  Amorphous complexions have recently been demonstrated to simultaneously
enhance the ductility and stability of certain nanocrystalline alloys. In this
study, three quinary alloys (Cu-Zr-Hf-Mo-Nb, Cu-Zr-Hf-Nb-Ti, and Cu-Zr-Hf-Mo-W)
are studied to test the hypothesis that increasing the chemical complexity of
the grain boundaries will result in thicker amorphous complexions and further
stabilize a nanocrystalline microstructure. Significant boundary segregation of
Zr, Nb, and Ti is observed in the Cu-Zr-Hf-Nb-Ti alloy, which creates a
quaternary interfacial composition that limits average grain size to 63 nm even
after 1 week at ~97% of the melting temperature. This high level of thermal
stability is attributed to the complex grain boundary chemistry and amorphous
structure resulting from multi-component segregation. High resolution
transmission electron microscopy reveals that the increased chemical complexity
of the grain boundary region in the Cu-Zr-Hf-Nb-Ti alloy results in an average
amorphous complexion thickness of 2.44 nm, approximately 44% and 32% thicker
than amorphous complexions previously observed in Cu-Zr and Cu-Zr-Hf alloys.
