Effect of annealing heat treatment on microstructure and mechanical properties of nonequiatomic CoCrFeNiMo medium-entropy alloys prepared by hot isostatic pressing

In this study, nonequiatomic Co28.5Cr21.5Fe20 Ni26Mo4 medium-entropy alloys (MEAs) were prepared using hot isostatic pressing. The effect of annealing heat treatment on microstructure and mechanical properties of MEAs was investigated. The results showed that the microstructure of as-sintered alloys was mainly composed of the face-centered cubic (FCC) phase and μ phase. The presence of the μ phase could improve the compressive strength of Co28.5Cr21.5Fe20Ni26Mo4 MEAs. Meanwhile, the ductile FCC phase matrix could effectively suppress the propagation of cracks to improve its ductility. Hence, as-sintered MEAs possessed excellent compression properties, and the average compressive strength value was 2,606MPa when the strain was 50%. Compared with as-sintered MEAs, the phase composition of as-annealed MEAs did not change. The micro-hardness of annealed MEAs was stable compared to assintered MEAs (342 HV), and its fluctuation was about ±30 HV. The compressive strength of the annealed MEAs did not alter greatly, and the maximum fluctuation value was only about 6.5%. Hence, Co28.5Cr21.5Fe20Ni26Mo4 MEAs had excellent thermal stability.


Introduction
Compared with traditional alloys [1,2], high entropy alloys (HEAs), which is a new alloy system, are easier to form simple solid solution such as FCC and bodycentered cubic (BCC) rather than intermetallic compound phases [3,4]. They possessed strong structural stability, good corrosion resistance, excellent wear resistance, and high strength [5,6]. Among HEAs, CoCrFeNi-based alloys with the FCC structure possessed excellent tensile ductility and fracture toughness at ambient temperature and low temperature, while their strength was poor. However, the optimization of their performance could be achieved by adding other metal elements [7]. For instance, for CoCrFeMnNi HEAs, the addition of the Mn element resulted in that their microstructure consisted of a single phase of the FCC structure. Hence, the HEAs had excellent tensile strength, fracture toughness, as well as good toughness at low temperatures [8,9]. The CoCrFeNiTa x HEAs composed of the γ phase of the FCC structure and C14 type Laves phase of the HCP structure exhibited relatively high strength and high plasticity [10]. The yield strength of CoCrFeNiNb 0. 25 HEAs increased sharply because of the addition of the Nb element, which was caused by the precipitation strengthening of ductile FCC precipitate with nano basket-weave microstructure, while the ductility of the HEAs slightly decreased [11].
Owing to the addition of the Al element, Al 0.3 CoCrFeNi HEAs produced nanocrystalline reaction with reduced contamination levels during mechanically activated annealing [12]. Experimental results about NiCoFeCrGa showed that the cooling rate could lead to a selfsimilarity of the solidified structure formed in the specimen and confirmed the grouping of Co, Fe, and Cr in the FCC phase [13]. Moreover, two elements were added to the CoCrFeNi-based alloys to form some novel HEAs [7], such as AlCoCrFeNbNi [14], Al 0.6 CrFeCoNiSi 0.3 [15], and Al 0.5 CoCrFexNiTi 0.5 [16], whose microstructure and mechanical properties had been extensively studied.
