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Effect of V and ball milling time on microstructure and thermal properties of CoCrCuFeNiVX by mechanical alloying
Physica B: Condensed Matter 571 (2019) 235–242
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Effect of V and ball milling time on microstructure and thermal properties of CoCrCuFeNiVX by mechanical alloying
T
Changqing Shu, Ke Chen, Hemei Yang*, Mingqiang Chen, Xiya He College of Engineering, Nanjing Agricultural University, 210031, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High-entropy alloy Mechanical alloying Microstructure Thermal properties
CoCrCuFeNiVx high-entropy alloy was prepared by mechanical alloying method. X-Ray Diffractometer (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive Spectroscopy (EDS) and Differential Scanning Calorimetry (DSC) were used for different samples. The crystal structure, microstructure, composition and thermal properties were studied with conclusions below. After ball milling time reaches 21 h, CoCrCuFeNiV0.6 consists of two phases (FCC1+FCC2). After adding V, the element distribution is more uniform and the complete melting temperature of the alloy decreases with increasing V content. Also, after CoCrCuFeNiV0.6 ball milling for 21 and 27 h, the alloy material powder showed the phenomenon of amorphous transition. Mechanical alloying degree of CoCrCuFeNiV0.8 is the highest when the ball milling time is 15 h. The mechanical alloying degree tends to be fixed after 21 h in the case of CoCrCuFeNiV0.6.
1. Introduction The concept of high-entropy alloy [1,2] broke the traditional alloy design concept [3,4]. The intense lattice distortion phenomenon [5] and the interaction between the elements affect the elemental bonding and diffusion during the phase transition, making it easier for the alloy to form nanocrystalline or amorphous structures at room temperature. Therefore, high-entropy alloys have excellent properties: high fracture toughness [6], high hardness [7,8], Excellent plasticity [9–11], high strength [12–14], Fatigue resistance [15], high temperature softening resistance [16], and high temperature oxidation resistance [17]. In some special occasions, it also has special advantages over traditional materials. Kumar [18] studied the irradiation resistance and found that significant suppression of FeNiMnCr in radiation-induced segregation compared with conventional irradiated austenite Fe–Cr–Ni alloys. Chou [19] found that Co1.5CrFeNi1.5Ti0.5Mox was not easy to be pitted in NaCl solution, and the alloy had excellent corrosion resistance in H2SO4 and NaOH aqueous solution when Mo was not added. However, the commonly used method, arc melting method, needs high heating temperature and quite difficult process, which imposes restrictions on the development of high-entropy alloys. The mechanical alloying method has been widely used in the field of high-entropy alloy because it can avoid the segregation or eutectic influence during the transition from molten liquid to solid, and obtain alloy powders with
stable microstructure, good room temperature processability and good chemical homogeneity [20–24]. CoCrCuFeNi is the most representative simple FCC structure highentropy alloy in the early stage. Due to its excellent performance, researchers have paid attention to it. Recently, Mao [25] team used the improved plasma arc discharge method to prepare CoCrCuFeNi spherical nanometer with average grain size of 80–120 nm particle. Although the Salishchev [26] team is based on CoCrFeNi, it has been found that V is less compatible with other elements which lead to significant expansion of FCC solid solution. The severe lattice distortion of high-entropy alloys makes the ratio limited to neither equimolar ratio nor near-molar ratio. Usually, elements with large difference in atomic radius are added to form a hard phase, which will improve the structure and mechanical properties. V has a large atomic radius and enhances solid solution. The Tabachnikova [27] team has a lower V content ratio in the CoCrFeNiMnVx system and high cryogenic strength and high plasticity are found in CoCrFeNiMnVx. The amorphous state of some alloys gives it better physical and chemical properties than the crystalline state, making the research of amorphous materials widely concerned. And thermodynamic theory analyzes the stability of the state without considering time and process. In materials research, the process of transforming a material from one state to another over time is called dynamics theory. The various methods of developing high-entropy alloys need to undergo this state of
*
Corresponding author. E-mail addresses: [email protected] (C. Shu), [email protected] (K. Chen), [email protected] (H. Yang), [email protected] (M. Chen), [email protected] (X. He). https://doi.org/10.1016/j.physb.2019.07.028 Received 7 May 2019; Received in revised form 14 July 2019; Accepted 15 July 2019 Available online 16 July 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
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change, so it is crucial to constantly improve the dynamics theory. The research in this paper is inspired by the fact which Wang [28] proved that CoCrCuFeNi has a slow diffusion property. CoCrCuFeNiVx highentropy alloy was prepared by mechanical alloying method, using diffusion theory to explain the state transition process of materials with time, providing an example for the development of high-entropy alloy dynamics theory. Through the detection of the microstructure of the powder, the good process parameters are summarized, which provides reference for powder post-treatment and application in various fields. Finally, thermal analysis was carried out to study the behavior of CoCrCuFeNiVX transition to amorphous. For the production of some complex parts, using high quality alloy powder can simplify the production process and reduce the cost. Therefore, it is particularly important to study the properties and process of CoCrCuFeNiVx high-entropy alloy powder to obtain high-quality powder, so as to further expand the application of high-entropy alloy.
2. Material and methods Alloy ingots with nominal compositions of CoCrCuFeNiVx(x: molar ratio, x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) were processed by mechanical alloying the mixture of pure powder Co(99.99% wt.%), Cr(99.99% wt.%), Cu(99.99% wt.%), Fe(99.99% wt.%), Ni(99.99% wt.%) and V(99.99% wt.%) under high-purity argon(99.9% wt.%) atmosphere. Setting the process parameters that ball-to-powder was 10:1 and rotational speed was 350r/min, rotating mode with ‘stop for 10 min every 30 min, change steering once at the same time’, six samples were prepared by alloying in a variable frequency planetary ball milling(XQM-4L type) for 15 h. Then, compositions of CoCrCuFeNiV0.6, the remaining parameters, were performed by the same step for y hours(y = 9, 18, 21, 27) to obtain four samples which are presented in Table 1 with the process illustrated in Fig. 1. The phase constitutional analysis of the alloys was carried out by using a Panalytical X'Pert powder type XRD analyzer with the Cu-Ka radiation stepped at 0.02°. The energy of the Cu X-ray was 40 kV,40 mA. The step frequency was 1s, and the scanning range was 2θ = 10°–90°. The microstructure of the alloy powder was followed up by a Hitachi S-4800 Scanning Electron Microscope (SEM), and the phase composition of the alloy was analyzed by X-ray EnergyDispersive Spectroscopy (EDS). Each group of 13–18 mg high-entropy alloy (HEA) powder was enclosed in a clean crucible and heated under the protection of high-purity argon(99.9% wt.%) by using a NETZSCH STA 449F3 Synchronous Thermal Analyzer to get Differential Scanning Calorimetry (DSC) Curves. It was first heated from 20 °C to 1400 °C at a rate of 20 K/min, and then cooled to room temperature at a rate of 50 K/min.
