Comparative Studies of Cr/Ti Additions for Cu40Mn25Al20Fe5Ni5 HEA on Microstructure and Corrosion Behavior in HNO3 Solution

The influence of Cr or Ti additions to Cu40Mn25Al20Fe5Ni5 high entropy alloy (HEA) on the microstructure and corrosion behavior in different concentrations of HNO3 solution were investigated. The microstructures of the Cu-rich HEAs are characterized and analyzed. Microstructural analysis of the HEAs indicates the formation of a dendritic structure. Also, the phases are identified by using X-ray diffraction (XRD). The corrosion behavior of the investigated HEAs in the HNO3 solution was studied. The corrosion rate (CR) for Cu40Mn25Al20Fe5Ni5Ti5 HEA (Ti5 HEA) has a lower value than Cu40Mn25Al20Fe5Ni5Cr5 HEA (Cr5 HEA). The morphology of the corroded HEAs was investigated by using SEM images, EDX analysis, and mapping for elemental distribution. Cr addition led to the formation of a Cr2O3 protective film on the dendrites. However, the Ti5 HEA has a good surface morphology with a homogenous distribution than Cr5 HEA which is associated with a decrease in the corrosion rate.


Introduction
High entropy alloys (HEAs) have been an interesting field in the last two decades. [1][2][3] HEAs consist of five or more elements which make them different from the traditional multi-components in all properties and used sometimes as reinforced materials. [4][5][6] The majority of studies on HEAs focused on Fe-based HEAs but there are few investigations on Cu-based HEAs. 2 Therefore, this research discusses novel non-equiatomic Cu 40 Mn 25 Al 20 Fe 5 Ni 5 Cr 5 HEA (Cr 5 HEA) and Cu 40 Mn 25 Al 20 Fe 5 Ni 5 Ti 5 HEA (Ti 5 HEA) with a comparison of the Cr/Ti additions effect on the corrosion resistance. A single-phase or multiphase can be designed to obtain the HEAs microstructure according to the required properties for any application. [7][8][9][10] CoCrCuFeNi, CoMo-FeNiCu, Cu 40 Mn 25 Al 20 Fe 5 Co 5 Ni 5 HEAs are composed of FCC solid solution and their microstructures are identified as dendrite and interdendritic structures. 11 HEAs improve their corrosion resistance in H 2 SO 4 , HNO 3 , and HCl acids due to the adding elements such as Cr, Ni, Mo, and/or Cu. [12][13][14] Ni addition with other metals improves the mechanical, oxidation, and corrosion resistance at elevated temperatures as in Ni-base superalloys. 15 As cast Cu 45 Mn 25 Al 15 Fe 5 Cr 5 Ni 5 HEA has good corrosion behavior in 3% NaCl solution by avoiding of intermetallic formation of Al-Ni compounds. 16 Minimizing segregation enhances the corrosion behavior such as adding Cr. 16 The Ti addition contains BCC structure on AlCoCrFeNiTi x which has good mechanical properties. 17 The addition of Cu to FeCo-NiCrCu x HEA increases the localized corrosion because of Cu-rich interdendritic and Cu-depleted dendrite formation. 18 The corrosion behavior of Al 0.8 CrFeCoNiCu x HEA coating is pitting and intergranular corrosion. 19 The Ti is an active metal that tends to contain intermetallic compounds. [20][21][22][23] To prevent galvanic corrosion in the Cu-Fe-based dualphase immiscible MEA, one approach is to increase the corrosion potential of the Fe-rich phase by adding Cr, which has been widely used in steels to improve corrosion resistance by forming a Cr passivation layer. [24][25][26] Because Cr is immiscible with Cu, 27 Cr should dissolve selectively into the Fe-rich phase in the Cu-Fe-based immiscible MEA. This allows the corrosion potentials of the Cu-and Fe-rich phases to be balanced. The formation of Cr-oxide layers on the Fe-rich phase initially converts the less noble phase from Fe-rich to Cu-rich. As a consequence, during the initial corrosion stage, fast Cu corrosion and the formation of the Cu-oxide layer take place. Meanwhile, an Al-Fe-Mn-rich passivation layer protects the Cu-rich phase's surface. Due to the outward diffusion of Cr through the existing Cr-oxide layer, the Cr-oxide layer forms on top of the inner Cu-oxide layer. The Cr-oxide layer formation changes the corrosion behavior of the alloy from active to passive. 28 This work aims to develop two different non-equiatomic Cu-rich high entropy alloys and study their characterizations. Cu 40 Mn 25 Al 20 Fe 5 Ni 5 Cr 5 HEA (Cr 5 HEA) and Cu 40 Mn 25 Al 20 Fe 5 Ni 5 Ti 5 HEA (Ti 5 HEA) corrosion behavior in (0.5, 1.0, 5, and 10%) HNO 3 solution was investigated. Studying the effect of obtained microstructures from casting on the corrosion behavior by weight-loss (WL) and potentiodynamic polarization (PP) measurements. This investigation was confirmed by using SEM, EDX analysis, and mapping for elemental distribution.

