Introduction

High-entropy alloy (HEA) is a novel material that was first defined in 2004 by Cantor et al. (Ref 1). Unlike traditional alloys, HEA was formed by more than five main elements, the concentration of each element ranging from 5 to 35 at.% (Ref 1,2,3). Because of the high mixing entropy effect (Ref 4), HEA tends to create solid solutions like face-center cubic (FCC), body-center cubic (BCC), hexagonal close-packed (HCP) rather than intermetallic phases (Ref 5,6,7). A large number of researches show that HEAs are suitable for use as coating materials, which resulted from the high hardness (Ref 8), high thermal stability (Ref 9), and excellent wear-corrosion resistance of HEAs (Ref 10,11,12). Recently, many methods are used to fabricate HEA coatings, such as plasma spray (Ref 13, 14), cold spray (Ref 15), magnetron sputtering (Ref 16), and laser cladding (Ref 17,18,19). Compared to other methods, the laser cladding technique shows many advantages, such as rapid cooling rate, good metallurgical bond, and refined microstructure. In addition, the process parameters can be flexibly adjusted to achieve coatings over a large area, and the repeat process can vary the thickness of the coating to suit the practical applications (Ref 20). Ye et al. (Ref 18) fabricated the AlxFeCoNiCuCr HEA coating on AISI 1045 substrate using laser cladding. They demonstrated that the coatings consisted of FCC and BCC solid solutions, and increasing aluminum content can impel the creation of the BCC structure. As a result, the alloy with higher Al atomic content exhibits higher hardness, better wear, and corrosion resistance. On the aluminum substrate, Shi et al. (Ref 19) also successfully fabricated a duplex-phase FCC/BCC AlCrFeNiCuCo HEA coating using the laser cladding technique. The microhardness of the coating reached 500 HV0.2, and the coating shown good corrosion resistance in the H2SO4 solution.

However, most of the designed HEAs contain a large amount of the expensive element Co, and thus, the increase in cost has limited their commercial application. Recently, low-cost Co-free HEAs also have been investigated. Ren et al. (Ref 21) investigated the corrosion behavior of CuCrFeNiMn HEA system and showed that its corrosion resistance increases with decreasing Cu content. The single FCC-phase Cu0.5CrFeNiMn alloy displays a better corrosion resistance in 1 mol/L H2SO4. Chen et al. (Ref 22) used Mn instead of Co in the AlCrCuFeCoNi HEA system. As a result, Mn impels the formation of the BCC structure and prevents Cu segregation into the interdendritic region.

Up to now, the Al-Fe-Mn-Ni-Cr-Cu HEA coating system has not been reported. In this work, we fabricated a novel low-cost Co-free AlxFeMnNiCrCu0.5 (x = 0.0, 0.5, 1.0) HEA coating using laser cladding technique. Previous studies have shown that copper is prone to segregation because the enthalpy value of binary between copper and other elements is more positive (Ref 23, 24). In order to relieve the compositional segregation in these alloys, we halve copper content compared to other elements. The effects of Al content on phase evolution, microstructure, hardness, corrosion resistance of the AlxFeMnNiCrCu0.5 HEA coatings were investigated.

Experimental Procedures

Material and Methods

The HEA coatings were synthesized on the 1045 steel plate with a size of 100×100×10 mm. The Al, Cu, Cr, Mn, Fe, Ni powder (purity ≥ 99.95%, particle size: 50 µm-150 µm) were used as raw materials and well mixed according to the molar ratio of AlxFeMnNiCrCu0.5. (x values are the molar ratio, x = 0.0, 0.5, 1.0. The samples were denoted as Al0.0, Al0.5, and Al1.0, respectively.) The powder mixture of 30 g was mixed for about 10 h in a MSK-SFM-1 planet ball milling machine (Hefei Kejing Materials Technology Co., Ltd., Anhui, China) with a ball-to-powder ratio of 3:5 and dried in a drying cabinet at 60° for 2 h. The dried powder mixture was pre-coated on the substrate with a layer thickness of 1.0 mm. The powder contents of each element in the powder mixture of 30 g are shown in Table 1. Figure 1 shows the morphology of Al0.5FeMnNiCrCu0.5 powder after milling. The schematic diagram of the laser cladding process is shown in Fig. 2(a), and a RC-LCD-1000-R laser processing system (Zhongke Raycham Laser Technology Co., Ltd., Nanjing, China) was used to fabricate a single-track, multi-track cladding layers on a steel substrate. Ar gas was used to prevent oxidation during the fabrication process. The optimized processing parameters were selected as follows: laser cladding power of 800 W, the scanning rate of 8 mm/s, the laser beam diameter of 3 mm, and track overlap ratio of 35%, respectively. Single-track layers with a length of 30 mm and multi-track layers with dimensions of 30 mm × 40 mm were fabricated in this research (Fig. 2b).

