Abstract: Al0.2CrFeNiCu1.5 high entropy alloy coating was successfully prepared on zirconium alloy rod by ultra-high speed laser cladding technology. The coating has good metallurgical bonding with the substrate and has no defects such as cracks and pores. The morphology and energy spectrum of the coating were analyzed by scanning electron microscopy, and the hardness and corrosion resistance of the coating were studied by micro-Vickers hardness tester and electrochemical workstation. The results show that there is mutual diffusion of elements between the bottom of the coating and the substrate, the heat affected zone is about 86 μm wide, and vertically growing columnar crystals can be observed at its interface. The coating structure is mainly composed of gray-black dendrite zone and gray-white interdendritic zone. The coating has uniform composition and the hardness can reach up to 690 HV. The unique solid solution strengthening, lattice distortion and slow diffusion effect of high entropy alloy are the main reasons for its high hardness. The self-corrosion current density of the coating is 6.241×10−8 A/cm2, the polarization resistance is 1.181×106 Ω/cm2, and the impedance is 2.326×106 Ω/cm2. The Al0.2CrFeNiCu1.5 high entropy alloy coating improves the corrosion resistance of the zirconium alloy substrate.
Keywords: laser cladding; high entropy alloy coating; corrosion resistance
Zirconium alloy is often used as a cladding material for nuclear fuel due to its advantages such as good processing performance, high corrosion resistance, high mechanical strength and low thermal neutron absorption rate [1]. In the past few decades, zirconium alloy cladding has been successfully used in light water reactors (LWRs) and has shown good radiation resistance and corrosion resistance. However, zirconium alloy cladding will react with water at high temperatures to accelerate the hydrogen absorption rate of the fuel cladding, causing hydrogen embrittlement and even “hydrogen explosion”, which is one of the main causes of major accidents in nuclear power plants.
At present, the traditional zirconium alloy cladding can no longer meet the requirements of the fourth generation reactor nuclear fuel elements. In view of the poor physical, chemical and mechanical properties of the existing zirconium alloy cladding in loss of coolant accidents and beyond design basis accidents, there are currently two main solutions: 1) Research and develop new cladding materials, such as stainless steel cladding, ceramic cladding, refractory metal cladding, etc., such as FeCrAl[2-3], Mo[4-5], MAX phase[6] materials and silicon carbide fiber reinforced silicon carbide composite ceramics (SiCf/SiC)[1] with high temperature resistance, corrosion resistance and oxidation resistance; 2) Surface coating strengthening coating, which can directly coat FeCrAl, MAX phase and silicon carbide materials on the surface of zirconium alloy, so that these coatings also have high temperature resistance, corrosion resistance and oxidation resistance. According to the latest research results at home and abroad, silicon carbide ceramic composite materials have excellent performance and can be directly applied to the cladding surface, but there are still technical obstacles in material preparation and welding process. The high-temperature mechanical properties of Mo alloy cladding are very good, but it is easily oxidized by water vapor at high temperature, and the neutron absorption rate of Mo alloy cladding is high, and the economic efficiency is poor. FeCrAl cladding has strong corrosion and oxidation resistance at high temperature, and also has radiation resistance, which can meet the performance requirements of accident-resistant materials, but the preparation process is relatively complex and the cost is high [7-9]. The research of new cladding materials requires a long time of exploration, and has high requirements for material performance, preparation process, and complex experimental environment. It also needs to consider economic factors. Considering the current composition and structural design of the coating, preparation and characterization methods, the preparation of the coating on the surface of the nuclear fuel zirconium alloy cladding tube has the advantage of not changing the nuclear reaction and the operating conditions in the reactor. Its technical feasibility and economy are the most likely effective way to improve the accident tolerance of the fuel cladding tube in a short period of time.
High entropy alloy is a new type of multi-principal alloy material composed of 5 or more elements in equimolar ratio or nearly equimolar ratio. It has attracted wide attention due to its unique composition design and excellent comprehensive mechanical properties. Since it was proposed in 2004, the number of related research papers has grown from dozens to tens of thousands, and various theoretical studies and new alloy compositions are increasing. High entropy alloy, bulk metallic glass and rubber metal are considered to be the three most promising alloys in recent decades [10]. Therefore, high entropy alloy has high academic research value and industrial application potential, and high entropy alloy coating is widely used on the surface of various parts and components. Laser cladding technology is currently one of the most commonly used methods for preparing high entropy alloy coating. Although there are many studies on high entropy alloy coating, due to the special requirements of nuclear fuel cladding for high temperature water vapor oxidation resistance, thermal shock resistance and thermal neutron absorption cross section, there is still little research on the application of high entropy alloy coating in nuclear fuel cladding accident tolerance.
