Low-angle [001] twist boundaries and [001](110) tilt boundaries in B-free and B-doped Ni-24at%Al were studied by means of electron microscopy, energy dispersive X-ray spectroscopy, annular dark-field imaging, and spatially resolved electron energy loss spectroscopy in a scanning transmission electron microscope. It was found that the interface structure consisted of periodically spaced pairs of a/2<110> partial dislocations that were linked by antiphase boundary. An analysis of the separation of the partials indicated antiphase boundary energies which were lower than those in bulk Ni3Al, and which decreased with increasing misorientation angle. The various imaging methods demonstrated that the antiphase boundaries were Ni-rich. The results led to the conclusion that Ni enrichment lowered the grain boundary energy by decreasing the number of high-energy Al-Al interactions across the antiphase boundary. The results for low-angle boundaries suggested that Ni-enrichment also decreased the number of high-energy Al-Al interactions across the interface at high-angle boundaries.

Structure, Chemistry and Bonding at Grain Boundaries in Ni3Al - II. The Structure of Small Angle Boundaries, Ni-Enrichment and its Influence on Bonding, Structure, Energy and Properties. Subramanian, S., Muller, D.A., Silcox, J., Sass, S.L.: Acta Materialia, 1996, 44[4], 1647-55