A method for constructing a designed nanocrystalline grain boundary network in a surface layer of a metal powder

Abstract

This invention provides a method for constructing a designable nanograin boundary network on the surface of metal powder, belonging to the fields of powder metallurgy and material surface engineering. The invention includes ultrasonic cavitation pretreatment to construct a high-defect nanoscale layer on the surface, external field-assisted annealing to control the surface grain boundary geometry, and cooling and post-treatment. By introducing a high-defect nanoscale layer on the powder surface and combining it with external field-assisted annealing to achieve selective recrystallization and grain orientation reconstruction, this invention obtains a nanograin boundary network structure with controllable grain boundary type, orientation, and curvature on the powder surface, thereby providing a stable and reproducible microstructural basis for improving powder surface properties.

Description

Technical Field

[0001] This invention relates to the fields of powder metallurgy and material surface engineering, and in particular to a method for constructing a designable nanocrystalline boundary network on the surface of metal powders. This method is especially suitable for applications where the surface microstructure and surface properties of metal powders with magnetic response characteristics need to be optimized simultaneously. Background Technology

[0002] Metal powders are widely used in powder metallurgy, soft magnetic components, additive manufacturing, and other fields. Their surface microstructure and grain boundary characteristics have a decisive influence on the magnetic properties, mechanical properties, and surface stability of the materials. Existing metal powders are mainly prepared by processes such as atomization, reduction, and mechanical pulverization. Although specific particle size ranges and compositional uniformity can be obtained, the surface grain size and grain boundary structure are usually spontaneously formed by the inherent process, making it difficult to achieve precise geometric design and controllable adjustment.

[0003] To optimize the microstructure of powder surfaces, existing technologies introduce high-density dislocations and nanocrystals onto the surface of powder or bulk materials through methods such as surface mechanical grinding, shot peening, and laser impaction, achieving surface nanostructuring. For example, Chinese patent application CN120205970A proposes using ultrasonic impaction to nanostruct the surface of zirconium alloy specimens, with an ultrasonic frequency of 15kHz to 30kHz and an amplitude of 15μm to 20μm; authorized patent CN114535591A discloses a ball milling surface nanostructuring method based on high-purity reduced iron powder. However, the grain boundary networks generated by such technologies generally exhibit significant randomness and anisotropy: grain boundary type, the proportion of high-angle grain boundaries, and grain boundary orientation and curvature are difficult to quantitatively design and stably reproduce according to application requirements, making it difficult to systematically establish and effectively utilize the structure-property relationship between surface microstructure and macroscopic properties.

[0004] In applications such as iron-based soft magnetic metal powders, surface grain size, grain boundary type, and defect state directly regulate the magnetic domain structure and magnetization process, thus having a crucial impact on coercivity, loss, and stability. The lack of a designable grain boundary network hinders the comprehensive performance optimization of low coercivity and low loss while maintaining high saturation magnetization. For other metal powders, the surface grain boundary structure is also closely related to properties such as surface hardness, wear resistance, and oxidation resistance.

[0005] Therefore, there is an urgent need to develop novel microstructure control technology for the surface of metal powders. Without changing the overall composition and preparation route of the powder, a geometrically designable nanocrystalline boundary network can be constructed on the powder surface. The type, orientation and curvature of the crystal boundaries can be precisely adjusted through controllable process parameters to specifically improve the key properties of the powder surface. Summary of the Invention

[0006] In view of this, to address the technical problem that the random, undesignable, and difficult-to-precise grain boundary structures of existing metal powder surface layers make it difficult to directionally optimize powder surface properties, this invention provides a method for constructing a designable nanocrystalline grain boundary network on the surface of metal powder. By introducing a high-defect nanoscale layer on the powder surface and combining it with external field-assisted annealing to achieve selective recrystallization and grain orientation reconstruction, a nanocrystalline grain boundary network structure with controllable grain boundary type, orientation, and curvature is obtained on the powder surface, thereby providing a stable and reproducible microstructural basis for improving powder surface properties.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for constructing a designable nanocrystalline boundary network on the surface of metal powder, comprising the following steps: Step (1): Ultrasonic cavitation pretreatment to construct a high-defect nano-layer on the surface. Metal powder is dispersed in a liquid medium and subjected to ultrasonic cavitation pretreatment under ultrasonic action. The mass ratio of metal powder to liquid medium is 1:(2~50), the ultrasonic frequency is 15~80kHz, the ultrasonic power density is 50~1000W / L, the treatment time is 1~180min, and the treatment temperature is 0~80℃. A high-defect nano-layer is constructed on the powder surface. Step (2): External field-assisted annealing to control the surface grain boundary geometry After separating and drying the metal powder that has undergone ultrasonic cavitation pretreatment in step (1) from the liquid medium, it is placed in an annealing equipment for external field assisted annealing. The annealing process temperature is 200-1000 ℃, the heating rate is 1-30 ℃ / min, and the holding time is 1 min-10 h. A geometrically designable nanocrystalline boundary network is constructed on the powder surface. Step (3), Cooling and Post-processing The annealed metal powder was subjected to slow cooling and post-treatment under a controlled atmosphere.

