Effects of Ultrasonic Shot Peening on the Corrosion Resistance and Antibacterial Properties of Al_0.3Cu_0.5CoCrFeNi High-Entropy Alloys

Abstract

Cu-bearing high-entropy alloys (HEAs) have been proposed for use as structural materials in the marine environment due to their superior mechanical and antimicrobial properties. However, the Al, Cu-enriched precipitations in HEAs damage their corrosion resistance. In this study, we used ultrasonic shot peening (USSP) technology to solve this problem. USSP caused severe plastic deformation of the Al_0.3Cu_0.5CoCrFeNi HEA surface and dispersed the long-strip Al, Cu-enriched phases into scattered dots, which reduced the galvanic corrosion of the HEA and enhanced passive film formation. The Al, Cu-enriched scattered precipitations also increased the number of Cu²⁺ ion dissolution sites, leading to the improvement of the alloy’s antibacterial properties.

1. Introduction

High-entropy alloys (HEAs), with their huge range of complex compositions and microstructures, are newly developed alloy systems [1]. Unlike traditional alloys, which contain one or two essential elements, HEAs are prepared with five or more principal elements in near-equal molar ratios, with each element in a concentration range of 5–35 at.% [2,3]. HEAs have been developing explosively in recent years—due in part to their multiple, fascinating properties such as their high strength and hardness [4], outstanding high temperature strength [5], and superior corrosion resistance [6].

Cu-bearing HEAs have been proposed as a new type of structural material for marine applications due to their strong antibacterial properties [7]. In marine environments, microbiologically influenced corrosion and seawater corrosion are the two main factors that cause material failure [8,9]. Zhou et al. [10] designed a novel Al_0.4CoCrCuFeNi HEA with superior antimicrobial properties; this Cu-bearing alloy inhibits the growth and formation of biofilms by releasing high concentrations of Cu²⁺ ions into environments where large numbers of marine bacterial species live. This high-Cu content HEA exhibits 99.99% antibacterial effectiveness against the marine gram-negative Pseudomonas aeruginosa and the gram-positive Bacillus vietnamensis. Ren et al. [11] prepared an antibacterial Cu_0.3Co_0.4FeCr_0.9 HEA without any intricate annealing treatment of the antimicrobial stainless steel (heat treatment, quenching, and severe cold-rolling); this alloy presented a 99.97% and 99.96% antibacterial rate against Escherichia coli and Staphylococcus aureus, respectively—which is significantly better than the rates of the classic antibacterial 304-Cu SS. A CuCoCrFeNi HEA fabricated by selective laser melting has also shown excellent broad-spectrum antibacterial abilities [12].

However, Cu-bearing alloys are more prone to pitting corrosion in seawater, which threatens the safe use of these materials [13,14]. The Al, Cu-enriched precipitations in AlCuCoCrFeNi HEAs could destroy their protective passive film and cause serious galvanic corrosion. Hsu et al. [15] found that increased Cu content in FeCoNiCrCu_x HEAs increases the risk of localized corrosion, due to the appreciable potential difference between the Cu-rich interdendrite and the Cr-rich dendrites in the alloys. The Cu elemental segregation in CuCrFeNiMn HEAs also deteriorates their general corrosion resistance [16]. It is necessary to improve the corrosion resistance of Cu-bearing HEAs for their practical application in the ocean. Ultrasonic shot peening (USSP) is a common method for causing the severe plastic deformation (SPD) of material surfaces [17,18]. Specifically, the USSP device drives tiny metal balls through an ultrasonic generator so that the balls hit the alloy surface at a great speed, resulting in the SPD of the alloy surface. This plastic deformation can induce the formation, accumulation, and evolution of defects such as dislocations, stacking faults, and twin crystals in the alloys—thus producing nanocrystals or ultrafine grains. Yuan et al. [19] reported that SPD can achieve the atomic-scale homogenization of the Al_0.3Cu_0.5CoCrFeNi HEA, producing an homogeneous, single-phase microstructure without the clustering of Cu, Al, and Ni—which is beneficial for alleviating the galvanic corrosion of this alloy. Perumal et al. [20] used the SPD method to homogenize the microstructure of the dual-phase MoNbTaTiZr HEA; the passive film formed on the homogenized MoNbTaTiZr was more stable. After SPD, the hydrophobicity and biocorrosion resistance of the MoNbTaTiZr alloy were greatly improved, compared with the as-cast alloy.

