Novel Frontiers in High-Entropy Alloys

Abstract

There is little doubt that there is significant potential for high-entropy alloys (HEAs) in cryogenic and aerospace applications. However, given the immense design space for HEAs, there is much more to be explored. This review will focus on four areas of application for HEAs that receive less attention. These focus areas include joining technologies, HEA nanomaterial synthesis, catalysis, and marine applications. The performance of HEAs as a filler metal for welding and brazing as well as their performance as a welded/brazed base metal will be discussed. Various methods for synthesizing HEA nanomaterials are reviewed with specifically highlighted applications in catalysis and energy storage. HEA catalysts, in particular, will be discussed in detail regarding their effectiveness, selectiveness, and stability. Marine applications are explored given the inherent corrosion resistance of HEAs as well as superior antifouling properties that make HEAs an intriguing marine-ready material.

1. Introduction

High-entropy alloys (HEAs) have existed for less than two decades, but their astounding breadth of impressive properties has spurred many studies to understand their properties and explore potential applications. HEAs are typically alloys that contain five or more elements in concentrations of 5–35 at% each. There are four core effects that govern their properties and separate them from other alloys: the high-entropy effect, sluggish diffusion kinetics, severe lattice distortion, and the cocktail effect [1,2,3]. The high-entropy effect refers to high mixing entropy ($\Delta S_{mix}$) which can be described using Equation (1), where $R$ $\mathrm{is}\mathrm{the}\mathrm{ideal}\mathrm{gas}\mathrm{constant}\mathrm{and}{x}_i$ is the mole percentage of element i.

$$\Delta S_{mix}=-R{{\displaystyle \sum }}_{i=1}^n{x}_{i}ln{x}_i$$

(1)

Note that for the HEA formulation to be stable, the enthalpy and the Gibbs free energy of mixing should also be taken into account. The enthalpy of mixing was found empirically by Zhang et al. to be between −10 and +5 kJ/mol in order to form a stable solid solution [4]. The equation for determining the enthalpy of mixing can be found in Equation (2) [5].

$$\Delta H_{mix}={{\displaystyle \sum }}_{i=1,i\ne j}^{n}4\Delta H_{AB}^{mix}{x}_i{x}_j$$

(2)

where $\Delta H_{AB}^{mix}$ is the enthalpy of mixing between components A and B (i.e., elements i and j). In general, the phase stability of a multicomponent system can be estimated using the Gibbs free energy.

$$\Delta G_{mix}=\Delta H_{mix}-T\Delta S_{mix},$$

(3)

where the delta G, delta H, and delta S are the change in Gibbs free energy, enthalpy, and entropy of mixing, respectively, and T is the thermodynamic temperature. For a stable single phase, the change in Gibbs free energy should be negative, while a positive change means a phase separation. For a single-phase solid solution, the delta Hmix is relatively small; therefore, the configuration entropy is the dominant factor for the phase stability of a multicomponent system at a sufficiently high temperature. Integrating more species in a near-equal molar can significantly increase the entropy and decrease the overall Gibbs free energy. Equation (4) defines a dimensionless parameter that, according to the studies of Zhang et al. [6] and Yang and Zhang [7], must be ≥1.1 for entropy to be the dominant thermodynamic driving factor for solid solution formation.

$$\mathsf{\Omega }=\frac{T_m\Delta S_{mix}}{\left|\Delta H_{mix}\right|}$$

(4)

where $T_m$ is the average melting point of the constituent elements as determined using the rule of mixtures.

HEA sluggish diffusion kinetics in a solid state is partially influenced by the severe lattice distortion caused by multiple elements with different atomic radii occupying the same lattice space. Keep in mind that most HEAs still have typical metal structures such as FCC [8,9,10,11], BCC [12,13,14,15], and occasionally HCP [16,17,18], as shown in Figure 1. Given that the constituent elements can have widely different radii, the lattice is more distorted than alloys with similar radii or has fewer elements occupying the same lattice space.

The lattice structure of HEAs is one of the characteristics that distinguishes it from bulk metallic glasses [19,20,21]. Lastly, the cocktail effect is an umbrella term used to describe the phenomenon when the properties of a material are determined by a multitude of factors dependent on the composition of the alloy on an elemental and microstructure level. According to the cocktail effect, even minor changes in the composition of a HEA can greatly affect the mechanical and chemical properties of the HEA. One of the mechanical consequences of the cocktail effect is that most HEAs to date report some level of solid solution strengthening [12,21,22].

As a result of the elemental requirements for classifying a HEA, the design space for HEAs consists of millions of possible combinations, and that is only considering the 38 transition metals. Additionally, with a massive design space, the properties of HEAs vary widely and already are competitive with more traditional alloys and superalloys. Some of the properties exhibited by HEAs include, but are not limited to, excellent high-temperature mechanical properties [23,24,25,26], cryogenic stability [12,27,28,29], corrosion resistance [30,31,32], and many more. Additionally, the properties can be further modified for superior performance using high-entropy superalloys [33,34,35,36,37], high-entropy steels [38,39], and high-entropy intermetallics [40,41,42], which use similar strengthening strategies as nickel superalloys such as Inconel and Monel [43,44,45,46]. Specific properties can be developed in the design of HEAs such as magnetism [47], hydrogen storage [48], and reinforcing composites [49].

With several impressive properties of HEAs, the possible applications are equally numerous. In this mini-review, we will discuss some of the emergent applications that are less frequently discussed: welding and brazing, nanomaterial synthesis, catalysis, and marine applications. We hope that this review will stimulate the relevant study in these active fields.

2. Welding and Brazing

2.1. Welding

Given that many HEAs are sought after for their structural potential and engineering applications, the welding of HEAs is of prime importance. The essential impact of welding HEAs has been significant in researching how the alloys behave in normal and extreme conditions including high temperatures, cryogenic temperatures, nuclear, and marine environments (discussed in Section 5) [50]. Stainless steels have received much more research in these particular environments, but HEAs are quickly making their way to the front of the line because of their corrosion resistance, ductility, and high weldability [7]. Most of the similar welding work has been performed in the AlCoCrFeNi alloy system. The CrMnFeCoNi alloy system, one of the first HEAs [1], is one of the other most popular alloys for welding research. Both alloy systems can be classified as 3D transition metal HEAs, which have received considerable attention since their discovery two decades ago [51].

