Microstructure and Properties of Laser Surface Remelting AlCoCrFeNi 2.1 High-Entropy Alloy

: In this study, laser surface remelting of an AlCoCrFeNi 2.1 high-entropy alloy was performed using a Yb:YAG laser. The effects of laser surface remelting on the phase structure, microstructure, Vickers hardness, frictional wear properties, and corrosion resistance of the high-entropy alloy were investigated. The remelted layer of the AlCoCrFeNi 2.1 high-entropy alloy was produced by remelting at 750 W laser power and formed a good metallurgical bond with the substrate. The X-ray diffraction results showed that the 750 W remelted layer consisted of face-centered cubic and body-centered cubic phases, which were consistent with the phases of the as-cast AlCoCrFeNi 2.1 high-entropy alloy, and a new phase was not generated within the remelted layer. Laser surface remelting is very effective in reﬁning the lamellar structure, and the 750 W remelted layer shows a ﬁner lamellar structure compared to the matrix. The surface hardness and wear resistance of the AlCoCrFeNi 2.1 high-entropy alloy were substantially improved after laser surface remelting. In a 3.5 wt.% NaCl solution, the laser-remelted surface had a larger self-corrosion potential and smaller self-corrosion current density, and the corrosion resistance was better than that of the as-cast high-entropy alloy.


Introduction
The conventional alloy design concept usually uses one or two metallic elements as the main constituent elements and adds a small amount of other metallic or nonmetallic elements for alloying to achieve the required properties for practical applications. However, this design approach with one or two elements as the main elements severely limits the number of alloy systems available. In 2004, Yeh et al. [1] broke away from the traditional alloy design concept and proposed the use of five or more elements in equal or near-atomic ratios to smelt the alloy, which was named a "high entropy alloy (HEA)".
The single-phase face-centered cubic (FCC) HEA (CoCrFeNiMn) has good plasticity but low strength [2], while the single-phase body-centered cubic (BCC) HEA has high strength but poor plasticity [3]. Lu et al. [4] combined the advantages of FCC and BCC phase high-entropy alloys and designed a two-phase AlCoCrFeNi 2.1 eutectic high-entropy alloy (EHEA) based on eutectic composition point ratios to achieve a balance of strength and plasticity. The remarkable combination of tensile fracture stress (1186 MPa) and tensile ductility (22.8%) was achieved in the cast EHEA. Bhattacharjee et al. [5] have studied the effect of low-temperature deformation conditions on the tensile properties of AlCoCrFeNi 2. 1 HEAs, in addition, a large number of researchers have studied the effects of cold rolling [6,7] and hot rolling [8] on the microstructure and mechanical properties of AlCoCrFeNi 2. 1 HEAs, but there are few studies on the surface modification of AlCoCrFeNi 2.1 HEAs.
Laser surface remelting (LSR) [9,10] is a surface modification technique that can refine the grain, homogenize the material organization, and reduces the defects and cracks so that the surface properties of the material can be greatly improved. Miao et al. [11] investigated the effects of laser surface remelting treatment and aging heat treatment on the microstructure, hardness, and wear resistance of AlCoCrFeNi 2.1 HEAs. Due to the laser quenching effect of laser surface remelting treatment and the precipitation of nanoscale particulate matter after aging, heat treatment enhanced the hardness and wear resistance of the alloy, but they did not investigate the surface corrosion resistance of the material, which is very important as it relates to the environment in which the material is applied. Hu et al. [12] examined the corrosion resistance of remelted layers by laser surface remelting of FeCrNiMnMo x (x = 0, 0.5, 1) alloys using a Nd:YAG laser. The results showed that the dendritic structure of the remelted alloy was significantly refined compared with the matrix due to the combined effect of Mo alloying and rapid solidification of the remelted surface, and the addition of Mo accelerated the formation of Cr 2 O 3 , improved the stability of the passivation film, and significantly improved the corrosion resistance in a 3.5% NaCl solution. In addition, Shuang et al. [13] investigated the corrosion resistance of the ascast and annealed FeCrNiCoNb 0.5 eutectic high-entropy alloy, and after 6 h of annealing treatment at 1200 • C, the nanoscale lamellar structure was significantly coarsened, leading to a decrease in corrosion resistance. Considering that laser surface remelting can refine the microstructure and whether it can further improve the corrosion resistance of the ascast AlCoCrFeNi 2.1 HEA, therefore, in this paper, we use laser remelting technology to treat the surface of the AlCoCrFeNi 2.1 HEA and to investigate the effects of laser surface remelting on the phase structure, microstructure, microhardness, friction wear properties and corrosion resistance of the high-entropy alloy.

