Ni20Cr alloy, preparation method and application thereof

Abstract

The application relates to the technical field of NiCr alloy surface modification, in particular to a Ni20Cr alloy and a preparation method and application thereof. The method comprises the following steps: annealing treatment is performed on a Ni20Cr alloy base material to obtain annealed Ni20Cr alloy, then surface pretreatment is performed to obtain recrystallized Ni20Cr alloy; subsequently, deep cold ultrasonic shot blasting treatment is performed on the recrystallized Ni20Cr alloy to obtain the Ni20Cr alloy. By constructing a gradient nanotwin structure on the surface of the Ni20Cr alloy, the mechanical properties and high-temperature oxidation resistance are synergistically improved. The gradient nanotwin structure not only endows the Ni20Cr alloy with excellent strength-plasticity matching, but also exhibits significant oxidation resistance in a high-temperature service environment, and the problems existing in the prior art are overcome.

Description

Technical Field

[0001] This invention relates to the field of NiCr alloy surface modification technology, specifically to a Ni20Cr alloy, its preparation method, and its application. Background Technology

[0002] Ni20Cr alloys are widely used in aerospace, energy, and power industries due to their excellent high-temperature strength, oxidation resistance, and corrosion resistance. However, with the increasing demands of modern industry on material performance, the performance of traditional Ni20Cr alloys in high-temperature oxidizing environments is no longer sufficient to meet the requirements of extreme working conditions.

[0003] During high-temperature service, Ni20Cr alloys undergo oxidation. If the resulting oxide film cannot effectively prevent oxygen diffusion inward, it will lead to severe inward degradation of the matrix. Meanwhile, according to the Hall-Petch relationship, there is often a trade-off between alloy strength and ductility; that is, while improving strength by refining grains or increasing dislocation density, ductile deformation capacity is usually sacrificed. This further limits the application of Ni20Cr alloys under complex stress conditions. Therefore, developing Ni20Cr alloys that combine high strength, high ductility, and excellent resistance to high-temperature oxidation is of significant practical importance.

[0004] To address the mismatch between strength and plasticity and insufficient resistance to high-temperature oxidation in Ni20Cr alloys, common surface modification techniques include thermal spraying, physical vapor deposition (PVD) and chemical vapor deposition (CVD), and elemental doping. Thermal spraying, in particular, uses flame or plasma spraying to form a ceramic coating (such as Al2O3 or Cr2O3) on the alloy surface to improve wear resistance and oxidation resistance. Its advantages include strong material adaptability and excellent coating performance; however, the high porosity of the coating makes it prone to becoming a channel for corrosive media penetration, and the high temperature during the spraying process can easily lead to a mismatch in the thermal expansion coefficients of the coating and the substrate, causing cracking or peeling. Physical vapor deposition (PVD) and chemical vapor deposition (CVD): These methods form dense ceramic or metallic coatings (such as TiN, CrN, and Al2O3) in a vacuum environment through vapor deposition. The advantages include dense coatings, excellent high-temperature oxidation resistance, and the ability to customize coating color and function (such as wear resistance and corrosion resistance). However, the coating thickness is thin (<10μm), making it prone to failure under long-term high-temperature wear. Especially since CVD process temperatures are often >800℃, it can easily lead to coarsening of the substrate grains, thus reducing mechanical properties. Element doping: Adding trace elements (such as Y, La, and Ti) to optimize grain boundary stability. Its advantages include grain refinement, synergistic improvement in strength and plasticity, and Y / La elements can promote the formation of a dense Cr2O3 oxide film, enhancing high-temperature oxidation resistance. However, the element ratio needs precise control; excessive addition can easily form brittle phases, and rare earth elements are expensive.

