  Complexions are phase-like interfacial features that can influence a wide
variety of properties, but the ability to predict which material systems can
sustain these features remains limited. Amorphous complexions are of particular
interest due to their ability to enhance diffusion and damage tolerance
mechanisms, as a result of the excess free volume present in these structures.
In this paper, we propose a set of materials selection rules aimed at
predicting the formation of amorphous complexions, with an emphasis on (1)
encouraging the segregation of dopants to the interfaces and (2) lowering the
formation energy for a glassy structure. To validate these predictions, binary
Cu-rich metallic alloys encompassing a range of thermodynamic parameter values
were created using sputter deposition and subsequently heat treated to allow
for segregation and transformation of the boundary structure. All of the alloys
studied here experienced dopant segregation to the grain boundary, but
exhibited different interfacial structures. Cu-Zr and Cu-Hf formed nanoscale
amorphous intergranular complexions while Cu-Nb and Cu-Mo retained crystalline
order at their grain boundaries, which can mainly be attributed to differences
in the enthalpy of mixing. Finally, using our newly formed materials selection
rules, we extend our scope to a Ni-based alloy to further validate our
hypothesis, as well as make predictions for a wide variety of transition metal
alloys.