Fig. 1. Schematic diagram of the research process.
3. Results and discussion 3.1. Phase analysis In the theory of thermodynamics
This formula illustrates the relationship among free energy of the high-entropy alloy system(G), entropy (S), enthalpy (H) and thermodynamic temperature (T). At a constant temperature, if the entropy of the system becomes higher or the system's enthalpy becomes lower, its free energy will be lower, and the system will be more stable at the same time. Table 2 lists enthalpy of different atom-pair, atomic size and melting point. As shown in Table 2, enthalpy of different atom-pair( ΔHijmix ) are relatively low among these four elements(Co, Cr, Fe, Ni). It should be pointed out from formula(3-2) that the mixing enthalpy values ( ΔHmix ) of CoCrFeNi is at a low level. n
ΔHmix =
∑
Ball-topowder ratio
Ball Milling Time(h)
Chemical formula
HEA1 HEA2 HEA3 HEA4 HEA5 HEA6 HEA7 HEA8 HEA9 HEA10
350 350 350 350 350 350 350 350 350 350
10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1
15 15 15 15 15 15 9 18 21 27
CoCrCuFeNi CoCrCuFeNiV0.2 CoCrCuFeNiV0.4 CoCrCuFeNiV0.6 CoCrCuFeNiV0.8 CoCrCuFeNiV1.0 CoCrCuFeNiV0.6 CoCrCuFeNiV0.6 CoCrCuFeNiV0.6 CoCrCuFeNiV0.6
(3–2)
If the total number of atoms in the alloy crystal is N, where n0 atoms are the same type of elements and n1 atoms are the same type of elements, …, nr atoms are of the same type. Then the formula of mixing entropy of alloy( ΔSmix ) is introduced below.
N! i=r
∏i = 0 ni !
(3-3)
“k” of the above formula is Boltzmann constant. 1 Whenn 0 = n1 = ...=nr = r N , the ΔSmix of system reaches the maximum. CoCrFeNi has an increase in entropy( ΔS > 0 ) compared to any case
Table 1 Processing parameters of samples. Rotational speed (r/min)
4ΔHijmix ci cj
i = 1, i ≠ j
ΔSmix = k ln
Sample
(3–1)
ΔG = ΔH − TΔS
Table 2 Enthalpy of different atom-pair(atomic size/melting point).
236
Element(atomic size/melting point)
Co
Cr
Cu
Fe
Ni
V
Co(1.25/1495 °C) Cr(1.27/1857 °C) Cu(1.28/1083 °C) Fe(1.27/1535 °C) Ni(1.24/1453 °C) V(1.35/1890 °C)
_
−4 _
6 12 _
−1 −1 13 _
0 −7 4 −2 _
−14 −2 5 −7 −18 _
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Fig. 2. The relationship between δ and Ω for multi-component alloys [32].
Fig. 3. XRD image of HEA1-6.
Fig. 5. Enlarged XRD image of all samples. Table 3 Chemical composition (in at.%) analysis results of HEA1-6 by EDS. Alloy
Area
Co(at.%)
Cr(at.%)
Cu(at.%)
Fe(at.%)
Ni(at.%)
V(at.%)
HEA1
Nominal Surface Nominal Surface Nominal Surface Nominal Surface Nominal Surface Nominal Surface
20 22.74 19.2 24.56 18.5 20.50 17.9 18.63 17.2 16.36 16.7 18.15
20 24.93 19.2 25.26 18.5 13.46 17.9 16.71 17.2 20.49 16.7 24.05
20 12.45 19.2 7.05 18.5 20.28 17.9 15.22 17.2 15.45 16.6 12.92
20 17.51 19.2 23.70 18.5 15.20 17.8 20.36 17.3 18.04 16.6 18.18
20 22.37 19.2 15.05 18.5 23.44 17.8 17.02 17.3 17.85 16.7 15.41
– – 4 4.39 7.5 7.12 10.7 12.07 13.8 11.82 16.7 11.30
HEA2 HEA3 HEA4 HEA5 HEA6
Fig. 4. XRD image of HEA4(HEA7-10).
