summary
CoCrFeNiAlxMo2-x (x = 0, 0.5, 1, 1.5 and 2) high-entropy alloy coatings are applied to Q235 steel, and the effect of x changes on microstructure evolution and properties is investigated in this study. It is found that with the increase of x, the crystal structure of the coating changes from the face-centered cubic (FCC) + body-centered cubic (BCC) + σ+B2 phase to the BCC+σ+B2 phase, and finally becomes the BCC stage. At the same time, the microhardness, wear resistance, corrosion resistance and high temperature oxidation resistance of the coating first increase and then decrease with the increase of X. The optimal value is reached when x = 1. The enhancement of microhardness and wear resistance of high-entropy alloy (HEA) coatings is attributed to changes in phase type, diffusion strengthening, and solution strengthening. The addition of the Al element creates a passivation film on the surface of the coating, which mitigates galvanic corrosion. In addition, the appropriate amount of Mo increases the thickness and compactness of the passivation film, thereby improving its corrosion resistance. In the early high-temperature oxidation process, Al undergoes rapid oxidation, while Mo promotes the formation of Al2O3, which has excellent high-temperature oxidation resistance. In addition, the results were fitted using mathematical model equations to gain insight into the effects of changes in Al and Mo content (x-value) on the wear resistance, corrosion resistance and high-temperature oxidation resistance of the coating.
introduction
With the progress of science and technology, the performance requirements for materials are getting higher and higher. Yeh et al. proposed the concept of high-entropy alloy (HEA), which is defined as an alloy containing at least 5 but no more than 13 major elements, each with an atomic fraction between 5%~35%. Due to the high entropy effect, HEAs are mainly composed of solid solution phases (FCC, BCC, or hexagonal densely packed HCPs). Due to the abundance of slip systems, the FCC phase has relatively good plasticity and toughness, while the BCC phase (a deformation mechanism dominated by helical dislocations) is stronger and harder. With a well-balanced composition and microstructure, HEA can have good resistance to abrasion, oxidation, and corrosion. As a surface modification material, HEA can effectively improve the mechanical properties and service life of parts, and has broad prospects in the industrial field.
Laser cladding technology offers higher energy density and faster cooling rates than other surface modification methods, enabling the production of coatings with controlled thicknesses, fine grains, excellent metallurgical bonding to the substrate, and superior wear and corrosion resistance. Therefore, it is a special method of surface modification of metal materials. Over the past few years, there has been a lot of research into laser cladding HEAs. The performance of the HEA coating during surface modification is significantly affected by the selected material system.
CoCrFeNi is a typical HEA system composed of a single FCC solid solution, which has good ductility. However, its poor microhardness and high-temperature oxidation resistance limit its further development. Adding an appropriate amount of Al with a large atomic radius to CoCrFeNi HEA will result in lattice distortion, resulting in low density and good ductility and formability. Presently. Researchers have extensively studied the microstructure and properties of AlCoCrFeNi HEA, such as Wang et al., who prepared AlCoCrFeNi HEA coatings with high hardness (about 500 HV) and compressive strength up to 3530 MPa, but the corrosion resistance of the coatings was reduced due to the introduction of the second phase (B2 phase). In addition, excellent high-temperature stability and oxidation resistance can be obtained by adding high-melting point elements, such as Liu et al. observed that with the increase of W content, the microhardness and high-temperature wear resistance of CoCrFeNiWx HEA coating were significantly improved, which was attributed to the diffusion strengthening and solution strengthening of the unmelted W phase. Cheng et al. found that adding an appropriate amount of Ta to CoCrFeNi can promote the formation of Laves phase in CoCrFeNiTax coating, thereby improving the strength and hardness of the coating. Zhi and Wu et al. found that adding an appropriate amount of Mo was helpful for the formation and repassivation of the passivation film of the HEA coating during electrochemical corrosion, thereby improving the pitting resistance of the CoCrFeNiMox HEA coating in Cl-containing solution. Obviously, both the high melting point element Mo and the light element Al can improve the performance of CoCrFeNi HEA coatings. Previous studies have explored the simultaneous addition of these two elements to CoCrFeNi HEA. Therefore, the addition of Al and Mo elements to CoCrFeNi HEA can improve the comprehensive mechanical properties and corrosion resistance of the alloy. With the increasing demand for corrosion performance in engineering, in recent years, CoCrFeNiAlMo HEA material has far-reaching application prospects in many fields such as transmission wheel hubs, aerospace engines, petrochemical pipelines, etc. For example, Juan et al. prepared a CoCrFeNiAlMox coating and observed that as the x value increased, the solid solution rich in Mo and Fe transformed into an intermetallic compound rich in Mo and Fe, resulting in an increase in hardness due to solution strengthening and solution strengthening. Dispersion intensification. These coatings are found to be about 3.5 times harder than 45# steel. At the same time, Hakan Gasan et al. found that the AlxCoCrFeNiMo coating contained relatively hard σ phases, B2 phases, and BCC phases. With the addition of the Al element, the hardness value of these coatings increased from 524.5 HV to 829.8 HV. Similarly, Sha et al. prepared AlxCoCrFe2.7MoNi coatings and noted that the increase in Al content promoted the formation of σ and B2 phases rich in Mo and Cr. As a result, the corrosion resistance, hardness, and wear resistance of the HEA coating were increased by 2.0 times, 4.2 times, and 4.0 times, respectively. However, in previous studies, the researchers examined the effects of another element while keeping the Al or Mo content constant. The effect of synergistic changes between the two elements has not been reported. In this study, CoCrFeNiAlxMo2-x (x = 0, 0.5, 1, 1.5, and 2) HEA coatings were prepared on Q235 steel mild steel surfaces using a laser cladding process. The researchers investigated the effect of synergistic changes in Al and Mo content on the microstructure and properties of HEA coatings.