Among CoCrFeNi-based HEAs, the study of equiatomic CoCrFeNiMo HEAs is extremely attractive. The properties of CoCrFeNi alloy could be effectively improved by precipitation strengthening caused by the addition of Mo element with a medium atomic size [17]. In terms of CoCrFeNiMo HEAs, their microstructure is mainly composed of solid solution phases of the FCC structure, σ phase and μ phase, and the regulation of these phases is the key to alter their properties. Wu et al. [18] added Mo element to CoCrFeNi alloy and found that the content of Mo element in CoCrFeNiMo x HEAs could affect the formation of the σ phase and μ phase, which could change the microstructure and properties of HEAs. Due to the addition of the Mo element, CrFeCoNiMo 0.2 HEAs got high activation energy (463 kJ mol −1 ), and their phases could be separated during thermal deformation [19]. In addition, during the process of dynamic recrystallization, Mo-rich phase particles formed in HEAs could restrict the migration on grain boundary by the pinning effect to improve the size of the recrystallized grains. Cai et al. [20] found that the annealed FeCoCrNiMo 0. 23 HEAs could produce Mo-rich intermetallic particles and reduced the stacking fault probability, resulting in the enhancement of their strength and the decrease of their ductility. It was indicated that the microalloying of Mo element could increase the strength of the alloys. The low ductility of FeCoCrNiMo 0. 23 HEAs was mainly attributed to the weak bond between the particles and the matrix interface as well as the reduction of the stacking fault probability, which could inhibit the formation of deformed twins. The microstructure of as-cast CoCrFeNiMo 0.85 HEAs was studied by Shun et al. [21], and their microstructure consisted of the FCC phase, a (Mo, Cr)-rich σ phase, and a minor (Mo, Cr)-rich μ phase. After CoCrFeNiMo 0.85 HEAs were processed by the aging treatment at 700°C, their hardness was optimal, which was caused by the precipitation of acicular σ phase in the FCC phase matrix. Similarly, in the study on CoCrFeNiMo x HEAs [22], it was found that the hardness of the FCC phase and σ phase was enhanced with the increment of Mo element in the alloys, which increased the hardness of HEAs. Meanwhile, with the increasing of Mo element, the compression resistance of HEAs was improved, while their ductility was reduced. Hence, it can be found that the equiatomic CoCrFeNiMo HEAs have high hardness and strength, but their ductility still needs to be further improved.
Recently, the novel nonequiatomic CoCrFeNiMo MEAs, whose microstructures were mainly composed of the FCC phase and μ phase, had been proposed, and they exhibited excellent tensile strength and ductility [23]. The properties of the Co 18 Cr 12.5 Fe 55 Ni 7 Mo 7.5 MEAs could be enhanced by the formation of the μ phase, which could result in the precipitation strengthening, the inhibition of grain growth, as well as recrystallization by the Zener pinning effect. Compared with other preparation techniques [24][25][26], hot isostatic pressing technology possesses the advantages of low forming temperature, compact product, excellent performance, etc. [27], and so it is applied to fabricate the Co 28.5 Cr 21.5 -Fe 20 Ni 26 Mo 4 MEAs in this experiment. At the same time, to explore the thermal stability of MEAs, the effect of the annealing heat treatment with diverse annealing temperatures (500, 550, 600, 800, 850, 900, 1,100, 1,150, and 1,200°C) on the microstructure and properties of the MEAs was investigated [28]. Subsequently, the microstructure and the mechanical properties of Co 28.5 Cr 21.5 -Fe 20 Ni 26 Mo 4 MEAs (as-sintered and as-annealed) were analyzed in detail. Furthermore, the formation mechanism of nonequiatomic Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs prepared by hot isostatic pressing and their thermal stability were discussed.
2 Experimental procedure

Materials preparation
In this study, the alloy ingot with the nominal composition of Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 was fabricated by the hot isostatic pressing technology (HIP) under an Ar atmosphere using the pre-alloying powder that was prepared by atomization. The original powder was supplied by SANDVIK Coromant. High-purity Fe, Co, Cr, Ni, and Mo (20:28.5:21.5:26:4) powders were melted in an inductionheated vacuum furnace to ensure the uniformity of powders, and the melt was dropped using a ceramic tube. The liquid droplets flew in the atomization chamber under high purity Ar (4 MPa), and then solidified to powders with the decrease of temperature. The Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs with high density and low porosity were successfully fabricated. First, the temperature was increased from 20 to 650°C at a rate of 15°C min −1 , and the pressure was increased to 20 MPa. Second, the temperature was increased to 11,90°C at a rate of 5°C min −1 , and the pressure was increased to 150 MPa. Third, the pressure was maintained at 150 MPa, and the temperature was maintained at 1,190°C with 2.5 h. Finally, the temperature was decreased to ambient temperature at a rate of 10°C min −1 , and the pressure was decreased to 40 MPa. After the pressure was released, the specimens were taken out of the chamber. The process curve is shown in Figure 1.

