Velimir R. Radmilović

Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000, Belgrade, Serbia

In this presentation we will discuss formation of core/shell ordered L12 Al3(LiSc) trialuminide precipitates embedded in Al-rich matrix by solid-state reaction using multi-stage heat treatment. The conventional high-resolution imaging reveals the fully ordered structure of the shell. High angel annular dark field scanning transmission electron microscopy (HAADF), energy filtered transmission electron microscopy spectrum imaging (EFTEM-SI) and field-ion microscopy (FIM) indicates that Sc is confined to the core, while Li is predominantly confined to the shell. The chemical composition of the core region is Al3(Li_0.40Sc_0.48Al_0.12), for which the mean atomic scattering factor of the Li, Sc, Al sublattice in the fully ordered L12 structure is very similar to that of Al. This explains why the contrast in the core is very similar to the contrast in the surrounding Al-matrix and how it is related to vanishing superlattice reflections. The presence of Li in the core is thermodynamically favorable. The phase of the exit wave reveals long-range order of the shell, in which Al columns can be distinguished easily from Li columns. Our approach demonstrates that using differential diffusivities and solubility of Li and Sc in an Al matrix it is possible to create monodisperse distribution of L12 ordered precipitates with exceptional resistance to coarsening.