Chemical Syntheses of Nanocrystalline Nickel Aluminides

Nanocrystalline nickel aluminides (nano-NiAl and nano-Ni<sub>3</sub>Al) are synthesized by reactions of NiCl<sub>2</sub> and LiAlH<sub>4</sub> in Et<sub>2</sub>O or 1,3,5-trimethylbenzene, followed by annealing of the reaction products at 550−750 °C. The reaction pathways followed in the two solvents differ, leading to nanocrystalline materials of different composition and microstructure. The nanocrystalline nickel aluminide powders are highly susceptible to adventitious oxidation, which produces Al<sub>2</sub>O<sub>3</sub>. Consolidates obtained by hot pressing the powders at 700−1200 °C and 350−650 MPa in an Ar atmosphere contain 8−23 wt % Al<sub>2</sub>O<sub>3</sub>. They possess densities of 80−98% of theory, and Vickers microhardnesses of 4.0−6.3 ± 0.3 GPa. Hot-pressing temperatures of ≥ 1200 °C are required to produce consolidates having good environmental stability. Consolidates are also produced by hot pressing admixtures of nano-Ni and nano-Al, with and without added nano-AlN as a second, reinforcement phase. The resulting nanocrystalline and nanocomposite nickel aluminides exhibit the highest purities, densities, and hardnesses (up to 7.4 ± 0.9 GPa) among the samples studied.