Heat treatment
The Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs prepared by hot isostatic pressing sintering were cut using the DK7740 CNC wire-cut electrical discharge machining (EDM) machine, and the dimension of the specimens was 15 mm × 15 mm × 5 mm. The surface of specimens was cleaned with acetone as well as alcohol and dried in air. Subsequently, the specimens were placed in an HVS-25 type vacuum sintering furnace that possesses the degree of vacuum of 6.6 × 10 −3 Pa to conduct the annealing heat treatment and prevent oxidation of the specimens. In this experiment, to study the influence of annealing temperature on the microstructure of the MEAs, the specimens were treated with different annealing temperatures under the identical holding time of 4 h. The annealing parameters are summarized in Table 1. First, the temperature was increased to 100°C at a heating rate of 5°C min −1 . Second, the temperature was increased to the required holding temperature at a heating rate of 10°C min −1 . Then, a holding time of 240 min was applied. Finally, the temperature was decreased to 800°C at a cooling rate of 10°C min −1 , and then, the specimens were taken out after the temperature naturally decreased to room temperature.

Materials characterization
Scanning electron microscopy (SEM; 10 kV, JEOL, JSM7800f) was used to observe and analyze the microstructure of the raw powder and Co 28 Figure 4. The phase composition of alloys mainly comprised the FCC phase and topologically close-packed μ phase (R3m). According to the JCPDS card number 29-0489, the structure of the μ phase is similar to rhombohedral Co 7 Mo 6 phase (a = 4.762 Å, b = 4.762 Å, c = 25.617 Å) [22]. Among as-sintered and as-annealed alloys, the dominant phase is the FCC phase. Due to the existence of the high entropy effect, the Gibbs free energy in the alloys can be reduced to promote the formation of FCC solid solution during solidification. The mixing entropy plays an important role in the process of preparing multicomponent alloys under high-temperature conditions. When the mixing entropy increases, the formation of solid solution phase can be promoted and the formation of intermetallic phases can be inhibited [5]. The calculation of the mixing entropy will be carried out in Section 4.
In the past studies, most of the binary components crystallize in the alloys with the FCC structure to form the FCC phase, such as CoNi, CoFe, and FeNi. Moreover, the stability of the FCC phase in the alloys can be enhanced by the complete dissolution of CoNi and FeNi [8,[31][32][33]. The phase transition of HEAs is not primarily influenced by individual elements, while it is largely controlled by the previous evolutionary binary components. It was found that CoCrFeNiMo HEAs contained binary components with the FCC structure, such as CoNi, NiCr, NiFe, and NiMo [34]. Therefore, the formation of the FCC phase in Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs can be stabilized by the mutual dissolution of similar binary  components. Liu et al. [17] found that the strength of CoCrFeNiMo HEAs could be significantly enhanced by the incoherent intermetallic phase (μ phase), and severe embrittlement of alloys did not occur. According to the previous studies, it was found that the μ phase was a (Mo, Cr)-rich phase in which the Mo content was greater than the Cr content [18]. Similarly, both Wang et al. [19] and Cai et al. [20] also found a Mo-rich phase in the Fe-Co-Ni-Cr-Mo alloys. According to the XRD patterns, compared with the as-sintered alloys, the position of the diffraction peak of each phase almost does not alter with the increase of the annealing temperature, indicating that the phase composition of the alloys does not change. Hence, the transition of the FCC phase to the BCC phase does not occur during quasi-equilibrium solidification. In addition, the FCC phase can be detected in the alloys annealed at high temperatures, indicating that the FCC phase can stably exist in the alloys over the range of high temperature [35]. Therefore, the annealing heat treatment does not promote