Compositional Design of High-Entropy Alloys: Advances in Structural and Hydrogen Storage Materials

Abstract

High-entropy alloys (HEAs) present a vast compositional design space, characterized by four core effects—high configurational entropy, sluggish diffusion, severe lattice distortion, and the cocktail effect—which collectively underpin their exceptional potential for both structural and hydrogen storage applications. This mini-review synthesizes recent advances in the compositional design of HEAs with emphasis on structural materials and hydrogen storage. Firstly, it provides an overview of the definition of HEAs and the roles of principal alloying elements, then synthesizes solid solution formation rules based on representative descriptors—atomic size mismatch, electronegativity difference, valence electron concentration, mixing enthalpy, and mixing entropy—together with their applicability limits and common failure scenarios. A brief introduction is provided to the preparation methods of arc melting and powder metallurgy, which have a strong interaction with the composition. The design–structure–property links are then consolidated for structural materials (mechanical properties) and for hydrogen storage materials (hydrogen storage performance). Furthermore, the rules for the combined design of control systems for HEAs and the associated challenges were further discussed, and the future development prospects of HEAs in structural materials and hydrogen storage were also envisioned.

1. Introduction

Materials innovation has historically driven technological advancement and human civilization. Metallic materials can be broadly categorized historically into three groups: single-principal-element traditional alloys [1,2], binary amorphous alloys [3], and multi-principal-element (≥5) high-entropy alloys (HEAs) [4]. Conventional alloy design involves adding one or more elements to a metal matrix to achieve desired properties. Metal matrix composites (MMCs) have been prominent in this evolution. Kim et al. [5] analyzed the impact of reinforcement volume fractions, particle size, and matrix grain size on the mechanical properties of particulate-reinforced Al matrix composites. Ye et al. [6] investigated the effect of SiC particle size on the mechanical properties and failure mechanisms of Al/SiC composites under compression. The results show that the bond strength between SiC particles and the Al matrix significantly affects compressive resistance. Key challenges in MMC fabrication include the dispersibility of the reinforcement phase [7] and its wettability with the matrix, which greatly affect material properties [8]. Amorphous alloys exhibit long-range disordered atomic structures, imparting excellent properties including corrosion and abrasion resistance, mechanical strength, and toughness [9,10]. However, most amorphous alloys exhibit brittleness with limited ductility [11]. Metallic glasses, a subset of amorphous alloys, face similar issues. Zhang et al. [12] demonstrated, via simulation, that metallic glasses are brittle macroscopically but ductile at the nanoscale, with oxygen potentially regulating the brittle-to-ductile transition; however, these findings require experimental validation.

High-entropy alloys, emerging over the last decade and a half [13,14], are novel structural materials. Unlike conventional alloys based on a primary metal element with minor additions for property enhancement, HEAs comprise multiple principal elements (typically ≥5) in high concentrations (5–35 at.%). HEAs exhibit excellent properties such as high strength and toughness [15,16,17,18], high strength and strong magnetic properties [19], high strength and high temperature oxidation resistance [20], high energy storage and long cycle life, and effective and stable catalytic ability, attributed to four core effects: high entropy, sluggish diffusion, severe lattice distortion [21], and cocktail effect. Concurrently, these effects collectively reshape phase stability, defect structures, transport properties, and interfacial chemistry. These effects do not operate in isolation; rather, they are manifestations of the underlying composition–structure landscape and therefore must be analyzed through a compositional design lens. Recent years have witnessed remarkable growth in the field of HEAs, with an expanding corpus of research dedicated to a wide spectrum of material systems, encompassing metallic alloys [16,17,18], ceramics [22], polymers [23], composites, and intermetallic compounds [24]. Extensive studies have been undertaken to elucidate their mechanical [16,17,18], electrical and magnetic [19,25], chemical [26,27], and functional properties [28]. These research endeavors underscore the significant potential of HEAs for multisectoral applications, including aerospace, transportation, advanced manufacturing, energy, defense, and biomedical engineering [29,30].

For example, HEAs can be used as structural materials, as they are typically strong and ductile. Průša et al. [31] prepared an equiatomic CoCrFeNiNb alloy by mechanical alloying (MA) and spark plasma sintering (SPS). The MA-processed alloy exhibited a uniform ultra-fine-grained microstructure and ultra-high strength (2412 MPa). Cheng et al. [32] investigated the effect of Al concentration on the microstructure and mechanical properties of FeCoCrNiMn alloy, achieving a compressive strength of 2552 MPa.

Furthermore, HEAs significantly expand the alloy design space, including for hydrogen storage applications. Their simple solid solution phases and severe lattice distortion provide suitable sites for H-atom occupation [33] and reduce H-atom diffusion distances, suggesting potential for hydrogen storage materials. The single-phase bcc HEA MgZrTiFe0.5Co0.5Ni0.5, prepared by high-energy ball milling under Ar, showed a maximum hydrogen capacity of 1.2 wt.% [34]. Zhang et al. [35] studied the hydrogen absorption capacity and hydride structure of TiZrNbTa HEA, finding decreased maximum absorption capacity and increased plateau pressure with temperature. Although some HEAs show promising hydrogen storage properties, performance varies significantly, often linked to absorption/desorption reversibility. Hydrogen absorption can induce phase segregation, hindering reversibility. However, TiZrHfMoNb alloy transforms to an FCC structure upon full hydrogenation and reverts to the original BCC structure upon dehydrogenation, indicating reversible phase transformation during cycling [36].

Despite these advances, a robust theoretical and data framework for HEA compositional design remains incomplete. Many models still extrapolate from binary/ternary interactions to truly multicomponent regimes with uncertain transferability. Predictions also remain processing-agnostic in many cases, even though non-equilibrium routes (mechanical alloying, rapid solidification) strongly influence defect populations, short-range order, and phase selection. These gaps slow progress in both structural and hydrogen storage directions.

To make design decisions tractable, this review structures the field around representative compositional descriptors that map composition to structure and hydrogen storage. The most widely used include atomic size mismatch, electronegativity difference, valence electron concentration, mixing enthalpy, and mixing entropy. These descriptors aid solid solution formation screening (FCC/BCC/dual-phase windows), identify ordering tendencies, and rationalize lattice distortion levels that control strengthening and diffusion. The design–structure–property links are then consolidated for structural materials and for hydrogen storage materials.

