EFFECT OF GRAIN-BOUNDARY AND INTERPHASE STRUCTURES ON MECHANICAL PROPERTIES OF STRUCTURAL MATERIALS

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Grain boundaries of single phase materials and interphase boundaries of composite materials are often responsible for the degradation of mechanical properties. In the present investigation, for molybdenum which has problems of grain boundary brittleness, ceramics of Si_3N_4, SiC and TiC which are hopeful as high-temperature structural materials, and micro-bonded Au-Al which is important in the IC industry, the grain boundary structure and interphase boundary structure have been observed by means of high-resolution transmission electron microscopy. Their boundary strength is discussed in connection with the boundary structure.Symmetric tilt-boundary in Molybdenum: The fracture strength strongly depends on the grain boundary character in molybdenum. SIGMA 1 low angle and near SIGMA 3 coincidence boundaries have a good coherency, whereas the boundaries of misorientation angle around 30ﾟ are poor in coherency. However, the structure in those boundaries was found to be described by the grain-boundary dislocation model, It is considered that the grain boundary structure in molybdenum with a high covalency in bonding is not greatly different from that in normal metals.Grain boundary structure of Si_3N_4, SiC and TiC: An amorphous-like layer was observed at grain boundaries in Si_3N_4 and SiC. That layer in SiC was analyzed by EELS using a micro beam technique. It is clarified that the layer is neither sintering aids nor impurity layer but a relaxed structure. A grain boundary layer such as observed in Si_3N_4 and SiC was not observed in TiC where the boundary structure was metal like.Interphase boundary of micro-bonded Au-Al: In the foils prepared with a ultra-microtome no amorphous phase was observed, whereas in the foils prepared by ion milling a large part of the intermetallic compound formed on the interface was amorphous. Using the foils prepared with the microtome, the contamination effect on the bonding was clarified.