[0008] For existing metal powders, especially those with magnetic response characteristics, such as iron-based soft magnetic metal powders, the surface grain size and grain boundary structure are mainly formed spontaneously by the powder preparation process. The grain boundary type, the proportion of high-angle grain boundaries, and the grain boundary orientation and curvature are difficult to design and precisely adjust according to application requirements. This makes it difficult to achieve predictable and reproducible optimization of surface-related properties such as magnetic properties, surface hardness, wear resistance, and surface stability. This invention aims to provide a general method for constructing geometrically designable nanocrystalline grain boundary networks on the surface of metal powders without changing the overall powder composition or powder preparation route. This allows for a controllable grain boundary network structure on the powder surface, thereby improving surface-related properties. Compared to existing technologies, it has the following beneficial effects: The present invention has significant technical advantages. First, it enables the design and precise control of the surface nanograin boundary network. A high-defect nano-layer is constructed through ultrasonic cavitation pretreatment, and selective recrystallization and grain orientation reconstruction are achieved by combining external field-assisted annealing. For example, in Example 1, the proportion of high-angle grain boundaries on the powder surface increased from about 15% to 75%, and in Example 2 it reached 65%. Moreover, the grain boundary network has good connectivity and adjustable orientation.

[0009] Secondly, this method does not change the overall composition of the powder or the preparation route; it only targets the surface treatment, requiring no adjustment to the overall powder composition or the original powder preparation process. Furthermore, it significantly improves the surface properties of the powder. For example, in Example 1, the coercivity of the iron-silicon soft magnetic powder is reduced by approximately 25%, the total loss by approximately 15%, and the initial permeability is increased. Similarly, in Example 2, even after parameter adjustments, the coercivity and total loss are still effectively reduced. In addition, it has a wide range of applications, suitable for various metal or alloy powders such as iron-based soft magnetic powders, stainless steel, and nickel-based powders. Especially for magnetically responsive powders, precise control can be achieved through magnetic field annealing.

[0010] Finally, the process parameters are controllable and the results are highly repeatable. The ultrasonic frequency (15-80kHz), power density (50-1000W / L), annealing temperature (200-1000℃) and other parameters have clear ranges. For example, after adjusting the parameters in Example 2, the grain boundary network was still successfully constructed and the performance was improved, ensuring the stability and reproducibility of the processing effect. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the process flow for constructing a designable nanocrystalline boundary network on the surface of metal powder according to the present invention. Figure 2 This is a comparative microstructure diagram of the surface grain boundary structure of metal powder before (a, c, d) and after (b, e, f) treatment in Example 2 of the present invention, including a comparison of the grain boundary distribution between untreated powder and powder treated by the method of the present invention. Figure 3 This is a comparison chart showing the test results of the powder sample of Example 2 of the present invention and the comparative powder sample in terms of typical surface-related properties (such as coercivity, total loss (50mT / 100kHz), magnetic permeability (50mT / 100kHz), and surface hardness). Detailed Implementation

[0012] like Figure 1 As shown, the present invention provides a method for constructing a designable nanocrystalline boundary network on the surface of metal powder, comprising the following steps: Step (1): Ultrasonic cavitation pretreatment to construct a high-defect nano-layer on the surface. Metal powder is dispersed in a liquid medium and subjected to ultrasonic cavitation pretreatment under ultrasonic irradiation. The mass ratio of metal powder to liquid medium is 1:(2–50), the ultrasonic frequency is 15–80 kHz, preferably 20–40 kHz, the ultrasonic power density is 50–1000 W / L, preferably 100–600 W / L, the treatment time is 1–180 min, preferably 5–60 min, and the treatment temperature is 0–80 °C, preferably 20–50 °C. A high-defect nanostructured layer is constructed on the powder surface. The liquid medium is an alcohol, preferably ethanol.