Table 1 The powder mass of each element in the powder mixture of 30 g, g
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Fig. 1
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The morphology of Al0.5FeMnNiCrCu0.5 powder mixture

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Fig. 2
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(a) Schematic diagram of the laser cladding process, (b) The image macrography of single-track and multi-track clad layer

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Microstructural Examination

The SEM and chemical composition of the coatings were carried out by SEM-FEI Quanta 250F system with energy-dispersive spectrometry (EDS). For microstructural observation, the metallographic samples were ground with SiC paper from 80 grit to 1000 grit, polished with a diamond polishing agent, and etched in an HNO3-HCl-CH3COOH-H2O solution. The coating crystalline structures were investigated using x-ray diffraction (XRD-6100, Shimadzu) at a scanning rate of 4°/min in the scan range from 30° to 100°.

Hardness and Electrochemical Testing

The Vickers microhardness was performed on the surface coatings using a Vickers hardness tester by applying a testing force of 1.96 N and a 15 s dwell time. The measured value is the average of the five different points.

The electrochemical properties of the coatings were analyzed in a 0.5  mol/L H2SO4 solution using an electrochemical workstation (IM6, Zahner Zennium, Germany) at room temperature. The samples were cut to a surface dimension of 1 cm2 for an electrochemical test. Then, they were mounted using epoxy resin with an exposed area of 1 cm2 at room temperature. Before the test, the sample surface was ground with SiC paper from 80 grit to 1000 grit, polished with a diamond polishing agent, and rinsed in acetone for 5 min. The potentiodynamic polarization test was achieved by a typical three-electrode system. The first electrode is the working electrode, which makes contact with a specimen; the second is a platinum foil (counter electrode); and the third is a saturated calomel (reference electrode). The open-circuit potential (OCP) was conducted for 30 min before the electrochemical test. The potential scan range is from − 1.0 V to + 1.2 V, and the scan speed of 1 mVs−1. The selected frequency range is 100 kHz to 0.01 Hz at an amplitude of 10 mV for the EIS measurement. The impedance data were analyzed using Zman software.

Results and Discussion

Phase and Microstructure Analysis

Figure 3 reveals the XRD profiles of the AlxFeMnNiCrCu0.5 HEA coatings with different Al contents. When there is the increase in Al content, the BCC structure was formed and gradually became the dominant phase. Only the FCC phase was observed when the Al is not added (x = 0.0). When x = 0.5, the diffraction peak of the FCC phase decreases, and a new BCC peak appears, forming a duplex-phase FCC/BCC structure. When x = 1.0, the coating fully consists of the BCC structure. Similar phenomena have been reported in as-cast FeCoNiCrCu0.5Alx HEA (Ref 25), laser-clad FeCoCrNiAlx (x = 0.3, 0.7) HEA coatings (Ref 26), and as-cast AlxCoCrFeNi HEA (Ref 27). They confirmed that the formation of the BCC phase resulted from an additional Al in HEAs. Thus, laser-clad AlxFeMnNiCrCu0.5 HEA coatings with the variation of Al content mainly consist of FCC phase, BCC phase, or mixed phases. This result was also observed for plasma-sprayed FeCoNiCrSiAlx HEA coatings, where there is a large amount of BCC phase with a small fraction of the FCC phase (Ref 28). However, it is worth noting that there seems to be no phase transition of plasma-sprayed FeCoNiCrSiAlx HEA coatings with the increase in Al content from 0.5 to 1.5.

Fig. 3
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X-ray diffraction analysis results of HEA coatings with different Al contents

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Figure 4 reveals the typical transverse sectional morphology of the single-track AlxFeMnNiCrCu0.5 HEA coatings. Four obvious zones could be observed, including the substrate, the heat-affected zone, the bonding zone, and the cladding zone. No cracks or any other defects were detected. The characteristics of the four regions are very different and easy to recognize.