237
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(a)
(b)
Fig. 6. (a), (b) SEM photographs of 10000-fold magnification of HEA3, HEA5. Table 4 Chemical composition (in at.%) analysis results of HEA4(HEA7-10) by EDS. Alloy
Area
Co(at.%)
Cr(at.%)
Cu(at.%)
Fe(at.%)
Ni(at.%)
V(at.%)
HEA7
Nominal Surface Nominal Surface Nominal Surface Nominal Surface Nominal Surface
17.9 18.57 17.9 18.63 17.9 17.35 17.9 18.81 17.9 18.40
17.9 17.10 17.9 16.71 17.9 18.61 17.9 17.77 17.9 16.78
17.9 16.56 17.9 15.22 17.9 15.38 17.9 17.67 17.9 16.72
17.8 20.89 17.8 20.36 17.8 20.54 17.8 18.38 17.8 18.99
17.8 16.61 17.8 17.02 17.8 14.52 17.8 16.63 17.8 19.38
10.7 10.28 10.7 12.07 10.7 13.58 10.7 10.74 10.7 9.74
HEA4 HEA8 HEA9 HEA10
form a face-centered cubic lattice structure when the valence electron concentration of the high-entropy alloy system is greater than 8. It must also be mentioned that the valence electron concentration of CoCrCuFeNiVx is greater than 8.17 while BCC phase exists in the spectrum shown in Fig. 3. This phenomenon seems contrary to previous studies, and is probably influenced by various process parameters such as ball milling time. In HEA2-3 and HEA6, the characteristic peak was detected at 2θ = 35.519°, and the NiV and CoV were judged, indicating that complex intermetallic compounds are formed under too low or too high V content. Fig. 4 displays the X-ray diffraction pattern of HEA4(HEA7-10). As the ball milling time increases, the metal powder continues to be cold welded and fractured, and the atoms in the powder particles are further diffused, which promotes the phase composition of the alloy to be simple. Such as HEA7, the metal powder is plastically deformed together, and numerous diffraction peaks exist. According to the diffraction relationship, the main phase is FCC1. Meanwhile, a small part of FCC2 phase, BCC phase and V element are present. The V elemental diffraction peak disappeared at 2θ = 42.065° in HEA4 and HEA8, and it is explained by the increase of ball milling time to promote V element's participation in mechanical alloying. However, the BCC diffraction peak disappeared in HEA9-10, and the alloy appeared to consisting of two face centered cubic phases (FCC1+FCC2), which confirmed the above conjecture. According to the X-ray diffraction pattern, the twophase lattice constants of HEA10 were 3.5330 Å and 3.5725 Å respectively, and the two-phase lattice lattice distortions were 0.379% and 0.432% respectively. The phase evolution of the alloy under different process parameters can be clearly observed in Fig. 5. With the increase of V content, FCC1 peak shifts towards the low 2θ angle and then towards the high 2θ
when the four elements are equal. It is stated that the quaternary system (CoCrFeNi) is very stable under formula(3-1). Another interesting finding from Table 2 is that Cu is mixed with other elements in the system, and has a large positive value, whereas V is the opposite exactly showing a large negative value. Their simultaneous action promotes ΔHmix to remain low, nevertheless, the ΔSmix is further enlarged( ΔS > 0 ) and the CoCrCuFeNiVx system becomes more stable at this moment. Therefore, CoCrCuFeNiVx can be prepared by mechanical alloying. Zhang [29–31] intensively studied the mechanism by taking the influence of mixed enthalpy and atomic radius difference into consideration in high-entropy alloy research, and summarized the schematic diagram of its phase formation, as shown in Fig. 2 [32], which was adopted by most scholars. Through calculation, each group of samples satisfies Ω ≥ 1.1, δ ≤ 6.6%, and can form a solid solution structure. The X-Ray Diffraction patterns of HEA1-6 were displayed in Fig. 3. It can be observed that solid solutions with a simple crystal structure have been formed. The composition of CoCrCuFeNiVx high-entropy phase is characterized by diversification. Initially, HEA1 is composed of FCC1 phase. Moreover, the phase structure of HEA2 was FCC1 phase and intermetallic compound phase. Then, HEA3 is composed of FCC1 phase, FCC2 phase, BCC phase and intermetallic compound phase. And in HEA4-5, the intermetallic compound phase disappears. Finally, HEA6 is composed of FCC1 phase, BCC phase and intermetallic compound phase. According to the correspondence relationship between the diffraction peaks, FCC1 is a Cu–Ni-based solid solution phase and FCC2 is a Co–V-based solid solution phase, while BCC is a Fe–Cu-based solid solution phase. The GUO S [32,33] team's research shows that it is beneficial to 238
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parameters larger and the spacing between the crystal planes larger. In Chen's study [34], it was found that the main peak of alloy shifted to the left in the same principle after adding V. In Li's study [35], Al with large atomic radius was added by AlxFeCrCoNi, and the same phenomenon appeared. On the basis of our findings, the FCC1 main peak shifts to the left can be concluded that the lattice distortion effect is intensified, and the lattice constant of the alloy becomes larger with the increase of V content. When V reaches a certain amount, it constitutes a suitable valence electron concentration, and V local segregation occurs, which causes the more FCC1 phase change in the alloy and the peak position to shift towards the high 2θ angle. The peaks of the two phases are blurred. Consequently, the alloy may have a single FCC structure as the ball milling time is further increased. 3.2. Composition analysis and microscopic morphology The reaction process of mechanical alloying is complex, and this paper can be explained by the mechanism of mechanical alloying reaction of Zhang [36]. They report these findings in the article: Firstly, the contact area of atomic reaction increases and the diffusion distance of atom decreases; Secondly, the diffusion coefficient increases, and crystal defects and diffusion couples are introduced during the reaction; Thirdly, the fresh powder is gradually consumed to form HEA. Adding V with a molar ratio of 0.4–0.8 can promote elemental diffusion and contribute to mechanical alloying which can be seen in Table 3. ‘at.%’ which is used below means atomic percent. Evidences below flesh out the conclusion. The content of almost all elements in HEA1-2 deviates significantly from the nominal composition because of the transition from simple plastic extrusion to more complex atomic diffusion between elements. The deviation of HEA3-5 is reduced, and the elements in HEA5 have the most uniform diffusion, their contents are relatively close to their nominal components. However, the deviation of the composition of HEA6 elements began to be serious. It can be seen from Table 2 that V has a negative mixing with Co, Cr, Fe, Ni. Hence, it is easier to dissolve or form intermetallic compounds between them. Interestingly, the atomic radius of V is large, and the degree of lattice distortion caused by solid solution is severe. What's more, it hinders the diffusion, and finally causes the composition of elements to be deviated to nominal composition on the surface. The powder particles were observed by Scanning Electron Microscope, in order to explain the microcosmic element distribution more fully. Fig. 6 displays the SEM backscattered images of HEA3 and HEA5. The color of the element with light(heavy) atomic mass is dark
Fig. 7. EDS Line analysis diagrams of HEA8 and HEA10.
angle, and FCC2 peak shifts towards the high 2θ angle. With the increase of the milling time, FCC1 peak shifts to the low 2θ angle. Notably, FCC2 peak position tends to be stable after shifting towards the high 2θ angle, and the two-phase peaks are increasingly blurred. It can be seen from Table 2 that the atomic radii of Fe, Cr, Ni, Co and Cu are between 1.24 and 1.28, while the V element is characterized by a larger atomic radius(1.35). The atomic size difference in the system becomes larger, aggravating the lattice distortion of the alloy, making the cell
Fig. 8. The cps of all elements in HEA10. 239
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Fig. 9. HEA1-6 DSC analysis curves in the range of 1050 °C–1400 °C.
Fig. 10. HEA4(HEA7-10) DSC analysis curves in the range of 1050 °C–1400 °C.
element is relatively complete after 21 h of ball milling. Observation was performed using EDS Line analysis to further analyze the alloy structure. As shown in Fig. 7, as the ball milling time is longer, the line analysis curve is more gradual, that is, the element distribution is more uniform. It is found from Fig. 8 that the strength of Cu in the CoCrCuFeNiVX alloy is low, and the reactivity of Cu in the system is relatively poor, which is determined by the positive mixing enthalpy of Cu with other elements.