2. Compositional Design of HEAs

2.1. Definition of HEAs

HEAs are typically composed of five or more principal elements, and the various elements are composed of equal atomic ratios or close to equal atomic ratios. The atomic fraction of each principal element of HEAs is between 5% and 35%. From the naming of HEAs, entropy is a key parameter for the preparation of these alloys. The increase in mixing entropy is positively related to the number of components. The more the number of components, the higher the entropy of the alloy, which is more favorable for the alloy to form a single-phase solid solution structure, which greatly limits the precipitation of intermetallic compounds. However, when the number of principal elements exceeds 13, the growth rate of the mixing entropy of the alloy begins to slow down, while the mixing entropy cannot be significantly improved by simply increasing the number of components [14]. Consequently, HEAs usually contain no more than six principal elements.

2.2. Effect of Alloying Elements

This new alloy will provide more flexible combination between elements to improve the properties of metallic materials. When designing the constituent elements of multi-principal high-entropy solid solution alloys, according to the idea of superimposed enhancement or complementary combination of element properties, the phase composition and microstructure of the alloy can be changed by adding certain elements to obtain the required comprehensive properties. The effects of various elements on the alloy microstructure are crucial for the compositional design of HEAs. Here the roles of various elements are discussed.

(1)

Al significantly influences phase transitions. Half of the current research examines Al content effects. Al governs phase selection and the strengthening pathway in Co/Cr/Fe/Ni-based HEAs. Thermodynamically, Al increases the atomic size mismatch δ and decreases the valence electron concentration, driving the matrix along the sequence FCC → FCC + BCC → BCC as Al rises. Kinetically, strong negative Al–(Ni, Co, Fe) pair enthalpies promote B2 short-range order and, at higher contents, ordered B2 precipitates or even a continuous B2 matrix. Xing et al. [37] demonstrated a transition from single-phase FCC to FCC + BCC + B2, and finally to BCC + B2 in FeCrNiMnAl_x HEAs (x > 0). Wear resistance improved with increasing Al content. In Al_xCoCrCuFeNi HEAs, superior high-temperature resistance was observed at higher Al content [38].

(2)

Fe addition to the Al_0.5CoCrFexNiTi_0.5 HEA inhibits σ phase formation while promoting the formation of the FCC phase [39]. The compressive strength of the alloys decreases with the increase in Fe content. Microstructurally, Fe raises the stacking fault energy (relative to Mn) and reduces chemical short-range order, which converts deformation from TWIP/TRIP or planar glide to more homogeneous dislocation slip. These effects explain the observed decline in compressive strength with higher Fe. However, increased Fe typically improves ductility and work-hardening stability, reduces casting hot-cracking, and enhances weldability.

(3)

Mo, with its large atomic radius, induces higher lattice distortion and solid solution strengthening. Mo addition promotes σ phase precipitation from the BCC phase in AlCrFeNiMo_x (x = 0, 0.2, 0.5, 0.8, 1.0) [40]. Furthermore, the μ phase appears with increasing Mo content in CoCrFeMo_xNi (x > 0.04) alloy [41].

(4)

Hf—a large-radius, strong carbide/boride former—markedly alters phase stability in CoCrFeNi-based HEAs. Hf addition to CoCrFeNi leads to the transformation of solid solution from single-phase FCC to C15 Laves and FCC phases [42]. Hf also has a strong affinity for C or B elements, producing HfC or HfB₂ dispersoids that refine grains and raising hardness and compressive yield strength.

(5)

Nb promotes the appearance of the Laves phase in CoCrFeNb_xNi (x = 0, 0.25, 0.45, 0.5, 0.75, 1.0, and 1.2), while the formation of the Laves phase leads to a decrease in the plasticity and increase in the Vickers hardness and the wear resistance [43]. The results by Fan et al. [44] show that the synergistic effect of Nb and Mo results in the formation of a new kind of Laves phase semi-coherent with FCC matrix, while the ration of Nb/Mo will affect the size of the lamellar structure of the Laves phase.

(6)

Co can promote the formation of the FCC phase, including the microstructure of the Al_0.4FeCrNiCo_x (x = 0, 0.25, 0.5, 1 mol) from FCC and BCC phases to the FCC phase with the increase in Co content [45]. From a deformation viewpoint, Co raises the stacking fault energy (SFE) relative to Mn-rich variants, shifting mechanisms from TWIP, TRIP, or planar glide toward more homogeneous dislocation slip. This typically improves work-hardening stability and uniform elongation at room temperature, while slightly lowering the extraordinary strain-hardening of low-SFE compositions.

(7)

Ti promotes the formation of the BCC phase. In Al₂CrFeNiCoCuTi_x HEAs, the corrosion resistance of this alloy coating is enhanced in 0.5 mol/L HNO₃ solution with increasing Ti content [46]. In terms of microstructure, Ti can achieve strong solid solution strengthening in both face-centered cubic and body-centered cubic matrix structures, thereby increasing the yield strength.

(8)

Ta has a similar phase evolution effect to Ti, promoting the formation of the BCC structure, while affecting the precipitate of the Cr₂Nb-Laves phase in NiTiCrNbTa_x HEAs [47]. Furthermore, Ta provides strong solid solution hardening and raises the Peierls barrier in BCC, giving superior hot strength and creep resistance relative to Ti-only variants.

(9)

W, a high melting point metal, typically stabilizes BCC solid solutions in HEAs. The incorporation of W leads to the formation of tungsten oxides, which prohibits the evolution of a protective oxide layer on the surface of W-containing HEAs, exhibiting good oxidation resistance [48].

(10)

Si induces complex phase (silicides) precipitation in (VNbTiTa)_100−xSi_x (x = 0, 2.5, 5, 5, 10) HEAs [49] and facilitates the BCC phase formation and hence stabilizes the BCC structure [50].

(11)

V enhances yield strength and ductility of V_xNbMoTa at room temperature, which can be contributed to solid solution strengthening and grain refinement [51]. V elevates the growth-restriction factor during solidification and tends to segregate weakly to boundaries, promoting finer as-cast grains; finer grains enhance both yield and uniform elongation.

