Thermal–Mechanical Processing and Strengthen in Al x CoCrFeNi High-Entropy Alloys

In this study high-entropy alloys (HEAs) were devised based on a new alloy design concept, which breaks with traditional design methods for conventional alloys. As a novel alloy, HEAs have demonstrated excellent engineering properties and possible combinations of diverse properties for their unique tunable microstructures and properties. This review article explains the phase transition mechanism and mechanical properties of high-entropy alloys under the thermal-mechanical coupling effect, which is conducive to deepening the role of deformation combines annealing on the structure control and performance improvement of high-entropy alloys, giving HEAs a series of outstanding performance and engineering application prospect. To reach this goal we have explored the microstructural evolution, formation of secondary phases at high and/or intermediate temperatures and their effect on the mechanical properties of the well known Al x CoCrFeNi HEAs system, which not only has an important role in deepening the understanding of phase transition mechanism in Al x CoCrFeNi HEAs, but also has important engineering application value for promoting the application of high-entropy alloys.


INTRODUCTION
The design and development of structural materials that are suitable for extreme environments such as high temperatures, cryogenic environments, or situations that involve irradiation and corrosion, etc., are a high priority in the fields of materials science and technology. Modern technological developments in engineering, particularly in the nuclear, turbine, and aerospace industries, require new structural materials to provide higher performance than the currently available commercial alloys. Cantor et al. (2004) and Yeh et al. (2004) took the lead in adapting the traditional design methodology of alloys in 2004 and proposed the novel concept of alloy design, in which five or more elements are mixed in equiatomic or near equiatomic concentrations, such that the high configurational entropy ΔS conf mixing could promote single-phase-disordered solid solutions formation, e.g., fcc-centered cubic (fcc), body-centered cubic (bcc) or hexagonal close-packed (hcp) disordered phase (Troparevsky et al., 2015;Yuan et al., 2017;Tan et al., 2018a;Lu et al., 2018). High-entropy alloys (HEAs), also called multi-principal element alloys (Senkov et al., 2015) or complex concentrated alloys (Gorsse et al., 2017), have attracted attention on account of their superior performance in many aspects and broad application prospects Wang et al., 2017c;Guo et al., 2017;Tan et al., 2017;Wang et al., 2018a;Tan et al., 2018b;Gao et al., 2019;George et al., 2020). The appearance of HEAs has made alloy design central to phase diagrams, making it possible to obtain more potentially unique microstructures and properties.
In the field of HEAs, solute and solvent cannot be applied in the conventional alloy and the theoretical strengthening system needs to be verified. The optimization of the mechanical properties of high-entropy alloys is thus challenging. Recently, studies have focused on overcoming the strength-ductility trade off, by introducing high-density nano-twins in the grains (Lu, 2016;Schneider et al., 2020), as processing fine grains combined with nanoscale precipitates even distribution within the grains (Zhao et al., 2020b;Guillot et al., 2020;Yu et al., 2020), controlling the lattice misfit between intermetallic nanoparticles and matrix (Miracle, 2015;He et al., 2016a;Zhang et al., 2019) and producing materials with a heterogeneous structure (Sun et al., 2018;Wu et al., 2019;Du et al., 2020). The successful preparation of eutectic high-entropy alloy (EHEA) provided a new way to optimize the phase structure of HEAs by combining the ductile phase (e.g., fcc solid-solution phase) with the hard phase (e.g., bcc solid-solution phase or intermetallics) Tan et al., 2019). This structure design could be realized through a simple and effective way of thermal-mechanical processing, offering a feasible strategy for a new generation of high-strength and high-toughness HEAs. Through cold rolling and annealing processing, the size, volume fraction, and distribution of the nano-sized particles (L1 2 -Ni 3 (Ti, Al), L2 1 -(Ni, Co) 2 TiAl, sigma (σ) or μ intermetallic phases) formed within the fcc phase matrix of HEAs could be controlled effectively, improving strain hardening ability by interacting with dislocations in plastic deformation (He et al., 2016b;Liu et al., 2016). An excellent balance between strength and ductility was obtained in the as-cast AlCoCrFeNi 2.1 EHEA . After extra heavily cold rolling and annealing, it showed significant improvement in tensile properties, with an ultimate tensile strength (UTS) of 1,200 MPa and elongation to failure (e f ) ∼12% (Wani et al., 2016). Through cold rolling and subsequent annealing, different types of precipitates were formed within the matrix of the FeCoNiCrTi 0.2 HEA, improving strength and ductility at the same time (Tong et al., 2019). It is expected that the mechanical properties of HEAs could be successfully tailored using simple thermo-mechanical processing and further extended to bulk engineer materials for industrial application.
Earlier studies have tended to focus on single-phase HEAs with equal atomic ratio. However, phase stability is easily be impacted by alloying or moderate temperature annealing, which has a further effect on mechanical properties Macdonald et al., 2019). In addition, there are often some fatal disadvantages concomitantly, in that poor fluidity and castability may cause considerable chemical inhomogeneity in single-phase HEAs Lu et al., 2017). It is hard to achieve excellent balanced tensile properties, e.g., the single fcc phase HEAs are usually soft while the single bcc phase HEAs are hard and brittle (Wang et al., 2012;Guo et al., 2013;Liu et al., 2019b). Thus, researchers have increasingly kept a watchful eye on not only the single-phase HEAs but also multiphase HEAs. By altering the composition of the alloying elements, the results of Stepanov et al. (2015) showed that the volume fraction of σ phase was related to the concentration of accumulated Cr and V elements. It was reported that the addition of Ni in AlCoCrFeMo 0.5 Ni x alloys promoted the formation of the fcc phase, and the Ni element was also found to be an effective fcc structural stable element (Juan et al., 2013). Among them, Al x CoCrFeNi HEAs have been perceived as potentially important engineering materials as the inherent crystal structure can evolve from fcc into bcc with the variation of Al concentration (Chou et al., 2009;Wang et al., 2012;Garlapati et al., 2020). It has attracted a wide range of research interest due to its excellent physical and mechanical properties (e.g., creep behavior, dynamical mechanical property, impact toughness, magnetic properties, and resistivity-temperature behavior) (Kao et al., 2011;Ma et al., 2014;Cao et al., 2016;Cieslak et al., 2018;Xia et al., 2018;Wang et al., 2019a;Zhao et al., 2020a;Wang et al., 2020).
This review reports on recent advancements in thermalmechanical processing and its effect on microstructural evolution and phase transformation in Al x CoCrFeNi HEAs. It also correlates insights into the mechanical properties and strengthening of HEAs, especially the evolution of hardness, plasticity, and strength under tension and compression at room temperature as well as at high temperatures with the phase structure, are also correlated.

