A critical review on mechanically alloyed high entropy alloys: processing challenges and properties

High entropy alloys are an innovative class of materials for a wide range of industrial applications due to their competitive properties such as improved mechanical properties, superior wear resistance characteristics, and excellent corrosion behavior, which are widely desired for a variety of applications considering several attributes such as economical, eco-friendly and safety. Thus, the quest for high-performance materials with exceptional properties is an unfading research topic for researchers, academia, and metallurgical scientists. HEA presents a novel alloy design idea focused on multi principal elements, a huge compositional space, and more opportunities to develop diverse alloys with exceptional properties. As universally acknowledged, the immense potential in compositions, microstructures, and properties has sparked a great interest in this field. Researchers primarily focused on equimolar HEAs, but the precedent eventually shifted to non-equimolar alloys. As the investigation over HEAs progressed, four core effects were identified as the most important aspects in enabling the distinct characteristics. Mechanical alloying (MA), followed by the sintering approach, has piqued the interest of all researchers focusing on HEA development. As a result, the main intent of this study is to examine mechanically alloyed HEAs critically for mechanical properties, tribological behavior, corrosion behavior, and functional properties. Furthermore, the predominant challenges and their conceivable prospects are also deliberated that offer novelty to this review article.


Introduction
Materials have played a vital role in nearly every aspect of human life throughout history. High-strength materials have been extensively sought after for structural components to enhance load-bearing capabilities while minimizing weight. Aside from strength, the material should have high ductility and hardness for shaping into diverse forms and preventing the catastrophic breakdown of components in service. For instance, one of the major objectives of the automobile sector is to enhance fuel efficiency and minimize ecologically damaging emissions by lowering the weight of components without compromising safety concerns [1]. Figure 1 represents the current scenario in the field of materials with time.
Material science and engineering have alloyed pure metals with two or three elements for decades [2,3] one of which is the major element in greater concentration and the other is the secondary element in a lower concentration. Several irons, copper, Nickel, titanium, magnesium, and aluminium alloys have been synthesized and examined for strength, thermal stability, and wear behavior, according to several research publications [4][5][6][7][8].
HEAs have piqued the interest of academia and scientists because of their remarkable properties over conventional alloys [9,10]. These characteristics are not limited to improved hardness [11], but also include substantially enhanced strength, higher fatigue [12], plasticity [13], stability at high temperatures, excellent resistance to corrosion [14], wear, and oxidation [15]. As a result, HEAs are widely recognized as the most innovative material family, with a broad array of applications spanning from automobile to maritime [16,17].
HEAs can be synthesized through liquid state processing, thin-film deposition techniques, additive manufacturing, and solid-state processing techniques. In liquid state processing techniques vacuum arc melting and induction melting are mainly reported. In liquid state processing, metallic alloy ingots or powder elements are heated in an electric furnace or by tungsten electric arc under an argon atmosphere to avoid oxidation [19]. However, there are certain limitations with liquid state processing are reported a) evaporation of low melting point b) due to low solidification rate formation of heterogenous compounds. Whereas thin film deposition technique is used to fabricate coatings of refractory elements over the substrate to enhance the properties [20]. Pulse laser deposition (PLD) [21], magnetron sputtering deposition (MSD) [20], and plasma spraying deposition [22] are some thin film deposition techniques. In additive manufacturing techniques, the development of intricate shape components takes place through the layer-by-layer deposition of molten metal [23]. This technique is adopted to develop internal cavity parts, complex geometrical shapes while maintaining a high level of accuracy. Different techniques of additive manufacturing explicitly reported are 3D plotting, selective laser    [57,58] melting (SLM) [24], electron beam melting [25], direct energy deposition (DED) [26]. Whereas the development of HEAs solid-state processing technique is widely adopted and explicitly reported. Figure 2 depicts the layout for the development of HEAs through different processing routes. In the processing of HEAs, through the SPS technique, the fine metallic powder is mixed at room temperature followed by consolidation at high temperature. The SSP technique includes powder development (atomization and mechanical alloying), followed by compaction (consolidation of fine HEA metallic powder) via different processes such as hot pressing [27], cold isostatic [28], and uniaxial pressing [29]. However, in the last decade, an exponential rise in research articles over mechanical alloyed high entropy alloys are reported. Table 1 also discusses the different HEAs synthesized using distinct methods, as well as their important properties and relevance. The present state of published scientific literature on MA synthesis of different HEAs is shown in figure 3. The impact of MA and its variables on HEA performance has gotten very little research. As a result, a thorough analysis of MA's significance in the development of HEAs, as well as their properties, has been made. This article's summary may be stated as follows: it began with a quick introduction to the various processing pathways used for HEAs, as well as fundamental definitions of HEAs. In brief, discussed mechanical alloying and phase