[0013] In step (1), the transient high-pressure impact and micro-jets generated by ultrasonic cavitation pretreatment act on the surface of the metal powder, causing severe local plastic deformation and strain accumulation on the surface of the metal powder, thereby forming a high-defect nanoscale layer with high-density dislocations, subgrain boundaries and nanocrystals.

[0014] In this invention, during the ultrasonic cavitation pretreatment process, physical or chemical dispersion methods are employed to improve the uniformity of powder dispersion in the liquid medium, thereby preventing severe powder agglomeration and improving treatment consistency. Physical dispersion methods include mechanical stirring or circulating flow. Depending on the powder type and surface tension requirements, dispersants or surfactants can be added to the liquid medium using chemical dispersion methods to improve powder wettability and dispersion stability.

[0015] Step (2): External field-assisted annealing to control the surface grain boundary geometry After the metal powder pretreated by ultrasonic cavitation in step (1) is separated from the liquid medium and dried, it is placed in an annealing apparatus for external field-assisted annealing. The annealing temperature is 200–1000 °C, preferably 500–800 °C, the heating rate is 1–30 °C / min, preferably 5–10 °C / min, and the holding time is 1 min–10 h, preferably 30 min–2 h, to construct a geometrically designable nanocrystalline boundary network on the powder surface. The annealing atmosphere can be an inert gas, a reducing gas, or a vacuum to suppress powder oxidation and contamination.

[0016] In step (2), the external field-assisted annealing can take different forms depending on the physical properties of the metal powder; for metal powders with magnetic response characteristics (such as iron-based soft magnetic metal powders), it is preferable to use an external directional magnetic field for magnetic field annealing. The magnetic field strength is 0.01 to 3 T, preferably 0.05 to 1 T. The orientation of the magnetic field relative to the powder packing direction or the sample bearing surface can be set as needed to control the selectivity of grain orientation during recrystallization.

[0017] During the external field-assisted annealing process, the high-defect nanoscale layer formed by ultrasonic cavitation pretreatment is used as an easily evolving structure. Under the combined action of annealing temperature and external field, selective recrystallization, grain growth, and orientation reconstruction occur on the surface of the metal powder, thereby forming a nanoscale grain boundary network dominated by high-angle grain boundaries on the surface of the metal powder. By adjusting parameters such as annealing temperature, holding time, external field intensity and direction, the surface grain boundary type, the proportion of high-angle grain boundaries, and the grain boundary orientation and curvature can be controlled and adjusted.

[0018] After being processed by the method of the present invention, the surface layer of the metal powder has nanocrystals with an average grain size of 10 to 500 nm within a certain depth range (e.g., 0.1 to 10 μm from the surface inward); the proportion of high-angle grain boundaries in the surface region is significantly increased, and the proportion of high-angle grain boundaries in the total length or total area of ​​grain boundaries can reach more than 70%, and the grain boundary network exhibits good connectivity and orientation adjustability.

[0019] Step (3), Cooling and Post-processing The annealed metal powder is subjected to slow cooling and post-treatment under a controlled atmosphere. The cooling method is slow cooling under a controlled atmosphere to control the stability of the surface grain boundary network structure. Subsequently, the metal powder is subjected to drying, sieving, and de-agglomeration treatments as needed to obtain a metal powder product with a designable nanocrystalline grain boundary network on the surface. Preferably, by controlling the ultrasonic cavitation pretreatment parameters, annealing temperature, and external field conditions in the above steps, the obtained metal powder can exhibit significant improvements in related properties such as magnetic properties (e.g., coercivity, loss, permeability), surface hardness, wear resistance, and surface stability compared to untreated powder or comparative powder not using the complete process of this invention.

[0020] In this invention, metal powder serves as the matrix. The metal powder is a metal or alloy powder, preferably iron-based or other metal powders with magnetic response characteristics. The average particle size of the metal powder can be 20–200 μm. The purity, oxygen content, and other impurity content of the metal powder are controlled within a preset range to ensure the repeatability and stability of subsequent processing. As needed, the metal powder can be pretreated by drying, sieving, and degassing.

[0021] The method provided by this invention is not limited to a specific metal or alloy system, but is applicable to iron-based soft magnetic metal powders, stainless steel powders, nickel-based powders, and other metal powders that require optimization of surface microstructure. For metal powders with magnetic response characteristics, fine control of grain orientation reconstruction and grain boundary network geometric parameters can be achieved by using magnetic field annealing.

