Effects of Mn addition on mechanical properties, fracture surface, and electrochemical corrosion resistance of CoCrFeNiAl high-entropy alloys

High-entropy alloys consisting of CoCrFeNiAl as the major elements and 2–5 at% Mn as the minor element were prepared using a vacuum arc melting method. The crystalline structures of the prepared alloys were identified by x-ray diffraction. Moreover, the mechanical properties of the alloys were examined under quasi-static (10−1, 10−2 and 10−3 s−1) and dynamic (3000, 4000, and 5000 s−1) loading conditions using a universal testing machine and split-Hopkinson pressure bar system, respectively. The experimental results showed that, for all of the HEA alloys, the flow stress and strain rate sensitivity coefficient increased with increasing strain rate. Among all the alloys, that with 3 at% Mn exhibited the best mechanical properties. A significant loss in plasticity was observed as the Mn content increased to 5 at%. The scanning electron microscope observations showed that the favorable mechanical properties of the alloy with 3 at% Mn were the result of a compact dimple structure, which enhanced the toughness. The HEA with 5 at% Mn showed the best electrochemical corrosion resistance among all the alloys due to the formation of dendritic structures at the grain boundaries.


Introduction
Metallic materials are widely used through engineering and industry, and hence their fracture resistance, deformation resistance, high and low temperature tolerance, corrosion resistance, tribological performance, and so forth, have been extensively examined. Most conventional alloys, e.g., steel and aluminum alloy, consist of a single major element combined with one or two other minor elements to improve the properties of the main element. However, in traditional metallurgy, the addition of more elements often leads to the formation of compounds with complex microstructures and poor mechanical properties, which weaken the material and cause embrittlement [1][2][3][4][5][6].
In 2004, Yeh et al [7,8] proposed a method for creating alloys consisting of five to six metallic elements added in equal ratios. Many studies [9][10][11][12][13][14] have shown that the properties of such alloys, referred to as high-entropy alloys (HEAs), are superior to those of conventional alloys since the different atoms are randomly distributed in the atomic layer, which results in a disorderly (i.e., high-entropy) effect and limits the formation of brittle compounds.
High-entropy alloys are applied in numerous fields, including temperature-resistant spray coatings, turning tool protective coatings, superconducting materials, biomedical tools, advanced nuclear applications, and many more. Many HEA systems have been developed in the past decade, where most of these systems consist of elements such as Cr, Fe, Co, Ni, and Al mixed in a fixed ratio. Various equi-atomic multi-component alloys have been reported in the literature [15][16][17][18]. While the mechanical assessment of such alloys has produced mixed results, these alloys invariably show improved toughness compared to that of traditional alloy systems. The present study investigates the mechanical properties, fracture features, and electrochemical corrosion resistance of CoCrFeNiAlMn x HEA alloy systems, where x is the atomic ratio of Mn and has a value in the range of 2.0 to 5.0 Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. at%. The crystalline structures of the prepared alloys are first identified by x-ray diffraction. The mechanical properties of the alloys are then investigated under quasi-static loading conditions (10 −1 , 10 −2 and 10 −3 s −1 ) on a universal testing machine. The dynamic impact response of the alloys is evaluated at strain rates of 3000, 4000, and 5000 s −1 using a split-Hopkinson pressure bar (SHPB) system. The fracture surfaces of the impacted specimens are observed by scanning electron microscopy. The observation results are then used to interpret the mechanical response behaviors of the various specimens. Finally, the electrochemical corrosion resistance of the alloys is evaluated via electrochemical impedance spectroscopy.

Experimental methods and material preparation
CoCrFeNiAlMn x=2,3,4,5 HEA samples were produced using a vacuum arc melting (VAM) technique. (Note that each raw material had a purity of at least 99.9 at%). Briefly, the alloy components were melted in a chamber filled with argon gas to prevent oxidation and were then inverted and remelted once again after they cooled. The melting and remelting processes were performed five times in order to improve the compositional homogeneity of the final ingots. After the final re-melting process, the molten metal was pumped into a copper mold and cast in the form of cylindrical rods with a diameter of 5 mm and length of 50 mm. The rods were then cut into cylindrical specimens with dimensions of 5 mm × 5 mm using a slow speed cutting machine. The structures of the alloy samples were examined using an x-ray diffractometer (XRD, Bruker D8 Discover), while the quasistatic and dynamic impact behaviors were evaluated using an MTS810 universal test machine and a split-Hopkinson pressure bar (SHPB) system, respectively. The fracture surface topologies of the various samples were examined by scanning electron microscopy (SEM, JEOL-6330TF; JEOL Ltd Japan) using standard metallographic techniques. Finally, the corrosion resistance of the alloys was evaluated by EIS tests performed in a 3 at% NaCl aqueous solution (3 at% NaCl + 97 at% H 2 O).