Materials and Methods
The AlCoCrFeNi 2.1 high-entropy alloy was prepared by vacuum arc melting, and the specimen was cut out of the block with a size of 50 mm × 20 mm × 10 mm by wire cutting, and the surface of the block was polished to remove the oxide skin, ultrasonically cleaned with anhydrous ethanol, and then blown dry with cold air. Laser surface remelting was carried out using a TruDisk 6002 laser with a laser power of 750 W and a scanning speed of 20 mm/s. Argon was used as the protective atmosphere, and the flow rate of the protective gas was 15 L·min −1 .
A wire cutter was used to cut out the block specimen after the laser surface remelting process, and the remelted layer cross-section and surface were inlaid using gum wood powder and gradually polished using 400#, 800#, 1000#, 1500#, and 2000# sandpaper. After the specimen was polished, it was rinsed with alcohol and the surface of the specimen was etched with aqua regia for 50 s. The crystal structure of the AlCoCrFeNi 2.1 HEA before and after laser remelting treatment was identified using a UIitimaIV X-ray diffractometer (XRD). Microstructure analysis of HEA was performed with an AXio Scope.A1 optical microscope and a JSM-6510LA scanning electron microscope (SEM) operating at 30 kV. The SEM was equipped to perform electron backscatter diffraction (EBSD, Bruker e-Flash) and energy dispersive spectroscopy (EDS). The EBSD scan step size was 75 nm, and a tolerance angle of 5 • was used for grain identification. Energy dispersive spectroscopy (EDS) was performed for elemental analysis. The average lamellar spacing of HEA was determined from SEM images using the average linear intercept method. For transmission electron microscopy (TEM, JEM-2100F), the acceleration voltage was 200 kV. The samples used for TEM investigations were prepared by mechanical polishing to around 70 µm thickness followed by electropolishing in an electrolyte of 10% perchloric acid + 90% ethanol. The hardness test was carried out on an HV-30Z/LCD type micro-Vickers hardness tester with a loading load of 200 g and a holding time of 15 s. The MFT-4000 multifunctional material surface property tester was used for friction and wear testing. The specimen surface was subjected to constant-load reciprocating friction with a specimen size of 10 mm × 10 mm × 5 mm, loading load of 15 N, speed of 100 mm/min, and wear scar length of 5 mm, which was subsequently analyzed for wear amount. The DH7003-2 electrochemical workstation was used for electrochemical analysis of the high-entropy alloy surface, and the test solution was a 3.5 wt.% NaCl solution. A three-electrode electrochemical cell system was used for the electrochemical study, in which a high-entropy alloy surface was used as the working electrode, a square platinum plate with an area of 1 cm 2 as the counter electrode, and a +0.241 V SHE saturated calomel electrode (SCE) as the reference electrode. The open-circuit potential (OCP) measurement time was 30 min, and the potentiodynamic polarization scan was performed at a scan rate of 1 mV/s and a scan range of −0.4 V (vs. SCE) to 1.2 V (vs. SCE). Electrochemical impedance spectroscopy (EIS) measurements were performed at an amplitude of 1 mV and from 0.01 Hz to 10 kHz.