Fig. 4
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The transverse sectional morphology of the single-track AlxFeMnNiCrCu0.5 HEA coatings

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Figure 5 illustrates the microstructure of AlxFeMnNiCrCu0.5 HEA coatings. A planar structure was at the bottom of the cladding zone in Fig. 4(a), (d), and (g), which was about 7-9 μm of width between the substrate and the coating. And then, the microstructure gradually changes from planar to columnar structure. According to the constitutional supercooling theory (Ref 29), the solidification rate and temperature gradient decide the microstructure characteristic of the cladding layer. At the bonding zone, the temperature gradient value is maximum, while the solidification rate is minimum, and a planar structure is formed between the substrate and the coating (Ref 30). With the increase in distance from the bottom of the molten pool, the solidification rate increased, while the temperature gradient decreased, which resulted in the columnar crystal structures. Near the middle of the cladding zone, the temperature gradient decreases quickly, and the heat dissipation direction distributes evenly in the molten pool. Thus, the dendritic structure formed in the middle and top areas of the coatings as shown in Fig. 5(b) and (c) for Al0.0 coating, in Fig. 5(e) and (f) for Al0.5 coating, and in Fig. 5(h) and (i) for Al1.0 coating, respectively. Besides, Fig. 5(e) and (f) shows alternately bright and dark microstructures in the Al0.5 coating. The distinct contrast of the phases appears in the image, indicating the existence of two phases.

Fig. 5
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Microstructures at the bottom, middle, and top of the cladding zone. Al0.0 coating (a, b, c); Al0.5 coating (d, e, f); and Al1.0 coating (g, h, i).

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Table 2 shows the EDS point analysis results at different zones of the coatings (marked with an arrow in Fig. 6). A and B points in Fig. 6(a) and (c), respectively, correspond to the dendritic and interdendritic position of these Al0.0 and Al1.0 coatings, while they correspond to the bright region and dark region of Al0.5 coating in Fig. 5b, respectively. The Cu element tends to segregate in the intergranular. This result also mentioned in previous studies of as-cast AlCrCuFeMnNi (Ref 31) and AlxCr0.4CuFe0.4MnNi (Ref 14) HEA systems. The positive mixing enthalpies between Cu and other elements can explain this phenomenon (Ref 32). Besides, due to the low melting point of Al, Al powder was partly burned and evaporated under irradiation of high-energy laser beams. As a result, the Al content is lower than the nominal value. The Fe content in the coating is higher than other elements, which can be attributed to the dilution of Fe in the steel substrate during the laser process.

Table 2 Chemical composition of the HEA coatings
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Fig. 6
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Microstructures at middle of the cladding zone with EDS analysis. (a) Al0.0 coating; (b) Al0.5 coating; and (c) Al1.0 coating.

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According to the EDS test results, the A area (dark phase) in a duplex phase of the Al0.5FeMnNiCrCu0.5 coating enriched Cr-Fe. The CrFe-rich phase has been confirmed as a disordered BCC phase-A2 structure by previous research in HEAs systems such as AlCrCuFeMnNi (Ref 31) and AlCrFeCuNix (Ref 33). Therefore, it can conclude that the A area (dark phase) with CrFe-rich is the BCC phase, and the B area (bright phase) is the FCC phase. It is noteworthy that the phase constitution of as-cast HEAs with increasing Al content often exists a Ni-Al-rich BCC phase, which is defined as ordered BCC phase-B2 structure (the diffraction peak usually appeared at 2\(\theta\) approximately 31°), for instance, as-cast AlxCoCrFeNi alloys (Ref 28) and as-cast AlCrCuFeMnNi alloy (Ref 31). However, the ordered BCC phase did not appear in the Al0.5 and Al1.0 coatings and only the disordered BCC phase was observed. This phenomenon is also reported by Chao et al. (Ref 34) for AlxCoCrFeNi (x = 0.3, 0.6, 0.85) HEA coatings. The compositional variation caused by dilution and distinct manufacturing conditions in laser cladding is probably responsible for the phase constitution difference. The dilution of composition in the clad coatings is shown in Table 2. Compared with the nominal design, it is evident that the Fe content is higher and the Al, Ni content is lower. In addition, the high solidification rate during laser processing resulted in formation of a non-equilibrium microstructure as a disordered phase within the coating (Ref 34, 35). Meanwhile, the solidification of as-cast HEA occurred under conditions of high temperature and long-time heating. These gave the elements enough time to diffuse and form an ordered phase.

Microhardness of HEA Coatings

The average microhardness of the surface HEA coatings and substrate is shown in Fig. 7. From these results, it can be seen that the microhardness of the AlxFeMnNiCrCu0.5 HEA coatings increases significantly from Al0.0 coating of 193 HV0.2 to Al1.0 coating of 541 HV0.2 with the increment of Al content. This trend is also reported in other laser-clad HEA coatings as FeCoCrNiAlx (Ref 26, 34), and as-cast HEAs as AlxCrCuFeMnNi (Ref 22), AlxCoCrFeNi (Ref 27). Previous studies have shown the increase in hardness when the addition of Al is through the phase transition and solid-solution strengthening. According to the XRD results indicated, the addition of Al content impels the FCC phase transition to the BCC phase, and the microhardness of the BCC phase is higher than that of the FCC phase (Ref 24). Thus, along with the increase in aluminum contents, the BCC phase fraction increases, leading to an increase in hardness. Also, the Al element has a larger atomic radius than other elements. The more aluminum content, the more severe the lattice distortion, thus causing a solid-solution strengthening effect and increasing the microhardness of coatings.

