Investigation on microstructure and properties of AlxCoCrFeMnNi high entropy alloys by ultrasonic impact treatment

Journal of Alloys and Compounds xxx (xxxx) xxx

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Investigation on microstructure and properties of AlxCoCrFeMnNi high entropy alloys by ultrasonic impact treatment Meiyan Li a, *, Qi Zhang a, Bin Han a, Lixin Song b, Jianlong Li a, Jie Yang a a b

School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong, 266580, China Offshore Oil Engineering (Qingdao) Co., Ltd., Qingdao, 266520, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 June 2019 Received in revised form 8 October 2019 Accepted 9 October 2019 Available online xxx

In the present study, ultrasonic impact treatment was applied to promote the as-cast AlxCoCrFeMnNi (x ¼ 0, 0.5, 1.0 and 1.5) high entropy alloys, and the microstructures, phase composition and surface properties were investigated. The increase in Al mole fraction from 0 to 0.5, 1.0 and 1.5 led to the different phase compositions from single FCC, duplex FCC þ BCC to single BCC phase as well as the increased microhardness and reduced corrosion resistance. Ultrasonic impact treatment did not change the phase compositions of AlxCoCrFeMnNi alloys, while the successive network structures were broken and the short rod-like structures formed rather than long strip-like structures in the impacted hardening layer. The microhardness of AlxCoCrFeMnNi alloys (x ¼ 0, 0.5 and 1.0) were improved by 92% (approximately 308.5 HV, 312.5 HV and 450.8 HV, respectively) after ultrasonic impact treatment due to the refine microstructures and precipitation strengthening mechanisms. Owing to the application of ultrasonic impact treatment the surface roughness of AlxCoCrFeMnNi alloys (Ra ¼ 0.54e0.57 mm) decreased. Additionally, the impacted hardening AlxCoCrFeMnNi alloys exhibited higher corrosion resistance than the as-cast alloys resulted from the lower surface roughness. © 2019 Published by Elsevier B.V.

Keywords: AlxCoCrFeMnNi High entropy alloys Ultrasonic impact treatment Microstructure Properties

1. Introduction High entropy alloys (HEAs), as a new design concept of alloy systems, contain at least five principle elements with individual elemental concentration ranging from 5 to 35 at.% [1,2], where multiple elements are in the leading positions. As we know, the high entropy alloys have many excellent properties compared with the traditional metal materials because of its single crystal structure, high entropy effect, severe lattice distortion effect, hysteretic diffusion effect and cocktail effect [3,4]. Recent investigations in the field of high entropy alloys have indicated that due to the high mixing entropy, HEAs intended to form face-centered cubic (FCC), body-centered cubic (BCC) and FCC þ BCC solid solutions rather than intermetallic compounds or other complex ordered phases [5e7]. J. Pi et al. [2] have reported the effect of the value of DSrDHh(sol.) on the tendency of forming single FCC or BCC solid solution by thermodynamic analysis of high entropy alloys. K. A.

* Corresponding author. School of Materials Science and Engineering, China University of Petroleum (East China), No. 66, West Changjiang Road, Huangdao District, Qingdao, 266580, China. E-mail address: [email protected] (M. Li).

Christofidou et al. [8] proved that Ni element was helpful for the stability of single FCC solid solution phase and suppressing the formation of the Cr-rich s phase in the CrMnCoFeNix high entropy alloys. In addition, Qi Chao et al. [9] found that with an increase in the Al mole fraction from 0.3 to 0.6 and 0.85, the crystal structure changed from FCC, to FCC þ BCC and BCC. Furthermore, owning to the higher mixing entropies than traditional alloys, HEAs possess versatile properties as well as a number of promising applications [10e12]. Usually, most of failures for the metallic parts occur on the surfaces, which relate to the structure and properties of the material surfaces. Therefore, an improved surface modification treatment is necessary for prolonging the service lifetimes [13]. Currently, one of the most effective methods to improve the properties of metallic parts is surface hardening treatment, such as shot peening [14], surface mechanical attrition treatment (SMAT) [15,16] and ultrasonic impact treatment [17,18], etc. Among the surface treatment methods, ultrasonic impact treatment (UIT) is a promising technique that brings out severe plastic deformation and allows fast modification of structure and composition of the surface layers. Moreover, ultrasonic impact treatment have the advantages of larger size of ball and higher frequencies while each impact can

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Please cite this article as: M. Li et al., Investigation on microstructure and properties of AlxCoCrFeMnNi high entropy alloys by ultrasonic impact treatment, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152626

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induce the plastic deformation with a high strain rate in the surface layers of the samples [18,19]. In addition, ultrasonic impact treatment is a decent method to introduce a gradient grain refinement near the surface while the properties of the bulk materials are preserved [20,21]. It was demonstrated that the microhardness and corrosion resistance of biomedical Coe28Cre6Mo alloy were enhanced obviously by ultrasonic impact treatment [20]. D. A. Lesyk et al. [22] revealed that applying ultrasonic impact treatment on the surfaces of laser hardening treated layer for AISI 1045 steel could provide more than triple increase in the surface hardness. Thanks to structural modifications by ultrasonic impact treatment, the improved hardness and corrosion resistance are expected. However, up to now, only a few studies have been carried out on the microstructural characteristics and properties of AlxCoCrFeMnNi alloys induced by ultrasonic impact treatment. In present work, we aimed to investigate the effect of ultrasonic impact treatment of AlxCoCrFeMnNi high entropy alloys on the phase composition and microstructures of the surface layers, and to study how much a treatment affects the microhardness and corrosion resistance of HEAs. Especially, analysis of the surface characteristics is very helpful in understanding and improving the HEAs’ performance.

