Grain boundaries in B-free and B-doped Ni-rich Ni3Al (76at%Ni) were examined using electron energy loss spectroscopy, X-ray fluorescence analysis, and annular dark field imaging in a UHV scanning transmission electron microscope, as well as conventional electron microscopy techniques. Ni enrichment was seen in a 0.5 to 1.0nm wide region at large angle boundaries, both in the absence and in the presence of B. Electron energy loss spectroscopy showed that B segregation varies along the interface, and examination of the Ni L2.3edge shows that the B-rich regions had a bonding similar to that in bulk Ni3Al. These results demonstrate that B segregation increases the cohesive strength of the boundary by making the bonding at the boundary similar to that in bulk Ni3Al. Small-angle boundaries were examined to address the question of why Ni enrichment at grain boundaries occurs. The interface structure in [001] twist and [001](110) tilt boundaries consists of periodically spaced pairs of a/2<110> partial dislocations, linked by an antiphase boundary. An analysis of the separation of the partials gives antiphase boundary energies which were lower than in bulk Ni3Al. Electron energy loss spectroscopy and annular dark field imaging demonstrate that the antiphase boundaries were Ni-rich. The observations on the antiphase boundary chemistry and energy lead to the conclusion that Ni enrichment occurs to lower boundary energy by decreasing the number of high-energy bonds across the antiphase boundary. The Ni enrichment at large-angle boundaries played a similar role.

The Structure, Bonding and Chemistry of Grain Boundaries in Ni3Al. Subramanian, S., Muller, D.A., Batson, P.E., Silcox, J., Sass, S.L.: Materials Science and Engineering A, 1995, 192-193[2], 936-44. See also: Materials Research Society Symposium – Proceedings, 1995, 364[1], 333-8