(12)

La alters grain morphologies from a dendritic structure to near-equiaxed structure and decreases grain size, while increasing the ultimate strength and yield strength. Moreover, the formation of La precipitates and the disilicide phase as the La content increases leads to a decrease in ductility [52].

(13)

Ni stabilizes the FCC structure and suppresses the formation of the σ phase in CrMnFeCoNi_x (0 ≤ x ≤ 1.5) [53]. Higher Ni contents promote homogeneous FCC matrices with fewer interdendritic compositional gradients and less partitioning of Cr and Mn. Ni elevates the SFE, thereby shifting deformation from TWIP, TRIP, or planar glide toward stable dislocation slip with good work-hardening capacity. As a result, yield strength and uniform elongation both remain high.

(14)

Cu favors FCC solid solution formation [54]. Due to positive enthalpy of mixing with most 3d transition elements [55], Cu readily precipitates from the FCC phase, forming a Cu-rich FCC phase. In AlCrFeNiTiCu_x HEAs, increasing Cu content promotes the segregation of Al, Ni, and Ti in dendrites, while Fe and Cr precipitate into particles distributed within dendrites and interdendritic regions [56]. Nanoscale Cu-rich precipitates can contribute modest precipitation strengthening while preserving matrix ductility. Moreover, Cu additions raise electrical/thermal conductivity of FCC-based HEAs—useful for heat-spreader or wear-resistant conductor coatings—provided segregation is controlled.

(15)

Cr promotes BCC phase formation [57]. And the formation of carbides on annealing and consolidation can enhance the mechanical properties of Cr-containing HEAs due to the precipitation strengthening effect [58,59].

(16)

Mg content had a positive effect on the formation the BCC phase, while it can significantly decrease the hardness of AlFeCuCrMg_x (x = 0.5, 1, 1.7) [60]. Mg provides relatively weak solid solution hardening, lowers the elastic modulus, and can soften Cu- or Mg-rich interdendritic pools if segregation is unchecked.

(17)

Zr facilitates BCC over FCC phase formation in TiMoNbZr_x HEAs [61]. But the study by Chen et al. [62] shows that the increase in Zr content leads to the volume fraction of the Laves phase increasing in AlCoCrFeNiZr_x alloy, while also proving that minor Zr addition can significantly improve the mechanical property of this HEA.

By summarizing the effects of the aforementioned elements, correspondence among the composition, microstructure, and properties was established, as shown in Table 1.

Table 1. Integrated roles of representative alloying elements in high-entropy alloys.

Element	Phase Tendency	Typical Microstructural Effects	Property Impact
Al	FCC → FCC + BCC + B2 → BCC + B2	Strong short-range order; B2 precipitation	Improve abrasion resistance and superior high-temperature performance
Fe	σ ↓; FCC ↑	Adjust stacking fault energy	Improve ductility and work-hardening stability; reduce compressive strength
Mo	σ ↑	Solution strengthening	Improve hardness and yield strength
Hf	FCC → C15 Laves + FCC	Refine grains	Improve hardness and yield strength
Nb	Laves ↑	Diffusion strengthening	Improve hardness and the wear resistance
Co	FCC + BCC → FCC	Adjust stacking fault energy	Improve work-hardening stability and uniform elongation
Ti	BCC ↑	Solution strengthening	Improve the yield strength
Ta	BCC ↑	Solution strengthening	Improve hot strength and creep resistance
W	BCC ↑	Solution strengthening	Improve oxidation resistance
Si	BCC ↑	Precipitation strengthening	Improve hardness and yield strength
V	BCC ↑	Solution strengthening/refine grains	Improve yield strength and uniform elongation
La	No change in matrix phase	Refine grains	Improve tensile strength and yield strength; reduce ductility
Ni	σ ↓; FCC ↑	Adjust stacking fault energy	Improve yield strength and uniform elongation
Cu	Segregates/second phase in FCC	Weak precipitation strengthening	Improve yield strength
Cr	BCC ↑	Precipitation strengthening	Improve yield strength
Mg	BCC ↑	Weak solution strengthening	Reduce hardness
Zr	BCC + FCC ↑	Refine grains	Improve tensile strength and yield strength

“→” indicates phase transformation; “↓” indicates inhibition of phase formation; “↑” indicates promotion of phase formation.

Here, an analysis was conducted to quantify the frequency of different elements utilized in HEA compositions across the periodic table, with the results presented in Figure 1. In recent decades, an increasing number of studies has been completed on HEAs composed of the four most fundamental elements, Co, Cr, Fe, and Ni, involving the microstructure, phase transition, and comprehensive properties of the alloys. Building on these first-generation HEAs, a series of CoCrFeNiM (M = Mo, Ti, Zn, Cu, Al, Zr, Si, V, Mn, Nb, Hf, and so on) HEAs was developed, and the effects of M element addition on phase evolution, mechanical properties, wear resistance, corrosion resistance, and oxidation resistance were investigated. But for alloys that consist of 5 to 13 from the 118 elements, the current work is just a fraction of that. Critically, the speed of developing new high-entropy alloys according to traditional research methods makes it difficult to meet the requirements. Recently, the method of simulation calculation is incorporated to develop and design new types of high-entropy alloys at an unpredictable speed, which will be one of the development trends of exploring HEAs in the future.

Figure 1. Element citation frequency in HEAs.

2.3. Solid Solution Formation Rules

HEAs favor simple solid solution formation, which is related to atomic size, electronegativity, valence electron concentration, enthalpy of mixing, and entropy of mixing, as shown in Figure 2.

Figure 2. Factors affecting the formation of solid solution in HEAs.

(1)

Atomic size

The atomic size difference (δ), namely the atomic size mismatch, affects the stability of solid solution phases in HEAs. A large atomic size difference, on the one hand, will increase the degree of lattice distortion in the HEAs, resulting in an increase in the strain of the crystal, thereby increasing the internal energy of the HEAs, which is not conducive to the stability of the solid solution phases. The slow diffusion between the HEAs reduces the phase transition rate and main element segregation, resulting in the local precipitation of amorphous phases and nanocrystals in the HEAs. δ is given as follows [63]:

$$\mathrm{\delta }=\sqrt{{\displaystyle \sum _{i=1}^n}{c}_i{(1-\frac{r_i}{{\displaystyle \sum _{i=1}^n}{c}_i{r}_i})}^2}$$

(1)

where n is a total number of components, c_i is atomic fraction of the ith element, and r_i is atomic radius of the ith element.

