Study on microstructure and corrosion resistance of CoCrFeNiTi0.2Mox high entropy alloy

Vacuum arc melting was employed by produting the high entropy alloys CoCrFeNiTi0.2Mox (x=0, 0.1, 0.3). We investigated the influence of Mo on the phase, microstructure, and corrosion resistance of CoCrFeNiTi0.2Mox (x=0, 0.1, 0.3). The introduction of Mo led the high entropy alloy to shift to a double-phase structure, with a rise in the volume percentage of σ phase. The corrosion resistance of high-entropy alloys decreases first and subsequently increases with increasing Mo content in 3.5 weight percent NaCl solution.


Introduction
High entropy alloy (HEA) is a compelling new kind of alloy material that has just appeared.Yeh et al. [1] proposed and called it for the first time in 2004.Cantor et al. [2] published a similar paper in the United Kingdom and dubbed the multi-principal element alloy in the same year.HEAs are made up of five or more elements, with each accounting for 5-35% of the alloy.The features of high entropy alloys are influenced by four important effects: the high entropy effect, the chronic dispersal effect, the lattice distortion impact, and the cocktail effect [1] .Solid solution phases formed by HEAs include face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close packed (HCP) [3] .The composition of the HEA may be changed to affect its qualities.Co, Ni, and Fe elements have high electrode potential in HEAs of the CoCrFeNi family, and Cr is the principal corrosion-resistant element.Co, Cr, Fe, and Ni elements are well miscible in the matrix with nearly minimal element segregation, resulting in high corrosion resistance [4] .Alloying elements will significantly affect the material's corrosion resistance, and the formation of five or more kinds of corrosion-resistant high-entropy alloys by adding alloying elements based on CoCrFeNi has become a research focus.Chen et al. observed the corrosion behavior of an AlCrFeNiMo 0.5 Ti x alloy in 3.5 weight percent NaCl solution.The passive coating on the alloy surface becomes more protective as the Ti percentage increases [5] .Therefore, this paper selected CoCrFeNiTi 0.2 Mo x (x= 0, 0.1, 0.3) as the research object to explore its corrosion resistance in 3.5 wt.% NaCl electrolytes.

Materials and methods
After Fe, Co, Cr, Ni, Ti, and Mo particles with mass fractions above 99.9% were scrubbed by ultrasonic water to eliminate impurities, argon gas was utilized as protection gas, and the vacuum arc melting technique was employed to make CoCrFeNiTi 0.2 Mo x (x=0, 0.1, 0.3) alloys ingots.Each alloy ingot was melted more than four times in the water-cooled copper crucible (when the melting times were 2, electromagnetic stirring was used during the melting process, and the stirring time was 2 minutes) to ensure chemical uniformity of the alloy, and the final obtained ingot was elliptical cake shape, about 15 mm thick and 40 mm diameter.Wire cutting was used to generate square samples sized 10 × 10 × 3 mm from the alloy ingot.
Phase structure features of samples analyzed using X-ray diffraction (XRD), and an autobiographic microscope and scanning electron microscope (SEM) coupled with an energy dispersion spectrometer were utilized for examing the microstructure and elemental distribution.The alloy ingots is welded with wire and then wrapped in epoxy resin, leaving a 1 cm 2 test area.Electrochemical tests were performed at ambient temperature utilizing a CHI660E electrochemical workstation in a 3.5 weight percent NaCl solution.A quintessential tri-electrode system was used, with the sample serving as the workingelectrode, the platinum plate serving as the auxiliary electrode, and saturated calomel electrode serving as the reference electrodes.The electrochemical impedance (EIS) test frequency is 10 5 ~10 -2 Hz, the amplitude is 10 mV, and the potentiodynamic polarization curve scanning rate is 5 mV/s.All samples were measured at least three times in the electrolyte solution to obtain a good repeatability of the polarization curve.

