Microstructure and corrosion behavior of AlCoCrFeNiSi0.1 high-entropy alloy

Intermetallics 114 (2019) 106599

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Microstructure and corrosion behavior of AlCoCrFeNiSi0.1 high-entropy alloy

T

C. Xianga,b, Z.M. Zhangb, H.M. Fuc, E.-H. Hanb,∗, H.F. Zhangc, J.Q. Wangb a

School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, China CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences (CAS), Shenyang, 110016, China c Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: High-entropy alloy Crystal structure Microstructure Segregation Corrosion Electrochemistry

The microstructure and corrosion behavior of AlCoCrFeNiSi0.1 high-entropy alloy are investigated. The alloy was prepared by vacuum arc melting and subsequent injection casting. Two different phases, including one disordered BCC (A2) phase and one ordered BCC (B2) phase, are identified in this alloy. The alloy shows a typical equiaxed dendritic microstructure, both of the dendritic and interdendritic regions have an (Al, Ni)-rich matrix with B2 structure and (Cr, Fe)-rich precipitates with A2 structure. The precipitates present different morphologies in the dendritic and interdendritic regions, whereas the chemical composition and crystal structure are very similar. With regard to the corrosion behavior in deaerated 3.5 wt% NaCl solution, the pits were formed at the dendrites, and the selective dissolution of the dendrites could be mainly ascribed to the lower Cr content and galvanic coupling between the dendrites and the interdendrites. The alloy exhibits an active-passive corrosion behavior in deaerated 0.5 mol/L H2SO4 solution. The preferential dissolution of the A2 phase is more severe than the B2 phase. The correlation between chemical composition, microstructure, and corrosion behavior is well illustrated.

1. Introduction The design of conventional alloys has been mainly based on one or two metallic elements. For example, steel is based on Fe, titanium alloys are based on Ti, aluminum alloys are based on Al, and so on. Meanwhile, other elements are often intentionally added to impart specific characteristics, such as mechanical strength or corrosion resistance, to the base metal. The limited number of principal elements in the periodic table hinders the exploration of new alloys designed by traditional strategies. Recently, a new kind of emerging advanced materials, which are termed as high-entropy alloys (HEAs) by Yeh et al. [1], and named by Cantor et al. [2] as multicomponent alloys, has drawn great attention from both academia and industries. HEAs are commonly defined as those alloys containing at least five major metallic elements with equimolar or near-equimolar concentrations between 5 and 35 at.% [1,3]. Since the possible combination of alloying elements in the periodic table is huge, this new alloy design strategy enables researchers to design and produce a vast amount of HEAs with desired properties such as unique microstructures [4–6], high strength [7–11], corrosion resistance [12–15], wear resistance [16–18] and irradiation resistance [19–23].

∗

It is noted that solid-solutions including body-centered cubic (BCC) structure [24,25] and face-centered cubic (FCC) structure [2,26] are often found in HEAs even with five or more elements. Among these reported HEAs, the microstructure and various properties of quinary AlCoCrFeNi alloy have been extensively studied [27–34]. The as-solidified AlCoCrFeNi alloy consists of a BCC phase with pronounced dendritic microstructure [28,35]. The effect of alloying elements like Ti [36], Mo [37], C [38] and Si [39] on the microstructure and properties of the AlCoCrFeNi alloy were investigated. The addition of Si significantly enhances the mechanical strength due to the solid solution of Si element and precipitation strengthening of nanoscale cellular structure but also reduces the ductility [39]. Nevertheless, the work on corrosion behavior of AlCoCrFeNi alloy system needs to be further investigated [40–44], which is crucial for its service in real conditions. In addition, the relationship between microstructure and corrosion behavior has not been elucidated comprehensively. Therefore, in this contribution, the authors focus on the microstructure and corrosion behavior of an AlCoCrFeNiSi0.1 HEA. The microstructure of the AlCoCrFeNiSi0.1 alloy was studied in detail. The corrosion behavior of the AlCoCrFeNiSi0.1 alloy was investigated and compared with a traditional 304 stainless steel (304ss). The relationship between the

Corresponding author. E-mail address: [email protected] (E.-H. Han).

https://doi.org/10.1016/j.intermet.2019.106599 Received 19 December 2018; Received in revised form 12 August 2019; Accepted 23 August 2019 0966-9795/ © 2019 Published by Elsevier Ltd.

