders yielded the same phases as the direct route from elemental powders. However, the dispersion step is more efficient with brittle NiMo than with ductile molybdenum, leading to a finer molybdenum dispersion size in the reacted powders. All mechanically alloyed powders are stable on exposure for 1 h at 1000 °C in a reducing atmosphere, but oxidize rapidly in air between 425 °C and 1000 °C.