Single-phase high-corrosion-resistant Fe-Cr-Ni medium-entropy alloy, preparation method and application thereof

Abstract

This invention relates to a single-phase, high-corrosion-resistant Fe-Cr-Ni medium-entropy alloy, its preparation method, and its applications. The alloy is composed of the following components by mass percentage: Fe: 31-38%, Cr: 23-31.5%, Ni: 33-40%, with the balance being unavoidable impurities, and the mass percentage content being less than 0.07%. The alloy is a single-phase face-centered cubic solid solution. After high-temperature oxidation, part of the alloy remains a single-phase FCC, while the remaining alloy precipitates a small amount of Cr-rich BCC phase. This alloy exhibits a good combination of strength and ductility from room temperature to 600 °C, and in a 3.5 wt.% NaCl solution, it has a high self-corrosion potential and a low self-corrosion current density. After isothermal oxidation at high temperature for 100 h, the weight gain does not exceed 2.5 mg·cm³. ‑2 This alloy has a simple composition, low cost, and a manufacturing process compatible with traditional stainless steel, making it suitable for load-bearing structural components in marine environments and high-temperature corrosive environments of 600~1000 ℃.

Description

Technical Field

[0001] This invention relates to the field of medium-entropy alloys, and specifically to a single-phase, highly corrosion-resistant Fe-Cr-Ni medium-entropy alloy. Background Technology

[0002] Fe-Cr-Ni alloys and austenitic stainless steels are among the most widely used structural materials in marine engineering, chemical equipment, power energy, and some high-temperature components. For example, commonly used austenitic stainless steels such as 304 and 316L rely on a certain amount of Ni to stabilize the γ phase and form a dense passivation film by adding alloying elements such as Cr, Mo, and N, thus achieving a balance between corrosion resistance at room and medium temperatures and certain mechanical properties. They have been widely used in seawater pipelines, heat exchangers, pressure vessels, and nuclear power equipment. However, as service conditions evolve towards higher temperatures, stronger corrosive media, and longer service lives, the limitations of traditional austenitic stainless steels are gradually becoming apparent: on the one hand, improving corrosion resistance and oxidation resistance often requires increasing the Cr and Mo content, and even introducing precious metal elements, significantly increasing material costs; on the other hand, during medium- and high-temperature service, high-Cr austenitic stainless steels are prone to complex phase decomposition behavior. Specifically, when the Cr content increases to approximately 22-25 wt.% or higher and is used for extended periods in the 600-900 °C range, traditional high-Cr austenitic stainless steel is highly prone to precipitating Cr-rich σ phases and M phases along grain boundaries or within grains. 23 Carbides such as C6. These σ phases are hard and brittle intermetallic compounds that cause severe stress concentration and Cr depletion zones near grain boundaries. This increases the yield strength of the alloy but drastically reduces its plasticity, while significantly decreasing corrosion resistance at grain boundaries and inducing failure modes such as intergranular corrosion, pitting corrosion, and stress corrosion cracking. For equipment serving in high-safety environments such as nuclear power and petrochemical plants, the aforementioned σ phase embrittlement and sensitized corrosion problems have become important factors limiting the further application of high-Cr stainless steel. In addition, traditional Fe-Cr-Ni alloys tend to form an outer oxide layer dominated by Fe2O3 and Fe3O4 in high-temperature oxidizing environments of 800~1100 ℃. The Cr2O3 protective layer is discontinuous or difficult to maintain integrity for a long time, resulting in a thick oxide scale with poor adhesion, thereby shortening the service life of the material.

[0003] On the other hand, traditional alloy design often relies on complex multi-element ratios and empirical optimization. The design approach is mostly to continuously add alloying elements to the existing stainless steel system to improve a certain performance. This "experience-based" design method not only increases the complexity and cost of material composition, but also makes it difficult to simultaneously consider single-phase microstructure stability and excellent corrosion resistance and high-temperature oxidation resistance from the composition source.