(bright) in backscattering observation. From the 10000-fold magnified view of the morphology, small white spots appeared in the surface of HEA3 powder particles. And the surface color of HEA5 powder particles (distribution of each element) was uniform, the white spots disappeared. This is in good agreement with the above composition analysis, there is almost no surface component segregation when x reaches 0.8. Mechanical alloying is a complex and versatile physicochemical change process demonstrated in Table 4. The EDS surface analysis results of two particles are very close to the nominal composition in HEA7. As a matter of fact, the alloy is simply plastically pressed together by each metal powder now. HEA4 has a content of Co, Fe, Ni, V which is significantly higher than the nominal composition. For this reason, the elements begin to diffuse but not completely at this time. In HEA8, only the content of Fe, Cr and V is higher than the nominal composition, and the phenomenon of deviation is obviously improved after 18 h of ball milling time. The diffusion coefficient reflects the thermal motion properties of atoms. The higher the melting point of a metal material is, the higher the bond energy of the metal is, and the more difficult it is for atoms to break free from the shackles of surrounding atoms. So the diffusion coefficient is smaller. To tell the truth, self-diffusion coefficients of V(1890 °C), Cr(1857 °C) and Fe(1535 °C) are very low due to higher melting point compared to other elements, and incomplete diffusion resulting from slow alloying rate causes surface element segregation. What's better, the elements of HEA9-10 are very close to the nominal composition. To sum up, the diffusion of each
3.3. Thermal analysis Fig. 9 is illustrated as a differential scanning calorimeter analysis pattern of HEA1-6. As shown in Fig. 9, CoCrCuFeNiVx alloy gets the expected thermal performance, with melting temperature is less than 1390 °C, respectively, which is lower than constituent elements of the alloy other than Cu shown in Table 2. Different from simple combination between melting points, the complete melting temperature of the alloy is affected by lattice distortion. For supplementary explanation, the alloy is not as stable as the former under the effect and less energy is needed to destroy its chemical bonds, so it shows a decrease in melting point. In HEA3-6, a new exothermic peak appears at 1070–1120 °C corresponding to the phase analysis above. The end points of six curves’ melting peak(1380 °C, 1390 °C, 1377 °C, 1365 °C, 1350 °C, 1303 °C) were analyzed by NETZSCH Proteus software, and the complete melting temperature of the alloy decreases with increasing V content. On the 240
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Acknowledgments
other hand, it also reflects the degree of lattice distortion. Fig. 10 shows the differential scanning calorimeter analysis pattern of HEA4 and HEA7-10. As shown in Fig. 10, only HEA9 and HEA10 have obvious crystallization exothermic behavior during 400–800 °C. HEA4 and HEA7-8 all have an exothermic peak at 1070°C–1130 °C which is close to the melting point of Cu (1083 °C), and the peak corresponds to copper-rich phase melting process judging from the XRD phase analysis (three-composite gold exists in the presence of Cu–Febased solid solution phase). It is worth noting that five curves have two exothermic peaks in the range of 1210°C–1390 °C, and the peak area decreases with the increase of the milling time, which corresponds to the melting peak of the FCC phase. On one hand, the metal powder is deformed, cold welded, fractured, and diffused during the mechanical alloying process. A large number of defects such as vacancies and dislocations are introduced, and the energy of the alloy system becomes very high. What's more, an amorphous state in the metastable state will be formed when the sum of the free energy of the crystalline state and the free energy of the crystal defect is greater than the amorphous free energy. On the other hand, the temperature of the alloy powder increased with the increase of ball milling time, and then the atom diffusion velocity increased, which promoted the crystal formation. Finally, the two trends act simultaneously, and the area of each peak (exothermic heat) is proportional to the content of the corresponding phase. In the case of ball milling for 21 h, the alloy system exhibits a large amount of amorphization behavior. Depending on the characteristics of amorphous alloy (uniform structure, corrosion resistance, high strength, high fracture toughness, etc.), the alloy should exhibit excellent comprehensive properties.
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Podolskiy, M.O. Laktionova, et al., Mechanical properties of the CoCrFeNiMnVx high entropy alloys intemperature range 4.2-300 K, J. Alloy. Comp. 698 (2017) 501–509. [28] R. Wang, W.M. Chen, J. Zhong, et al., Experimental and numerical studies on the sluggish diffusion in face centered cubic Co-Cr-Cu-Fe-Ni high-entropy alloys, J. Mater. Sci. Technol. 34 (2018) 1791–1798. [29] Y. Zhang, Y.J. Zhou, J.P. Lin, et al., Solid-solution phase formation rules for multicomponent alloys, Adv. Eng. Mater. 10 (2018) 534–538. [30] Y. Zhang, Y.J. Zhou, et al., Solid solution formation criteria for high entropy alloys, Mater. Sci. 561–565 (2007) 1337–1339. [31] X. Yang, Y. Zhang, et al., Prediction of high-entropy stabilized solid-solution in
4. Conclusion (1) After adding V, the phase composition of CoCrCuFeNi high-entropy alloy tends to be diversified, and the change process is complicated. It consists of a single phase (FCC1), two phases (FCC1+FCC2) or three phases (FCC1+FCC2+BCC). BCC phase changes to FCC2 phase in CoCrCuFeNiV0.6 with the increase of ball milling time, and the alloy consists of two phases (FCC1+FCC2) after 21 h. (2) When x reaches 0.8, there is almost no surface component segregation. Increasing the ball milling time makes the alloy powder more uniform in element distribution. (3) The complete melting temperature of the alloy decreases with increasing V content. After CoCrCuFeNiV0.6 ball milling for 21 and 27 h, the alloy powder showed the phenomenon of amorphous transition. (4) Mechanical alloying degree of CoCrCuFeNiV0.8 is the highest when the ball milling time is 15 h. The mechanical alloying degree tends to be fixed after 21 h in the case of CoCrCuFeNiV0.6.
Conflicts of interest We wish to draw the attention of the editor to the following facts may be considered as potential conflicts of interest and to significant financial contribution to this work. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that we have followed the regulations of our institutions concerning intellectual property. We confirm that we have provided a current, correct email address which is accessible by the corresponding author and which has been configured to accept email.
