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

TL;DR

This study demonstrates that increasing chemical complexity at grain boundaries in nanocrystalline alloys leads to thicker amorphous complexions and significantly enhances thermal stability, with potential implications for alloy design.

Contribution

It provides experimental evidence that multi-component segregation at grain boundaries results in thicker amorphous complexions and improved stability in nanocrystalline alloys.

Findings

Amorphous complexion thickness increased by 44-32% in complex alloys.

Grain size stabilized at 63 nm after one week at high temperature.

Multi-component segregation enhances thermal stability of nanocrystalline grains.

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.