According to the above discussion, USSP was used here to solve the contradiction between the corrosion resistance and the antibacterial properties of an Al_0.3Cu_0.5CoCrFeNi HEA. This paper compares the surface microstructure features of as-cast and USSP-processed Al_0.3Cu_0.5CoCrFeNi HEAs, analyzes the composition and structure of the passive films formed on the HEAs, and comprehends the relevant mechanisms of the effects of USSP on the corrosion resistance and antimicrobial properties of the Al_0.3Cu_0.5CoCrFeNi alloy—serving as a case for the further development of Cu-bearing HEAs with unique levels of corrosion resistance and antibacterial abilities.

2. Materials and Methods

2.1. Materials

The Al_0.3Cu_0.5CoCrFeNi HEA in this study was prepared using vacuum arc-melting technology. The raw materials—high purity (≧99.99 wt.%) Al, Cu, Co, Cr, Fe, and Ni metals—were melted under a vacuum (0.01 atm) environment repeatedly for 7 times; each smelting time was 5 min. In the melting process, the electromagnetic stirring function was turned on to improve the uniformity of the alloy composition. The atom ratio of Cu/(AlCoCrFeNi) was 0.5/(0.3 + 1 + 1 + 1 + 1). The specimens for the USSP process were cut into 1 cm × 1 cm × 0.3 cm slices and mechanically ground with 600#, 1000#, and 2000# sandpapers. The USSP device generated energy using high-frequency and high-power ultrasonic generators. A piezoelectric vibrating transducer was used to convert an ultrasonic vibration into a longitudinal mechanical vibration of the same frequency. The mechanical vibration was amplified by the vibrating horn, producing a high-energy wave to impact the HEA surface. The main parameters of the USSP process were designed as follows: the vibration frequency of the chamber was 15 kHz, the diameter of the GCr15 balls was 1.5 mm, and the processing duration was 300 s.

2.2. Surface Analysis

The surface hardness of the USSP-processed Al_0.3Cu_0.5CoCrFeNi HEA specimen was measured using an Anton-Paar NHT³ nano-indenter (Graz, Austria). The test mode was the load–unloading method and the indenter displacement was 500 nm. The surface microstructure of the HEA specimens was characterized by X-ray diffraction (XRD, D2 PHASER, BRUCKER, Billerica, MA, USA) with Cu Kα radiation. The scanning scope of the XRD was 20–90° and the scanning speed was 1°/min. The passive film compositions of the Al_0.3Cu_0.5CoCrFeNi HEA were analyzed by an X-ray photoelectron spectrometer (XPS, ESCALAB 250XI, Thermo Scientific, Waltham, MA, USA) with a routine monochromatic Al anode source. The XPS measurements were performed after the HEA specimens were immersed for 7 days in 3.5 wt% NaCl solution. The spectra results were fitted by Avantage software. The phase structures and corrosion morphologies of the HEAs were observed using a scanning electron microscope (SEM, Sigma 300, ZEISS, Jena, Germany) equipped with an energy-dispersive X-ray spectrometry (EDS) module. The elemental compositions of the HEA phases were detected by an EDS point scan.

2.3. Electrochemical Tests

The as-cast and USSP-processed HEAs were polished with a 1.5 μm-size diamond polishing paste. Epoxy resin was used to encapsulate and prepare the standard electrochemical specimens for the tests. The electrochemical tests were carried out using a Reference 600 Plus Gamry electrochemical workstation with a typical three-electrode electrochemical cell. The saturated calomel electrode (SCE) served as the reference electrode and the platinum sheet served as the auxiliary electrode. The area of the working electrodes surface area was 1 cm². Both potentiodynamic polarization and EIS tests were performed in a 3.5 wt.% NaCl solution at 22 ± 1 °C. All the HEA specimens were corroded freely to reach the quasi-stationary values of the open circuit potential (OCP) before tests. The potentiodynamic polarization tests were carried out at a scan rate of 100 mV/min, from −0.65 V_SCE to 0.60 V_SCE. Electrochemical impedance spectroscopy (EIS) tests were performed at the OCP; the sinusoidal potential amplitude was 10 mV and the frequency range was 100 kHz–10 mHz. These tests were carried out at least three times to ensure the data’s reproducibility.