Phase Analysis
The XRD pattern of the as-cast AlCoCrFeNi 2.1 alloy and the 750 W remelted layers are shown in Figure 1. As indicated by the results, the 750 W remelted layer consisted of FCC and BCC phases, which were consistent with the phases of the as-cast AlCoCrFeNi 2.1 HEA, and a new phase was not generated within the remelted layer. Interestingly, the (100) and (200) diffraction peaks of the BCC phase in the LSR HEA disappeared and the intensity of the (110) diffraction peak was significantly weakened, suggesting that the volume fraction of the two phases may have changed before and after the laser surface remelting treatment. In addition, the peak intensity of the FCC phase diffraction peak is significantly higher than that of the BCC phase diffraction peak, indicating that the volume fraction of the FCC phase in the specimen is large. After laser surface remelting, the full width at half maximum of the highest diffraction peak of the as-cast AlCoCrFeNi 2.1 HEA and 750 W remelted layer are 0.166 and 0.223, respectively, and according to the Debyb-Scherrer formula it is known that the grain size is inversely proportional to the full width at half maximum of the diffraction peak, and the narrower the full width at half maximum of the diffraction peak, the larger the grain size. After laser surface remelting treatment, the grains are significantly refined, which is due to the great cooling rate of the melt pool after laser removal, up to 10 5 -10 8 K/s, and the grains do not have enough time to grow in order to solidify [14].   Figure 2a,b illustrate the OM images of the as-cast AlCoCrFeNi 2.1 HEA and the remelted layers produced by remelting at 750 W, respectively. The results show that the microstructure of the cast AlCoCrFeNi 2.1 high-entropy alloy was composed of alternating black and white lamellar and white bulk tissues, where the white phase was the FCC phase and the black phase was the BCC phase, and the white bulk tissues gradually increased with the Ni content in the HEA system [15]. After LSR, a remelted layer appeared on the surface of the AlCoCrFeNi 2.1 HEA, and a smooth interface zone existed between the remelted layer and the substrate, which demonstrated the development of a favorable metallurgical bond, and the thickness of the 750 W remelted layer was evaluated to be~644.5 µm. To elucidate the changes in microstructure after laser surface remelting, observations were made using scanning electron microscopy. The SEM images of the as-cast AlCoCrFeNi 2.1 HEA show distinct grain boundaries, with lamellar structures within each cell growing in different directions, and the microstructure consists of typical lamellar as well as labyrinth-like organization with an alternating alignment of the two phases (Figure 2c,e).

Microstructure
Figure 2d,f show SEM images of the interface and remelted layers after remelting at 750 W. LSR is very effective in refining the lamellar structure, the remelted layer of the AlCoCrFeNi 2.1 HEA shows a finer lamellar structure compared to the matrix, and the lamellar spacing of cast AlCoCrFeNi 2.1 HEA and 750 W remelted layer are calculated to be 4 µm and 0.476 µm, respectively. Figure 2g,h show the EBSD phase diagram of the AlCoCrFe 2.1 HEA and the 750 W remelted layer. The microstructure shows a homogeneous microduplex morphology consisting of FCC (green) and BCC (red) lamellar regions; notably, the nanoscale dotted BCC phase precipitated inside the FCC phase in the 750 W remelted layer. In addition, the fraction of FCC in the 750 W remelted layer (~76.8%) determined from the EBSD phase diagram is slightly higher than that of as-cast AlCoCrFe 2.1 HEA (~65.2%). Ocelík et al. [16] performed LSR on Al0.7CoCrFeNi (Al_0.7) and AlCoCrFeNi (Al_1) prepared by arc melting, and the volume fraction of the BCC phase in the remelted layer increased to 7.5%, compared to 3% in the non-remelted Al_0.7 sample, and the volume fraction of the FCC phase in the non-remelted treated Al_1 HEA was 65.3%. After LSR, the Al_1 remelted layer was completely composed of the BCC phase. This difference in the volume fraction of the two phases from the vacuum arc melting sample arises from the high solidification rate after LSR.