High entropy alloys
HEAs were firstly reported by yeh et al [59] and cantor et al [60] individually in the year 2004. However, the term 'high entropy alloys' was coined by yeh et al. In general, alloys consist of two to three major elements whereas, the design approach of HEAs are different from conventional alloys which are built on their composition, alloy design strategy, configurational entropy, microstructure, diffusion, properties, solid solution strengthening and different aspect discussed in table 2. In terms of content, HEAs are described as alloys containing five or more principal elements in an equiatomic ratio of up to thirteen [59,61]. In contrary to conventional alloys, the term 'high entropy alloys' was coined since it was believed that random phases formed sooner than intermetallic compounds owing to the high entropy of mixing, which stabilized the phases and formed a basic crystal structure [59]. Researchers have introduced terminologies like     Core effects: The four core effects that distinguish HEAs from the rest of such bulk alloys [68,69] are responsible for their remarkable properties. The effect has an impact on HEAs' shape, microstructure, and desired properties. Figure 4 depicts the relation amongst the four fundamental effects and the respective interaction with composition, microstructural and related functionalities.
The thermodynamic characteristic of alloys is acknowledged as one of the four core effects known as the high entropy effect. The state with the least Gibbs free energy is named an equilibrium state as per the second rule of thermodynamics [9,68]. The sustainability of solid-solution phase development is shown to be enhanced by high entropy. The high entropy influence is essential to prevent the development of intermetallics, that are rigid and tough to evaluate [67]. The sluggish diffusion effect, on the other hand, hinders phase change and results in the development of slower grain growth, finer precipitates, amorphous structure, increased creep resistance, and higher recrystallization temperature [71]. It also improves the microstructure and morphology of HEAs while also improving their functionality [72,73]. In HEAs, extreme lattice distortion occurs due to the presence of several principal elements in the alloy of the solid's solution phase., severe lattice distortion occurs. Each atom has a distinct sort of atom next to it, with different atomic sizes, which causes lattice stress and strain. Lattice distortion reduces the thermal impact of significant solutions stiffening in a severely distorted lattice, allowing for increased strength and hardness [74,75]. Ranganathan et al [76] endorsed the cocktail effect, stating that the cocktail effect is heavily emphasized in HEAs since at least five key elements are employed to increase the materials' performance. HEAs can be separated into one or more phases, depending on their composition and processing [77]. The capacity to contribute particle morphology, individual phase characteristics, and phase distribution leads to the properties that are obtained. Intrinsic features, collective interaction among elements, and excessive lattice distortion all contribute to the reported traits [78,79].

Mechanical alloying
Vara Lakshmi et al [80] were the first to describe the production of HEAs employing MA, having produced nanocrystalline AlCrCuFeTiZn HEA. Following that, there has been an upsurge in the number of MA-related research papers. There are currently just three review studies on mechanical alloyed HEAs available [81][82][83]. As a result, the goal of this study is to give the most recent information on mechanically alloyed HEAs and the mechanical, tribological, and functional properties that permit potential applications.
Milling's primary aims are reduction in particle size, blending and mixing, and particle reconfiguration [84,85]. The rates of welding and fracture reach steady-state equilibrium after a certain period of milling time [ 26,86,87]. The development of supersaturated solid solutions and the homogeneous dispersion of the strengthening process is required by this methodology. The welding process, which produces equiaxed particles, takes dominance in the following stage. At this point, directed welding lines have been identified; after that, the welding and fracture mechanisms have reached equilibrium, and spontaneously aligned welding lines dominate particle development. The last stage is defined by a steady-state phase wherein the microstructure refining occurs but particle distribution and sizes keep generally constant [88][89][90]. The MA technique is depicted in its totality in figure 5.
MA is influenced by a number of crucial factors that play an important role in the formation of uniform compounds. These variables influence ultimate product features such as final stoichiometry, degree of disorder, and, amorphization. Superior particle formation arises from improved administration of these variables. Figure 6 depicts the existing scenario of research publications dealing with MA-related variables. Table 4 further discusses different variables and their impacts on HEAs lattice strain and crystal size in detail.