[0005] In addition, although conventional mechanical shot peening or laser shot peening can improve fatigue life by introducing residual compressive stress, the shot peening process is accompanied by an increase in temperature (>200℃), which can easily induce dynamic recovery and recrystallization, resulting in grain coarsening, reduced strength, and difficulty in accurately controlling the shot peening intensity and coverage. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a Ni20Cr alloy, its preparation method, and its applications. Using Ni20Cr alloy as a substrate, this invention employs a cryogenic ultrasonic shot peening process to construct a gradient nanotwin structure on the surface of the Ni20Cr alloy, thereby obtaining the Ni20Cr alloy. Specifically, after cryogenic ultrasonic shot peening, the mechanical properties of the Ni20Cr alloy are significantly improved (its surface hardness increases from 150HV to 400HV, an increase of 160%; its yield strength increases from 200MPa to over 300MPa, an increase of over 50%; its tensile strength is ≥580MPa, and its elongation is ≥60%). Simultaneously, its high-temperature oxidation resistance is greatly improved (after oxidation at 1000℃ for 100h, the oxidation rate is ≤0.006mg / cm). 2 •h, with an increase of over 30% in high-temperature oxidation resistance, while achieving an excellent balance of strength and plasticity, further enhancing the oxidation resistance of Ni20Cr alloy in high-temperature service environments.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The purpose of this invention is to provide a method for preparing Ni20Cr alloy, comprising the following steps: S1. The Ni20Cr alloy substrate is annealed to eliminate residual stress, promote microstructure homogenization and recrystallization, and obtain annealed Ni20Cr alloy. Commercially available Ni20Cr alloys are usually smelted and forged to eliminate solidification defects such as shrinkage cavities and porosity generated during casting. However, the forging process introduces microstructural defects such as grain refinement, dislocation multiplication, and deformation twinning, as well as residual stress, causing the Ni20Cr alloy to be in a non-equilibrium state. Therefore, this invention, through annealing, can effectively eliminate residual stress, promote microstructure homogenization and recrystallization, and obtain a reference state with uniform grain size and stable microstructure, providing a standardized reference state for subsequent cryogenic ultrasonic shot peening.

[0008] S2. The annealed Ni20Cr alloy is subjected to surface pretreatment to remove the oxide layer on the surface of the annealed Ni20Cr alloy, so as to obtain the recrystallized Ni20Cr alloy.

[0009] S3. The recrystallized Ni20Cr alloy is subjected to cryogenic ultrasonic shot peening to form a gradient nanotwin structure with high-density dislocations and small-angle grain boundaries on the surface of the recrystallized Ni20Cr alloy, thus obtaining the Ni20Cr alloy.

[0010] Preferably, the cryogenic ultrasonic shot peening process involves immersing the recrystallized Ni20Cr alloy in liquid nitrogen. Once the recrystallized Ni20Cr alloy reaches the liquid nitrogen temperature (77K), a cryogenic environment is maintained with the assistance of a cryogenic chamber. Simultaneously, an ultrasonic transducer drives a high-frequency ultrasonic probe to vibrate, causing the shot to impact the surface of the recrystallized Ni20Cr alloy at high speed. This subjects the surface of the recrystallized Ni20Cr alloy to strong indentation and shear plastic deformation. Through the synchronized action of cryogenic treatment and ultrasonic shot peening, the dynamic recovery and recrystallization processes are effectively suppressed, constructing a gradient nanotwin structure with a depth exceeding 1mm on the surface of the recrystallized Ni20Cr alloy. This structure consists of high-density surface nanotwins, small-angle grain boundaries, and high-density dislocations, forming a continuous gradient distribution from the surface to the core, resulting in a surface-treated Ni20Cr alloy.

[0011] Preferably, the cryogenic ultrasonic shot peening is performed in a liquid nitrogen environment at an ambient temperature of 77K. The shot material is GCr15 high-strength bearing steel with a shot diameter of 1.0mm to 4.0mm, an ultrasonic frequency of 20kHz to 40kHz, a distance of 10mm to 70mm between the shot and the surface of the recrystallized Ni20Cr alloy, an oscillator amplitude of 15μm to 75μm, and a treatment time of 4min to 20min.

[0012] More preferably, in order to simultaneously obtain a deeper reinforcement layer, good surface quality and low roughness, the diameter of the projectile is 3 mm, the ultrasonic frequency is 25 kHz, and the processing time is 12 min.