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multi-component alloys, Mater. Chem. Phys. 132 (2012) 233–238. [32] S. Guo, C.T. Liu, et al., Phase stability in high entropy alloys: formation of solidsolution phase or amorphous phase, Prog. Nat. Sci.: Mater. Int. 21 (2011) 433–446. [33] S. Guo, C. Ng, J. Lu, et al., Effect of valence electron concentration on stability of fcc or bcc phase in high entropy alloys, J. Appl. Phys. 109 (2011) 103505. [34] Y.X. Chen, S. Zhu, et al., Influence of V and Ce micro addition on microstructure and mechanical properties of Al0.5CoCu0.5NiSi light-weight high entropy alloys, Hot
Work. Technol. 47 (2018) 1–5. [35] Y.D. Cheng, Y.H. Yao, et al., Study on microstructure and mechanical properties of AlxFeCrCoNi high entropy alloys, J. Xi’an Technol. Univ. 38 (2018) 140–146. [36] X.S. Zhang, G. Ran, et al., Review of the reactive mechanism in mechanical alloying, Heat Treat. Met. 6 (2003) 28–32.
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Effect of V and ball milling time on microstructure and thermal properties of CoCrCuFeNiVX by mechanical alloying
T
Changqing Shu, Ke Chen, Hemei Yang*, Mingqiang Chen, Xiya He College of Engineering, Nanjing Agricultural University, 210031, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High-entropy alloy Mechanical alloying Microstructure Thermal properties
CoCrCuFeNiVx high-entropy alloy was prepared by mechanical alloying method. X-Ray Diffractometer (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive Spectroscopy (EDS) and Differential Scanning Calorimetry (DSC) were used for different samples. The crystal structure, microstructure, composition and thermal properties were studied with conclusions below. After ball milling time reaches 21 h, CoCrCuFeNiV0.6 consists of two phases (FCC1+FCC2). After adding V, the element distribution is more uniform and the complete melting temperature of the alloy decreases with increasing V content. Also, after CoCrCuFeNiV0.6 ball milling for 21 and 27 h, the alloy material powder showed the phenomenon of amorphous transition. Mechanical alloying degree of CoCrCuFeNiV0.8 is the highest when the ball milling time is 15 h. The mechanical alloying degree tends to be fixed after 21 h in the case of CoCrCuFeNiV0.6.
1. Introduction The concept of high-entropy alloy [1,2] broke the traditional alloy design concept [3,4]. The intense lattice distortion phenomenon [5] and the interaction between the elements affect the elemental bonding and diffusion during the phase transition, making it easier for the alloy to form nanocrystalline or amorphous structures at room temperature. Therefore, high-entropy alloys have excellent properties: high fracture toughness [6], high hardness [7,8], Excellent plasticity [9–11], high strength [12–14], Fatigue resistance [15], high temperature softening resistance [16], and high temperature oxidation resistance [17]. In some special occasions, it also has special advantages over traditional materials. Kumar [18] studied the irradiation resistance and found that significant suppression of FeNiMnCr in radiation-induced segregation compared with conventional irradiated austenite Fe–Cr–Ni alloys. Chou [19] found that Co1.5CrFeNi1.5Ti0.5Mox was not easy to be pitted in NaCl solution, and the alloy had excellent corrosion resistance in H2SO4 and NaOH aqueous solution when Mo was not added. However, the commonly used method, arc melting method, needs high heating temperature and quite difficult process, which imposes restrictions on the development of high-entropy alloys. The mechanical alloying method has been widely used in the field of high-entropy alloy because it can avoid the segregation or eutectic influence during the transition from molten liquid to solid, and obtain alloy powders with
stable microstructure, good room temperature processability and good chemical homogeneity [20–24]. CoCrCuFeNi is the most representative simple FCC structure highentropy alloy in the early stage. Due to its excellent performance, researchers have paid attention to it. Recently, Mao [25] team used the improved plasma arc discharge method to prepare CoCrCuFeNi spherical nanometer with average grain size of 80–120 nm particle. Although the Salishchev [26] team is based on CoCrFeNi, it has been found that V is less compatible with other elements which lead to significant expansion of FCC solid solution. The severe lattice distortion of high-entropy alloys makes the ratio limited to neither equimolar ratio nor near-molar ratio. Usually, elements with large difference in atomic radius are added to form a hard phase, which will improve the structure and mechanical properties. V has a large atomic radius and enhances solid solution. The Tabachnikova [27] team has a lower V content ratio in the CoCrFeNiMnVx system and high cryogenic strength and high plasticity are found in CoCrFeNiMnVx. The amorphous state of some alloys gives it better physical and chemical properties than the crystalline state, making the research of amorphous materials widely concerned. And thermodynamic theory analyzes the stability of the state without considering time and process. In materials research, the process of transforming a material from one state to another over time is called dynamics theory. The various methods of developing high-entropy alloys need to undergo this state of
*
Corresponding author. E-mail addresses: [email protected] (C. Shu), [email protected] (K. Chen), [email protected] (H. Yang), [email protected] (M. Chen), [email protected] (X. He). https://doi.org/10.1016/j.physb.2019.07.028 Received 7 May 2019; Received in revised form 14 July 2019; Accepted 15 July 2019 Available online 16 July 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
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change, so it is crucial to constantly improve the dynamics theory. The research in this paper is inspired by the fact which Wang [28] proved that CoCrCuFeNi has a slow diffusion property. CoCrCuFeNiVx highentropy alloy was prepared by mechanical alloying method, using diffusion theory to explain the state transition process of materials with time, providing an example for the development of high-entropy alloy dynamics theory. Through the detection of the microstructure of the powder, the good process parameters are summarized, which provides reference for powder post-treatment and application in various fields. Finally, thermal analysis was carried out to study the behavior of CoCrCuFeNiVX transition to amorphous. For the production of some complex parts, using high quality alloy powder can simplify the production process and reduce the cost. Therefore, it is particularly important to study the properties and process of CoCrCuFeNiVx high-entropy alloy powder to obtain high-quality powder, so as to further expand the application of high-entropy alloy.
