  Building on the recent discovery of tough nanocrystalline Cu-Zr alloys with
amorphous intergranular films, this paper investigates ternary nanocrystalline
Cu-Zr-Hf alloys with a focus on understanding how alloy composition affects the
formation of disordered complexions. Binary Cu-Zr and Cu-Hf alloys with similar
initial grain sizes were also fabricated for comparison. The thermal stability
of the nanocrystalline alloys was evaluated by annealing at 950 {\deg}C (>95%
of the solidus temperatures), followed by detailed characterization of the
grain boundary structure. All of the ternary alloys exhibited exceptional
thermal stability comparable to that of the binary Cu-Zr alloy, and remained
nanocrystalline even after two weeks of annealing at this extremely high
temperature. Despite carbide formation and growth in these alloys during
milling and annealing, the thermal stability of the ternary alloys is mainly
attributed to the formation of thick amorphous intergranular films at high
temperatures. Our results show that ternary alloy compositions have thicker
boundary films compared to the binary alloys with similar global dopant
concentrations. While it is not required for amorphous complexion formation,
this work shows that having at least three elements present at the interface
can lead to thicker grain boundary films, which is expected to maximize the
previously reported toughening effect.