The two phases were further investigated using high-resolution TEM, as shown in Figure 3a, where the average thicknesses of FCC and BCC lamellae (denoted by a1 and a2 in Figure 3a, respectively) were~340 nm and 110 nm, respectively, in good agreement with the results measured by scanning electron micrographs ( Figure 2f). As shown in Figure 3b,c, no superlattice diffraction spots were found in the selected area diffraction patterns (SADPs), thus indicating the presence of disordered FCC and BCC phases.  Table 1 lists EDS analysis of the FCC and BCC phases of the AlCoCrFeNi 2.1 HEA before and after laser surface remelting. For the as-cast AlCoCrFeNi 2.1 HEA, the sum of the percentages of Al and Ni atoms in the BCC phase is close to 70%, which is due to the mostly negative mixing enthalpy between the Al and Ni elements, which can be easily combined to generate the AlNi phase [17][18][19]. After laser surface remelting, it can be found that the contents of Al, Co, Cr, Fe, and Ni in the BCC and FCC phases of the 750 W remelted layer are close to each other, which is due to the fact that the high-energy laser beam melts the surface of the high-entropy alloy and the solidification process is very rapid. The elements in the molten pool do not have enough time to diffuse, resulting in the relatively uniform distribution of each element [20][21][22][23].  Figure 4 shows the comparative Vickers hardness of the remelted zone, heat-affected zone, and the substrate zone of the 750 W laser surface remelted AlCoCrFeNi 2.1 HEA cross-section. The average Vickers hardness of the as-cast AlCoCrFeNi 2.1 HEA is 293 HV, and the average Vickers hardness of the 750 W remelting zone is 351 HV. The surface Vickers of the remelted zone after laser remelting treatment has increased by 20% compared with the Vickers of the base material zone. Based on [24,25], Miao et al. [11] derived the equations for hardness and lamellar spacing, as follows:

Hardness Analysis
where H is the Vickers hardness, H 0 is a constant with its value related to the nanoscale precipitated phase in the lamellar structure, k is the fitting constant, and d is the lamellar spacing. The refinement of the lamellar structure of the remelted layer after laser surface remelting treatment results in a smaller lamellar spacing, leading to an increase in hardness.  Figure 4 shows the comparative Vickers hardness of the remelted zone, heat-affected zone, and the substrate zone of the 750 W laser surface remelted AlCoCrFeNi2.1 HEA cross-section. The average Vickers hardness of the as-cast AlCoCrFeNi2.1 HEA is 293 HV, and the average Vickers hardness of the 750 W remelting zone is 351 HV. The surface Vickers of the remelted zone after laser remelting treatment has increased by 20% compared with the Vickers of the base material zone. Based on [24] and [25], Miao et al. [11] derived the equations for hardness and lamellar spacing, as follows:

Hardness Analysis
where H is the Vickers hardness, H0 is a constant with its value related to the nanoscale precipitated phase in the lamellar structure, k is the fitting constant, and d is the lamellar spacing. The refinement of the lamellar structure of the remelted layer after laser surface remelting treatment results in a smaller lamellar spacing, leading to an increase in hardness.
