Materials Science and Engineering A

The structure, bonding and chemistry of grain boundaries in Ni3Al

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Grain boundaries in B-free and B-doped Ni-rich Ni3Al (76 at.% Ni) were examined using electron energy loss spectroscopy (EELS), X-ray fluorescence analysis (XRF), and annular dark field (ADF) imaging in a UHV scanning transmission electron microscope, as well as conventional electron microscopy techniques. Ni enrichment is seen in a 0.5-1.0 nm wide region at large angle boundaries, both in the absence and in the presence of B. EELS shows that B segregation varies along the interface, and examination of the Ni L2.3 edge shows that the B-rich regions have 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 (APB). An analysis of the separation of the partials gives APB energies which are lower than in bulk Ni3Al. EELS and ADF imaging demonstrate that the APBs are Ni-rich. The observations on the APB 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 APB. Ni enrichment at large-angle boundaries plays a similar role. © 1995.