Key findings
• The best approach for creating nano-structured alloys with greater chemical uniformity.
• MA features comprise shortened time, decreased power intakes, improved coating adherence, and greater versatility in the formation of diverse functional coatings [92].
• It improves configurational consistency and can effectively alleviate the segregation challenges.
• It has a better rate of diffusion and is perfectly suited for developing various kinds of materials, such as quasicrystalline, amorphous, and oxide permeation reinforced materials, among others [93].

Phase evolution in HEAs
Some of the central themes of extensive study in HEAs include phase stability and their evolution [91,130]. The primary factor that permits the development of the crystalline phase in HEAs is the configurational entropy [80].
The DSconfig effect is established on the second rule of thermodynamics for in an irreversible process that proceeds exclusively in a single path for a minimum free energy state depicted by Some of the core aspects of extensive investigation in HEAs entail phase development and sustainability [87,118]. The main determinant that facilitates the development of solid crystalline phases in HEAs is configurational entropy [80]. For the irreversible process that only advances in one way for a comparatively lower energy state indicated by the DSconfig effect.
Where DHmix is the enthalpy of mixing, DGmix is Gibbs free energy and DSmix denotes the entropy of mixing, that is described in the given expression.; It is evident from the following equation that when DSmix increases, the free energy of solid solutions diminishes. According to Boltzmann's analysis, DSconfig can be calculated from the given expression, Where R denotes the universal gas constant and Xi denotes the mole fraction, The number of elements is denoted by n.DSconfig is not an adequate factor to preserve the solid solution; key determinants must be considered to determine the solid solution formation criteria. According to Zhang et al [119], definite criteria must be fulfilled in the quest for a solid solution to emerge that depicted in figure 7.
MA is an effective approach for HEAs to achieve improved performance by altering microstructure and phases. Elements like Cu, Co, Ni, and Mn encourage the growth of the FCC phase, whereas Al, V, Cr, and Ti promote the advent of the BCC phase [120]. Al is important for controlling phase forming and influencing mechanical, thermal, and tribological aspects [57]. Chung et al investigated AlxCoCuNiFe (x=0 and 0.3) HEAs and found that as the Al content increased, the BCC phase became more prevalent, resulting in better hardness. In a similar study, Al was evidenced to be a BCC stabiliser and a solid solution hardening agent [121]. The alloying behavior of distinct HEA elements is accomplished during milling.
Rituraj chandrakar et al [49] discussed the effect of Si content on phase evolution of AlCoCrCuFeNiSi x . Figure 8(i) XRD pattern proved that the BCC is a prominent phase in comparison to FCC owing to the lesser value of VEC. The mixing enthalpy becomes more negative with the addition of Si which is also favorable for the formation of BCC phase. With the milling time, peaks get broadened which indicated the refinement of crystal size, increase in lattice strain, and the solid solution evolution. Figure 8(ii) demonstrated the SEM micrograph of the AlCoCrCuFeNiSi x HEAs figure 8(iii) shows the EDS spectrum of the region. With the addition of Si large volume fraction of BCC is observed in which BCC phase is in rich Fe-Al-Si elements whereas sigma phase is rich in Cr element.
Guofeng wang et al [114] examined the influence of milling duration on MoNbTaTiV HEA and found that lattice strain increases while grain size decreases as milling time increases. Furthermore, the milling time XRD pattern showed that Ti and V peaks disappeared after milling time of 5 h and 15 h. However, after 20 h of milling, nearly all of the peaks were disappeared. Elements with a higher melting point seem to have a lower diffusion coefficient and vice versa. Figure 9 depicts the dissolution sequence of MoNbTaTiV.
Mohanty et al [122] discussed the microstructure of equiatomic AlCoCrFeNi HEA where the XRD pattern obtained in figure 1(a) revealed that FCC and BCC solid solution formed in which fcc is predominant peak. Figure 10(b) SEM micrograph shows two phases contrast which indicated the presence of two-phase in MA HEA powder. In figure 10(c) TEM image showed the grain of the HEA particle for 25 h milling figure 10(d) confirm the nature of the particle by SAD of fcc solid solution whereas figure 10(e) represented the SAD of another particle which indexed due to bcc solid solution.

