Preparation of nanocrystalline high-entropy alloys via cryomilling of cast ingots

The advancement of nanotechnology demands large-scale preparation of nanocrystalline powder of innovative materials. High-entropy alloys (HEAs) exhibit unique properties: mechanical, thermal, magnetic etc., making them potentials candidates for applications in energy, environment and biomaterials etc. Thus, there is a need to develop novel synthesis methods to prepare nanocrystalline high-purity HEAs in large quantity. Conventional mechanical alloying of the multicomponent metallic powder mixture requires larger milling time and it is prone to contaminations and phase transformation. The present investigation reports a unique approach, involving casting followed by cryomilling, leading to formation of nanocrystalline HEAs powder, which are relatively contaminations free with narrow size distribution. Using examples of two FCC and one BCC single-phase HEAs, it has been shown that large-scale nanocrystalline HEAs powder can be prepared after few hours of cryomilling at 123 K. The formation of nanocrystalline HEAs during cryomilling has been discussed using theoretically available approaches.


Introduction
The exploration of novel nanomaterials for various applications in the field of nano-science and nanotechnology has slowly been maturing with discovery of new and advanced materials. It is well known that, the exceptional properties of nano-materials having surface area -to-volume ratio make these materials suitable for various applications [1], such as catalytic [2,3], optical [4,5], anti-bacterial [6,7], magnetic [8], biomedical applications [9] etc. In this regard, alloy nanoparticles found to exhibit exceptional properties for these applications due to the possibility of different combination of element being used. Hence, the alloy nanoparticles possess distinctly different properties from the constituent elements as well as bulk alloys, allowing them to be exploited for various applications. These include Ag/Au bimetallic nanoparticle for cancer detection [10], Sn-Ag-Cu for low temperature lead free inter-connect [11], Fe/Cu nanocrystalline multilayers for soft magnetic application [12]. Furthermore, the properties of the alloy nanoparticles can further be tailored by tuning the number and type of constituent elements, the composition, degree of chemical ordering and morphology. However, continuous advancement in the nanotechnology requires design and development of novel alloy nanoparticles including multi-metallic high entropy alloys [13][14][15][16][17][18][19]. Therefore, we require devising novel strategies to prepare these nanoparticles in large quantity.
High entropy alloys are considered as multi-metallic cocktail of at least five elements in equal or near equal proportions and thus termed as concentrated multi-principal element alloys [20,21]. Discovered in 2004, the research activities on these alloys have been vigorous across the globe [22,23,17,[24][25][26][27]. They exhibit unique set of mechanical (combination of strength, ductility and toughness) [28,29], physical, thermal, magnetic properties due to their intrinsic nature and hence, are deemed to be potential candidates for applications in energy, biomedical, catalysis sectors [30][31][32][33][34]. However, preparation of nanocrystalline HEA in large quality is a challenge to the material science community.
Conventionally, nanocrystalline HEAs powder has been prepared using mechanical alloying of the elemental mixture. There exist two experimental pathways to obtain nanocrystalline HEAs via ball milling [35,36,16]. First, individual powders of required elements are fed into a ball mill and milled sufficiently long until HEA phases form. This pathway is known as mechanical alloying (MA). Second approach is related to crushing of the HEA ingot conventionally cast via ball milling at extremely low temperature. In the first case, NPs of HEAs may form but it requires longer milling time, which can impart unwanted impurity or contamination from the milling tools as well as atmospheric oxygen and nitrogen. The multicomponent HEAs are prone to oxidation and hence pure nanocrystalline HEAs are difficult to achieve via conventional ball milling route. Being multicomponent, the HEA NPs, obtained via mechanical alloying of individual powder mixture may lead to formation of metastable phases [37][38][39]. On the other hand, the second route, i.e, casting followed by cryomilling, requires the material to be hard and brittle at low temperature. The consideration of high ductility (>40%) and toughness (>250 2 1 .m MPa ) of FCC HEA phases from high to low temperature (800 to -196 o C) makes the formation of NPs of HEA via cryomilling difficult. The high ductility and toughness lead to easier cold welding, which is considered an obstacle to the formation of NPs via cryomilling [40].
However, cryomilling has many advantages, including low contamination, reduced oxidation, suppression of recovery and recrystallization etc [41][42][43][44][45]. Therefore, if successful, cryomilling is expected to deliver the solution to synthesize of nanocrystalline HEAs in large quantity, useful for variety of applications [46]. using X-ray diffraction (XRD), transmission electron microscope (TEM), surface plasmon resonance. The mechanism of the formation of HEAs-NPs has also been discussed in detail.

