Correlation between grain boundary (GB) structure, GB fracture strength and GB energy in metals and covalentbonded materials

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The grain boundary (GB) property depends strongly on its character. Also, the mechanical properties such as the GB sliding and/or GB migration at high temperatures are affected by the GB character. In this investigation, the orientation controlled bicrystals of Al, Mo, TiC and Si were prepared, and the GBs have been systematically observed by the high resolution transmission electron microscopy to clarify the effect of the GB character or atomic bonding on GB structures. In addition to the TEM observation, the GB energy and its stregth were measured, then we have discussed the correlation between GB strength, GB energy and GB structure.1.Grain boundary structure : The grain boundaries were found to be consisted of the primary dislocation (lattice dislocations) array and the secondary one (DSC dislocation array) irrespective of the GB character and of atomic bonding except the case that the GB phase, which was often observed in covalent-bonded ceramics such as Si_3N_4, existed along the boundaries. These periodic structures would be probably described by the geometric models based on the O-lattice and the coincidence site lattice theories.2.Correlation between the GB energy and the GB dislocation density (GB structure) : It was seen that there existed good ocrrelation between the misorientation-dependence of GB energy and that of GB dislocation density, suggrsting that the main part of the GB energy must have been contributed by the elastic energy of GB dislocations. According to the Read-Shockley relation, the GB energy can be uniquely determined by the misorientation angle, though the relation is only valid in the range of small angle GBs. However, we found that it was possible to extend the relation to the range of high angle GBs by plotting against the GB dislocation density instead of the misorientation angle.Also, it was seen that the core energy of the GB dislocation was less dependent of the magnitude of its Burgers vector.