Development of Al x CoCrFeNi HEAs
In the initial stage of the HEA research field, the HEA systems containing Cu were widely exploited due to the improvement of plasticity (Cantor et al., 2004;Chen et al., 2004;Yeh et al., 2004;Wu et al., 2006;Tung et al., 2007;Sriharitha et al., 2013). However, Cu tended to segregate at grain boundary for its high enthalpy of mixing with other elements. Meanwhile, the Al x CoCrCuFeNi HEA system has been carefully studied and the variation of Al content was found to bring significant changes in microstructure and mechanical properties of Al x CoCrCuFeNi (Tong et al., 2005a;Tong et al., 2005b;Wu et al., 2006;Tung et al., 2007). The compressive properties of AlCoCrFeNi alloy without Cu were investigated, showing a combination of excellent compressive strength (∼2004 MPa) and plastic strain (∼32.7%) (Wang et al., 2008). The Cu-free Al x CoCrFeNi HEA system has subsequently received increasing attention in the last decade.

Effects of Al Addition on Microstructure of Al x CoCrFeNi HEAs
The resultant phases in all the investigated as-cast Al x CoCrFeNi HEA system (x 0-3) were: only solid-solution structures, mainly fcc, bcc or their mixture (Kao et al., 2009;Li et al., 2009;Ma et al., 2014;Wang et al., 2014;Zuo et al., 2014;Niu et al., 2016;Xia et al., 2016;Gangireddy et al., 2018a;Gangireddy et al., 2018b;Liu et al., 2018;Zhou et al., 2018;Annasamy et al., 2019;Li et al., 2019;He et al., 2020). Alloys with low Al contents (x 0-0.3) exhibited the simple fcc structure, as x exceeded ∼0.5, the bcc phase formed out of the fcc solid solution phase. Spinodal decomposition occurred later with an increase in Al content, leading to the formation of ordered (NiAl-type) and disordered bcc (A2) phases. Rao et al. (2016) also reported a fcc-bcc crystallographic orientation relationship between the precipitation and fcc matrix: (1) (1−10) bcc // (200)  √ ] fcc . The mechanism of bcc phase formation in Al x CoCrFeNi has received much attention. Kao et al. (2009) investigated the microstructural evolution of as-cast, -homogenized (1,100°C/ 24 h), and -deformed (1,100°C/24 h + 50% CR) Al x CoCrFeNi (0 ≤ x ≤ 2) high-entropy alloys. They reported that the structure of as-cast was sequentially single fcc (x < 0.45), duplex fcc-bcc (0.45 ≤ x ≤ 0.88), and single bcc (x > 0.88). While the x intervals for the existence of duplex phase in -homogenized and -deformed Al x CoCrFeNi alloys are 0.30 ≤ x ≤ 1.17 and 0.30 ≤ x ≤ 0.875, respectively, indicating the widening of the range of two-phase regions after homogenization treatment. The Al element was found to be a bcc stabilizer in Al x CoCrFeNi alloys. Jasiewicz et al. (2015) elucidated the phase stability of Al x CoCrFeNi (x ≤ 3) HEAs based on experimental data and computational results and found that the fcc phase was more stable when x < 0.62, and the range of fcc/bcc two-phase region was 0.5 ≤ x ≤ 1.0. Ogura et al. (2017) also illustrated the effect of Al addition on phase transition in Al x CoCrFeNi alloys via first principles electronic structure calculations and an increase in Al content lowered the total energy difference between fcc and bcc.

Thermodynamic Appraisal of Al x CoCrFeNi HEAs
To accelerate the characterization of the composition-structureproperty relationship of Al x CoCrFeNi HEAs, a calculation of phase diagrams (CALPHAD) approach was developed to predict the phase relationship in previous studies (Xia et al., 2016;Butler and Weaver, 2017;Rao et al., 2017;Gwalani et al., 2018;Shi et al., 2018;Komarasamy et al., 2019). Xia et al. (2016) successfully predicted the phase diagram of the Al x CoCrFeNi (x 0-2) alloy system through the CALPHAD approach using the PanHEA database. The experimental observations of Al x CoCrFeNi (x 0.1, 0.75 and 1.5, respectively) were consistent with the predicted equilibrium phases diagrams. The fcc phase was stable over a wide temperature range of 645-1434 C for Al 0.1 CoCrFeNi alloy, whereas decomposition occurred at lower temperatures below 500 C. In addition, the predicted primary phase is the fcc phase and the B2 phase for the Al 0.75 CoCrFeNi alloy and Al 1.5 CoCrFeNi alloy, respectively. Komarasamy et al. (2019) chose the aging condition based on the pseudo-binary phase diagram using the PANDAT HEA database to develop the Ni 3 Al (L1 2 -type), B2 (Al-Ni based), and Al 3 Ni particles to strengthen the Al 0. 5 CoCrFeNi alloy. The existence of B2 precipitates was predicted for all heat treatment conditions except for 550°C, where L1 2 phases formed. The experimental phase relationship agreed well with the calculated result. To understand the phase stability and transformation pathway of Al 0.3 CoCrFeNi alloy under different thermal treatments, Gwalani et al. (2018) calculated the molar fraction of equilibrium phases as a function of the temperature for the alloy. Based on the calculated equilibrium phase data, the fcc-based Al 0.3 CoCrFeNi alloy was metastable at 620°C, and thus providing the possibility for the formation of new phases. All of these suggest that this way of calculating equilibrium phase diagrams provides useful guidance for the prediction of phases in the Al x CoCrFeNi HEAs.

