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 compare the influence of increased chemical complexity on thermal stability of the nanocrystalline microstructure, in addition to grain boundary structure. Significant boundary segregation is observed for Zr, Nb, and Ti in the Cu-Zr-Hf-Nb-Ti alloy, creating a quaternary interfacial composition which limits grain growth even after 1 week at ~97% of the melting temperature. This level of thermal stability is attributed to the high levels of chemical complexity at the grain boundary that is a consequence of the multi-component segregation. High resolution electron microscopy of grain boundary structure reveals that the complex grain boundary chemistry in the Cu-Zr-Hf-Nb-Ti alloy is associated with a 44% and 32% increase in the average amorphous complexion thicknesses found in previously studied Cu-Zr and Cu-Zr-Hf alloys.

High-entropy grain boundaries

“…We subsequently showed that HEGBs can be utilized to stabilize nanocrystalline alloys (nanoalloys) at high temperatures 9 . While that original work 9 , particularly the idea of stabilizing nanoalloys with high-entropy effects, has caught substantial interests [10][11][12][13][14][15][16][17][18][19][20] , a rigorous thermodynamic theory of HEGBs has not been presented. This article elaborates the relevant concepts and present a complete thermodynamic framework for the first time, and subsequently discusses the future perspective.…”

Computing grain boundary “phase” diagrams

“…[81,117] A most recent report also showed the formation of a thick amorphous complexion in Cu-Zr-Hf-Nb-Ti and similar quinary nanoalloys. [91] GB phase-like transitions of HEGBs are scientifically interesting. Modeling such HEGBs, including computing GB diagrams to represent their thermodynamic stability and properties, is worth pursuing for future studies.…”

“…The discovery of interfacial phase‐like behaviors provided new insights into the understandings of a spectrum of long‐standing scientific mysteries, for example, origins and atomic mechanisms of activated sintering of ceramics and refractory metals, [ 5–11 ] liquid metal embrittlement of Ni–Bi and Al–Ga [ 52,53,63–65 ] as well as the classical GB embrittlement of Bi versus S‐doped Ni (Figure 2), [ 52–54 ] and abnormal grain growth in Al 2 O 3 and Ni–S. [ 54,56,57 ] IGFs and other GB complexions are also known to affect the toughness, strength, fatigue, and wear resistance of Si 3 N 4 , SiC, and Al 2 O 3 and other ceramics, [ 9,23,24,27,44,66,67 ] the hot strength and creep and oxidation resistance of various structural ceramics, [ 68–76 ] superplasticity of zirconia, [ 77 ] grain growth and mechanical properties of WC‐based cermets, [ 55,78–80 ] the stability and mechanical properties of nanocrystalline alloys, [ 81–91 ] corrosion of synroc, [ 92 ] the electrical resistance of ruthenate thick‐film resistors, [ 93 ] the coercivity of Nd–Fe–B magnets, [ 94 ] the nonlinear I–V character of ZnO‐based varistors, [ 1,42,43 ] the critical current of YBCO superconductors, [ 95 ] the ionic conductivity of solid electrolytes, [ 96–98 ] and performance of various battery electrode materials, [ 99–102 ] amongst other structural and functional properties. [ 12,13,27,50,103 ]…”