High entropy alloys: PVD has a major role to play
Over the last five millennia, alloys have aided and shaped the advancement of society. A remarkable progression from native metals and native alloys to the inadvertent discovery of arsenical bronzes. Numerous combinations of alloying elements, typically based on a single primary element, were tested. In alloys which including high-tin bronzes and ultrahigh-carbon steels, greater quantities of alloying were utilized. From such concentrated binary alloys to multicomponent alloys signified a significant advancement in the previous century, resulting in alloy steels and superalloys. Nonetheless, all these alloys had one metal component in a disproportionately large amount. In 2004, Jien-Wei Yeh and Brian Cantor separately revealed multicomponent equiatomic or near-equiatomic alloys, marking a new milestone in alloying. Surprisingly, several of these alloys were solid solutions comparable to bronzes from the third millennium BCE. They have revitalized the material world, offering an incredibly diverse family of alloys.
Alloying is the most useful discovery in the history of metallurgy. Although in metallurgy, where pure metals are of little utility, but many alloys have many applications, the English literatures insist on unalloyed joys, meaning that the sense of pleasure must be pure and not admixed with other emotions. This concept of alloying is not just useful for metals, though. Alloying is a technique that may be used to both polymers and ceramics. Further development is possible by combining different kinds of materials into composites.
The society's civilizational journey began with the discovery of pure metals such as gold and copper. We now have access to an enormous amount and variety of materials. The Ashby map (Ashby, 2011) depicted in the following figure provides a panoramic picture of the evolution of material consumption over 10 millennia. A visual representation of the many groups of materials, ranging from ceramics through metals, polymers, and, more recently, composites, is depicted. It is possible to trace the path from discovery through development to material design. Ashby's (2011) chart of strength against density, illustrated in the following figure, vividly depicts the filling of material-property space from 50,000 BCE to the present scenario. In terms of time, the most filling has happened in the last 50 years, during which metal, ceramic, and composite envelopes have expanded significantly, and new envelopes of synthetic polymers and foam materials have taken up substantial space. However, the filled region appears to be approaching some basic boundaries beyond which it is difficult to progress any farther.
Alloys have grown from simple to complicated compositions based on mankind's capacity to produce the materials. The increased functions and performances of alloys that arise allow civilizations to develop. Significant evolution and improvement over the previous century have resulted in the development of specific alloys such as stainless steels, high-speed steels, and superalloys. Although alloys made of many elements have higher mixing entropy than pure metals, the better qualities are mostly due to mixing enthalpy, which allows for the inclusion of appropriate alloying components to boost strength and improve physical and/or chemical properties. More complicated compositions with greater mixing entropies have been introduced since the turn of the century. Such complex compositions do not always imply a complex structure and microstructure, as well as the associated brittleness. In contrast, complex compositions with considerably higher mixing entropy might simplify the structure and microstructure while still imparting appealing features to the alloys. The HEAs that have been reported till today have either the FCC or BCC structure and no HCP structured HEAs, as shown in figure 3. For example, CoCrCuFeNi HEA alloys have an FCC crystalline structure, and Al3CoCrCuFeNi HEA have a BCC crystalline structure. Guo et al. in 2011 suggested to use the valence electron concentration (VCE) to predict the BCC and FCC structured solid solutions of HEAs. Figure 4 illustrates the relationship between VEC and the FCC, BCC structure according to Gu et al. in the paper “Effect of valence electron concentration on stability of fcc or bcc phase in high entropy alloys”, Journal of Applied physics 109, 103505, 2011.
In reports published in 2004, Jien-Wei Yeh and Brian Cantor separately reported the viability of high-entropy alloys and equiatomic multicomponent alloys. Across the last decade, this breakthrough in alloying principles has propelled research on these novel materials all over the world.
Even though bulk HEAs were discovered to be synthesizable and had promising qualities, high-entropy alloy (HEA) coatings in the form of thin or thick films on substrates have been investigated for last two decades. As the HEA idea and their primary effects are equally relevant to ceramic materials, research on HEA coatings includes nitrides, carbides, and oxides as well as their mixed forms. The standard thin-film and thick-film processes might both be used for HEA coatings and have proven to be effective in creating high-quality films. Furthermore, when optimal HEA formulations and procedures are devised, promising qualities and properties superior to those of traditional coatings might be obtained.
