  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 compare the influence of increased chemical complexity on
thermal stability of the nanocrystalline microstructure, in addition to grain
boundary structure. Significant boundary segregation is observed for Zr, Nb,
and Ti in the Cu-Zr-Hf-Nb-Ti alloy, creating a quaternary interfacial
composition which limits grain growth even after 1 week at ~97% of the melting
temperature. This level of thermal stability is attributed to the high levels
of chemical complexity at the grain boundary that is a consequence of the
multi-component segregation. High resolution electron microscopy of grain
boundary structure reveals that the complex grain boundary chemistry in the
Cu-Zr-Hf-Nb-Ti alloy is associated with a 44% and 32% increase in the average
amorphous complexion thicknesses found in previously studied Cu-Zr and Cu-Zr-Hf
alloys.