Welded AlCoCrFeNi-based HEAs tend to perform favorably, often experiencing little to no welding defects and an increase or minimal decrease in yield strength and ultimate tensile strength (UTS) [30]. For instance, Shen et al. reported that no defects were observed when welding as-cast AlCoCrFeNi_2.1 eutectic HEA using gas tungsten arc (GTA) welding. The HEA melted three times before welding, but no other work was performed prior to welding. There was a 28% increase in yield strength due to the refinement in the lamellar structure in the fusion zone (FZ) and less than a 2% decrease in UTS as a result of welding. Elongation, however, reduced from 20.6% to 13.0% [52].

When introducing Cu into the AlCoCrFeNi system, some of the welding challenges with HEAs become apparent. Martin et al. welded Al_xCoCrCu_yFeNi HEA using GTAW and linked Cu segregation to hot cracking in fusion welds when y > 0.1. When x ≥ 0.5, this HEA forms a Cu-rich FCC matrix with BCC dendrites, and the BCC phase becomes more dominant as Cu composition decreases. When there is little to no Cu in the HEA, cracking occurs due to brittle intergranular cracking in the high-hardness (>500 HV) BCC structure. However, this is mitigated when x ≤ 0.5 and y = 0.1 by promoting the formation of the more ductile FCC structure [53,54]. Lin et al.’s study confirmed Martin et al.’s findings, where friction stir-welded Al_0.3CoCrCu_0.3FeNi was found to have improved ductility due in part to the partially recrystallized microstructure in the stir zone. The yield strength of the as-welded joint was 50 MPa higher than the base metal, with the stir zone exhibiting high yield, tensile strength, and high ductility [55]. As demonstrated by Lin et al. [54] and Martin et al. [53], some welding processes can disrupt the single-phase equilibrium of some HEA compositions.

The CrMnFeCoNi HEA system, in sharp contrast, tends to have more issues with similar welding compared to the AlCoCrFeNi system. Oliviera et al. reported that as-rolled CrMnFeCoNi HEA sheets were welded with little defects using GTA welding. However, there was a significant decrease in hardness from the base metal into the heat-affected zone (HAZ) and fusion zone (FZ). Additionally, the yield strength and ultimate tensile strength decreased from 587 MPa and 943 MPa to 247 MPa and 519 MPa, respectively. Part of the reduced strength was grain coarsening in the HAZ and FZ [55,56].

The CrMnFeCoNi HEA system, in sharp contrast, tends to have more issues with similar welding compared to the AlCoCrFeNi system. Oliviera et al. reported that as-rolled CrMnFeCoNi HEA sheets were welded with little defects using GTA welding. However, there was a significant decrease in hardness from the base metal into the heat-affected zone (HAZ) and fusion zone (FZ). Additionally, the yield strength and ultimate tensile strength decreased from 587 MPa and 943 MPa to 247 MPa and 519 MPa, respectively. Part of the reduced strength was grain coarsening in the HAZ and FZ [55,56].

The CrMnFeCoNi HEA system, however, has shown to have more favorable welded properties when using 308L and 410 stainless steel filler metals [57,58]. Additionally, adding an additional element such as Cu [59] or Al [60] was found to enhance the weldability or strength of CrMnFeCoNi HEAs. Interestingly, Nam et al. demonstrated CrMnFeCoNi HEAs had better weldability when involved in a pseudo-dissimilar welding study between cast and rolled CoCrFeMnNi HEAs. The dissimilar weld from the cast side of the weld exhibited larger dendrite spacing and dendrite packets, leading to greater tensile properties at cryogenic temperatures [61].

2.2. Dissimilar Welding

As for dissimilar welding cases, dissimilar welding with stainless steels is very popular. In two studies, CrMnFeCoNi HEA was laser welded to 316 [62] and duplex stainless steel [40,63]. Interestingly, in both studies, welding did not produce any simple (two–three-element) intermetallic compounds (IMCs). In some dissimilar welding studies, such as steel–aluminum welding [64,65,66], IMCs are detrimental to welding performance and are always a cause for concern due to their brittle nature. Of course, the controlled precipitation of IMCs, carbides, oxides, and other hard phases can increase strength, creep resistance, and other mechanical properties, but this is typically performed either using special heat treatment or externally introducing the hard phase into the base metal using welding [67,68,69,70]. In Adomako et al., IMCs were not formed in the joint between CrMnFeCoNi and duplex stainless steel; however, CrMn oxides were shown on the HEA side of the HEA, but not the duplex stainless steel side [63].

In the same HEA system, CoCrFeNi, CoCrNi, and CoNiV were laser welded to 304 stainless steel without defects. The CoNiV-304SS weld, in particular, had excellent strength–ductility synergy with a UTS of 686 MPa and 28.9% elongation. The fusion zones of all joints were relatively well-mixed with a very clear drop-off in certain elements when transitioning from the fusion zone into the base metal. The grains on the 304SS side of the fusion zone were columnar and grew more refined and equiaxed as they became closer to the HEA [71].

The AlCoCrFeNi HEA system also exhibits dissimilar welding potential. For example, Arab et al. [72] welded AlCoCrFeNi to 6061 Al using an explosive welding technique, albeit with some cracking observed due to the high-velocity collision.

2.3. Brazing

At the time of writing this article, there has been somewhat limited research on brazing HEAs, which makes sense given the age of HEA technology. However, the currently existing efforts are quite fascinating. Some HEAs have been brazed with conventional nickel-based brazing filler metals (FMs) such as BNi-2 and MBF601 (

Table 1. Composition of commercial fillers used in Lin et al. for reference. Values are listed in at%.

Material	Ni	Cr	B	Si	Fe	P	C	S	Ti	Al	Zr	Other
BNi-2	Bal.	7	3.12	4.59	3	0.02	0.06	0.02	0.05	0.05	0.10	0.6
MBF601	Bal.	16	0.5	1.5	32	6	0	0	0	0	0	1.5

). In the case of Li et al. [73], Al_0.3CoCrFeNi was brazed to FGH98 using BNi-2. Like with nickel superalloys, boron diffused into the HEA substrate to form Cr-rich boride and Ni-rich HEA. However, borides and voids in the FZ disappeared as the brazing temperature increased. Intergranular Cr-rich borides and carbide precipitation in FGH98 increased as the temperature reached 1090 and 1110 °C, respectively. The max strength (454 MPa) was achieved at 1070 °C for 10 min.