Preparation of HEAs-NPs
The HEA ingots have been parted in smaller pieces utilizing diamond saw (Bueheler, USA).
Further, these pieces were milled in a custom built cryomill [47] at-160±10 o C.The single ball cryomill (schematic is shown in Figure 1) was cooled using liquid nitrogen (LN 2 ). The detailed design and working principle of cryomill ware reported elsewhere [47]. The cryomill is a vibratory mill having a single ball with diameter 5 cm and vial 9.5 cm with volume of 250 cc.
Thus, ball amplitude 2 mm was maintained through milling using tungsten carbide (WC) ballvial. The milling chamber was continuously purged using argon (Ar) gas to protect the powder from oxidation during milling. After six hours of cryomilling, the HEAs-NPs were collected for characterization.

Characterization of HEA-NPs
The X-ray diffractometer (PanalyticalXPert make PANalytical) was used with Cu K radiation (=1.54056 nm) to study the phases in the bulk as well as cryo-milled powder. The annealed strain free Si powder was used to determine instrumental broadening for crystallite size measurement. The small quantity of nanocrystalline powder was dispersed in ultrapure methanol (CH 3 OH) using sonication. The few drops of the prepared solution was placed over carbon

X-ray diffraction analysis
The formation of single phase HEA -NPs has been analyzed using X-ray diffraction (XRD), analyses as shown in Figure 2(a-c). The as cast and homogenized alloys,  Table 1. However, there is large difference in the crystallite size estimated using TEM and XRD. The powder prepared by cryomilling is expected to produce nanocrystals having irregular shape. In the present case, the crystallite size of different HEA nanoparticles has been estimated using average value of the K [50]. In addition, crystallite size has been estimated by direct measurement using TEM images.
However, there is large difference in the crystallite size estimated using TEM and XRD. There are several reasons for this discrepancy.
1. The shape parameter the crystallite size can from crystallite to crystallite due to irregular shape.
2. The HEAs subjected to intrinsic lattice distortion as well as different atomic sizes (all 5 elements), which is might be a reason of abnormal peak broadening causes large difference in crystallite size compared to other techniques [20]. However, the high entropy alloys have very distorted lattices and different atom sizes, which are the reasons to contribution in broadening.
3. In case of nanoparticles preparation by top down method, the broadening in the XRD peaks contributed from other sources (dislocation density, stacking fault or defects generated by cold work in the crystal system).

Size and morphology
The size, shape and morphology of the HEAs-NPs have been studied using transmission electron microscope (TEM) to decipher the nature of the cryomilled HEA powder. Figures 3 (a-b) show the typical bright field image of Fe 0.2 Cr 0.2 Mn 0.2 Ni 0.2 Co 0.2 alloys nanoparticles. Figure 3b is the low magnification micrograph revealing distribution of the cryomilled HEA-NPs. The higher magnification bright field (Fig. 3a) and dark field image (Fig. 3(c)) show the irregularly shaped NPs having size ranging from 5-10 nm. Table 1 shows the average crystallite size measured using TEM images. Fig. 3(e) shows bright field image of particles, where oleylamine has been used as capping agent during dispersion of the nanoparticle prior to the sample preparation for TEM investigation to estimate size of large number of the NPs (~ 500 particles). The histogram has been generated as shown in Fig. 3d. The average size is found to be 41 nm. The selected area diffraction patterns (SADP) clearly reveals the single-phase FCC HEAs. Detailed analysis indicates that the SADP pattern is similar to XRD patterns ( Fig. 2(a)). Similarly, the bright field TEM image of Cu 0.2 Ag 0.2 Au 0.2 Pt 0.2 Pd 0.2 alloys nanoparticles is shown in Figure 4(a-b) indicating the average crystallite size 95 nm (distribution is shown in Figure 4d).The XRD and SADP (Figure 4(e)) indicate that this is also single phase FCC. The as cast, BCC HEA and the HEA-NPs has also been investigated using TEM and found to be single phase with average crystallite size of nanoparticles to be 62 nm as shown in Figure 5(a-e).

