The Cocktail Effect in High-Entropy Alloys: A Comprehensive Overview
High-entropy alloys (HEAs), also known as multi-principal element alloys, have emerged as a novel class of materials, captivating the academic and industrial worlds alike. These materials, characterized by their unique "cocktail" effect, are revolutionizing materials science with unprecedented combinations of properties.
Typical microstructure of a High-Entropy Alloy
What are High-Entropy Alloys?
High-entropy materials (HEMs), including high-entropy alloys (HEAs), high-entropy oxides (HEOs), and other high-entropy compounds, have garnered significant interest in recent years. HEAs are multicomponent solid solution materials that contain five or more elements in near-equiatomic proportions. This unique composition stabilizes their structures by maximizing the configurational entropy.
The term "high-entropy alloys" was coined by Taiwanese scientist Jien-Wei Yeh because the entropy increase of mixing is substantially higher when there is a larger number of elements in the mix, and their proportions are more nearly equal. These unconventional structures provide opportunities for achieving unprecedented combinations of phase stability and mechanical performance, especially overcoming the strength-ductility trade-off.
High-entropy materials (HEMs) including the above compounds, are defined as any solid solution materials that consist of quasi-equimolar multicomponent.
The Four Core Effects of HEAs
It is believed that the excellent performances of HEAs are promoted by four “core effects”:
- The high-entropy effect
- The lattice distortion effect
- The sluggish diffusion effect
- The “cocktail” effect
All of which are pivotal to structure-property studies in this field.
For our view, the most unique structural character of HEAs is the antisite disordering of atomic types within an ordered crystal. This character could be extended to compounds and other applications.
In addition to thermodynamic stability, the uniform configuration of elements with different sizes through a single-phase solid solution also leads to severe lattice distortion, which then increases the activation barrier of diffusion (the sluggish diffusion effect) and improves kinetic stability.
Synthesis of HEAs
There are many physical or chemical synthetic routes being developed in the past 20 years to fabricate HEMs. For energy-related applications, HEAs usually adopt nanostructures as nanoporous or NPs to maximize the surface area. Dealloying of bulk alloy is a common method to fabricate nanoporous HEA (such as AlMoCuPdAu, AlMnCoIrMo, AlNiCuPtPdAu, etc.) with enhanced surface area and uniform pore structures, ensuring high catalytic activity.
Wet-chemistry is the general method to fabricate NPs through reduction of metallic precursors in solution. The general solution-based common chemical synthesis might not be adequate for obtaining homogeneous nano-HEA with multi-components, and often severe decomposition or shock syntheses or reduction reactions are required.
Liu et al. take advantage of ultrasonication to develop a facile ultrasonication-assisted wet-chemistry for preparing HEA NPs. In another case, Yao et al. developed the CTS method to prepare multicomponent HEA NPs.
(A) Carbothermal synthesis of HEA-NPs on carbon supports. Reproduced with permission from Yao et al. (2018). (B) Nanodroplet-mediated electrodeposition synthesis controlling nanoparticle stoichiometry and microstructure. Reproduced with permission from Glasscott et al. (2019).
Precursor of metal salt mixtures was thermally shocked at ∼ 2,000 K within 55 ms (as shown in Figure 2A), alloying up to eight dissimilar elements into single-phase solid-solution NPs (PtPdCoNiFeCuAuSn, etc.) loaded onto carbon support. This method can synthesize a wide range of multicomponent NPs with desired compositions and proper sizes by controlling the CTS parameters as well as supports, which may be a general route toward high performance and cost-effective catalysts.
However, productivity of HEA NPs via CTS is very low. Future work to enhance the productivity for industrial applications is needed. Furthermore, Gao et al. developed a fast-moving bed pyrolysis (FMBP) method to prepare HEAs with diverse supports. Mixed metal precursors were rapidly heated to ∼ 923 K (higher than their pyrolysis temperature) within 5 s.