Mechanical Properties of Al x CoCrFeNi HEAs
The Al content has a significant impact on the mechanical properties of the Al x CoCrFeNi HEA system. With the Al addition, the as-cast Al x CoCrFeNi alloys become much stronger, arising from the formation of bcc structures. The transition from fcc to bcc led to the hardness values varying from ∼120 to ∼570 Hv (Wang et al., 2012). The as-forged Al x CoCrFeNi alloys exhibited excellent mechanical properties at both room and cryogenic temperatures. The yield strength and tensile strength of the Al 0.1 CoCrFeNi alloy reached 412 and 1,042 MPa, while the yield strength and tensile strength of the Al 0.3 CoCrFeNi alloy were 515 and 1,010 MPa at 77 K, respectively. Different from conventional alloys, the plasticity of HEAs increased significantly with the decrease of temperature .
Optimizing the thermal-mechanical processing of the low-Al Al x CoCrFeNi alloys is a promising method of enhancing mechanical properties. Recent studies have indicated that the combined strength-ductility results from the complex interplay between multiple strengthening mechanisms, which are created by cold rolling and subsequent annealing (Gwalani et al., 2020). A strong Hall-Petch hardening effect with refining grains could be obtained in single-phase Al 0.3 CoCrFeNi alloy (Gwalani et al., 2017). Aging the alloy at 550-900°C led to the precipitation of nano-scaled B2 and/or σ intermetallic precipitates. Complex reinforcement effects resulting from fine fcc grains, intermetallic B2, and/or σ grains resulted in the unusually high strength of Al 0.3 CoCrFeNi HEA. Furthermore, the atomic distribution of Al-rich nano-clusters within the fcc phase brought about additional strength in the CR-550 condition (Gwalani et al., 2020). The significant changes in the phase structure and mechanical properties induced by the Al addition make this alloy system a desirable candidate material for structural materials with tunable strength.

Thermodynamic Ways of Annealing
The fcc-type CoCrFeNi HEA was generally considered to be thermally stable in the as-cast condition. However, after annealing at 750°C for 800 h, composition decomposition occurred and the minor addition of Al could further shorten the duration (He et al., 2017). The annealing temperature is one of the key factors that determine phase compositions. Prolonged annealing could also promote the second phase to precipitate from the matrix or along the grain boundary. Therefore, the investigation for the phase equilibria and phase stabilities of a series of Al x CoCrFeNi HEAs is significant, since they could influence the material design for applications at an intermediate temperature and expected component lifetime.
The room and high temperature XRD (from 573 to 1,373 K) patterns of the Al x CoCrFeNi (x 0-1.8) alloys demonstrated that crystal structure remained stable, except in Al 0.9 -Al 1.2 alloys . The (Ni, Al)-rich acicular precipitates formed within the dendritic region of Al 0.5 , while Al 0.3 remained a simple fcc phase with uniform composition after aged at 1,173 and 1,373 K for 2 h. A spinodal formed two-phase microstructure, consisting of disordered (Fe, Cr)-rich phase and ordered (Ni, Al)-rich phase, was observed in both as-cast Al 0.9 and Al 1.5 alloys. For the Al 0.9 -Al 1.2 alloys, in line with the DSC analysis, the phase transition from bcc to fcc took place at 873 K and the appearance of σ phase was also observed at the temperature range of 873-1173 K, which depended on Al content. Butler and Weaver (2017) investigated the phase equilibria and phase stabilities of Al x CoCrFeNi HEAs through methodically coupling the experimental results with predictions based on CALPHAD models. The coarsening of microstructures that consist of fcc, bcc, and/or ordered B2 phases occurred in all the alloys after annealing at 1,050°C. Annealing at 700°C conduced to stabilizing the mixed structure of fcc, bcc, B2, and/or σ phases depending on the relative concentration of Al. EDS analysis showed the Al depletion and Cr enrichment in fcc and bcc phases, respectively, while the B2 phase was enriched in Al and Ni, and the σ phase was enriched in Fe and Cr, respectively. The phase prediction by modeling coincided with the experimental results in Al 10 Co 22.5 Cr 22.5 Fe 22.5 Ni 22.5  The discrepancies were mainly related to destabilized components of the fcc phase at 700and 1,000°C. This indicated that CALPHAD based models were useful for predicting the phases formed in HEAs with complex components, but that the current thermodynamic database needs to be improved. Nartu et al. (2020) investigated the effect of heat treatment on the microstructural evolution and mechanical properties of laser additively manufactured Al 0.3 CrFeCoNi HEA. The formation of the nano-scaled (Al, Ni)-rich clusters resulted from in-situ heat treatment during the part building and could act as the heterogeneous nucleation sites for the precipitates that form at high temperature aging. Nanometer-sized L1 2 or c′ type precipitation with spherical or rod-like morphology was predominantly revealed in the grain interior after being aged at 620°C for 50 h. Larger precipitates were also found at the grain boundaries of this condition and confirmed to be Alrich B2 and Cr-rich σ phases. Niu et al. (2017a) investigated the influence of heat treatment on the microstructure of a fcc + bcc dual-phase Al 0.5 CoCrFeNi alloy. The alloy changed from dendritic into spherocrystal after being heat-treated at 650°C while rod-shaped phases and elliptoid phases rich in Al-Ni were precipitated after heat treatment at 1,150°C. The Al-Ni rich nano-sized B2 phase was also found to precipitate from the interdendritic (bcc) region after heat treatment at 650°C . Grain coarsening occurred after heat treatment at 850°C, and the fine bcc phase emerged and distributed uniformly in the matrix, as seen in Figure 1 ( Wang et al., 2017a). Another study reported the precipitation of a plate-like hexagonal closed-packed phase with high cobalt content located at the interdendritic region of the Al 0.5 CoCrFeNi HEA after 650°C/8 h heat-treatment . Though a simple shear, the bcc phase could transform into the hcp phase, and the crystal orientation relationship between these two phases has also been to been established as [11][12][13][14][15][16][17][18][19][20] hcp || [111] bcc (000-1) hcp || (10-1) bcc . Liu et al. (2014) investigated the phase completion and stability of the AlCoCrFeNi alloy. They observed the phase transition from bcc and B2 to bcc and fcc after heat treatment at 950°C, indicating that the solid-solution phases of the as-cast alloy were unstable due to quenched-in chemical segregation. These studies suggest that the phase stability of Al x CoCrFeNi HEA should be carefully evaluated, especially at the intermediate temperature.

