  Grain size engineering, particularly reducing grain size into the
nanocrystalline regime, offers a promising pathway to further improve the
strength-to-weight ratio of Al alloys. Unfortunately, the fabrication of
nanocrystalline metals often requires non-equilibrium processing routes, which
typically limit the specimen size and require large energy budgets. In this
study, multiple dopant atoms in ternary Al alloys are deliberately selected to
enable segregation to the grain boundary region and promote the formation of
amorphous complexions. Three different fully dense bulk nanocrystalline Al
alloys (Al-Mg-Y, Al-Fe-Y, and Al-Ni-Y) with small grain sizes were successfully
fabricated using a simple powder metallurgy approach, with full densification
connected directly to the onset of amorphous complexion formation. All the
compositions demonstrate densities above 99% with grain sizes of <60 nm
following consolidation via hot pressing at 585 oC. The very fine grain
structure results in excellent mechanical properties, with nanoindentation
hardness values in the range of 2.2-2.8 GPa. Detailed microstructural
characterization verifies the segregation of all dopant species to grain
boundaries as well as the formation of amorphous complexions, which suggests
their influential role in aiding effective consolidation and endowing thermal
stability in the alloys. Moreover, nanorods with a core-shell structure are
also observed at the grain boundaries, which likely contribute to the
stabilization of the grain structure and high strength. Finally, intermetallic
particles with a sizes of hundreds of nanometers form. As a whole, the results
presented here demonstrate a general alloy design strategy of segregation and
boundary evolution pathway that enables the fabrication of multiple
nanocrystalline Al alloys with hierarchical microstructures and improved
performance.
