Structure, chemistry and bonding at grain boundaries in Ni 3Al—I. The role of boron in ductilizing grain boundaries

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Boron segregation is known to change the fracture mode from intergranular to transgranular in Ni-rich Ni 3Al. This paper addresses the question of why boron segregation at grain boundaries improves their resistance to fracture. Grain boundaries in B-free and B-doped Ni-rich Ni 3Al (76 at. % Ni) were examined using spatially resolved electron energy loss spectroscopy (EELS), energy dispersive X-ray spectroscopy (EDS) and annular dark field (ADF) imaging in an UHV scanning transmission electron microscope, as well as conventional electron microscopy techniques. Ni-enrichment was seen in a 0.5 – 1.0 nm wide region at large angle boundaries, both in the absence and presence of B. Using EELS, B segregation to the boundary was observed to vary along the interface. EELS of the Ni L 2,3 edge showed that the B-rich regions have a bonding similar to that in bulk Ni 3Al, while the B-free regions have a bonding similar to the more Ni-like character of undoped boundaries. These results demonstrate that boron segregation increases the cohesive strength of grain boundaries in Ni 3Al by making the bonding at the boundary similar to that in the bulk and, in this manner, increases their fracture resistance.