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2. Experimental procedures

were prepared to ensure the reproducibility. After the potentiodynamic polarization measurement, the specimen was removed from the test solution and cleaned with distilled water. Then, the morphology of the corroded surface was observed by SEM.

2.1. Materials preparation and microstructural characterization

2.3. Scanning Kelvin probe force microscopy (SKPFM) measurements

The AlCoCrFeNiSi0.1 alloy ingots were prepared by vacuum arc melting in a water-cooled copper crucible under a Ti-gettered argon atmosphere. A mixture of the ultrasonically cleansed Al, Co, Cr, Fe, Ni and Si elements with purity higher than 99.5 wt % were weighted to obtain the nominal composition. The alloy ingots were flipped over and re-melted at least 5 times to ensure the chemical homogeneity. Each ingot has a weight of 80 g. Then a proper amount of ingots was put in a quartz tube and remelted using an induction heating coil under high vacuum argon atmosphere, and injected into a copper mold with dimension about 12 × 12 × 50 mm3. The crystal structure was identified with an X-ray diffractometer (X’PERT PRO) using Cu Kα radiation, operated at a voltage of 40 kV and a current of 40 mA. The scanning range is from 20 to 90° with a scanning rate of 4°/min. The microstructure was analyzed using a FEI XL30 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). Prior to SEM observation, the sample was mechanically ground with SiC abrasive papers, polished with diamond paste, and then etched with 1.5% HNO3 + 2.5% H2SO4 + 46% HCl +50% ethanol (mass percent) for 2 min. The detailed microstructure and localized chemical composition were further studied using a JEOL 2100 F transmission electron microscopy (TEM) operated at 200 kV equipped with an EDS. The TEM specimen was first mechanically ground down to a thickness of ~40 μm and then ion milled using a Gatan 691 ion polishing machine.

SKPFM techniques, which are based on scanning probe microscopy (SPM), can provide accurate measurement of the local contact potential difference (VCPD) between the atomic force microscopy (AFM) tip and the sample surface. The VCPD is defined as: VCPD = (Φtip − Φsample )/e, where Φtip and Φsample are the work functions (WFs) of the tip and the sample, and e is electronic charge [45]. Then the work function (WF) differences among various phases in an alloy can be obtained. The sample with a dimension of 8 × 8 × 2 mm3 for the SKPFM measurements was first mechanically ground with abrasive paper up to 3000# and then mechanically polished with 0.5 μm diamond paste, and finally polished with SiO2 polishing solution for 2 h. The SKPFM measurements were performed using a Dimension Icon AFM (Bruker Corporation, CA, USA) at room temperature in air. A Pt–Ir coated, electrically conductive tip (Model: SCM-PIT-V2, Bruker Corporation, CA, USA) with a resonant frequency of 75 kHz and a force constant of 3.0 N/m was used in the SKPFM measurements. During the SKPFM measurements, a dual-scan mode was used. The surface topography of the sample was recorded in a tapping mode during the first scan, and then the tip was lifted up 50 nm to obtain the VCPD signal. The scan rate was 0.5 Hz.

chemical composition, microstructure, and corrosion behavior was discussed.