Various attributes of mechanical alloying
Achieving fine particles having specified morphology and microstructure is a difficult task. This can be accomplished while optimization of process variables. The following are the process variables and their impact on MA:

Ball to powder weight ratio (BPR)
The degree of amorphization is significantly influenced by the kinetic energy of the ball milling, resulting in reaction and interdiffusion. The amorphization rate rises as BPR rises. The amorphous phase proportion is noticed in the first 48 h of ball milling, and subsequently, as the heat produced rises, a crystalline phase is found. Small particles are produced at a higher BPR, thus, the use of a high weight ratio has a drawback in the form of contamination [123]. However, at lower BPR, the volume of the crystallites fluctuated, suggesting a change in the efficacy of the milling operation & insufficient phase development [124].

Rotational speed
Rotational speed has an impact on particle shape as well. The ball strikes the ball or container surface with significant force at greater speeds, which favors welding. The particle size rose as the rotational speed grew. The maximum impact energy is obtained at higher RPMs, however, it is observed that milling time efficiency is shown to be low at lower speeds [125]. Ciprian et al [19] produced HfNbTiTaZr HEAs & explored how rotating speed affects them. They observed that the ideal crystallite size was achieved at 300 RPM when the RPM was controlled between 200 and 300. Khailo et al [126] pioneered the notion of high-intensity MA in the synthesis of HEAs. A single-phase FCC CoCrNiFe HEAs is developed in 2 h, whereas typical milling takes about 10 h. According to Salemi et al [127], suitable alloying was not accomplished after 50 h of milling time & a rotating speed of 300 RPM.
Single-phase FCC was achieved when the rotating speed was raised to 350 RPM. This is stated that with a rise in rotating speed, transfer of energy to the HEA metallic powder is predicted to be enhanced nearly by 1.7 times (at approx. 350 RPM). Tan et al [128] produced Al 2 NbTi 3 V 2 Zr at 350 and 150 RPM, respectively. High-energy milling was shown to be significantly better efficacious compared to low-energy MA.

Milling time
The fracture cold-welding process occurred during milling. Cold-welding prevails over fracturing in the early stages of milling, resulting in a dramatic rise in particle size, accompanied by a reduction in particle size once the fracturing process gets hold. An XRD pattern at various milling durations can be used to evaluate the level of alloying [114]. Figure 11 depicts the influence of milling time on the microstructure and morphology of HEAs  after a particular period of time when cold welding and fracturing are in balance. Furthermore, the milling period should not be excessively long because longer milling times diminish diffraction peaks. This suggests that a prolonged milling duration has a detrimental effect on phase recognition of the powder. It's also been discovered that when milling duration goes up, the crystal size reduces, and lattice strain rises [86]. The extreme plastic deformation of HEA particles in milling is attributable to structural imperfections including dislocation motion and deficiencies, which result in increased lattice strain [129]. Undesired phases and impurity concerns might emerge when milling time is extended [130]. Milling is undesirable and fails to obtain acceptable attributes at shorter milling durations [65]. Table 5 shows the influence of milling time affecting the desirable attributes of HEAs specifically.

Process control agent (PCA)
Maintaining particle shape and size is usually a difficult job, and milling environments are vital [139,140]. Surface-active materials, usually referred to as process control agents [141], are one of the measures being used to prevent severe welding. These PCA get deposited on the particles' surfaces and significantly lower surface tension [142], limit agglomeration, and interfere with cold welding. Toluene, methanol, stearic acid, ethyl  alcohol, and propyl glycol, are common control agents described in MA [143]. PCA alters the inherent character of materials by interacting with surface particles, affecting dissolving degrees, refining crystal size, and influencing ultimate mechanical behavior [144].

Ambient conditions
The milling ambient circumstances are among the most important aspects that influence compositional metallic powder [145]. While milling, a very finer powdered particle with a high specific surface area is formed, which is extremely reactive with oxygen and forms oxidised compound, as well as other gases such as H 2 , N 2 or, S, etc [77].

Summary and parameters recommendations of MA
The operational parameters must be tailored to obtain the utmost possible properties. In MA, the spherical morphology of elemental powder progressively evolves depending on several factors including a low ball to powder ratio (10:1) and a medium speed of 300 RPM during the optimal duration that can considerably retain the morphology of alloyed particles [146][147][148]. Milling duration must not be quite long; longer milling times diminish diffraction peaks. This reveals that a prolonged milling duration has a detrimental impact on phase characterization and identification of the material. Stearic acid is the most suited surface-active material in the MA consistent fracturing-welding process to achieve an appropriate balance amid fracturing & welding. Yating Qiao et al [149] noticed that refractory metals cause extreme cold welding during MA in the Solid PCA (stearic acid), and absence of PCA, whereas the presence of liquid PCA efficaciously relieved the cold welding, raising the recovery proportion from 5% to 90% by limiting direct contact between metal powder and metal powder. The attributes with varied ranges and suggested values are detailed in table 6 derived from the prior discussion.

