Keywords

Synthesis; processing; melting and casting; mechanical alloying; spark plasma sintering; sputtering; cladding; combinatorial materials science

5.1 Introduction

A variety of processing routes has been adopted for the synthesis of HEAs. HEAs have been synthesized in different forms like dense solid castings, powder metallurgy parts, and films. The processing routes can be broadly classified into three groups, namely, melting and casting route, powder metallurgy route, and deposition techniques.

Melting and casting techniques, with equilibrium and nonequilibrium cooling rates, have been used to produce HEAs in the shape of rods, bars, and ribbons. The most popular melt processing techniques are vacuum arc melting, vacuum induction melting, and melt spinning. Mechanical alloying (MA) followed by sintering has been the major solid-state processing route to produce sintered products. Sputtering, plasma nitriding, and cladding are the surface modification techniques used to produce thin films and thick layers of HEAs on various substrates.

This chapter gives a brief description of the different synthesis and processing routes adopted for HEAs. Processing routes are similar for both equiatomic and nonequiatomic HEAs.

5.2 Melting and Casting Route

The most widely adopted route for the synthesis of HEAs is the melting and casting route. Figure 5.1 gives an idea of the number of papers published on HEAs, grouped according to different synthesis routes. It is very clear from Figure 5.1 that the casting route (bulk) dominates the processing routes, with almost 75% of the papers published so far on HEAs being produced by this route.

A vast majority of HEAs that have been reported so far has been produced by vacuum arc melting and a few by vacuum induction melting. Arc melting has been the most popular technique for melting HEAs as the temperatures that can be achieved during arc melting are high (close to about 3000°C), which is sufficient to melt most of the metals used for making HEAs. However, the disadvantage of this technique is the possibility of evaporation of certain low-boiling point elements during the alloy preparation thus making compositional control more difficult. In such cases, induction and resistance heating furnaces have been adopted for making the alloys.

One of the constraints faced in the melting and casting route is the heterogeneous microstructure developed due to various segregation mechanisms caused by the slow rate of solidification. The typical solidification microstructure of the HEAs produced by arc melting and casting is dendritic (DR) in nature with interdendritic (ID) segregation as shown in Figure 5.2 (Hemphill et al., 2012). Similar DR structure can also be seen in a number of other alloys prepared by induction melting and casting, as was observed in AlCoCrCuFeNi alloy (Singh et al., 2011b). The microstructure in this alloy showed DR and ID regions with a number of ordered precipitates (B2 and L1₂). Incidentally, when the same alloy was produced by a faster cooling route (splat cooling), it showed a BCC matrix with ordered (B2) precipitates. Cantor et al. (2004) also showed that melt spinning of a number of equiatomic quinary and sexinary alloys such as CoCrFeMnNi, CoCrFeMnNiNb, CoCrFeMnNiTi, CoCrFeMnNiV, CoCrFeMnNiCu, and CoCrFeMnNiGe leads to predominantly single phase FCC structure.

This demonstrates that faster cooling suppresses the precipitation of secondary phases leading to the formation of predominantly single-phase alloys. Among the melting and casting techniques, those that lead to faster solidification rates such as splat quenching, melt spinning, injection casting, suction casting, and drop casting have also shown similar microstructures with predominantly single-phase microstructures. This brings an important point to focus whether the single-phase structures obtained in some of the HEAs are kinetically favored or thermodynamically stabilized.

Cui et al. (2011a and 2011b) have directionally solidified AlCoCrFeNi and CoCrCuFeNi alloys, by vertical Bridgman technique. They observed that the microstructure of the alloy changes from planar to cellular and to DR on increasing the growth rate. Their results indicate that directional solidification can lead to finer DR structure with a decrease in the concentration difference between the DR and ID regions due to rapid growth rate and high-temperature gradients. Zhang et al. (2012b) observed, in the case of AlCoCrFeNi alloy, that the microstructure changes from DR to equiaxed when the alloy is prepared by Bridgman solidification in contrast to copper mold casting. Ma et al. (2013a) used extremely low withdrawal velocity of 5 μm/s to produce single crystals of FCC Al_0.3CoCrFeNi by Bridgman technique. In contrast, the equiatomic AlCoCrFeNi HEA under the same conditions yields columnar BCC grains, and single crystal could not be obtained in this case. The reason for this difference is yet to be understood.