2. Experimental procedures Four AlxCoCrFeMnNi high entropy alloys samples with different Al contents (x ¼ 0, 0.5, 1.0 and 1.5 in mole fraction) were produced by vacuum arc-melting technique. The purity of each of the raw materials was at least 99.9%. These specimens were exposed to ultrasonic impact treatment. A schematic of the experimental setup for ultrasonic impact treatment is shown in Fig. 1. It consisted of an ultrasonic generator with a frequency of 28.5 KHz connected to an ultrasonic instrument. The diameter of the ultrasonic impactor was 6 mm while the impact velocity of 150 mm/min and the static pressure of 0.3 MPa were applied during ultrasonic impact treatment. After ultrasonic impact treatment, the analysis of phase composition was performed by using the X-ray diffractometer (XRD), with Cu-Ka radiation. The work voltage and current were set to be 40 kev and 40 mA. The cross-sections and surfaces of the produced specimens were finely polished and then etched with a mixture of hydrochloric acid and nitric acid in the ratio of 3:1 (by volume). The microstructures of the impacted hardening specimens were characterized by means of scanning electron microscopy (SEM, JEOL-7200F), equipped with the energy dispersive spectrometer (EDS). In addition, the comparative analysis on the surface roughness of the AlxCoCrFeMnNi high entropy alloys before and after ultrasonic impact treatment were conducted by means of TR300 Surface Roughness Shape Measuring Instrument. Microhardness of HEAs and impacted specimens were evaluated using a

MH-3 Vickers microhardness tester at a load of 100 g and a dwell time of 10 s (hardness measurement at least three times), and the average values were used for drawing microhardness curves along the depth from the surface. Furthermore, the corrosion resistance of the HEAs specimens by ultrasonic impact treatment was analyzed in the solution of 3.5% NaCl using CS310 electrochemical test system. According to the Tafel curves, the values of Ecorr and Icorr were obtained and the corrosion resistance was discussed.

3. Thermodynamic calculation of AlxCoCrFeMnNi high entropy alloys As we know, the Gibbs free energy is different between liquid and crystal or intermetallic compounds. In an undercooled alloying liquid, the less DG1-s means the more stable and the more easily formed corresponding phase. According to Hume-Rothery rule, the value of DG1-s can be estimated by the following equation:

DG1-s ¼ DHmix-TDSmix

(1)

where, DG1-s is the Gibbs free energy difference between the liquid phase and the solidified phase, DHmix is the mixing enthalpy, and T is the temperature of the undercooled alloying liquid. It can be seen that the more negative value of DSmix means the obvious trend of forming the solid solution rather than intermetallic compounds. However, high value of DSmix is not the only factor controlling the formation of solid solution in the isoatomic multicomponent alloys [23]. In order to estimate the phase compositions of HEAs, Zhang Y. et al. [24] promoted that the following conditions should be met simultaneously for forming solid solutions: 22 DHmix 7 kJ/mol, 0 d  8.5 and 11 DSmix 19.5 J/(K$mol). Additionally, both Guo S. et al. [25] and C. T. Liu et al. [26] studied the effect of valence electron concentration (VEC) value on the phase stability for HEAs. They found it easy to form a single FCC phase when the value of VEC was above 8 and a single BCC phase was generated when the value of VEC was below 6.87. However, the mixed solid solution structures of duplex FCC þ BCC phases formed with the condition of 6.87  VEC <8. The mixing enthalpy DHmix (kJ/mol) can be expressed by:

DHmix ¼

n X

Uij cj ci

(2)

i¼1;isj mix here, Uij ¼ 4DH mix AB ; DH AB is the mixing enthalpy of binary liquid AB alloy. The mixing enthalpy DH mix ij (kJ/mol) of binary liquid between arbitrary alloying elements i and j could be calculated according to Miedema model, as listed in Table 1. Following Boltzmann’s hypothesis, the mixing entropy (DSmix) of N-element regular solution alloys can be expressed as Eq. (3), which will reach the maximum value when the alloy is of equalatomic ratio (as shown in Eq. (4)):

Table 1 Values of DHmix ij (Kj/mol) calculated by Miedema’s model for atomic pairs between the elements involved in the papers [27,28].

Fig. 1. Schematic diagram of ultrasonic impact treatment for the HEAs samples.