Compositional measurements
The compositions of ingot and cryomilled powder have been estimated using Electron Probe Micro-Analyzer (EPMA) based Wavelength dispersive spectroscopy (WDS) and Energy dispersive spectroscopy (EDS). The qualitative WDS spectra has shown in Figure 6. The qualitative analyses of the spectra clearly indicate that oxygen is only impurity present in the sample and clearly reveal the absence of N. It is to be noted here the carbon arises from the carbon tape as well as resins utilized for mounting the samples during EMPA analysis.
In addition, quantitative analyses of the composition of the ingot and powder have been carried out using EPMA. The results are shown in Table 2

Surface plasmon resonance
The past groundbreaking research on biosensors for detection of single viruses, interaction between different bio-molecules shows that metallic nanoparticles are highly sensitive to optical method using SPR technique [51]. Therefore, the HEA -NPs containing multiple metallic elements could be a promising candidate for the devices required plasmonic as well as magnetic properties [52]. It is to be noted that the nanoparticles show surface plasmon resonance (SPR) band and which is results of electrons collective oscillation [53]. The dielectric of the surrounding medium can have major effect on peak position and intensity [54]. In addition, the shape and size also affect the peak position. As size decreases the SPR resonance band exhibit blue shift (blue shift: peak shift to lower wavelength and red shift: peak shift to higher wavelength  HEAs-NPs. Hence, it is not possible to discuss the peak position for HEA-NPs. In addition, the HEAs-NP dispersions is measured quite stable in methanol as shown in Figure 7(b) which might be due to nanocrystalline nature and electrostatic stabilization [55,56] of the NPs in methanol.