For example, solvothermal, co-sputtering, and deposition have also been successfully developed to prepare HEA NPs. Electrodeposition can be used to produce multi-component HEMGs as PtCoFeLaNi, by confining multiple metal salt precursors to water nanodroplets emulsified in dichloroethane within 100 ms electroshock as shown in Figure 2B. The contents of nanodroplet are entirely reduced during collision, enabling disordered co-deposition of different metal precursors.
CrMnFeCoNi NPs with an averaged diameter of 1.7 nm were synthesized by combinatorial co-sputtering into the [Bmim]-[Tf2N]. It was proposed that the key factor to synthesize HEA NPs is to control the cooling rate. A proper cooling rate, ranging from several seconds to several minutes depending on the system or synthesis method, facilitated the formation of single-phase solid solutions with certain crystal structures, enabling HEA structures.
Crystal Structures of HEAs
HEAs can have cubic crystal structure with fcc lattice (Figure 3A) and bcc lattice (Figure 3B), hexagonal crystal structure (Figure 3C), and other structures. Fcc lattice is by far the most extensively investigated structure of HEAs.
For energy-related applications, nano-HEAs such as CoMoFeNiCu and CoFeLaNiPt with fcc lattice have a lattice parameter of a ≈ 3.6 Å.
HEAs containing heavy hcp metals (mainly Os, Re, Ru, and Zr) could form hcp structure. Yusenko et al. synthesized Ir0.19Os0.22Re0.21Rh0.20Ru0.19 HEA with space group P63/mmc and lattice parameters а = 2.728 Å and c = 4.338 Å.
High-Entropy Oxides (HEOs)
Rost et al. discovered the entropy-stabilized oxides and named them as HEOs. High-entropy compounds can be synthesized from binary metal compound precursors. For HEO in bulk form, traditional methods including solid-state sintering, spark plasma sintering, and reactive flash sintering have been successfully developed.
With proper solvent, heating/cooling rates, and reaction temperatures, shocking methods as CTS, FMBP, and pulsed laser deposition (PLD) might be adequate to fabricate HEO NPs from bulk HEOs similar with HEA. Films of (Mg0.2(1-x)CoxNi0.2(1-x)Cu0.2(1-x)Zn0.2(1-x))O (x = 0.2, 0.27, 0.33) were fabricated by solid-state sintering combined with PLD method.
Wet-chemistry methods such as solution combustion, flame spray pyrolysis, nebulized spray pyrolysis (NSP), reverse co-precipitation, etc., have been successfully applied to the fabrication of (CoCuMgNiZn)Ox HEO NPs. Among these methods, the NSP method might have broad prospect because it can directly synthesize nanocrystalline single phase.
HEOs with fcc lattice constitute a large portion of energy-related applications such as catalysis and battery electrode materials. Among fcc structures, rock-salt HEO of (MgCoNiCuZn)Ox was synthesized by Rost et al. (Figure 3D). Extended X-ray absorption fine structure and scanning transmission electron microscopy (STEM) analyses reveal that the first-near-neighbor cation-to-anion distances of this compound are identical.
With a substitution of Li+ up to 16.6%, this HEO still keeps its rock-salt structure, shedding potential for energy-storage applications. Recently, Jimenez-Segura et al. reported that this kind of rock-salt HEO exhibits a long-range magnetic order coexisted with the component disorder, and Zhang et al.
Berardan et al. studied the distortion and atomic disordering of this HEO using X-band electron paramagnetic resonance spectroscopy. They proposed that post-annealing treatment can adjust the local environment of Cu2+, from octahedral in air-quenched Cu-sub-stoichiometric samples to rhombic in Cu-enriched HEOs, and then adjust the Jahn-Teller distortion and the dielectric property. The results agreed with the density functional theory (DFT) calculations by Rák et al.
HEOs composed of multicomponent rare earth elements as (Ce,La,Pr,Sm,Y)2O3+x synthesized by NSP method have CaF2-type structures. Interestingly, Djenadic et al. suggested that they cannot maintain single phase without Ce (such as (Ga, La, Nd, Pr, Sm, Y)O). In addition, Chen et al. found that (HfZrCeTiSn)O2 with CaF2 structure exhibits a single phase at high temperature and multiphase at low temperature, which is believed as an evidence of entropy stabilization.