Element

Al

Co

Cr

Fe

Mn

Ni

Al Co Cr Fe Mn Ni

19 10 1 19 22

19 4 1 0

10 4 1 2 7

1 1 1 2

19 2 -

22 0 7 2 -

Please cite this article as: M. Li et al., Investigation on microstructure and properties of AlxCoCrFeMnNi high entropy alloys by ultrasonic impact treatment, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152626

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DSmix ¼  R

N X

ci Lnci

(3)

i¼1

DSmix ¼ RLnN

(4)

where R is gas constant, ci is mole precent of component i, and N P ci ¼ 1. i¼1 The value of valence electron concentration (VEC) can be defined by:

VEC ¼

n X

ci ðVECÞi

(5)

i¼1

here,(VEC)i is the valence electron concentration for the ith element. The atomic size difference (d) is determined by Eq. (5):

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N . uX d ¼ 100t Ci （1  ri r）2

(6)

i¼1 N P where r ¼ ci ri , ci and ri are the atomic percentage and atomic i¼1 ith element. The numerical factor 100 is used to radius of the amplify the data for clarity. For the AlxCoCrFeMnNi HEAs, the value of x was chosen as 0, 0.5, 1.0 and 1.5, respectively. Based on the Eqs. (1)e(4), the parameters of thermodynamic calculation for AlxCoCrFeMnNi HEAs concerning atomic size difference (d), enthalpy of mixing (DHmix), entropy of mixing (DSmix) and the valence electron concentration (VEC) were calculated in Table 2. It can be seen that the AlxCoCrFeMnNi (x ¼ 0, 0.5, 1.0, 1.5) HEAs system satisfied the following conditions of 22 DHmix 7 kJ/mol, 0 d  8.5 and 11 DSmix 19.5 J/(K$mol). That meant AlxCoCrFeMnNi HEAs tended to form solid solution rather than intermetallic compounds. Furthermore, when the x was 0, the VEC value was 8, which indicated the formation of single FCC phase. When x was 0.5 and 1.0, the VEC values were less than 8 and a mixture of dual FCC and BCC phases would be formed. As the value of x was 1.5, VEC ¼ 6.85 < 6.87, which meant the formation of single BCC phase solid solution.

4. Results and discussion 4.1. XRD analysis Fig. 2 depicts the X-ray diffraction patterns of the examined AlxCoCrFeMnNi high entropy alloys, and the phase structures are listed in Table 3. It can be seen that Al element addition plays an important role in the phase structures of the AlxCoCrFeMnNi high entropy alloys. Only diffraction peaks corresponding to a FCC crystal structure are detected in the CoCrFeMnNi and Al0.5CoCrFeMnNi alloys specimens. As the content of Al increases, the BCC phase occurs, and the examined Al1.0 CoCrFeMnNi alloys present a mixture of dual phases FCC þ BCC. For Al1.5CoCrFeMnNi alloy, it is mainly composed of BCC solid solution. According to the researches

3

of F. J. Wang et al. [29], the phase transformation of AlxCoCrFeMnNi HEAs from single FCC to BCC phase with increase in the Al content was explained by the atomic packing efficiency. The atomic packing efficiency of FCC phase is about 74%, which is higher than that of BCC phase (68%). Therefore, the solid solution of Al element tends to improve the lattice strain and lattice distortion energy in the FCC structure [27]. Consequently, in order to relax the lattice distortion energy, the metastable FCC phase may transform to relatively stabilized BCC phase with the increment of Al element. In addition, it is worth mentioning that except of the Al0.5CoCrFeMnNi alloy, the phase structures of other AlxCoCrFeMnNi specimens are in good accordance with the results of thermodynamic calculation, as listed in Table 3. Only diffraction peaks corresponding to a FCC crystal structure is detected in the Al0.5CoCrFeMnNi alloy specimen while the mixed dual phases FCC þ BCC are expected by thermodynamic calculation. It is supposed that the BCC phase transformation is inhibited due to the rapid cooling rate during solidification process. The superiority of solid solutions over intermetallic in this alloy is attributed to the effects of the high mixing entropy [30,31] and atomic size difference factor [24]. According to the Bragg Equation (Eq. (7)), the formula of crystal plane spacing for cubic crystals (Eq. (8)) and the lattice constants of AlxCoCrFeMnNi high entropy alloys can be estimated (Eq. (9)), as listed in Table 4. 2dsinq ¼ nl

(7)

a d ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 h þ k2 þ l2

(8)

a¼

nl

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h2 þ k2 þ l2 2sinq

(9)

here, d is the crystal plane spacing (nm), q is Bragg diffraction angle ( ), l is the wavelength (nm), ¼ 0.15406 nm. From Table 4, we can see that the lattice constant of FCC phase in the CoCrFeMnNi alloy is estimated to be 0.36012 nm. The reflections of Al0.5CoCrFeMnNi alloy shift to the leftwards slightly, and the lattice constant of FCC phase is larger than that of CoCrFeMnNi alloy. It can be supposed that the increment of the lattice constant is resulted from the solid solution of Al element with large atomic radius than any other atoms (except Ti), which lead to the lattice deformation and lattice expansion [27]. However, the lattice constant of FCC phase (0.36195 nm) in the Al1.0CoCrFeMnNi alloy is larger than that of CoCrFeMnNi alloy (0.36012 nm) but smaller than that of Al0.5CoCrFeMnNi alloy (0.36258 nm). It is supposed that as an important element affecting phase composition, the increase of Al content leads to the appearance of BCC phase. At this time, the solid solution of Al and other alloying elements causes the serious lattice distortion of FCC and BCC phases. Therefore, with the increment of Al element content the increase of FCC lattice constant is not linear, but decreases. Furthermore, the lattice constant of BCC phase in the Al1.5CoCrFeMnNi alloy is estimated to be 0.28895 nm larger than that of Al1.0CoCrFeMnNi alloy (0.28880 nm). Clearly, the

Table 2 Parameters of thermodynamic calculation for AlxCoCrFeMnNi HEAs. Alloys

VEC

d

DHmix/(kJ/mol)

DSmix/(J$K1$mol1)

Structure

CoCrFeMnNi Al0.5CoCrFeMnNi Al1.0CoCrFeMnNi Al1.5CoCrFeMnNi

8 7.55 7.18 6.85

3.27 4.71 5.56 5.99

2.08 6.37 13.74 11.29

13.38 14.69 14.89 14.79

FCC FCC þ BCC FCC þ BCC BCC

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Fig. 2. X-ray diffraction patterns of HAEs samples. (a) CoCrFeMnNi; (b) Al0.5CoCrFeMnNi; (c) Al1.0CoCrFeMnNi; (d) Al1.5CoCrFeMnNi.