(2)

Electronegativity

The electronegativity of an alloying element indicates the ability of the element’s electrons to capture electrons in other molecules. The greater the difference in electronegativity between alloy elements, the easier it is for elements with high electropositivity to lose extranuclear electrons, and the easier it is for elements with high electronegativity to gain electrons to form intermetallic compounds [64]. Thus, the formation of a simple solid solution phase requires that the electronegativity differences (Δχ) of the elements are close to each other. Δχ is defined as follows [65]:

$$\Delta \chi =\sqrt{\sum _{i=1}^n{c}_i{({\chi }_i-\sum _{j=1}^n{c}_j{\chi }_j)}^2}$$

(2)

where χ_i and χ_j are Pauling electronegativity for ith and jth elements, respectively.

(3)

Valence electron concentration (VEC)

When the valences of alloy elements are close to each other, the solid solution of the elements is relatively stable. When the VEC changes or exceeds a certain limit, the bond between elements will be disordered, which reduces the stability of the solid solution and promotes the formation of intermetallic compounds. The VEC is given by Guo et al. [66].

$$\mathit{VEC}=\sum _{i=1}^n{c}_i{(\mathit{VEC})}_i$$

(3)

where (VEC)_i is the VEC of the ith element.

(4)

Enthalpy of mixing (ΔH_mix)

ΔH_mix is expressed as the affinity between alloy elements. ΔH_mix > 0, with the increase in the ΔH_mix the solid solubility of the alloy decreases, which promotes the precipitation of the phase. ΔH_mix < 0, as the negative value of the ΔH_mix increases, the bonding force between the alloying elements is stronger, resulting in the formation of intermetallic compounds. The ΔH_mix is given by [67]

$$\Delta H_{\mathit{mix}}=\sum _{i=1,j\ne 1}^{n}4\Delta H_{\mathit{ij}}^{\mathit{mix}}{c}_i{c}_j$$

(4)

where Δ$H_{ij}^{mix}$ is the enthalpy of mixing of the binary liquid alloy composed of the ith element and the jth element in a regular melt.

(5)

Entropy of mixing (ΔS_mix)

Higher ΔS_mix will increase the disorder of the alloy system and significantly reduce the free energy of the alloy, resulting in the disorderly distribution of different alloy elements in the lattice position of the crystal, inhibiting the generation of ordered phases and phase separation, and promoting the formation of solid solution phases. The ΔS_mix is calculated using

$$\Delta S_{\mathit{mix}}=-R\sum _{i=1}^n{c}_i\ln c_i$$

(5)

where R is the gas constant (8.314 J/mol∙K).

The basic rule to obtain the solid solution is governed by the empirical Hume-Rothery rule, which includes atomic sizes, electronegativity and valence electron concentration, chemical compatibility (namely enthalpy of mixing), and entropy of mixing [4,63]. In short, the above five factors can be summarized into three criteria, namely ΔH_mix–δ, VEC, and Ω.

(1)

ΔH_mix–δ

The relationship between ΔH_mix and δ is shown in Figure 3, and as can be seen, when the enthalpy of mixing and atomic radius differences are within the appropriate range (−12.32 kJ/mol < ΔH_mix < 0 kJ/mol, 2.5% < δ < 6.56%), it is easier for the solid solution phase to form, which is similar to the findings of Guo et al. [68].

Figure 3. Relationship between atomic radius difference and enthalpy of mixing. Data from Ref. [69].

(2)

VEC

The VEC determines the favored solid solution structure (FCC or BCC) in HEAs. Chen et al. [70] investigated the relationship between VEC and the stability of FCC and BCC solid solution phases in HEAs. Higher VEC is beneficial to the formation of the FCC phase and improves the ductility of the alloy, while lower VEC is beneficial to the formation of the BCC phase that improves the strength of the alloy [71]. Thus, the VEC is used to control the contradictory relationship between the plasticity and strength of high-entropy alloys, and then high-entropy alloys with high strength and toughness are designed and developed. Moreover, Guo et al. [66] more precisely delineated the stable region of the solid solution phase in different VEC ranges. When VEC > 8, the FCC solid solution phase is relatively stable, and when VEC < 6.87, the BCC solid solution phase is relatively stable.

(3)

Ω

The ΔH_mix–δ and VEC rules described above have limitations in the prediction of solid solution formation. To accurately evaluate the formation ability of the solid solution phase, a new parameter (Ω) was proposed by Yang and Zhang [72].

$$\Omega =|T_m\Delta S_{\mathit{mix}}/\Delta H_{\mathit{mix}}|$$

(6)

$$T_m=\sum _{i=1}^n{c}_i{(T_m)}_i$$

(7)

where T_m is the melting point of the alloy, and (T_m)_i is the melting point of the ith element.

According to the study of the phase composition of high-entropy alloys, Ω = 1.1 can be regarded as the critical condition for the formation of the solid solution phase [73]. Ω ≥ 1.1, the effect of ΔS_mix on the formation of the solid solution exceeds that of ΔH_mix during alloy solidification, and it is easy from the alloy to form a solid solution. Ω ≤ 1.1, the effect of ΔS_mix is weaker than that of ΔH_mix, the nucleation of the solid solution will be inhibited, and intermetallic compounds or phase separations will be preferentially formed.

In summary, high VEC values tend to stabilize FCC phases, whereas low VEC values favor the formation of BCC phases. Moderate Ω parameters and weakly negative ΔH_mix values generally promote solid solution formation, while strongly negative ΔH_mix values facilitate the precipitation of intermetallic compounds. However, it must be noted that the synthesis route inevitably influences the final microstructure of HEAs. Furthermore, the appropriate introduction of intermetallic precipitates can significantly enhance the overall performance of HEAs. For instance, Han et al. [19] designed a non-equiatomic Fe–Co–Ni–Ta–Al multicomponent alloy incorporating a high density of coherent L1₂-structured intermetallic nanoparticles. Their research demonstrated that these precipitates, with an optimal size of approximately 91 nm and a volume fraction of ~55%, simultaneously enhance mechanical strength and preserve excellent soft magnetic properties. The nanoparticles effectively impede dislocation motion, leading to a high tensile strength of 1336 MPa and a ductility of 54%. Consequently, the compositional design and synthesis of HEAs should adopt a performance-driven approach. Utilizing appropriate processing techniques to design a series of dual-phase HEAs, combining solid solutions with intermetallic compounds, holds promise for achieving unexpected performance enhancements, thereby meeting the demanding requirements for both structural mechanical properties and hydrogen storage capabilities.