2. Material and methods Alloy ingots with nominal compositions of CoCrCuFeNiVx(x: molar ratio, x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) were processed by mechanical alloying the mixture of pure powder Co(99.99% wt.%), Cr(99.99% wt.%), Cu(99.99% wt.%), Fe(99.99% wt.%), Ni(99.99% wt.%) and V(99.99% wt.%) under high-purity argon(99.9% wt.%) atmosphere. Setting the process parameters that ball-to-powder was 10:1 and rotational speed was 350r/min, rotating mode with ‘stop for 10 min every 30 min, change steering once at the same time’, six samples were prepared by alloying in a variable frequency planetary ball milling(XQM-4L type) for 15 h. Then, compositions of CoCrCuFeNiV0.6, the remaining parameters, were performed by the same step for y hours(y = 9, 18, 21, 27) to obtain four samples which are presented in Table 1 with the process illustrated in Fig. 1. The phase constitutional analysis of the alloys was carried out by using a Panalytical X'Pert powder type XRD analyzer with the Cu-Ka radiation stepped at 0.02°. The energy of the Cu X-ray was 40 kV,40 mA. The step frequency was 1s, and the scanning range was 2θ = 10°–90°. The microstructure of the alloy powder was followed up by a Hitachi S-4800 Scanning Electron Microscope (SEM), and the phase composition of the alloy was analyzed by X-ray EnergyDispersive Spectroscopy (EDS). Each group of 13–18 mg high-entropy alloy (HEA) powder was enclosed in a clean crucible and heated under the protection of high-purity argon(99.9% wt.%) by using a NETZSCH STA 449F3 Synchronous Thermal Analyzer to get Differential Scanning Calorimetry (DSC) Curves. It was first heated from 20 °C to 1400 °C at a rate of 20 K/min, and then cooled to room temperature at a rate of 50 K/min.
Fig. 1. Schematic diagram of the research process.
3. Results and discussion 3.1. Phase analysis In the theory of thermodynamics
This formula illustrates the relationship among free energy of the high-entropy alloy system(G), entropy (S), enthalpy (H) and thermodynamic temperature (T). At a constant temperature, if the entropy of the system becomes higher or the system's enthalpy becomes lower, its free energy will be lower, and the system will be more stable at the same time. Table 2 lists enthalpy of different atom-pair, atomic size and melting point. As shown in Table 2, enthalpy of different atom-pair( ΔHijmix ) are relatively low among these four elements(Co, Cr, Fe, Ni). It should be pointed out from formula(3-2) that the mixing enthalpy values ( ΔHmix ) of CoCrFeNi is at a low level. n
ΔHmix =
∑
Ball-topowder ratio
Ball Milling Time(h)
Chemical formula
HEA1 HEA2 HEA3 HEA4 HEA5 HEA6 HEA7 HEA8 HEA9 HEA10
350 350 350 350 350 350 350 350 350 350
10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1
15 15 15 15 15 15 9 18 21 27
CoCrCuFeNi CoCrCuFeNiV0.2 CoCrCuFeNiV0.4 CoCrCuFeNiV0.6 CoCrCuFeNiV0.8 CoCrCuFeNiV1.0 CoCrCuFeNiV0.6 CoCrCuFeNiV0.6 CoCrCuFeNiV0.6 CoCrCuFeNiV0.6
(3–2)
If the total number of atoms in the alloy crystal is N, where n0 atoms are the same type of elements and n1 atoms are the same type of elements, …, nr atoms are of the same type. Then the formula of mixing entropy of alloy( ΔSmix ) is introduced below.
N! i=r
∏i = 0 ni !
(3-3)
“k” of the above formula is Boltzmann constant. 1 Whenn 0 = n1 = ...=nr = r N , the ΔSmix of system reaches the maximum. CoCrFeNi has an increase in entropy( ΔS > 0 ) compared to any case
Table 1 Processing parameters of samples. Rotational speed (r/min)
4ΔHijmix ci cj
i = 1, i ≠ j
ΔSmix = k ln
Sample
(3–1)
ΔG = ΔH − TΔS
Table 2 Enthalpy of different atom-pair(atomic size/melting point).
236
Element(atomic size/melting point)
Co
Cr
Cu
Fe
Ni
V
Co(1.25/1495 °C) Cr(1.27/1857 °C) Cu(1.28/1083 °C) Fe(1.27/1535 °C) Ni(1.24/1453 °C) V(1.35/1890 °C)
_
−4 _
6 12 _
−1 −1 13 _
0 −7 4 −2 _
−14 −2 5 −7 −18 _
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Fig. 2. The relationship between δ and Ω for multi-component alloys [32].
Fig. 3. XRD image of HEA1-6.
Fig. 5. Enlarged XRD image of all samples. Table 3 Chemical composition (in at.%) analysis results of HEA1-6 by EDS. Alloy
Area
Co(at.%)
Cr(at.%)
Cu(at.%)
Fe(at.%)
Ni(at.%)
V(at.%)
HEA1
Nominal Surface Nominal Surface Nominal Surface Nominal Surface Nominal Surface Nominal Surface
20 22.74 19.2 24.56 18.5 20.50 17.9 18.63 17.2 16.36 16.7 18.15
20 24.93 19.2 25.26 18.5 13.46 17.9 16.71 17.2 20.49 16.7 24.05
20 12.45 19.2 7.05 18.5 20.28 17.9 15.22 17.2 15.45 16.6 12.92
20 17.51 19.2 23.70 18.5 15.20 17.8 20.36 17.3 18.04 16.6 18.18
20 22.37 19.2 15.05 18.5 23.44 17.8 17.02 17.3 17.85 16.7 15.41
– – 4 4.39 7.5 7.12 10.7 12.07 13.8 11.82 16.7 11.30
HEA2 HEA3 HEA4 HEA5 HEA6
Fig. 4. XRD image of HEA4(HEA7-10).
