Properties and processing technologies of high-entropy alloys

As of now, many works have reported various preparation technologies to fabricate different forms of HEAs, such as general melting, additive manufacturing, film and coating technologies. To clearly summarize the preparation process of HEAs with different forms, we try to classify the HEAs in terms of different dimensions. As shown in figure 1, it mainly classified into three-dimensional (3D) bulk materials, two-dimensional (2D) film and sheet materials, one-dimensional (1D) fiber materials, and zero-dimensional (0D) powder materials. The 3D HEAs, mainly containing bulk alloys, are generally fabricated by alloy melting, additive manufacturing, and sintering methods, with a weight from several grams to tens of kilograms. According to thickness, the 2D HEAs also can be divided into thin films, coating, sheet, and thin strip. For 1D HEAs, it mainly refers to high-entropy wire and fiber, which is generally obtained by the drawing method and glass covered spinning method. Due to its unique performance combinations, high-entropy wire and fiber are one of the recent research hotspots. Moreover, the high-entropy powders, a kind of 0D HEA, are generally used as raw materials for powder metallurgy and additive manufacturing, as well as a novel catalyst for functional applications [40, 41].

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The unique characteristics of HEAs in thermodynamics, kinetics, structure and performance have inspired many excellent properties that can break through the limits of traditional materials. As shown in figure 2, Yan and Zhang summarized the five advantages of HEAs which have been proven by many published works [36]. Here, we want to briefly discuss the related mechanisms for these attractive performances.

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2.2.1. Overcoming the strength–ductility trade-off.

2.2.2. Low temperature ductility.

2.2.3. Thermal stability.

2.2.4. Corrosion resistance.

2.2.5. Irradiation resistance.

Compared with the conventional alloys, the composition design of HEAs is more complex and diverse. Based on the reported works, we summarize the alloy systems of HEAs that have been developed so far. Taking the 'component number' and 'frequency of research' as Y axis, respectively, 47 different alloy systems have been counted and classified by Python, as shown in figure 3. For HEAs, the five-component alloy systems account for the largest proportion, followed by quaternary alloys. Among them, CoCrFeMnNi (i.e. the Cantor alloy) has been studied and published the most times, more than 7000 times. The AlCoCrFeNi and CoCrFeNi alloy systems are also two of the most researched alloy systems. It should be noted that, each alloy system usually contains many different compositions, as well as various structures and performances. How to choose ideal alloys in such a large compositional space is a huge challenge for researchers.

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Compared with conventional alloys, the composition design of HEAs is more complex and diverse due to their entropic characteristic. In fact, in addition to relying on experimental verification, machine learning, calculation, and simulation are also effective methods for designing HEAs. Here, we propose a systematic design pathway of HEAs, namely 'trilogy of composition design', including (a) machine learning, (b) empirical models and calculations, and (c) combinatorial experimental approaches, as shown in figure 4. It is expected that 'trilogy of composition design' can help us obtain interested HEAs in a short time span and pave the way for designing and synthesizing entropic alloys with intriguing properties.

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The machine learning is generally conducted based on the reported results. Data analytics and machine learning can help effectively achieve rapid screening in vast compositional space. Steingrimsson et al have successfully predicted temperature-dependent ultimate strengths of bcc HEAs via machine learning [46]. They proposed a bilinear log model for predicting the ultimate strengths of HEAs under different temperatures, evaluating effectiveness by 21 compositions. Briefly, machine learning can help us effectively organize the data in a meaningful fashion and extract complex, hidden relationships.

For calculation and simulation, it mainly contains empirical models, first-principles calculations, and calculation of phase diagrams (CALPHAD), which can help for predicting the phase formation and properties in various conditions [47, 48]. The CALPHAD method is based on Gibbs–Helmholtz free-energy formulations to derive equilibrium-phase of each phase. The CALPHAD method allows the identification of co-existing phases as well as their compositions and volume fractions. Gorsse and Senkov have examined the limitations of CALPHAD databases applied to HEAs, and concluded that it is still sufficient to correctly predict thermodynamic properties of quaternary alloy systems [47]. Ye et al have successfully predicted the formation possibility of a (Zr_0.25Nb_0.25Ti_0.25V_0.25)C high-entropy ceramics by the first-principles calculations and thermodynamical analysis [49]. The calculation results showed that the high-entropy ceramics are thermodynamically stable above 959 K, which is consist with the experimental results fabricated by hot pressing sintering technique.