.  Figure 5 shows the friction coefficient versus time curves and wear weight loss images of the surface layer of AlCoCrFeNi2.1 HEA before and after laser remelting. From Figure 5a, it can be seen that the average friction coefficient of cast AlCoCrFeNi2.1 highentropy alloy is about 0.462, and the average friction coefficient after 750 W laser remelting is about 0.395. The large fluctuation in the friction coefficient of the two alloys is due to the difference in the hardness of the two phases. Hasannaeimi et al. [26] measured the friction coefficients of the B2 and L12 phases in AlCoCrFeNi2.1 using micro-indentation, and the average friction coefficient was about 0.87 for the high hardness B2 phase and about 0.82 for the low hardness L12 phase. From Figure 5b, it can be concluded that the wear weight loss of the two high-entropy alloys before and after laser surface remelting treatment is 0.0231 mm 3 and 0.0179 mm 3 after 30 min reciprocal friction, respectively. It can be seen that the laser surface remelting treatment can reduce the average friction  Figure 5 shows the friction coefficient versus time curves and wear weight loss images of the surface layer of AlCoCrFeNi 2.1 HEA before and after laser remelting. From Figure 5a, it can be seen that the average friction coefficient of cast AlCoCrFeNi 2.1 high-entropy alloy is about 0.462, and the average friction coefficient after 750 W laser remelting is about 0.395. The large fluctuation in the friction coefficient of the two alloys is due to the difference in the hardness of the two phases. Hasannaeimi et al. [26] measured the friction coefficients of the B2 and L1 2 phases in AlCoCrFeNi 2.1 using micro-indentation, and the average friction coefficient was about 0.87 for the high hardness B2 phase and about 0.82 for the low hardness L1 2 phase. From Figure 5b, it can be concluded that the wear weight loss of the two high-entropy alloys before and after laser surface remelting treatment is 0.0231 mm 3 and 0.0179 mm 3 after 30 min reciprocal friction, respectively. It can be seen that the laser surface remelting treatment can reduce the average friction coefficient and wear amount of the surface layer of AlCoCrFeNi 2.1 high-entropy alloy and improve the wear resistance of the alloy surface. SEM images of the as-cast AlCoCrFeNi 2.1 high-entropy alloy and 750 W laser remelting layer after frictional wear are shown in Figure 6. Figure 6a,c show the SEM images of the cast AlCoCrFeNi 2.1 high-entropy alloy after frictional wear. The presence of a large number of deposited layers and spalling pits on the surface of the wear marks is typical of adhesive wear, in addition, there are some furrows and the wear mechanism is abrasive wear. Figure 6b,d show the SEM images after frictional wear of the 750 W laser-remelted layer. There are deposited layers, abrasive chips, and shallow furrows on the surface of the wear marks; the wear mechanism is adhesive wear and abrasive wear; the surface of the wear marks is smoother than that of the cast high-entropy alloy, and the wear morphology is significantly improved. Figure 6e,f show the EDS images of the wear scar of the highentropy alloy in the as-cast and remelted states, respectively. A large number of O elements exist on the surface of the wear scar, especially the deposited layer at the edge of the wear scar has a higher content of O elements, which indicates that the material also undergoes oxidative wear. This is due to the reciprocal frictional motion of the grinding ball on the surface of the alloy, leading to the oxidation of the alloy surface at elevated temperatures, which is in agreement with the research of Kafexhiu [27].

Electrochemical Characteristics
We measured the open-circuit potential (OCP) of the AlCoCrFeNi 2.1 HEA in a 3.5 wt.% NaCl solution with increasing voltage and gradually increasing potential during immersion. The OCP stabilized after 30 min with an open-circuit potential of −0.162 V for the cast AlCoCrFeNi 2.1 HEA and −0.102 V for the LSR AlCoCrFeNi 2.1 HEA. Figure 7 shows the potentiodynamic polarization curves of the as-cast and laser remelting AlCoCrFeNi 2.1 highentropy alloy in a 3.5 wt.% NaCl solution. The specific electrochemical parameters are shown in Table 2. It can be seen from the diagram that the polarization curves of the two alloys have the same trend, there is no activation-passivation transition process, and there are obvious passivation intervals (∆E) and pitting potential (E pit ). The self-corrosion potential (E corr ) and self-corrosion current density (I corr ) of as-cast AlCoCrFeNi 2.1 high-entropy alloy are −354 mV and 1.731 × 10 −7 A·cm −2 , respectively. The self-corrosion potential and self-corrosion current density of remelting layer are −308 mV and 1.362 × 10 −7 A·cm −2 , respectively. The corrosion resistance of remelting layer is obviously enhanced after laser surface remelting. In addition, the pitting potential of the remelting layer (281 mV) was significantly higher than that of the as-cast high-entropy alloy (132 mV), and the laser surface remelting treatment improved the pitting resistance of the material. The passivation interval of the remelting layer is about 1.4 times that of the as-cast high-entropy alloy. The width of the passivation interval is usually related to the stability of the passivation film formed on the material surface [28,29]. The relatively uniform distribution of each element after laser surface remelting improves the stability of the passivation film.

