NANOCRYSTALLINE INTERMETALLIC COMPOUNDS - STRUCTURE AND MECHANICAL-PROPERTIES

Nanocrystalline intermetallic compounds (Ni3Al, NiAl, TiAl) with crystallite sizes in the range 5-20 nm were prepared by means of an inert gas condensation technique. As-prepared nanocrystalline Ni3Al samples (24 at.% Al) exhibited no superlattice reflection in X-ray diffraction (XRD) and transmission electron microscopy. Ordering occurred during annealing starting at 400-degrees-C. In contrast, as-prepared nanocrystalline NiAl samples (50 at.% Al) were at least partially ordered. XRD spectra of as-prepared nanocrystalline TiAl samples (54 at.% Al) suggested that these samples consisted of a mixture of alpha(2)-Ti3Al and gamma-TiAl. Vickers hardness measurements showed that nanocrystalline samples were substantially harder than polycrystalline indicating that grain refinement caused strengthening. Hot pressing at temperatures up to 650-degrees-C under high vacuum conditions results in retarded grain growth compared with annealing.