Keywords

Property goal; high-entropy alloy patent; medium-entropy alloy; materials genome

10.1 Introduction

From previous chapters, it is evident that HEA concept has opened up an entirely new alloy field, which encompasses a wide range of microstructure and properties. Not only their compositions could be designed but also conventional (ingot metallurgy, powder metallurgy, and coating technology) processes or even new processes could be selected to generate unique alloys with different properties. The new processes include thermomechanical treatments, rapid solidification, mechanical alloying, spray forming, semisolid forming, equal channel angular extrusion, reciprocating extrusion, superplastic forming, high-strain-rate superplastic forming, stir friction welding, spark plasma sintering, and nanoscale materials technology. As a result, numerous combinations and possibilities exist. From current HEA-related literature, it is clear that a suitable alloy composition and proper processing might obtain outstanding properties for intended applications. Besides HEAs research for scientific curiosity, researchers expect that HEAs can substitute conventional materials in difficult and stringent operating conditions by providing superior performance with increased service. This ultimately improves energy saving, materials saving, performance efficiency, cost reduction, resources conservation, environment, and health. This chapter includes the property goals pursued, advanced applications demanding new materials, and a few examples. This chapter also reviews the current state of patents related to HEAs and points out future directions of HEAs.

10.2 Goals of Property Improvement

Materials selection involves seeking the best match between the property profiles of the materials in the universe and that required by design (Ashby, 2011). But from engineering and commercial points of view, materials properties and cost are the main consideration in selection of materials to fit the requirements. In other words, the main aim is to minimize cost while meeting product performance goals. In addition, manufacturability, availability, appearance, and recyclability are also factors (Kalpakjian and Schmid, 2014) to be taken into account.

There are many properties which are investigated in materials research. It is often noted that some properties are emphasized and pursued in order to attain higher levels than the existing ones. For example (Yeh, 2006):

10.3 Advanced Applications Demanding New Materials

Although many advanced applications presently use conventional alloys, there always exists the demand of materials with higher and higher performance. There are various driving forces to push the development of new and improved materials including market pull and competition, curiosity-driven research, miniaturization, multifunctionality, environmental consciousness, and increased product liability (Ashby, 2011). For example, longer lifetime of components and systems means lower replacement cost and resource saving. In addition, higher energy conversion efficiency at higher temperature and higher pressure operation conditions in engines improves efficiency, which reduces fuel consumption, cost and pollution. In the current state, a number of advanced applications demanding new and improved materials include the following (Yeh, 2006):

However, the development of conventional alloys and new alloys based on one or two major elements gradually approached their limit at the end of the twentieth century. This saturation has left difficulties in creating new materials to cater to the anticipated jump in performance. Under these circumstances, HEAs and related materials provide a new and huge opportunity. Most importantly, the reported outstanding properties of HEAs have shown that suitable composition design and process selection might yield HEAs replacing traditional materials for such applications. In fact, almost all the above items have been researched with HEAs, HEA-based ceramics, HEA-binder carbides, and HEA-binder cermets in recent years.

10.4 Examples of Applications

Several examples are introduced in this section to demonstrate the promising applications of HEAs.

10.4.1 Al₅Cr₁₂Fe₃₅Mn₂₈Ni₂₀ HEA

This alloy has a simple FCC structure with spherical NiAl-rich precipitates. In its cast state or homogenized state it displays a very good workability (Yeh, 2006). No intermediate annealing during cold rolling is required in contrast to the frequent annealing in 304 stainless steels. Figure 10.1 shows a foil of 70 μm thick which was cold rolled to an extension of 4257% without any edge cracking. Its work hardening curve indicates that its hardness approaches saturation around 360 HV at large deformation. At this state, the foils still can be folded without cracking. This indicates that its bendability is excellent. The alloy also shows good corrosion resistance in salt spray test. Insulating thin film of SiN_x of 800 nm in thickness was deposited on the polished surface of this foil for testing the minimum bend radius without causing permanent deformation of foil. Adhesion and current leakage of SiN_x thin film were also assessed. The minimum radius is 10 mm at which no spalling occurs. The leakage current after bending test is 8×10⁻⁹ A/cm². This easily rollable alloy thus has potential application as flexible substrates for solar cells and displays.