Experimental setup
Q235 steel is widely used in coal mining, construction and other fields because of its good plasticity and low raw material cost. However, harsh working environments can lead to severe wear and corrosion, reducing the service life of materials. Therefore, Q235 surface treatment is an important way to improve material performance and reduce maintenance costs. The raw materials for the laser coating are composed of Co, Cr, Ni, Al and Mo powders, each of which is more than 99.5% pure and has a particle size ranging from 75 to 150 μm. The Fe element is obtained from the molten base metal. The powder was mixed in a V-shaped powder mixer for 2 hours, and a 1 mm thick layer of HEA powder was formed on the surface of 70 mm × 50 mm × 6 mm Q235 steel using water glass as the binder. After drying, the coating (x:atom ratio, x = 0, 0.5, 1, 1.5, and 2) was prepared using a YLS-10 kW laser processing system for CoCrFeNiAlxMo2-x HEA. The selected process parameters are shown in Table 1 according to the optimization test of the process parameters.
To analyze the phase composition, the prepared samples were cut into 10 mm × 10 mm blocks, then ground and polished, and phase analysis was performed using X-ray diffraction (XRD). XRD analyzed a tube voltage of 40 kV, a tube current of 40 mA, a scan range of 20°–85°, and a scan rate of 2°/min. The microstructure was characterized using scanning electron microscopy (SEM, JEOL-7800F) and the chemical composition of different typical regions was analyzed using energy dispersive spectroscopy (EDS).
The microstructure of HEA coatings was investigated by electron backscatter diffraction (EBSD, Nordly Max3, Oxford). Particle size was calculated using Image-Pro Plus 6.0 software (average over 10 measurements). Selected microregions were characterized by transmission electron microscopy (TEM, FEI Talos F200X).
The microhardness of the cross-section of the sample was measured using a micro-Vickers hardness tester (HXD-1000TMC) with a test load of 200 g and a duration of 15 s. In order to ensure the accuracy of the results, each sample was tested 3 times, and the average value was taken for comparative analysis.
For room-temperature dry friction properties, the coated samples and Q235 steel were tested using a tribo-wear tester (HSR-8M) with a round-trip length of 235 mm. WC carbide balls with a diameter of 4 mm and a microhardness of 1424 HV0.2 (Co content of 6%) were used as the friction pair, and the test time was 30 min.
After 20 minutes of ultrasonic cleaning with absolute ethanol, the width and depth of the wear marks and the amount of frictional wear were measured using the Contour Elite K, and the topography of the worn surfaces was observed using SEM. X-ray photoelectron spectroscopy was used to identify the change of elemental valence state in coating wear fragments. (XPS, using AXIS SUPRA+). The wear rate (W) is calculated using the following formula:
where W is the wear rate (mm3/(N⋅m)), V is the amount of wear (mm3), D is the normal load (N), and L is the sliding distance (m).
Electrochemical tests were performed at room temperature using an electropotentiostat (Gamary Interface 1000) in a 3.5 wt% NaCl solution. The coating and Q235 steel as a working electrode have an exposed area of 1 cm2. The auxiliary electrode (CE) is a platinum sheet electrode and the reference electrode is a saturated calomel electrode. The AC impedance spectroscopy (EIS) test is performed at an open-circuit potential (OCP) with a frequency range of 105 to 10−2 Hz and an amplitude of 10 mV. The potentiodynamic polarization curve has a scan range of -0.5 V to 1 V (relative to OCP) and a scan rate of 0.5 mV/s. To study the high-temperature oxidation resistance, the samples were placed in a corundum crucible and heated at 800 °C for 50 hours in a muffle furnace (XL-1200C). At specific time intervals (1, 3, 5, 10, 20, 30, 40 and 50 hours), the oxides are weighed using an electronic balance. XRD was used to analyze the phase information of oxides, and SEM and EDS were used to characterize their microstructure and elemental composition.
