Abstract: The purpose of the present work is to study the NiO cluster formation in Ni3Al alloys by field ion microscope and atom probe (AP-FIM). A polycrystal Ni3Al (B-doped) was heat-treated in atmospheres, the surface adsorption of air (hydrogen, oxygen) moisture etc.) occured on the Ni3Al surface and then these absorbents diffused into the interior of alloy through groin boundaries. AP-FIM studies found that the NiO and AlO clusters appeared in the local regions and amount of NiO clusters is much more than that of AlO. Moreover the hydrogen was simultaneously detected in the identical region.These results provided an experimental evidence that the formation of NiO and AlO clusters is the result of reaction of Ni (or Al) with residual moisture in Ni3Al, i.e. Ni+H2 O→NiO+2H. But the samples of B-free Ni3Al and B-doped single crystal Ni3Al have low concentration of NiO and H. This result shows that the diffusion of H2O molecule was promoted by genie boundaries containing boron. In addition, the boron suppresses environmental emvironmental was discussed, which suggested that the formation of Ni-O bonding has influence on bonding character of Ni-Al atoms and benefits the ductility of alloy.

Key words: Ni-3Al alloy, NiO cluster, AP-FIM