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(a)
(b)
Fig. 6. (a), (b) SEM photographs of 10000-fold magnification of HEA3, HEA5. Table 4 Chemical composition (in at.%) analysis results of HEA4(HEA7-10) by EDS. Alloy
Area
Co(at.%)
Cr(at.%)
Cu(at.%)
Fe(at.%)
Ni(at.%)
V(at.%)
HEA7
Nominal Surface Nominal Surface Nominal Surface Nominal Surface Nominal Surface
17.9 18.57 17.9 18.63 17.9 17.35 17.9 18.81 17.9 18.40
17.9 17.10 17.9 16.71 17.9 18.61 17.9 17.77 17.9 16.78
17.9 16.56 17.9 15.22 17.9 15.38 17.9 17.67 17.9 16.72
17.8 20.89 17.8 20.36 17.8 20.54 17.8 18.38 17.8 18.99
17.8 16.61 17.8 17.02 17.8 14.52 17.8 16.63 17.8 19.38
10.7 10.28 10.7 12.07 10.7 13.58 10.7 10.74 10.7 9.74
HEA4 HEA8 HEA9 HEA10
form a face-centered cubic lattice structure when the valence electron concentration of the high-entropy alloy system is greater than 8. It must also be mentioned that the valence electron concentration of CoCrCuFeNiVx is greater than 8.17 while BCC phase exists in the spectrum shown in Fig. 3. This phenomenon seems contrary to previous studies, and is probably influenced by various process parameters such as ball milling time. In HEA2-3 and HEA6, the characteristic peak was detected at 2θ = 35.519°, and the NiV and CoV were judged, indicating that complex intermetallic compounds are formed under too low or too high V content. Fig. 4 displays the X-ray diffraction pattern of HEA4(HEA7-10). As the ball milling time increases, the metal powder continues to be cold welded and fractured, and the atoms in the powder particles are further diffused, which promotes the phase composition of the alloy to be simple. Such as HEA7, the metal powder is plastically deformed together, and numerous diffraction peaks exist. According to the diffraction relationship, the main phase is FCC1. Meanwhile, a small part of FCC2 phase, BCC phase and V element are present. The V elemental diffraction peak disappeared at 2θ = 42.065° in HEA4 and HEA8, and it is explained by the increase of ball milling time to promote V element's participation in mechanical alloying. However, the BCC diffraction peak disappeared in HEA9-10, and the alloy appeared to consisting of two face centered cubic phases (FCC1+FCC2), which confirmed the above conjecture. According to the X-ray diffraction pattern, the twophase lattice constants of HEA10 were 3.5330 Å and 3.5725 Å respectively, and the two-phase lattice lattice distortions were 0.379% and 0.432% respectively. The phase evolution of the alloy under different process parameters can be clearly observed in Fig. 5. With the increase of V content, FCC1 peak shifts towards the low 2θ angle and then towards the high 2θ
when the four elements are equal. It is stated that the quaternary system (CoCrFeNi) is very stable under formula(3-1). Another interesting finding from Table 2 is that Cu is mixed with other elements in the system, and has a large positive value, whereas V is the opposite exactly showing a large negative value. Their simultaneous action promotes ΔHmix to remain low, nevertheless, the ΔSmix is further enlarged( ΔS > 0 ) and the CoCrCuFeNiVx system becomes more stable at this moment. Therefore, CoCrCuFeNiVx can be prepared by mechanical alloying. Zhang [29–31] intensively studied the mechanism by taking the influence of mixed enthalpy and atomic radius difference into consideration in high-entropy alloy research, and summarized the schematic diagram of its phase formation, as shown in Fig. 2 [32], which was adopted by most scholars. Through calculation, each group of samples satisfies Ω ≥ 1.1, δ ≤ 6.6%, and can form a solid solution structure. The X-Ray Diffraction patterns of HEA1-6 were displayed in Fig. 3. It can be observed that solid solutions with a simple crystal structure have been formed. The composition of CoCrCuFeNiVx high-entropy phase is characterized by diversification. Initially, HEA1 is composed of FCC1 phase. Moreover, the phase structure of HEA2 was FCC1 phase and intermetallic compound phase. Then, HEA3 is composed of FCC1 phase, FCC2 phase, BCC phase and intermetallic compound phase. And in HEA4-5, the intermetallic compound phase disappears. Finally, HEA6 is composed of FCC1 phase, BCC phase and intermetallic compound phase. According to the correspondence relationship between the diffraction peaks, FCC1 is a Cu–Ni-based solid solution phase and FCC2 is a Co–V-based solid solution phase, while BCC is a Fe–Cu-based solid solution phase. The GUO S [32,33] team's research shows that it is beneficial to 238
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parameters larger and the spacing between the crystal planes larger. In Chen's study [34], it was found that the main peak of alloy shifted to the left in the same principle after adding V. In Li's study [35], Al with large atomic radius was added by AlxFeCrCoNi, and the same phenomenon appeared. On the basis of our findings, the FCC1 main peak shifts to the left can be concluded that the lattice distortion effect is intensified, and the lattice constant of the alloy becomes larger with the increase of V content. When V reaches a certain amount, it constitutes a suitable valence electron concentration, and V local segregation occurs, which causes the more FCC1 phase change in the alloy and the peak position to shift towards the high 2θ angle. The peaks of the two phases are blurred. Consequently, the alloy may have a single FCC structure as the ball milling time is further increased. 3.2. Composition analysis and microscopic morphology The reaction process of mechanical alloying is complex, and this paper can be explained by the mechanism of mechanical alloying reaction of Zhang [36]. They report these findings in the article: Firstly, the contact area of atomic reaction increases and the diffusion distance of atom decreases; Secondly, the diffusion coefficient increases, and crystal defects and diffusion couples are introduced during the reaction; Thirdly, the fresh powder is gradually consumed to form HEA. Adding V with a molar ratio of 0.4–0.8 can promote elemental diffusion and contribute to mechanical alloying which can be seen in Table 3. ‘at.%’ which is used below means atomic percent. Evidences below flesh out the conclusion. The content of almost all elements in HEA1-2 deviates significantly from the nominal composition because of the transition from simple plastic extrusion to more complex atomic diffusion between elements. The deviation of HEA3-5 is reduced, and the elements in HEA5 have the most uniform diffusion, their contents are relatively close to their nominal components. However, the deviation of the composition of HEA6 elements began to be serious. It can be seen from Table 2 that V has a negative mixing with Co, Cr, Fe, Ni. Hence, it is easier to dissolve or form intermetallic compounds between them. Interestingly, the atomic radius of V is large, and the degree of lattice distortion caused by solid solution is severe. What's more, it hinders the diffusion, and finally causes the composition of elements to be deviated to nominal composition on the surface. The powder particles were observed by Scanning Electron Microscope, in order to explain the microcosmic element distribution more fully. Fig. 6 displays the SEM backscattered images of HEA3 and HEA5. The color of the element with light(heavy) atomic mass is dark
Fig. 7. EDS Line analysis diagrams of HEA8 and HEA10.
angle, and FCC2 peak shifts towards the high 2θ angle. With the increase of the milling time, FCC1 peak shifts to the low 2θ angle. Notably, FCC2 peak position tends to be stable after shifting towards the high 2θ angle, and the two-phase peaks are increasingly blurred. It can be seen from Table 2 that the atomic radii of Fe, Cr, Ni, Co and Cu are between 1.24 and 1.28, while the V element is characterized by a larger atomic radius(1.35). The atomic size difference in the system becomes larger, aggravating the lattice distortion of the alloy, making the cell
Fig. 8. The cps of all elements in HEA10. 239
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Fig. 9. HEA1-6 DSC analysis curves in the range of 1050 °C–1400 °C.