[0013] Among them, ultrasonic frequencies in the range of 20kHz-40kHz correspond to higher strain rates, which can effectively suppress dynamic recovery during deformation. This dynamic recovery, as a thermally activated process, is prone to occur at lower strain rates (below 20kHz), leading to dislocation cancellation and weakening the strengthening effect. Therefore, setting the frequency above 20kHz ensures the processing is within a high strain rate range, promoting defect accumulation and forming a deep, fine-grained strengthening layer. ① When the vibration frequency is below 20kHz, the number of impacts per unit time decreases, resulting in a lower strain rate on the material surface. This relatively enhances the dynamic recovery effect during deformation, with some plastic work dissipated as heat, making it difficult for the dislocation density to reach the critical saturation value in a short time. Experiments and theory show that the grain refinement efficiency is significantly reduced in this state, making it difficult to obtain an ideal nanoscale surface structure and matching high-amplitude residual compressive stress. Therefore, 20kHz is a necessary technical threshold for effectively suppressing dynamic recovery and ensuring efficient grain refinement.

[0014] ② Setting the upper limit of the frequency range to 40kHz is based on a comprehensive consideration of technical feasibility and effect stability: Energy attenuation and skin effect: In the ultrasonic band, as the frequency increases, the attenuation of ultrasonic waves propagating in media such as amplitude transformers and projectiles intensifies, and the high-frequency skin effect causes energy to concentrate on the surface, making it difficult to drive a projectile of sufficient mass to achieve a uniform impact effect; System resonance stability: Ultrasonic processing systems need to operate efficiently in a resonant state. Currently, commercially available high-performance ultrasonic transducers and supporting generation technologies face challenges in power capacity and long-term resonance stability above 40kHz, making it difficult to guarantee the uniformity and consistency of shot peening intensity in industrial applications. Finally, excessively high frequencies pose a greater risk of material strengthening and failure: Excessively high frequencies (>40kHz) can lead to insufficient single impact energy or an increased risk of microscopic defects (such as microcracks) on the workpiece surface, which is detrimental to improving material performance.

[0015] Preferably, the annealing conditions are: annealing at 800℃~1000℃ for 90min~180min in a vacuum environment.

[0016] Preferably, the grain size of the annealed Ni20Cr alloy is 150μm~420μm, and the average grain size is about 350μm.

[0017] Preferably, the surface pretreatment operation is as follows: after sanding with 100#, 600#, 1200# and 2000# sandpaper in sequence, mechanical polishing is performed with silica nano polishing liquid with a particle size of 80nm.

[0018] Preferably, based on the influence of reinforcement depth on the comprehensive mechanical properties of the material, and through continuous parameter optimization, the maximum gradient nanotwin structure depth is obtained as 1 mm.

[0019] A second objective of this invention is to provide a Ni20Cr alloy prepared by the above-described method, wherein the Ni20Cr alloy comprises a Ni20Cr alloy substrate and a gradient nanotwin structure on the surface of the Ni20Cr alloy substrate; the gradient nanotwin structure comprises nanotwins, dislocations, and small-angle grain boundaries.

[0020] Preferably, the depth of the gradient nanotwin structure is at least 1 mm.

[0021] Preferably, the surface roughness Ra of the Ni20Cr alloy is less than 0.3 μm.

[0022] Preferably, after the Ni20Cr alloy is oxidized at a constant temperature of 1000°C, a composite oxide film composed of dense NiCr2O4 spinel and Cr2O3 is formed on the surface of the Ni20Cr alloy.

[0023] Preferably, the high-strength, high-plasticity, and high-temperature oxidation-resistant Ni20Cr alloy has a surface hardness ≥400HV, yield strength ≥300MPa, tensile strength ≥580MPa, and elongation ≥60%.