3. Preparation Methods

3.1. Arc Melting (AM)

AM produces HEAs with fine grains, uniform microstructures, compositional homogeneity, and high density. Wan et al. [74] fabricated the WReTaMo refractory HEA with high strength by vacuum arc melting. Hou et al. [75] prepared the AlFeCoNiBx (x = 0, 0.05, 0.10, 0.15, 0.2) HEAs by using AM, and investigated the effect of boron content on the microstructure and mechanical properties of the alloys. The results show that a suitable amount of B can refine eutectic structure and grain size and obtain excellent mechanical properties (ultimate compression strength reaches 2293 MPa). The reason lies in B increasing the growth-restriction factor and forming numerous heterogeneous nucleating agents (such as nanoscale borides/boride oxides), which transform coarse dendrites into near-isometric grains and reduce the size of eutectic clusters.

3.2. Powder Metallurgy

Mechanical alloying (MA), a key powder metallurgy technique, has been successfully used for metal matrix composites [76], intermetallics [77], amorphous alloys [78], hydrogen storage alloys [79], and so on. MA is a non-equilibrium solid-state alloying technique that involves repeated cold welding, fracturing, and rewelding of powder particles in a high-energy ball mill. In the MA process, the powder particles are broken into finer particles under the action of shear force produced by the impact of the milling ball [80]. During mechanical ball milling, powder particles undergo intense shear and collision, leading to severe plastic deformation. This process not only generates extremely high dislocation densities but also continuously exposes fresh interfaces. Meanwhile, diffusion distances are reduced to the sub-micron scale, which enables significant interdiffusion of atoms even at ambient or relatively low temperatures. For many high-entropy alloy systems, the structural evolution typically follows this pathway: initial blending of elements forms layered composite architectures, which gradually develop into supersaturated solid solutions and then evolve into single-phase FCC or BCC (or dual-phase FCC+BCC) microstructures. If the energy input during ball milling is extremely high, the material may further progress toward amorphization. Short-term annealing treatments subsequent to milling promote recrystallization and ordering transitions (such as B2/L1₂ intermetallic phases) while avoiding pronounced coarse segregation. Shear forces fragment particles, enabling alloying of elements with large melting point differences, preparing nanocrystalline materials with uniform microstructure and composition, and enhancing solid solubility. Thus, MA is a suitable process for the preparation of HEAs [69,81]. Wu et al. [82] synthesized a MgTiVZrNb high-entropy alloy via mechanical alloying. After prolonged mechanical ball milling, the phase structure transformed into a body-centered cubic structure, which contributed to favorable hydrogen absorption kinetics at room temperature and a hydrogen storage capacity of 1.196 wt.%.

Figure 4 illustrates the powder preparation and consolidation processes. As can be seen from Figure 4a, the powder preparation technology can be divided into mix, mechanical alloying, and gas atomization, where mechanical alloying contributes the most to powder preparation, accounting for 79%. In the sintering process of powder particles, one can choose various methods including hot-pressing sintering (HP), uniaxial pressing (UP), spark plasma sintering (SPS) [83], cold isostatic pressing (CIP), and hot isostatic pressing (HIP). The main feature of SPS technology is the use of bulk heating and surface activation to achieve ultra-fast densification and sintering of materials, which has the advantages of fast heating rate, short sintering time, low sintering temperature, and uniform heating. The comprehensive effect of elevated temperature sintering inhibits the growth of grains and retains the microstructure of the original particles, making the microstructure of the material fine and uniform and high in density. As shown in Figure 4b, the HEAs prepared by the SPS process accounted for 68%. To summarize, the combination of MA and SPS combines the advantages of their respective processes and plays a crucial role in the preparation of HEAs [84,85].

Figure 4. (a) Powder processing process and (b) consolidation process (A: mix, B: mechanical alloying, C: gas atomization). Data from Ref. [69].

4. Structural Materials

Mechanical properties are paramount for structural materials. Phases critically determine the strength and plasticity of HEAs. Sun et al. [86] reported an equiatomic CoCrNiCuZn alloy prepared by MA and SPS, where two kinds of FCC phases co-existed in the samples after sintering. The CoCrNiCuZn HEA shows a high ultimate compressive strength with 3121 MPa. The superior mechanical properties are primarily attributable to (i) the retention of nanocrystalline grains facilitated by the rapid heating and cooling cycles inherent to the SPS process, (ii) the presence of chemical SRO, which enhances lattice friction and thereby impedes dislocation motion, and (iii) the minimization of intergranular defects, such as pores and cracks, owing to the near-full densification achieved during sintering. Shabani et al. [87] produced FeCrCuMnNi HEA using vacuum induction melting. The microstructure illustrated that the alloy exhibited a typical cast dendritic structure, where rich-Cr and Fe were distributed in dendrite regions (BCC), while rich-Cu and Ni were distributed in interdendritic regions. The HEA showed an excellent ultimate tensile strength of 950 MPa and elongation of 14%.

Moreover, HEAs with nanocrystalline structures can achieve high mechanical properties, especially at high temperatures [88]. Cheng et al. [89] prepared a quinary alloy FeCoCrNiMn, which showed the nanocrystalline structures composed of FCC and BCC and amorphous phases, while the ultimate compressive strength of the alloy reaches 2129 MPa. The report by Yurkova et al. [90] showed that the BCC solid solutions with nanocrystalline structures can be obtained by the MA process. The nanocrystalline structure was retained after sintering under high pressure and low temperature, which improved the yield strength of 4100 MPa. Zhou et al. [91] reported that the mechanical properties of nanostructured FeCoCrNi films are dictated by their nanoscale architecture in the form of nano-twinned nanolaminates. A transition from “smaller is stronger” behavior to a high-hardness plateau was achieved by varying the layer thickness, which controls deformation mechanisms such as detwinning and an FCC-to-HCP phase transformation.