the occurrence of phase transition in Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs, and similar phenomena had been found in other results [23,36]. In addition, a small amount of Cr oxides (Cr 3 O 4 ) are discovered, which may be produced during hightemperature sintering. A small amount of Mo solid solution is found, which is mainly attributed to the low diffusivity of Mo. The lattice parameters of the FCC matrix for assintered and as-annealed Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs are calculated by using Maud software, and the results are shown in Figure 5. The lattice parameter for assintered MEAs is 3.5974 Å, and the lattice parameter for L-50 to M-85 alloy is found to increase gradually from 3.5907 to 3.599 Å. However, the lattice parameter for M-90 alloy significantly decreases to 3.5888 Å, and then, the lattice parameter for H-110 alloy increases to 3.6003 Å. In terms of H-115 alloy and H-120 alloy, the lattice parameter is similar to H-110 alloy. To further study the change of the lattice parameter, the rule of mixtures (Equation (1)) is used to calculate the theoretical lattice parameter of the FCC matrix for MEAs, and the results are also shown in Figure 5 [37]. Compared with the lattice parameter calculated by applying Maud software, the theoretical lattice parameter of the FCC matrix for all MEAs is relatively low. Because Equation (1) is just used to obtain a rough theoretical lattice parameter, the calculation of the theoretical lattice parameter of the FCC phase can be better finished by the usage of first principle [38]. Similarly, the theoretical lattice parameter of the FCC matrix for M-85 alloy is the highest (2.9693 Å), and the theoretical lattice parameter for M-90 alloy also significantly decreases to 2.9490 Å.
where c i is the atomic fraction of the ith element and a i is the lattice parameter of the ith element.
The change of the lattice parameter is caused by the change of the ratio among the solid solution atoms in the FCC phase during the heat treatment. Since the atomic radius of the Mo element is larger than the atomic radius of the other elements, the movement of Mo atoms is more likely to cause lattice expansion during heat treatment [22]. Furthermore, the diminishing of the lattice parameter for as-annealed alloys is caused by desolution and renovation in the solid solution phase during the sintering. The precipitation of the metal component is too violent, so that the deformation energy is released, resulting in a decrease in lattice parameters [ MEAs is observed by the SEM-BSE analysis, and the results are shown in Figure 6. It is found that the distribution of the μ phase grains is uneven, and the grain size of the μ phase is greatly different from each other. Certain regions are composed of small and dense grains (red dotted circle), and most areas mainly consist of large and sparse grains. The chemical composition of the FCC phase and μ phase is analyzed by the SEM-EDS. The results show that the FCC phase (region II) is mainly composed of Fe (21.3 ± 0.6 at%), Co (28.2 ± 0.7 at%), Ni (25.6 ± 0.9 at%), Cr (21.7 ± 0.4 at%), and Mo (3.2 ± 0.3 at%) elements, while the μ phase (region I) mainly consists of Fe (11.0 ± 0.5 at%), Co (20.6 ± 0.6 at%), Ni (15.6 ± 0.8 at%), Cr (20.6 ± 0.5 at%), and Mo (32.2 ± 0.4 at%) elements. At high temperature, an element possessing high melting point has high diffusion activation energy and low diffusion rate. Hence, the diffusion ability of the Mo element with a high melting point is poor, which results in the occurrence of a serious segregation phenomenon of Mo element as well as the generation of a large number of small μ phase grains [40]. Similarly, the aggregated nanoparticle phase was found in the AlCrFe-NiTi 0.5 HEAs, and the nanoparticle phase disappeared with the increase in the annealing temperature [41]. As shown in Figures 8 and 9, this aggregation phenomenon is not found in the alloys at diverse annealing temperatures. Therefore, the aggregated μ phase grains appearing in the as-sintered alloy can be eliminated by the annealing heat treatment.