Kinetic Way of Tailoring Phases
The kinetic way is also important to the mechanism of phase evolution for HEAs. It is a way of establishing the reasonable heat treatment parameters to enhance performance. Wang et al. have inspected the phase transformation kinetics of the fcc Al 0.25 CoCrFeNi  and fcc + bcc Al 0.5 CoCrFeNi  HEAs during an experiment on isochronal heating by thermal dilation. The estimated activation energy and kinetic exponent were almost stable, indicating a slow and steady phase transition in an Al 0.25 CoCrFeNi HEA ( Figure 2A). The phase transition was also a growth-controlled process without nucleation during the main transformation process, according to the decreasing trend from 1.5 to one of the determined kinetic exponent. The ordering transition from fcc to L1 2 occurred during the heat treatment at 650°C.
Compared with Al 0.25 CoCrFeNi HEA, three stages of phase transformation were characterized in Al 0.5 CoCrFeNi during the continuous heating process, and the third one corresponded to the fcc-bcc phase transition. Careful research into the microstructural evolution indicated that the first two stages of phase transformation involved nanoparticle precipitation Niu et al., 2017a), whereas the transition from fcc to bcc took place in the third phase transformation. The three stages of the third phase transition were divided according to the determined Avrami exponent n ( Figure 2B). Note that the third stage was divided into two parts, Ⅲ 1 and Ⅲ 2 . The decrease of n indicated a reduced nucleation rate in Ⅲ 1 . In Ⅲ 2 FIGURE 2 | (A) The local Avrami exponent (blue circle) as a function of phase transformed fraction of Al 0.25 CoCrFeNi HEA (4 K/min). The green zone represents the range of n from 1 to 1.5 . (B) Variation in n with the increasing volume fraction of Al 0.5 CoCrFeNi HEA (3 K/min). .
Frontiers in Materials | www.frontiersin.org January 2021 | Volume 7 | Article 585602 the formation of bcc structure led to a decrease of the concentration of bcc forming elements at the interface, thus further lowering the nucleation rate. The activation energy for Al 0.25 CoCrFeNi (198 ± 1 KJ/mol) and Al 0.5 CoCrFeNi (144-284 KJ/mol) HEAs was quite different, mainly due to the phase transition type differences between the two alloys, for the former it was order-disorder transition while the latter was an fccbcc transition. The kinetics of phase transformation in HEAs requires further exploration in future studies. In particular, it would be helpful to understand more about the phase selection through kinetic phase transformation and how this relates to annealing. Niu et al. (2016) reported that annealing at 650°C for 0.5 to 8 h of Al 0.5 CoCrFeNi HEA had a significant hardening effect, exhibiting the yield strength (YS) varying between 355 and 834 MPa strongly depending on the annealing time. The excellent tensile properties of alloy heat-treated for 8 h showed a YS of ∼834 MPa with an elongation of 25%, which could be ascribed to the dispersed nano-sized B2 phase (∼70 nm). After heat treatment at 850°C, the nano-sized B2 phase with a structure of thick slat and bcc phase with a structure of thin slat was formed in the fcc matrix. After heat treatment, a good combination of strength and plasticity was obtained by increasing the ultimate tensile strength up to 1,143 MPa and an elongation of 21.5% . In Al 0.3 CoCrFeNi HEA single crystals, only coherent particles of the c′-phase were detected during aging 50 h at 893 K. The interaction of slip dislocations with c′-phase particles occurred via a shearing mechanism and the contribution to the hardening from c′-phase particles was 25-30 MPa. The absence of grain boundaries also suppressed the formation of B2phase particles in Al 0.3 CoCrFeNi HEA single crystals (Kireeva et al., 2020). In another example for AlCoCrFeNi HEA, heat treatment at 850°C caused the phase transformation of the bcc matrix to the σ phase, drastically increasing the micro-hardness of the interdendritic region. The brittle σ phase then transformed back to the bcc matrix when heat-treated at 975°C, causing the alloy to soften, with lower micro-hardness but larger elongation. Homogenization treatment was conducted at a temperature above 1,200°C, leading to an increase both in compressive strength and compressive contraction (Munitz  Zhao et al. (2020a) reported that with increasing temperature from 950to 1,200°C, the increased bcc phase and disappeared σ phase could improve the compressive properties of AlCoCrFeNi with both the YS and strain increased from ∼1,069 to 1,254 MPa and ∼17-∼21%, respectively. Phase distribution was found to strongly influence the initiation and propagation of the cracks. They observed that cracks could propagate and terminate in both the dendrite (Figures 3B,C,E) and interdendritic region (Figures 3A,D,F) of the two samples. Besides, cracks changed the propagation route and then terminated at grain boundaries in the interdendritic region, which consisted of bcc matrix and B2 precipitates ( Figures 3A,F), rather than stopping at the fcc grain boundaries.