Each high-entropy alloy consists of a number of elements, frequently five or more in equiatomic or nearly equiatomic ratios, as well as minor elements (Yeh et al., 2004). The fundamental idea underlying HEAs is that solid solution phases with noticeably large mixing entropies are more stable than intermetallic complexes, especially at high temperatures. This improvement makes it possible for us to quickly synthesize, process, analyze, modify, and use them. HEAs are generally understood to be alloys of at least five primary elements, each of which has an atomic proportion between 5% and 35%. Thus, each minor element's atomic proportion, if any, is less than 5%.
Due to the blending of at least five basic elements, high entropy alloys are a novel class of materials with exceptional physical features. Figure 6 shows the hardness of some reported HEAs in the descending order with stainless steel as reference. Figure 7 is a classification of materials according to their yield strength and density. It is shown that high entropy alloys have densities close to the steel but have higher specific strength (yield strength/density). Furthermore, these substances have straightforward crystal structures that are extremely stable at high temperatures (often in the bcc or amorphous phase). Figure 8 shows a schematic illustration of a HEA BCC crystal structure.
Interstitial high entropy alloys (nitrogen, carbon, oxygen, and boron) have undergone extensive investigation due to their superior mechanical, physical, and chemical characteristics. The creation of nitride films and coatings based on alloys with high entropy and various inherent architectures is gaining special consideration. Various deposition techniques determine the composition of space, the structural-phase state, microstructure, and physico-mechanical characteristics of high entropy nitrides. The summary of the link between microstructures and characteristics opens compositional possibilities for developing novel materials with distinctive properties that will meet the needs of contemporary material sciences.
For example, Yi Xu et al. have obtained super-hard (AlCrTiVZr)N HEAs nitrides (48 GPa) using high power impulse magnetron sputtering (HiPIMS) at a bias voltage of -150V that resulted in low average grain size, very dense microstructure, very smooth surface, as well as optimized compressive residual stress.
Figure 9.a shows the hardness of the coating (AlCrTiVZr)N obtained as a function of the bias voltage and the corresponding microstructure. (Source: Effect of bias voltage on the growth of super-hard (AlCrTiVZr)N high-entropy alloy nitride films synthesized by high power impulse magnetron sputtering, Applied Surface Science, Volume 564, 2021, 150417)
Since their discovery, high entropy alloys (HEAs) have mostly been studied for their mechanical and structural characteristics. A rising number of areas, including magnetic, magnetocaloric, hydrogen storage, corrosion resistance, thermoelectric, superconducting behaviours, irradiation, catalysis, and biological characteristics, are being investigated, and this has increased interest in creating high entropy functional materials. These characteristics are connected to the particular structural characteristics of HEAs and their fundamental characteristics, such as lattice distortion and the development of multicomponent solid solutions stabilised by entropy.
HEA for hydrogen storage
One of the biggest issues facing human society's future is ensuring a sufficient and safe source of energy. Given its availability, high energy density, and environmental friendliness of the oxidation product, hydrogen has emerged as one of the most plausible prospective contenders to replace the current fossil fuel-based economy. The relatively low density of hydrogen at standard temperature and pressure, the energy-intensive liquefaction procedures, and the safety concerns associated with handling pressurized hydrogen canisters provide the major obstacles to its usage as an energy carrier for fuel cell applications.
Due to the high pressure or extremely low temperatures required, the traditional storage choices of compressed gaseous storage, liquid storage, etc., are somewhat less appealing, which in turn raises questions about safety and the need for energy-intensive processes for storage.
Although it is conceivable, the low gravimetric hydrogen storage capacity of these materials makes physical storage of molecular hydrogen inside the porous cavities of various high surface area materials—such as carbon-based materials, metal-oxide frameworks, zeolites, etc.—unsuitable for practical applications. Because of their large storage capacity per volume and reversibility, metal hydrides are now recognized as a top choice for hydrogen storage applications. Metal hydride batteries employ lanthanum nickel metal hydrides, which have been made commercially available. However, one of the main disadvantages of using these hydrides as hydrogen storage materials in fuel cell vehicle applications is their limited gravimetric storage capacity.