[0022] In summary, the method provided by this invention has the following characteristics: It has the advantages of enabling the design and precise control of surface nanocrystalline boundary networks, without changing the overall powder composition and preparation route, significantly improving powder surface properties, having a wide range of applications, being compatible with multiple metal systems, and having controllable process parameters and high repeatability.

[0023] The technical solution of the present invention will be clearly and thoroughly described below with reference to specific embodiments.

[0024] Example 1 The method of this invention is applied to iron-silicon soft magnetic metal powders. In this embodiment, iron-silicon soft magnetic alloy powder is used as the matrix. The method of this invention is used to construct a designable nanocrystalline boundary network on the surface of the powder and to verify its influence on soft magnetic properties.

[0025] 1. Selection and pretreatment of metal powder matrix Iron-silicon soft magnetic alloy powder was produced using a gas atomization method as the metal powder matrix. The nominal composition of the alloy was Fe–6.5 wt% Si, with the remainder being unavoidable impurities. The powder was prepared via gas atomization and screened to obtain a particle size distribution of 10–75 μm, with a D50 of approximately 35 μm. The obtained powder was placed in a vacuum drying oven and dried at 80 °C for 2 h to remove surface-adsorbed moisture and volatile impurities. After drying, the powder was sealed and stored in an argon atmosphere for later use.

[0026] 2. Ultrasonic cavitation pretreatment to construct a high-defect nanostructured layer on the surface. Weigh 100 g of dried iron-silicon soft magnetic powder and add it to 2000 g of liquid medium, which is a mixed solution of 50% deionized water and 50% anhydrous ethanol (volume fraction), i.e., the mass ratio of metal powder to liquid medium is 1:20. Add 0.5 g of a nonionic surfactant suitable for water-alcohol systems to the mixed solution to improve the wettability and dispersion stability of the powder.

[0027] The powder-containing system was placed in an ultrasonic treatment tank equipped with a mechanical stirrer. The mechanical stirrer was turned on at a speed of 300 r / min to ensure uniform suspension of the powder in the solution. Ultrasonic treatment was performed using an immersion ultrasonic probe at a frequency of 25 kHz and a power density of approximately 400 W / L. The treatment temperature was controlled at 30±2 ℃ using a cooling circulation system, and the continuous treatment time was 30 min.

[0028] Under ultrasonic cavitation, a large number of cavitation bubbles and microjets are generated in the solution, producing transient high-pressure impact and shearing on the surface of the suspended iron-silicon soft magnetic powder. This causes severe local plastic deformation and strain accumulation on the powder surface, resulting in a high-defect nanoscale layer with a thickness of approximately 0.5–2 μm. This layer contains high-density dislocations, subgrain boundaries, and submicron or even nanoscale grains. Transmission electron microscopy and X-ray diffraction peak broadening analysis confirm that the surface grains are significantly refined and the degree of lattice distortion is increased.

[0029] After ultrasonic treatment, ultrasonication and stirring were stopped, and the suspension was filtered to collect the powder. The powder was then washed twice with anhydrous ethanol to remove residual surfactants and solution components. Subsequently, the powder was placed in a vacuum drying oven and dried at 60 °C for 4 h to obtain iron-silicon soft magnetic powder pretreated by ultrasonic cavitation.

[0030] 3. Magnetic field annealing modulates the surface grain boundary geometry. The pretreated and dried iron-silicon soft magnetic powder, after ultrasonic cavitation, was loaded into a high-temperature resistant non-magnetic quartz boat, spreading it evenly to a thickness of approximately 5 mm. The quartz boat was then placed in a tubular annealing furnace, with a water-cooled electromagnet coil installed outside the furnace tube to provide a directional magnetic field.

[0031] A protective atmosphere consisting of 95% high-purity nitrogen and 5% hydrogen (by volume) was introduced at a total flow rate of 500 sccm to purge air from the furnace tubes. The heating phase was conducted without a magnetic field, with the temperature rising from room temperature to 750 °C at a rate of 5 °C / min. After reaching the annealing temperature, a magnetic field with a strength of 0.5 T was applied, parallel to the furnace tube axis. The annealing temperature and magnetic field strength were maintained constant for 1 hour.