Table 3 Phase structures and grain sizes of AlxCoCrFeMnNi HEAs by UIT (nm). Alloys

Phase structures

CoCrFeMnNi Al0.5CoCrFeMnNi Al1.0CoCrFeMnNi Al1.5CoCrFeMnNi

Thermodynamic calculation FCC FCC þ BCC FCC þ BCC BCC

Grain sizes As-cast FCC FCC FCC þ BCC BCC

x

0

0.5

1.0

1.5

a/nm

0.36012

0.36258

afcc ¼ 0.36195 abcc ¼ 0.28880

abcc ¼ 0.28895

Al addition has an important influence on the lattice distortions of FCC and BCC phases as well as the lattice constants. Furthermore, it can also be found that the phase structures of AlxCoCrFeMnNi alloys are not changed after ultrasonic impact treatment, as shown in Fig. 2aec and Table 3. The grain sizes of AlxCoCrFeMnNi alloys specimens before and after ultrasonic impact treatment were calculated according to the Debye-Scherrer formula (Eq. (10)) and XRD patterns (Fig. 2aec). As can be seen from Table 3, the grain sizes of AlxCoCrFeMnNi alloys after ultrasonic impact treatment decline by 25.8 %e40.88%.

Kg bcosq

As-cast 82.56 99.65 87.836 82.672

By UIT 61.257 58.9084 55.0104 -

half maximum (FWHM) of diffraction peak (rad), q is Bragg diffraction angle ( ).

Table 4 Lattice constants of AlxCoCrFeMnNi HEAs.

D¼

By UIT FCC FCC FCC þ BCC BCC

(10)

here, D is the grain size along the direction of crystal plane, K is Scherrer constant, K ¼ 0.89, g is 0.154056 nm, b is the full wave at

4.2. Microstructural analysis of HEAs samples Fig. 3 shows the microstructures of the as-cast CoCrFeMnNi specimens by ultrasonic impact treatment. As can be seen, typical dendrites are formed and obvious plastic deformation occurs at the top region resulting from ultrasonic impact treatment (Fig. 3a). The enlarged photograph exhibits the deflection of dendrites, as shown in Fig. 3b. Fig. 4 shows the surface morphologies of CoCrFeMnNi alloys. The solidification boundaries are clearly defined at low magnification (Fig. 4a) while the dendritic structures exist within the boundaries, as shown in Fig. 4b. Moreover, the refined grains are formed at the surface after ultrasonic impact treatment. Fig. 5 reveals the microstructures of as-cast Al0.5CoCrFeMnNi specimens. The solidification boundaries are clearly visible with parallel solidification direction within the grains, as shown in Fig. 5b. After ultrasonic impact treatment, the impact traces and refined grains are found on the surface, as indicated in Fig. 5c and d. Fig. 6 shows the cross-sectional morphologies of Al0.5CoCrFeMnNi alloys by ultrasonic impact treatment. A plastic

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Fig. 3. Microstructures of CoCrFeMnNi HEAs specimens after ultrasonic impact treatment using SEM (JEOL-7200F). (a) Cross-sectional morphology; (b) enlarged photograph of impact hardening layer.

Fig. 4. Surface morphologies of CoCrFeMnNi HEAs specimens by means of SEM (JEOL-7200F). (a) the as-cast specimen at low magnification; (b) dendritic characteristics at high magnification; (c) specimens by ultrasonic impact treatment.

deformation layer generated (Fig. 6a) and the very fine grains are obtained at the top region of impact hardening layer (Fig. 6b). Fig. 7 exhibits the microstructures of Al1.0CoCrFeMnNi alloys. We can see that typical successive network structures are generated (Fig. 7a) and the fine precipitated particles disperse along the grain boundaries (Fig. 7b). Further analysis has proved that the fine long rod-like microstructures distribute within the grains, as shown in Fig. 7c, where plenty of fine particles are located between the rod-like structures (Fig. 7d). Furthermore, enlarged photograph reveals that the particles with nano-scale size are arranged densely (Fig. 7e). After ultrasonic impact treatment, the microstructural characteristics near the surface layer are changed and the successive network structures are broken as shown in Fig. 8a. Compared with the microstructures shown in Fig. 7d, the short rod-like structures form rather than long strip-like structures in the impacted hardening layer (Fig. 8b). Fig. 9 indicates the surface morphologies of Al1.0CoCrFeMnNi alloys by ultrasonic impact treatment. The original successive