10.4.2 Co_1.5CrFeNi_1.5Ti_0.5 HEA

This alloy has a simple FCC structure in the as-cast state. It can be heat-treated to obtain gamma prime and eta precipitates in the FCC matrix. The as-cast hardness of the alloy is 378 HV and a peak hardness of 513 HV can be obtained by aging at 800°C for 5 h. It shows smaller magnetization and higher resistivity than Stellite 6. It has a very small eddy current loss under alternating magnetic field. It also shows better abrasion wear resistance tested by the pin-on-belt method with alumina sand belt and a higher corrosion resistance in 0.5 M H₂SO₄+0.5 M NaCl solution than Stellite 6. Figure 10.2 shows the cast and machined bearings made by lost-wax cast method. It can be used in severe conditions such as underground electrical pump components used in oil well system. Because of its high temperature strength and oxidation resistance, this alloy has been used for components such as connecting rods and holders for high-temperature tensile testing machine up to 1000°C at which its hot hardness is still high, around 250 HV, which is higher than that of most commercial superalloys.

10.4.3 Profile Hardening of Al_0.3CrFe_1.5MnNi_0.5 HEA

Al_0.3CrFe_1.5MnNi_0.5 HEA has shown significant age hardening at 600–800°C. The hardening and thus improved wear resistance is due to the formation of ρ-phase which is a Cr-, Fe-, and Mn-rich phase. By exploiting the merit of high-temperature aging, the alloy parts could be profile hardened by simple heating. Figure 10.3 shows a comb-shaped part which was profile-hardened by simple heating in air at 550°C for 2 h (Chuang et al., 2013). All the surfaces get uniform hardened layer around 74 μm in thickness. The surface layer has a hardness of 1090 HV whereas the substrate has a hardness of 338 HV. The abrasion wear resistance is about 1.45 times that of SKD61 tool steel (520 HV) and 1.3 times that of SUJ2 bearing steel (723 HV). This demonstrates that the HEA is unique in providing an effective route for surface hardening instead of shot peening, carburizing and nitriding treatment. It could be used for shafts, and also complex components requiring high strength and wear resistance.

10.4.4 HEA Coatings for Antisticky Molds and Solar Cells

Because HEA coatings easily form amorphous structure with very low roughness, they can be used for antisticky coating and diffusion-barrier applications. Figure 10.4 compares the hard-Cr coating and HEA coating on the fine-sand-blasted surface of a SKD61 steel mold for IC (integrated circuit) package. It can be seen that hard-Cr coating is rougher than AlCoCrCuFeNiTi HEA coating. As a result, the release force of HEA coating required to separate molded IC components (packaged with the molding compound of epoxy resin, graphite, and silica particles) from the mold is only half that of the hard-Cr coating. This is also seen in the molding of fuel cell bipolar plates at 200°C; HEA coating is easy to release the molded plates as shown in Figure 10.5 whereas hard-Cr coating would cause warping and even tearing of molded plate. In addition, HEA film has been tested for CuInGaSe (CIGS) thin-film solar cell as back electrode in view of its thermally stable amorphous structure, better diffusion-barrier property, and higher reflectivity and conductivity as compared with Mo back electrode. The energy conversion efficiency is effectively increased by 9% based on the undisclosed data of a CIGS developing company (Yeh, 2013b).

10.5 Patents on HEAs and Related Materials

In the huge scope of HEAs and HEA-related materials, a number of patents have been applied and granted. The first patent with the title “High-entropy multicomponent alloys” concerning the composition range of HEAs from five to eleven principal elements was filed as early as 1998 for a Taiwan patent which was granted in 2003 as Taiwan patent number 193729. It was also granted by China (patent number 00133500.6) and Japan (patent number 4190720) but with a smaller composition range. However, the patent was not granted by the United States. After this patent, there are more patents filed and granted. Huang (2009) reviewed the status of HEA patents in 2009. He divided the patents into two groups. One consists of HEAs having some specific functions, the other consists of composite materials with HEAs and other reinforcements. The first group includes hard-facing alloy, soft-magnetic thin film inductor and magnetic multicomponent alloy film, HEA catalyst, sprayed HEA coating, high-temperature HEAs with low concentration of cobalt and nickel. The second group includes the cemented carbide with HEA binders, and the HEA-based complex materials.