Laser-engineered net shaping (LENS) in the technology of rapid prototyping can fabricate HEAs in bulk form directly by injecting metal powders into the area focused with high-powered laser beam. This technology was developed by Sandia National Laboratories for manufacturing solid metallic components from powder using a high-powered laser with a help of computer-aided design (CAD) model. Figure 5.3 shows a schematic of LENS technology. In this technique, the metal powder is fed through a deposition head placed coaxially to a focused laser beam. The X–Y table and the deposition head move with a number of degrees of freedom in order to generate the component with the required shape and size. An inert gas is used as a shield to prevent oxidation of the powder and the melt pool formed during the process. In developing HEAs, this technique has been used to produce gradient HEA rods layer by layer with changed compositions. For example, Al content can be varied from 0 to 3 segmentally in a grown Al_xCoCrCuFeNi alloy rod (Welk et al., 2013). Similarly, other elements could be varied to produce segmentally gradient rods.

5.3 Solid-State Processing Route

A small fraction of about 5% of the reports on HEAs so far deal with synthesis of HEAs by solid-state processing, which involves MA of the elemental blends followed by consolidation. MA is a process of high-energy ball milling of elemental powder blends, which involves diffusion of species into each other in order to obtain a homogeneous alloy. This technique was first developed by Benjamin and his coworkers as a part of the program to produce oxide dispersion strengthened Ni base superalloys (Benjamin, 1970). In 1990, Fecht and his coworkers gave a first systematic report on the synthesis of nanocrystalline metals by high-energy ball milling (Fecht et al., 1990). Figure 5.4 shows schematically the ball to powder interaction during high-energy ball milling that involves continuous deformation, fracture, and welding of particles finally leading to the nanocrystallization or even amorphization. MA has been demonstrated over the past four decades as a viable processing route for the development of a variety of advanced materials such as nanomaterials, intermetallics, quasicrystals, amorphous materials, and nanocomposites (Murty and Ranganathan, 1998; Suryanarayana, 2001; Venugopal and Murty, 2011).

The research group of Murty is the first to develop nanostructured HEAs using MA (Varalakshmi et al., 2008) and demonstrated high thermal stability and good mechanical properties of such alloys. Figure 5.5 (Varalakshmi et al., 2008) demonstrates the formation of nanocrystalline single-phase BCC after MA in a sexinary AlFeTiCrZnCu equiatomic elemental blend. One of the advantages of MA is its ability to produce excellent homogeneity in the alloy composition. Each of the nanoparticles obtained by MA is equiatomic in its composition, which has been confirmed by EDS and atom probe tomography.

These HEAs obtained by powder metallurgy route need to be sintered to achieve dense components. Conventional sintering of nanocrystalline alloy powders can lead to significant grain growth during the exposure of the alloy powders to high temperatures for long periods. In order to avoid this, nanocrystalline alloys obtained by MA are usually consolidated by spark plasma sintering (SPS). SPS involves application of high amperage pulsed current (up to 5000 A) through the sample kept usually in a graphite die, while simultaneously applying pressures to the tune of about 100 MPa as shown in Figure 5.6. The pulsed currents lead to the formation of spark plasma at the particle–particle interface in short periods causing almost instantaneous heating of the powder particles. This leads to sintering being completed within a short period of a few minutes in contrast to conventional sintering which takes a few hours to reach similar sintering densities. Thus, the time available for grain growth during SPS is extremely small, which helps in retaining the nanostructures in the mechanically alloyed powder compacts. Retention of nanocrystallinity in CoCrFeNi alloy was observed even after annealing at 700°C up to 25 days subsequent to consolidation by SPS at 900°C.

5.4 HEA and HEA-Based Coatings

Figure 5.1 shows that almost 20% of the papers on HEAs reported so far have been obtained in thin film/coating form by various techniques involving vapor and liquid.

5.4.1 HEA and HEA-Based Coatings from Vapor State

Among the vapor-based surface modifications, two techniques have been quite popular, namely, magnetron sputtering and plasma nitriding. The attempts by various investigators were to produce thin films or layers of HEA on the surfaces of substrates such as mild steels, Al alloys, and HEAs in order to improve corrosion resistance, oxidation resistance, and wear resistance.

Sputter deposition is a standard technique of depositing thin film onto a substrate by sputtering away atoms from a target under the bombardment of charged gas ions. DC sputtering shown in Figure 5.7 (Chen et al., 2013a) is the simplest of sputtering techniques wherein a DC bias is applied between the target and the substrate to aid the deposition. The deposition rates can be controlled by controlling power, the bias voltage, and the argon pressure. Radio frequency (RF) sputtering shown in Figure 5.7 (Chen et al., 2013a) is used for sputter deposition of insulating materials. In DC sputtering, if one attempts to sputter deposit an insulating film, a very high voltage to the order of 10¹² V is required. This can be avoided in RF sputter deposition. In case of RF sputter deposition, the plasma can be maintained at a lower argon pressure than in DC sputter deposition, and hence fewer gas collisions leading to more lines of sight deposition.