3. Results and discussion 3.1. Crystal structure Fig. 1 shows the X-ray diffraction pattern of the as-cast AlCoCrFeNiSi0.1 alloy. Two different phases can be identified from the diffraction peaks. One is a disordered BCC (A2) phase with a lattice parameter of 2.877 Å. Another is an ordered BCC (B2) phase. The presence of superlattice peaks indicates that phase separation may exist in the AlCoCrFeNiSi0.1 alloy. Several studies already demonstrated that the ascast AlCoCrFeNi alloy consists of an A2 phase plus a B2 phase [28,31,46,47]. The current result indicates that the adding of a small amount of Si does not change the alloy's crystal structure. Wang et al. [28] also investigated the effect of Al addition on the microstructure evolution of AlxCoCrFeNi HEAs, and it is found that the structure changes from FCC to FCC + BCC, and finally to BCC with increasing Al content. Moreover, the addition of Al element promotes the formation of the ordered B2 phase in the AlxCoCrFeNi HEAs with increasing Al

2.2. Electrochemical measurements The samples for electrochemical tests with a dimension of 12 × 12 × 1 mm3 were cut from the bulk material by electric discharge machine. Each test specimen was welded with a copper line on the back and then cold mounted with epoxy resin. The exposed surface area was about 1.4 mm2 and 1.0 mm2 for the AlCoCrFeNiSi0.1 alloy and 304ss, respectively. The chemical composition of 304ss is given in Table 1. Prior to electrochemical measurement, the specimen was mechanically ground using SiC abrasive paper up to 2000# (Ra ~0.1 μm) and then cleaned with ethanol and distilled water. The electrochemical measurements were conducted in both 3.5 wt% NaCl and 0.5 mol/L H2SO4 solutions using a typical three-electrode system with a platinum sheet as the counter electrode, a saturated calomel electrode (SCE, −0.241 V versus the standard hydrogen electrode) as the reference electrode and a test specimen as the working electrode. The electrochemical measurements were performed at room temperature. The test solution was deaerated by bubbling high purity nitrogen gas for 30 min before the electrochemical measurements to eliminate the effect of dissolved oxygen. Potentiodynamic polarization measurements were made at a scan rate of 1 mV/s from an initial potential of −0.5 V to a final potential of 1.5 V versus the open circuit potential (OCP). Prior to the polarization measurement, the OCP was recorded for 30 min to reach a steady-state potential. Then, the specimen was cathodically polarized at −0.5 V versus the OCP for 5 min to remove the surface oxides. The potential was controlled and the current was measured using a potentiostat (Gamry Reference 600). In each test, at least three specimens Table 1 Chemical composition (wt.%) of the 304 stainless steel. Cr

Ni

Mn

Si

C

P

S

Fe

18.48

9.43

1.13

0.53

0.019

0.020

0.008

Bal.

Fig. 1. X-ray diffraction pattern of the as-cast AlCoCrFeNiSi0.1 alloy. 2

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Fig. 2. SEM images of the AlCoCrFeNiSi0.1 alloy; (a) the lower magnified image showing the equiaxed dendritic microstructure; (b)–(d) the higher magnified images showing the grain boundary adjacent to the interdendritic region, the cuboidal precipitates in the dendritic region and the intertwined precipitates in the interdendritic region.

distributed in both dendrites and interdendrites. Due to the small size of these precipitates, it is difficult to determine the accurate chemical composition by SEM-EDS. Hence, a detailed characterization of the chemical composition and crystal structure of the fine precipitates in the AlCoCrFeNiSi0.1 alloy was conducted by TEM. Figs. 3a and 4a show the scanning transmission electron microscopy (STEM) bright-field images for the dendritic and interdendritic regions, respectively. The corresponding selected area diffraction patterns (SADP) for these two regions are shown in Figs. 3b and 4b. The superlattice (as labeled by the yellow circle in Figs. 3b and 4b) was observed in both regions, which indicates the existence of an ordered BCC (B2) phase in the alloy. Since the SADP was taken from the area containing the two phases and the lattice parameters of the two phases are very close, the diffraction patterns of the A2 phase are overlapped with the B2 phase. These results are in good agreement with the literature [35,46,48,49]. The results show that both of the dendritic and interdendritic regions have a disordered A2 phase plus an ordered B2 phase. Furthermore, elemental mappings for Al, Co, Cr, Fe, Ni and Si in the dendritic and interdendritic regions are given in Fig. 3c–h and Fig. 4c–h, respectively. It can be clearly seen that the precipitates in the dendrites and the interdendrites, even with different morphologies, are enriched with Cr and Fe, while Al and Ni are preferentially segregated into the matrix. By comparison, Co is almost uniformly distributed in

content from x = 0.7 to x = 2.0. Therefore, the formation and structure of the B2 phase in the AlCoCrFeNiSi0.1 alloy are affected significantly by the Al element.