In another study, CoCrFeMnNi HEA was IR brazed to 316 stainless steel using BNi-2 and MBF601 [74]. The borides formed in the joint remained a weakness in the joint when brazed with BNi-2. Likewise, the joints brazed with MBF601 fractured at the phosphides that formed along the grain boundaries in the HEA. These studies demonstrated that, like nickel superalloys that share many of the same applications and properties of HEAs, such as those in the AlCoCrFeNi and CrMnFeCoNi systems, care must be taken to ensure that the boron and other melting point depressants (MPDs) used in traditional brazing FMs are diffused sufficiently into the base metals to avoid boride and other unfavorable compounds [75,76].

Some novel FMs have emerged in the last few years that allow HEAs to be brazed without MPDs. For example, Lei et al. brazed Al_0.3CoCrFeNi using Ni/Nb/Ni interlayers, resulting in the dissolution of the HEA into the Ni-Nb liquid and reaching 592 MPa (95% of the base metal strength). The strength was attributed to proeutectic γ cellular growth and eutectic γ lamellae. Rupture also primarily occurred in the softer proeutectic γ phase [77]. In another study, Song et al. used graphene nanoplate-reinforced AgCuTi filler metal to braze SiC to Al_0.3CoCrFeNi. The addition of only 0.3 wt% graphene nanoplates and the active element Ti impeded heavy diffusion towards SiC, which would have formed detrimental brittle compounds. TiC reinforcements helped refine the microstructure, relieve residual stress, and achieve shear strength of 36.7 MPa [78].

2.4. Brazing Filler Metals and Welding Interlayers

A significant portion of the progress made in joining technologies involving HEAs is in the development of HEA brazing FMs and welding FMs. As previously stated, HEAs have a massive design space and innumerable possibilities. This massive design space has allowed for several FMs to be developed to accommodate various dissimilar brazing/welding combinations. A commonality among several studies on HEA FMs is that at least one of the HEA constituent elements is one of the principal elements in one or both base materials, as seen in Figure 2. Sharing a constituent element with the base metal is one way to ensure metallurgical compatibility; however, this need not be a requirement [79]. The underlying requirement for HEA FMs is to avoid forming IMCs using welding or brazing processes.

In another study, 6061 Al was welded to UNS S33207 duplex stainless steel using Fe₅Co₂₀Ni₂₀Mn₃₅Cu₂₀ HEA. Interestingly, Fe₅Co₂₀Ni₂₀Mn₃₅Cu₂₀ HEA FM was originally developed and tested by Gao et al. and Bridges et al. [80] for nickel alloys, but it was proven effective by Mohan et al. as a welding filler metal for 6061 Al and UNS S33207 duplex stainless steel. The HEA FM successfully hindered the formation of Fe-Al IMCs and successfully achieved high strength (237 MPa) and ductility. In previous studies, successful steel–aluminum welding was achieved when the Fe-Al IMC layer was <8 µm [81]. The limiting factor in the strength was attributed to the formation of Al₃V IMCs, where the vanadium diffused from the steel [82].

In a different study, an Al_0.5FeCoCrNi interlayer facilitated the spot welding of 6061-T6 aluminum to St-12 steel. With the addition of the interlayer, complex IMCs were formed instead of the simple Fe-Al IMCs that limit the effectiveness of steel–aluminum welding. The complex IMCs formed from the reaction with the HEA interlayer prevented cracking on the Al side but increased the tendency of hot tearing. Failures were attributed to hot tearing, according to Azhari-Saray et al. [83].

GH3536 and SS304 were brazed using eutectic Co₂₅Fe₂₅Mn₅Ni₂₅Ti₂₀ HEA filler at 1180° for 10 min. Few Ti-based intermetallic compounds were observed. The maximum strength was 568 MPa [84].

Another interesting method for which HEAs have been gaining prominence is ceramic–metal brazing. Ceramic–metal brazing is highly desirable for high-temperature electronics and high-temperature aerospace applications. Unfortunately, both conventional Ag-based and Ni-based FMs present obstacles pertaining to high-temperature suitability and formation of IMCs, respectively [85,86]. With the versatility and design space for HEAs, it is possible to overcome these obstacles in addition to developing an FM with high-temperature stability and favorable kinetic properties.

TiAl was brazed to Ti₂AlNb using TiZrHfCoNiCu HEA FM with a shear strength of 157 MPa at RT and 123 MPa at 650 °C. The HEA reacted at both interfaces to produce other intermetallic and solid solution phases [87].

HEA FMs need not be composed of a single alloy prior to welding or brazing, as demonstrated by Wu et al., who used an Al/FeCo/NiCr multilayer structure with the assistance of Ni/Al nano-multilayer films to braze Al_0.1CoCrFeNi HEA to 304 stainless steel. In the study, a maximum strength of (156.8 MPa) was achieved when 40 MPa of external pressure was applied. Furthermore, the produced microstructure consisted of BCC NiAl due to the exothermic reaction of the nano-multilayer film and disordered FCC + BCC AlCoCrFeNi from the Al/FeCo/NiCr multilayer structure [88].

SiC-based ceramics are among the most popular substrates for investigating ceramic–metal. Note that with SiC and other ceramics, brazing and soldering are used to prevent unfavorable reactions between the ceramic and the second base metal. HEA FMs have been investigated to help prevent this. The sluggish diffusion kinetics make HEAs extremely attractive as a brazing FM and/or diffusion barrier, as illustrated in Figure 3. For example, Wang et al. brazed ZrB₂-SiC ceramic to Nb using CoFeNiCrCu FM at 1160 °C for 60 min, with a maximum strength obtained of 216 MPa at room temperature and 94 MPa at 650 °C [77]. This is a substantial success considering that it is difficult to achieve shear strength above 100 MPa even at room temperature [41,42,43,44]. Luo et al. were also successful in brazing SiC to Zr using CoCrFeNiCuSn + Cu foam at 1040 C for 20 min with a shear strength of 221 MPa at RT and 207 MPa at 600 °C, demonstrating substantial high-temperature strength [89]. Zhao et al. were also successful in brazing silicon carbide fiber/SiC composites to GH536 brazed using CoFeNiCrCu filler, which is of interest in the aerospace industry [90]. It should be noted that a common occurrence in brazing SiC or other carbon-containing substrates with HEAs is that carbides of Cr and Ti are very likely. However, if the carbide formation is controlled, it can be beneficial, as in the case of Liu et al. and Hu et al. [91,92].

