HIGH-RESOLUTION ELECTRON-MICROSCOPY OF INTERFACES IN FCC MATERIALS

Modern high-resolution electron microscopy (HREM) instruments, which are capable of a point-to-point resolution of better than 0.2 nm, have allowed atomic-scale observations of a variety of intemal interfaces. The application of the HREM technique to fcc model systems for the purpose of addressing a number of interface issues will be examined in this paper. Atom.ic structure observations for heterophase interfaces of metal/metal and metal/metal-oxide systems as well as HREM studies of grain boundaries in NiO and Au will be discussed with emphasis on generic structural features and the role of the interface plane. Comparisons between observed interface structures and atomistic computer modeling results have shown agreements for some interfaces, and certain differences in others. A number of structural features are common to both metal and oxide grain boundafies, as well as certain heterophase boundaries. Of particular importance in close-packed solids appears to be the tendency to preserve, as much as possible, local atomic coordination, giving rise to atomically well-matched regions that altemate along the interface with regions of misfit. It is commonly observed that heterophase interfaces are preferentially formed on dense-packed planes. Low-index planes are also frequently observed in asymmetric grain boundaries. In addition to the observation of misfit dislocations in heterophase boundaries, misfit-dislocation-like defects have also been found in asymmetric, incommensurate grain boundaries. The tcndency for maintaining coherence between dense-packed planes across the interface has resulted in the formation of novel three-dimensional grain boundary structures. HREM observations have brought new insights into the coffelations between macroscopic geometry, interfacial energy, and microscopic atomic relaxations.