Mechanical characteristics
Mechanical characteristics are the main significant deciding aspect in terms of material performance and durability, and phases play a prominent part. FCC-based HEAs, for example, have a low yield strength but great plasticity, while BCC has a high yield strength but poor plasticity. The microhardness of mechanically alloyed HEAs varied between 375 and 699 HV. The advantage of MA accompanied by various consolidation processes was the formation of nano-twins, precipitates, and numerous hardening mechanisms, all of which improved the strength of HEAs. The hardness of CoCrFeNi treated with MA and SPS is 570 HV [150], which is approximately four times that of the as-cast alloy (150 HV) [151]. The compressive strength of AlCrCoFeNi synthesized by MA and SPS was 2.6 GPa [122], which was approximately twice that of as-cast HEA [152]. When AlCoFeMoNiTi HEA was processed via MA, nanoprecipitates were produced that had better strength over as-cast HEAs [134]. Figure 11. Characterization of MoNbTaTiV HEA milled powder (a) particle size distribution (b) average particle size (c) cross-section HEA SEM images. Reproduced from [114], with permission from Elsevier. The compressive yield strength and ductility of NbMoTaW HEA produced using MA and SPS reported as 2612 MPa and 8.8% of NbMoTaW HEA produced using MA and SPS. Similar HEAs were produced by arc melting in another study, with ductility and strength of 1.7% and 1246 MPa, respectively [153].
Mechanical properties of non-equiatomic Co 1.5 FeCrNi 1.5 Ti 0.5 HEAs prepared by MA and SPS were studied, and bend strength of 2600 MPa, a tensile strength of 1400 MPa, hardness of 4400 MPa, and ductility of 4% was found, those were significantly superior to typically as-casted HEA [97]. A substantial FCC phase was generated in an AlCoNiFeTi HEA processed by MA and SPS, along with B 2 phase precipitates and Al 3 Ti particles. The inclusion of TiO 2 particles and Ni 3 Ti precipitates in the FCC phase of Co 1.5 CrFeNi 1.5 Ti 0.5 resulted in a superior combination of 1460 MPa strength and 14.5 percent ductility [154]. Al 7.5 Fe 25 Co 25 Cu 17.5 Ni HEA exhibited higher hardness (454 HV) and superior strength (1795 MPa) [155]. Furthermore, various HEAs were processed through MA and different processing techniques listed in table 7.