For some applications, a diffusion barrier would still be beneficial such as in Way et al., who used a ZnGaCu-(AuSn) HEA to join skutterudite (CoSb_2.75Sn_0.05Te_0.20) to a copper electrode with a 0.7-micron Ni diffusion barrier applied to the skutterudite. The Ni diffusion barrier was applied to skutterudite to prevent unfavorable IMC formations with the Cu in the HEA FM. The Ni diffusion barrier most likely also helped with the HEA compatibility with the skutterudite [93].

In summary, HEAs demonstrate excellent weldability and can be readily brazed. When it comes to metal–metal joining, it can be seen that most of the HEA FM choices can form solid solutions with the base metal and vice versa. Hence, various HEAs have robust uses as welding interlayers and brazing FMs by avoiding several of the obstacles associated with existing dissimilar welding and brazing techniques. In the case of ceramic brazing, as the filler material, HEAs display extensively designing freedom for brazing different base materials. HEAs effectively join ceramics to metals or other substrates while preventing unfavorable reactions between the base materials. The criteria for HEAs to (usually) have five or more elements also enables them to have high compatibility with other alloys and nonmetallic materials.

3. Nanomaterial Synthesis

The synthesis of high-entropy alloy (HEA) nanomaterials presents unique challenges due to the inherent complexity of these materials, which arise from their high-entropy effect. The main target of HEA nanoparticle synthesis is to produce nanoparticles with uniform composition, single-phase structures, and controlled size and dispersion. The high-entropy effect can result in difficulties when controlling oxidation, maintaining single-phase structures, and avoiding multiphase formations. To address these challenges and meet the increasing demand for efficient, single-phase, and high-purity HEA nanomaterials, researchers have developed various synthesis techniques. These methods produce a range of nanostructures, from individual nanoparticles to nanoporous structures embedded in bulk materials. In this section, we will delve into the various approaches used to synthesize HEA nanomaterials, highlighting their advantages, limitations, and potential applications.

We begin our discussion with techniques that primarily yield individual nanoparticles, including carbothermal shock, mechanical alloying, microwave heating, wet chemistry, laser ablation synthesis in liquid, and other unique methods. Each of these methods offers distinct advantages in terms of process control, scalability, processing time, amount yielded, and the ability to tailor the properties of the resulting nanoparticles. However, they also come with inherent limitations, such as potential contamination, energy consumption, and the need for post-processing steps.

Following our exploration of nanoparticle synthesis, we will examine alternative approaches that result in nanoporous structures within bulk HEA materials. These techniques offer the advantage of creating interconnected porous networks, which can enhance properties such as surface area and catalytic activity. We will discuss the most notable methods for creating such structures, with a particular focus on a recently developed technique that has demonstrated promising results for producing nanoporous bulk materials.

3.1. Carbothermal Shock

The carbothermal shock method is a synthesis technique for producing high-entropy alloy nanoparticles. The process involves mixing metal precursors with a carbon source and heating the mixture rapidly in a furnace to induce a sudden thermal shock. The method is versatile and can be used to produce a wide range of HEA nanoparticles with different compositions and properties.

In their study, Xu et al. investigated the production of quinary high-entropy alloy nanoparticles using two methods: carbothermal shock and pyrolysis. They dissolved polyacrylonitrile in N,N-dimethylformamide to create a 10 wt% solution for the electrospinning solution. The electrospun precursor was then treated in a blast air oven and temperature-programmed furnace to obtain aligned carbon nanofiber substrates. The precursor solutions were made by dissolving metal chlorides and nitrates in ethanol and deionized water, respectively. By adding these solutions to CNFs and guanine mixtures, the authors synthesized HEA-NPs using carbothermal shock and pyrolysis techniques. The samples were named according to the metal chloride or nitrate used. The Xu et al. study offers a detailed account of HEA-NP synthesis, which could be valuable to researchers creating new materials with better properties. The authors’ use of various metal chlorides and nitrates in precursor solutions and their application of different synthesis techniques could give insights into developing HEA-NPs with unique structures and compositions [94].

Yao et al. provided a detailed account of their novel carbothermal shock synthesis method for producing high-entropy alloy nanoparticles. The process involved flash heating and cooling of metal precursors on an oxygenated carbon support, reaching temperatures of approximately 2000 K, shock durations of around 55 ms, and ramp rates on the order of 10⁵ K/s. The rapid heating and cooling cycles promoted particle “fission” and “fusion” events, which resulted in uniform mixtures of multiple elements, and enabled the formation of crystalline solid-solution nanoparticles by facilitating kinetic control over the thermodynamic mixing regimes [95]. Although carbothermal shock is an ultrafast synthesis, it is reasonable to expect that a small amount at gram levels can be processed by taking the safety issue into account.

3.2. Mechanical Alloying

In the mechanical alloying technique, powders of different elements are placed inside a high-energy ball mill, where they are subjected to intense grinding and impact. The process is performed for tens of hours. This action causes the elements to repeatedly fracture and weld, resulting in a uniform blend of the constituent elements, as seen in Figure 4. The final product is a homogenous mixture, forming nanoparticles with unique properties ideal for various applications. Mechanical alloying promotes improved compositional homogeneity by breaking down particle agglomerates and facilitating the diffusion of atoms. This method also allows for precise control over the microstructure and grain refinement, resulting in fine-grained structures with enhanced properties. Furthermore, mechanical alloying is effective at achieving a uniform blend of alloying elements, even when positive and negative enthalpies of mixing are involved. With appropriate processing parameters, hundreds of gram-level nanoparticles can be fabricated using mechanical powdering.

In their study, Nam et al. demonstrated a detailed synthesis method for producing NbMoTaW refractory high-entropy alloy films using a novel powder deposition system with mechanically alloyed powders. The authors first fabricated equimolar NbMoTaW refractory high-entropy alloy (RHEA) powders with mechanical alloying, using an attrition mill at 500 rpm for 36 h under an Ar atmosphere, with ball-to-powder ratios of 10:1 and 3 wt% stearic acid as a process control agent. After milling, the powders were heat-treated at 500 °C for 20 min to remove the stearic acid. The as-milled RHEA powders were then placed in a powder cartridge, and a Si substrate was heated to 70 °C to form a single body-centered cubic phase of the RHEA film. Finally, the 36-hour-milled NbMoTaW RHEA powders were sprayed onto the Si wafer using gas pressure under low vacuum, resulting in a film with an area of 10 mm × 10 mm and a thickness of 1–2 μm [96].

