Figure 3a shows the entropy of mixing, calculated according to Equation (2) depending on the number of elements in equimolar alloys
[30]. Thus, for equimolar two-, four-, and five-component alloys, the entropy of mixing was equal to 5.76, 11.52, and 13.37 J/K mol, respectively. Based on the entropy approach, alloys can be roughly divided into three categories according to their entropy of mixing on the supposition of random distribution of elements in a solid solution, namely (i) low-entropy alloys (traditional alloys) with one or two main elements with
ΔSmix≤5.76 or
0.69 R, (ii) medium-entropy alloys with two to four base elements with entropy of mixing in the range of
0.69 R<ΔSmix<1.61 R, and (iii) high-entropy alloys with five or more base elements with
1.61 R≤ΔSmix, as shown in the inset in
Figure 3a.
An increase in the number of components in an alloy increases the configurational entropy, which reduces the Gibbs energy, which is also facilitated by an increase in temperature. The competing factor is the change in enthalpy; however, at a high entropy of mixing and a total negative change in the Gibbs energy, a homogeneous disordered solid solution is formed instead of a multiphase system. In this case, possible processes of spinodal decomposition, the formation of intermetallic compounds, and the appearance of ordered structures and secondary phases are suppressed. It is also noted in
[30] that in HEAs the percentage of individual components in the HEA can vary from 5 to 35%, which significantly expands the range of materials under consideration. It is worth noting the surprising result obtained by Senkov et al. in
[31]. The authors evaluated 134,547 alloy systems using a calculated phase diagram (CALPHAD) method and found that the formation of solid solution (SS) or intermetallic alloys (IM) became less likely as the number of elements increased, while the probability of the formation of mixed SS + IM phases increased (
Figure 3b). As the number of elements increases,
ΔSmix rises slowly while the probability of at least one pair of elements favoring IM formation increases more rapidly, explaining this apparent contradiction with the major principle of HEAs.
where
is the electronegativity of the
i-th element, and
χ¯¯ is an average electronegativity. The low value of the
Δχ criterion ensures a uniform ability to attract electrons across the lattice, leading to a stable solid solution
[36]. Polletti and Battezzati assigned the values of
Δχ<6% and
δ<6% as a guideline for the selection of elements to predict new HEAs
[37]. The stability of a high-entropy state in HEAs is also affected by such parameters as the density of valence (VEC) and free electrons (e/a), i.e., their number per atom
[37][38].
In addition to the configuration component, the total value of entropy also includes temperature-dependent contributions, vibrational, electron, and magnetic. Using finite-temperature ab initio methods, Ma et al.
[39] revealed that entropy contributions beyond the configurational contribution were crucial for determining phase stability (for hcp, fcc, and bcc structures) and such properties of the Cantor alloy (CoCrFeMnNi) as thermal expansion and bulk modulus. According to their results, electronic and magnetic entropies can contribute up to 50% of the configurational entropy value. Shuo Wang et al.
[40], when considering electronic and thermodynamic properties of this alloy in nonmagnetic (NM) and ferrimagnetic (FIM) states, demonstrated that the magnetic properties of the constituent atoms play an important role in the thermodynamics of this HEA. Calorimetric measurements of thermal entropy of the series of the HEAs, including the Cantor alloy by Haas et al.
[41], showed that it was a few folds higher than the configurational contribution. However, for alloys with the same crystal structure, the thermal contributions did not depend on the number and concentration of the alloying elements; therefore, it was directly demonstrated that despite the thermal entropy being significantly higher than the configurational entropy, it does not increase the thermal stability of HEAs relative to simple alloys or pure metals.
It is worth mentioning that an increasing number of recent studies have been revealing that the formation of single-phase solid solutions in HEAs show a weak dependence on maximization of
ΔSmix through equiatomic ratios of elements. Moreover, the entropy maximum has been shown to be not the most significant parameter in the creation of multicomponent alloys with high functional properties. On the basis of these studies, the review by Li and Raabe
[42] demonstrated that by changing the strategy of creating alloys from single-phase equiatomic to two- or multi-phase non-equiatomic, it is possible to obtain high-strength and superplastic HEAs.