Spinel HEOs (as shown in Figure 3E) such as (Cr,Fe,Mg,Mn,Ni)3O4 and (Co,Mg,Mn,Ni,Zn) Al,Co,Cr,Fe,Mn)2O4 with a space group of Fd3¯m can be synthesized by solid-state sintering. Based on NiFe2O4, (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)Fe1.9(Dy0.02Er0.02Gd0.02Ho0.02Tb0.02)O4 were synthesized by sol-gel method enhanced by entropy stabilization.
HEMs with perovskite structure (ABX3) have fruitful physical properties with a broad range of energy-related applications, which contains 12-fold coordinated A-site cations, 6-fold coordinated B-site cations, and X anion octahedra (Figure 3F). Methods similar to preparation of HEOs can be used to synthesize other high-entropy compounds.
Sure et al. synthesized nanoscale HEC of (TiNbTaZrHf)Cx with fcc lattice via a facile electrochemical process. Liu et al. developed a facile borothermal reduction method to produce HEB of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2 in hexagonal crystal structure with average particle size of 310 nm.
Table 1 summarizes the synthetic methods for the construction of efficient nano-HEMs with well-controlled composition, size, and uniformity, including both top-down and bottom-up methods. For the top-down methods, proper cooling/heating rate and appropriate duration time are required to be well controlled to prepare HEMs with adequate size. For the bottom-up methods based on wet-chemistry, low cost and high productivity are key factors for industrial production.
HEMs provide a large amount of surface sites with different atomic environments associated with various adsorption energies. For example, heterogeneous catalysts may speed up chemical reactions at active sites on the surface. HEM-based catalysts provide plentiful possible combinations of surface structure (then electronic structure) and the opportunity to tune catalytic performances.
Crystal structures of HEAs and HEOs: (a) fcc lattice, (b) bcc lattice, (c) hcp lattice, (d) rock-salt structure, (e) spinel structure, (f) perovskite structure.
The Cocktail Effect: Enhancing Alloy Properties
The cocktail effect is used to emphasize the enhancement of the alloy's properties by at least five major elements. Because HEAs might have one or more phases, the whole properties are from the overall contribution of the constituent phases. Besides, each phase is a solid solution and can be viewed as a composite with properties coming not only from the basic properties of the constituent, but by the mixture rule also from the interactions among all the constituents and from severe lattice distortion.
Early Research and Development
Although HEAs were considered from a theoretical standpoint as early as 1981 and 1996, and throughout the 1980s, in 1995 Taiwanese scientist Jien-Wei Yeh came up with his idea for ways of actually creating high-entropy alloys, while driving through the Hsinchu, Taiwan, countryside. Soon after, he decided to begin creating these special alloys in his lab, being in the only region researching these alloys for over a decade.
A few months later, after the publication of Yeh's paper, another independent paper on high-entropy alloys was published by a team from the United Kingdom composed of Brian Cantor, I. T. H. Chang, P. Knight, and A. J. B. Vincent. Yeh was also the first to coin the term "high-entropy alloy" when he attributed the high configurational entropy as the mechanism stabilizing the solid solution phase.
Phase Formation and Stability
Phase formation of HEA is determined by thermodynamics and geometry.
where is defined as enthalpy of mixing, is temperature, and is entropy of mixing respectively. and continuously compete to determine the phase of the HEA material. Disordered solids form when atomic size difference is small and is not negative enough. This is because every atom is about the same size and can easily substitute for each other and is not low enough to form a compound. More-ordered HEAs form as the size difference between the elements gets larger and gets more negative. When the size difference of each individual element become too large, bulk metallic glasses form instead of HEAs.
The multi-component alloys that Yeh developed also consisted mostly or entirely of solid-solution phases, contrary to what had been expected from earlier work in multi-component systems, primarily in the field of metallic glasses. Yeh attributed this result to the high configurational, or mixing, entropy of a random solid solution containing numerous elements.