[0004] In recent years, the concept of medium-entropy / high-entropy alloys has provided new ideas for the development of the Fe-Cr-Ni system. Multiple studies have shown that multi-principal solid solutions such as Fe-Cr-Ni, after medium-entropy or high-entropy alloying, can stably form single-phase FCC structures within a relatively wide composition range, exhibiting good corrosion resistance and certain high-temperature oxidation resistance potential. However, existing Fe-Cr-Ni medium-entropy alloys mostly employ equiatomic or near-equiatomic ratios, focusing more on trial-and-error exploration around the central region of the phase diagram, lacking a generalizable quantitative relationship between composition, structure, and properties. While some studies have reported that Fe-Cr-Ni medium-entropy alloys exhibit superior corrosion resistance in chloride solutions and oxidation resistance at 800–900 °C compared to traditional stainless steel, systematic analysis of phase stability under medium-temperature long-term service conditions (e.g., the evolution of second phases such as α-Cr and σ phases), the formation mechanism of the Cr2O3 protective layer, and its coupling relationship with composition is still lacking.

[0005] Although some alloys exhibit good corrosion resistance and oxidation resistance in short-term tests, they may still undergo unfavorable microstructural evolution such as σ phase precipitation, α-Cr coarsening, or Cr depletion under medium-temperature long-term conditions that are closer to actual working conditions, leading to a decline in overall service performance.

[0006] In summary, existing Fe-Cr-Ni alloys and austenitic stainless steels exhibit significant contradictions in their microstructural stability, corrosion resistance, and high-temperature oxidation resistance. Traditional empirical composition design methods struggle to systematically balance these multiple performance requirements within the Fe-Cr-Ni system. (1) While ensuring a high Cr content to support a stable Cr2O3 protective layer, the precipitation of brittle phases such as σ phase is suppressed, resulting in a single-phase FCC matrix and good long-term phase stability. (2) It has better corrosion resistance than traditional 316L stainless steel in room temperature and medium temperature chloride environments; (3) Under high-temperature oxidation conditions of 800~1100 ℃, a dense and continuous Cr2O3 protective layer is formed, exhibiting a lower oxidation rate and a thinner oxide layer thickness; (4) The overall alloy composition is simple, the cost is controllable, and the preparation process is compatible with existing stainless steel production lines, which facilitates engineering promotion and application. Summary of the Invention

[0007] To address the technical challenge of simultaneously achieving structural stability, corrosion resistance, and high-temperature oxidation resistance in high-Cr stainless steel and existing medium-entropy alloys, this invention successfully achieves synergistic optimization of single-phase FCC microstructure, high corrosion resistance, and excellent high-temperature oxidation resistance by limiting the specific composition range of Fe-Cr-Ni medium-entropy alloys and combining it with conventional smelting-heat treatment processes. Without adding elements such as Mo and N, the resulting alloy exhibits a single-phase FCC structure after annealing at 1100℃, demonstrates superior corrosion resistance compared to 316L stainless steel in 3.5wt.% NaCl solution, and forms a dense Cr2O3 protective layer upon oxidation at 800~1100℃. Crucially, some components completely suppress σ-phase precipitation during long-term service at 800℃ / 400h, while others exhibit ultra-low corrosion current density, thus meeting differentiated engineering requirements for long-term structural stability or service in extreme corrosive environments within the same alloy system.

[0008] The first aspect of this invention protects a high corrosion-resistant Fe-Cr-Ni medium-entropy alloy, which is composed of the following components by mass percentage: Fe: 31~38%, Cr: 23~31.5%, Ni: 33~40%, with the balance being unavoidable impurities, and the mass percentage content being less than 0.07%; preferably Fe: 31.33~37.71%, Cr: 23.23~31.03%, Ni: 33.03~39.51%.

[0009] Within the above composition range, after recrystallization annealing at 1100~1200℃ (preferably 1100℃), all embodiments exhibited a single-phase FCC structure; after oxidation in air at 800~1100℃ for 100h, a dense oxide scale mainly composed of Cr2O3 was formed on the surface, with a weight gain per unit area of ​​less than 1.2 mg·cm³. - ²; The alloy exhibits good mechanical properties from room temperature to 600℃, and its corrosion resistance in 3.5wt.% NaCl solution is superior to that of 316L stainless steel. No σ phase precipitation was detected under long-term oxidation conditions of 800℃ / 400h, and a small amount of α-Cr precipitate phase was observed in some components, thus achieving a comprehensive improvement in phase stability, corrosion resistance and high-temperature oxidation resistance.