[0024] The third objective of this invention is to provide the application of the above-mentioned high-strength, high-plasticity, high-temperature oxidation-resistant Ni20Cr alloy in the preparation of key components for aero-engine blades and nuclear reactor internals in extreme high-temperature environments.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention provides a method for preparing Ni20Cr alloy, wherein the Ni20Cr alloy substrate is annealed to eliminate residual stress, promote microstructure homogenization and recrystallization, and obtain annealed Ni20Cr alloy; the annealed Ni20Cr alloy is subjected to surface pretreatment to remove the oxide layer on the surface of the annealed Ni20Cr alloy, and obtain recrystallized Ni20Cr alloy; the recrystallized Ni20Cr alloy is subjected to cryogenic ultrasonic shot peening to form a gradient nanotwin structure on the surface of the recrystallized Ni20Cr alloy, and obtain Ni20Cr alloy.

[0026] This invention achieves a synergistic improvement in mechanical properties and high-temperature oxidation resistance by constructing a gradient nanotwin structure on the surface of Ni20Cr alloy. This gradient nanotwin structure not only endows the Ni20Cr alloy with excellent strength-ductility matching but also exhibits significant oxidation resistance advantages under high-temperature service environments: after isothermal oxidation at 1000℃ for 100 hours, a composite oxide film composed of dense NiCr2O4 spinel and Cr2O3 forms on the surface of the Ni20Cr alloy, effectively blocking further diffusion of oxygen into the matrix. Compared with untreated Ni20Cr alloy, the Ni20Cr alloy treated by the processing method of this invention shows an improvement of over 30% in high-temperature oxidation resistance, a significant increase of over 50% in surface strength, and maintains a high uniform elongation of 60%, providing a reliable guarantee for long-life service under extreme high-temperature environments.

[0027] Compared to existing technologies, this invention employs cryogenic ultrasonic shot peening to construct a surface gradient nanotwin structure on Ni20Cr alloy. The synergistic effect of high-density nanotwins, small-angle grain boundaries, and high-density dislocations significantly enhances the alloy's strength and hardness while preserving the high plasticity of the matrix. The cryogenic process provides crucial support for the introduction of even higher-density nanotwins and the suppression of dynamic recovery processes, achieving an excellent surface strengthening effect.

[0028] 2. After cryogenic ultrasonic shot peening, the Ni20Cr alloy of the present invention has excellent properties of high strength, high plasticity and high temperature oxidation resistance. Its surface hardness is ≥400HV, yield strength is ≥300MPa, tensile strength is ≥580MPa and elongation is ≥60%, achieving an excellent match between strength and plasticity.

[0029] 3. The Ni20Cr alloy of this invention is suitable for critical components in extreme high-temperature environments such as aero-engine blades and nuclear reactor internals. Its surface features high-density gradient nanotwin boundaries, small-angle grain boundaries, and high-density dislocations, providing rapid diffusion channels for Cr, promoting selective oxidation of Cr, and quickly forming a dense NiCr2O4 spinel and a dense Cr2O3 oxide film. This effectively inhibits further oxygen diffusion into the material and internal oxidation, improving high-temperature oxidation resistance by more than 30%.

[0030] 4. The processing method of the present invention can be extended to other nickel-based alloys, and can be adapted to different application scenarios by adjusting the composition and ultrasonic shot peening parameters. Attached Figure Description

[0031] Figure 1 The images show the electron backscattering diffraction (EBSD) pattern and the X-ray diffraction (XRD) pattern of a single-phase face-centered cubic structure of a commercial Ni20Cr alloy after vacuum annealing at 1000℃ for 90 min. (a) is the EBSD pattern of the annealed Ni20Cr alloy; (b) is the XRD pattern of the Ni20Cr alloy before cryogenic ultrasonic shot peening; and (c) is the XRD pattern of the Ni20Cr alloy after cryogenic ultrasonic shot peening. Both (b) and (c) show a single-phase face-centered cubic structure.

[0032] Figure 2 This is a schematic diagram of a cryogenic ultrasonic shot peening device.

[0033] Figure 3 The images show SEM, TEM, high-resolution atomic images of nanotwin boundaries and small-angle grain boundaries of the gradient nanotwin Ni20Cr microstructure after cryogenic ultrasonic shot peening. (a) is an SEM image, (b) is a TEM image, (c) is a high-resolution atomic image of nanotwin boundaries (with insets showing magnified views), and (d) is a high-resolution atomic image of small-angle grain boundaries (with insets showing magnified views).

