Production of fine-microstructural nickel aluminides by wet mechanical alloying process

A Ni₃Al(γ′) and NiAl(β) two phase nickel aluminide (Ni-33.9 mol%Al-0.1 mol%B) powder material was fabricated from Ni and NiAl raw powders by mechanical alloying (MA) process followed by hot pressing. The effects of process control agents for milling on the suppression of oxidation and microstructure refinement in powders as well as compaction have been investigated. It was found that hexane as a milling agent was very effective to suppress the oxidation. Especially, a very fine γ′ and β two phase microstructure was obtained by hot pressing after a wet milling process using a large quantity of hexane enough to separate the powders by the organic agent. The average grain size was about 1.2 μm. Oxygen content was as low as about 0.37 mass%. The change of X-ray diffraction patterns with the heating of the wet MA powders indicated that the (γ′ + β) two phase microstructure was formed at above 773 K by the reaction between the finely dispersed Ni and NiAl phases in the powders. Compression tests were carried out for the compacted specimens at temperatures of 1173 and 1273 K at an initial strain rate range from 1.4 × 10^-4 s^-1 to 5.6 × 10^-2 s^-1. The strain rate sensitivity exponent m was estimated as high as 0.36 to 0.38 at 1173 K and 1273 K, respectively. These results suggest that the compacted specimen can deform superplastically at the strain rate range higher than 10^-2 s^-1.