[0010] More preferably, the alloy satisfies any of the following preferred embodiments: 1. The Fe content is 34.38~37.57 wt.%, the Cr content is 23.23~26.24 wt.%, and the Ni content is 36.20~39.42 wt.%. Alloys within this composition range retain a single-phase FCC structure in the matrix after oxidation at 800℃ for 400h, with no detected σ phase or α-Cr precipitation, making them suitable for applications requiring extremely high long-term structural stability. 2. The Fe content is 31.33~34.51 wt.%, the Cr content is 29.17~31.03 wt.%, and the Ni content is 35.02~39.51 wt.%. Alloys within this composition range exhibit a self-corrosion current density ≤1×10⁻⁶ in a 3.5 wt.% NaCl solution. -7 A·cm - ², and after oxidation at 1050~1150℃ (preferably 1100℃) for 90~110h (preferably 100h), it has the lowest weight gain and the thinnest oxide scale, making it suitable for high-temperature and highly corrosive environments.

[0011] The alloy of the present invention is prepared by vacuum melting, homogenization heat treatment, rolling deformation and recrystallization annealing, and a single-phase FCC structure is obtained after recrystallization annealing at 1050~1150℃ (preferably 1100℃).

[0012] The alloy of the present invention was subjected to room temperature and high temperature tensile tests, microhardness tests, electrochemical tests in 3.5 wt.% NaCl solution, and high temperature oxidation tests at 800~1100 °C to obtain its mechanical properties, corrosion resistance and oxidation resistance.

[0013] The stability of the matrix phase and the precipitation behavior of α-Cr / σ phase in the alloy of the present invention were analyzed by long-term oxidation test at 800 ℃ / 400 h. Another type, such as Fe5Cr5Ni6, precipitates α-Cr with a lower volume fraction in the matrix, which acts as a Cr reservoir to maintain the stability of the Cr2O3 protective layer. In contrast, 316L precipitates the harmful σ phase.

[0014] Another aspect of this invention protects the preparation method of the alloy described above, comprising the following steps: obtaining an alloy ingot by vacuum induction melting; subjecting the alloy ingot to homogenization heat treatment; subjecting the homogenized alloy to cold rolling deformation; and subjecting the cold-rolled alloy to solution treatment.

[0015] For the technical solution described above, a further preferred embodiment of the preparation method includes the following steps: (1) Weigh high-purity Fe, Cr and Ni raw materials according to the composition ratio, and prepare alloy ingots by vacuum induction melting method; (2) The ingot is homogenized at 1100~1200℃ (preferably 1150℃), and then cooled to room temperature in the furnace; cold rolled; (3) The ingot is cold-rolled and deformed, with the total deformation controlled at 32-38% (preferably 35%), and the reduction per pass not exceeding 5%; (4) The cold-rolled alloy is solution treated at 1050~1150℃ (preferably 1100 °C) and then cooled to obtain the alloy.

[0016] For the technical solution described above, it is further preferred that the processing time of step (2) or (3) is 2~3.5h respectively.

[0017] In another aspect, this invention protects the application of the single-phase high corrosion-resistant Fe-Cr-Ni medium-entropy alloy described above in equipment with high safety requirements for corrosion-resistant load-bearing components in nuclear power, petrochemical, and marine engineering; it is particularly preferred for use in key components that need to simultaneously meet the requirements of "no σ phase precipitation" or "corrosion resistance better than 316L".

[0018] More preferably, the single-phase high corrosion-resistant Fe-Cr-Ni medium-entropy alloy has a tensile strength of not less than 493.5 MPa at room temperature and a corrosion current density of not more than 1×10⁻⁶ in a 3.5 wt.% NaCl solution. -7 A·cm -2 It has extremely broad application prospects in manufacturing corrosion-resistant load-bearing components with higher safety requirements.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The obtained single-phase, high-corrosion-resistant Fe-Cr-Ni medium-entropy alloy has a simple composition and stable structure. In Fe... 5-6 Cr 4- 5.3 Ni 5-6 A high Cr content and single-phase FCC structure were achieved within the compositional range. Experiments showed that the alloys were all single-phase FCC after recrystallization annealing at 1100 °C; after isothermal oxidation at 800 °C for 400 h, Fe6Cr4Ni6 and Fe6Cr... 4.5 Ni 5.5 The matrix remains a single-phase FCC, which solves the problem of σ-phase precipitation easily induced by high Cr content in traditional high-Cr austenitic steel.