network structures generated in the as-cast Al1.0CoCrFeMnNi HEA (Fig. 7a) disappear and intermittent structures form, as shown in Fig. 9a. In addition, compared with the microstructures without ultrasonic impact treatment, original long rod-like microstructures (Fig. 7ced) are disturbed into finer and shorter precipitations inside grains (Fig. 9b and c) while lots of fine particles are located between these precipitations (Fig. 9c). Enlarged photograph indicates the rough surface of the short rod-like precipitations, as shown in Fig. 9d. Fig. 10 illustrates the microstructures of Al1.5CoCrFeMnNi alloys. It indicates that the structural characteristics is different from the other AlxCoCrFeMnNi high entropy alloys, as shown in Fig. 10a. From Fig. 10b and c, we can find that the precipitated phases distribute along the grain boundaries and cluster together. Enlarged photograph in Fig. 10d shows the long rod-like precipitated phases with the smooth surface while the internal structures of grains are adhered to the precipitated phases. Additionally, the composition detection indicates that the long rod-like phases are rich in Cr element, as shown in Figs. 10d and 11.

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Fig. 5. Microstructures of Al0.5CoCrFeMnNi HEA specimen by means of SEM (JEOL-7200F). (a) as-cast specimen; (b) parallel solidification directions within the solidification boundary; (c) surface morphology by ultrasonic impact treatment; (d) refined grains.

Fig. 6. Cross-sectional microstructures of Al0.5CoCrFeMnNi HEAs after ultrasonic impact treatment by means of SEM (JEOL-7200F). (a) Cross-sectional morphology; (b) refined grains at the top region.

Based on the above analysis results, the high strain rate as well as local plastic deformation during UIT gives rise to the refined microstructures, which plays an important role in promoting the properties of HEAs. 4.3. Microhardness tests Fig. 12 shows the surface hardness of the AlxCoCrFeMnNi (x ¼ 0, 0.5, 1.0, 1.5) high entropy alloys samples. Based on the measurements made, the following results can be noted: surface hardness of HEAs increase with the increase of the Al element content, which reaches 572 HV when the value of x is 1.5. Firstly, the improvement of hardness is resulted from the increment of BCC phase from 0 to 1.5. Secondly, based on the microstructural examination, the dispersion of nano-crystallites also provides an effective precipitation strengthening effect (Figs. 7e10). Nevertheless, as we know the hardness and strength of BCC

structure are higher than those of FCC phase while its plasticity and toughness are relatively lower. In this experiment, when the ultrasonic impact treatment was performed on the surfaces of the Al1.5CoCrFeMnNi specimens with single BCC structure the fracture phenomenon occurred. In addition, plenty of long rod-like precipitations distributing along the grain boundaries induced the embrittlement of Al1.5CoCrFeMnNi alloy, as shown in Fig. 10. Therefore, ultrasonic impact treatment processing was not applied for the strengthening of Al1.5CoCrFeMnNi alloy in this research. Fig. 13 indicates the microhardness curves of AlxCoCrFeMnNi HEAs samples by ultrasonic impact treatment along the depth from the surface. We can see that the microhardness of the as-cast CoCrFeMnNi and Al0.5CoCrFeMnNi alloys are about 160 HV and 175 HV, which are improved by 83.2% and 74.4%, respectively (approximately 308.5 HV and 314 HV) after ultrasonic impact treatment (Fig. 13a and b). The refined microstructures and precipitation strengthening mechanisms are expected to be

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Fig. 7. Microstructures of Al1.0CoCrFeMnNi HEA by means of SEM (JEOL-7200F. (a) Successive network structures; (b) fine precipitated particles dispersed along the grain boundaries; (c) fine lath-like microstructures; (d) plenty of fine particles; (e) particles at high magnification.

Fig. 8. Microstructure of Al1.0CoCrFeMnNi HEA by ultrasonic impact treatment by means of SEM (JEOL-7200F. (a) Cross-sectional morphology; (b) short rod-like structures.

responsible for the increased hardness. Furthermore, as the increase of the depth, the microhardness decreases gradually, and there are no sharp interfaces between the matrix and the impacted hardening layer. As for the Al1.0CoCrFeMnNi high entropy alloy, the

microhardness reaches to 450.8 HV after ultrasonic impact treatment. However, the depth of impacted hardening layer is smaller than that of CoCrFeMnNi and Al0.5CoCrFeMnNi high entropy alloys. It is deduced that the strengthening contribution is both the

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Fig. 9. Surface morphologies of Al1.0CoCrFeMnNi high entropy alloys by ultrasonic impact treatment using SEM (JEOL-7200F. (a) Surface structural characteristics; (b) refine structures within the grains; (c) precipitations and fine particles; (d) enlarged photograph of the precipitations.

Fig. 10. Microstructures of the as-cast Al1.5CoCrFeMnNi high entropy alloy by means of SEM (JEOL-7200F). (a) at low magnification; (b) morphologies of grains; (c) precipitations; (d) long rod-like precipitates at high magnification.

original hardness of the as-cast high entropy alloys and hardness after ultrasonic impact treatment. High hardness for Al1.0CoCrFeMnNi high entropy alloys means strong ability to resist local deformation during ultrasonic impact treatment. In addition, the

hardness of impacted hardening layer is higher than that of CoCrFeMnNi alloy which brings more difficulties in deformation during overlapping impact. Obviously, the results show that ultrasonic impact treatment

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Fig. 11. Composition analysis of point A by means of energy dispersive spectrometer (EDS) attached with SEM (JEOL-7200F).