Cheng et al. explored the preparation of FeCoCrNiAl 3D transition metal high-entropy alloy and HEA-based nanocomposite powders with tungsten carbide diffusion distribution using high-energy ball milling. Their study demonstrated that ball milling synthesizes BCC-HEA after 10 h, with WC particles adhering to the HEA matrix powder’s surface. As the ball-milling duration increased, BCC gradually transformed into the FCC phase, generating FCC-BCC biphasic HEA-based WC nanocomposite powders. The authors observed that the dynamic process between reciprocal cold welding and fracture ensured stable powder size and suppressed WC particle agglomeration. Furthermore, the intrinsic properties of the HEA acted as catalysts for the formation of spherical composite powders as the ball-milling time increased, demonstrating the potential for improved composite materials in various applications [97].

Zhang et al. synthesized FeCoNiCrAl high-entropy alloy nanoparticles using a high-energy ball milling technique with Fe, Co, Ni, Cr, and Al powders in an equal molar ratio. XRD analysis confirmed that the as-prepared samples had a single-phase, face-centered cubic structure. The SEM images showed that the as-prepared HEA nanoparticles had an average particle size of 50–100 nm [98].

3.3. Microwave Heating

Microwave heating is a method for heating materials using electromagnetic waves. Microwave radiation causes rapid molecular vibration and localized heating, which can trigger nucleation and growth to form nanoparticles.

Qiao et al. produced a scalable method for HEA nanoparticle synthesis using microwave heating. The microwave-heating method uses partially reduced graphene oxide film (rGO-570) as a model substrate, maintaining a balance between functional group defects for microwave absorbability and thermal conductivity. This balance enables uniform heating of metal salt precursors, which decompose into liquid metals and reduce the rGO-570. For synthesizing FeCoNiPdPt HEA-NPs, 0.1 mmol of each metal salt is dissolved in deionized water to prepare the mixed precursor. The precursor is drop cast onto the rGO-570 film, dried in ambient conditions, and sealed in a glass bottle filled with argon. The bottle is heated in a microwave oven for 10 s, producing HEA-NPs. Alternative carbon substrates, such as carbon nanofibers and carbonized wood, can induce temperatures of >1400 K, providing size control of HEA-NPs. This method is compatible with the roll-to-roll process for scalable nanomaterial manufacturing [99], with the potential to synthesize nanoparticles at kilogram levels.

Tang et al. developed a novel synthesis method for preparing porous CoCrFeNiMo high-entropy alloy (HEA) catalysts for electrochemical water splitting. The authors first blended Co, Cr, Fe, Ni, and Mo powders using a planetary ball mill to achieve homogeneity. Next, they added Mg powders as a space holder agent to the mixture, followed by cold pressing the blended powders into green compacts. The green compacts were then sintered using microwave heating in a high-purity argon atmosphere to prevent oxidation. The resulting CoCrFeNiMo-xMg HEA catalysts exhibited a highly porous structure with abundant stacking faults and twins, which increased the active specific surface area of the catalysts and exposed more active sites [100].

3.4. Wet Chemistry

Wet chemistry, in the context of HEA nanoparticle synthesis, involves the use of chemical reactions that occur in a liquid medium to form nanoparticles. This process typically involves the reduction of metal salts using a reducing agent and the use of stabilizing agents to prevent agglomeration of the nanoparticles, as seen in Figure 5. The properties of the resulting HEA nanoparticles can be tailored by adjusting the composition and reaction conditions. This process is scalable and can vary from a few hours to days. Dependent on the reactor volume, gram- to kilogram-level nanoparticles can be easily synthesized using wet chemistry.

The synthesis of ultrafine (<10 nm) high-entropy alloy nanoparticles without separate phases is a challenging task due to the vast difference in chemical and physical properties of the mixing elements. Feng et al. developed a suitable and scalable synthetic strategy that ensured the simultaneous reduction of mixed metal salts. Carbon supports were added to the reaction solution before the co-reduction of metal salts to prepare ultrasmall NPs with uniform dispersion. A series of comparative samples with different loadings were obtained, and a 5 wt% loading, in theory, was determined to be the most appropriate. The resulting ultrasmall HEA/C NPs had a face-centered cubic structure like that of pure Pt and an average diameter of 1.68 nm, which is much smaller than those of other reported HEAs. The synthesis method has been demonstrated as a general strategy for the synthesis of high-entropy alloy NPs with different compositions. The method allows for the controllable synthesis of single-phase HEA NPs with ultrasmall size and uniform dispersion, providing direct evidence of alloy formation. Overall, the developed synthesis strategy provides a scalable and general approach to the synthesis of ultrafine HEA NPs without separate phases [101].

By adding a nearly equal atomic ratio mixture of metal precursors into preheated triethylene glycol containing polyvinylpyrrolidone as a protective agent at 230 °C, Wu et al. were able to produce a quasi-spherical, single-phase, homogeneous HEA nanomaterial. The resulting powder exhibited a size distribution of 4.1 ± 1.2 nm [102].

Zhu et al. demonstrated the synthesis of structurally ordered high-entropy alloy nanoparticles on nitrogen-rich mesoporous carbon nanosheets. The synthesis method involved the preparation of organic–inorganic 2D superstructures using a metal–catecholamine functionalized poly(ethylene oxide)-b-poly(methyl methacrylate) composite, and subsequent heat treatment of the superstructure in NH3 atmosphere to achieve the chemical order of HEA NPs and nitrogen enrichment of the mesoporous framework [103].

In a research study conducted by Liu et al., high-entropy alloy nanoparticles and activated carbon nanocomposites were synthesized with a wet chemistry impregnation–adsorption method using metal nitrates as precursors. The precursors were dissolved in ethanol and mixed in equal atomic ratios and then combined with washed and dried AC, which was subsequently immersed in the precursor solution for 4 h. After removing excess ethanol, the precursor-loaded AC was heated to 1273 K for 3 h under a protective gas mixture of Ar:H₂ (95:5) to form the final HEA NP-AC nanocomposites [104].