THERMO-MECHANICAL COUPLING EFFECT Microstructure Evolution After Thermo-Mechanical Treatment
Since the effect of heat treatment on mechanical properties was limited, numerous methods including rolling, forging, tension, compression and torsion, were then adopted to strengthen the materials by inducing the plasticity of the HEAs (Yu et al., 2016;Hou et al., 2017a;Guo et al., 2018). Optimized thermal-mechanical processing by deformation and subsequent annealing could lead to the grain refinement and formation of the strengthening phase. Figure 4 exhibits the microstructures of ultrafine grains and/or nanostructured precipitates generated simply through cold deformation and annealing under different conditions (Guo et al., 2018;Gwalani et al., 2018;Shi et al., 2018;Hou et al., 2019;Wu et al., 2019;He et al., 2020). In the meantime, Wang et al. (2018c) investigated the effect of cold rolling on the fcc to bcc transformation kinetics of the Al 0.5 CoCrFeNi HEA and concluded that pre-deformation induced defects could facilitate the nucleation and growth process during the phase transition compared with the as-cast sample.

Effect of Cold Deformation and Annealing on the Microstructure
In general, thermo-mechanical processing is usually used to break and refine the coarse as-cast grains and/or second phases. However, the increase in strength was almost always along accompanied by decreased work hardenability due to the reduced dislocation storage capability of the refined structure. Therefore, a rational design of the thermo-mechanical processing is needed, to improve both strength and ductility. This section describes the effect of cold deformation and subsequent annealing on the microstructure of Al x CoCrFeNi HEAs. It reviews the recrystallization and grain growth of Al x CoCrFeNi alloys, and  2019) have proposed a promising way to enhance the comprehensive mechanical properties of bulk high-entropy alloys by simple cold rolling and subsequent annealing at an intermediate temperature of 873 K. A heterogeneous structure with three types of grains was successfully generated including deformed grains, partially recrystallized grains (volume fraction of ∼37%), and fully recrystallized-equiaxed grains, with a volume fraction of ∼10, 37, and 53%, respectively, as shown in Figure 4A. The phase remained in a single fcc phase after the cold-rolled Al 0.25 CoCrFeNi was annealed at 1,100°C for 10 h (Hou et al., 2017b). The grain size was reduced from 230 to ∼13 μm through multiple cold rolling and annealing processes (He et al., 2020), as shown in Figure 4B. The grain size could also be adjusted in a wide range in a single fcc phase Al 0.3 CoCrFeNi alloy (Gwalani et al., 2017). The fcc-based Al 0.3 CoCrFeNi HEA was dramatically transformed to a duplex fcc-ordered B2 (Choudhuri et al., 2018b) and even a tri-phase consisting of fcc, B2, and σ phases ( Figure 4C) by tuning the cold rolling and annealing at a temperature of 800°C and 620°C, respectively (Gwalani et al., 2018). The strength enhancement due to grain boundary strengthening obeyed the Hall-Petch relationship and the Hall-Petch coefficient in Al 0.25 and Al 0.3 alloy was ∼784 and 824 MPa μm 1/2 , respectively. The high Hall-Petch coefficient observed in the Al x CoCrFeNi (x 0.1-0.3) alloy indicated that dislocation motion is more difficult in Al x CoCrFeNi (x 0.1-0.3) HEA than conventional alloys. Hou et al. (2019) investigated the grain growth behavior of cold-rolled Al 0.45 CoCrFeNi alloy after heat treatment at 700-1,100°C. Shear bands could still be retained after annealing at 800°C due to recrystallization. Ultrafine recrystallized fcc grains with an average grain size of 0.62 μm were generated, as shown in Figure 4D. The grain-growth exponent (n) is estimated to be 3.4, which was attributed to the fact that the grain corner-distributed B2 precipitation effectively hindered grain growth. The microstructure of the cold-rolling ( Figure 4E) and subsequent annealing of Al 0.5 CoCrFeNi HEA has also been investigated (Guo et al., 2018;Yang et al., 2019a). The reported recrystallization temperature (∼0.81 T m ; T m referring to the melting point) in the alloy was attributed to coarse as-cast grains, severe lattice distortion effect, and sluggish diffusion effect. The dispersed bcc particles at grain boundaries could effectively slow the grain growth when annealing at 1,200°C with a holding time extended to 16 h.
After cold rolling, a large number of crystal defects such as dislocations are produced in the material, which store high deformation energy. Through subsequent annealing, the stored energy could be released and the deformed structure recrystallizes to refine the grains. The ideal structures could be obtained via reasonable heat treatment, i.e., ultrafine-grain structure, or heterogeneous structure. Meanwhile, high-entropy alloys exhibited a rather high recrystallization temperature and strong resistance to grain growth during annealing, mainly due to the precipitates in the vicinity of the grain boundaries effectively suppressing the grain coarsening of the fcc matrix.