Fig. 12. Surface hardness of the as-cast AlxCoCrFeMnNi HAEs specimens by means of HR-150 hardness tester.

can promote the AlxCoCrFeMnNi HEAs appreciably. According to the previous researches on pure metals [32,33], the gradient surface layers appeared to be highly effective and especially did not have sharp interfaces, thus the load transfer and distribution over the deforming gauge section could be quite smooth. 4.4. Surface roughness The comparative analysis on the surface roughness of the AlxCoCrFeMnNi high entropy alloys before and after ultrasonic impact treatment are conducted, as shown in Table 5. It should be noted that the surface roughness of the as-cast AlxCoCrFeMnNi high entropy alloys is about 0.62e0.64 mm when they are grinded with 2000 mush sandpaper while the smaller values of surface roughness (Ra ¼ 0.54e0.57 mm) are obtained by ultrasonic impact treatment.

Fig. 13. Microhardness curves of AlxCoCrFeMnNi HEAs samples by ultrasonic impact treatment using Vickers microhardness tester at a load of 100 g and a dwell time of 10 s. (a) x ¼ 0; (b) x ¼ 0.5; (c) x ¼ 1.0.

Table 5 Surface roughness of AlxCoCrFeMnNi HEAs samples by UIT (Ra/mm). Treatment

CoCrFeMnNi

Al0.5CoCrFeMnNi

Al1.0CoCrFeMnNi

Fine grinding UIT

0.64 0.56

0.64 0.54

0.62 0.57

4.5. Corrosion resistance Fig. 14 shows the dynamic potential polarization curves of the AlxCoCrFeMnNi high entropy alloys before and after ultrasonic impact treatment in 3.5% NaCl solution. The corrosion kinetic parameters obtained by linear fitting are summarized in Table 6. It can be seen that the corrosion current densities of the as-polished AlxCoCrFeMnNi high entropy alloys samples increase with the increment of Al element addition, which indicates the reduction of corrosion resistance in 3.5% NaCl solution. The corrosion of AlxCoCrFeMnNi high entropy alloys in NaCl

solution is considered to be accomplished by the electron transfer [34]. The corrosion process mainly consists of the dissolution of anode and the reduction of cathode [35]. For the CoCrFeMnNi high entropy alloy, the reactions occurring in the anode and cathode are expressed as follows: Feþ2H2O þ Cl/Fe3þ$2H2O þ Clþ3e

(11)

O2þ4eþ2H2O/4OHþ

(12)

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Fig. 14. Dynamic potential polarization curves of AlxCoCrFeMnNi HEAs samples by ultrasonic impact treatment using CS310 electrochemical test system in the solution of 3.5% NaCl. (a) x ¼ 0; (b) x ¼ 0.5; (c) x ¼ 1.0; (d) x ¼ 1.5.

Therefore, the corrosion of AlxCoCrFeMnNi high entropy alloys accelerated due to the participation of Al element.

Table 6 Corrosion kinetic parameters of different HEAs samples. HEAs

Treatment

Ecorr/mV

Icorr/mA$cm2

CoCrFeMnNi

without UIT with UIT without UIT with UIT without UIT with UIT without UIT

343.4 285.1 255.8 233.0 391.7 285.3 397.34

2.823 1.902 4.519 2.329 5.351 4.518 5.984

Al0.5CoCrFeMnNi Al1.0CoCrFeMnNi Al1.5CoCrFeMnNi

Ni/Niþþ2e

(13)

Niþþ2H2O/Ni(OH)2þ2Hþ

(14)

Cr/Crþþ3e

(15)

Crþþ3H2O/Cr(OH)3þ3Hþ

(16)

During the corrosion process, the ions entered the corrosion solution and the electrons transferred from the anode to the cathode. In the cathode, oxygen reached the surface of the cathode and he residual electrons in the adsorbed to form OH (Eq.(11) and (12)). Meanwhile, the Ni and Cr elements dissolved in the solution (Eq.(13)e(16)). When the Al element was introduced, it involved in the reactions of the anode and cathode, which was similar to the Fe element (Eq. (11)). In addition, the sodium chloride was dissociated and the corrosion products formed, as shown in Eq.(17)e(19).

Alþ2H2O þ Cl/Al3þ$2H2O þ Clþ3e

(17)

Al3þ$2H2Oþ3Cl-/AlCl3$2H2O

(18)

Al3þþ3OHþ/Al(OH)3

(19)

On the other hand, the as-cast AlxCoCrFeMnNi high entropy alloys samples displayed an evolution of crystal structure from FCC, to FCC þ BCC and BCC with an increase in the Al mole fraction from 0 to 1.5. As we know, the austenitic stainless steels of FCC structure normally have greater corrosion resistance than the ferritic stainless steels with BCC structure [36]. This fact could explain why the corrosion resistance of AlxCoCrFeMnNi alloys with FCC phase (x ¼ 0, 0.5) was better than other HEAs with FCC þ BCC or BCC phase [35]. Furthermore, it is clearly seen from Table 6 that in comparison with the initial state the corrosion current densities of all the alloys specimens decrease obviously after ultrasonic impact treatment, which means the improvement of corrosion resistance. Preliminary experimental results show that the use of the ultrasonic impact treatment for the as-cast high entropy alloys facilitates the roughness lowering (Table 5). As for the impacted hardening AlxCoCrFeMnNi high entropy alloys by ultrasonic impact treatment, their surfaces with smaller roughness tend to bring out the smaller effective contact areas and the larger apparent contact angles [37], which are contributed to the decrease of corrosion behavior in comparison with the surfaces without ultrasonic impact treatment.