The irregular black-gray region is found in Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs, as shown in Figure 6c. This region mainly consists of dark-gray phase and lightgray phase. The SEM-EDS tests are performed on these phases, and the results are shown in Figure 7. It is found that the irregular black-gray region is composed of O, Cr, and Mo elements. O and Cr elements mainly concentrate in the dark-gray phase, and the Mo element is mainly distributed in the light-gray phase. Combined with the previous XRD analysis, the black-gray phase is Cr oxide (Cr 3 O 4 ). Karlsdottir et al. [34] also found Cr oxides in CoCrFeNiMo HEAs. Hence, the irregular region is mainly composed of Cr oxides and a small amount of Mo solid solution. The Cr 3 O 4 may be generated by the existence of a little oxygen during sintering. The existence of Cr 3 O 4 can effectively prevent the occurrence of serious wear when the alloy is applied in industry and can lubricate the friction process between the alloy surface and the friction material [42].    grain distribution of the as-annealed alloys is uniform, and there is no obvious aggregation region of fine μphase grains, indicating that the low-temperature annealing can regulate the size and the uniformity of the grains. In addition, as for the alloys annealed at these temperatures, the size and uniformity of the μ phase grains in these alloys is almost consistent with each other, indicating that annealing at low temperature has similar effects on grains. As shown in Figure 8d-f, the irregular black-gray regions are still found in these as-annealed alloys, and these regions display a streamlined shape at 550 and 600°C. Combined with the previous results of the XRD analysis, the irregular region is still mainly comprised Cr 3 O 4 . However, compared with the irregular black-gray regions in as-sintered alloys, the Mo solid solution virtually disappears, indicating that annealing heat treatment is beneficial for the diffusion of the Mo element into the matrix. The SEM-EDS tests are conducted on as-annealed alloys, and the chemical compositions of the FCC phase and μ phase are presented in Table 2. Meanwhile, the change situation of Fe, Co, Ni, Cr, and Mo elements is shown in Figure 9. Compared with as-sintered alloys, the contents of Fe, Co, Ni, Cr, and Mo elements in the FCC phase and μ phase do not change much, indicating that annealing at low temperature has no obvious influence on the chemical composition of microstructure inCo 28 Figure 10a-c. Compared with as-sintered alloys, as the temperature increases from 800 to 900°C, the grains of as-annealed alloys gradually become denser and more uniform. The alloys annealed at 800, 850, and 900°C are subjected to the SEM-EDS analysis. The elemental compositions of the FCC phase and μ phase are presented in Table 2, and the change situation of Fe, Co, Ni, Cr, and Mo elements is shown in Figure 9. Compared with as-sintered alloys, the content of each element in the FCC phase does not change much. The Ni element has a slight increase at 850°C (∼1.5 at%), and the Co element has a slight decrease at 850°C (∼1.7 at%). Similarly, compared with as-sintered alloys, the content of each element in the μ phase still does not alter much. The Mo element has a slight decline at 800°C (∼3.6 at%), the Fe element has a slight decline at 850°C (∼1.5 at%), and the Ni element has a slight increase at 800°C (∼2.2 at%). In addition, there is no significant change in Cr and Co elements.