[0034] Figure 4 The tensile stress-strain curves of CG-Ni20Cr and USSP-Ni20Cr in Example 1 are shown.

[0035] Figure 5The image shows the thickness of the oxide layer cross section of CG-Ni20Cr and USSP-Ni20Cr after oxidation at 1000℃ for 100h in Example 1, where (a) is CG-Ni20Cr and (b) is USSP-Ni20Cr alloy.

[0036] Figure 6 The oxidation kinetics curves of CG-Ni20Cr and USSP-Ni20Cr in Example 1 at 1000℃ are shown.

[0037] Figure 7 This is a diagram showing the difference in oxide layer composition between CG-Ni20Cr and USSP-Ni20Cr in Example 1 at 1000°C. Detailed Implementation

[0038] The technical solution of the present invention will be clearly and completely described below with reference to the data in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0039] It should be noted that the technical terms used in this invention are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased on the market or prepared by existing methods. The Ni20Cr alloy has a composition of 80wt% Ni and 20wt% Cr.

[0040] In existing technologies, surface strengthening methods for Ni20Cr alloys mainly include thermal spraying, physical / chemical vapor deposition, elemental doping, and conventional shot peening. However, these methods all have significant limitations: thermally sprayed coatings have high porosity and are prone to peeling due to thermal expansion mismatch; PVD / CVD coatings are thin, easily fail at high temperatures, and the high-temperature CVD process leads to coarsening of the matrix grains; elemental doping is costly and difficult to control in terms of proportion; conventional shot peening, due to temperature rise-induced dynamic recovery and recrystallization, makes it difficult to achieve deep and uniform strengthening. More importantly, none of the above technologies can simultaneously solve the dual problems of strength-plasticity mismatch and insufficient high-temperature oxidation resistance of Ni20Cr alloys, severely restricting their application in extreme high-temperature environments such as aero-engine blades and nuclear reactor internals.

[0041] To address the problems existing in the prior art, this invention provides a method for preparing Ni20Cr alloy, comprising the following steps: annealing a Ni20Cr alloy substrate to eliminate residual stress, promote microstructure homogenization and recrystallization, thereby obtaining an annealed Ni20Cr alloy; performing surface pretreatment on the annealed Ni20Cr alloy to remove the oxide layer on the surface of the annealed Ni20Cr alloy, thereby obtaining a recrystallized Ni20Cr alloy; and subjecting the recrystallized Ni20Cr alloy to cryogenic ultrasonic shot peening to form a gradient nanotwin structure on the surface of the recrystallized Ni20Cr alloy, thereby obtaining the Ni20Cr alloy.

[0042] This invention constructs a gradient nanotwin structure on the surface of Ni20Cr alloy through cryogenic ultrasonic shot peening, significantly improving the strength and plasticity of the Ni20Cr alloy and its resistance to high-temperature oxidation. The high-density nanotwin boundaries and small-angle grain boundaries in the gradient nanotwin structure hinder dislocation movement, increasing material strength. Simultaneously, the coarse-grained core of the Ni20Cr alloy in the gradient nanotwin structure undergoes synergistic deformation, maintaining good plasticity and enabling the alloy to withstand greater loads and deformations. The high-density nanotwin boundaries, small-angle grain boundaries, and high-density dislocations on the surface of the Ni20Cr alloy in the gradient nanotwin structure form rapid diffusion channels for elements such as Cr at high temperatures, accelerating the selective oxidation of Cr. This leads to the rapid formation of a dense oxide film of NiCr2O4 spinel and dense Cr2O3 oxides on the alloy surface, effectively inhibiting further oxygen intrusion into the material and providing favorable conditions for the formation of a dense oxide film. This improves the high-temperature oxidation resistance of the Ni20Cr alloy, allowing it to operate stably for longer periods at 1000℃, thus extending the alloy's service life.

