THE STRUCTURE, BONDING AND CHEMISTRY OF GRAIN-BOUNDARIES IN NI3AL

Grain boundaries in B-free and B-doped Ni-rich Ni3Al (76 at.% Ni) were examined using electron energy loss spectroscopy (EELS), X-ray fluore scence analysis (XRF), and annular dark field (ADF) imaging in a UHV s canning transmission electron microscope, as well as conventional elec tron 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 pres ence of B. EELS shows that B segregation varies along the interface, a nd examination of the NiL(2,3) edge shows that the B-rich regions have a bonding similar to that in bulk Ni3Al. These results demonstrate th at B segregation increases the cohesive strength of the boundary by ma king the bonding at the boundary similar to that in bulk Ni3Al. Small- angle boundaries were examined to address the question of why Ni enric hment at grain boundaries occurs. The interface structure in [001] twi st and [001](110) tilt boundaries consists of periodically spaced pair s of a/2(110) partial dislocations, linked by an antiphase boundary (A PB). An analysis of the separation of the partials gives APE energies which are lower than in bulk Ni3Al. EELS and ADF imaging demonstrate t hat the APBs are Ni-rich. The observations on the APE chemistry and en ergy lead to the conclusion that Ni enrichment occurs to lower boundar y energy by decreasing the number of high-energy bonds across the APE. Ni enrichment at large-angle boundaries plays a similar role.