To evaluate the thermal stability of as-sintered Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs at high temperature, as-sintered alloys are annealed at 1,100, 1,150, and 1,200°C, respectively. As shown in Figure 10d-f, with the increase of annealing temperature, the grains in asannealed alloys gradually become sparse, indicating that annealing at a high temperature can contribute to the formation of the FCC phase. In terms of the alloys annealed at high temperature, the size and distribution of the μ phase grains change significantly compared to as-sintered alloys. Furthermore, it can be found that there are no aggregation regions of fine μ phase grains. Then, alloys annealed at 1,100, 1,150, and 1,200°C are subjected to the SEM-EDS analysis. The results are presented in Table 2, and change situation of elements is shown in Figure 9. For the alloys annealed at high temperature, compared with as-sintered alloys, there is no significant change in the content of various elements in the FCC phase and μ phase. Hence, the MEAs have excellent thermal stability at high temperature. The elemental distribution of Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs annealed at 1,150°C is studied by the map scanning analysis of EDS, and the results are shown in Figure 11. It is found that the C element mainly distributes in black particles, which is mainly caused by the improper processing during polishing. Mo element is concentrated in the μ phase, while Fe, Co, Ni, and Cr elements are mainly distributed in the FCC phase. In addition, some Co, Ni, and Cr elements are detected in the μ phase, and a small amount of the Fe element can also be detected in the μ phase. Hence, the μ phase is the Mo-rich phase composed of Mo, Cr, Ni, Co, and Fe elements, and the FCC phase is the solid solution phase mainly composed of Fe, Cr, Ni, and Co elements.
As for as-sintered Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs, both the FCC phase and μ phase are saturated in alloys. After  undergoing annealing heat treatment, the change in the size of μ phase grains is mainly related to the solid solubility of elements at different temperatures. Hence, when as-sintered Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs are annealed, the solute will be redistributed, which results in alteration of the content of the μ phase and FCC phase [43]. In this work, it is necessary to study the ratio change of the μ phase and FCC phase in as-sintered and as-annealed alloys. By using the image processing software Image J [44], the contents of the μ phase and FCC phase in assintered and as-annealed alloys are analyzed, and the results are presented in Table 3. It can be found that annealing at low temperature (500, 550 and 600°C) has the similar influence on the content of the μ phase, and the amount of the FCC phase is decreased by 4-5% compared to that of as-sintered alloys. However, compared with assintered alloys, when the annealing temperature increases from 800 to 900°C, the content of the FCC phase gradually decreases, and the content of the FCC phase reaches to the minimum value at 900°C (68.38%). This phenomenon indicates that the formation of the μ phase can be promoted at 900°C, and Bae et al. [23] have the similar results in their experiments. In addition, the content of the FCC phase increases with the increase of temperature, and then, its content reaches to the maximum value at 1,200°C (82.42%). Therefore, as presented in Table 3, the fluctuation of the content of the FCC phase and μ phase is relatively large at the annealing temperature of 900 and 1,100°C. The ratio of the FCC phase and μ phase is relatively stable at other temperatures. In particular, the content of the FCC phase does not basically alter compared with that of as-sintered alloys when the annealing temperature is 1,200°C.  Figure  12 shows the effect of different annealing temperatures on the hardness of Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs. Compared with the conventional alloys, MEAs can obtain higher hardness at a lower cost [45]. As shown in Figure 12, the hardness value of as-sintered alloys is 342 HV. When the annealing temperature is 500 and 600°C, compared with as-sintered alloys, the value of hardness does not change significantly. However, when the annealing temperature is 550°C, the value of hardness increases to 355 HV, which is attributed to the enhancement of the proportion of the μ phase at this temperature ( Table 3). The presence of the μ phase can boost the hardness of the material, and its hardness can reach 15 GPa [17]. The value of hardness gradually increases with the increment of annealing temperature from 800 to 900°C, and the value of hardness reaches the highest at 900°C (370 HV) because of the gradual increase of the μ phase. In the study on Fe-Co-Cr-Ni-Mo x multielement alloy claddings, Chen et al. [46] found that the Mo element in the μ phase could enhance the hardness of the alloys by precipitation strengthening and solid solution strengthening. As the annealing temperature gradually increases from 1,000 to 1,200°C, the value of hardness gradually decreases and the minimum value of 312 HV is obtained at 1,200°C, which is caused by the gradual diminishing of the μ phase (Table 3). However, as for alloys annealed at 1,100°C (as shown in Table 3), although the amount of the μ phase is 23.89%, the value of hardness does not significantly change compared to as-sintered alloys (μ phase: 18.44%). Similarly, compared with as-sintered alloys, although the amounts of the μ phase in the alloys annealed at 1,150 and 1,200°C are 19.28% and 17.58%, respectively, the hardness of these alloys is lower. This abnormal phenomenon is mainly caused by the growth of grains at high temperature. Therefore, although the content of the μ phase can affect the hardness of alloys, the hardness of asannealed alloys does not change much, indicating that the annealing heat treatment has no significant influence on the hardness of Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs.

