| The mechanical properties and oxidation behavior of nanocrystalline NiAl are evaluated and compared with coarse-grained NiAl to determine the effect of microstructure on these properties. Synthesis of the nanocrystalline NiAl, involved mechanical alloying of Ni and Al powders followed by shock-wave compaction, while the coarse-grained NiAl was prepared by conventional arc melting. The mechanical property studies on the nanocrystalline NiAl intermetallic show that the microhardness increases with decreasing grain size and is higher than the coarse-grained NiAl. Hall-Petch strengthening appears to contribute to the strengthening of the nanocrystalline NiAl, and a Hall-Petch slope of approximately 6 GPa(nm)1/2 was obtained for the nanocrystalline NiAl, consistent with the mechanism involving dislocations motion inhibited by grain boundaries due to dislocations generation at ledges. The oxidation behavior of nanocrystalline NiAl (3 to 30 nm) was compared with that of the conventional arc-melted NiAl with a coarse grain size of 130 ± 80 μm. Kinetic studies were performed via thermogravimetric analysis, and accompanying x-ray diffraction experiments and plan view and cross-sectional SEM and TEM were employed to yield information on the oxidation mechanism. Results indicate that the oxidation of nanocrystalline NiAl is somewhat faster than the oxidation of coarse-grained NiAl, yet the activation energy of diffusion is unchanged. Thus for a given time and temperature, the oxide scale is thicker on the nanocrystalline material. Several mechanisms to account for these effects are proposed and evaluated. |
