Structure and growth of core-shell nanoprecipitates in Al-Er-Sc-Zr-V-Si high-temperature alloys
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2018, Springer Science+Business Media, LLC, part of Springer Nature. Lightweight Sc-containing aluminum alloys exhibit superior mechanical performance at high temperatures due to coreshell, L12-ordered trialuminide nanoprecipitates. In this study, the structure of these nanoprecipitates was studied, using different transmission electron microscopy (TEM) techniques, for an AlErScZrVSi alloy that was subjected to a two-stage overaging heat treatment. Energy-dispersive X-ray spectroscopy of the spherical Al3(Sc, Zr, Er ,V) nanoprecipitates revealed a coreshell structure with an Sc- and Er-enriched core and a Zr-enriched shell, without a clear V outer shell. This structure is stable up to 72% of the absolute melting temperature of Al for extended periods of time. High-angle annular dark-field scanning TEM was used to image the {100} planes of the nanoprecipitates, demonstrating a homogeneous L12-ordered superlattice structure for the entire nanoprecipitates, despite the variations in the concentrations of solute atoms within the unit cells. A possible growth path and compositional trajectory for these nanoprecipitates was proposed using high-resolution TEM observations, where different rod-like structural defects were detected, which are considered to be precursors to the spherical L12-ordered nanoprecipitates. It is also hypothesized that the structural defects could consist of segregated Si; however, this was not possible to verify with HAADF-STEM because of the small differences in Al and Si atomic numbers. The results herein allow a better understanding of how the AlSc alloys coreshell nanoprecipitates form and evolve temporally, thereby providing a better physical picture for future atomistic structural mappings and simulations.