Analysis of compression properties of Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs
In this study, since the specimens do not show the trend of fracture during the trial compression tests, the alloys possess excellent compression properties. To avoid damaging the test equipment, compressive strength of 50% strain is selected to conduct comparative analysis [47,48]. Figure 13 shows the compressive strength of asannealed MEAs and as-sintered MEAs. As shown in Figure  13a-c, compared with as-sintered alloys, as-annealed alloys have a small fluctuation about compressive strength. Figure 13d shows that the average compressive strength of as-sintered alloys is about 2,606 MPa. When the annealing temperature is 600°C, the average compressive strength increases to the highest value of 2,640 MPa. When the annealing temperature is 1,150°C, the average compressive strength of as-annealed alloys decreases to the lowest value of 2,436 MPa. However, when the alloys are annealed at 500, 550, 800, 850, 900, 1,100, and 1,200°C, the compressive strength of as-annealed alloys is almost the same as as-sintered alloys. Hence, there is a small difference in the compressive strength between assintered alloys and as-annealed alloys, indicating that Co 28

Discussion
The purpose of this study is to analyze the effect of annealing heat treatment on the microstructure and  8.295, which confirms the presence of the FCC phase. The formula for calculating the VEC is as follows [39,52]: where VEC i is the VEC of each component and c i is the concentration of each component in Co 28.5 Cr 21.5 Fe 20 -Ni 26 Mo 4 MEAs.
In addition, the formation and the stability of the solid solution can be predicted by the enthalpy of mixing, and the previous studies found that the formation of phases and the segregation of the elements could be determined by the enthalpy of mixing between the elements [53,54]. The enthalpy of mixing between the elements is presented in Table 4. Therefore, the formula (3) is used to calculate the enthalpy of mixing.
where n is the number of elements in the alloys, C i and C j are the atomic percentages of the ith and jth elements, respectively, R i is the atomic radius of the ith element, H Δ AB mix is the enthalpy of mixing of the binary liquid alloys, and R is the gas constant, H is the mixed enthalpy, S is the mixed entropy, and T is the absolute temperature. In this study, the enthalpy of mixing of the alloys is −3.9448 kJ mol −1 . Yang and Zhang [56] found that when the ΔH mix < 0, one or more compounds can be stably produced in the alloy. The more negative the enthalpy of mixing is, the greater the binding force between the elements is. Therefore, an intermetallic compound (μ phase) is produced in the Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs. Zhang et al. [57] found that when −15 kJ mol −1 < ΔH mix < 5 kJ mol −1 , the solid solution can be formed in the alloy. When enthalpy of mixing becomes more negative, dual phase is easier to be produced. At the same time, Bae et al. [23] studied nonequiatomic Co 18 Cr 12.5 Fe 55 Ni 7 Mo 7.5 MEAs and Co 17.5 Cr 12.5 Fe 55 Ni 10 Mo 5 MEAs. The entropy of mixing of the alloys was 10.6238 and 10.5904 J mol −1 K −1 , and the enthalpy of mixing of the alloys was −2.282 and −2.335 kJ mol −1 , respectively, as presented in Table 5. Furthermore, the microstructure of these alloys is composed of the FCC phase and μ phase. In contrast, the enthalpy of mixing in this alloy is small. Therefore, in this study, the FCC phase and μ phase are more likely to be produced. It can be seen from the calculation of formula (4) that the entropy of mixing of this MEA is 12.3795 J mol −1 K −1 . Compared with Co 18 Cr 12.5 Fe 55 Ni 7 Mo 7.5 MEAs and Co 17.5 Cr 12.5 Fe 55 Ni 10 Mo 5 MEAs, the entropy of mixing of Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs increases and the high entropy effect is stronger, which can inhibit the formation of brittle intermetallic compounds to promote the production of the FCC phase. The high entropy of mixing can significantly reduce the free energy of the alloy, which can reduce the order degree of the macroscopic solid solution to make the alloy more stable [58]. In addition, it can be known from the formula (5) that since this MEA has the larger entropy of mixing and the smaller enthalpy of mixing, the Gibbs free energy becomes smaller and the alloy is more stable at high temperatures. Therefore, in this work, the FCC phase and μ phase in Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs are stable.