[0032] During magnetic field annealing, the high-defect nanostructured layer formed by ultrasonic cavitation pretreatment undergoes recrystallization and orientation reconstruction. Under the combined action of the magnetic field driving force and the annealing heat driving force, grains with specific orientations preferentially grow on the surface of the iron-silicon soft magnetic powder, forming a nanograin boundary network dominated by high-angle grain boundaries. After annealing, the heating power supply is turned off, and the magnetic field is maintained until the temperature drops below 300 ℃. Then, the magnetic field is turned off, and the furnace is subsequently cooled to room temperature under a protective atmosphere.

[0033] 4. Cooling, post-treatment, and performance testing After cooling to room temperature, the powder was removed and loosened by gentle vibration to prevent agglomeration. A small amount of large, sintered particles were removed by sieving to obtain the target product. Electron backscatter diffraction (EBSD) was used to characterize the surface grain boundary structure of the treated powder cross-section. The results showed that within approximately 2 μm from the powder surface inwards, the average grain size was approximately 80–250 nm. High-angle grain boundaries (grain boundary orientation difference greater than 15°) accounted for approximately 75% of the total grain boundary length, significantly higher than the approximately 15% in the untreated powder. The grain boundary network exhibited good connectivity and a clear orientation concentration characteristic.

[0034] Iron-silicon soft magnetic powder treated by the method of this embodiment and untreated raw powder were respectively pressed into ring samples, and their soft magnetic properties were tested under the same pressing density and heat treatment conditions. The test results show that at a working frequency of 1 kHz, the coercivity of the powder sample of this embodiment is reduced by about 25% and the total loss is reduced by about 15% compared with the raw powder sample, and the initial permeability is improved. This indicates that by constructing a designable nanograin boundary network on the powder surface, the soft magnetic properties of iron-silicon soft magnetic powder can be effectively improved.

[0035] Example 2 Iron-silicon soft magnetic powder processing under parameter adjustment This embodiment is similar to Embodiment 1, except that the parameters for ultrasonic cavitation pretreatment and magnetic field annealing are adjusted to illustrate the applicable range of parameters for the method of the present invention.

[0036] Using iron-silicon soft magnetic powder with the same composition and particle size distribution as in Example 1, the mass ratio of powder to liquid medium during ultrasonic cavitation pretreatment was 1:10, the liquid medium was deionized water, the ultrasonic frequency was 20 kHz, the ultrasonic power density was about 300 W / L, the treatment time was 20 min, and the treatment temperature was 25 ℃.

[0037] After drying, magnetic annealing was carried out in a tubular annealing furnace of the same type, with nitrogen as the protective atmosphere, annealing temperature of 700 ℃, heating rate of 3 ℃ / min, holding time of 2 h, and magnetic field strength of 0.3 T.

[0038] The results of electron backscatter diffraction (EBSD) show that, Figure 2-3As shown, in this embodiment, the average grain size of the iron-silicon soft magnetic powder in the 1–3 μm range is approximately 100–300 nm, and the proportion of high-angle grain boundaries is approximately 65%, which is significantly higher than that of the untreated powder. The surface grain boundary network also exhibits good connectivity. Soft magnetic performance test results show that the coercivity and total loss of the powder sample treated in this embodiment are significantly lower than those of the original powder sample, proving that within a certain range of parameter variations, the method of this invention can also achieve the construction of a designable nanocrystalline grain boundary network on the surface and the improvement of soft magnetic performance.

[0039] Example 3 Iron-silicon soft magnetic powder processing under preferred low-end conditions This embodiment is similar to Embodiment 1, except that the ultrasonic cavitation pretreatment and magnetic field annealing parameters are adjusted to verify that a nanocrystalline network can also be constructed on the powder surface to improve soft magnetic properties.

[0040] Selection and pretreatment of metal powder matrix A soft magnetic iron-silicon alloy powder with the same composition and particle size distribution as in Example 1 was selected as the metal powder matrix. The nominal alloy composition was Fe–6.5 wt% Si, with the remainder being unavoidable impurities. The particle size distribution of the powder was controlled by sieving, with an average particle size of approximately 50 μm and a particle size range mainly concentrated between 20 and 80 μm. The obtained powder was placed in a vacuum drying oven and dried at 80 °C for 2 h to remove surface-adsorbed moisture and volatile impurities. After drying, the powder was sealed and stored in an argon atmosphere for later use.

[0041] Ultrasonic cavitation pretreatment constructs a high-defect nano-layer on the surface. Weigh 100 g of dried iron-silicon soft magnetic powder and add it to 2000 g of liquid medium, which is a mixed solution of 50% deionized water and 50% anhydrous ethanol (volume fraction), i.e., the mass ratio of metal powder to liquid medium is 1:20. Add 0.5 g of a nonionic surfactant suitable for water-alcohol systems to the mixed solution to improve the wettability and dispersion stability of the powder.