To further improve the mechanical properties of HEAs, many researchers used the strategy of adding a small quantity of other elements. Liu et al. [92] added a small amount of Ti and C to FeCoCrNiMn HEAs to form carbides, thereby refining the grains to improve the mechanical properties. Xie et al. [93] investigated the effects of N addition with 0.1 at.% on the microstructure and mechanical properties of CoCrFeNiMn alloy. The results show that the samples after VHPS had FCC solid solutions and carbides and Cr₂N, while the ultimate compression strength reached 2141 MPa. Ye et al. [94] fabricated novel [FeNi]_75−xCr₁₅Mn₁₀Al_x (x = 0, 5, 10, 15, 20, 25) HEAs by nonconsumable vacuum melting (VM). With the increase in Al content, the phases undergo a transition from the FCC to FCC + B2, finally to BCC + B2 phases, while the alloy exhibits a high compressive strength of 1660 MPa when the Al content reaches 25 at.%. Table 2 compares the phase and mechanical properties from different HEAs processed through varying techniques.

Table 2. Mechanical properties and preparation process through varying techniques.

HEA	Preparation	Phase(s)	Ultimate Compressive Strength/MPa	HV	Ref.
CoCrFeNiNb	MA + SPS	HCP + FCC	2412	798 ± 9	[31]
FeCoCrNiMnAl0.7	MA + HP	FCC + BCC	2552	622	[32]
Al0.5CoCrFe0.5NiTi0.5	AM	FCC + BCC + σ	2240	748	[39]
AlCrFeNiMo0.2	AM	BCC1 + BCC2	3222	911.5	[40]
(VNbTiTa)90Si10	AM	BCC + M5Si3	1671	-	[49]
V1.0NbMoTa	AM	-	1233	-	[51]
NbMoTiVSi0.2	AM	BCC + eutectic structure + MSi2	2091	-	[52]
(NbMoTiVSi0.2)99.7La0.3	AM		2130	-	[52]
TiZrNbMoTa	MA + SPS	BCC + ZrO2	3759	-	[84]
AlCuFeMnTiV	MA + SPS	BCC + B2 + HCP	2630	618.44	[85]
CoCrNiCuZn	MA + SPS	FCC1 + FCC2	2121	615	[86]
FeCoCrNiMn	MA + VHPS	FCC + BCC + Amorphous	2129	332	[89]
FeCoCrNiMnTi0.1C0.1	MA + VHPS	FCC + carbides	1652	461	[90]
CoCrFeNiMnN0.1	MA + VHPS	FCC + Cr2N	2141	468	[93]
[FeNi]50Cr15Mn10Al25	VM	BCC + B2	1660	-	[94]
AlCuSiZnFe	MA + SPS	FCC + BCC	1987	-	[95]
ZrTiNbMoCr	AM	BCC + Laves	1479	564	[96]
ZrTiNbMo	AM	BCC	2045	-	[96]
(ZrTiNbMo)98Cr2	AM	BCC + Laves	2528	-	[96]
CoCrFeMnNi	MA + SPS	FCC + carbides	3000	-	[97]
Re0.5NbMoTaW	AM	BCC + particle phase	1465 ± 18	567 ± 9	[98]

HEAs exhibit exceptional mechanical properties as advanced structural materials, particularly demonstrating ultra-high compressive strength. In general, HEAs with a BCC structure possess higher strength than those with an FCC structure. The incorporation of refractory elements such as Nb and Ta, precipitation-promoting elements like Al and Ti, and interstitial elements including C and Si can significantly enhance the strength and hardness of the material. Therefore, by employing high-density consolidation processes and designing compositions that lead to dual-phase microstructures or structures containing intermetallic precipitates, it is promising to develop novel HEAs with superior mechanical performance.

5. Hydrogen Storage Materials

Hydrogen storage alloy is an important functional material. HEAs have great application potential in hydrogen storage alloys due to their solid solution phase and severe lattice distortion, which can provide more suitable sites for the accommodation of H atoms [33] and significant void space for hydrogen. Compared with classical AB5/AB2/A2B intermetallic hydrides, HEAs allow the fine tuning of both the host lattice (site size, elasticity) and the hydride thermodynamics (plateau pressure, hysteresis) by mixing elements that strongly form hydrides with those that destabilize them. The HEAs studied for hydrogen storage can be roughly divided into three categories: BCC HEAs, lightweight HEAs, and intermetallic HEAs [99]. Among these, the HEAs with a BCC structure may be considered the most promising for hydrogen storage. Ti, V, and Cr are the most basic constituent elements in BCC HEAs, and the hydrides formed by the reaction of these elements with hydrogen have very low formation enthalpy, which means that these elements can be used as A elements requiring elements with high affinity with hydrogen in the composition of hydrogen storage alloys. Liu et al. [100] investigated the hydrogen storage properties of HEAs with a Ti-V-Cr-based BCC structure. Lightweight HEAs are primarily achieved by incorporating elements such as Mg/Al/Ti with a small fraction of transition metals to reduce density while retaining workable thermodynamics. Nanostructuring is usually required to overcome sluggish bulk diffusion in Mg-rich systems. For intermetallic-based HEAs (such as Laves-lean), they can store hydrogen in specific sublattices; they generally offer lower diffusion rates but improved cycling stability when finely dispersed or partially amorphized. Moreover, the surface electronic structure also affects the hydrogen storage properties of HEAs. In conventional hydrogen storage alloys, the process of hydrogen absorption is accompanied by the formation of surface sub-oxides [101]. For the HEAs, there are two stages in the process of hydrogen absorption: the transformation of high-valence oxides to sub-oxides and the transformation of sub-oxides to sub-hydroxides. The interaction between different elements in HEAs may lead to changes in the electronic structure of the alloy surface, which may reduce the stability of sub-oxides and promote the reaction from sub-oxides to sub-hydroxides. The OH⁻ ions in hydroxides can act as a fast pathway for the diffusion of hydrogen atoms, while oxides are generally considered to be barriers to hydrogen diffusion, so the surface hydroxides are more conducive to the penetration of hydrogen into the matrix than oxides [102]. Thus, the formation of sub-hydroxides on the surface may be the main reason for the significant improvement in hydrogen absorption performance of HEAs after activation [103].