Materials
Cu 40 Mn 25 Al 20 Fe 5 Ni 5 Cr 5 HEA (Cr 5 HEA) and Cu 40 Mn 25 Al 20 Fe 5 Ni 5 Ti 5 HEA (Ti 5 HEA) were developed via the casting route. The size of each heat was designed to be 200gm, and pure elements of Cu, Al, Ni, Mn, Fe, Ti, and Cr were used as raw materials for the casting process of the two HEAs. The ingots were prepared via arc melting technique under vacuum, and the ingots were melted and solidified in water-cooled copper mold. Each heat was remelted at least three times to ensure homogeneity. After finishing the casting process, the pressure inside the melting chamber is equalized by the atmospheric pressure, then the furnace is opened to extract the samples. HEAs ingots were cut via a wire cutting machine. The chemical composition of the casted ingot was analyzed. The chemical composition was determined via XRF mobile analyzer. The performance required for quick alloy grade identification and precise chemistry of a wide range of materials is delivered by the X-MET8000 range of handheld X-ray fluorescence (HHXRF) analyzers (solid and powder metals, polymers, wood, solutions, soil, ores, minerals, etc). Optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the microstructure and phase identification. The chemical compositions of the two casted HEAs are shown in Table 1.

Electrochemical Corrosion Characterization
The corrosion resistance of the HEAs was evaluated by chemical immersion test, and electrochemical techniques to assess the impact of the addition of Cr or Ti on the corrosion behavior of Cu 40 Mn 25 Al 20 Fe 5 Ni 5 HEA. In a glass vessel, 20 ml of 0.5, 1, 5, and 10% HNO 3 were used to measure the weight loss (WL) of the HEAs.
The WL was immersed for 120 h at 20 ± 1°C. HEAs were ground up to a 1000-grit emery paper then polished by alumina paste, degreased in acetone, washed in doubly distilled water, dried, and then weighed on an analytical scale. The weighted HEAs are dispersed in 20 mL of HNO 3 solutions of various concentrations. Following immersion, the samples were taken out, cleaned with acetone and distilled water, dried, and weighed. The test was repeated several times, with the average value of WL being recorded. After that, the average corrosion rate (CR) can be calculated. 29 where K, T, A, DW, and D are a constant (8.76 9 10 4 ), time in h, section area (cm 2 ), loss of sample weight (grams), and density (g/cm 3 ), respectively.
With a scan rate of 0.5 mV/s, the PP curves were used to obtain the corrosion potential and corrosion current density in the region of -0.8 to 0.8 V. The cell consisted of a working electrode made of test specimens, an auxiliary electrode made of platinum wire, and a reference electrode made of SCE (Hg/Hg 2 Cl 2 -Sat. KCl) coupled to a typical electrolytic cell with a capacity of 25 ml. It is possible to   where z is the ionic charge, M is the metal atomic, d is the density g/cm 3 , and I corr is the current density of corrosion expressed in a unit of mA/cm 2 . In the presence of different concentrations of HNO 3 , the intersection of the linear cathodic and anodic branches of Tafel plots was used to calculate the values of current and potential of corrosion (I corr and E corr ).
SEM and EDX analysis were used to analyze the surface morphology of the corroded HEAs, as well as mapping for elemental distribution.