[0042] The powder-containing system was placed in an ultrasonic treatment tank equipped with a mechanical stirrer. The mechanical stirrer was turned on at a speed of 300 r / min to ensure that the powder was uniformly suspended in the solution. Ultrasonic treatment was performed using an immersion ultrasonic probe at a frequency of 20 kHz and a power density of approximately 100 W / L. The treatment temperature was controlled at 20±2 ℃ using a cooling circulation system, and the continuous treatment time was 5 min.

[0043] Under ultrasonic cavitation, a large number of cavitation bubbles and microjets are generated in the solution, which generate transient high-pressure impact and shearing on the surface of the suspended iron-silicon soft magnetic powder, causing local plastic deformation and strain accumulation on the powder surface, forming a high-defect nanoscale layer with a thickness of about 0.3 to 1 μm on the powder surface. This layer contains high-density dislocations, subgrain boundaries and submicron or even nanoscale grains.

[0044] After ultrasonic treatment, ultrasonication and stirring were stopped, and the suspension was filtered to collect the powder. The powder was then washed twice with anhydrous ethanol to remove residual surfactants and solution components. Subsequently, the powder was placed in a vacuum drying oven and dried at 60 °C for 4 h to obtain iron-silicon soft magnetic powder pretreated by ultrasonic cavitation.

[0045] Magnetic field annealing modulates the surface grain boundary geometry The pretreated and dried iron-silicon soft magnetic powder, after ultrasonic cavitation, was loaded into a high-temperature resistant non-magnetic quartz boat, spreading it evenly to a thickness of approximately 5 mm. The quartz boat was then placed in a tubular annealing furnace, with a water-cooled electromagnet coil installed outside the furnace tube to provide a directional magnetic field.

[0046] A protective atmosphere consisting of 95% high-purity nitrogen and 5% hydrogen (by volume) was introduced at a total flow rate of 500 sccm to purge air from the furnace tubes. The heating phase was conducted without a magnetic field, with the temperature rising from room temperature to 500 °C at a rate of 5 °C / min. After reaching the annealing temperature, a magnetic field with a strength of 0.05 T was applied, parallel to the furnace tube axis. The annealing temperature and magnetic field strength were maintained constant for 2 hours.

[0047] During magnetic field annealing, the high-defect nanoscale layer formed by ultrasonic cavitation pretreatment undergoes recrystallization and orientation reconstruction. Under the combined action of the magnetic field driving force and the annealing heat driving force, the surface of the iron-silicon soft magnetic powder preferentially grows oriented subgrains and grains, forming a nanoscale grain boundary network dominated by high-angle grain boundaries. After annealing, the heating power supply is turned off, and the magnetic field is maintained until the temperature drops below 300 ℃. Then, the magnetic field is turned off, and the furnace is subsequently cooled to room temperature under a protective atmosphere.

[0048] Cooling and post-treatment and performance testing After cooling to room temperature, remove the powder and loosen it by gentle shaking to prevent agglomeration. Sieve to remove any large, sintered particles to obtain the target product.

[0049] Electron backscatter diffraction was used to characterize the surface grain boundary structure of the treated powder cross section. The results showed that the average grain size of the iron-silicon soft magnetic powder in this embodiment was in the nanometer range of about 1 to 2 μm. The proportion of high-angle grain boundaries (grain boundary misorientation angle > 15°) in the total grain boundary length was significantly higher than that of the untreated powder. The grain boundary network showed a continuous mesh structure, but the degree of grain boundary refinement and the proportion of high-angle grain boundaries were slightly lower than those in Example 1.

[0050] Iron-silicon soft magnetic powder treated by the method of this embodiment and untreated raw powder were respectively pressed into ring-shaped samples, and their soft magnetic properties were tested under the same pressing density and heat treatment conditions. The test results show that the coercivity and total loss of the powder sample of this embodiment are significantly lower than those of the raw powder, while the initial permeability is improved. This indicates that even under the low end of the preferred parameter range, it is still possible to construct a surface-designable nanograin boundary network and significantly improve the soft magnetic properties.

[0051] Example 4 Based on Example 1, this embodiment adjusts the ultrasonic cavitation pretreatment and magnetic field annealing parameters to demonstrate that a stable nanocrystalline network on the powder surface can still be constructed under high input energy conditions, and evaluates its impact on soft magnetic properties.