3.2. Microstructure analysis The SEM images of the as-cast AlCoCrFeNiSi0.1 alloy are shown in Fig. 2. The alloy has an equiaxed dendritic microstructure, and the bight dendritic regions and the grey interdendritic regions could be seen clearly (Fig. 2a). The different contrast should be ascribed to the elemental segregation between the dendritic and interdendritic regions. Fig. 2b presents the grain boundary which is adjacent to the interdendritic regions. Fig. 2c and d shows the higher magnified images of the dendrites and the interdendrites, respectively. It can be seen that fine precipitates with cuboidal shape are embedded within the dendrites, while the alternating precipitates about 50 nm in width are intertwined with the matrix in the interdendritic regions. The total composition of this alloy and the compositions for the dendrites and interdendrites are given in Table 2. It is noted that the overall composition of the AlCoCrFeNiSi0.1 alloy is very close to the nominal composition. The dendrite is rich in Al and Ni, while the interdendrite is enriched with Cr and Fe. The content of Si in the interdendrites is slightly higher than that in the dendrites. Co is almost uniformly Table 2 Chemical composition (at. %) of the AlCoCrFeNiSi0.1 alloy by SEM-EDS analysis. Region

Al

Co

Cr

Fe

Ni

Si

Nominal Overall Dendrites Interdendrites

19.60 19.35 ± 0.17 22.50 ± 0.48 17.83 ± 1.27

19.60 19.79 ± 0.24 19.63 ± 0.41 19.13 ± 0.20

19.60 19.45 ± 0.40 16.21 ± 0.43 21.34 ± 0.62

19.60 19.11 ± 0.37 17.95 ± 0.15 20.08 ± 0.45

19.60 20.31 ± 0.70 22.01 ± 0.53 18.93 ± 0.16

2.00 2.00 ± 0.10 1.70 ± 0.26 2.71 ± 0.16

3

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Fig. 3. (a) STEM bright-field image for the dendritic region of the AlCoCrFeNiSi0.1 alloy; (b) selected area diffraction pattern showing that the dendrite has a disordered BCC (A2) structure plus an ordered BCC (B2) structure. (c)–(h) corresponding elemental mappings of Al, Co, Cr, Fe, Ni, and Si.

bond between Al and Ni is very strong, therefore, Al and Ni tend to segregate together to form the matrix in the dendritic and interdendritic regions. The observed periodic, coherent microstructure suggests that a spinodal decomposition of a high temperature B2 phase may be responsible for the formation of the (Cr, Fe)-rich A2 phase and (Al, Ni)rich B2 phase in the dendrites and interdendrites [28,47,54]. However, Tang et al. [49] suggested that the formation of a nano-scale mixture of A2 and B2 phases may result from eutectic reaction rather than the spinodal decomposition based on the microstructure analysis and thermodynamic modeling. The accurate mechanism for the formation of this interesting nanoscale A2/B2 phases in the AlCoCrFeNi alloy system needs to be further investigated in future work. Furthermore, the different morphologies of the (Cr, Fe)-rich precipitates with similar compositions and structures in the dendritic and interdendritic regions may be resulting from the domains with different order and strain distribution [55].

these two distinct regions. The chemical composition determined from at least three points by TEM-EDS is listed in Table 3, and it is interesting to note that the compositions of the precipitates in the dendrites are almost the same with the precipitates in the interdendritic regions. Moreover, the (Al, Ni)-rich precipitates are often found to have an ordered BCC crystal structure as demonstrated in Refs. [35,46,50–52]. Combining the XRD, TEM-EDS results and SADP analysis, it is concluded that the precipitate rich in Cr and Fe has an A2 structure and the matrix rich in Al and Ni has a B2 structure. As mentioned above, the SEM-EDS results show that the contents of Cr and Fe in the interdendritic regions are higher than those in the dendritic regions, this could be attributed to the fact that the (Al, Ni)rich dendrites solidify first while Cr and Fe preferentially segregate into the liquid phase during the solidification process in the AlCoCeFeNi alloy [49]. The mixing enthalpy between Al and Ni is −22 kJ/mol [53], which is the most negative between Al and other alloying elements in the AlCoCeFeNiSi0.1 alloy. The most negative value implies that the