Based on the protection of zirconium alloy cladding, this paper selects Al, Cr, Fe, Ni, and Cu, five low thermal neutron absorption cross-section elements, as the components of high entropy alloy coatings for design research. Ultra-high-speed laser cladding technology is used to prepare Al0.2CrFeNiCu1.5 high entropy alloy coatings on the surface of zirconium alloy, and the microstructure and corrosion resistance of the coatings are analyzed and studied.
1 Experiment and method
1.1 Preparation of powder and coating
The raw materials used in the experiment are all metal powders: Al powder, Cr powder, Fe powder, Ni powder, Cu powder (content ≥99.95%, particle size <50 μm), all powders are from Tianjin Haoknas Alloy Welding Material Co., Ltd. The powder was prepared by high-energy ball milling, which is mainly divided into three steps: 1) Weigh the original powder and grinding balls. First, weigh 1 mol of Al0.2CrFeNiCu1.5 alloy powder. Al powder, Cr powder, Fe powder, Ni powder, and Cu powder were weighed in a molar ratio of 0.2:1:1:1:1.5. The masses of the powders after weighing were: Al powder 5.39 g, Cr powder 52.00 g, Fe powder 55.85 g, Ni powder 58.69 g, and Cu powder 95.32 g. Then the grinding balls were weighed (the ball-to-material ratio was 10:1 by mass). In order to increase the number of effective collisions between the grinding balls and the powder during ball milling and make the grinding balls and the powder contact more fully, 5 kinds of stainless steel grinding balls with diameters (φ = 15, 12, 10, 8, 5 mm) were selected in the experiment; 2) Canning of powders and grinding balls. In order to prevent oxidation of powders during ball milling, the canning process was carried out in a glove box filled with argon (99.99%). The powders and the weighed 5 kinds of stainless steel grinding balls with different diameters were placed in a stainless steel ball milling jar and sealed; 3) Ball milling. A full-range planetary ball mill (QXQM-4, China) was used for ball milling. The speed was 300 r/min and the ball milling time was 30 h. In order to meet the particle size requirements of laser cladding powders, the powders after ball milling were sieved, and the powders between 15 and 50 μm were collected, dried, and sealed in bags.
The ball-milled high-entropy alloy powder was placed in a vacuum drying oven (DZF-6090AB, China) at 100 ℃ for 6 h and then placed in the feeding barrel of laser cladding for standby use. A φ45 mm zirconium alloy rod was selected as the substrate. The composition of the zirconium alloy rod is listed in Table 1. It was polished before use, and acetone was used to remove impurities and oil stains on the surface. The dried zirconium alloy rod was placed on a laser cladding lathe. Argon was used as the protective atmosphere to prepare the Al0.2CrFeNiCu1.5 high-entropy alloy coating on the zirconium alloy rod. By changing the parameters such as the power and scanning speed during cladding, a composite high-entropy alloy coating with good bonding with the zirconium alloy rod was obtained.
1.2 Test and characterization
Several sample blocks were cut from the zirconium alloy rod with high entropy alloy coating by wire cutting method. The upper surface of the sample blocks was ground and polished. In order to observe its microstructure, the polished surface was etched with an etchant
(aqua regia, etch time was 20 s) and observed with a scanning electron microscope (Regulus8100, Hitachi) with an energy dispersive spectrometer. The cross section of the coating after grinding and polishing was tested with a micro Vickers hardness tester (MH-6, China). The measurement was carried out from the upper surface of the coating to the substrate in sequence. Five points were tested in parallel each time, with a load of 1.96 N and a dwell time of 15 s. The corrosion resistance of the coating was tested with an electrochemical workstation (Gamry1010, China). Before the test, the five surfaces except the coating were wrapped. A three-electrode battery system was used, in which the platinum electrode, saturated AgCl solution and the sample were used as the auxiliary electrode, reference electrode and working electrode respectively. The samples were electrochemically measured at room temperature, with 3.5% NaCl solution as the electrolyte (simulating corrosion resistance in seawater), and the potentiodynamic polarization curve of the coating was measured at a voltage range of −1 V to 1 V and a scanning speed of 0.5 mV/s. The electrochemical impedance spectroscopy test was carried out at a stable open circuit potential, with a frequency range of 0.01 to 100 000 Hz and a voltage amplitude of 0.01 V. The samples were tested three times in parallel under the same environment.
