STRUCTURE, CHEMISTRY AND BONDING AT GRAIN-BOUNDARIES IN NI3AL .2. THESTRUCTURE OF SMALL-ANGLE BOUNDARIES, NI-ENRICHMENT AND ITS INFLUENCE ON BONDING, STRUCTURE, ENERGY AND PROPERTIES

Small angle [001] twist boundaries and [001](110) tilt boundaries in B -free and B-doped Ni-rich Ni3Al (76 at. % Ni) were examined using conv entional electron microscopy techniques as well as Energy Dispersive X -ray Spectroscopy (EDS), annular dark field (ADF) imaging and spatiall y resolved electron energy loss spectroscopy (EELS), in an UHV scannin g transmission electron microscope. The interface structure consists o f periodically spaced pairs of a/2[110] partial dislocations, linked b y an antiphase boundary (APE). An analysis of the separation of the pa rtials gives APE energies which are lower than in bulk Ni3Al and which decrease with increasing misorientation angle. EDS, EELS and ADF imag ing demonstrate that the APBs are Ni-rich. The observations on the APE chemistry and energy taken together lead to the conclusion that Ni-en richment lowers grain boundary energy by decreasing the number of high energy Al-Al interactions across the APE. These results on small angl e boundaries lead to the suggestion that Ni-enrichment also decreases the number of high energy A-Al interactions across the interface at la rge angle boundaries.