2.2. HEA Synthesis Methods
A wide range of synthesis techniques has been developed for HEAs. Methods of synthesis, both bulk and powder-based, have been reviewed in a number of review papers
[10][43][44]. The most commonly used solid-state method to obtain HEA powders is that of metal alloying as it is the most simple and most suitable method due to its increased solid solubility, high level of homogeneity, and room-temperature processing
[45]. The method includes high-energy planetary ball milling (usually using stainless steel balls) in a protective milling media (
Figure 4a). Mechanical alloying is followed by the consolidation of HEAs using, for example, spark plasma sintering (SPS)
[46]. The second most commonly used method to obtain full pre-alloyed HEA powders suitable, for example, for additive manufacturing is that of atomizing, using different gas or liquid streams (
Figure 4b)
[47]. HEAs can also be manufactured via liquid-state synthesis methods using melting and casting techniques such as conventional arc melting
[48][49] (
Figure 4c), as well as vacuum induction melting, directional solidification, infiltration (
Figure 4d), etc. Gas-state HEAs synthesis methods, through the deposition of thin HEAs films, include the plasma spray process (
Figure 4e), thermal spraying, and magnetron sputtering (
Figure 4f), molecular beam epitaxy, vapor deposition, etc. More detailed descriptions of liquid and gas-state methods are given in the recent reviews
[50][51]. Among the most cost-effective methods to form HEA coatings is electrochemical deposition which, it should also be noted, does not require complicated equipment
[52]. This provides the opportunity to control the film thickness and content simply by regulating the deposition parameters such as current density and applied potential.
Figure 4. HEA synthesis methods: (
a) metal alloying; (
b) atomization; (
c) arc melting; (
d) infiltration; (
e) plasma spraying; (
f) sputtering; (
g) carbothermal shock technique; (
h) fast-moving bed pyrolysis ((
a,
g,
h) are reprinted with permission from
[28], Copyright (2020) American Chemical Society; (
b,
d,
f) are reproduced from
[50] with permission of Springer Nature; (
e) is reproduced from
[51] with permission of the Royal Society of Chemistry).
Nanosized HEAs are reported to have superior performance in the fields closely related to the fuel cell technology, namely catalysis, energy storage, and conversion
[28][53][54]. However, their production is challenging due to the critical conditions that are required for the synthesis, such as heating up to extremally high temperatures followed by rapid cooling to “freeze” the nonequilibrium state. There are a few synthesis techniques which allow HEA nanoparticles to be obtained: a carbothermal shock technique
[55] (
Figure 4g), sputter deposition
[56], solvothermal synthesis
[57], and fast-moving bed pyrolysis method
[58] (
Figure 4h), a microwave heating method that utilizes carbon substrates
[59], etc.
Kinetically controlled laser synthesis was reported as a new, highly reproducible, and scalable method to obtain HEA’s nanoparticles. Due to fast kinetics, it allows the formation of a large number of isolated ultrafine nanoparticles with properties close to those of the ablation target used
[60]. Recently, Wang et al. proposed using laser scanning ablation as a simple and general approach to synthesizing both high-entropy alloy and ceramic nanoparticles
[61]. The advantage of this method is that it can be implemented at atmospheric temperature and pressure, both for synthesis where there is a substrate or where no substrate is used. The ultrarapid process ensures the synthesis of HEA from up to nine metallic elements regardless of their thermodynamic solubility.
2.3. HEAs Applications
Although conventional alloys are presently used in modern advanced applications, it should be noted that the development of new alloys based on one or two major elements gradually approached the limit of feasible combinations at the end of the twentieth century
[62]. This saturation created certain difficulties in meeting material requirements in the face of the anticipated technology-driven performance leap. Under these circumstances, HEA and related materials can provide new advanced features. Studies undertaken in HEA high temperature applications have shown that appropriate composition design and process selection can lead to HEA replacing traditional alloys for such energy-related applications as energy conversion and storage
[63], hydrogen storage
[27][64], catalysis
[28], electrocatalysis
[65][66], electrocatalysis for hydrogen evolution, oxygen evolution, and oxygen reduction reaction
[67][68], surface electrocatalysis
[69], nuclear power
[70][71][72], lithium and sodium batteries
[73][74], and coatings for energy applications
[75].