[0052] Selection and pretreatment of metal powder matrix A soft magnetic iron-silicon alloy powder with the same composition and particle size distribution as in Example 1 was selected as the metal powder matrix. The nominal alloy composition was Fe–6.5 wt% Si, with the remainder being unavoidable impurities. After sieving, the average particle size of the powder was approximately 50 μm, with the particle size mainly concentrated in the range of 20–80 μm. The powder was placed in a vacuum drying oven and dried at 80 °C for 2 h to remove surface-adsorbed moisture and volatile impurities. After drying, the powder was sealed and stored in an inert atmosphere for later use.

[0053] Ultrasonic cavitation pretreatment constructs a high-defect nano-layer on the surface. Weigh 100 g of dried iron-silicon soft magnetic powder and add it to 2000 g of anhydrous ethanol, i.e., the mass ratio of metal powder to liquid medium is 1:20. Add 0.5 g of a nonionic surfactant suitable for alcohol systems to the solution and use mechanical stirring to suspend the powder uniformly in the solution.

[0054] Mechanical stirring (300 r / min) and immersion ultrasonic probe were turned on. The ultrasonic frequency was 40 kHz, the ultrasonic power density was about 600 W / L, the treatment temperature was controlled at 50±2 ℃ by the cooling circulation system, and the continuous treatment time was 60 min.

[0055] Under the above conditions, the powder surface undergoes strong cavitation impact and shearing, forming a high-defect nanoscale layer with a thickness of about 1 to 3 μm. The layer contains high-density dislocations, subgrain boundaries and a large number of nanocrystals, and the degree of lattice distortion of the surface layer is significantly improved.

[0056] After ultrasonic treatment, ultrasonication and stirring were stopped, the powder was recovered by filtration, and rinsed twice with anhydrous ethanol. Then, it was vacuum dried at 60 °C for 4 h to obtain iron-silicon soft magnetic powder pretreated by ultrasonic cavitation.

[0057] Magnetic field annealing modulates the surface grain boundary geometry The pretreated and dried iron-silicon soft magnetic powder, after ultrasonic cavitation, was loaded into a high-temperature resistant non-magnetic quartz boat and spread evenly to a powder layer thickness of approximately 5 mm. The quartz boat was then placed in a tubular annealing furnace equipped with a water-cooled electromagnet coil.

[0058] A protective atmosphere of high-purity nitrogen / hydrogen mixture (95% N2 and 5% H2 by volume) was introduced at a total flow rate of 500 sccm to purge air from the furnace tubes. The heating phase was conducted without a magnetic field, with the temperature increased from room temperature to 800 °C at a rate of 10 °C / min. After reaching the annealing temperature, a magnetic field with a strength of 1 T was applied, parallel to the furnace tube axis. The annealing temperature and magnetic field strength were maintained constant for 30 min.

[0059] During magnetic field annealing, the highly defective nanostructured layer on the powder surface undergoes intense recrystallization and grain rearrangement. Under the coupling effect of thermal and magnetic driving forces, the grains exhibit significant preferred orientation growth, forming a nanograin boundary network dominated by high-angle grain boundaries and characterized by obvious orientation concentration. After annealing, the heating power supply is turned off, and the magnetic field is maintained until the temperature drops below 300 °C. Then, the magnetic field is turned off, and the furnace is subsequently cooled to room temperature under a protective atmosphere.

[0060] Cooling and post-treatment and performance testing After cooling to room temperature, the powder is removed and loosened by gentle vibration. The powder is then sieved to remove sintered and adhered particles, thus obtaining the target product.

[0061] Electron backscatter diffraction was used to characterize the surface grain boundary structure of the treated powder cross-section. The results showed that the average grain size of the iron-silicon soft magnetic powder in this embodiment was in the nanometer range of approximately 1–3 μm, the proportion of high-angle grain boundaries was significantly higher than that of the untreated powder, and the grain boundary network was more continuous and the orientation concentration was more obvious. Compared with Example 1, the proportion of high-angle grain boundaries and grain boundary connectivity were further improved or basically equivalent, depending on the actual test results.

[0062] The powder treated by the method of this embodiment, the untreated powder, and the powder of Example 1 were respectively pressed into ring-shaped samples, and their soft magnetic properties were tested under the same pressing density and subsequent heat treatment conditions. The test results show that the coercivity and total loss of the powder sample of this embodiment are significantly lower than those of the untreated powder, and the initial permeability is significantly improved. Compared with Example 1, it shows a better or comparable level in terms of the reduction in coercivity and the initial permeability, indicating that significant performance improvement can also be obtained under the high end of the preferred parameter range.