For the combinational experiments, many different combinatorial methods have been applied on the parallel preparation of HEAs, such as supergravity, 3D printing, and combinatorial thin-film materials libraries. Zhang et al have made many efforts on the parallel preparation and screening of HEAs, mainly including Al–CrFeNi–Ti [50], WTa–FeCrV [51], and Zr–Ti–Nb [52] alloy systems. For example, the parallel preparation of a ternary Ti–Nb–Zr system alloy was achieved by co-sputtering deposition method combined with physical masking, and successfully screened a bcc HEAs with excellent mechanical properties [52, 53]. Based on combinatorial thin-film materials libraries, Xing et al have also investigated the solar absorptivity of HEAs with the change of phase structural and microstructures [51]. In addition to obtaining different alloy compositions at one time, different microstructures can also be designed by combinational experiments. Li et al have applied the supergravity method to prepare the graded Al–Zn–Li–Mg–Cu alloys [54]. They found that the morphology of the alloy significantly changes from the bulk intermetallic to eutectic structures along the supergravity force direction.

Excellent compressive mechanical properties under a wide temperature range is one of the most representative advantages of HEAs. There are many salient works that have developed HEAs with high strength and good ductility, which show great application potential in a new generation of structural materials. Li et al have reported that Fe_80-x Mn_x Co₁₀Cr₁₀ (at. %) alloy systems successfully overcome the strength–ductility trade-off by regulating metastable phase [30]. Transformation induced plasticity and dual-phase strengthening are two key contributions for such breakthrough. Lei et al proposed a novel strengthen mechanism by inducing ordered oxygen complexes in HEAs [29]. They found that the tensile strength is enhanced and ductility is substantially improved when doping a model TiZrHfNb HEA with 2.0 at. % oxygen, which can be attributed to the ordered interstitial complexes changing the dislocation shear mode from planar slip to wavy slip, and promoting double cross-slip. Yang et al reported a strategy to break this trade-off by controllably introducing high-density ductile multicomponent intermetallic nanoparticles in complex alloy systems [55]. Results show that the AlCoCrFeNiTi alloy possess ultrahigh strengths of 1.5 GPa and ductility of 50% in tensile test.

Yan et al [31] have summarized an Ashby diagram to declare the general relationship between the yield strengths and tensile strains of reported HEAs with different structure, as shown in figure 5. In this work, HEAs were classified into six types, including fcc, bcc, hcp, and dual-phase structures between each other. Among them, bcc alloys show the highest strengthen, and dual-phase structure is slightly inferior, while fcc alloys show relatively low yield strengths. Amorphous alloys generally have high strengths but fail to undergo tensile deformation. Here, we would like to discuss the correlation between strength and atomic radius difference (δ) of HEAs, intermetallic and amorphous, as shown in figure 6. Overall, there is a positive correlation between strength and δ, which can be attributed to significant solid-solution strengthening caused by large lattice distortion. In general, the bcc structural alloys have a larger atomic size mismatch accompanied with higher strength, fcc structural alloys show a low size mismatch level with relatively low strength, while alloy with bcc and fcc dual phase occupy the middle region. The amorphous materials (AM) and intermetallic that exhibit high hardness and high strength generally have higher atomic mismatches. Therefore, the parameter δ can give a direct guidance to the phase structure and strength of the HEAs.

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The excellent thermal stability of HEAs can be attributed to a multi-component alloy system which inspires complex interaction between the different atoms, and significantly limits the diffusion rate in HEAs. In general, the refractory bcc-HEAs with larger lattice distortion are more stable than fcc-HEAs with small lattice distortion during long-time thermal annealing. As we all good known, larger lattice distortion causes higher level atomic interaction force which meaning higher atomic migration barrier. In this case, the bcc-HEAs generally show great potential in refractory materials. Zou et al have investigated high temperature performances of NbMoTaW thin films and small-sized pillars, which is annealed at 1100 °C for 3 d [33]. In contrast with the conventional superalloy W, the NbMoTaW HEAs shows a good microstructure stability as well as the mechanical stability, as shown in figure 7. The pure W alloy showed obvious structural instability with the morphology changing from needle-like shapes to equiaxed-crystal structures, while the post-annealed NbMoTaW alloy retained uniform needle-like morphology without obvious grain growth.

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Senkov et al have also investigated the phase structure of NbMoTaW and NbMoTaWV [70]. After annealing at 1400 °C for 19 h, results proved that alloys are stable upon heating at least up to 1400 °C. There is no evidence for an order–disorder transition in these alloys under high temperature. Zhang et al have also studied the heat-softening resistance of WTaFeCrV alloys, as shown in figure 8 [14]. It is found that the yield strength of the FeCrV(WTa)_0.1 and FeCrV(WTa)_0.2 alloys are higher than those of superalloys with temperature up to 800 °C.

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The unique characteristics of HEAs in dynamics, thermodynamics, and structure make it possible to develop special functional properties. Recently, much research on HEAs has focused on their potential applications on functional materials, such as radiation resistant materials [34], superconducting materials [71], soft magnetic materials [26], catalysts [28] and photothermal conversion materials [15]. It is worth noting that the outstanding mechanical performances of HEAs in extreme environments are also a huge driving force for making potential functional materials. Developing HEAs with a unique combination of mechanical properties and physical properties is a key point for future development.