Results and discussion
3.1. XRD structural analysis Figure 1 presents the XRD patterns of the four HEA alloys with different levels of Mn addition. The results reveal that all of the alloys have BCC in the (110) and FCC structures in the (111) and (220) planes, respectively [19,20]. As the Mn content increases from 2 at% to 3 at%, the BCC peak intensity increases. However, as the Mn content further increases to 5 at%, the intensity of the BCC peak reduces. The Mn content has no effect on the intensity of the FCC peaks; however, the peaks gradually broaden as the Mn content increases. Furthermore, the kurtosis of the FCC peaks gradually shifts toward a lower angle since the atomic radius of Mn is greater than that of the other elements.  content increases. According to the composition analysis showed that the dendrites consist mainly of Fe, Cr, Mn, and Co. Since only the Mn content varied among the dendrites formed in the different specimens, it was inferred that the formation of dendrites was correlated mainly with the level of Mn addition.  Figures 3 and 4 present the stress-strain curves obtained for the various alloys in the quasi-static and dynamic impact tests, respectively. (Note that the tests were performed at room temperature in both cases). Furthermore, the dynamic compression examination has dispersive effect during the stress wave propagation. So, we use the fast Fourier transformation algorithm which yields a discrete Fourier transformation to correct the stress wave and let the stress strain look smoother. The detailed description was reported by the current authors in a previous study [21]. Tables 1 and 2 summarize the maximum stress and strain values of the alloys under each of the tested strain rates. In general, the results show that for both strain rate regimes, the maximum stress increases with an increasing strain rate, while the fracture strain decreases. This phenomenon is particularly pronounced in the specimens tested under dynamic deformation conditions due to the greater generation rate and travel speed of the dislocations at higher strain rates. Notably, the generation of a greater number of dislocations  inhibits the formation of a slip plane within the crystal structure, and therefore reduces the plastic deformation (i.e., increases the toughness) of the alloy samples under high strain rate conditions. Figures 3 and 4 show that the flow stress of the CoCrFeNiAlMn x alloys increases as the Mn content is increased from 2 to 3 at%, but decreases as the Mn content is further increased to 5 at%. This trend is consistent with the XRD patterns shown in figure 1, which reveal that the intensity of the BCC peaks reaches its maximum value in the sample with a Mn content of 3 at%, but reduces in the samples with 4 at% and 5 at% Mn addition, respectively. It was known that the slip directions of BCC structure is more difficult than FCC [22,23].

Figures 5(a) and (b)
compare the stress-strain responses of the CoCrFeNiAlMn x alloys under the lowest and highest strain rates of 10 −1 s −1 and 5 × 10 3 s −1 , respectively. The results confirm that, for a given strain rate, adding a small quantity of Mn to the HEA system improves the mechanical properties, while an excessive Mn addition reduces the yield strength and maximum stress. The results also confirm that, for a constant strain rate, the fracture strain of all the alloys decreases as the Mn content increases [24]. Figure 6 shows the variation of the flow stress with the strain rate as a function of the true strain in the quasistatic and dynamic strain rate regimes, respectively. In general, the results reveal that, for a given strain, a higher strain rate results in a higher flow stress. Figure 6(a), corresponding to the quasi-static strain rate range, shows that for both values of the true strain (0.15 and 0.12), the flow stress increases with increasing strain rate. Moreover, for a constant strain rate, the flow stress increases with increasing strain. A similar tendency is found in the dynamic strain rate regime, as shown in figure 6(b). In other words, in both strain rate regimes, the strain and strain rate have a significant effect on the flow stress, yield stress, material ductility, and fracture strain of the CoCrFeNiAlMn x alloys.