Phase transition law of CoCrFeNiTi
The XRD pattern of CoCrFeNiTi 0.2 Mo x (x= 0, 0.1, 0.3) alloys as cast is revealed in Fig. 1.The three alloys are represented by Ti 0.2 , Ti 0.2 Mo 0.1 , and Ti 0.2 Mo 0.3 , respectively.As seen in the diagram, Ti 0.2 and Ti 0.2 Mo 0.1 alloys have a single face-centered cubic solid solution (FCC) structure.When Mo content is 0.3, it can be seen that there is an apparent characteristic peak of the second phase, namely σ phase, at face-centered cubic (111), and the diffraction peak intensity at face-centered cubic (111) is significantly reduced, which may result from the precipitation of σ phase.The power of the face-centered cubic diffraction peak decreases.To further determine the distribution of elements in CoCrFeNiTi 0.2 Mox (x= 0, 0.1, 0.3) alloys, EDS component analysis was carried out for different regions in Fig. 2, as shown in Table 1.Structure A is the dendrite region (DR), Structure B is the interdendritic region (ID), and Structure C is the generated second phase (σ phase).As can be seen from the table, element segregation occurs in the alloy.The dendrite zone of Ti 0.2 high-entropy alloy is rich in Cr and Fe elements, whereas the interdendritic area is rich in Ni and Ti elements.After adding the Mo element, Co, Fe, and Ni are more evenly distributed between the dendrites and dendrites and yet Cr and Mo elements are more distributed in the σ phase (structure C).This σ phase isn't checked by XRD when the Mo content is low because the σ phase exists in a small amount, as presented in Fig. 3(c  Table 2 reveals the anode and cathode features of the alloy derived by fitting the polarization curve.As shown in the Table, the affixion of the Mo element strengthens the alloy's corrosion propensity, while the corrosion current density increases and subsequently drops, with values of 1.124 × 10 -6 A/cm 2 , 1.129 × 10 -6 A/cm 2 , and 6.982 × 10 -7 A/cm 2 , respectively.Lower I corr indicates higher corrosion resistance, and Ti 0.2 Mo 0.3 alloy has a much lower I corr than the other two alloys, indicating that Ti 0.2 Mo 0.3 alloy has the best corrosion resistance.Figs.4(b) and (c) show the Nyquist and Bode diagrams obtained by testing alloy samples.Because the corrosion resistance electrochemical curve of alloys can be conveyed by trigonometric function, a half ring can usually be seen in the high-frequency portion of the Nyquist illustration, and the bigger the semidiameter of the half ring is, the stronger the corrosion resistance of the alloy is.The Nyquist diagram shows that all samples have partial single-arc resistance in the high-frequency range.The arc radius of the arc resistance is greatest when the Mo concentration is 0.3, suggesting the optimum corrosion resistance.The Bode diagrams of all samples demonstrate that only a onetime constant is observed, and the maximum phase Angle values are all less than 90°, which may be attributed to the material surface's non-uniformity or mass transfer, resulting in a divergence from the ideal capacitance.Hence, as depicted in Fig. 4(d), the R (QR) model is chosen as the equivalent circuit design for fitting the EIS.The figure shows that the theoretical model and the test results have high consistency over the whole frequency range, showing that the equivalent circuit model is credible.In the equivalent circuit design, R 1 means solute resistance, R 2 means charge transport impedance (polarization resistance of the crosslinked film), and CPE means constant phase angle element.Its analogous element is Q, which is frequency independent and is referred to as the continuous phase Angle element.Q's impedance is defined as:

MATMA-2023
where Y 0 is the conductance of the CPE element, j is an unreliable figure, ω is the corner frequency, and n is the dispersion index.When n = 0, Q equals resistance R; when n = 1, Q equals ideal capacitance C; when n = −1, Q equals inductance L; when n = 0.5, Q equals Warburg impedance.As an outcome, the value of n varies from 0 to 1. Table 3 reveals that the dispersion index n number of the three high-entropy alloys are all between 0.8 and 1, indicating that rather homogeneous passivation coatings form on the three alloys' surfaces.Table 3 displays the equivalent circuit fitting findings.When the chi-square (χ 2 ) value of all fitted data is less than 10 -3 , the matched data is considered reliable.R 2 values may be used to assess the corrosion resistance of materials.A higher R 2 value suggests greater corrosion resistance.The R 2 values of the three high-entropy alloys are 4.83 × 10 5 , 9.38 × 10 4 , and 5.99 × 10 4 Ω•cm 2 , respectively.The measured findings of EIS are under the polarization curve.(2) When the electrolyte is 3.5 weight percent NaCl solution, the corrosion resistance of the alloy initially drops and then increases with the rise in Mo content.

Fig. 2
Fig. 2 briefed the microstructure morphology of CoCrFeNiTi 0.2 Mo x (x=0, 0.1, 0.3) alloys difference is that when the Mo content ≥ 0.1, there is a σ phase (fine sheet structure) in the dendrite region, increasing with a rise in Mo content.Fig. 3 shows the microstructure of a CoCrFeNiTi 0.2 Mo x alloys in backscattered electron (BSE) mode.The graphic shows that the volume percentage of the σ phase go up with the Mo content arise.No σ phase characteristic peaks were identified in the XRD plot above when the Mo concentration was raised to 0.1.The scanning electron microscope backscattering pattern and metallographic microstructure map, on the opposite hand, revealed the presence of phase.

Table 1 .
).With the increase of Mo propotion, it is confirmed by XRD analysis.EDS analysis results of CoCrFeNiTi 0.2 Mo x HEAs for different regions (at.%).

Table 2 .
E corr and I corr of CoCrFeNiTi 0.2 Mo x HEAs in 3.5 weight percent NaCl solution.

Table 3 .
Equivalent circuit value of CoCrFeNiTi 0.2 Mo x HEAs in 3.5 weight percent NaCl solution.In CoCrFeNiTi 0.2 Mo x (x= 0, 0.1, 0.3) alloys, Ti 0.2 alloy is FCC phase, Ti 0.2 Mo0.1 and Ti 0.2 Mo0.3 alloys are dominated by FCC phase and σ phase, and the volume proportion of the σ phase escalates as go up the Mo content.