Fig. 4. (a) STEM bright-field image for the interdendritic region of the AlCoCrFeNiSi0.1 alloy; (b) selected area diffraction pattern showing that the interdendrite also has a disordered bcc (A2) structure plus an ordered BCC (B2) structure; (c)–(h) corresponding elemental mappings of Al, Co, Cr, Fe, Ni, and Si. 4

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Table 3 The chemical composition of the precipitates and the matrix in the dendrites (DR) and interdendrites (ID) of the AlCoCrFeNiSi0.1 alloy. The data was obtained by TEM/EDS analysis from at least three points. Region

Al

Co

Precipitates (DR) Matrix (DR) Precipitates (ID) Matrix (ID)

0.83 ± 0.13 15.49 ± 0.03 0.58 ± 0.06 13.08 ± 0.30

21.39 23.67 21.49 24.84

± ± ± ±

0.89 0.45 0.25 0.15

Cr

Fe

39.89 ± 0.11 6.04 ± 0.43 39.70 ± 0.06 5.29 ± 0.37

28.90 15.74 29.46 17.08

± ± ± ±

0.42 1.17 0.33 0.35

Ni

Si

5.93 ± 0.37 36.41 ± 0.93 6.48 ± 0.05 37.75 ± 0.28

3.07 2.67 2.30 1.98

± ± ± ±

0.86 0.25 0.13 0.19

Fig. 5. (a) Potentiodynamic polarization curves of the AlCoCrFeNiSi0.1 alloy and 304ss in deaerated 3.5 wt% NaCl solution; (b)–(c) corroded surface after the electrochemical test showing that the pits were formed at the dendrites, and the interdendritic regions remain unaffected.

solution. Fig. 5b shows that a large number of pits were formed after the potentiodynamic polarization test. The corroded regions (Fig. 5c) exhibit a flower-like morphology, which is similar to the microstructure shown in Fig. 2a. In addition, this corroded morphology is quite similar to the corroded AlCoCrFeNi alloy in 3.5 wt% NaCl solution in which the Cr-depleted dendrites were suffered from severe corrosion attack [41]. Fig. 6 depicts the elemental mappings of Cr, Fe, O, Al, and Ni from the corroded surface. Fig. 6b–d shows that the unaffected area is enriched with Cr, Fe and O, but depleted of Al and Ni, while Fig. 6e and f shows that Al and Ni are absent in this region. As stated previously in Section 3.2, the chemical composition analysis shows that the dendrite is enriched with Al and Ni, whereas the interdendrite is rich in Cr and Fe. Based on these results, it is concluded that the dendrites suffer serious corrosion attack in 3.5 wt% NaCl solution, while the interdendritic regions remain unaffected. The different corrosion behavior may result from the microstructural and compositional differences between the dendritic and interdendritic regions. Shi et al. [58] investigated the effect of Al content on the stable/metastable pitting of AlxCoCrFeNi (x = 0.3, 0.5 and 0.7) HEAs in 3.5 wt% NaCl solution, and the results

