Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking. High-entropy alloys have greatly expanded the compositional space for alloy design. In this Review, the authors discuss model high-entropy alloys with interesting properties, the physical mechanisms responsible for their behaviour and fruitful ways to probe and discover new materials in the vast compositional space that remains to be explored.

High-entropy alloys

Open literature publications, in the period from 2010 to the end of January 2018, on refractory high entropy alloys (RHEAs) and refractory complex concentrated alloys (RCCAs) are reviewed. While RHEAs, by original definition, are alloys consisting of five or more principal elements with the concentration of each of these elements between 5 and 35 at.%, RCCAs can contain three or more principal elements and the element concentration can be greater than 35%. The 151 reported RHEAs/RCCAs are analyzed based on their composition, processing methods, microstructures, and phases. Mechanical properties, strengthening and deformation mechanisms, oxidation, and corrosion behavior, as well as tribology, of RHEA/RCCAs are summarized. Unique properties of some of these alloys make them promising candidates for high temperature applications beyond Ni-based superalloys and/or conventional refractory alloys. Methods of development and exploration, future directions of research and development, and potential applications of RHEAs are discussed.

Development and exploration of refractory high entropy alloys—A review

High entropy alloys: A focused review of mechanical properties and deformation mechanisms

High-entropy alloys (HEAs) are a relatively new class of materials that have gained considerable attention from the metallurgical research community over recent years They are characterised by their unconventional compositions, in that they are not based around a single major component, but rather comprise multiple principal alloying elements Four core effects have been proposed in HEAs: (1) the entropic stabilisation of solid solutions, (2) the severe distortion of their lattices, (3) sluggish diffusion kinetics and (4) that properties are derived from a cocktail effect By assessing these claims on the basis of existing experimental evidence in the literature, as well as classical metallurgical understanding, it is concluded that the significance of these effects may not be as great as initially believed The effect of entropic stabilisation does not appear to be overarching, insufficient evidence exists to establish the strain in the lattices of HEAs, and rapid precipitation observed in some HEAs sug

https://www.tandfonline.com/doi/pdf/10.1080/09506608.2016.1180020

High-entropy alloys: a critical assessment of their founding principles and future prospects

As human improve their ability to fabricate materials, alloys have evolved from simple to complex compositions, accordingly improving functions and performances, promoting the advancements of human civilization. In recent years, high-entropy alloys (HEAs) have attracted tremendous attention in various fields. With multiple principal components, they inherently possess unique microstructures and many impressive properties, such as high strength and hardness, excellent corrosion resistance, thermal stability, fatigue, fracture, and irradiation resistance, in terms of which they overwhelm the traditional alloys. All these properties have endowed HEAs with many promising potential applications. An in-depth understanding of the essence of HEAs is important to further developing numerous HEAs with better properties and performance in the future. In this paper, we review the recent development of HEAs, and summarize their preparation methods, composition design, phase formation and microstructures, various properties, and modeling and simulation calculations. In addition, the future trends and prospects of HEAs are put forward.

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Science and technology in high-entropy alloys

A hexagonal close-packed high-entropy alloy: The effect of entropy

This study reports the design and development of ductile and strong refractory single-phase high-entropy alloys (HEAs) for high temperature applications, based on NbTaV with addition of Ti and W. Assisted by CALPHAD modeling, a single body-centered cubic solid solution phase was experimentally confirmed in the as-cast ingots using X-ray diffraction and scanning electron microscopy. The observed elemental segregation in each alloy qualitatively agrees with CALPHAD prediction. The Vickers microhardnesses (and yield strengths) of the alloys are about 3 (and 3.5–4.4) times that those estimated from the rule of mixture. While NbTaTiVW shows an impressive yield strength of 1420 MPa with fracture strain of 20%, NbTaTiV exhibits exceptional compressive ductility at room temperature.

NbTaV-(Ti,W) refractory high-entropy alloys: Experiments and modeling

Simultaneous enhancement of strength and ductility in a NiCoCrFe high-entropy alloy upon dynamic tension: Micromechanism and constitutive modeling

The AlCrCuFeNi 2 high-entropy alloy with a mixed face-centered cubic (FCC) plus body-centered cubic (BCC) structure is investigated. The as-cast sample shows a double-phase-eutectic microstructure. Quasi-static and dynamic compression tests with varying strain rates of 1×10 −3 –3×10 3  s −1 at ambient temperature are performed on the current system. The dynamic yielding strength approximately presents a linear relationship with the strain rate on the designated scale. The strain hardening rates upon dynamic loading are significantly enhanced specially at the initial plastic stage. The Johnson–Cook (J–C) plasticity model is employed to model the dynamic flow behavior, and the constitutive relationship is obtained as σ = ( 855 + 1436 e 0.32 ) ( 1 + 0.004 ln e * ) .

Strain rate effects on the dynamic mechanical properties of the AlCrCuFeNi2 high-entropy alloy

The mechanical properties and microstructure evolution of dual-phase Al0.6CoCrFeNi high-entropy alloys (HEAs) upon dynamic loading are investigated. Dynamic impact tests with varying strain rates of 2800–4000 s−1 at room temperature are performed by the split-Hopkinson pressure bar (SHPB). The yielding strengths evidently increase with increasing the strain rate for the present system. The high strain-rate sensitivity (SRS) parameter is extracted from the observed stress-strain responses. A modified Johnson-Cook (J-C) plasticity constitutive model is proposed to characterize the dynamic flow behavior. Nanoscale deformation twins induced by dynamic loading, accompanied by high density dislocation substructures, realize the excellent strength-ductility combination. According to the critical stress theory, the stacking fault energy (SFE) of the FCC phase can be estimated to be about 36 mJ/m2, which is evidently lower than the overall SFE for the Al0.6CoCrFeNi HEA.

Shufang Ma

Papers

A hexagonal close-packed high-entropy alloy: The effect of entropy

NbTaV-(Ti,W) refractory high-entropy alloys: Experiments and modeling

Simultaneous enhancement of strength and ductility in a NiCoCrFe high-entropy alloy upon dynamic tension: Micromechanism and constitutive modeling

Strain rate effects on the dynamic mechanical properties of the AlCrCuFeNi2 high-entropy alloy

Mechanical response and deformation behavior of Al0.6CoCrFeNi high-entropy alloys upon dynamic loading