Electrochemical Characteristics
We measured the open-circuit potential (OCP) of the AlCoCrFeNi2.1 HEA in a 3.5 wt.% NaCl solution with increasing voltage and gradually increasing potential during immersion. The OCP stabilized after 30 min with an open-circuit potential of −0.162 V for the cast AlCoCrFeNi2.1 HEA and −0.102 V for the LSR AlCoCrFeNi2.1 HEA. Figure 7 shows the potentiodynamic polarization curves of the as-cast and laser remelting AlCoCrFeNi2.1 high-entropy alloy in a 3.5 wt.% NaCl solution. The specific electrochemical parameters are shown in Table 2. It can be seen from the diagram that the polarization curves of the two alloys have the same trend, there is no activation-passivation transition process, and there are obvious passivation intervals (ΔE) and pitting potential (Epit). The self-corrosion potential (Ecorr) and self-corrosion current density (Icorr) of as-cast AlCoCrFeNi2.1 highentropy alloy are −354 mV and 1.731 × 10 −7 A·cm −2 , respectively. The self-corrosion potential and self-corrosion current density of remelting layer are −308 mV and 1.362 × 10 −7 A·cm −2 , respectively. The corrosion resistance of remelting layer is obviously enhanced after laser surface remelting. In addition, the pitting potential of the remelting layer (281 mV) was significantly higher than that of the as-cast high-entropy alloy (132 mV), and the laser surface remelting treatment improved the pitting resistance of the material. The passivation interval of the remelting layer is about 1.4 times that of the ascast high-entropy alloy. The width of the passivation interval is usually related to the stability of the passivation film formed on the material surface [28,29]. The relatively uniform distribution of each element after laser surface remelting improves the stability of the passivation film. Liu et al. [30] investigated the corrosion resistance of laser-selected melted AlCoCrFeNi2.1 HEA in a 3.5 wt.% NaCl solution. The Ecorr and Icorr of cast AlCoCrFeNi2.1 HEA were −0.197 V and 1.1 × 10 −6 A·cm −2 , respectively, and the Ecorr and Icorr of SLM AlCoCrFeNi2.1 HEA were −0.196 V mV and 1.0 × 10 −6 A·cm −2 , respectively. Additionally, the corrosion resistance of AlCoCrFeNi2.1 HEA prepared by laser-selective melting was not greatly improved. In this study, the self-corrosion potential of LSR AlCoCrFeNi2.1 HEA was increased by about 50 mV and the self-corrosion current was reduced by 23% compared with that of the as-cast alloy. In addition, the unit of self-corrosion current in this study is one order of magnitude smaller.
The grain and lamellae structure is significantly refined after laser surface remelting treatment, and the grain and phase boundaries are increased, which are susceptible to corrosion, leading to the deterioration of the corrosion resistance of the material [25,31], but the results of our study are contrary to this. Related studies have shown [32] that when the average spacing of anode sites is greater than the critical perturbation wavelength, it leads to electrochemical instability and decreased pitting resistance. Shuang et al. [13] studied the corrosion resistance of an FeCrNiCoNb0.5 eutectic high-entropy alloy in 1 M NaCl solution, which is a lamellar structure composed of a nanoscale C14 Laves phase and FCC phase, with an Nb-poor FCC phase as the anode site, and the average spacing of FCC phase is 100 nm, much less than the critical perturbation wavelength of 6 um, the pitting pits cannot be extended to grow, so the FeCrNiCoNb0.5 alloy has good pitting resistance. After 6 h of 1200 °C annealing treatment, the nanoscale lamellar structure was significantly coarsened, the average spacing of the FCC phase was 6-10 μm, slightly larger than the critical perturbation wavelength of 6 μm, and larger pitting pits were formed on the alloy surface. The improved corrosion resistance after the laser surface remelting treatment in this study may be due to the obvious refinement of the BCC phase as the anode site and the relatively uniform distribution of each element leading to the improved stability of the passivation film.   Liu et al. [30] investigated the corrosion resistance of laser-selected melted AlCoCrFeNi 2.1 HEA in a 3.5 wt.% NaCl solution. The E corr and I corr of cast AlCoCrFeNi 2.1 HEA were −0.197 V and 1.1 × 10 −6 A·cm −2 , respectively, and the E corr and I corr of SLM AlCoCrFeNi 2.1 HEA were −0.196 V mV and 1.0 × 10 −6 A·cm −2 , respectively. Additionally, the corrosion resistance of AlCoCrFeNi 2.1 HEA prepared by laser-selective melting was not greatly improved. In this study, the self-corrosion potential of LSR AlCoCrFeNi 2.1 HEA was increased by about 50 mV and the self-corrosion current was reduced by 23% compared with that of the as-cast alloy. In addition, the unit of self-corrosion current in this study is one order of magnitude smaller.