[0020] 2. Good strength and ductility matching at room temperature to intermediate temperature. By adjusting the proportions of Fe, Ni, and Cr, the alloy of this invention exhibits high yield strength and tensile strength at room temperature while maintaining good elongation; it also maintains high strength and considerable ductility at 400-600 °C, wherein Fe5Cr5Ni6 and Fe 5.4 Cr 5.3 Ni 5.3 The composition achieves a good balance between strength and plasticity, making it suitable as a material for load-bearing components in medium temperatures.

[0021] 3. The corrosion resistance is significantly superior to 316L. In a 3.5 wt.% NaCl solution, the self-corrosion potential of the alloy of this invention is more positive than that of 316L, and the self-corrosion current density is significantly lower than that of 316L. The polarization curve shows a broad and stable passivation region, indicating that the alloy of this invention has a lower uniform corrosion rate and stronger resistance to pitting corrosion. Without introducing expensive alloying elements such as Mo or elements that increase processing difficulty such as N, superior corrosion resistance to 316L is achieved through medium-entropy alloying to increase the Cr content and optimize the Fe / Ni ratio, and the process is simple.

[0022] 4. Excellent high-temperature oxidation resistance and stable single-phase structure. Under isothermal oxidation conditions of 800 and 1100 °C in air for 100 h, the surface of the high corrosion-resistant Fe-Cr-Ni medium-entropy alloy of this invention mostly forms a dense oxide scale mainly composed of Cr2O3, with significantly lower weight gain and oxide scale thickness than 316L. Furthermore, after long-term oxidation at 800 °C for 400 h, the alloys Fe6Cr4Ni6 and Fe6Cr... 4.5 Ni 5.5 The matrix remains a single-phase FCC, while the other alloys precipitate a BCC (α-Cr) phase with a low volume fraction on the matrix; however, 316L precipitates a σ phase and forms a thick and loose Fe oxide layer under the same conditions, resulting in a significant deterioration in overall performance.

[0023] 5. Balancing the dual requirements of complete single-phase stability and optimal high-temperature oxidation resistance. Within the composition range defined in this invention, by adjusting the proportions of Fe, Cr, and Ni, two types of alloys with engineering application value can be obtained: one type is based on Fe6Cr4Ni6, Fe6Cr... 4.5 Ni 5.5 One type, represented by Fe5Cr5Ni6, maintains a completely single-phase FCC structure in the matrix under isothermal oxidation conditions of 800 ℃ / 400 h, making it suitable for applications requiring extremely high long-term structural stability. Another type, represented by Fe5Cr5Ni6, exhibits minimal weight gain and the thinnest and densest Cr2O3 oxide scale under isothermal oxidation conditions of 800 ℃ and 1100 ℃, combining excellent high-temperature oxidation resistance and corrosion resistance. It can be used as a preferred material in high-temperature corrosive environments, achieving the design goal of "complete single-phase stability" and "optimal high-temperature oxidation resistance" in the same alloy system.

[0024] 6. The alloy composition is simple, the cost is low, and the process is compatible with existing stainless steel. This invention uses only three elements: Fe, Cr, and Ni, avoiding the use of large amounts of precious metals and various trace elements. The composition system is simple and the raw material cost is low. The preparation process adopts conventional vacuum melting, homogenization, rolling, and recrystallization annealing, which has high compatibility with existing stainless steel production lines and is conducive to industrial scale-up and engineering applications. Attached Figure Description

[0025] Figure 1XRD pattern of a high corrosion-resistant Fe-Cr-Ni medium-entropy alloy.

[0026] Figure 2 Metallographic image of a high corrosion-resistant Fe-Cr-Ni medium-entropy alloy.

[0027] Figure 3 SEM image of the surface of a high corrosion-resistant Fe-Cr-Ni medium-entropy alloy.