Tribological characteristics
It is challenging to develop a high wear-resistant material without compromising mechanical properties such as compressive and tensile strength [172]. However, HEAs present a novel design approach to develop a material with exceptional tribological properties as well as good mechanical properties, a proper balancing between alloy matrix and self-solid lubricating element or hard particles must be established. The HEA solid solution achieved significant chemical disorder and topological order, overcoming the strength-ductility trade-off unconventionally [45].
The coefficient of friction (COF) and rate of wear is used to assess the tribological behaviour of materials. Three configurations, namely block on block, ball on disc, and pin on disc were employed to evaluate the tribological behaviour of newly designed HEAs [173]. Various aspects such as applied load, rotation speed of the disc, and time or total distance impact the wear rate in the methods described above. Although the majority of HEA research is focused on structural properties, tribological behavior research has lately garnered attention. Cantor alloys have been extensively studied for their tribological properties; revealing the intrinsic wear characteristics of cantor alloys is critical. Nagarjuna et al [174] examined the wear behaviour of CoCrFeMnNi HEA utilizing a ball on disc. The surface of the HEAs was scratched with some adherent debris at the start of the wear testing, as shown in figure 12. When the applied load rises, the stress at the contact surfaces increases, causing surface cracking and delamination. So, the result indicated that the transition occurred from adhesive and abrasive wear to delamination as the duration of wear is continued and examined the variation of CoF with load, sliding speed, and time [175]. Dollmann et al [176] investigated the wear behaviour of CoCrFeMnNi HEAs in a separate study. A soft fcc HEA has a low hardness value for the undeformed surface, while the hardness value increases significantly to 500 HV for the deformed surface. HEAs with exceptional wear resistance qualities have received a lot of attention. Ceramics and carbides are commonly used in the strengthening phase of HEAs, but oxides are also used. Jiang et al [177]observed in their investigation that the addition of TiC particles to CoCrFeCuAl HEA lowered the wear rate and CoF as shown in figure 13. The hardness increased to 10.8 GPa with the addition of 50% TiC, resulting in a decreased wear rate of 9.6 10 −5 mm 3 N −1 m −1 . Guo et al [178] also observed that the inclusion of Cr 3 C 2 greatly improved the wear performance of single-phase CoCrFeMnNi HEA. Cai et al [179] investigated the pinning effect of TiC particles on the strengthening of CoCrFeMnNi HEAs, which resulted in grain boundaries and grain size refinement and reduction from 76 um to 26 um for a 15% TiC-CoCrFeMnNi Composite. The use of TiC improves hardness from 213 HV to 288 HV. TiC particles reduce The thickness of the surface hardening layer protects against wear. However, researchers investigate the role of self-lubrication on the tribological behavior of HEAs. It has been found that solid lubricant plays a key role in controlling wear rate. The several kinds of solid lubricants utilized in high entropy alloys such as graphite, MoS 2 , BaF 2 , Cu, and Ag are reported. The soft phase Ag and Cu, which have low shear strength, are purportedly employed in CoCrFeNi HEA, which improves HEA wear resistance. Verma et al [180]evaluated the tribological properties of CrCoFeNiCu x (x = 0 to 1, at 0.2 mean intervals) HEAs. Cu particle segregation at the grain boundaries of CrCoFeNiCu x HEAs reduces wear rate at room temperature and 600 C. The wear rate of CrCoFeNiCu x HEAs bearings at higher temperature is stated to be lower than at room temperature. To achieve self-lubricating capabilities, HEAs are designed with a tailored composition that includes lubricating additives with self-lubricating properties. Kumar et al [181] carried out a wear investigation of Al 0.4 FeCrNiCo x (x = 0, 0.25, 0.5, 1.0 M) HEAs under oil lubrication by varying the normal load, sliding distance, and sliding speed. The effect of varying Co was also carefully examined. It was stated that maximal wear was observed for Co = 1 HEA for two reasons: (i) when cobalt concentration increases, hardness decreases, and (ii) an oil film breakdown occurs. The wear surface revealed peeling off, flow material, and cracks seen perpendicular to the sliding direction. Another work used demineralized water with and without a 3.5% NaCl solution to conduct wear testing. The wear mode reported during the investigation were combined effect of adhesion with delamination, plastic flow, abrasive, and oxidation [182]. Furthermore, other articles on wear characteristics of HEAs are enlisted in table 8.

Corrosion behavior
The corrosion behavior is evaluated by electrochemical/chemical interaction between material and its surrounding. Corrosion affects the proper functioning of materials that leads to failure during its functionality corrosion. Corrosion costs more than 3% of the world's GDP [196]. Therefore, the study of corrosion behavior has great significance. The corrosion resistance of HEAs depends on a different set of attributes that are primarily dependent on chemical homogeneity for elemental distribution through grain boundaries and crystalline phases. However, passive films stability plays a vital role in this aspect [197]. Figure 14 illustrates design guidelines for corrosion resistance HEAs.
The corrosion behavior of HEAs depends upon several factors such as follows: • The impact of alloying materials • The effect of microstructure • Environmental stability & passivation mechanism of passive films formed on HEAs.
• Influence of processing methods on corrosion resistance.
The passive layer has a great influence on the corrosion behavior of the alloys. Shi et al [198] used XPS and electro-chemical impedance spectroscopy to explore the influence of Al on passive films of HEAs and show the corrosion process of AlxCoFeCrNi alloys in 3.5 wt% NaCl. The passive layer on the surface of the Al x CoFeCrNi alloy reduced even as the concentration of Al rose, whereas the passive layer on the surface dropped as the Cr concentration grew. The overall composition of the passive coating on the surface of stainless steel is closely linked to its corrosion resistance, with Cr oxide playing the most important role [199]. The concentration of Cr above 12% ensures passivation. Cr is commonly utilized as one of the key elements of high-performance alloys (HEAs) that improve corrosion resistance. Chai et al [200] investigated the effect of Cr concentration on the resistance to corrosion of FeCoNiCr X alloys exhibiting fcc phase in 3.5 wt% NaCl and 0.5 M H 2 SO 4 in 3.5 wt percent NaCl and 0.5 M H 2 SO 4 , respectively. The columnar structure of the FeCoNiCr 0.5 alloy was preserved in both treatments, but the passive area and breakage potential of the alloy increased dramatically. Shi et al [201] shown that a decrease in short-range elemental segregation and a variation in phase composition might have a substantial impact on the corrosion rate of Al X CrCoFeNi (x=0.3, 0.5, 0.7) HEAs. After a 1000 h treatment at 1250°C, the grain size of the Al 0.3 CrCoFeNi alloy increased to five times that of the as-forged state; also, some annealing twins emerged. The work function of the as-equilibrated Al 0.3 CoCrFeNi alloy was increased by 50 meV due to the decrease in imperfections and short-range elemental segregation of HEA during hightemperature homogenization, implying that the Al 0.3 CrCoFeNi alloy has greater corrosion resistance (figures 15(a) and (b)). The disordered Cr-Co-Fe-rich BCC phase solubilized into the Al-Ni-rich B2 matrix over the same thermal processing in the Al 0.5 CrCoFeNi alloy, lowering the composition distinction in between BCC   and FCC phases and thereby lessening potential gradients across the interphase ( figure 15(e)). Ultimately, when referred to the as-forged Al 0.7 CrCoFeNi alloy, the volume fraction of the Cr-Co-Fe-rich bcc phase in the asequilibrated Al 0.7 CrCoFeNi alloy lowered from 45% to 22%, along with the microstructural modification of the Cr-Co-Fe-rich BCC phase from mesh-like to block-like, resulting in a substantial decrease in the potential difference among different microstructures figures 15(f)-(h).