2.4. Antibacterial Tests

P. aeruginosa and S. aureus were cultivated for linear polarization resistance (LPR) and antibacterial property tests. Sterile Marine 2216E (PH = 7.6 ± 0.2) and Luria Bertani (LB; pH = 7.2 ± 0.1) media were prepared for the inoculation of P. aeruginosa and S. aureus, respectively. The LPR tests were taken at a scanning rate of 0.125 mV/s, from −10 mV_SCE to 10 mV_SCE, in the 2216E medium with P. aeruginosa. The initial inoculum concentration of P. aeruginosa was 10⁶ cfu/mL. The culture flasks of the bacterium were preserved in an incubator at 37 °C. For the antibacterial tests, all the relevant experimental instruments were sterilized at 121 °C for 20 min before use. The HEA specimens were immersed in 75% ethanol for 1 h and dried naturally in an ultra-clean cabinet. The sterile HEA specimens were placed in conical flasks with P. aeruginosa and S. aureus, respectively, for 72 h. Then, the unattached bacteria on the HEA specimens were washed away by 0.1 M phosphate buffer solution (PBS). The washed HEA specimens were placed in 5 mL EP tubes with 3 mL PBS. The EP tubes were vibrated for 3–5 min to transfer the bacteria from the specimens to the PBS. The planktonic bacteria solutions in the EP were diluted and plated on 2216E and LB agars, respectively. Finally, the number and morphology of the bacteria were observed.

3. Results and Discussion

3.1. XRD and Hardness

XRD was used to identify the phase composition of the as-cast and USSP-processed Al_0.3Cu_0.5CoCrFeNi HEA specimens. As Figure 1a shows, both of the two specimens showed one set of obvious diffraction peaks corresponding to a face-centered cubic structure. The peaks of the Al₂Cu and A_l4Cu₉ phases in the USSP-processed specimen were weakly present compared with the as-cast HEA, indicating that the surface severe plastic deformation of the USSP may have dispersed the precipitated phases of the alloy; therefore, the partially precipitated phases could not be detected by XRD. The surface hardness of the USSP-processed specimen was measured as in Figure 1b. The hardness in the surface of the USSP-processed Al_0.3Cu_0.5CoCrFeNi is 4095 MPa was much higher than in the matrix (3300 MPa). The deformed layer thickness of the USSP-processed specimen was within 60 μm.

3.2. Electrochemical Results

Potentiodynamic polarization tests were carried out to characterize the corrosion properties of the Al_0.3Cu_0.5CoCrFeNi HEA specimens. As illustrated by the curves in Figure 2, the as-cast HEA specimen only showed a single Tafel region, and the USSP-processed specimen showed a more significant passive region above the Tafel region. This feature indicates that a protective film formed on the USSP specimen at the corrosion potential. USSP was able to improve the surface activity of the alloy, promote the dissolution of Fe and the oxidation of Cr, and reduce the electronic work function of this alloy—leading to the formation of this compact passive film. The relevant potentiodynamic polarization parameters are summarized in

Table 1. Potentiodynamic polarization parameters for the as-cast and USSP-processed Al_0.3Cu_0.5CoCrFeNi specimens. Values within parentheses are standard deviations.

HEA Specimens	Ecorr (mVSCE)	Icorr (nA/cm2)	Ipass (nA/cm2)	Ep (mVSCE)
As-cast	−251.8 (±7.2)	740.6 (±50.4)	-	-
USSP	−249.4 (±9.7)	143.2 (±6.2)	270.2 (±12.3)	−85.5 (±5.8)

. The corrosion potential (E_corr) was defined as the potential of the HEA specimens in an open-circuit situation. The corrosion current density (I_corr) was calculated by the extrapolation of the polarization curve. The critical pitting potential (E_p) was the potential of the passive film breakdown. The I_pass was the current density in the passivation area of the material. The I_pass value was taken from the middle of the passive region in the polarization curve [21]. As Table 1 shows, the I_corr of the Al_0.3Cu_0.5CoCrFeNi HEAs was reduced from 740.6 nA/cm² to 143.2 nA/cm² after the USSP process, and the value of the E_corr changed little. The I_pass and E_p of the USSP-processed HEA specimen were 270.2 nA/cm² and −85.5 mV_SCE, respectively. The potentiodynamic polarization result shows that the USSP process contributed to the generation of a more protective passive film than in the as-cast alloy.