[0028] Figure 4 Hardness-temperature curves of high corrosion-resistant Fe-Cr-Ni medium-entropy alloys.

[0029] Figure 5 Room temperature and high temperature tensile curves of high corrosion-resistant Fe-Cr-Ni medium-entropy alloy.

[0030] Figure 6 Electrochemical Tafel polarization curves of high corrosion-resistant Fe-Cr-Ni medium-entropy alloys.

[0031] Figure 7 Oxidation weight gain per unit area of ​​high corrosion-resistant Fe-Cr-Ni medium-entropy alloy and 316L alloy at 800 ℃ / 400 h and 1100 ℃ / 100 h.

[0032] Figure 8 XRD patterns of high corrosion-resistant Fe-Cr-Ni medium-entropy alloy and 316L alloy surfaces after oxidation at 800 ℃ and 1100 ℃ for 100 h, and XRD patterns of the surfaces after grinding after oxidation at 800 ℃ for 400 h and 1100 ℃ for 100 h.

[0033] Figure 9 Fe5Cr5Ni6 and Fe 5.5 Cr5Ni 5.5 Schematic diagram of oxide scale morphology and EDS element distribution on alloy cross section after oxidation at 1100 ℃.

[0034] Figure 10 EDS elemental diagram of the alloy matrix after Fe6Cr4Ni6 and Fe5Cr5Ni6 were oxidized at 800 °C for 400 h. Detailed Implementation

[0035] The following embodiments will further illustrate the present invention.

[0036] In Fe 5-6 Cr 4-5.3 Ni 5-6 Within the composition range, various alloy compositions were obtained by adjusting the contents of Fe, Cr, and Ni. The compositions of the examples are shown in Table 1.

[0037] Table 1. Ingredients of the Examples

[0038] 1. Alloy Preparation Raw materials: High-purity Fe, Cr, and Ni particles (purity ≥ 99.9%).

[0039] Process: (1) Weigh the raw materials according to the composition in Table 1; (2) Alloy samples were prepared using an FJ21-02 non-consumable vacuum induction furnace.

[0040] (3) The ZMF-1200C-M vacuum box furnace was used to heat the sample at 1150 ℃ for 3 hours in a vacuum environment and then slowly cooled to room temperature with the furnace.

[0041] (4) The sample was subjected to cold rolling deformation treatment using a twin-roll mill. The total deformation was controlled at around 35% through multiple rolling passes, and the single-pass pressing was strictly limited to within 10%. The surface of the sample was smoothed after each rolling pass.

[0042] (5) The cold-rolled sample was placed in a vacuum furnace again and annealed for 2.5 hours in a vacuum environment at 1100 °C, and then air-cooled in the furnace.

[0043] In the specific implementation process, the "balance of unavoidable impurities" in the alloy composition refers to inclusion elements that may be introduced in trace amounts during the smelting of high-purity raw materials and alloy preparation, and are difficult to completely remove by conventional industrial means. Based on industrial practice and material performance considerations, the total mass percentage of unavoidable impurities in this invention should be less than 0.07%. "Impurities < 0.07%" is a conventional industrial purity control and does not contradict the composition design; generally speaking, the impurities in this application mainly include phosphorus (P) and sulfur (S), and their mass percentage content is preferably controlled as follows: P ≤ 0.040%, S ≤ 0.030%. Strict control of impurity content helps to ensure the purity of the alloy, thereby ensuring that its high-temperature mechanical properties, corrosion resistance, and structural stability are not impaired.

[0044] The alloy microstructure characterization method involved in this application is as follows: Phase analysis was performed using an Empyrean X-ray diffractometer (PANACO, Netherlands) to compare and study the phase composition evolution of as-cast and final-state samples. Key experimental parameters were set as follows: the radiation source was a Cu target. KAlpha rays (λ = 0.15406 nm); tube voltage / current 40 kV / 40 mA; scanning angle 2θ 20° ~ 90°, scanning speed 2° / min. Background subtraction and smoothing were performed on the diffraction patterns using MID Jade 6 analysis software, and phase identification was performed based on a database. Phase content was calculated using the Rietveld refinement method with full-spectrum fitting; the determination result was reliable when the matching factor Rwp < 5%.