Functional properties
HEAs are novel designed materials that are being studied for their microstructure and mechanical characteristics. There has been some progress in the study of the functional characteristics of HEAs. Soft magnetic properties, resistance against Irradiation, catalytic properties, and thermoelectric capabilities are all included in table 9. Their specific structural property, multi-principal element solid solution, has an impact on these characteristics. In applications such as Automotive industries, Nuclear irradiation components, solar plates, and energy sectors possess functional characteristics that are very critical.

Potential applications
In addition to the commonly used superalloys, the need for structural and functional materials is surging in various sectors, including extraction, railway, mining [229], defense [230], and aviation in science and engineering. Figure 16 depicts the existing and anticipated uses of various HEAs in various industries.
1. Ti 15 V 3 Cr 3 Sn 3 Al HEAs are employed as embedded aircraft equipment with their exceptional strength ratio and outstanding cold formability [231]. Ti 3 Al 8 V 6 Cr 4 Zr 4 Mo, on the other hand, has a foothold in the aviation sector since of a combination of properties including high cycle fatigue high strength, and plasticity, high cycle fatigue maintainability, and outstanding fracture toughness [232]. Ti 10 V 2 FAle has been effectively used in the landing gear systems of A-40-500/600, and Boeing 777 [233] due to its better ductility and lightweight ratio.
2. Improved mechanical properties at extremely high temperatures, as well as resistance against corrosion, are attained in boilers, both of which are necessary for optimal performance [234,235]. Currently, HR3C steel is typically utilized for these purposes; however, Al x CoCrFeNi HEA, which can withstand supercritical temperatures, might be an effective replacement [236].
3. Material customization for maritime applications is still a substantial task that requires a comprehensive insight into tribological properties, anti-fouling characteristics, anti-corrosion potentials, & better mechanical behavior. Yuan et al [237] developed AlCoCrNiFeCu 0.5 HEAs, which have excellent antifouling characteristics as well as resistance against corrosion & wear. Zhou et al [238] revealed that Al0.4CoCrNiFeCu x is antibacterial.
4. HEAs are well explored for components of automobile industries due to their high strength, improved ductility, and toughness. HEAs enhance the load-bearing capacity and reduce the weight of components [239]. The reduction of weight without compromising safety leads to improvising fuel efficiency and reduces harmful gas emissions [240].
5. The remarkable properties of HEAs nitrides and carbides are similarly fascinating. Amorphous solid solution morphologies, as well as high mechanical strength and toughness, are possible. On instrument steels and high-speed steels, they could be used as coating materials and dissemination shields. Highentropy nitrides and carbides have also been shown to have enough potential to be used as biomedical coatings in recent investigations [150].  • In regards to annealing temperature, crystallization temperature, and equilibrium temperature decreases. [227,228] • Advocated for high temperature applications

