| The use of ordered intermetallic compounds for structural applications has largely been limited due to their low room temperature ductility and propensity for intergranular fracture; behavior that has been attributed to both poor intergranular cohesion and environmental embrittlement. In this study, grain boundaries in two intermetallic compounds, Ni{dollar}sb3{dollar}Al and Ni{dollar}sb3{dollar}Si, are examined, using analytical and conventional transmission electron microscopy techniques to obtain insight into the effects of microalloying on boundary structure, chemistry and bonding. The relationship between the observed changes in bonding and boundary properties is discussed.; Microalloying with B improves the ductility of Ni{dollar}sb3{dollar}Al and changes the fracture mode from intergranular to transgranular. To understand the effect of B, grain boundaries in B-free and B-doped Ni-rich Ni{dollar}sb3{dollar}Al were examined in a scanning transmission electron microscope using energy dispersive X-ray spectroscopy and annular dark field imaging to study the chemistry, and electron energy loss spectroscopy to study the local electronic structure. 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{dollar}sb{lcub}2,3{rcub}{dollar} edge showed that the bonding at B-rich regions is similar to that in bulk Ni{dollar}sb3{dollar}Al, while the bonding at B-free regions is similar to that at undoped Ni-rich boundaries. The changes in bonding and their relationship to the observed fracture mode are discussed. It is shown that segregation of B results in an increase in the cohesive strength of grain boundaries consistent with the observed change in the fracture mode from intergranular to transgranular. Similar studies were conducted on grain boundaries on Zr-doped Ni{dollar}sb3{dollar}Al and Ni{dollar}sb3{dollar}Si with and without B and the results were found to correlate with the observed mechanical properties.; The dislocation structure and antiphase boundary energy at small angle grain boundaries in Ni-rich Ni{dollar}sb3{dollar}Al were studied using conventional and scanning electron microscopy techniques. Ni-enrichment at APBs was shown to result in a lower APB energy at grain boundaries relative to the bulk, by replacing high energy Al-Al interactions with low energy Ni-Al and Ni-Ni interactions. The implications of lower APB energy for boundary properties are discussed. |