The grain and lamellae structure is significantly refined after laser surface remelting treatment, and the grain and phase boundaries are increased, which are susceptible to corrosion, leading to the deterioration of the corrosion resistance of the material [25,31], but the results of our study are contrary to this. Related studies have shown [32] that when the average spacing of anode sites is greater than the critical perturbation wavelength, it leads to electrochemical instability and decreased pitting resistance. Shuang et al. [13] studied the corrosion resistance of an FeCrNiCoNb 0.5 eutectic high-entropy alloy in 1 M NaCl solution, which is a lamellar structure composed of a nanoscale C14 Laves phase and FCC phase, with an Nb-poor FCC phase as the anode site, and the average spacing of FCC phase is 100 nm, much less than the critical perturbation wavelength of 6 um, the pitting pits cannot be extended to grow, so the FeCrNiCoNb 0.5 alloy has good pitting resistance. After 6 h of 1200 • C annealing treatment, the nanoscale lamellar structure was significantly coarsened, the average spacing of the FCC phase was 6-10 µm, slightly larger than the critical perturbation wavelength of 6 µm, and larger pitting pits were formed on the alloy surface. The improved corrosion resistance after the laser surface remelting treatment in this study may be due to the obvious refinement of the BCC phase as the anode site and the relatively uniform distribution of each element leading to the improved stability of the passivation film. Figure 8a,b show the Nyquist plots and fitted circuit diagrams of AlCoCrFeNi 2.1 high-entropy alloy in a 3.5 wt.% NaCl solution before and after the laser surface remelting treatment. From Figure 8a, it can be seen that the Nyquist curves of both alloys before and after laser surface remelting are semicircular, and the size of the diameter of the semi-circular arc reflects the strength of the corrosion resistance of the specimens, and the larger the diameter of the semi-circular arc, the stronger the corrosion resistance. This is consistent with the results of the previous analysis of the polarization curves. The fitted circuit diagram in Figure 8 consists of the solution resistance (Rs), polarization resistance (Rp), and constant phase element (CPE). According to [33][34][35][36], Y CPE is the conductance of the CPE and is given by the following equation.
where Y 0 and n are the magnitude and exponential terms of the CPE, j is an imaginary number, and ω is the angular frequency. The CPE behaves as an ideal resistance at n = 0 and an ideal capacitance at n = 1. In addition, the size of the n value reflects the roughness of the working electrode surface, and a larger n value indicates a smoother working electrode surface. The value of Rp is related to the passivation film, and a larger Rp value indicates a stronger corrosion resistance of the alloy [37,38]. The electrochemical impedance fitting data of the AlCoCrFeNi 2.1 HEA before and after laser surface remelting treatment are shown in Table 3. The Rp value of laser surface remelting AlCoCrFeNi 2.1 HEA is twice the Rp value of cast high-entropy alloy, which indicates that laser surface remelting treatment improves the corrosion resistance of high-entropy alloys.