The multicomponent high entropy alloys provide us the chance to change their structure and characteristics to suit our needs. It has recently been noted that body-centered cubic or Laves phase high entropy alloys have a propensity to absorb hydrogen at room temperature. High entropy alloys' capacity to absorb hydrogen offers considerable potential for the development of several metal-hydrogen systems with practical applications, including hydrogen storage materials for fuel cell cars and metal hydride batteries, among others.
Other applications of HEA
High entropy alloys have several extremely amazing functional characteristics, including magnetic properties, magnetocaloric applications, hydrogen storage capabilities, corrosion resistance features, thermoelectric applications, and superconducting behavior. It has been noted that improving functional characteristics does not automatically favor increasing entropy. High entropy alloy functional qualities can also be influenced by the elemental composition and electronic structure of the alloy, in addition to being dependent on the fundamental structural characteristics of high entropy alloys.
In the case of magnetic materials, lattice distortion results in the pinning of domains, which increases coercivity at the price of electrical conductivity. High entropy alloys, however, actually have lower saturation magnetization and lower Curie point temperatures due to lower ferromagnetic component fractions.
It has been discovered that for magnetocaloric applications, rare earth-free high entropy alloys have a higher Curie point temperature whereas rare earth-containing high entropy alloys have higher magnetic entropy and refrigeration capacity. The goal of next study should be to strike a balance between them.
The influence of environmental factors, elemental additions, synthesis techniques, etc. on the corrosion-resistance characteristics of high entropy alloys are explored. High entropy alloys have corrosion characteristics that are equivalent to those of standard alloys and may be employed in a variety of settings. Figure 10 shows the performance of HEAs in corrosion resistance with respect to other alloys, in two different mediums.
It has been highlighted that surface passive coatings are crucial for improving the behavior of high entropy alloys in terms of corrosion resistance. Molybdenum addition and inhibitor inclusion could be helpful to increase corrosion resistance. However, improving corrosion resistance does not necessarily result from adding elements. Due to elemental segregation, the addition of elements like aluminum, copper, etc. causes the production of non-uniform passive films, which enhances galvanic corrosion and causes the films to disintegrate.
Figure 11 shows the effect of Al addition of AlCrFeMnNi HEAs where the corrosion behavior of the HEA as a function of aluminum contents is investigated. It is seen that aluminum content decreases the corrosion potential and increases the corrosion current density. (Source: Qiu, Yao; Thomas, Sebastian; Gibson, Mark A.; Fraser, Hamish L.; Birbilis, Nick; 2017 Corrosion of high entropy alloys npj Materials Degradation 15; 2397-2106)
High entropy alloys have improved corrosion resistance due to homogeneous elemental distribution and homogenous microstructure development. Due to the quick cooling that occurs during certain synthetic processes, such as laser cladding, electro-spark deposition, and magnetron sputtering, the corrosion resistance can be improved. The removal of elemental segregations and achievement of homogenous microstructure are two additional benefits of heat treatment at increased temperature.
Only a few hydrogen storage property analyses of high-entropy alloys exist. Titanium–vanadium–zirconium (Ti–V–Zr)-based BCC and Laves phase high entropy alloys absorb 1.8–2% hydrogen. The TiVZrHfNb alloy stores the most (2.7 wt.%).
High crystal symmetry and single-phase production in high-entropy alloys increase power factor, whereas lattice distortion and large crystal size diminish lattice thermal conductivity, making thermoelectric materials more efficient. Thus, thermoelectric material entropy engineering is trending. Entropy engineering yielded a figure of merit of 1.61 in (Sn0.5Ge0.4875)0.5Pb0.5Te, which was previously impossible in polycrystalline SnTe systems.
High-entropy alloys may be able to match superconductors' high-pressure stability, however their Curie point temperature is low and study is ongoing. Moving towards high-entropy chalcogenides and hydrides could be a wise approach.
Physical vapor deposition (PVD) is a well-known route for manufacturing hard coatings. In recent years, PVD has also been applied to create high-entropy alloy (HEA) thin films. HEAs are multicomponent alloys with nearly equal atomic fractions. To achieve the desired composition, typically alloy targets are used in the deposition process.
The geometry of the sputtering system results in a well-defined component gradient and the nanocrystalline HEA film has a higher hardness compared to its bulk counterpart.