3.3. Potentiodynamic polarization measurements Fig. 5a presents the potentiodynamic polarization curves of the AlCoCrFeNiSi0.1 alloy and 304ss tested in deaerated 3.5 wt% NaCl solution. Both of the AlCoCrFeNiSi0.1 alloy and 304ss exhibit pseudopassive behavior in deaerated 3.5 wt% NaCl solution since the curves do not show a distinct primary passivation potential, whereas a pitting potential (Epit) and a transpassive region were observed [56]. The Epit is determined to be the potential at which the current density exceeds 100 μA/cm2 [57]. The corrosion potential (Ecorr) of the AlCoCrFeNiSi0.1 alloy is −372 mVSCE, which is lower than that of 304ss with an Ecorr of −331 mVSCE. The corrosion current density (icorr) for the AlCoCrFeNiSi0.1 alloy is 0.32 μA/cm2, which is slightly lower than that of 304ss with an icorr of 0.41 μA/cm2. In addition, the Epit of the AlCoCrFeNiSi0.1 alloy (62 mVSCE) is much lower than that of 304ss (372 mVSCE), which indicates that the pitting corrosion resistance of the AlCoCrFeNiSi0.1 alloy is inferior to 304ss in deaerated 3.5 wt% NaCl solution. Fig. 5b and c presents the corroded morphologies of the AlCoCrFeNiSi0.1 alloy after the potentiodynamic polarization test in deaerated 3.5 wt% NaCl 5

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Fig. 6. The elemental mappings of Cr, Fe, O, Al, and Ni from the corroded surface after the potentiodynamic polarization test in deaerated 3.5 wt% NaCl solution.

Fig. 7. (a) Potentiodynamic polarization curves of the AlCoCrFeNiSi0.1 alloy and 304ss in deaerated 0.5 mol/L H2SO4 solution; (b) corroded surface indicating the uniform corrosion of the AlCoCrFeNiSi0.1 alloy; (c)–(d) higher magnified images showing the preferential dissolution of the (Cr, Fe)-rich precipitates in the dendrites and the interdendritic regions. Table 4 Electrochemical parameters of the AlCoCrFeNiSi0.1 alloy and 304ss in deaerated 0.5 mol/L H2SO4 solution. Alloy

Ecorr (mVSCE)

icorr (A/cm2)

ipass (A/cm2)

Epp (mVSCE)

icri (A/cm2)

Eb (mVSCE)

ΔE (mVSCE)

AlCoCrFeNiSi0.1 304ss

−453 −438

304.20 × 10−6 105.12 × 10−6

19.35 × 10−6 4.47 × 10−6

−425 −336

161.21 × 10−6 1292.80 × 10−6

925 925

1350 1261

6

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Fig. 8. (a) and (c) are the AFM topography image and line profile of the interdendritic regions in the AlCoCrFeNiSi0.1 alloy, which show the intertwined microstructure. The A2 and B2 phases could be identified according to the results and discussions in Section 3.2. (b) and (d) are the VCPD map and line profile of the interdendritic regions in the AlCoCrFeNiSi0.1 alloy, the B2 phase has higher VCPD value than that of the A2 phase.

and passive current density, respectively [62]. It is noted that the AlCoCrFeNiSi0.1 alloy presents two distinct anodic peaks at −425 and −228 mVSCE in the active-passive transition region. This phenomenon is quite similar to the potentiodynamic polarization curves of the duplex stainless steel in the acidic solutions [64–66]. It is suggested that the peaks are related to the different dissolution behavior of the ferritic and austenitic phases at different potentials. With regard to the AlCoCrFeNiSi0.1 alloy, the distinct anodic peaks may relate to the different dissolution behavior between the A2 and B2 phases. It can be seen from Table 4 that the Ecorr of AlCoCrFeNiSi0.1 alloy is similar to that of 304ss, but the icorr is about three times higher than that of the 304ss. Moreover, the ipass of the AlCoCrFeNiSi0.1 alloy is also larger than that of 304ss. The breakdown potential with a value of 925 mVSCE for both alloys is the same. In addition, the AlCoCrFeNiSi0.1 alloy has a wider passive region than that of the 304ss. An attempt was immediately made to observe the corroded surface of the AlCoCrFeNiSi0.1 alloy after the potentiodynamic polarization tests. Fig. 7b shows that the uniform corrosion occurred and no pits were formed on the surface. Fig. 7c and d shows the higher magnified images of the dendrites and the interdendritic regions, respectively. Discontinuous, particulate corrosion products were found to present on the surface within the dendritic regions. Preferential dissolution occurred at the cuboidal sites in the dendrites as the arrows labeled. Likewise, the (Cr, Fe)-rich precipitates in the interdendritic regions are also preferentially dissolved rather than the (Al, Ni)-rich matrix. As discussed in Section 3.2, these cuboidal and alternating precipitates correspond to the (Cr, Fe)-rich precipitates with A2 structure. The different dissolution characteristics of the precipitates and the matrix are mainly attributed to the chemical inhomogeneity and different phase structures. Fig. 8 shows the AFM topography image and VCPD map of the