2 Results and discussion
2.1 Analysis of the microstructure of the coating
Figure 1a shows the cross-sectional morphology at the interface between the coating and the zirconium alloy substrate after laser cladding. It can be seen that there is a good metallurgical bond between the coating and the zirconium alloy substrate, and there are no defects such as cracks and pores. Due to the high laser energy during cladding, there is a heat-affected zone with an average width of about 86 μm between the coating and the substrate. The energy spectrum analysis of the interface between the coating and the heat-affected zone is performed. As shown in Figure 1b, there is mutual diffusion of elements between the substrate and the coating. This is because the high laser energy during cladding has a certain dilution effect on the coating and the substrate [11]. According to the mixing enthalpy between elements, the more negative the enthalpy value, the easier it is for the two to combine and diffuse [12]. The mixing enthalpy between each element and zirconium is listed in Table 2 [13]. It can be seen that the diffusion capacity of the elements in the zirconium alloy matrix is: Cr＜Cu＜Fe＜Al＜Ni, so Ni and Al elements diffuse deeper downward, while the mixing enthalpy of Cr, Cu, Fe and Zr is small, the diffusion is slow, and it is easy to be enriched at the top of the heat-affected zone. At the same time, vertically growing columnar crystals can be seen at the interface between the coating and the heat-affected zone. It is known that the solidification structure of alloys mainly depends on the temperature gradient (G) and the solidification rate (R), and with G as the driving force, a larger G/R is usually more likely to form columnar crystals, while a smaller G/R is more likely to form equiaxed crystals [14]. During the laser cladding process, the heat dissipation at the junction of the bottom of the molten pool and the substrate is faster, and the temperature gradient is larger. From the bottom of the molten pool to the top of the coating, the G/R gradually decreases. Therefore, columnar crystals are more likely to form at the bottom of the coating, and equiaxed crystals are more likely to form in the middle and upper parts of the coating. The cross-sectional morphology of the middle and upper parts of the coating is shown in Figure 2a, and there are no defects such as cracks. When a part of the coating is enlarged, it can be seen that its structure is mainly composed of gray-black dendrites and gray-white interdendritic regions. This is mainly due to the different bonding forces between atoms, resulting in a certain degree of segregation between the elements in the alloy. Solid solution strengthening is one of the four major effects of high entropy alloys. In the process of forming a solid solution, due to the large number of constituent elements and the large binding energy between different elements, the total internal energy of the alloy is high. Therefore, the elements will segregate in the solid solution to reduce the internal energy [15]. Generally, the more negative the mixing enthalpy between elements, the easier it is to form a segregation zone of heterogeneous elements. The more positive the mixing enthalpy, the more likely it is to repel each other and form a segregation zone of homogeneous elements [13]. According to Table 2, the mixing enthalpy of Ni and Al is −22 kJ/mol, so the two are easily enriched together to form a stable NiAl phase; similarly, Fe and Cr are easy to form FeCr-enriched phases due to their negative mixing enthalpy and strong electronegativity. The mixing enthalpy between Cu and other elements is positive, and it is usually enriched at the grain boundary when forming a solid solution. The area (b) in the white frame on the upper part of the coating in Figure 2a and the area in the middle of the coating (c) are enlarged and analyzed, as shown in Figures 2b and 2c, respectively. It can be seen that the coating is composed of gray-black dendrites and gray-white interdendritic regions. Scanning them separately shows that Fe and Cr are enriched in the dendrite region, Al and Ni are enriched in the interdendritic region, and Cu is enriched at the interface between dendrites and dendrites, which is consistent with the analysis results of the mixing enthalpy above. It is worth noting that the remelting overlap area of the coating can also be seen in Figure 2a, because the ultra-high-speed laser cladding coating is prepared by overlapping in a “domino”-like manner, with a cladding ratio of up to 70% to 90%. Since this area will be heated multiple times, the temperature gradient here is small, and equiaxed crystals are usually formed.
2.2 Hardness analysis of coating
Figure 3 shows the microscopic Vickers hardness distribution from the top of the Al0.2CrFeNiCu1.5 high entropy alloy coating to the substrate. From top to bottom, it is divided into the coating area, heat-affected zone and substrate area, and the hardness of each area is quite different. The overall hardness fluctuation in the coating area is not large, indicating that the coating composition is uniform. Due to the dilution effect of laser cladding on the coating and substrate, the hardness begins to decrease in the heat-affected zone. The hardness of the coating is significantly higher than that of the substrate, up to 690 HV, which is about 1.53 times that of the substrate. The high hardness of the coating is related to the unique strengthening mechanism of high entropy alloys. First, solid solution strengthening is the main strengthening mechanism of high entropy alloys, and the resulting coating generally has higher strength and hardness [16]. Secondly, due to the large difference in atomic radius between the components of the coating, it may cause serious lattice distortion, hinder the movement of dislocations, and thus strengthen the coating; the slow diffusion effect of high entropy alloys will inhibit the diffusion between atoms. When the solidification rate is greater than the diffusion rate between atoms, it will promote the formation of solid solution [11]. Moreover, the characteristics of rapid heating and cooling of ultra-high-speed laser cladding promote the slow diffusion effect, enhance the structural stability of the solid solution, and improve the hardness of the coating.