During activation, high-entropy alloys develop a rich population of crystal defects [104,105], including dislocations, vacancies and vacancy clusters, sub-grain and grain boundaries, and, in FCC matrices, stacking faults and twins. Together these defects constitute a percolating network of fast diffusion pathways [106,107] that lowers the migration barrier for hydrogen and shortens the transport distance from surface to bulk, thereby eliminating induction periods and markedly enhancing apparent kinetics and diffusion coefficients. Mechanistically, dislocation cores and grain boundaries act as one- and two-dimensional “short-circuit” routes, while shallow traps (weakly bound vacancies and strained interfacial regions) transiently accommodate hydrogen, smooth pressure plateaus, and accelerate desorption; concomitantly, conversion of surface sub-oxides to sub-hydroxides introduces OH⁻-mediated channels that further reduce the entry barrier. Excessive defect densities or the formation of deep traps (coarse vacancy agglomerates, continuous interfaces with brittle second phases), however, can increase hysteresis, reduce reversible capacity, and promote pulverization. To further improve the hydrogen storage properties of HEAs, hydrogen storage alloys with a nanocrystalline structure were developed. Relative to their coarse-grained counterparts, nano-treated HEAs possess a much higher density of defects (dislocations, vacancies) and interfaces (grain/sub-grain boundaries), together with locally distorted or partially amorphous surface/interfacial layers. This hierarchical defect network provides abundant pathways for the rapid ingress/egress of hydrogen and lowers the activation barrier for dissociation and diffusion. Size reduction also increases the specific surface area and shortens the average diffusion length to particle centers, thereby accelerating both absorption and desorption. In addition, after the alloy is nanosized, the particle volume is reduced, which also shortens the diffusion distance of hydrogen to the center of the particle, thereby increasing the hydrogen storage rate of the alloy. Practically, nanostructuring can be achieved by mechanical alloying, severe plastic deformation, or additive manufacturing followed by controlled heat treatment. However, the caveat is important: excessive amorphization or intergranular embrittlement can cause decrepitation on cycling, and regrowth of dense surface oxides can partially negate the kinetic gains. The summarized features and properties of each of these HEAs are shown in Table 3.

Table 3. Hydrogen storage properties of HEAs.

HEA	Hydrogen Absorption Temperature/K	Phase(s)	H2 Absorption Capacity/wt%	H/M	Onset Desorption Temperature/K	Peak Desorption Temperature/K	Ref.
TiVZrNbHf	572	BCC	2.7	2.5	-	-	[33]
MgZrTiFe0.5Co0.5Ni0.5	623	BCC	1.2	0.7	-	-	[34]
TiZrNbTa	293	BCC	1.67	-	-	-	[35]
Ti0.2Zr0.2Nb0.3Mo0.1Hf0.2	373	BCC	1.54	-	605	-	[36]
Ti0.2Zr0.2Nb0.2Mo0.2Hf0.2	373	BCC	1.18	-	575	-	[36]
Ti0.2Zr0.2Nb0.1Mo0.3Hf0.2	373	BCC	1.40	-	437	-	[36]
TiZrNbCrFe	473	C14 + BCC	1.9	1.32	-	-	[108]
TiZrFeMnCrV	303	C14	1.80	-	-	-	[109]
TiZrNbHfTa	573	BCC	-	2.5	-	-	[110]
Ti0.2V0.2Zr0.2Nb0.2Hf0.2	573	BCC	2.1	1.94	623	-	[111]
MgAlTiFeNi	598	BCC	0.94	-	-	-	[112]
TiZrCrMnFeNi	293	C14	1.7	1.0	-	-	[113]
CoFeMnTi2.5VZr	353	C14	1.3	-	-	-	[114]
Mg0.10Ti0.30V0.25Zr0.10Nb0.25	298	BCC	2.70	1.72	523	563	[115]
(ZrTiVFe)90Al10	293	C14 + tetragonal + HCP	1.3	-	-	-	[116]
TiZrHfMoNb	523	BCC	1.041	-	486.2	503.709	[117]
TiZrHfMoNbPt0.0025	523	BCC	1.421	-	475.547	505.3	[117]
TiZrHfMoNbPd0.0025	523	BCC	1.604	-	461.523	473.355	[117]

6. Challenges

HEAs represent a novel multi-principal-element material class. The intrinsic properties of the constituent elements mean that high-entropy alloys have four core effects including high-entropy effect, sluggish diffuse, severe lattice distortion effect, and cocktail effect, which expand the application range of HEAs from structural materials to functional materials. However, high-entropy alloys also face numerous challenges, and more in-depth research is needed to address these issues.

In the realm of structural materials, the challenges are particularly complex. Firstly, the complexity of compositional design represents a fundamental bottleneck. While the multi-principal-element concept offers near-infinite elemental combinations, the precise screening of compositions that simultaneously possess high strength, high toughness, excellent creep resistance, and irradiation resistance remains a daunting task. Traditional trial-and-error approaches are inadequate for this vast compositional space, and the accuracy and generalizability of emerging predictive methods, such as machine learning and high-throughput calculation, still require substantial improvement. Secondly, the contradiction between performance enhancement and stability is a critical concern. For instance, when interstitial atoms are introduced to strengthen alloys, the segregation behavior of these atoms within different local chemical environments and their impact on stacking fault energy and grain boundary stability are not yet fully understood, posing significant challenges for precisely tailoring mechanical properties.