Results and Discussion
Microstructure and Chemical Analysis The microstructures of the two examined HEAs are demonstrated in Figure 1, which indicate the founding of two phases. As seen in Figure 1, the Cr 5 HEA has a dendritic structure. However, the microstructure of the Ti 5 HEA consists of a matrix with dispersed cuboidal particles.  Moreover, the micrographs reveal the existence of flowerlike precipitates. Surprisingly, the EDX analysis of Cr 5 HEA reveals no remarkable difference in the chemical analysis of both the dendritic and inter-dendritic zones. On the other hand, the chemical analysis of Ti 5 HEA demonstrates three different zones. Firstly, the copper-rich matrix, which depleted in Fe, Ti, and Ni. This result can be attributed to the fact that the positive enthalpy of mixing between Cu-Fe and Cu-Ti pairs. Therefore, the segregation phenomenon is expected to be promoted, resulting in the existence of both copper-rich and lean phases. Figure 2 and Table 2 represent the EDX analysis of the investigated alloys. Moreover, the chemical analysis of spot 4 suggests that the formed precipitates might be C14 laves phase Ti (Fe 1-x Mn x ) 2 . Finally, the chemical analysis of the cuboidal particles reveals a high Al content compared to the other two phases. Nonetheless, this chemical analysis suggests that these particles could be solid solutions, not intermetallic ones. The elemental distribution of the Cr 5 HEA is presented in Figure 3. Figure 4 illustrates the EDX results for Ti 5 HEA, whereas Figure 5 provides the elemental distribution of Ti 5 HEA.
The elemental distribution of the Cr 5 HEA is presented in Figure 3. The figure shows the complete distribution of all elements all over the Cr 5 HEA and appears to be a dendritic structure. However, the elemental distribution of elements all over Ti 5 HEA shows the existence of Ti cuboids as given in Figure 5. Figure 6 shows the XRD pattern of Cr 5 HEA, where it can be seen that there are two BCC solid solutions. Moreover, there are two minor peaks associated with the ordered AlCu 2 Mn phase. Unlike the previously investigated Cr 5 HEA, increasing Al content from 15 to 20 is accompanied by the appearance of super-lattice minor peaks. For the Ti 5 HEA, a similar diffraction pattern with two minor superlattice peaks is present, as seen in Figure 7. However, there is an interesting feature; the peaks of the two solid solutions are overlapped as shown in Figure 8. This could be attributed to maintaining nearly the same lattice constant.

Electrochemical Corrosion Characterization
Weight-Loss Measurement Figure 9 and Table 3  influence their corrosion behavior in the investigated conditions. 1 The visual images of HEAs in different concentrations of nitric acid, after 120 h of immersion testing experiments are shown in Figure 11.

Potentiodynamic Polarization
The PP curves of Cr 5 HEA and Ti 5 HEA in numerous concentrations of HNO 3 acid are provided in Figure 12.
The current change in HNO3 diluted solutions is connected to the applied potential, E. The electrochemical parameters for the HEAs are listed in Table 5. It was noticed from this table that decreasing the polarization resistance with increased acid concentration increases the CR of Cr 5 HEA.  The CR depends on the composition and the passive film structure produced in the solution. 29

Corrosion Mechanism
The corrosion mechanism was that of the anodic dissolution of the investigated HEAs by the strong HNO 3 acid used due to the hydrogen content getting higher which led to the alloys oxidizing. The dissolution anodic characteristic of the HEAs and HNO 3 acid is very complex. The anodic reaction for Cu is commonly thought to be as follows 31 : where Cu(I) ads are the specimen that is absorbed into the copper surface and not diffused into the solution.
Increasing the Al content in the HEA might lead to increasing the possibility of pitting corrosion. 32 The Cr addition in the FeCoNi-based HEA has revealed good corrosion resistance; however, excessive Cr addition leads to deterioration in corrosion resistance due to Cr-induced segregation. 33 In a 0.5 M HNO 3 solution, the Ti addition greatly enhances the corrosion behavior of Al 2 CrFeCo-CuNiTi x (x = 0, 0.5, 1.0, 1.5, and 2.0 molar fraction). 28,30,34 The Ti improves the corrosion resistance of Ti 5 HEA in Cu-based HEA more than Cr does in Cr 5 HEA.

Surface Morphology
The surface morphologies and EDX results of the corroded Cr 5 HEAs in 0.5% and 1% HNO 3 are provided in Figure 13a, b, respectively. While the results for 5% and 10% HNO 3 are shown in Figure 14a, b, respectively. The EDX analysis of corroded Cr 5 HEAs in at.% is listed in Table 6. The elemental distribution of the corroded Cr 5 HEAs in different concentrations of HNO 3 is depicted in Figure 14.
As shown in Figure 15 and Table 6, the structure of the dendrite has higher corrosion resistance which mainly is Cr-and Mn-rich. This agrees with the elemental distribution in Figure 15. Adding Cr led to protective passive film formation on the surface, which prevents further corrosion of the alloys underneath. 35,36 From mapping, oxygen exists with Cr which may be due to the formation of Cr 2 O 3 protective film on the dendrites.
The surface morphology and the EDX result of the corroded Ti 5 HEAs for 10% HNO 3 are provided in Figure 16. The EDX result in at.% is 44.2%O, 12.5%Al, 11.8%Ti, 7.5%Mn, 10.8%Fe, 6.0%Ni and 7.2%Cu. The elemental distribution of the corroded Ti 5 HEAs is seen in Figure 17. The Ti 5 HEA has a good surface morphology with a homogenous distribution than Cr 5 HEA which is associated with a decrease in the corrosion rate.

Conclusion
In

Funding
Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). This research received no external funding.

Data Availability
The authors confirm that this article contains all the data supporting the findings of this study.
Conflict of interest The authors declare that they have no conflict of 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.

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