Results & Discussion
Figure 1. (a) Top view and (b) cross-sectional topography of the CoCrFeNiAlMo HEA coating, (c) microscopic topography near the fusion zone and (d) corresponding EDS line scan results
Figure 2. XRD results of CoCrFeNiAlxMo2-x HEA coating
Figure 3 Microstructure of CoCrFeNiAlxMo2-x HEA coating: (a) x = 0, (b) x = 0.5, (c) x = 1, (d) x = 1.5 and (e) x = 2
Figure 4. EBSD analysis of CoCrFeNiAlMo HEA coatings: (a) reverse pole diagram (IPF), (b) kernel mean orientation error (KAM)
Figure 5. The average grain size of the HEA coating
Figure 6 TEM image of CoCrFeNiAl0.5Mo1.5 HEA coating: (a) brightfield image, (b) selective electron diffraction (SAED) for region c
Figure 7. Microhardness distribution of a cross-section of a CoCrFeNiAlxMo2-x HEA coating
Figure 8. (a) coefficient of friction, (b) width and depth of wear marks, (c) and (d) wear rate of HEA coating and Q235 steel after 30 minutes of wear
Fig.9 Fitting results of the relationship between the wear rate of HEA coating and the function of x-value
Figure 10. Wear curves for CoCrFeNiAlxMo2-x HEA coatings: (a) Q235 steel, (b) x = 0; (c) x = 0.5, (d) x = 1, (e) x = 1.5, and (f) x = 2
Fig. 11 XPS spectra of (a) Al, (b) Mo, (c) Fe, (d) Cr in abrasive grains on x=0, 1, and 2 coating surfaces
Figure 12. Potentiodynamic polarization curves of CoCrFeNiAlxMo2-x HEA coating and Q235 steel in 3.5 wt% NaCl solution
Fig.13. Fitting results of the relationship between corrosion current density and x-value function
Figure 14. EIS results of CoCrFeNiAlxMo2-x HEA coating and Q235 steel in 3.5 wt% NaCl solution: (a) and (b) Nyquist plots, (c) impedance amplitude and (d) Bode plots of phase angle versus frequency
Figure 15. Equivalent circuits of (a) RsQdlRct and (b) Rs(QdlRct)(QfRf).
Fig.16. Fitting results of charge transfer resistance as a function of x-value
Figure 17 Schematic diagram of the corrosion mechanism: (a) x = 0, (b) x = 0.5, (c) x = 1 and 1.5, and (d) x = 2
Figure 18. (a) oxidative weight gain curve and (b) oxidation kinetics curve of HEA coating
Figure 19 Fitting of the parabolic velocity constant as a function of x
Fig.20 XRD pattern of the oxide layer formed on the coating after 50 h of high-temperature oxidation
Figure 21 Cross-sectional micromorphology and EDS elements of HEA-coated and Q235 steel after 50 h of high-temperature oxidation: (a) Q235 steel, (b) x = 0, (c) x = 0.5, (d) x = 1, (e) x = 1.5 and (f) x = 2
Figure 22. Gibbs free energy of CoMoO4, NiO, CoO, Fe2O3, Cr2O3, Al2O3 as a function of temperature
Figure 23. Schematic diagram of the high-temperature oxidation behavior of an HEA coating: (a) x = 0, (b) x = 0.5, (c) x = 1, (d) x = 1.5, and 2
conclusion
CoCrFeNiAlxMo2-x (x=0, 0.5, 1, 1.5 and 2) HEA laser coatings were prepared on the surface of Q235 steel, and its microstructure and properties were studied. The following conclusions were drawn:
1. When x = 0 and 0.5, the coating consists of BCC + FCC + σ + В2 phases. As the Al content increases, the FCC and σ phases decrease, while the BCC and B2 phases increase. When x=2, only the BCC phase is present in the coating.
2. The microhardness, wear resistance, corrosion resistance and high temperature oxidation resistance of HEA coating all increase first and then decrease with the increase of x, and reach the best when x = 1.
3. With the increase of x, the FCC phase is transformed into the BCC hard phase, and the σ phase and B2 phase are separated from the FCC phase and BCC phase, respectively, and the diffusion strengthening effect is significant. In addition, the strengthening of Al and Mo elements with large radius increases the degree of lattice distortion and enhances the effect of solution strengthening. The synergistic variation of the content of these elements affects the microhardness and wear resistance of the alloy.
4. In the process of electrochemical corrosion and high-temperature oxidation, an appropriate amount of Mo element is conducive to the formation of Al2O3, resulting in the formation of a denser protective oxide film on the surface of HEA coating. This hinders ion diffusion, which enhances the coating's resistance to corrosion and oxidation at high temperatures.
Thesis information
Microstructure and properties of CoCrFeNiAlxMo2-x high-entropy alloy coating by laser cladding
https://doi.org/10.1016/j.intermet.2023.108169
The copyright of this article belongs to the original author, only for communication and learning, and the final interpretation right belongs to this official account (laser manufacturing research).
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