Fig. 7
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Average microhardness chart of AlxFeMnNiCrCu0.5HEA coatings

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The microhardness of the laser-clad Al1.0FeMnNiCrCu0.5 coating (541 HV) is nearly three times higher than that of the steel substrate (182 HV), which is obviously higher than those of the other HEA coatings with relatively similar compositions, for instance, plasma-sprayed AlCrFeCoNi (421 HV) (Ref 13, 36), FeCoNiCrSiAl1.0 (439 HV) (Ref 28), cold-sprayed AlCoCrFeNi (388 HV) (Ref 37). As mentioned above, Al1.0 coating contains high aluminum content, and its constituent phase is completely hard BCC phase, which is the main reason for the high hardness of this coating. This suggests that the laser-clad Al1.0FeMnNiCrCu0.5 coating is potential for use as a wear-resistant coating.

Electrochemical Properties

The Tafel curves of AlxFeMnNiCrCu0.5 HEA coatings and steel substrate are shown in Fig. 8, and the electrochemical data are given in Table 3. The results clearly show that the 1045 steel substrate exhibits the worst corrosion resistance because it has the most negative corrosion potential (Ecorr) and the highest corrosion current (Icorr) value. For the AlxFeMnNiCrCu0.5 HEA coating samples, the Al0.0 coating has the smallest Icorr value (1.39 × 10−5 A cm−2) and the highest Ecorr value (− 0.25 V); the Al0.5 coating has the highest Icorr value (5.06 × 10−4 A cm−2) and the smallest Ecorr value (− 0.42 V); and the electrochemical parameter values of the Al1.0 coating are between two alloys. Therefore, the corrosion resistance of three coatings in descending order is Al0.0 coating (Icorr = 1.39 × 10−5 A cm−2), Al1.0 coating (Icorr = 1.22 × 10−4 A cm−2), and Al0.5 coating (Icorr = 5.06 × 10−4 A cm−2), respectively. Furthermore, the HEA coatings reveal a passive region in the H2SO4 solution, which shows that the coatings have a passive tendency.

Fig. 8
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Tafel curves of AlxFeMnNiCrCu0.5 HEA coating, and steel substrate

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Table 3 Electrochemical parameters of AlxFeMnNiCrCu0.5 HEA coatings and steel substrate
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The anticorrosion of the duplex phase of Al0.5 coating is much lower than that of the single-phase Al0.0 and Al1.0 coatings. This phenomenon could be attributed to the formation of galvanic corrosion between the FCC phase and BCC phase. In traditional stainless steel, the corrosion resistance of ferritic stainless steels (such as 409 and 430 SS with BCC α - iron structure) is generally lower than that of austenitic stainless steel (such as 316 and 304 SS with FCC γ - iron structure) (Ref 38). This may explain why Al0.0 alloy with the FCC phase structure exhibits good corrosion resistance compared to Al1.0 alloy with a BCC phase structure. Furthermore, an as-cast alloy of the same composition Cu0.5CrFeNiMn (Ref 21) also shows good corrosion resistance in the H2SO4 solution. Thus according to the results above, the corrosion resistance of coating first decreases and then increases with increasing aluminum content.

The surface morphologies of three HEA coatings after the potentiodynamic polarization test in 0.5 mol/L H2SO4 solutions are shown in Fig. 9. It can be seen that all surfaces of the samples were eroded at the grain boundaries. Figure 9(b) and (c) shows the pitting corrosion phenomenon on the Al0.5 and Al1.0 coating surface, especially on the Al0.5 coating. For the Al0.0 coating, its surface is smooth without pitting corrosion. This is more clearly confirmed in the cyclic polarization curve of the Al0.0 coating, as shown in Fig. 10. The Al0.0 coating does not have pitting corrosion in 0.5 mol/L H2SO4 solution due to a negative hysteresis loop appearing on the curve (Ref 39). These results show the outstanding corrosion resistance of the Al0.0 coating compared to the other coatings.

Fig. 9
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The SEM images of AlxFeMnNiCrCu0.5 HEA coating after the potentiodynamic polarization. (a) Al0.0 coating, (b) Al0.5 coating, (c) Al1.0 coating

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Fig. 10
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Cyclic polarization curve for Al0.0 HEA coating in 0.5 mol/L H2SO4 solution.