Figure 3 shows the Nyquist and Bode diagrams of the EIS results for the Al_0.3Cu_0.5CoCrFeNi specimens immersed in 3.5 wt% NaCl solution. The Nyquist plots show the semicircular arc features. The diameter of the depressed semicircle for the USSP-processed specimen was significantly larger than that for the as-cast one, which indicates the high charge-transfer resistance of the USSP-processed alloy [22]. The Bode plots show that both the capacitance resistance and phase angle of the USSP specimen were larger than those of the as-cast alloy in the whole-frequency region, indicating that the protective ability of the passive film formed on the USSP-processed specimen was enhanced. The inset of Figure 3a shows the equivalent electrical circuit (EEC) that was designed as the best fit for the measured results. This EEC consisted of two (R/CPE) parallel circuit elements. R_s is the solution resistance, R₁ and C₁ are the charge transfer resistance and the double layer capacitance—which reflect the electrochemical activity of the film/solution interface—and R₂ and C₂ are the film resistance and capacitance [23]. The CPE (constant phase element) replaced pure capacitance in the EEC to compensate for the deviation between the double-layer capacitor and pure capacitance. Z_CPE is the impedance of the CPE and is expressed as follows [24]:

$${\mathrm{Z}}_{\mathrm{CPE}}=\frac{{\left(jw\right)}^{-n}}{Y}$$

(1)

where Y is a parameter of the CPE, j is the imaginary unit, w is the angular frequency, and n is a dimensionless exponential. The capacitance C was calculated using Y and n [25]:

$$\mathrm{C}=Y^{1/n}{R}^{\left(1-n\right)/n}$$

(2)

The fitting parameters of the EIS results for the Al_0.3Cu_0.5CoCrFeNi HEA specimens are summarized in

Table 2. Equivalent-circuit element values for the EIS data corresponding to the as-cast and USSP-processed Al_0.3Cu_0.5CoCrFeNi specimens.

HEA Specimens	RS (Ω·cm2)	R1 (Ω·cm2)	R2 (Ω·cm2)	CPE1 Parameters		CPE2 Parameters
				Y1 (μF/cm2)	n1	Y2 (μF/cm2)	n2
As-cast	3.46	8.74 × 105	1.14 × 104	540	0.81	56.9	0.95
USSP	3.34	8.13 × 106	2.07 × 106	174.9	0.78	24.7	0.98

. It can be seen that the USSP process greatly improved R₁ from 8.74 × 10⁵ Ω·cm² to 8.13 × 10⁶ Ω·cm² and R₂ from 1.14 × 10⁴ Ω·cm² to 2.07 × 10⁶ Ω·cm², and decreased the Y₁ and Y₂ values—indicating that the film formed on the USSP-processed Al_0.3Cu_0.5CoCrFeNi was more compact and protective.

Figure 4 shows the corrosion morphologies of the Al_0.3Cu_0.5CoCrFeNi specimens after potentiodynamic polarization tests in 3.5 wt% NaCl solution. The surface of the as-cast HEA shows severe corrosion pits, possibly caused by galvanic corrosion as they are mainly located on the precipitation phases of this alloy. These pits can be distributed as long strips or be dotted. The length of the long-strip pits is about 4–70 μm and their width is 3–6 μm. The diameter of dotted pits is about 5 μm. For the USSP-processed HEA, the number of pits was clearly decreased compared with the as-cast HEA. Most of these pits were dotted. The corrosion depth of these pits was slight. The SEM micrographs of the specimens were consistent with the results of the potentiodynamic polarization and EIS curves.

3.3. Antibacterial Properties

In addition to corrosion resistance, the USSP process also affected the antibacterial properties of the Al_0.3Cu_0.5CoCrFeNi HEA. Figure 5 shows the variation trend of the linear polarization resistance R_p of the as-cast and USSP-processed Al_0.3Cu_0.5CoCrFeNi specimens that were immersed in 2216E medium and inoculated with P. aeruginosa for 14 days. The R_p values of both of the two specimens decreased with a prolonged immersion time, indicating that P. aeruginosa caused the corrosion of the Al_0.3Cu_0.5CoCrFeNi HEAs [26]. The R_p values of the USSP-processed alloy specimen were commonly higher than those of the as-cast alloy, which shows that the USSP process significantly reduced the corrosion of the Al_0.3Cu_0.5CoCrFeNi HEA in the P. aeruginosa culture medium. This conclusion was further demonstrated by the antibacterial tests; Figure 6 shows the results of the plate count tests for the as-cast and USSP-processed Al_0.3Cu_0.5CoCrFeNi HEA specimens; it can be observed clearly that the antibacterial effects of this alloy for P. aeruginosa (gram-negative bacteria) were better than for S. aureus (gram-positive bacteria), which was attributed to the thicker cell-wall structure of the gram-positive bacteria impeding the diffusion of copper ions. Moreover, it can be easily noted that the USSP-processed Al_0.3Cu_0.5CoCrFeNi HEA exhibited faster and more effective antibacterial abilities in S. aureus and P. aeruginosa culture mediums compared with the as-cast HEA specimen. The USSP process improved the corrosion resistance and antibacterial properties of the Al_0.3Cu_0.5CoCrFeNi HEA, which was further analyzed by XPS and SEM.