Fig. 10. HEA4(HEA7-10) DSC analysis curves in the range of 1050 °C–1400 °C.
element is relatively complete after 21 h of ball milling. Observation was performed using EDS Line analysis to further analyze the alloy structure. As shown in Fig. 7, as the ball milling time is longer, the line analysis curve is more gradual, that is, the element distribution is more uniform. It is found from Fig. 8 that the strength of Cu in the CoCrCuFeNiVX alloy is low, and the reactivity of Cu in the system is relatively poor, which is determined by the positive mixing enthalpy of Cu with other elements.
(bright) in backscattering observation. From the 10000-fold magnified view of the morphology, small white spots appeared in the surface of HEA3 powder particles. And the surface color of HEA5 powder particles (distribution of each element) was uniform, the white spots disappeared. This is in good agreement with the above composition analysis, there is almost no surface component segregation when x reaches 0.8. Mechanical alloying is a complex and versatile physicochemical change process demonstrated in Table 4. The EDS surface analysis results of two particles are very close to the nominal composition in HEA7. As a matter of fact, the alloy is simply plastically pressed together by each metal powder now. HEA4 has a content of Co, Fe, Ni, V which is significantly higher than the nominal composition. For this reason, the elements begin to diffuse but not completely at this time. In HEA8, only the content of Fe, Cr and V is higher than the nominal composition, and the phenomenon of deviation is obviously improved after 18 h of ball milling time. The diffusion coefficient reflects the thermal motion properties of atoms. The higher the melting point of a metal material is, the higher the bond energy of the metal is, and the more difficult it is for atoms to break free from the shackles of surrounding atoms. So the diffusion coefficient is smaller. To tell the truth, self-diffusion coefficients of V(1890 °C), Cr(1857 °C) and Fe(1535 °C) are very low due to higher melting point compared to other elements, and incomplete diffusion resulting from slow alloying rate causes surface element segregation. What's better, the elements of HEA9-10 are very close to the nominal composition. To sum up, the diffusion of each
3.3. Thermal analysis Fig. 9 is illustrated as a differential scanning calorimeter analysis pattern of HEA1-6. As shown in Fig. 9, CoCrCuFeNiVx alloy gets the expected thermal performance, with melting temperature is less than 1390 °C, respectively, which is lower than constituent elements of the alloy other than Cu shown in Table 2. Different from simple combination between melting points, the complete melting temperature of the alloy is affected by lattice distortion. For supplementary explanation, the alloy is not as stable as the former under the effect and less energy is needed to destroy its chemical bonds, so it shows a decrease in melting point. In HEA3-6, a new exothermic peak appears at 1070–1120 °C corresponding to the phase analysis above. The end points of six curves’ melting peak(1380 °C, 1390 °C, 1377 °C, 1365 °C, 1350 °C, 1303 °C) were analyzed by NETZSCH Proteus software, and the complete melting temperature of the alloy decreases with increasing V content. On the 240
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Acknowledgments
other hand, it also reflects the degree of lattice distortion. Fig. 10 shows the differential scanning calorimeter analysis pattern of HEA4 and HEA7-10. As shown in Fig. 10, only HEA9 and HEA10 have obvious crystallization exothermic behavior during 400–800 °C. HEA4 and HEA7-8 all have an exothermic peak at 1070°C–1130 °C which is close to the melting point of Cu (1083 °C), and the peak corresponds to copper-rich phase melting process judging from the XRD phase analysis (three-composite gold exists in the presence of Cu–Febased solid solution phase). It is worth noting that five curves have two exothermic peaks in the range of 1210°C–1390 °C, and the peak area decreases with the increase of the milling time, which corresponds to the melting peak of the FCC phase. On one hand, the metal powder is deformed, cold welded, fractured, and diffused during the mechanical alloying process. A large number of defects such as vacancies and dislocations are introduced, and the energy of the alloy system becomes very high. What's more, an amorphous state in the metastable state will be formed when the sum of the free energy of the crystalline state and the free energy of the crystal defect is greater than the amorphous free energy. On the other hand, the temperature of the alloy powder increased with the increase of ball milling time, and then the atom diffusion velocity increased, which promoted the crystal formation. Finally, the two trends act simultaneously, and the area of each peak (exothermic heat) is proportional to the content of the corresponding phase. In the case of ball milling for 21 h, the alloy system exhibits a large amount of amorphization behavior. Depending on the characteristics of amorphous alloy (uniform structure, corrosion resistance, high strength, high fracture toughness, etc.), the alloy should exhibit excellent comprehensive properties.
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4. Conclusion (1) After adding V, the phase composition of CoCrCuFeNi high-entropy alloy tends to be diversified, and the change process is complicated. It consists of a single phase (FCC1), two phases (FCC1+FCC2) or three phases (FCC1+FCC2+BCC). BCC phase changes to FCC2 phase in CoCrCuFeNiV0.6 with the increase of ball milling time, and the alloy consists of two phases (FCC1+FCC2) after 21 h. (2) When x reaches 0.8, there is almost no surface component segregation. Increasing the ball milling time makes the alloy powder more uniform in element distribution. (3) The complete melting temperature of the alloy decreases with increasing V content. After CoCrCuFeNiV0.6 ball milling for 21 and 27 h, the alloy powder showed the phenomenon of amorphous transition. (4) Mechanical alloying degree of CoCrCuFeNiV0.8 is the highest when the ball milling time is 15 h. The mechanical alloying degree tends to be fixed after 21 h in the case of CoCrCuFeNiV0.6.
Conflicts of interest We wish to draw the attention of the editor to the following facts may be considered as potential conflicts of interest and to significant financial contribution to this work. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that we have followed the regulations of our institutions concerning intellectual property. We confirm that we have provided a current, correct email address which is accessible by the corresponding author and which has been configured to accept email.
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