[0043] To enable those skilled in the art to more clearly understand the technical solution of the present invention, the following will provide a detailed description in conjunction with specific embodiments: The cryogenic ultrasonic shot peening equipment used in this invention integrates a liquid nitrogen cooling system and an ultrasonic vibration platform, such as... Figure 2 The uppermost component is a cooling chamber. An annular sealing gasket is placed between the cooling chamber and the plate sample. The cooling chamber and the recrystallized Ni20Cr alloy are fixed to the main structure of the ultrasonic shot peening by screws. Before the experiment, the cooling medium is continuously added to the cooling chamber (i.e., liquid nitrogen medium to control the temperature). The sample temperature is measured and recorded. After the temperature stabilizes, the ultrasonic transducer amplitude, ultrasonic frequency, distance between the shot and the sample surface, shot diameter, and processing time are adjusted to obtain a gradient nanotwin structure with uniform deformation thickness and a smooth and clean surface.

[0044] Example 1 A method for preparing a Ni20Cr alloy includes the following steps: S1. Commercial Ni20Cr alloy was annealed in a vacuum environment at 1000℃ for 90 min. After annealing, it was cooled to room temperature in the furnace to obtain a uniform annealed microstructure with an average grain size of 350 μm; annealing twins were formed in some grains. Figure 1 ), to obtain annealed Ni20Cr alloy.

[0045] S2. The annealed Ni20Cr alloy is wire-cut into circular pieces with a diameter of 40mm and a thickness of 3mm. The upper and lower surfaces and sides of the circular pieces are polished step by step using 100#, 600#, 1200# and 2000# silicon carbide wet sandpaper to obtain recrystallized Ni20Cr alloy, denoted as CG-Ni20Cr.

[0046] S3. The recrystallized Ni20Cr alloy was placed in liquid nitrogen at 77K. When the temperature of the recrystallized Ni20Cr alloy was consistent with that of the liquid nitrogen, the recrystallized Ni20Cr alloy was fixed in the ultrasonic peening chamber of the cryogenic ultrasonic peening equipment. With the assistance of the cryogenic chamber device, a continuous low temperature condition was obtained. At the same time, in the liquid nitrogen environment, the ultrasonic transducer drove the ultrasonic probe to vibrate at high frequency, driving a 3mm diameter GCr15 high-strength mold steel shot to impact the surface of Ni20Cr alloy at high speed for 12 minutes. Finally, mechanical polishing was performed with a silica nano polishing liquid with a particle size of 80nm to maintain the same recrystallized Ni20Cr alloy conditions as before cryogenic ultrasonic peening, and the Ni20Cr alloy was obtained, which was denoted as USSP-Ni20Cr.

[0047] The parameters for ultrasonic shot peening are as follows: oscillator amplitude is 30 μm, frequency is 20 kHz, distance between the shot and the surface of the recrystallized Ni20Cr alloy is 48 mm, and processing time is 12 min.

[0048] Example 2 A method for preparing Ni20Cr alloy is the same as the processing method in Example 1, except that the ultrasonic shot peening parameters are: oscillator amplitude of 75μm, frequency of 40kHz, distance between the shot and the surface of the recrystallized Ni20Cr alloy of 10mm, and processing time of 20min, to obtain Ni20Cr alloy.

[0049] Example 3 A method for preparing Ni20Cr alloy is the same as the processing method in Example 1, except that the ultrasonic shot peening parameters are: oscillator amplitude of 15μm, frequency of 40kHz, distance between the shot and the surface of the recrystallized Ni20Cr alloy of 70mm, and processing time of 4min, to obtain Ni20Cr alloy.

[0050] Example 4 A method for preparing a Ni20Cr alloy is the same as the processing method in Example 1, except that the vacuum annealing at 1000°C for 90 min in S1 is replaced with vacuum annealing at 800°C for 180 min to obtain the Ni20Cr alloy.