In magnetron sputtering, electric and magnetic fields are used to increase the electron path length, thus leading to higher sputter deposition rates at lower argon pressures. The basic principle of magnetron sputtering is demonstrated in Figure 5.8. Magnetron sputter deposition uses both DC and RF for sputtering. Magnetron sputtering has been the most widely used coating technique for the HEAs (Tsai et al., 2013a).

Chang et al. (2008) have shown that DC magnetron sputtering of AlCrMoSiTi HEA in pure argon leads to amorphous thin film (Figure 5.9A). In the presence of nitrogen, the film turns out to be a simple NaCl-type FCC nitride (Figure 5.9B) mainly due to high-entropy effect. No phase separation of nitrides into TiN, AlN, Si₃N₄ is observed in this system. An octonary AlMoNbSiTaTiVZr alloy has been successfully deposited as an amorphous film by Tsai et al. (2008b) which has a high performance as a diffusion barrier between Cu and Si in IC interconnects. Huang and Yeh (2010) have been able to develop superhard nitride coatings of AlCrNbSiTiV with hardness more than 40 GPa. Interestingly, it was found that the hardness of these coatings does not decrease with temperature even up to 1000°C. Grain coarsening was also observed to be minimal in these coatings up to 1000°C.

Similarly, sputtering (both RF and DC magnetron sputtering) of AlCrSiTiV alloy nitrides on mild steel substrate has shown a hardness of about 30 GPa and the grain size and hardness of these coatings were found to be quite stable even at 1173 K for 5 h (Lin et al., 2007). Similar results were observed by Chang et al. (2008) in case of AlCrMoSiTi nitrides. Braic et al. (2012) have recently developed HfNbTaTiZr nitride and carbide coating on Ti6Al4V alloy by DC magnetron sputtering for biomedical applications. They also observed that these coatings not only have excellent wear resistance but also have good biocompatibility in simulated body fluids.

Plasma nitriding is not as widely used as magnetron sputtering for making surface hardened layer for protection. Very few studies have been reported so far on this technique. However, this technique has been reported to produce thicker layer (50–100 µm) than magnetron sputtering (<1 µm). Plasma nitriding of Al_0.3CrFe_1.5MnNi_0.5 alloy has led to the formation of nitrided surface layer (Figure 5.10). The nitride layer has been analyzed as a mixture of various nitrides (AlN, CrN, and (Mn,Fe)₄N) and having a peak surface hardness around 1300 HV. By pin-on-disk adhesion wear test with an SKH-51 steel disc, the nitrided samples of HEAs with different prior processing have higher wear resistance than the unnitrided ones by 49 to 80 times and also than nitrided samples of conventional steels by 22 to 55 times (Tang et al., 2012). Feng et al. (2013) have recently combined magnetron sputtering with plasma-based ion implantation to produce NbTaTiWZr HEA-based nitrides with a better control on the thickness. They have also observed that this combination helps in improving the adhesion of the coating with the substrate.

5.5 Combinatorial Materials Synthesis

Combinatorial chemistry uses chemical synthesis methods that make it possible to prepare a large number (up to even millions) of compositions in a single process. Combinatorial chemistry also includes strategies that allow identification of useful components of the libraries for such large-scale synthesis. Over the last two decades, combinatorial chemistry has altered the drug development process to discover new drugs (Pandeya and Thakkar, 2005). By this encouragement, materials scientists can also apply this methodology to accelerate the discovery of new compounds for high-T_c superconductors, luminescent materials, catalysts, and polymers (Xiang et al., 1995). They used thin-film technology to deposit substances sequentially in different amounts layer by layer onto a gridded substrate and then to mix the elements and create a stable compound by heating. The physical properties of interest are then measured on each composition to find out the outstanding composition. Basically under little guidance to predict new materials, this is a very efficient method to discover new materials in contrast to the conventional one-composition-at-a-time approach, which is time consuming.

For the development of multicomponent alloys by this method, the concept involves development of techniques that can fabricate large number of alloy specimen with continuous distribution of binary and ternary compositions across the surface, called the “alloy library.” This technique saves the time, energy, and expense in alloy design and can help the development of new HEAs with improved properties. Figure 5.11 shows a schematic of the development of alloy library coupon using combinatorial materials science. In Figure 5.11, three controlled geometry thin films are deposited and annealed to develop one coupon with continuous distribution of elements. This high-throughput synthetic route holds great promise for further development of HEAs.