Morphologies of metastable inert gas precipitates in aluminum observed with in situ HRTEM

Inert gases of Xe, Ar and Ne were ion-implanted into Al at room temperature under the observation in a transmission electron microscope (TEM). The precipitation and the structure of inert gas precipitates were studied with conventional and high-resolution transmission electron microscopy (CTEM and HRTEM). An in situ observation revealed that Xe ions aggregate as atomic clusters, and the clusters change to crystalline particles in the size around 2.0 nm. It is a minimum size for the observable crystalline Xe precipitates, below which the Xe ion clusters are not stable either in shape or in structure. The Xe precipitates have sizes ranging from about 2.0 to 10 nm in specimens implanted with Xe to a dose of 5×1019 ions m−2. The precipitates larger than about 6 nm are usually in non-crystalline state, but the maximum size of crystalline precipitates also depends on the surrounding environment. Ar precipitates began to be visible with a size of about 1.4 nm at an implantation dose of 8×1018 ions m−2. The solid Ar precipitates in the specimen implanted with Ar to a dose of 1.9×1020 ions m−2 range from 1.4 to 5 nm in size and have a truncated cuboctahedron shape faceted by {1 1 1} and {1 0 0} of Al. Ne precipitates in Al are in non-crystalline state, with sizes ranging from about 1.5 to 15 nm. The Ne precipitates are also in a truncated cuboctahedron shape, similar to the Ar precipitates. Analysis of growth dynamics of Ar and Ne precipitates suggests that dissolution of clusters and precipitates take place during the implantation, and that the growth rate of the precipitates increased with the implanted dose.