Please cite this article as: M. Li et al., Investigation on microstructure and properties of AlxCoCrFeMnNi high entropy alloys by ultrasonic impact treatment, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152626

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5. Summary In the current work, ultrasonic impact treatment was applied to promote the as-cast AlxCoCrFeMnNi (x ¼ 0, 0.5, 1.0 and 1.5) high entropy alloys. After that, the microstructures, phase structures and surface properties were investigated. The following conclusions can be draw from this study: (1) The increase in Al mole fraction from 0 to 0.5, 1.0 and 1.5 led to the different phase structures from single FCC, dual FCC þ BCC and single BCC phase, respectively, as well as the increased microhardness and reduced corrosion resistance. Ultrasonic impact treatment did not change the phase structures of AlxCoCrFeMnNi alloys, while the microhardness was improved and the grain sizes were refined obviously. (2) For CoCrFeMnNi and Al0.5CoCrFeMnNi alloys, typical dendrites formed while typical successive network structures generated and the fine precipitated particles dispersed along the grain boundaries for Al1.0CoCrFeMnNi. Fine long rod-like microstructures distributed within the grains, and plenty of fine particles were located between the rod-like structures. After UIT, the successive network structures were broken and the short rod-like structures formed in the impacted hardening layer. In the case of Al1.5CoCrFeMnNi alloys, long rodlike phases with smooth surfaces clustered along the grain boundaries while the internal structure of grains were adhered to the precipitated phases. (3) Owing to the application of ultrasonic impact technology, the microhardness of the as-cast CoCrFeMnNi and Al0.5CoCrFeMnNi alloys were improved by 83.2% and 74.4%, respectively. The microhardness decreased gradually along the depth and there was no sharp interfaces between the matrix and the impacted hardening layer. As for the Al1.0CoCrFeMnNi alloy, the microhardness reached to 450.8HV after ultrasonic impact treatment. (4) The surface roughness of Ra ¼ 0.54e0.57 mm for the AlxCoCrFeMnNi alloys can be obtained by ultrasonic impact treatment. Furthermore, the corrosion resistance of AlxCoCrFeMnNi improved after ultrasonic impact treatment resulting from the refined microstructure and lower surface roughness. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (No.51801234 and No.51771228), the Natural Science Foundation of Shandong Province (No. ZR2019MEM047) and the Fundamental Research Funds for the Central Universities (No.18CX02089A). References [1] J.W. Yeh, S.K. CHEN, S.J. LIN, et al., Nanostructured high entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299e303. [2] Ji Pi, Y. Pan, Thermodynamic analysis for microstructure of high entropy alloys, Rare Metal Mater. Eng. 42 (2013) 232e237. [3] Z. Wang, Q. Wu, W. Zhou, et al., Quantitative determination of the lattice constant in high entropy alloys, Scr. Mater. 162 (2019) 468e471. [4] Y. Zhang, T.T. Zuo, Z. Tang, et al., Microstructures and properties of highentropy alloys, Prog. Mater. Sci. 61 (2014) 1e93. [5] X. Yang, Y. Zhang, Prediction of high-entropy stabilized solid solution in multi-