[0051] Through systematic optimization of various experimental parameters and deformation temperatures, the preparation of a gradient nanotwinned Ni-20Cr alloy with a surface roughness Ra of less than 0.3 μm and a uniform deformation layer depth exceeding 1 mm was successfully achieved. During cryogenic ultrasonic shot peening, high-speed projectiles continuously impacted the surface of the recrystallized Ni-20Cr alloy, inducing strong indentation and shear plastic deformation. This introduced different defect types onto the recrystallized Ni-20Cr alloy surface: high-density nanotwins, small-angle grain boundaries, and dislocations, forming a gradient nanotwinned structure with a continuous gradient distribution from the surface to the core. Figure 3 As shown. Among them, Figure 3 (b) shows that there are a large number of nanotwins and high-density dislocations on the surface; Figure 3 (c) further reveals that there are a large number of dislocation pile-ups near the nanotwin boundaries. The interaction between these dislocations and the twin boundaries significantly improves the work hardening ability and microstructure stability of the Ni20Cr alloy.

[0052] High-Temperature Oxidation Resistance Experiment: The oxidation kinetics of Ni20Cr alloy before and after cryogenic ultrasonic shot peening were studied using an isothermal oxidation method. The experimental standard HB5258-2000 was followed. Before testing, the muffle furnace temperature was calibrated using a standard thermocouple, and the corundum crucible was placed in a 1200℃ muffle furnace until the crucible mass remained unchanged. To ensure accuracy and repeatability, at least three parallel samples were used for each experiment. Before oxidation, the length × width × thickness of the sample was measured using a vernier caliper with an accuracy of 0.01 mm, and the surface area of ​​the sample was calculated. The total weight of the sample and the alumina crucible was recorded using an electronic balance with an accuracy of 0.01 mg. Then, the sample and crucible were placed together in a furnace at 1000℃ and held for a certain time. Afterward, the sample was removed and air-cooled to room temperature, and the total mass of the sample and crucible was weighed and recorded. This process was repeated until the experiment was completed, and oxidation kinetic curves were plotted.

[0053] Table 1 shows the mechanical property test results of commercial Ni20Cr alloy and USSP-Ni20Cr from Example 1. By employing cryogenic ultrasonic shot peening technology and optimizing methods such as extreme ultrasonic frequencies and extremely low deformation temperatures, a gradient nanotwin structure was constructed on the surface of Ni20Cr alloy at a depth of 1 mm. This achieved an excellent balance between strength and plasticity in the Ni20Cr alloy. At the same time, the high density of nanotwin boundaries and small-angle grain boundaries on the alloy surface provides a rapid channel for the diffusion of Cr elements, promoting the selective oxidation of Cr and forming a thicker, continuous, and dense Cr2O3 oxide film, which significantly enhances its resistance to high-temperature oxidation.

[0054] like Figure 4 As shown, after ultrasonic shot peening, the yield strength of USSP-Ni20Cr increased from 182 MPa to 280 MPa (an increase of 54%), and the tensile strength increased from 562 MPa to 574 MPa, while maintaining a high elongation at break of 59.7%. This indicates that cryogenic ultrasonic shot peening significantly enhances the yield strength of USSP-Ni20Cr, maintains excellent work hardening ability, and achieves an excellent balance between high strength and high plasticity in USSP-Ni20Cr.

[0055] Depend on Figure 5 The results showed that the average oxide layer thickness of CG-Ni20Cr was approximately 33.4 µm, with a dense Cr2O3 layer thickness of approximately 5 µm, accounting for about 15% of the total oxide layer thickness. In contrast, the average oxide layer thickness of USSP-Ni20Cr was only 10.4 µm, with a dense Cr2O3 layer thickness of approximately 8.5 µm, accounting for as much as 82%. Compared to CG-Ni20Cr, the oxide layer thickness of USSP-Ni20Cr obtained after ultrasonic shot peening was significantly reduced by approximately 68.9% after high-temperature oxidation, indicating its excellent resistance to high-temperature oxidation.

