HIGH-RESOLUTION ELECTRON MICROSCOPY OF GRAIN BOUNDARIES IN FCC MATERIALS

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Abstract
The atomic structure of and large-angle tilt grain boundaries(GBs) has been studied in Au and NiO by high-resolution electron microscopy. In addition to symmetric boundaries a number of asymmetric facets are found in both Au and NiO. Asymmetric GBs are important, not only because of the much greater multitude of possible asymmetric configurations, but also because asymmetric facets often may be low in energy. The rigid-body translation normal to the Σ=5 symmetric GBs in NiO is smaller than predicted from computer simulation, while GBs in Au largely appear to have greater expansions than expected from calculations based on embedded atom potentials. The total excess volume in the oxides may be substantially larger than deduced from the rigid-body translations alone, due to the presence of vacancy-type defects at the GB. Structural multiplicities and a tendency for maintaining coherence across the interface have been observed in both NiO and Au grain boundaries. A number of novel GB structures that assume a 3-dimensional character have been observed in Au. These observations underline the importance of misfit localization and the tendency in close-packed solids to maintain, whenever possible, atomically well-matched regions

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