Compared to structural materials, the challenges for HEAs in hydrogen storage applications focus more intensely on optimizing thermodynamic and kinetic properties and mitigating cost. Currently, research on the hydrogen storage performance of HEAs is still in its nascent stages. A prominent issue is that the hydrogen absorption in many HEAs is characterized by excessive thermodynamic stability, necessitating high temperatures for hydrogen desorption. This compromises the overall energy efficiency of the storage system and limits practical application. Tuning the enthalpy of hydride formation through compositional design to achieve an optimal balance between high storage capacity and favorable thermodynamics is a major research focus and difficulty. Concurrently, the kinetics of hydrogen diffusion within the alloy bulk need urgent enhancement. Slow hydrogen absorption/desorption rates critically impact the system’s efficiency, demanding a deeper understanding of hydrogen diffusion pathways and energy barriers within the complex lattice and strain fields of HEAs. From an industrialization perspective, cost and scalable synthesis present another significant barrier. The global market for HEA-based hydrogen storage materials is projected to grow rapidly but remains small. Many compositions demonstrating superior performance often incorporate costly elements (e.g., Co, V), and their synthesis routes—requiring high homogeneity, activation treatments, and controlled nanostructures—are complex, leading to high costs that hinder competitiveness with established storage technologies. Table 4 summarizes the key challenges facing high-entropy alloys in the application domains of structural and hydrogen storage materials.

Table 4. Challenges facing high-entropy alloys in structural and hydrogen storage applications.

Application Field	Core Challenge	Specific Manifestations
Structural materials	Composition design and performance prediction	The immense compositional space poses a significant challenge for efficient screening, and the reliability of theoretical and machine learning predictions remains limited, hindering rapid and reliable discovery.
	Microstructure–property manipulation	The introduction of interstitial elements enhances strength; however, their distribution, interaction with defects, and effects on ductility and toughness are typically highly complex and difficult to precisely control.
	Long-term stability in extreme environments	Mechanisms of creep resistance, phase evolution, and performance degradation under high temperature, strong radiation, etc. are not sufficiently studied.
Hydrogen storage materials	Thermodynamic and kinetic properties	Overly strong metal–hydrogen bonds lead to high dehydrogenation temperatures; slow hydrogen diffusion kinetics within the complex lattice affect system efficiency and cycling rates.

7. Summary and Future Work

HEAs represent a significant paradigm shift in alloy design, moving beyond traditional single-principal-element systems. This review has comprehensively analyzed the compositional design strategies, phase formation rules, processing techniques, and application potentials of HEAs, with a particular focus on their roles as structural and hydrogen storage materials. The core of HEA design lies in leveraging the four fundamental effects—high-entropy effect, severe lattice distortion effect, sluggish diffusion effect, and cocktail effect—to achieve unprecedented property combinations. A critical achievement elucidated in this work is the establishment of robust criteria, such as the Ω parameter, valence electron concentration (VEC), and the interplay between atomic size difference (δ) and mixing enthalpy (ΔH_mix), for predicting the formation of stable solid solutions. This provides a vital theoretical framework for navigating the vast compositional space of HEAs. In structural applications, HEAs processed via techniques like mechanical alloying and spark plasma sintering have demonstrated exceptional mechanical properties, including ultra-high compressive strength exceeding 3 GPa in alloys like CoCrFeNiNb and TiZrNbMoTa, attributed to mechanisms such as solid solution strengthening, nanocrystalline formation, and strategic precipitation. For hydrogen storage, body-centered cubic (BCC)-structured HEAs have emerged as particularly promising, with compositions like TiVZrNbHf achieving remarkable capacities up to 2.7 wt.% H₂. Their performance is closely tied to the severe lattice distortion providing abundant interstitial sites and the catalytic effect of the surface sub-oxide to sub-hydroxide transformation during activation, which facilitates hydrogen dissociation and penetration. The key findings from this review are presented in Table 5.

Table 5. Key research findings on high-entropy alloys presented in this review.

Research Aspect	Key Finding	Example
Compositional Design and Elemental Effects	Systematic analysis of how individual alloying elements influence phase formation (FCC/BCC/Laves), microstructure, and properties.	Al promotes BCC phase; Ni stabilizes FCC phase; Mo/Nb promote Laves phase formation.
Solid Solution Formation Rules	Established predictive criteria (Ω ≥ 1.1, VEC, δ-ΔHmix) for forming stable solid solutions, crucial for navigating the vast composition space.	The Ω parameter incorporates Tm, ΔSmix, and ΔHmix to accurately predict solid solution stability.
Advanced Processing Techniques	Mechanical alloying (MA) combined with spark plasma sintering (SPS) is a highly effective route for producing HEAs with refined, homogeneous microstructures.	MA + SPS produced TiZrNbMoTa alloy with a compressive strength of 3759 MPa.
Structural Material Performance	HEAs can achieve a combination of ultra-high strength and good ductility, often through nanocrystalline structures and secondary phase strengthening.	CoCrFeNiNb (2412 MPa), AlCrFeNiMo0.2 (3222 MPa), and CoCrNiCuZn (3121 MPa) exhibit exceptional compressive strength.
Hydrogen Storage Material Performance	BCC-structured HEAs show great promise for hydrogen storage, with high capacity and tunable thermodynamics/kinetics, enhanced by surface activation.	TiVZrNbHf alloy achieved a hydrogen capacity of 2.7 wt.%; surface sub-hydroxide formation aids hydrogen absorption kinetics.

Despite ongoing research, applying these alloys practically remains challenging. Their extreme compositional complexity fundamentally slows down and complicates the search for the best-performing alloys. Current computational methods, often reliant on limited binary or ternary databases, struggle to precisely predict phase formation under non-equilibrium processing conditions like MA. For hydrogen storage, key issues include cycling stability—where repeated hydriding/dehydriding induces massive volume changes, lattice strain, and micro-cracking, leading to pulverization and capacity decay—and the thermodynamic tailoring of hydride stability to lower dehydrogenation temperatures. Looking ahead, the advancement of HEAs is critically dependent on the quality and quantity of available data resources. Currently, the limited data call for integrated approaches combining CALPHAD databases, high-throughput experimentation, and machine learning to accelerate the design and optimization of new alloy compositions. Furthermore, the segregation behavior of atoms within HEAs, which varies across complex local chemical environments, poses significant analytical challenges that cannot be adequately addressed by conventional characterization techniques. Therefore, achieving precise control over the microstructure of high-entropy alloys requires the deep integration and synergistic collaboration of computational alloy design, advanced synthesis techniques, and high-end characterization methods. This approach is expected to further expand the application prospects of high-entropy alloys, promote their large-scale adoption in next-generation structural components and solid-state hydrogen storage systems, and thereby provide solid technological support for building a sustainable energy future.