3.5. Laser Ablation Synthesis in Liquid

Laser ablation synthesis involves using a pulsed laser to bombard the surface of bulk HEA, removing part of the bulk material and causing nanoparticles to form suspended in a liquid, as demonstrated in Figure 6. This method is scalable based on laser power and can provide roughly 1 g per minute.

Jahangiri et al. used the femtosecond pulsed laser ablation in liquid method to synthesize HfTaTiNbZr refractory high-entropy alloy nanoparticles. They immersed the as-cast HfNbTaTiZr sample in three different media: distilled water, ethanol, and n-hexane, and subjected it to laser bombardment using a Spectra-Physics Spitfire Ace system, which generated 120 fs pulses near the wavelength of 800 nm at a 1 kHz repetition rate. The experiments were performed at three different average powers (20 mW, 38 mW, and 54 mW) and corresponding laser fluences of 0.1 mJ/cm², 0.16 mJ/cm², and 0.23 mJ/cm², respectively. The XRD patterns of the fabricated RHEA NPs revealed that they all crystallized predominantly in the cubic phase, specifically the Fm-3m space group. However, the surrounding liquid medium had a significant impact on the formation of oxide phases in the NPs. While NPs produced in n-hexane did not show any oxide or carbide phases, Rietveld refinement estimated that about 89% and 91% of RHEA NPs were oxidized during the ablation process in ethanol and distilled water, respectively. It was also observed that NPs produced in n-hexane had the smallest crystallite size, as evidenced by broader peaks with higher full width at half maximum values in comparison to those produced in water and ethanol [105].

In a study by Rawat et al., a two-step laser ablation synthesis method was used for non-equiatomic high-entropy alloy nanoparticles using an Nd: YAG laser at a wavelength of 1064 nm. The target material, Al₄₀(SiCrMnFeNiCu)₆₀, was first ablated in deionized water with a fluence of 80 J/cm² and a spot size of 1.5 × 10⁻³ cm² for 40 min. The synthesized colloidal solution was then subjected to secondary processing with a 532 nm laser for 15 min, using a fixed energy of 60 mJ and a fluence of 40 J/cm², resulting in HEA nanoparticles [106].

Waag et al. successfully synthesized high-entropy alloy nanoparticles using picosecond-pulsed laser ablation of CoCrFeMnNi HEA targets in ethanol, resulting in a brownish-colored, transparent colloid. Analytic disc centrifugation and analytical ultracentrifugation analyses revealed a bimodal volume-weighted size distribution of NPs, with hydrodynamic diameters primarily at 2.8 nm and a secondary dominant size mode at 7.9 nm. The presence of different size fractions in the colloids can be attributed to distinct particle formation mechanisms during the picosecond-pulsed laser synthesis process [107].

3.6. Other Notable Synthesis Methods

Yang et al. presented a groundbreaking aerosol droplet-mediated approach for the large-scale production of high-entropy alloy nanoparticles, featuring an atomic-level blend of otherwise immiscible metal elements. By nebulizing an aqueous solution of metal salts, they created aerosol droplets measuring approximately 1 μm in size. The droplets were then exposed to rapid heating and cooling processes, which facilitated the decomposition of precursor compounds and the formation of zero-valent metal atoms (Figure 7) [108].

In their work, Zhao et al. developed an innovative method for synthesizing high-entropy alloy nanoparticles using a spray-drying technique combined with thermal decomposition reduction. Using a homemade solution of precursors and graphene oxide as carriers, they successfully synthesized Pt-based HEA NPs with desirable properties. Their approach demonstrates a scalable and cost-effective method for preparing HEA NPs, which have potential applications in various catalytic processes [109].

Having explored various synthesis techniques for producing individual HEA nanoparticles and nanoporous structures within bulk materials, we now shift our focus to a unique and promising method known as chemical dealloying. This process is particularly adept at creating nanoporous structures in the context of high-entropy alloys, often by selectively dissolving aluminum from the alloy in a chemical solution. Chemical dealloying involves the selective dissolution of specific elements from an alloy, leaving behind a nanoporous structure. When applied to high-entropy alloys, this technique results in the formation of nanoparticles that exhibit unique properties, such as high surface area and enhanced catalytic activity. In the following section, we will discuss the intricacies of chemical dealloying, its potential applications, and how it leverages the selective dissolution of aluminum to create high-performance HEA nanomaterials.

Fang et al. developed a highly bifunctional, active AlFeCoNiCr-based nanoporous high-entropy alloy/high-entropy oxide catalyst for use in aqueous and solid-state Zn-air batteries. The precursor alloy ribbons were made by melting corresponding melts followed by melt-spinning. Chemical dealloying in a NaOH solution was used to obtain the multicomponent nanoporous alloys, which were coated on a glassy carbon electrode. The bifunctional catalyst ink was prepared by mixing np-HEA/HEO, carbon powder, isopropanol, and Nafion, which was coated on the electrode for the electrochemical tests. The incorporation of Cr was found to enhance the bifunctional activity of the catalyst, and the np-AlFeCoNiCr-based battery outperformed the Pt/C-IrO2-based battery [110].

4. Catalysis

Energy and the environment are two crucial issues for the sustainability of human society. Catalysts are extremely important in effective energy harvest and environmental remedies. This applies to numerous catalytic reactions, such as the oxygen reduction reaction (ORR), CO₂ reduction reaction (CO₂RR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and organic oxidization reaction (OOR). Currently, the most widely representative catalysts are precious metal (e.g., Pt, Pd, Ru, Ir, Ag, and Au) alloys or their alloys with transition metals (e.g., Fe, Co, Ni, and Cu). In general, these components have no more than three elements. To improve catalytic efficiency, researchers need to optimize the adsorption energy of reaction intermediates using the coordination environment. However, simple alloy composites limit the design of catalysts for effective and selective catalysis [111,112].

HEAs possess numerous unique advantages as catalysts, including (1) a multielement composition space for the discovery of new catalysts and fine-tuning of surface adsorption (i.e., activity and selectivity), reducing the requirement of noble metals, (2) diverse active sites derived from the random multielement mixing that are especially suitable for multifunctional and tandem catalysis, and (3) a high-entropy-stabilized structure that improves structural durability in harsh catalytic environments. All these properties are desired for a long catalyst life [113]. Figure 8 shows the unique features of HEA catalysis. We will now briefly review the progress in ORR, CO₂RR, OER, and OOR catalysis.