Effect of Hot Deformation and Annealing on the Microstructure
The investigation of hot deformation and dynamic recrystallization mechanisms in Al x CoCrFeNi alloys is vital, considering the current interest in employing HEAs for critical high temperature applications. Haghdadi et al. (2020b) investigated the hot-worked microstructure of an Al 0.3 CoCrFeNi alloy. Most deformation bands were poorly developed even at high strains due to the intrinsic resistance of the lattice to dislocation movement in HEA. The alloy softened through dynamic recrystallization, mainly occurring at the initial grain boundaries through the grain boundary migration caused by strain then followed by multiple twinning chains and the formation of micro-shear bands. Hot deformation at 1,030°C under the strain rate of 0.1 s −1 was mainly conducted through the matrix phase in Al 0.6 CoCrFeNi and Al 0.9 CoCrFeNi alloys (Haghdadi et al., 2020a). Discontinuous dynamic recrystallization (DDRX) occurred at the interphase boundary mantle regions in the fcc matrix of Al 0.6 CoCrFeNi alloy, along with local subgrain coalescence. With further deformation, the DDRX regions expanded and covered most of the microstructure at a strain of 1.5. While in the Al 0.9 CoCrFeNi alloy, the bcc matrix mainly underwent continuous dynamic recrystallization. No evidence of DDRX in the BCC phase of Al 0.9 CoCrFeNi was observed even at the highest strain level of 1.5. Rao et al. (2017) investigated the formation of the second phase in the forged Al x CoCrFeNi (x 0.3, 0.5, 0.7) alloys via an in-situ TEM heating study. The forged alloys mainly consisted of fcc and B2 phases (Ni, Al)-rich L1 2 nanoprecipitates were observed in the fcc matrix of forged Al 0.5 CoCrFeNi, while the Cr-rich bcc phase existed within the B2 phase. Many twins also formed within the fcc phase in all the as-forged alloys due to the low stacking fault energy (Kumar et al., 2015;Choudhuri et al., 2019), whereas high local deformation was present in the B2 region. Two types of precipitates were observed using an in-situ TEM study, a Cr-rich σ phase, and an Al-rich θ phase (an Al 45 Cr 7 type). They also described the σ phase precipitation in detail using a thermodynamic calculation. One observation was that the σ phase could nucleate at the bcc/B2 interface, where the Cr-rich bcc phase forming in the B2 region would then decompose. The other observation was that the σ phase directly from the fcc matrix. The homogenization effect of 1,250°C heat treatment on the forged Al 0.7 CoCrFeNi HEA was investigated by Shi et al. (2018). A net-like A2 (disordered) phase and nano-sized spherical A2 particles were distributed in the B2 matrix in the as-forged Al 0.7 CoCrFeNi alloy. After being heat-treated at 1,250°C for 1,000 h, the asequilibrated Al 0.7 CoCrFeN showed a morphological transformation of the A2 phase, changing from being net-like nano-particles to cuboidal participates that were randomly distributed in the B2 matrix, as shown in Figure 4F.
Frontiers in Materials | www.frontiersin.org January 2021 | Volume 7 | Article 585602 8 Research on the hot deformation of Al x CoCrFeNi highentropy alloys is still rare. The softening mechanism within the fcc and/or bcc phases could be influenced by the varying of Al concentration. Meanwhile, the heat treatment after hot deformation at homogenization temperature was certified to have a profound influence on simplifying the microstructure and reducing the elemental segregation. Research exploring the hot deformation behavior of high entropy alloy is at a preliminary stage and further studies on the microstructure evolution and property improvement of the alloy after deformation are needed.

Precipitation at Intermediate Temperatures
Recent studies indicate that Al x CoCrFeNi HEAs display a significant age-hardening phenomenon. Precipitation at intermediate temperatures of the famous fcc-based Al 0.3 CoCrFeNi alloy has been reported by many researchers. An aging treatment that was used to fully-recrystallize Al 0.3 CoCrFeNi HEA at 550°C for 150 h and 700°C for 50 h, resulted in the formation of ordered L1 2 Ni 3 (Ti,Al)-type precipitates dispersed in the interior of the grains and B2 precipitates forming along the grain boundaries, respectively (Gwalani et al., 2017). The lath-like B2 precipitates exhibited an orientation relationship with the fcc matrix. The stable nanoscale B2 precipitates were also found along the twin boundary in close proximity to the fcc matrix . A combination of both nano-scaled ordered L1 2 precipitates and grain boundary B2 precipitates were obtained after being aged at 620°C for 50 h (Gwalani et al., 2017).
Concurrent recrystallization and precipitation occurred after being directly aged at 550°C for 24 h and 620°C for 50 h of 90% cold-rolled Al 0.3 CoCrFeNi alloy and microstructures consisting of tri-phase (fcc + B2 + σ) were obtained. A dual-phase (fcc + B2) structure was produced after being aged at 900°C for 0.5 h, and only the fcc phase was detected after being aged at 1,100°C (Gwalani et al., 2020). The grain size had a significant impact on the annealing-induced phase transformation in Al 0.3 CrFeCoNi HEA. In the coarse-grained alloy, the fcc phase transformed to the ordered L1 2 phase and finally to the B2 phase with increasing temperature, while the No L1 2 phase formed during the annealing process of the nanocrystalline alloy . The grain boundaries in the nanocrystalline HEA could provide a channel for the rapid diffusion of elements to form the high-temperature stable B2 phase at a relatively low temperature, thereby inhibiting the formation of the L1 2 ordering.
The fcc phase remained disordered after being heat treated at 1,100°C in cold-rolled Al 0.7 CoCrFeNi HEA, while the B2 to B2+bcc transformation occurred in the B2 lamellae. Subsequent low temperature annealing treatment at 580°C for 24 h introduced nano-scaled order L1 2 precipitates in the fcc lamellae. B2/bcc phases were also found in the fcc + L1 2 region . The formation of similar conjoint B2/bcc phase during the process of annealing was also reported in Al 0.5 CoCrFeNi alloy, as shown in Figure 5  . The compositional differences are visible within the Al-Ni rich B2 regions ( Figure 5A), suggesting the presence of Cr rich nanoprecipitates, resulting from the coherent interfaces between the two phases and driven by the concentration profiles (Rao et al., 2017).
In general, the low-Al CoCrFeNi-based high-entropy alloy shows striking phase stability during annealing at an elevated temperature after cold rolling for a long time. However, intermediate temperature annealing for a long time could lead to phase decomposition and then form a dual-phase or tri-phase structure. During the thermo-mechanical processing of duplex Al x CoCrFeNi alloys, the fcc grains show slow growth during annealing for a long time, which is ascribed to the pinning effect resulting from the precipitated bcc phase in the grain boundaries. Besides, ordered phases are amenable to precipitation in fully recrystallized Al x CoCrFeNi alloys after a prolonged annealing time at an intermediated temperature. Prior deformation and subsequent annealing could substantially alter the morphology and distribution of precipitates. The dislocations introduced by pre-deformation could also act as heterogeneous nucleation sites for the secondary phase.