number, and ω is the angular frequency. The CPE behaves as an ideal resistance at n = 0 and an ideal capacitance at n = 1. In addition, the size of the n value reflects the roughness of the working electrode surface, and a larger n value indicates a smoother working electrode surface. The value of Rp is related to the passivation film, and a larger Rp value indicates a stronger corrosion resistance of the alloy [37,38]. The electrochemical impedance fitting data of the AlCoCrFeNi2.1 HEA before and after laser surface remelting treatment are shown in Table 3. The Rp value of laser surface remelting AlCoCrFeNi2.1 HEA is twice the Rp value of cast high-entropy alloy, which indicates that laser surface remelting treatment improves the corrosion resistance of high-entropy alloys.     Figure 9 shows the swept electron microscopy images of the corroded surface of the AlCoCrFeNi 2.1 high-entropy alloy after the as-cast and LSRed. From Figure 9a, it can be seen that the BCC phase in the as-cast AlCoCrFeNi 2.1 high-entropy alloy underwent dissolution compared to the FCC phase, and the dissolution of the large BCC phase led to the collapse of the lamellar structure to form corrosion pits, which is due to the significantly higher content of corrosion-resistant elements Cr and Co in the FCC phase compared to the BCC phase, which has better corrosion resistance. In addition, the as-cast highentropy alloy also exhibited selective corrosion of the differently oriented grains, with only slight corrosion of the grains above Figure 9a, which is consistent with the study of Hasannaeim [25]. Figure 9b shows the corrosion morphology of AlCoCrFeNi 2.1 highentropy alloy after remelting treatment, it can be seen that the lamellar structure of the remelted layer is fine and dense, the dissolution of the BCC phase leads to the formation of some small cavities, and the corrosion morphology is significantly improved compared to that of the as-cast high-entropy alloy.
Metals 2022, 12, 1590 12 Figure 9 shows the swept electron microscopy images of the corroded surface o AlCoCrFeNi2.1 high-entropy alloy after the as-cast and LSRed. From Figure 9a, it ca seen that the BCC phase in the as-cast AlCoCrFeNi2.1 high-entropy alloy under dissolution compared to the FCC phase, and the dissolution of the large BCC phase l the collapse of the lamellar structure to form corrosion pits, which is due to significantly higher content of corrosion-resistant elements Cr and Co in the FCC p compared to the BCC phase, which has better corrosion resistance. In addition, the as high-entropy alloy also exhibited selective corrosion of the differently oriented gr with only slight corrosion of the grains above Figure 9a, which is consistent with the s of Hasannaeim [25]. Figure 9b shows the corrosion morphology of AlCoCrFeNi2.1 h entropy alloy after remelting treatment, it can be seen that the lamellar structure o remelted layer is fine and dense, the dissolution of the BCC phase leads to the form of some small cavities, and the corrosion morphology is significantly improved comp to that of the as-cast high-entropy alloy.

Conclusions
The AlCoCrFeNi2.1 HEA was successfully treated by laser surface remelting. microstructure, hardness, wear resistance, and corrosion resistance properties

Conclusions
The AlCoCrFeNi 2.1 HEA was successfully treated by laser surface remelting. The microstructure, hardness, wear resistance, and corrosion resistance properties were intensively investigated. The main conclusions of this work are as follows.
The laser surface remelted AlCoCrFeNi 2.1 high-entropy alloy was examined by X-ray diffraction, and it was found that no new phases were generated in the remelted layer after laser remelting, it was only composed of FCC and BCC phases.
The LSRed AlCoCrFeNi 2.1 HEA has a significantly finer lamellar structure and increased hardness of 351 HV. In addition, due to the rapid solidification, the elements do not have enough time to diffuse, resulting in a more uniform distribution of elements within the FCC and BCC phases.
Laser surface remelting treatment significantly improved the friction and wear performance resistance of the high-entropy alloy. Wear weight loss reduced from 0.0231 mm 3 to 0.0179 mm 3 .
In a 3.5 wt.% NaCl solution, the laser-remelted surface had a larger self-corrosion potential and smaller self-corrosion current density, and the corrosion resistance was better than that of the as-cast HEA.