From this it can be seen that alloys in which the components are present in equal proportions will have the highest entropy, and adding additional elements will increase the entropy. However, entropy alone is not sufficient to stabilize the solid-solution phase in every system. The enthalpy of mixing (ΔH) must also be taken into account.
Zhang et al. found, empirically, that in order to form a complete solid solution, ΔHmix should be between -10 and 5 kJ/mol.
The atomic radii of the components must also be similar in order to form a solid solution.
The multi-element lattice in HEAs is highly distorted because all elements are solute atoms and their atomic radii are different. helps evaluating the lattice strain caused by disorder crystal structure. When the atomic size difference (δ) is sufficiently large, the distorted lattice would collapse and a new phase such as an amorphous structure would be formed.
Deformation and Strengthening Mechanisms
In terms of deformation, dislocation slip is the primary mode of plasticity. In FCC HEAs such as the CrMnFeCoNi Cantor alloy, slip often occurs on {111} planes and produces long pile-ups, while cross-slip is relatively difficult. At cryogenic temperatures, deformation twinning is activated, which greatly enhances work-hardening and ductility.
Strengthening in HEAs arises from several sources. Solid solution strengthening is particularly strong because near-equiatomic compositions generate significant lattice distortion that impedes dislocation motion. The stacking-fault energy (SFE) is another key factor: high-SFE alloys deform mainly by slip, whereas low-SFE alloys can activate twinning (TWIP) or martensitic transformation (TRIP), both of which improve strength-ductility combinations.
Predicting Crystal Structure
For those alloys that do form solid solutions, an additional empirical parameter has been proposed to predict the crystal structure that will form. HEAs are usually FCC (face-centred cubic), BCC (body-centred cubic), HCP (hexagonal close-packed), or a mixture of the above structures, and each structure has their own advantages and disadvantages in terms of mechanical properties. There are many methods to predict the structure of HEA.
Valence electron concentration (VEC) can be used to predict the stability of the HEA structure. When HEA is made with casting, only FCC structures are formed when VEC is larger than 8. When VEC is between 6.87 and 8, HEA is a mixture of BCC and FCC, and while VEC is below 6.87, the material is BCC. In order to produce a certain crystal structure of HEA, certain phase stabilizing elements can be added.
Solid-State Processing
Solid-state processing is generally done by mechanical alloying using a high-energy ball mill. This method produces powders that can then be processed using conventional powder metallurgy methods or spark plasma sintering. The conventional method of mechanical alloying mixes all required elements in one step, where elements A, B, C, and D get milled together to form ABCD directly.
Vaidya et al. proposed a new method of creating HEA with mechanical alloying called sequential alloying, where elements are added step by step. In order to create AlCrFeCoNi HEA, Vaidya's team first formed binary CoNi alloy, then added Fe to form tertiary FeCoNi, Cr to form CrFeCoNi, and Al to from AlCrFeCoNi. The same alloy composition can be produced through different sequences, and different sequences lead to different proportions of BCC and FCC phases, showing a path dependence on the method.
Computational Modeling
The atomic-scale complexity presents additional challenges to computational modelling of high-entropy alloys. Thermodynamic modeling using the CALPHAD method requires extrapolating from binary and ternary systems. Most commercial thermodynamic databases are designed for, and may only be valid for, alloys consisting primarily of a single element. Thus, they require experimental verification or additional ab initio calculations such as density functional theory (DFT).
However, DFT modeling of complex, random alloys has its own challenges, as the method requires defining a fixed-size cell, which can introduce non-random periodicity. This is commonly overcome using the method of "special quasirandom structures", designed to most closely approximate the radial distribution function of a random system, combined with the Vienna Ab initio Simulation Package.
| Property | Description |
|---|---|
| High Entropy Effect | Promotes the formation of solid solutions and simplifies microstructure. |
| Lattice Distortion Effect | Caused by atomic size differences, leading to lattice strain and stress. |
| Sluggish Diffusion Effect | Diffusion vacancies are surrounded by different element atoms, affecting diffusion rates. |
| Cocktail Effect | Enhancement of alloy properties due to the combination of multiple elements. |