Mechanical Properties and Deformation Behavior
The tensile yield strength of the fcc-based Al 0.25 CoFeCrNi and Al 0.3 CoFeCrNi HEAs can be easily tuned via different thermalmechanical processes, e.g., cold rolling and subsequent annealing (CRSA), as shown in Figure 6 (Gwalani et al., 2017;Hou et al., 2017a;Gwalani et al., 2020;He et al., 2020). Hou et al. (2017a) carried out cold rolling processing on Al 0.25 CoFeCrNi, resulting in a substantial strengthening of the alloy, with yield strength approaching 1,280 MPa ( Figure 6A); however, this was at the expense of low ductility (ε ∼2.3%). The annealing of the deformed alloy triggered recrystallization of the alloy, introducing an effective Hall-Petch strengthening (He et al., 2020). A further increase in YS could result from the precipitation of L1 2 , B2, and/ or σ nanoparticles. A high Hall-Petch strengthening, caused by direct annealing after deformation, led to a high yield strength (∼1,060 MPa) with acceptable ductility (∼19%) ( Figure 6A). The increase in YS of the fully crystallized Al 0.3 CoFeCrNi alloy after the precipitation of the ordered phases is shown in Figure 6B, comparing the CR-1150, CRSA-550, CRSA-620, and CRSA-700 conditions. The combination of both the L1 2 and B2 type precipitates within the fcc matrix leading to optimal enhancement, with strengthening contributions calculated as ∼126 MPa for the L1 2 precipitates, and ∼56 MPa for the B2 precipitates, respectively (Gwalani et al., 2017). The multiple strengthening effects of grain boundary strengthening, composite reinforcement effect caused by ultrafine B2 and σ grains, and precipitation strengthening by Al-Ni rich nanoclusters give the CR-550 alloy high strength (∼1,862 MPa) (Gwalani et al., 2020).

Grain Boundary Strengthening
With the grain refinement in Al 0.1 CoCrFeNi alloy twinning in recrystallized alloys could be severely inhibited, resulting in a lower value of strain hardening rate compared to as-cast alloy . The average grain size at a different scale (4.7, 7.5, 15.2, 32.5, and 59.5 μm) also affected the deformation behavior of the Al 0.1 CoCrFeNi alloy. The alloy with fine grains revealed a good balance of strength and ductility compared to those with coarse grain size. A mixed deformation mode of dislocation slip and twinning occurred in the alloys, certified by the high density of dislocation entanglements in association with the generation of deformation twin bundles ( Figures  7A-F-F). However, more active twinning behavior was observed in coarse grain alloys with wider twin thickness and smaller twin spacing compared to the fine grain alloys during tensile deformation (Figures 7G-I-I), which promoted the hardening of the alloy and postponed the necking and thus enhanced the ductility (Yang et al., 2019c). Liu et al. (2019a) fabricated an ultra fine grained triplex Al 0.3 CoCrFeNi high-entropy alloy through cold rolling and annealing at 700°C for 36 h. The alloy exhibited an extraordinary yield strength of 900 MPa and a total plastic strain of ∼25%, as shown in Figure 8A. Grain boundary strengthening was calculated to be ∼780 MPa, and the difference between the computed and experimental value of yield strength could be attributed to the fine B2 and σ precipitates that formed at the grain interior and boundaries during annealing treatment. The formation of extensive deformation nano twins during plastic deformation gave rise to the enhanced dislocation storage ability of the fcc phase and thus led to large ductility ( Figure 8B). Even though the Hall-Petch strengthening contribution of Al 0.3 CoCrFeNi HEA increased from ∼160 to ∼850 MPa can be attributed to the extremely fine grains, the extent of Hall-Petch strengthening could nevertheless be reduced after the precipitation of the B2 and σ phases (Gwalani et al., 2020). These results indicate that subtle control of the grain size in high-entropy alloys is a strategy  (Hou et al., 2017a;Gwalani et al., 2017;Gwalani et al., 2020;He et al., 2020).
Frontiers in Materials | www.frontiersin.org January 2021 | Volume 7 | Article 585602  Frontiers in Materials | www.frontiersin.org January 2021 | Volume 7 | Article 585602 11 of strengthening and toughening. However, the interplay of multiple strengthening mechanisms should also be taken into consideration to obtain the best performance.

