Structure, Growth, and Properties of Twist and Tilt Grain Boundaries in Bicrystals of NaCl

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Abstract
Grain boundaries having from 2° to 45° mismatch across the interface were grown for pure tilt and pure twist misorientation in NaCl by the Czochralski technique. The method of crystal growth and the techniques required for good control of the misorientation and the planarity of the grain boundary are described. The substructure of twist and tilt NaCl bicrystals was examined using both optical and electron‐microscopic techniques. The tilt grain boundaries were found to be strong mechanically and not easily cleaved apart. The twist grain boundaries were very weak mechanically, especially at mismatch angles greater than 20°, and exhibited anomalously rapid diffusion or capillary flow effects along the grain boundary. Grain boundaries in the bicrystals were examined by: (1) pulling the crystals apart at the grain boundary and (2) by polishing away one side of the bicrystal to within 0.5 mm of the grain boundary. (The grain boundary surface could then be examined by focusing through the layer of NaCl.) Both of these techniques showed that the twist bicrystals were held together by small regions or spots of continuity at the grain boundary. The spot density ranged from 500/cm2 to 20 000/cm2 for 45° and 10° twist bicrystals, respectively. No substructure was observed for tilt grain boundaries. This may explain not only the extreme mechanical weakness of twist grain boundaries, but may also help to explain brittle fracture in polycrystalline ionically bonded solids generally, including ceramics and other alkali halides, and phenomena, such as the solution embrittlement of AgCl.

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