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Grain size engineering, particularly reducing grain size into thenanocrystalline regime, offers a promising pathway to further improve thestrength-to-weight ratio of Al alloys. Unfortunately, the fabrication ofnanocrystalline metals often requires non-equilibrium processing routes, whichtypically limit the specimen size and require large energy budgets. In thisstudy, multiple dopant atoms in ternary Al alloys are deliberately selected toenable segregation to the grain boundary region and promote the formation ofamorphous complexions. Three different fully dense bulk nanocrystalline Alalloys (Al-Mg-Y, Al-Fe-Y, and Al-Ni-Y) with small grain sizes were successfullyfabricated using a simple powder metallurgy approach, with full densificationconnected directly to the onset of amorphous complexion formation. All thecompositions demonstrate densities above 99% with grain sizes of <60 nmfollowing consolidation via hot pressing at 585 oC. The very fine grainstructure results in excellent mechanical properties, with nanoindentationhardness values in the range of 2.2-2.8 GPa. Detailed microstructuralcharacterization verifies the segregation of all dopant species to grainboundaries as well as the formation of amorphous complexions, which suggeststheir influential role in aiding effective consolidation and endowing thermalstability in the alloys. Moreover, nanorods with a core-shell structure arealso observed at the grain boundaries, which likely contribute to thestabilization of the grain structure and high strength. Finally, intermetallicparticles with a sizes of hundreds of nanometers form. As a whole, the resultspresented here demonstrate a general alloy design strategy of segregation andboundary evolution pathway that enables the fabrication of multiplenanocrystalline Al alloys with hierarchical microstructures and improvedperformance.

Bulk nanocrystalline Al alloys with hierarchical reinforcement structures via grain boundary segregation and complexion formation