The Design and Development of Eutectic High Entropy Alloys with Excellent Mechanical and Functional Properties


Student thesis: Doctoral Thesis

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Award date22 Jul 2020


Eutectic high entropy alloys (EHEA) are a class of alloys based on the multi-principal element system or high entropy alloy system which shows a eutectic microstructure, i.g. the lamellar microstructure. By combining one hard/brittle phase and soft/ductile phase, EHEAs can reach a balanced set of strength and ductility. To date, many reported EHEAs have shown superior mechanical properties and thus attracted a great deal of attentions. However, at the fundamental level, several issues are still open for EHEAs, such as the lack of phase diagram, the poor understanding of the deformation mechanisms.

In relatively simple binary or ternary eutectic systems, the eutectic compositions can be derived either from the governing thermodynamic Schroeder-Van-Laar equations or directly from the phase diagrams. However, given the absence of a phase diagram in multicomponent systems, it remains a non-trivial task to locate a eutectic composition. To address this issue, we first proposed a strategy that enables an efficient way to design EHEA. Through searching the data reported, we firstly classified the constituent elements in an EHEA into two categories: the high entropy base elements such as Co, Cr, Cu, Fe, Ni; the eutectic forming elements such as Nb, Al, Mo, Ti, etc. By doing this, one EHEA can be generally reformulated by a sum of eutectic binaries. In turn, this formulation can enable us to design EHEAs, as validated by a number of EHEAs we developed.

To study the mechanical behavior of EHEA, one CoNiFeNb0.5 eutectic alloy was carefully studied. The nanocomposite possesses a high volume fraction (> 50%) of a cubic Laves phase but shows superb strength and excellent malleability at room temperature. This high mechanical performance results from the formation of an in-situ nano-scale lamellar structure that joins the hard cubic Laves phase and soft medium entropy face-centered cubic(FCC) phase. When the size of the lamellar structures is tuned below a critical value, this nanocomposite exhibits strong and sustainable strain hardening, leading to a fracture strain over 20% and fracture strength over 3.5 GPa in conventional compression. The mechanism for the unusual strain hardening in the Laves-phase rich nanocomposite is explored afterward with micromechanical experiments and theoretical modeling, which unveils a size-controlled transition in the plasticity mechanism from dislocation slip to twinning in the nano-scale Laves phase.

For the complex Laves phase contained EHEAs, brittle fracture is also an important issue which induce strain softening and failure. Conventional Laves phase is extremely brittle at room temperature. However, through a systematic study, we showed that a quinary Co-CrFe-Ni-Nb Laves phase in a eutectic high entropy alloy (EHEA) can be plastically deformed at room temperature when its size is refined below 200 nm. Based on our current results, we propose a general micromechanical model that not only explains the brittle-to-ductile transition in the high entropy Laves phase but also enables the extraction of its fracture toughness even when the high entropy Laves phase in a dual-phase microstructure.

Finally, we would also like to demonstrate EHEAs could show great potentials in functional applications, such as working as electrochemical catalysis. For oxygen evolution reaction (OER) catalysts, the figures of merit include high reaction activity, high stability, and low cost. By dealloying the FeCoNiCrNb0.5 EHEA, we obtained a porous structure with amorphous high entropy oxide ultrathin films wrapped around the nano-sized intermetallic ligaments. This porous structure exhibits an extraordinarily large active surface area, fast dynamics, and superb long-term durability, outperforming the existing alloy- and ceramic-based OER electrocatalysts. The outcome of this research suggests that the paradigm of “high entropy” design could be also used to develop high-performance catalytic materials.