Transmission electron microscopy of Al-Li control rod pins

Abstract

This report discusses transmission electron microscopy employed to characterize the microstructures of both cast and mechanically alloyed powder Al-Li control rod pins. The results indicated that microstructural differences existed between the ingot and powder metallurgy materials. In general, the cast specimens contained large, dendritic {alpha} grains separated by regions of much smaller a grains and AlLi {beta} particles. The grain interiors, except in the immediate vicinity of the {beta} particles, contained a high number density of Al{sub 3}Li ({delta}{prime}) precipitates. Several specimens also contained a widely distributed and unidentified Si-rich phase. In contrast, the microstructure of powder metallurgy samples consisted of small, randomly oriented {alpha} grains containing a dispersion of rod-like Al{sub 4}C{sub 3} (aluminum carbide) particles - confirmed by both TEM and X-ray diffraction. No TEM evidence was found for the presence of the {delta}{prime} or {beta} phases in the as-received powder material, although the {beta} phase was identified with X-ray diffraction. The {beta}{prime} phase was observed in the powder metallurgy pin after the material was given a solutionizing heat treatment. This observation suggests that Li may have been segregated to other phases, most likely either the {beta} or LiAlO{sub 2} phases, that subsequently dissolved during the ``solution treatment`` freeing-up Li to form {delta}{prime}. The possibility exists that the microstructural differences between these two materials could be decreased by further heat treatments or thermo-mechanical processing. However, without further experimentation, it is unknown which material (microstructure) is better suited for control rod pin applications.