Excellent processing and deformation ability is one of the most important factors for HEAs to achieve their industrial applications. Rolling is a direct way to obtain high-entropy sheets. In much reported research, it can be found that the performance improvements of most HEAs are based on the deformation processing and subsequent heat treatment, such as ZrNbTiAl [31], CoCrFeNi [39], and AlCoCrFeNi [19] alloy systems.

Moreover, based on the deformation processing, Zhang et al have developed a novel high-entropy material: HEA wires. Hiromoto et al [38] have successfully prepared Al_0.3CoCrFeNi wires with a diameter of 1 mm, which displayed remarkable tensile strength and ductility (1207 MPa and 7.8%) at 298 K, increasing to 1600 MPa and 17.5% at 77 K, as shown in figure 9. Furthermore, micron-level HEA wires with a diameter of 60 µm through a drawing process have also been prepared. The room temperature tensile strength of these wires reached as high as 2.8 GPa, and together with an elongation of 2.4%. Liu et al also reported a novel CoCrNi medium-entropy alloy wire with a diameter of 2 mm, and tested its low temperature mechanical properties [21]. The tensile strength of these CoCrNi wires was 1.8 GPa at liquid nitrogen temperature, with an elongation of 37.4%. The high-entropy wires with excellent comprehensive mechanical properties show great development potential in various applications.

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Lu et al have successfully developed eutectic HEAs, namely AlCoCrFeNi_2.1 alloy [72]. The novel eutectic alloys effectively solve the obstacles for their technological applications, such as inferior castability and compositional segregation of HEAs. The microstructure of AlCoCrFeNi_2.1 alloy is composed of soft fcc and hard bcc phases, which is in the form of fine lamellar fcc/B2 microstructure, as shown in figure 10. Results showed that the AlCoCrFeNi_2.1 alloy possessed pretty good compressive mechanical properties at room temperature, with a true fracture strength and ductility of 1186 MPa and 22.8%, respectively. Wani et al have investigated the cold-rolled and annealed treatment of the AlCoCrFeNi_2.1 eutectic alloy [73]. The alloy successfully suffered from a severe cold rolling with a reduction of 90%. The strength of rolled alloy reached 1800 MPa, but at the expense of elongation (∼6%). Overall, the eutectic HEAs is quite suitable for the casting process due to its good fluidity and formability, and can be readily adapted to large-scale industrial production of HEAs.

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Additive manufacturing has provided new opportunities for fabricating geometrically complex HEAs with the possibility of in situ tailoring of their microstructure features. Zhou et al have proposed a precipitation-strengthened Ni_2.1CoCrFeNb_0.2 alloy which was particularly easy to print for use in additive manufacturing [74]. The Ni_2.1CoCrFeNb_0.2 alloy showed an excellent combination of strength and ductility. Especially after heat-treatments, the tensile strength of the alloy is 1127 MPa with a tensile elongation of 17%, which is the best performance in the reported additive manufactured HEAs. Moreover, Chew et al have fabricated the CoCrFeMnNi alloy by laser aided additive manufacturing [75]. Results proved that graded microstructures were obtained, in which the dendritic columnar grains formed close to the melt-pool boundaries and transited to equiaxed grains further away from the boundaries. The mechanical properties of additive-manufactured CoCrFeMnNi alloy are better than conventionally cast HEAs, with a yield strength of 518 MPa and ultimate tensile strength of 660 MPa, which can be attributed to finer grains obtained by the additive manufacturing process. In fact, many efforts have been carried out to verify the feasibility of additive manufacturing technology to prepare HEAs [76], which is expected to open up new possibilities for the further development of additive manufacturing fabricated HEAs.

Electrodeposition is an effective method for obtaining nanocrystalline HEAs, but has seen little attention to date. In comparison to the most commonly-used techniques, such as mechanical alloying, high-pressure torsion, and sputtering-based techniques, electrodeposition offers itself as a relatively low-cost and low-energy method for nanocrystalline alloy synthesis that can be applied to various substrates. However, it is truly difficult to deposit multiple elements simultaneously, due to different potential of components and complexity of electrolytes. To date, Haché et al have developed two model ternary alloy systems to serve as starting points for the eventual development of five-element HEAs, as shown in figure 11 [77]. In this work, two important issues have been evaluated, including the ability of forming and maintaining a single solid-solution phase and standard reduction potential. Based on these evaluations, ternary NiFeCo and NiFeCr alloys were selected to investigate the electrodeposition behavior, and the ternary alloys show a nano grain size of 1–360 nm. Particularly, these alloys show enhanced thermal stability compared to pure metal and binary alloy electrodeposits, and even nanocrystalline CoCrFeNiMn fabricated by high-pressure torsion. It could be inferred that electrodeposition is a viable route towards the synthesis of strong and highly stable nanocrystalline alloys. The research route to develop suitable ternary alloys as 'stepping-stones' for future design of HEAs is worthy of expectation.

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