Chemical syntheses of nanocrystalline nickel aluminides

Nanocrystalline nickel aluminides (nano-NiAl and nano-Ni3Al) are synthesize d by reactions of NiCl2 and LiAlH4 in Et2O or 1,3,5-trimethylbenzene, follo wed by annealing of the reaction products at 550-750 degrees C. The reactio n 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 Al2O3 Consolidates obtained by hot pressing the powders at 7 00-1200 degrees C and 350-650 MPa in an Ar atmosphere contain 8-23 wt % Al2 O3. They possess densities of 80-98% of theory, and Vickers microhardnesses of 4.0-6.3 +/- 0.3 GPa. Hot-pressing temperatures of greater than or equal to 1200 degrees C are required to produce consolidates having good environ mental stability. Consolidates are also produced by hot pressing admixtures of nano-Ni and nano-Al, with and without added nano-AIN as a second, reinf orcement phase. The resulting nanocrystalline and nanocomposite nickel alum inides exhibit the highest purities, densities, and hardnesses (up to 7.4 /- 0.9 GPa) among the samples studied.