Strain rate effect
The effect of the strain rate on the mechanical response of the alloy samples was further investigated using the strain rate sensitivity coefficient β, which is defined as [25,26].
In other words, for a given strain rate within a given interval, a greater change in stress results in a higher strain rate sensitivity coefficient. Figures 7(a) and (b) show the variation of the strain rate sensitivity coefficient with the strain for the CoCrFeNiAlMn alloys tested under static compression and dynamic loading conditions, respectively. For all of the specimens, and both strain rate ranges, the strain rate sensitivity coefficient increases with increasing strain rate. In other words, all of the CoCrFeNiAlMn x HEAs are influenced by the strain rate.  Figures 8 and 9 present SEM images of the fracture surfaces of the HEA alloys with different Mn additions after the static and dynamic impact tests, respectively. All of the specimens exhibit a mixture of brittle fracture features (intergranular fracture) and ductile fracture features (dimple fracture), regardless of the strain rate under which deformation was performed. However, the dimple features in the specimens tested under static deformation conditions are generally smaller and more compact than those formed in the specimens tested under dynamic strain rates. For both strain rate ranges, the fracture features transform from dimple features to dimple features with granules and lamellas as the Mn content increases. The stress-strain curves presented in figures 3 and 4 have shown that the fracture strain of the alloy samples decreases with an increasing Mn content, which suggests a loss in plasticity at higher levels of Mn addition. This inference is supported by the fracture feature observations in figures 8 and 9, which show that the density of the dimple structures decreases at higher levels of Mn addition. The observations also show that the density of the dimple structures is generally lower in the samples subjected to dynamic loading than those tested under static compression loading. This tendency is again consistent with the results presented in figures 3 and 4, which show that, for a given alloy composition, the fracture strain of the specimens tested under dynamic strain rates is lower than that of the specimens compressed under quasi-static conditions.

Electrochemical corrosion analysis
The corrosion resistance of the various alloys was evaluated by means of electrochemical impedance spectrometry (EIS) tests in a 3 at% NaCl aqueous solution (3 at% NaCl + 97 at% H 2 O). (Note that the NaCl concentration of the aqueous solution is similar to that of sea water. As shown in figure 10, the corrosion current reduces continuously with an increasing Mn content. Futhermore, the table 3 presents the electrochemical properties for the different content Mn of HEA. From the table 3 and figure 10, we can find the CoCrFeNiAlMn 3 has the better value than other alloys. It has the higher corrosion voltage (E corr ) is −0.39 V, the low corrosion current (I corr ) is 8.5 × 10 −8 A cm −2 and the higher corrosion resistance(R p ) is 2.01 × 10 6 Ω. In other words, the results are consistent with those of previous studies, which showed that a higher Mn content is beneficial in improving the corrosion resistance of HEAs [27,28]. The metallographic observations (as shown in figure 2) suggest that the improved corrosion resistance is due to the replacement of the corrosion holes at the grain boundaries with dendritic precipitates.

Conclusions
This study has evaluated the mechanical, microstructural, and electrochemical corrosion resistance properties of CoCrFeNiAlMn HEAs with Mn contents of 2∼5 at%. The XRD analysis results have shown that all of the alloys contain both BCC and FCC structures. The maximum intensity of the BCC peak in the (110) plane occurs in the alloy with a Mn content of 3 at%. The peak intensity of the FCC structure in the (111) and (220) plane is insensitive to the Mn content; however, the peaks broaden and shift toward a lower angle as the Mn content increases. The mechanical response of the CoCrFeNiAlMnx alloys has been investigated under quasi-static and dynamic deformation conditions. In general, the results have shown that the alloy with 3 at% Mn provides the best mechanical response (i.e., the highest flow stress). For all of the specimens, the flow stress increases with an increasing strain rate. Moreover, regardless of the change in strain rate, a higher strain results in a higher strain rate sensitivity coefficient. In other words, a higher strain has a greater effect on the strain rate. The SEM observations have shown that, as the Mn content increases, the fracture surface changes from dimple-like features to a smoother surface with brittle characteristics. Thus, the toughness of the alloy samples reduces at higher levels of Mn addition, as shown in the stress-strain curves. Finally, the EIS results have shown that as the Mn content increases, the corrosion current reduces. The higher corrosion resistance is attributed to the gradual formation and spreading of dendritic structures at the grain boundaries of the samples with a greater Mn content.