The ORR is an important reaction for metal–air batteries and fuel cells [114]. In an acidic solution, the ORR typically involves four proton–electron transfers to reduce oxygen to water or two proton–electron transfers to hydrogen peroxide. In alkaline media, the four-electron process yields hydroxide ions, while the two-electron process produces peroxide ions [115]. Qiu et al. developed a nanoporous AlNiCuPtPdAu using dealloying. The effective ORR of this nanoporous HEA decayed by only 7.5% after 100,000 electrochemical cycles [116]. The same group also reported Al-Ni-Co-Ru-Mo nanowires for Zn-air batteries. The high open circuit voltage and a high energy density of 851.3 Wh/kg_Zn were achieved at 20 mA/cm² [117]. To develop a strategy to explore effective ORR HEA catalysts, Schumann applied a simple model to build a relationship between the adsorption energy and the electrochemical curves. They predicted a tuned composition for an enhanced ORR [118]. Hu et al. developed a high throughput synthesis combined with a droplet electrochemical characterization to construct a composite–catalysis relationship. A machine learning (ML) technique was used to facilitate the search and design of an effective HEA catalyst [119].

For the CO₂RR, there are pioneer reports using CoCuGdNiZn and AgAuCuPdPt for electrocatalytic reduction of CO₂ and CO [120,121], but the experimental results displayed only gaseous products of methane, ethylene, H₂, and CO. No methanol was synthesized. However, a 100% faradic efficiency (FE) and a current efficiency of 81.8% of CO2 conversion were achieved [121]. In one recent work [122], superior durable catalysis of CO₂ hydrogenation using CoNiCuRuPd HEA nanoparticles, compared to Pd, was also successfully exhibited at 300 °C to 400 °C, which was elucidated to be due to the unique cocktail effect and the sluggish diffusion of HEAs. The highly selective CO₂ hydrogenation to methanol has been recently predicted theoretically using the density functional theory (DFT) calculation for the surface-adsorption energy of reactive intermediates, and about 34 catalysts of CuCoNiZnSn-based HEAs have been identified with high-throughput screening, using machine learning based on a series of 36,750 catalysts [123,124]. However, there are no experiments to validate these results and realize the highly efficient conversion of CO₂ to methanol using HEA catalysts.

The OER is an anodic semi-reaction of electrocatalytic decomposition of water and a cathodic reaction during the charging of metal–air batteries. Like the ORR, the OER also involves four proton-couple electron transfers, where molecular oxygen is produced in acidic or alkaline media [115]. The sluggish electron transfer in the OER is the key barrier to effective renewable-energy conversion and storage. During the OER, a high positive potential tends to oxidize the surface of metal catalysts and form metal oxides or hydroxides [125]. Therefore, a metal–core–oxide shell is found to exhibit a highly effective OER in nanoporous FeCoNiCrNb [41]. Qiu et al. studied the OER performance of nanoporous AlNiCoFeX (X = Mo, Nd, Cr) after dealloying Al [126]. AlNiCoFeMo displayed the best OER activities. Recently, using electrochemical deposition, Fan et al. reported that an optimized NiFeCoMnAl catalyst exhibits an overpotential of 190 mV at 10 mA/cm² in 1 M KOH solution, which was much superior to the NiFeAl ternary and NiFeCoAl quaternary counterparts [127]. The origin of the enhanced OER could be attributed to numerical factors as follows: (i) the incorporation of Mn can construct an electron-rich environment of active Ni centers, and the relatively lower oxidation state of Ni facilitates the self-construction of β-NiOOH intermediates, resulting in a promoted deprotonating step and lowering the required overpotential. (ii) The doped Co and the formation of Mn⁴⁺ ions can enhance conductivity, which helps the charge transfer process to boost the OER rate. (iii) The dealloying of Al can form a nanoporous structure with abundant defects, promoting the generation and exposure of active sites. (iv) The amorphous structure facilities the self-construction of the β-NiOOH phase and provides abundant catalytic activities.

The performances of HEAs as both ORR and OER catalysts for bifunctional oxygen electrocatalysts are very attractive. This is particularly crucial for metal–air batteries, which involve the ORR during discharge and the OER during charge.

The OOR is involved in both fuel cells and environmental remedies. Direct methanol fuel cells (DMFCs) and direct ethanol fuel cells (DEFCs) are highly promising renewable energy technologies for energy transportation and storage, high-energy conversion efficiency, and environmental friendliness. However, the commercialization of DAFCs/DEFCs has been hindered by the lack of highly efficient and stable anode electrocatalysts. The main catalysts are still Pt-group noble metals, including Ru, Rh, Pd, Os, Ir, and Pt. While the OOR is a slow multistep reaction, the catalyst poison caused by the reaction intermediate CO is a serious issue. An effective strategy has been investigated to form alloying between Pt-group metals and multiple oxyphilic metals (e.g., Ni, Cu, Ru, etc.). This brings opportunities for HEA with tunable metal sites for the OOR.

Using a polyol method, Li et al. [128] fabricated Pt₁₈Ni₂₆Fe₁₅Co₁₄Cu₂₇ HEA nanoparticles. The higher OOR of this composite Pt/C was shown.

Their study proved that HEA provides multiple effective sites: (1) Pt promotes electron transfer on the surface of HEAs; (2) Fe can produce stable adsorption to OOR intermediates and has a strong resistance to CO poisoning; (3) Co is beneficial for improving the electron transfer efficiency of the OOR; (4) Ni sites show a relatively stable d-band center; and (5) Cu improves the HEA electroactivity. Therefore, the enhanced OOR performance is attributed to the synergistic effect of numerous active sites on the HEA surface. Recently, Fan et al. reported noble metal-free CoNiCuMnMo HEA nanoparticles for glycerol oxidation in an alkaline solution [127]. A long-term catalytic stability to 12 days running at 50 mA/cm² with a faradic efficiency of 92% for formate production in the anode was observed. This reaction is also promising for electrocatalytic degradation of organic contamination for environmental remedies.

In summary, HEA alloys are easier to prepare in nanoparticle/nanoporous types for various chemical reactions. The search and design for effective HEAs are still tedious. The general strategy is to combine high throughput synthesis, machine learning for HEA design, and local electrochemical characterization. However, many investigations are needed on the structural complexity of HEAs and on catalytic mechanisms for practical applications of HEA in various electrocatalytic devices.