Challenges and the opportunities
It is noticeable out of this article that MA is widely employed in the tailoring of HEAs, although it still has certain shortcomings that must be overcome. As a result, this section adequately outlines the contemporary obstacles along with their conceivable ramifications. In addition, figure 17 schematically depicts the needs and challenges.
1. Despite the fact that HEAs have exceptional mechanical properties like increased strength at higher temperatures, better plasticity, enhanced hardness, and better toughness under very low temperature, they have still to find a practical application in which they would substitute well recognized conventional alloys such as nickel & Titanium alloys, and steel, that includes firmly anchored mechanical properties such as higher strength & stiffness, with suitable deformability, resistance against creep, still, they have to find a practical application The majority of HEA research has mainly focused on their morphology and microstructure, as well as their mechanical and tribological properties. As a result, HEAs may be constrained to replicating the capabilities of their prior class of materials in the future.
2. HEA's continued development should focus on improving mechanical and functional properties such as weldability, hydrogen embrittlement resistance, stress corrosion resistance, anti-fouling surface oxides layer, a combination of magnetic and invar response, piezoelectric energy utilizing capabilities, superconductivity, and anti-bacterial resistance, among others. As a result, HEAs must be developed to have improved mechanical and functional capabilities while maintaining a low overall cost.
3. In a significant advance, HEAs examined microstructure, mechanical and tribological properties. The emphasis should be transitioned toward the development of application-based HEAs. HEAs ought to develop explicitly for functional attributes that might lead to the development of new items and processes in response to forthcoming generation needs. It must be tailored to develop multifunctional elements with distinct characteristics that are challenging to achieve with a sole element. Our metallurgical academics, materials scientists, and engineers are still investigating HEAs with improved mechanical properties in order to achieve qualities such as wear-resistance to oxidation besides stress corrosion. These difficulties overlay the pathway for new materials to be designed and produced to suit future demands in aerospace, railways, maritime, power distribution equipment, transportation, and structural applications.
4. Because of their wide compositional range, nanocrystalline HEAs have a greater potential as catalysts. Nonetheless, the coherent and controlled development of these inherently intricate materials is a critical job associated with them [241]. The ability to quickly and effectively synthesize HEAs at elevated temperatures along with good thermal steadiness is now largely owing to developments in computational-assisted rational design [242,243]. 5. Weldability is an essential factor that cannot be neglected while using HEAs in any application; accordingly, special attention should be focused on knowing more about HEA weldability. Although work on CoCrFeNiMn HEA is promising, process parameters must be optimized in order to achieve an extremely effective joint having remarkable mechanical performance. The research shows that HEA may absorb impurities, yielding in unanticipated phases in non-equilibrium densification and high functional joint with superior mechanical characteristics. Impurities, on either hand, do not necessitate improved characteristics, thus microstructure homogenization is vital.
6. Grain refinement (via severe plastic deformation techniques and thermo-mechanical synthesis) and deformation attributes such as strain rate and temperature have a significant impact on mechanical properties. As a result, considerable attention should be dedicated to thermal stability, which governs the morphology and microstructure of HEAs, thereby improving their properties.

Concluding remarks
Mechanical alloying is a widely used method for synthesizing HEAs. This detailed evaluation explores the synthesis of HEAs using MA while considering its unique aspects. The primary goal of this research is to explore how to thoroughly assess phase development during HEA MA. Instead of a sophisticated, multiphase crystalline structure, several principal elements may form a single-phase crystalline structure, contrary to popular belief. Hardness, thermal stability, high-temperature strength, oxidation resistance, corrosion resistance, and wear endurance were all significantly improved when MA with fine HEAs powder disseminated evenly in the solid solution was used. Variations in MA times provide a wide range of crystalline forms and properties. As adequate alloying doesn't occur when milling time is too short, and When the milling period is too long, since it is difficult to distinguish phases of powder particles, the milling duration should be decreased. The particle's crystallite size reduces as milling time rises, yet lattice strain between the particles increases. It's crucial to develop & synthesize HEAs with superior mechanical performances under a variety of environmental conditions, as well as phase stability and strengthening processes. According to the research, MA only forms crystalline phases with multiple phases and complex structures. Phases evolve into an intricate structure during consolidation and annealing, and numerous solid solution phases appear, signifying the advent of metastable phases. Since it provides high retention and densification, SPS is observed to be an extensively employed technique for HEA metal powder consolidation. Due to their hardness, HEAs have better wear resistance. Furthermore, HEA coatings have better corrosion resistance, according to various papers on corrosion behavior.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).

Declaration of competing interest
No conflict of interest exists for this manuscript.

Funding
No Funding is available.