[0056] Combination Figure 6 The oxidation rate of CG-Ni20Cr was calculated to be (9.0 ± 1.6) × 10⁻⁶. -3 mg / (cm 2 The oxidation rate of USSP-Ni20Cr decreased to (5.9 ± 0.8) × 10⁻⁶ h, while the oxidation rate of USSP-Ni20Cr decreased to (5.9 ± 0.8) × 10⁻⁶ h. -3 mg / (cm 2 The temperature drop (·h) exceeded 30%. This indicates that cryogenic ultrasonic shot peening effectively slowed down the high-temperature oxidation kinetics of USSP-Ni20Cr.

[0057] like Figure 7As shown, the oxide layer of CG-Ni20Cr, from the surface to the substrate, consists of: NiO (~12µm), NiCr2O4 (~3µm), a NiO / NiCr2O4 mixed layer (~10µm), and a dense Cr2O3 layer (~5µm). The oxide layer structure of USSP-Ni20Cr is simplified to: NiCr2O4 (~3µm) and a dense Cr2O3 layer (~10µm). Cryogenic ultrasonic shot peening introduces high-density nanotwin boundaries and small-angle grain boundaries on the surface of USSP-Ni20Cr. These defects provide rapid channels for the surface diffusion of Cr, promoting the selective oxidation of Cr and thus altering the composition and formation kinetics of the oxide layer, significantly enhancing the high-temperature oxidation resistance of USSP-Ni20Cr.

[0058] It should be noted that when numerical ranges are involved in this invention, it should be understood that both endpoints of each numerical range, as well as any value between the two endpoints, can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described here to avoid redundancy. Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this invention.

Claims

1. A method for preparing a Ni20Cr alloy, characterized in that, Includes the following steps: The Ni20Cr alloy substrate was annealed to eliminate residual stress, promote microstructure homogenization and recrystallization, and obtain annealed Ni20Cr alloy. The annealed Ni20Cr alloy was subjected to surface pretreatment to remove the oxide layer on the surface of the annealed Ni20Cr alloy, and a recrystallized Ni20Cr alloy was obtained. The recrystallized Ni20Cr alloy was subjected to cryogenic ultrasonic shot peening to form a gradient nanotwin structure on the surface of the recrystallized Ni20Cr alloy, thus obtaining the Ni20Cr alloy.

2. The preparation method according to claim 1, characterized in that, Cryogenic ultrasonic shot peening is performed in a liquid nitrogen environment at a temperature of 77K. The shot material is GCr15 high-strength bearing steel with a diameter of 1.0mm to 4.0mm. The ultrasonic frequency is 20kHz to 40kHz, and the treatment time is 4min to 20min.

3. The preparation method according to claim 1, characterized in that, The annealing conditions are as follows: annealing in a vacuum environment at 800℃~1000℃ for 90min~180min.

4. The preparation method according to claim 1, characterized in that, The grain size of the annealed Ni20Cr alloy is 150μm~420μm.

5. The preparation method according to claim 1, characterized in that, The surface pretreatment process is as follows: after sanding with 100#, 600#, 1200# and 2000# sandpaper in sequence, mechanical polishing is performed with silica nano polishing slurry with a particle size of 80nm.

6. A Ni20Cr alloy prepared by the method according to any one of claims 1 to 5, characterized in that, The Ni20Cr alloy includes a Ni20Cr alloy substrate and a gradient nanotwin structure on the surface of the Ni20Cr alloy substrate, wherein the gradient nanotwin structure includes nanotwins, dislocations and small-angle grain boundaries.

7. The Ni20Cr alloy according to claim 6, characterized in that, The depth of the gradient nanotwin structure is at least 1 mm.

8. The Ni20Cr alloy according to claim 6, characterized in that, The surface roughness Ra of the Ni20Cr alloy is less than 0.3 μm.

9. The Ni20Cr alloy according to claim 6, characterized in that, After being oxidized at a constant temperature of 1000℃, the surface of the Ni20Cr alloy forms a composite oxide film composed of NiCr2O4 spinel and Cr2O3.

10. The application of the Ni20Cr alloy of claim 6 in the preparation of key components for aero-engine blades and nuclear reactor internals in extreme high-temperature environments.

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