Local Electronic Structure and Cohesion of Grain Boundaries in Ni3Al

Abstract

AbstractOne of the fundamental questions concerning Ni3Al is why doping with boron improves the room temperature ductility of the polycrystalline material. Boron is thought to prevent environmental embrittlement and increase the cohesive strength of grain boundaries since it changes the fracture mode from intergranular to transgranular. This change in cohesive energy must be reflected in the bonding changes at the grain boundary which can be probed using spatially resolved electron energy loss spectroscopy (EELS). We have examined grain boundaries in both undoped and boron doped Ni0.76Al0.24using EELS, EDX and ADF imaging in a UHV STEM. Ni-enrichment is seen in a 0.5–1 nm wide region at large angle grain boundaries, both in the absence and presence of B. EELS shows that B segregation can vary along the interface. The Ni L2, 3core edge fine structure which is sensitive to the filling of the Ni d-band, shows only the boron rich regions of the grain boundary to have a bonding similar to that of the bulk material. These results demonstrate that boron segregation increases the cohesive energy and hence improves the fracture resistance of the grain boundary, by making the bonding at boundaries similar to that in the bulk. The measured changes in d band filling may also affect the local solubility of hydrogen.