[0045] Typical samples were cut using a wire EDM machine under continuous coolant flushing, ensuring the cut surface was perpendicular to the original surface at a 90° angle. Immediately after cutting, the samples were immersed in anhydrous ethanol for ultrasonic cleaning (15 min) to remove cutting residue. Finally, the sample surface was polished using SiC water-resistant sandpaper of grades 240#, 400#, 600#, 800#, 1000#, 1200#, 1500#, and 2000#. Each pass of polishing was performed until only scratches in one direction were visible before rotating the sample 90° to the next grade of sandpaper, preventing confusion between scratches from different directions and ensuring complete removal. The samples were then polished sequentially with 1.5 μm diamond polishing paste until a mirror-like, scratch-free finish was achieved. Water polishing for 4 min removed residue, followed by alcohol cleaning and drying. When preparing the etching solution, it should be done in a constant temperature and humidity experimental environment. Mix the chemical reagents according to the formula: 3g FeCl3 + 2ml HCl + 95ml C2H5OH, and stir thoroughly to prepare the etching solution. Due to the volatility of ethanol, immediately use a dropper to apply a small amount of the etching solution to the sample surface. After waiting 30 seconds, immediately rinse with deionized water for 15 seconds and use a drying device to quickly dry the sample surface. Observe the phase composition and structure of the alloy sample using a Leica DMi8 metallurgical microscope and a SUPRA 55 scanning electron microscope.

[0046] The alloy performance testing method involved in this application is: The hardness of the sample was tested using a ZD-HVZHT-10 high-temperature Vickers hardness tester with a load of 200g and a holding time of 15s. Before testing, the sample was placed in the experimental chamber, and then the chamber was purged. First, the mechanical pump was turned on to reduce the vacuum level inside the chamber, and then high-purity argon gas was introduced. This purging process was repeated three times to maintain a low oxygen content in the chamber and prevent oxidation of the sample surface during the high-temperature experiment. The temperature program was then set on the computer, with the following temperatures: 20℃, 100℃, 200℃, 300℃, 400℃, 500℃, 600℃, 700℃, 800℃, 900℃, and 1000℃, each held for 30 minutes. The sample was fixed in the center of the test platform, ensuring a flat surface. The experimental parameters were then adjusted, and the focal length was quickly adjusted until the sample surface could be clearly observed. The current coordinates were then input into the device to ensure the lens automatically adjusted to the current position. Finally, switch to the indenter and apply pressure. After 60 seconds, once the edge of the prismatic indentation on the sample is clearly visible through the lens, use the four-point method to collect the hardness value of the current area. During measurement, each alloy sample is measured three times at the same temperature, and the average value is taken.

[0047] The C45-105EY microcomputer-controlled electronic universal testing machine was used to test the room temperature and high temperature tensile properties of alloy samples. The thickness of the sample for room temperature tensile testing was 2 mm. Before the room temperature tensile test, the cut tensile sample was polished with 80# and 240# sandpaper to remove scratches and oxide scale, ensuring accurate tensile test results. The polished sample was placed in the testing machine and pre-tightened with clamps to prevent sample slippage from affecting the experiment. The tensile speed was set to 0.5 mm / min in the experimental parameters on the computer, and all values ​​were reset to zero before the experiment began. During the experiment, the surrounding environment was kept quiet to reduce experimental errors. The experiment was stopped immediately after the sample broke, and the current stress-engineering strain test data was saved. For high temperature tensile testing, a high temperature tensile fixture was prepared in advance. The prepared sample was fixed in the fixture and ensured that the sample was in a stress-free state. Then, three thermocouple wires were connected to the upper and lower ends of the fixture and the middle area of ​​the sample, respectively. Finally, the furnace was closed tightly, and asbestos was stuffed into the gaps of the furnace to ensure that the temperature inside the furnace remained stable within a certain range. Set the experimental temperature in advance, start the heating program, and keep the temperature for 10 minutes after all three thermocouples have reached the set temperature. After ensuring that the extensometer output is stable, start the tensile test. Immediately turn off the heating after the sample breaks, stop the experiment, save the data, and wait for the heating furnace to cool to room temperature before removing the sample.