11

component alloys, Mater. Chem. Phys. 132 (2012) 233e238. [6] F. He, Z. Wang, P. Cheng, et al., Designing eutectic high entropy alloys of CoCrFeNiNbx, J. Alloy. Comp. 656 (2016) 284e289. [7] L.M. Martyushev, V.D. Seleznev, Maximum entropy production principle in physics, chemistry and biology, Phys. Rep. 426 (1) (2006) 1e45. [8] K.A. Christofidou, T.P. McAuliffe, P.M. Mignanelli, et al., On the prediction and the formation of the sigma phase in CrMnCoFeNix high entropy alloys, J. Alloy. Comp. 770 (2019) 285e293. [9] Q. Chao, T. Guo, T. Jarvis, et al., Direct laser deposition cladding of AlxCoCrFeNi high entropy alloys on a high-temperature stainless steel, Surf. Coat. Technol. 332 (2017) 440e451. [10] R. Anand Sekhar, Srinivasa Rao Bakshi, Microstructural evolution of TieAleNi(Cr,Co,Fe)-Based high-entropy alloys processed through mechanical alloying, Trans. Indian Inst. Met. 72 (2019) 1427e1430. [11] S. Singh, N. Wanderka, B.S. Murty, et al., Decomposition in multi-component AlCoCrCuFeNi high-entropy alloy, Acta Mater. 59 (2011) 182e190. [12] Y. Dong, L. Jiang, H. Jiang, et al., Effects of annealing treatment on microstructure and hardness of bulk AlCrFeNiMo0.2 eutectic high-entropy alloy, Mater. Des. 82 (2015) 91e97. [13] J. Hou, M. Zhang, H. Yang, et al., Surface strengthening in Al0.25CoCrFeNi highentropy alloy by boronizing, Mater. Lett. 238 (2019) 258e260. [14] Angel L. Ortiz, Jia-Wan Tian, Leon L. Shaw, et al., Experimental study of the microstructure and stress state of shot peened and surface mechanical attrition treated nickel alloy, Scr. Mater. 62 (2010) 129e132. [15] N. Tao, H. Zhang, J. Lu, et al., Development of nanostructures in metallic materials with low stacking fault energies during surface mechanical attrition treatment (SMAT), Mater. Trans. 44 (2003) 1919e1925. [16] K. Lu, J. Lu, Nanostructured surface layer on metallic materials induced by surface mechanical attrition treatment, Mater. Sci. Eng. A 375e377 (2004) 38e45. [17] D.A. Lesyk, S. Martinez, B.N. Mordyuk, et al., Effect of laser heat treatment combined with ultrasonic impact treatment on the surface topography and hardness of carbon steel AISI 1045, Opt. Laser. Technol. 111 (2019) 424e438. [18] W.Y. Tsai, J.C. Huang, Y.J. Gao, et al., Relationship between microstructure and properties for ultrasonic surface attrition treatment, Scr. Mater. 103 (2015) 45e48. [19] D.A. Lesyk, S. Martinez, B.N. Mordyuk, et al., Microstructure related enhancement in wear resistance of tool steel AISI D2 by applying laser heat treatment followed by ultrasonic impact treatment, Surf. Coat. Technol. 328 (2017) 344e354. [20] Yu N. Petrov, G.I. Prokopenko, B.N. Mordyuk, et al., Influence of microstructural modifications induced by ultrasonic impact treatment on hardening and corrosion behavior of wrought Co-Cr-Mo biomedical alloy, Mater. Sci. Eng. C 58 (2016) 1024e1035. [21] S.P. Chenakin, V.S. Filatova, I.N. Makeeva, et al., Ultrasonic impact treatment of CoCrMo alloy: surface composition and properties, Appl. Surf. Sci. 408 (2017) 11e20. [22] S.P. Chenakin, B.N. Mordyuk, N.I. Khripta, Surface characterization of a ZrTiNb alloy: effect of ultrasonic impact treatment, Appl. Surf. Sci. 470 (2019) 44e55. [23] Y. Zhang, Z.P. Lu, S.G. Ma, et al., Guidelines in predicting phase formation of high-entropy alloys, MRS Commun. 4 (2014) 57e62. [24] Y. Zhang, Y. Zhou, J. Liu, et al., Solid-solution phase formation rules for multicomponent alloys, Adv. Eng. Mater. 10 (2008) 534e538. [25] Sheng Guo, Chun Ng, Jian Lu, et al., Effect of valence electron concentration on stability of fcc or bcc phase in high entropy alloys, J. Appl. Phys. 109 (2011), 103505-1-5. [26] C.T. Liu, Physical metallurgy and mechanical properties of ductile ordered alloys, Int. Met. Rev. 29 (1984) 168e194. [27] K.B. Zhang, Z.Y. Fu, Y.J. Zhang, et al., Microstructure and mechanical properties of CoCrFeNiTiAlx high-entropy alloys, Mater. Sci. Eng. A 508 (2009) 214e219. [28] Y.J. Zhou, Y. Zhang, T.N. Kim, et al., Microstructure characterizations and strengthening mechanism of multi-principal component AlCoCrFeNiTi0.5 solid solution alloy with excellent mechanical properties, Mater. Lett. 62 (2008) 2673e2676. [29] F.J. Wang, Y. Zhang, G.L. Chen, Atomic packing efficiency and phase transition in a high entropy alloy, J. Alloy. Comp. 478 (2009) 321e324. [30] S. Ranganathan, Alloyed pleasures: multimetallic cocktails, Curr. Sci. 85 (2003) 1404e1406. [31] C.J. Tong, Y.L. Chen, S.K. Chen, et al., Microstructure characterization of AlxCoCrCuFeNi high-entropy alloy system with multiprincipal elements, Metall. Mater. Trans. A 36 (2005) 881e893. [32] L. Chen, F. Yuan, P. Jiang, et al., Mechanical properties and deformation mechanism of Mg-Al-Zn alloy with gradient microstructure in grain size and orientation, Mater. Sci. Eng. A 694 (2017) 98e109. [33] X. Yang, X. Ma, J. Moering, et al., Influence of gradient structure volume fraction on the mechanical properties of pure copper, Mater. Sci. Eng. A 645 (2015) 280e285. [34] X. Qiu, Corrosion resistance of Al2CrFeCoxCuNiTi high-entropy alloy coating in alkaline solution and salt solution, Results Phys 12 (2019) 1737e1741. [35] C.P. Lee, C.C. Chang, Y.Y. Chen, et al., Shih. Effect of the aluminium content of AlxCrFe1.5MnNi0.5 high-entropy alloys on the corrosion behaviour in aqueous environments, Corros. Sci. 50 (2008) 2053e2060. [36] William Fortune Smith, Structure and Properties of Engineering Alloys, McGraw Hill, New York, 1993. [37] C.M.H. Hagen, A. Hognestad, O.Ø. Knudsenb, et al., The effect of surface roughness on corrosion resistance of machined and epoxy coated steel, Prog. Org. Coat. 130 (2019) 17e23.

Please cite this article as: M. Li et al., Investigation on microstructure and properties of AlxCoCrFeMnNi high entropy alloys by ultrasonic impact treatment, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152626