Xe precipitates at grain boundaries in Al under 1 MeV electron irradiation

Crystalline nanoprecipitates of Xe have been produced by ion implantation i nto mazed bicrystalline Al at 300 K, in which the matrix grain boundaries a re mainly 90 degrees lilt boundaries. Within Al grains, Xe nanocrystals are fee, isotactic with the Al and cuboctohedral in shape with {111} and {100} facets. With an off-axial imaging technique, the nanocrystals were structu re imaged against a relatively featureless matrix background. In contrast t o metal precipitates in Al, such as Pb, Xe precipitates straddling a matrix grain boundary are bicrystals as small as approximately 2 nm in diameter. Larger Xe precipitates tend to avoid boundaries that are inclined away from asymmetrical orientation and which have a significant twist component. Und er the 1 MeV electron irradiation employed for HREM observation, small Xe n anocrystals near a grain boundary may migrate to the boundary and coalesce with other Xe precipitates. The structural observations are rationalized on a simple geometrical interpretation.