Discussion
The results of the present investigation categorically show that HEAs-NPs can be prepared using cryomilling of the cast ingots, which are relatively ductile and toughness. These NPs retain crystals structure of the phases in the bulk ingot. The detailed TEM measurements reveal narrow size distribution of HEA-NPs. Further, we shall explain the mechanisms of formation of nanocrystalline HEA via cryomilling.
It is to be noted that cryo-milling is a type of milling in which, powder particles are crushed/milled at temperature below 123K. It suppresses the recrystallization, dynamic recovery, processes leading to refinement of the grain and formation of nanocrystalline materials in short period of time. Accordingly, it reduces the milling duration and hence suppresses the contamination or milling media debris from the milling tools and atmosphere. Thus, cryomilling takes advantage of the extremely low temperature (boiling temperature 77 K) of the liquid nitrogen (cooling media) as well as those related with room temperature ball milling. Cooling of the ball-vial and milling powders is an efficient way to increase the fracturing process, leading to a nano-grained structures and early grain refinement. In addition, the powder coarsening (cold bond) and coating (sticking) to the milling media can effectively be suppressed leading to more efficient product outcome like higher yield and powder particles refinement. The milling with argon atmosphere and extremely low temperature reduce the oxidation of the powder.
Because the formation of nanoparticles using room temperature milling is requir es very long time of milling, it can easily impart unavoidable contamination from the milling tools [57]. In case of highly ductile and tough materials (HEA), it is required to be milled for very long time to obtain nanoparticle via ball milling at room temperature. Therefore, the contaminations are mixed to milling powder atomically and hence it might alter the crystal structure as well multiatomic pre-calculated proportion. Therefore, the extremely low temperature during cryomilling helps in many ways [58,59]. In the following, we should discuss the various advantages of cryomilling process, leading to formation of nanocrystalline.  [17,53]. To the best of the authors' knowledge, mechanical property of Cu 0.2 Ag 0.2 Au 0.2 Pt 0.2 Pd 0 .2 is not available in the literature. The HEAs phases exhibit high ductility and toughness even at LN 2 temperature and thus making it difficult to suppress the deformation and accelerate the fracturing process.
According to Fecht et al. grain refinement via mechanical milling process occurs in five stages [40]: (i) flattening, (ii) cold welding, (iii) fracturing, (iv) equiaxed particles formation followed by random welding and (v) steady state particle formation. In the initial stage, severe plastic deformation is effectively applied to the particles, which is lead to the creation of the shear bands due to localized severe deformation. The deformation generally accumulates high density of localized dislocation tangled networks, resulting in the increase of plastic strain. Further, the second stage involves formation of sub-grains separated by low-angle grain boundaries. In the third stage, repeated deformation of the particles creates additional shear bands with further fracturing of the sheets of particles formed due to random cold welding. In the last stage, a steady state is reached between cold welding and fracturing. Therefore, the phenomenon of cold welding is extensively observed during ball milling of the ductile metallic material including HEAs. This can be vigorously suppressed by reducing temperature (cryomilling). Therefore, the mechanical milling extremely low temperature can effectively shorten five-stage process to three stage process, flattening of particles (sheet formation), fracturing of sheets and steady state nanoparticle formation as shown in Figure 8. It might be possible that the flattened powder particles are slightly cold welded but that bond strength of the cold welds is very low, which is easily breakable during further cryomilling. In addition, the low temperature reduces the dynamic recovery in HEAs [60,61], which can accelerate the fracturing of particles. In addition, the extremely low temperature reduces the rate of oxidation [62], which is beneficial to protect the nano-crystals from oxidation during milling.
The formation of HEAs by casting and pulverized them into nanoparticles by cryomilling is the easiest way to synthesized HEAs-NPs, which is capable to prepare large scale nanomaterials.
The cryomilling is advantageous due to short time of milling to form nanoparticles as it vigorously reduces debris from milling tools [47,56,63]. Secondly, this process can be utilized to prepare HEA-NPs in large quantity. In order to estimation of the quantity of HEA-NPs, the equation 1 has been used [47,64].
Percent Yield = (Weight of nanoparticles / total weight of milling powder) ×100 (1) Earlier study shows that the yield of the process is about 97% with only 3% approximately of the milled powder is lost due to coating on the ball and vials. The present cryomill [47] having 250 cc vial and 150 cc ball can mill maximum 25 g at a time i.e. is with ball to powder ratio of 40:1.
It requires 6 hours to synthesize NPs having size less than 10 nm. Thus, it is possible to obtain about 97 g HEA-NPs/day. This amount is substantially large as compared to available green synthesis techniques such as wet chemical synthesis, pulsed discharge arc discharge, plasma induced chemical synthesis or even conventional mechanical alloying. The detailed literature survey reveals that yield of the similar process (commination or arc dressing during minerals beneficiation) is about 90% for industrial scales of production several tones. Hence the yield is true for industrial scale.

Conclusion
The present investigation demonstrates the large scale preparation of HEAs-NPs successfully by cryomilling bulk high entropy alloy ingots. In general, the following conclusions can be made i) the cryomilling can prepare HEAs-NPs less than10 nm size with high yield ii) it allows retaining equiatomic proportion of composition of HEAs.
iii) Cryomilling imparts negligible contamination from milling tools and atmosphere. iv) the bulk HEAs and nanoparticles HEAs are retain their crystal structure in cryomilling v) large scale preparation of HEA-NPs can be achieved by cryomilling.