show that the increment of Al content deteriorates the localizes corrosion resistance for the increasing volume fraction of the Cr-depleted phase resulting in the thicker/dispersive passive films. The Al0.1CoCrFeNi alloy in the as-cast state [44] or as-equilibrated state [43] shows a higher Ecorr value and a lower icorr value in 3.5 wt% NaCl solution than the AlxCoCrFeNi (x = 0.3, 0.5 and 0.7) alloys, which indicated that the addition of Al deteriorates the corrosion resistance in 3.5 wt% NaCl solution. In addition, Kao et al. [27] also found that the addition of Al harms the corrosion resistance of AlxCoCrFeNi (x = 0, 0.25, 0.5 and 1.0) alloys in H2SO4 solution. Furthermore, it is noted that Al has the most negative standard electrode potential of −1.66 V with respect to the standard hydrogen electrode (SHE) [59]. The dissolution of the dendrite could be mainly attributed to the galvanic coupling between the dendritic and interdendritic regions, where the (Cr, Fe)rich interdendritic phase acts as the cathode and the (Al, Ni)-rich dendrites act as the anode. As a result, the dendrites dissolved gradually and the pits finally formed as indicated by Fig. 5b and c. The dendritic microstructure is frequently found in the as-cast HEAs, and the galvanic coupling between the dendritic and interdendritic regions often results in the selective dissolution in NaCl solution [41,60,61]. The potentiodynamic polarization curves for the AlCoCrFeNiSi0.1 alloy and 304ss in 0.5 mol/L deaerated H2SO4 solution is shown in Fig. 7a and the relevant electrochemical parameters are summarized in Table 4. Both alloys exhibit an active-passive corrosion behavior. A passive region and a transpassive region were observed. The primary passivation potential (Epp) is determined as the potential at which the anodic current density reaches the maximum value in the active region [62]. The breakdown potential (Eb) is defined as the potential at which the current density suddenly increases above the passive region [12]. The passive region width (ΔE) was calculated from the difference between Eb and Epp [63]. The icri and ipass represent critical current density 7

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Fig. 9. (a) and (c) are the AFM topography image and line profile of the dendrites in the AlCoCrFeNiSi0.1 alloy. The bright particulate phase is the A2 phase. (b) and (d) are the VCPD map and line profile of the dendritic regions in the AlCoCrFeNiSi0.1 alloy.

occurred at a higher potential (−228 mVSCE), while the dissolution of B2 phase occurred at a lower potential (−425 mVSCE). When the potentials are above −228 mVSCE, both phases turn into the passive state. It is noted that the anodic current density at −228 and −425 mVSCE is 2.98 and 0.16 mA/cm2, respectively. The considerable higher anodic current density at higher potential indicates that the preferential dissolution of the (Cr, Fe)-rich precipitates is more severe than that of the (Al, Ni)-rich matrix. That is exactly what has been observed in the dendrites (Fig. 7c) and the interdendritic regions (Fig. 7d) after the passive film is broken down when the potential is higher than the breakdown potential. The corrosion behavior of the AlCoCrFeNiSi0.1 alloy in both deaerated 3.5 wt% NaCl and 0.5 mol/L H2SO4 solutions are intensively influenced by the different chemical compositions and phase structures. The results are similar to the corrosion behavior of Tibased metallic glass matrix composites [72,73], which exhibits a dendritic microstructure containing a (Zr, Cu)-rich amorphous matrix and a (Ti, V)-rich BCC dendrite. Based on the aforementioned results, it is imperative to enhance the alloy's chemical homogeneity for improvement of its corrosion resistance.