[0048] The corrosion resistance of the samples was tested using a Gamry Reference 600+ electrochemical workstation. To simulate a seawater environment, the three-electrode electrochemical system was immersed in a 3.5% NaCl solution. The anode in the three-electrode electrochemical system was a copper sheet in contact with the sample, the cathode was a platinum electrode, and the reference electrode was a saturated calomel electrode. After immersing the samples in the solution for 10 minutes, the open-circuit potential was measured. Corrosion resistance testing was then performed when a stable potential was reached. The Tafel polarization test was selected, with a scan rate of 1 mV / s and a scan range of -0.5 to 1.3 V relative to the open-circuit potential, from negative to positive. The Tafel polarization curves of the samples were then obtained.

[0049] High-temperature oxidation was carried out in a box-type muffle furnace under an air atmosphere. Fe-Cr-Ni alloy and 316L samples (approximately 20 mm × 10 mm × 2 mm) were placed in an alumina crucible and isothermally oxidized at 800 °C and 1100 °C for 100 h. Samples were removed at 25 h, 50 h, 75 h, and 100 h intervals, weighed using an analytical balance with an accuracy of 0.01 mg, and the weight gain per unit area was calculated. To investigate long-term oxidation behavior, a 400 h oxidation test was conducted on samples under the same conditions at 800 °C. Weights were measured at predetermined time intervals to obtain the weight gain-time relationship within 400 h. After oxidation, a selection of samples were analyzed for surface and cross-section: the oxide layer composition was characterized using an Empyrean X-ray diffractometer (Cu Kα, 2θ = 20°–100°, scanning speed 2° / min). Cross-sections were inlaid, polished, and observed using SEM / EDS to evaluate the oxide layer thickness, morphology, and elemental distribution.

[0050] Figure 1 , Figure 2 , Figure 3 The XRD, metallographic, and SEM results for the seven groups of medium-entropy alloys show that all examples exhibit a face-centered cubic single-phase structure after solution treatment, with no second phase such as σ phase or ferrite observed. Optical microscopy and SEM observations reveal that the microstructure consists of fully recrystallized equiaxed grains.

[0051] according to Figure 4 It can be seen that at room temperature, the Vickers hardness of the alloy in this embodiment is approximately 140 HV. As the temperature increases, the hardness of the alloy decreases, and at 600 °C, the Vickers hardness of the alloy in this embodiment is approximately 65 HV. The rate of hardness decrease is relatively fast before 800 °C, and then slows down. After 800 °C, the Vickers hardness of the alloy in this embodiment is approximately 50 HV.

[0052] Figure 5Figures (a)-5(d) show the engineering stress-strain curves of the alloy of Example 1 at 25 °C, 400 °C, 500 °C, and 600 °C, respectively. At room temperature, the tensile strength of the alloy of this example is approximately 500 MPa, the yield strength is approximately 160 MPa, and the elongation is approximately 45%. At 400 °C, 500 °C, and 600 °C, the tensile strength remains above 360 ​​MPa, 340 MPa, and 266 MPa, respectively, and the elongation is not less than 27%, indicating that the alloy of this example still has good strength-plasticity matching in the mid-temperature range. In the high-temperature tensile test, the alloy of the example exhibits dynamic strain aging phenomenon (serrated stress-strain curve) below 600 °C.

[0053] Figure 6 The image shows the polarization curves obtained from potentiodynamic polarization tests of the alloy in this embodiment using a three-electrode system in a 3.5 wt.% NaCl solution. The alloy in this embodiment exhibits significant spontaneous passivation behavior; the self-corrosion potential of Fe5Cr5Ni6 is approximately 0.62 V, a positive shift of approximately 0.58 V compared to 316L, and the self-corrosion current density is approximately 10... -7 A·cm -2 Significantly lower than 10 for 316L -6 A·cm -2 Furthermore, the self-corrosion potential of the alloys in all seven examples is higher than 316L, indicating that the alloys in this example have a lower corrosion tendency and a lower corrosion rate.