5. Marine Applications

HEAs containing aluminum and/or copper are particularly favorable for marine applications [129]. Due to the cocktail effect, HEAs can serve as multifunctional materials, which is crucial to a marine environment.

HEAs have been researched for marine coating applications to create a barrier to protect the integrity of structural materials such as steel and aluminum alloys [130,131]. Aluminum alloys perform an excellent job by forming their own protective oxide layer when exposed to air and water compared to some steels. However, in the presence of seawater, this oxide layer has a limited corrosion resistance [132]. Therefore, additional coatings are required. The types of coatings applied to Al alloys include paint, ceramic reinforcement, and cold sprays [130,131,132,133]. Copper-containing and nickel-containing HEAs are viable cold spray candidates, as demonstrated in other studies [134,135,136]. Copper alloys, in other studies, have been shown to be excellent for marine coatings due to their antifouling [137], hydrodynamic, and corrosion properties [130]. Such properties also appear in copper-containing HEAs. Yu et al., for instance, tested a Cu-doped AlCoCrFeNi HEA (AlCoCrFeNiCu_0.5) in a marine environment. Their study demonstrated an excellent combination of mechanical, antifouling, corrosion resistance, and wear resistance properties. Furthermore, the HEA did not display significant Cu segregation and thereby avoided brittle fracture mechanisms [136,137,138,139,140]. Hydrophobic coatings can help maintain the surface of an aluminum alloy by maintaining air retention and blocking chloride ions from attacking the surface [133]. Antifouling is the fight against basibionts, which are the hosts to epibionts, causing epibiosis. The formulation of this can be detrimental to the alloys, causing problems with movement and breaking down the overall integrity [137]. Continuing to test these coatings and various alloys can help defend against corrosion, which would allow a longer life and require less maintenance. With less maintenance, there are less costs regarding the vessel.

Aluminum-containing HEAs are also viable candidates for marine environments. Ayyagaru et al. conducted a study on the wear resistance of two single-phased, face-centered cubic HEAs in dry and marine environments using equimolar CoCrFeMnNi and Al_0.1CoCrFeNi. In their research, the wear morphologies in dry and marine environments were used to identify the synergistic degradation mechanisms, and the authors found that there was a negative synergy between wear and corrosion. In other words, both HEAs were more wear resistant in a marine environment than a dry environment. This was attributed to the greater degree of passivation in the marine environment. Furthermore, Al_0.1CoCrFeNi proved to be more corrosion- and wear-resistant in both environments than CoCrFeMnNi, which correlates to a greater degree of passivation compared to steels and other iron alloys [141]. It can be assumed that the addition of aluminum is beneficial to the performance of HEAs in a marine environment as well. To date, relatively little research has been performed on HEAs in marine environments, and there are still many alloy compositions that can be tested to improve HEAs in marine applications.

In terms of corrosion resistance, larger grain sizes can be a cause for concern compared to smaller grain sizes [129]. As previously stated, copper-containing high-entropy alloys are also great candidates for future marine applications due to their low cost and corrosion resistance. For example, Xue et al. conducted a study on the influence of grain size on the corrosion properties of an Al₂Cr₅Cu₅Fe₅₃Ni₃₅ HEA. In their study, the HEA formed a passive layer that was integral to its corrosion resistance in a marine environment, and it was found that corrosion resistance increased as grain size decreased [127,142].

In addition to corrosion resistance in saline-based solutions such as seawater, a few HEAs have been identified so far as having cavitation corrosion resistance including Al_0.1CoCrFeNi [143,144] and CoCrFeNiTiMo [145]. Interestingly, Cao et al. reported developing a ternary Al₁₀Cr₂₈Co₂₈Ni₃₄ HEA with high cavitation corrosion resistance due to the simultaneous absorption of mechanical impact energy and thermal energy released when the bubbles collapse. This was achieved by inducing a martensitic transformation under cavitation load (mechanical impact), and the cavitation heat induced grain growth and structural relaxation. The result was a cavitation erosion resistance 2 times higher than that of AlCrCoFeNi HEA [146].

In summary, copper-containing and aluminum-containing HEAs bring significant benefits to marine applications compared to aluminum alloys and steels. Given the current technical difficulties in mass-producing HEAs for structural purposes, the fact that most of these HEAs can be applied as a coating to existing steels and aluminum alloys is extremely encouraging.

6. Conclusions

In conclusion, this comprehensive mini-review explored various novel applications for HEAs. In terms of material joining, several HEAs have been tested in cases of similar welding, dissimilar welding, and brazing and as filler metals in a variety of cases. Recent research exceedingly demonstrates the potential of HEA FMs to address some of the issues with brazing and dissimilar welding by acting as a barrier to unfavorable phase transformations.

In nanomaterials, synthesis methods for HEA nanoparticles and nanoporous structures have displayed significant progress, showcasing their unique properties and potential applications. By examining techniques such as carbothermal shock synthesis, mechanical alloying, microwave heating, wet chemistry, laser ablation synthesis in liquid, and chemical dealloying, we illustrated the diverse range of approaches for producing HEA nanoparticles with tailored properties. As researchers continue to develop and refine these synthesis methods for facile large-amount and low-cost synthesis, the potential for high-entropy alloy nanoparticles in fields such as catalysis, energy storage, and other materials science fields continues to grow.

In the area of catalysis, the wide design space of HEAs has proven to be an indispensable asset in creating novel catalysis solutions to the ORR, CO₂RR, OER, and OOR processes, which will vastly improve the state of energy production, energy storage, and environmental concerns.

Lastly, marine experimentation on HEAs has proven to be very fruitful as various HEAs have been tested or developed to have superior mechanical, antifouling, and corrosion resistance properties compared to competing aluminum alloys and steels. Given the comparatively little research on HEAs and the arduous task of developing or reorganizing manufacturing paradigms for mass-producing HEAs, it is encouraging to see that HEA coatings in marine applications are sufficient for improving the effectiveness of cheaper aluminum alloys and steels. It is, however, only a matter of time before the cost of manufacturing HEAs decreases, which will serve to encourage the more liberal use of HEAs in various fields including, but by no means limited to, those discussed in this review. These innovative applications may predict the bright future of HEA since unique structure-property relevance and designing freedom have been unveiled in HEA.