Thick amorphous complexion formation and extreme thermal stability in ternary nanocrystalline Cu-Zr-Hf alloys

Abstract

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 grain boundary 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, followed by detailed characterization of the grain and grain boundary structure. All of the ternary alloys exhibited exceptional thermal stability and remained nanocrystalline even after two weeks of annealing at this extremely high temperature. Even though some carbides do form and grow in these alloys during milling and annealing, the thermal stability is mainly attributed to the formation of thick amorphous intergranular films at high temperatures. Our results show that higher levels of dopant segregation and thicker boundary films exist in the ternary alloys, as compared to the binary alloys with similar 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 will maximize the previously reported toughening effect.