Among the Co 18 Cr 12.5 Fe 55 Ni 7 Mo 7.5 MEAs processed by heat treatment and the Co 17.5 Cr 12.5 Fe 55 Ni 10 Mo 5 MEAs, the maximum ratio of the μ phase in alloys was up to 8.7% [23]. In the present study, the minimum ratio of the μ phase was 17.58% among the as-sintered and asannealed Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs ( Table 3). Yang and Zhang [56] found that combining the ratio (Ω) of T m ΔS mix and ΔH mix as well as the atomic size mismatch effect (δ) between atoms could judge the formation ability of solid solution and intermetallic compounds in alloys [59,60]. The relevant equations can be written as equations (6)- (8).When the value of δ is in the range of 0.8%-6.6% and the value of Ω is in the range of 1.1 to 229.8, the solid solution will be easily formed in alloys. In addition, it is also found that when the value of Ω is larger and the value of δ is smaller, the solid solution in alloys possesses better stability. As the diminishing of the value of Ω, the intermetallic phases and amorphous phases are apt to be produced. According to equations (6)- (8), the value of δ and Ω can be calculated, and results are listed in Table 5. Compared with Co 18    where T m is the melting point temperature of the alloy, (T m ) i is the melting point of the ith component in the alloy, R i is the atomic radius of the ith element, and n is the number of elements in the alloy.
In this study, when the annealing temperature is 900°C, the hardness of the Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs increases to the largest value of 370 HV. When the annealing temperature increases to 1,200°C, the hardness of the alloys decreases to the lowest value of 312 HV. However, compared with the hardness of as-sintered alloys (342 HV), there is no significant difference and the fluctuation value of hardness is just about 30 HV. Therefore, the hardness of Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs is relatively stable. In addition, when the annealing temperature is 1,150°C, the compressive strength of Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs decreases to the lowest value of 2,436 MPa. However, when the annealing temperature is 600°C, the compressive strength increases to the highest value of 2,640 MPa. Hence, compared with as-sintered alloys (2,606 MPa), the compressive strength of as-annealed alloys has no significant fluctuation, which demonstrates that the compressive strength of Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs is relatively stable. Compared with as-sintered alloys, the phase composition of microstructure in as-annealed alloys is still composed of the FCC phase and μ phase. Therefore, the hardness and compressive strength of the as-annealed alloys do not change heavily. Meanwhile, as shown in Figure 9, the alteration of chemical composition of the phase in the asannealed alloys is just small, indicating that Fe, Co, Ni, Cr, and Mo elements possess low diffusivity at low, medium, and high temperatures [61]. Hence, according to the aforementioned analysis, the Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs have excellent thermal stability.
with as-sintered alloys, the compressive strength of asannealed alloys did not change heavily, and the maximum value of fluctuation was just about 6.5%. Therefore, the thermal stability of Co 28.5 Cr 21.5 Fe 20 Ni 26 Mo 4 MEAs was excellent.