Structure, chemistry and bonding at grain boundaries in Ni 3Al—II. The structure of small angle boundaries, Ni-enrichment and its influence on bonding, structure, energy and properties

Abstract

Small angle [001] twist boundaries and [001] (110) tilt boundaries in B-free and B-doped Ni-rich Ni 3Al (76 at. % Ni) were examined using conventional electron microscopy techniques as well as Energy Dispersive X-ray Spectroscopy (EDS), annular dark field (ADF) imaging and spatially resolved electron energy loss spectroscopy (EELS), in an UHV scanning transmission electron microscope. The interface structure 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 Ni 3Al and which decrease with increasing misorientation angle. EDS, EELS and ADF imaging demonstrate that the APBs are Ni-rich. The observations on the APB chemistry and energy taken together lead to the conclusion that Ni-enrichment lowers grain boundary energy by decreasing the number of high energy AlAl interactions across the APB. These results on small angle boundaries lead to the suggestion that Ni-enrichment also decreases the number of high energy AlAl interactions across the interface at large angle boundaries.