3.4. XPS and SEM Analyses

The corrosion resistance and antibacterial properties of an alloy are related to the passive film on the alloy’s surface. XPS was used to investigate the chemical compositions of the film. The film detection area was about 0.5 mm × 0.5 mm, and the detection depth was within the nano-meter scale. The spectra of the Al 2p, Cr 2p3/2, Fe 2p3/2, and Cu 2p in the passive films formed on the as-cast and USSP-processed Al_0.3Cu_0.5CoCrFeNi specimens were separated. As Figure 7 shows, the Al and Cr spectra were separated as Al at 72.3 eV, Al³⁺ ox/hy at 74.8 eV, Cr at 573.5 eV, and Cr³⁺ ox/hy at 576.7 eV, respectively. The Fe spectra were separated into metallic-state Fe at 706.7 eV, ionic state Fe²⁺ at 708.0 eV, and Fe³⁺ at 712.2 eV. The Cu 2p spectra peaks were separated into Cu at 932.2 eV and Cu²⁺ ox at 933.5 eV. The quantitative results of these oxide film compositions were calculated using the following expression [27]:

$$M_i=\frac{100A_i}{{{\displaystyle \sum }}_{i=1}^n{A}_i}$$

(3)

where M_i is the percentage of each composition, i represents the composition, n is the number of peaks, and A_i is the intensity of each peak. Figure 8 shows the percentages of the Al, Cr, Fe, and Cu species in passive films formed on the as-cast and USSP-processed HEAs. It can be seen that the content of Al³⁺ and Cu²⁺ in the passive film decreased from 98.4% to 93.5% after USSP processing, and the contents of Cr³⁺ and Fe^2+,3+ in the film changed little—indicating that the USSP did not affect the formation of the main protective oxides (Cr₂O₃ and Fe₂O₃) or the bi-layer structure of the passive film [28]. However, the lower content of Al, Cu oxides and hydroxides was beneficial for preventing the formation of porous films on the USSP-processed Al_0.3Cu_0.5CoCrFeNi specimen [29], resulting in better corrosion resistance. According to the XPS results, it seems that the USSP process should not produce such an obvious effect on the corrosion resistance and antibacterial properties of Al_0.3Cu_0.5CoCrFeNi via the strengthening of its passive film; this issue was further clarified by microstructure observations and compositional analyses of the Al_0.3Cu_0.5CoCrFeNi HEAs. As Figure 9 shows, both the as-cast and USSP-processed Al_0.3Cu_0.5CoCrFeNi HEAs presented with significant precipitated phases. The EDS results showed the precipitations as being enriched with Al, Cu elements and lacking Cr elements, which would lead to severe galvanic corrosion between the Cr-depleted precipitation and the alloy matrix. The USSP process changed the precipitation morphology from continuously distributed long strips to scattered dots, which clearly reduced its galvanic corrosion—as shown in Figure 4. Meanwhile, the Al, Cu-enriched scattered dots increased the dissolution areas of the Cu²⁺ ions, leading to better antibacterial abilities in the USSP-processed alloy.

4. Conclusions

The USSP technique was used to ameliorate the corrosion resistance and antibacterial properties of Al_0.3Cu_0.5CoCrFeNi high-entropy alloys. The electrochemical tests showed that USSP promoted the formation of a passive film on the Al_0.3Cu_0.5CoCrFeNi surface and reduced the I_corr of this alloy from 740.6 nA/cm² to 143.2 nA/cm². Both the values of the transfer resistance R₁ and the passive film resistance R₂ of the USSP-processed Al_0.3Cu_0.5CoCrFeNi HEAs were much higher than those of the as-cast alloy. The antibacterial tests indicated that P. aeruginosa accelerated the corrosion of the Al_0.3Cu_0.5CoCrFeNi HEAs. The USSP process reduced the microbiologically influenced corrosion of this HEA, which presented as reduced R_p values and inhibition of the activity of P. aeruginosa and S. aureus on agar plates. The XPS and EDS results suggest that the USSP process dispersed the large-size Al, Cu-enriched precipitations in the Al_0.3Cu_0.5CoCrFeNi alloy, making the passive film more uniform and compact and preventing the galvanic corrosion of this alloy. Meanwhile, all the dispersed Al, Cu-enriched phases were able react as Cu²⁺ dissolution sites to inhibit the activity of the bacteria. Thus, the USSP-processed Al_0.3Cu_0.5CoCrFeNi HEA showed better antibacterial properties.