2.3 Analysis of corrosion resistance of coating
In order to study the corrosion resistance of Al0.2CrFeNiCu1.5 high-entropy alloy coating, 3.5% NaCl solution was used to electrochemically test the coating and substrate. Figure 4 shows the potentiodynamic polarization curve.
In general, corrosion resistance is positively correlated with the self-corrosion potential and negatively correlated with the self-corrosion current density [17]. The self-corrosion potential of the coating is about −0.508 V, which is about 1.723 V positively shifted compared with the self-corrosion potential of the substrate of −2.231 V; the self-corrosion current density of the coating is 6.241×10−8 A/cm2, which is 2 orders of magnitude lower than that of the substrate, indicating that the coating has better corrosion resistance than the substrate. The Tafel linear extrapolation method can be used to obtain the anode constant (βa) and cathode constant (βc) of the polarization curve, and then the polarization resistance Rp of the coating can be calculated according to the Stern-Geary formula: (1).
Where Icorr is the self-corrosion current density. The calculated electrochemical parameters of the coating and the substrate are listed in Table 3. According to the calculation results, the polarization resistance of the coating is 1.181×10’6Ω/cm2, which is much larger than the polarization resistance of the substrate and 2 orders of magnitude higher than that of the substrate. A passivation region can also be observed in the polarization curve, indicating that a passivation film is formed on the surface. The wider the passivation region, the better the corrosion resistance of the coating[18]. As can be seen from Figure 4, the coating has a wider passivation region than the substrate, indicating that its corrosion resistance is better.
To further reveal the difference in corrosion resistance between the coating and the substrate, electrochemical impedance analysis of the coating and the substrate was performed, as shown in Figure 5. The impedance spectrum is usually composed of a capacitance loop and an inductance loop. The capacitance loop is related to the transfer of charge, and the inductance loop is mainly related to the corrosion product or the passivation film on the surface[19]. The coating mostly exhibits the characteristics of a capacitive loop in the solution, and the corrosion resistance of the coating is positively correlated with the radius of the capacitive arc in the impedance spectrum [20]. As shown in Figure 5a, the radius of the capacitive arc of the coating is significantly larger than that of the substrate, proving that the coating has good corrosion resistance. The Bode plot consists of an amplitude-frequency characteristic curve (Figure 5b) and a phase-frequency characteristic curve (Figure 5c). In Figure 5b, the impedance of the coating is higher than that of the substrate, indicating that the coating has good corrosion resistance in the NaCl solution. In the phase-frequency characteristic curve, the peak value of the phase angle is related to the passivation film of the coating. The larger the peak value, the stronger the protective ability of the passivation film. The peak value of the phase angle of the coating is higher than that of the substrate, and the peak value of the Al0.2CrFeNiCu1.5 high entropy alloy coating is the largest at around 1 Hz, with a phase angle of about 86°. Impedance is also an important indicator for characterizing the corrosion resistance of the coating [21]. The equivalent circuit fitting of the electrochemical impedance curve is shown in Figure 5d, where Rs is the solution resistance, Rp is the polarization resistance, and CPE is the constant phase element. After fitting, the impedance of the coating is 2.326×106 Ω/cm2, and the impedance of the substrate is 2.753×105 Ω/cm2, which is 1 order of magnitude higher than that of the substrate. The comprehensive electrochemical impedance analysis results are consistent with the conclusion of the polarization curve. The Al0.2CrFeNiCu1.5 high entropy alloy coating prepared by ultra-high-speed laser cladding has excellent corrosion resistance.
3 Conclusion
In this paper, the Al0.2CrFeNiCu1.5 high entropy alloy coating with no defects such as cracks and good metallurgical bonding with the substrate was successfully prepared on the zirconium alloy rod by ultra-high-speed laser cladding technology. The microstructure morphology, hardness and corrosion resistance of the coating were studied, and the conclusions were as follows.
1) The microstructure of the coating is mainly composed of gray-black dendrites and gray-white interdendritic regions, in which Fe and Cr are enriched in the dendrites, Al and Ni are enriched in the interdendritic regions, and Cu is mainly enriched at the grain boundaries.
2) The microscopic Vickers hardness of the coating is up to 690 HV. The solid solution strengthening, lattice distortion and slow diffusion effect of the high entropy alloy are the main reasons for its high hardness.
3) Compared with the zirconium alloy substrate, the coating has better corrosion resistance. The self-corrosion current density in 3.5% NaCl solution is 6.241×10−8 A/cm2, the polarization resistance is 1.181×10’6 Ω/cm’2, the capacitive reactance arc radius is large, showing the characteristics of a capacitive loop, and the impedance is 2.326×10’6 Ω/cm’2.