interdendritic regions in the AlCoCrFeNiSi0.1 alloy. The dark A2 phase and the bright B2 phase could be recognized according to the aforementioned results and discussions in Section 3.2. Fig. 8b and d shows that the VCPD value of the B2 phase is higher than that of the A2 phase. The topography image and VCPD map of the dendrites in the AlCoCrFeNiSi0.1 alloy are shown in Fig. 9. The A2 and B2 phases could be recognized in Fig. 9a. However, the VCPD difference between the A2 and B2 phases could not be clearly distinguished from the VCPD map (Fig. 9b), which may be ascribed to the small size of the cuboidal precipitates, and/or the limited AFM tip resolutions. Recently, Shi et al. [13] studied the homogenization effect of heat treatment on the microstructural evolution and corrosion behavior of AlxCoCrFeNi (x = 0.3, 0.5 and 0.7) HEAs. The SKPFM measurements were carried out to characterize the relative nobility of A2 and B2 phases in the Al0.5CoCrFeNi and Al0.7CoCrFeNi alloys. Shi et al. [13] found that the measured VCPD values of the (Co, Cr, Fe)-rich A2 precipitates are lower than that of the (Al, Ni)-rich B2 matrix. Based on the results shown in Fig. 8b and Ref. [13], it is believed that the A2 phase has a lower VCPD value than that of the B2 phase in the current AlCoCrFeNiSi0.1 alloy. Since a lower value of the VCPD implies a higher WF, which reflects the electronic energy level of the alloy surface [67]. The WF is closely related to the Volta potential in ultra-high-vacuum [68]. Stratmann et al. [69,70] and Schmutz et al. [71] found that the Volta potential measured in air has a linear relationship with the corrosion potential in aqueous solution. Therefore, the WF is suggested to be a measurement of the relative nobility of the various phases in an alloy. The lower WF of a phase, the lower the corrosion potential is. The phase with a lower WF is prone to corrosion than a phase with a higher one. The abovementioned results show that the A2 phase has a higher WF of the B2 phase, indicating the higher corrosion potential of the A2 phase. This result is in agreement with the potentiodynamic polarization curve (Fig. 7a) and the corroded morphologies (Fig. 7c and d). The distinct anodic peak (Fig. 7a) indicates that the dissolution of the A2 phase

4. Conclusion The microstructure and corrosion behavior of AlCoCrFeNiSi0.1 highentropy alloy were studied in the present work, and the following conclusions can be reached: (1) The AlCoCrFeNiSi0.1 alloy consists of a disordered BCC phase plus an ordered BCC phase. (2) The AlCoCrFeNiSi0.1 alloy exhibits a typical equiaxed dendritic microstructure, both of the dendritic and interdendritic regions have an (Al, Ni)-rich matrix with ordered BCC structure and (Cr, Fe)-rich precipitates with disordered BCC structure. The precipitates present cuboidal shape and intertwined morphology in the 8

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dendritic and interdendritic regions, respectively. The chemical composition and crystal structure are similar for the precipitates independent of the locations. (3) The AlCoCrFeNiSi0.1 alloy and 304ss exhibit pseudo-passive behavior in deaerated 3.5 wt% NaCl solution. The pits formed at the dendrites, which are mainly attributed to the lower content of Cr and thus the galvanic coupling with the interdendrites. (4) The AlCoCrFeNiSi0.1 alloy shows active-passive corrosion behavior in deaerated 0.5 mol/L H2SO4 solution. Two distinct anodic peaks exist in the active-passive transition region which is associated with the dissolution characteristics of the (Cr, Fe)-rich precipitates at a higher potential and the (Al, Ni)-rich matrix at a lower potential. The corroded morphologies of the precipitates and the matrix are well related to the respective magnitude of the anodic current densities. The different dissolution characteristics of the precipitates and the matrix are mainly attributed to the chemical inhomogeneity and different phase structures.

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