Precipitation Strengthening
The fcc phase of Al 0.3 CoCrFeNi alloy was transformed into an ordered B2 phase through tuning the cold rolling percentage and subsequently annealing at 800°C. The B2 grains acted as Eshelby type inclusion and could raise local stress levels during the deformation to trigger twinning (Choudhuri et al., 2018b). The yield strength of Al 0.3 CoCrFeNi could be enhanced from ∼260 to 490 MPa by introducing the c′ phase and grain boundary B2 phase. c′ precipitation led to a ∼78% enhancement in YS while B2 precipitation led to a ∼35% enhancement in YS (Gwalani et al., 2017). The boundaries of coherent nano-scaled L1 2 precipitate were inferred as thermal obstacles . Introduction of the L1 2 precipitate effectively strengthened the dual-phase (fcc + B2) Al 0.7 CoCrFeNi alloy and the strengthening contribution was modeled using Orowan bowing and by-pass mechanism . Komarasamy et al. (2019) modified the hot-rolled Al 0.5 CoCrFeNi alloy via high-temperature severe plastic deformation (HTSPD) and subsequent aging. The dissolution of the thermodynamically stable B2 phase during HTSPD then resulted in the precipitation of fine L1 2 and B2 phases as well as a thermodynamically unfavorable Al 3 Ni phase, showing a combination of high ultimate tensile strength (1,400 MPa) and good elongation. Both the HTSPD and aged condition alloys with hierarchical microstructural features exhibited increasing strength and sustained work hardening, which were attributed to the precipitation of the thin B2 phase and increased dislocation storage caused by the Al 3 Ni phase, respectively. It has been reported that the homogeneous distribution of the ordered L1 2 phase occurred at 650°C in fully-recrystallized Al 0.5 CoCrFeNi alloy and that this can strengthen the fcc matrix by inducing substantial low-misfit nanoscale interfaces that resist dislocation motion effectively (Yang et al., 2019b). Heterogeneous plastic strain could accumulate near the fcc-bcc/B2 phase boundaries in Al-rich fcc-bcc Al x CoCrFeNi HEAs. B2/bcc precipitates could prevent the formation of distinct deformation bands and their propagation in the fcc region of heat-treated Al 0.5 CoCrFeNi alloy during room temperature deformation. The work hardening rate of the Al 0.5 CoCrFeNi alloy proved to be high, and a large number of deformation twins were produced in the fcc matrix (Bonisch  . HETEM image of sample compressed at 600°C, (C) a typical deformation twin, and (D) the enlarged image of the white dashed square .  , 2018). A large number of deformation twins generated by plastic deformation in hot-forged Al 0.7 CoCrFeNi alloys could also be used for strengthening and toughing the alloy. However, when the volume fraction of the second phase exceeds a specific value, this could have a very obvious effect on the mechanical properties of the alloy, making it high strength but low plasticity. In summary, it is interesting that the alloying of Al to fcc-based CoCrFeNi alloy could induce not only the transition from fcc to bcc but also fascinating phase transitions, including the ordering transition, bcc-hcp transition, and the formation of B2 and σ phases. Such phase transitions usually contribute to the substantial strengthening of the alloys. Synergy could be achieved in a variety of strengthening-toughening mechanisms through the appropriate thermal-mechanical process, which is expected to obtain the desired mechanical properties.

Deformation Behavior
Most of the fcc-based HEAs are reported to have rather low stacking fault energies (6-30 mJ/m −2 ), and can form extensive twins during deformation at cryogenic temperature but minimal twinning at room temperature (Otto et al., 2013;Zhang et al., 2014;Choudhuri et al., 2019;Chen et al., 2020). For example, the Al 0.1 CoCrFeNi alloy formed only a small fraction of nanoscale (∼2 nm thick) deformation twins under quasistatic tension at room temperature, which provided an additional plastic deformation mode and improved the work hardening of the Al 0.1 CoCrFeNi alloy (Choudhuri et al., 2018a). Deformation twins, as another fundamental plastic deformation mode, are relevant to internal and external factors such as crystal structure, strain rate, and temperature (Wang et al., 2018d). Severe plastic deformation could introduce higher internal stresses, which is expected to promote the emergence of deformation twins (Kumar et al., 2015;Choudhuri et al., 2018b;Chen et al., 2019). Choudhuri et al. (2019) investigated the deformation mechanisms of Al 0.3 CoCrFeNi HEAs. TEM observation of the deformed behavior of the coarse-grained single-phase alloy indicated that the formation of the {111} slip band and generation of the nanotwin were the predominant deformation mechanisms. The presence of the hard B2 and σ intermetallics directly promoted the strain hardening of the fcc matrix phase. Within the fcc-B2 clusters, thicker deformation twins and dislocation plasticity were the main deformation mechanisms, while within the fcc-B2-σ clusters, dislocation plasticity, nano-twinning, plus limited crack formation in and around the σ grains were the primary deformation mechanisms. Zhang et al. (2016) explored the hot deformation behavior of Al 0.5 CrFeCoNi HEA in both the as-cast and homogenized conditions, where the flow stress heightened with increasing strain rate and lower temperature. Moreover, under the same deformation conditions, the flow stress in the homogenized state was higher than that in the as-cast state. The average activation energy was estimated to be ∼300 KJ/mol. Serrations in flow behavior have also been reported for a few HEAs. Zhang et al. (2018) reported that an abnormal tendency happened in the serration curves during hot compression, indicating a conversion of the deformation mechanism at elevated temperatures. A large number of parallel deformation twins formed in the Al 0.3 CoCrFeNi HEA after being compressed at 600°C, accounting for the continuous work-hardening phenomenon that occurred below 700°C ( Figures 9A,C,D). As the temperature rose to 800°C, the escalation of dislocation mobility and SFE led to the transformation of the deformation mechanism from {111} <11-2> twinning to slip ( Figure 9B). The transformation of the serration type was also reported in Al 0.5 CrFeCoNi HEA . With increasing temperature (200-500°C) and decreasing strain rate (10 −3 -10 -4 s −1 ), the serration type changed from type A to type A+ B, and finally to type B+ C. The allow became brittle, which was attributed to dynamic strain aging since solute atoms could block the dislocation movement by interacting with dislocations.

CONCLUDING REMARKS
This article reviewed the effect of thermal-mechanical processes on Al x CoCrFeNi high-entropy alloys, emphasizing the correlation of the microstructure and how this corresponds to mechanical performance. There is tremendous scope to further explore HEAs with hierarchical microstructure to achieve enhanced mechanical properties. By controlling the hierarchical grain structure or precipitation of various intermetallic compounds, the complex interplay of various strengthening mechanisms within the HEA results in a large range of strength-ductility combinations. However, such cumulative strengthening effects should be further systematically quantified. In addition, the temperature has a very significant impact on the stacking fault energy. Therefore, an alloy with relatively low stacking fault energy regulates the stacking fault energy, triggering the TWIP and trip effects of alloys, which is also expected to play a guiding role in further optimization of the strength and toughness of highentropy alloy.

AUTHOR CONTRIBUTIONS
HY made a first draft, with substantial input, writing efforts, and discussions from JL, WW, HK, and JW.