After 2009, there has been a large increase in patent applications. Among these, most are related to cemented carbides, cermets, and spray-deposited hard-facing materials. Like the patent CN100526490C (China patent publication number is CN1827817A) disclosed in 2006, three patent applications, CN102787266A, CN102787267A, and CN102796933A are related to hard metals and cermets with HEA binders. US patent 8075661 deals with ultrahard composite materials and their manufacturing method. China patent CN100535150C deals with hard-facing HEAs. CN102828139A concerns with HEA powders for spray coatings. CN103255414A, CN103255415A, and CN103276276A deal with carbide-reinforced HEA spray coatings deposited by plasma spray technique.

On the other hand, a composition range of hydrogen-storage HEAs has been granted in Taiwan I402357 and applied in China and the United States for a patent. Two composition ranges of HEA brazing fillers has been granted as China patents CN101554686B and CN101590574B, respectively. Refractory HEAs patent entitled with “multicomponent solid solution alloys having high mixing entropy” has been applied in the United States (US20130108502 A1). HEA piston ring patent entitled with “multicomponent alloy base piston ring” has been granted as Taiwan patent I403594. Based on this trend, it can be predicted that a large number of HEA-related material patents will be generated by more and more research and development in the future.

10.6 Future Directions

In the last decade, more than 500 HEA journal and conference papers have been published, Nevertheless the understanding of the whole HEA world is still in its infancy. Several future research trends can be foreseen (Yeh, 2013b).

More fundamental and basic studies are required. Because materials science and solid state physics are mainly based on conventional materials with one or two principal elements, what happens in HEAs would be interesting for better understanding of materials. In the whole-solute matrix, different contributions to mixing entropy, mutual interactions in all unlike atomic pairs, short range order, lattice distortion, electrical and thermal conductivity, thermal expansion, vacancy concentration, diffusion coefficients, phase transformation, Young’s modulus, dislocation energy, staking fault energy, grain boundary energy, slip, twinning, serration behavior, strengthening, toughening, fracture, fatigue, creep, wear, corrosion, and oxidation are all needed to be understood with their mechanisms. Whether these mechanisms are simple extensions from those of conventional alloys or not is a matter of inquiry.

More research on composites of HEAs with ceramic reinforcements and high-entropy ceramic (HEC) reinforcements is required. Such a combination would generate numerous composites among which many opportunities could be found for critical applications not easily attained by traditional composites.

More research on medium-entropy alloys (MEAs) is also required. It is recognized that there still exists a large space in MEAs. This is reasonable since the view from the high-entropy side and that from conventional low-entropy side could be linked up to generate more sparks in the undeveloped area of MEAs which could be Fe-base, Ni-base, Co-base, Cr-base, and Cu-base. In addition to different routes of alloy design strategy, suitable compositions of MEAs might be easily obtained from those phases that are rich in some specific elements during the investigation of microstructure and its correlation with properties for HEAs. This new finding can be regarded as a by-product from HEAs research.

Assessment of existing database to find possible applications is required. It is believed that HEAs, HECs, or their composites could solve many bottlenecks encountered by conventional materials. Although the database is still limited, many suggestions of their potential applications are seen from the literature.

One foresees more synthesis based on combinatorial methods which allow high-throughput exploration of the composition space. In addition to the preparation of the alloys, this allows the measurement of a variety of properties across the composition spectrum. Optimum composition with required properties could be found more efficiently. Moreover, their structure and microstructure can be effectively investigated. For example, the lattice distortion arising from the simultaneous presence of multiple solutes needs to be measured. The effect of this distortion on dislocation movement needs to be understood. More research with modelling and simulations are required. Remarkable progress can be anticipated as new computational tools, integrated computational materials engineering (ICME) and materials genome approach are employed.

In conclusion, HEAs and HE-related materials have potential applications in different fields and are expected to replace traditional materials in many sectors. In just a decade from 2004, extraordinary progress has been made. This research theme has caught global attention. A bright future is seen.