Multi-principal element grain boundaries: Stabilizing nanocrystalline grains with thick amorphous complexions

Abstract: 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.