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The EIS test further confirmed the corrosion properties of the HEA coatings. Figure 11 shows the Nyquist plot of three samples. It can be seen that the Nyquist plot of Al0.0 coating reveals a big capacitive loop at the high frequency and a tilted line at the lower-frequency range correspond to the shape of a finite-length Warburg. Meanwhile, those of Al0.5 and Al1.0 coatings reveal a capacitive loop at the high frequency and an inductive loop at the lower-frequency range. The presence of a capacitive loop has corresponded to the electrolyte-coating double layer (Ref 40), the inductive loop shows the occurrence of pitting corrosion on the surface of coating (Ref 41), and the tilted line is related to a diffusion process (Ref 42).

Fig. 11
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The Nyquist plots of AlxFeMnNiCrCu0.5 HEA coatings

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Figure 12 shows the Bode lines and fitted plots of HEA coatings. The value of \(\left|Z\right|\) at low frequency (f = 0.01 Hz) is directly proportional to the corrosion resistance (Ref 43). The value of \({\left|Z\right|}_{f=0.01\mathrm{Hz}}\) is 410 Ω·cm2 of Al0.0 coating, 107 Ω·cm2 of Al1.0 coating, and 18 Ω·cm2 of Al0.5 coating, respectively. The EIS results show that the Al0.0 coating with FCC structure has the best corrosion resistance, followed by the Al1.0 coating with BCC structure of, and the worst is the Al0.5 coating with the duplex FCC/BCC structure. This result is also consistent with the polarization test results.

Fig. 12
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Bode lines and fitted plots of AlxFeMnNiCrCu0.5 HEA coatings (a) Al0.0 coating, (b) Al0.5 coating, (c) Al1.0 coating.

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Figure 13 shows the equivalent circuit models of HEA coatings, and the fitting parameters are revealed in Table 4. The Al0.0 coating corresponds to the circuit model in Fig. 13(a). The Al0.5 and Al1.0 coatings have the same circuit model (Fig. 13b). Rs is the solution resistance, CPE is the constant phase element related to the electrolyte-coating double layer, and Rct is the charge-transfer resistance. As the above results are obtained, the diffusion process in the Al0.0 coating is fitted by the equivalent circuit where Rd is the diffusion resistance, and W is the Warburg impedance (Fig. 13a), while the equivalent circuit fits the pitting corrosion process in the Al0.5 and Al1.0 coatings where L is the inductor element, and RL is its resistance. The Cdl in both circuits is the double-layer capacitance of the interface at or near the coating–substrate interface (Ref 44). Rct is used to characterize the resistance of reactions occurring on the electrode surface. The Al0.0 coating possesses the Rct value of 189.67 Ω·cm2, which is about 2-5 times that of other coatings. This means that the resistance to electrode reaction occurring on the surface of Al0.0 coating is better than that of other coatings. The RL values of Al0.5 coating and Al1.0 coating are 28.8 Ω·cm2 and 88.7 Ω·cm2, respectively. A smaller RL value of the Al0.5 coating indicates that the Al0.5 coating is more sensitive to pitting corrosion. This result is consistent with the outcome of SEM images after the electrochemical test in Fig. 9.

Fig. 13
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Equivalent circuits of (a) Al0.0 coating; (b) Al0.5 and Al1.0 coating.

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Table 4 Equivalent circuit fitting parameters for EIS measurements
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Conclusions

AlxFeMnNiCrCu0.5 HEA coating could be successfully prepared on the 1045 steel substrate using a laser cladding technique with mixed powders. The phase structure of the AlxFeMnNiCrCu0.5 HEA coatings mainly consists of FCC or BCC structures. The addition of Al content could promote the process of forming the BCC phase. The AlxFeMnNiCrCu0.5 HEA coatings from the FCC phase (x = 0.0) transform into a duplex-phase FCC/BCC (x = 0.5) and then to the BCC phase (x = 1.0). The microhardness of HEA coatings increased with increasing the Al content. The maximum microhardness of Al1.0FeMnNiCrCu0.5 HEA coating reached up to 541 HV0.2, which was about three times higher than that of 1045 steel. The electrochemical results show that in a 0.5 mol/L H2SO4 solution, the AlxFeMnNiCrCu0.5 HEA coatings exhibited a good corrosion resistance. In this investigation, the FeMnNiCrCu0.5 and the Al1.0FeMnNiCrCu0.5 HEA coatings showed the best corrosion resistance and highest hardness, respectively.