[0054] The samples were subjected to isothermal oxidation in air at 800 ℃ and 1100 ℃ for 100 h, respectively. The weight gain was measured periodically, and the oxide scale on the cross-section was observed. The oxidation weight gain curves are shown below. Figure 7 As shown, the weight gain per unit area of ​​the alloy in the examples after oxidation at 800 °C and 1100 °C for 100 h was less than 1.2 mg·cm³. -2 The weight gain of 316 L at 1100 °C is significantly higher than that of the alloy in this embodiment. Figure 8 (a)-(b) show that the XRD pattern on the alloy surface of the examples after oxidation at 800 °C and 1100 °C for 100 h indicates that the oxide scale is mainly composed of Cr2O3. Figure 9 (a)-(b) show the cross-sectional SEM / EDS analysis of Fe5Cr5Ni6 and Fe6Cr5Ni5, respectively. The analysis shows that the oxide scale is thin and uniformly distributed, with a straight interface with the substrate and no obvious penetrating oxide channels.

[0055] The sample was isothermally oxidized in air at 800 °C for 400 h, and after cooling, the oxide scale was mechanically removed. The matrix was then characterized by XRD and cross-sectional SEM / EDS. Figure 8 (c) XRD results show that the alloy is still dominated by FCC diffraction peaks, and no σ phase was detected. Figure 10(a)-(j) EDS phase distribution diagrams and local composition analysis show that BCC (α-Cr) precipitates with low volume fractions appear on the matrix of some components such as Fe5Cr5Ni6, while the alloy retains the FCC main phase.

[0056] The above-described embodiments are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A single-phase, high-corrosion-resistant Fe-Cr-Ni medium-entropy alloy, characterized in that: It consists of the following components by mass percentage Composition: Fe: 31~38%, Cr: 23~31.5%, Ni: 33~40%, balance being unavoidable impurities, with a mass percentage content of less than 0.07%.

2. The single-phase high corrosion-resistant Fe-Cr-Ni medium-entropy alloy according to claim 1, characterized in that: It consists of the following components by mass percentage composition: Fe: 31.33~37.71%, Cr: 23.23~31.03%, Ni: 33.03~39.51%, with the balance being unavoidable impurities, and the mass percentage content is less than 0.07%.

3. The single-phase high corrosion-resistant Fe-Cr-Ni medium-entropy alloy according to claim 1, characterized in that: The single-phase, high-corrosion-resistant Fe-Cr-Ni medium-entropy alloy exhibits a tensile strength of not less than 493.5 MPa at room temperature and a corrosion current density of not more than 1 × 10⁻⁶ MPa in a 3.5 wt.% NaCl solution. -7 A·cm -2 .

4. The method for preparing a single-phase high corrosion-resistant Fe-Cr-Ni medium-entropy alloy according to claim 1, characterized in that: Includes the following steps: The raw materials are proportioned according to the mass percentage of the ingredients as described in claim 1; an alloy ingot is obtained by vacuum induction melting; and the alloy ingot is subjected to homogenization heat treatment. The homogenized alloy is subjected to cold rolling deformation; the cold-rolled alloy is then subjected to solution treatment.

5. The method according to claim 4, characterized in that: Includes the following steps: (1) Weigh high-purity Fe, Cr and Ni raw materials according to the composition ratio, and prepare alloy ingots by vacuum induction melting method; (2) The ingot is homogenized at 1100~1200 ℃, and then cooled to room temperature in the furnace; cold rolled; (3) The ingot is cold-rolled and deformed, with the total deformation controlled at 32-38% and the single-pass reduction not exceeding 5%; (4) The cold-rolled alloy is solution treated at 1050~1150 °C and then cooled to obtain the alloy.

6. The method according to claim 5, characterized in that: The processing time for steps (2) or (3) is 2 to 3.5 hours, respectively.

7. The application of the single-phase high corrosion-resistant Fe-Cr-Ni medium-entropy alloy as described in claim 1, characterized in that: The aforementioned single-phase high corrosion-resistant Fe-Cr-Ni medium-entropy alloy is used in equipment with high safety requirements for corrosion-resistant load-bearing components in nuclear power, petrochemical, and marine engineering applications.

8. The application according to claim 7, characterized in that: The single-phase high corrosion-resistant Fe-Cr-Ni medium-entropy alloy is used to prepare components that simultaneously meet the requirements of no σ-phase precipitation or corrosion resistance superior to 316L.

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