[0063] Comparative Example 1 Magnetic field annealing without ultrasonic cavitation pretreatment This comparative example illustrates the changes in the surface grain boundary structure and soft magnetic properties of iron-silicon soft magnetic powder when only magnetic field annealing is used without ultrasonic cavitation pretreatment, thus demonstrating the important role of ultrasonic cavitation pretreatment in the method of this invention.

[0064] Selection and pretreatment of metal powder matrix A soft magnetic iron-silicon alloy powder with the same composition and particle size distribution as in Example 1 was selected as the metal powder matrix. The nominal alloy composition was Fe–6.5 wt% Si, with the remainder being unavoidable impurities. After sieving, the average particle size of the powder was approximately 50 μm, with the particle size mainly concentrated in the range of 20–80 μm. The obtained powder was placed in a vacuum drying oven and dried at 80 °C for 2 h to remove surface-adsorbed moisture and volatile impurities. After drying, the powder was sealed and stored in an inert atmosphere for later use.

[0065] Unlike Example 1, this comparative example does not perform an ultrasonic cavitation pretreatment step, and the dried iron-silicon soft magnetic powder is directly used for subsequent magnetic field annealing.

[0066] Magnetic field annealing The dried iron-silicon soft magnetic powder was loaded into a high-temperature resistant non-magnetic quartz boat, spreading it evenly to a thickness of approximately 5 mm. The quartz boat was then placed in a tubular annealing furnace, with a water-cooled electromagnet coil installed outside the furnace tube to provide a directional magnetic field.

[0067] A protective atmosphere consisting of 95% high-purity nitrogen and 5% hydrogen (by volume) was introduced at a total flow rate of 500 sccm to purge air from the furnace tubes. The heating phase was conducted without a magnetic field, with the temperature rising from room temperature to 750 °C at a rate of 5 °C / min. Once the annealing temperature was reached, a magnetic field with a strength of 0.5 T was applied, parallel to the furnace tube axis. The annealing temperature and magnetic field strength were maintained constant for 1 hour. After annealing, the heating power was turned off, and the magnetic field was maintained until the temperature dropped below 300 °C. The magnetic field was then turned off, and the furnace was subsequently cooled to room temperature under the protective atmosphere.

[0068] Cooling and post-treatment and performance testing After cooling to room temperature, the powder was removed and loosened by gentle vibration. A small amount of sintered and agglomerated particles were removed by sieving to obtain a comparative powder sample.

[0069] Electron backscatter diffraction was used to characterize the surface grain boundary structure of the comparative sample powder cross section. The results showed that the average grain size of the iron-silicon soft magnetic powder in the comparative sample was still in the submicron range within the 1-3 μm range. The proportion of high-angle grain boundaries (grain boundary misalignment angle > 15°) in the total grain boundary length only increased slightly, for example from about 15% of the original powder to about 20-25%. The connectivity of the grain boundary network was generally poor, and no obvious orientation concentration characteristics were observed.

[0070] The comparative example powder, the untreated original powder, and the powder from Example 1 were pressed into ring-shaped samples, and their soft magnetic properties were tested under the same pressing density and heat treatment conditions. The test results showed that, compared with the original powder, the coercivity and total loss of the comparative example powder decreased only to a limited extent, and the initial permeability increased only slightly. Compared with Example 1, the reduction in coercivity and total loss was significantly insufficient. This indicates that without the high-defect nanoscale layer constructed by ultrasonic cavitation pretreatment, magnetic field annealing alone is insufficient to form a geometrically designable nanocrystalline boundary network on the powder surface, resulting in limited improvement in soft magnetic properties.

[0071] Comparative Example 2 (without field-assisted annealing) Ultrasonic cavitation pretreatment without field-assisted annealing This comparative example illustrates the changes in the surface grain boundary structure and soft magnetic properties of iron-silicon soft magnetic powder when only ultrasonic cavitation pretreatment is performed without external field-assisted annealing, thus demonstrating the important role of external field-assisted annealing in the method of this invention.

[0072] Selection and pretreatment of metal powder matrix The same iron-silicon soft magnetic alloy powder with the same composition and particle size distribution as in Example 1 was selected as the metal powder matrix. The composition and pretreatment method were the same as in Example 1. That is, the average particle size of the powder after sieving was about 50 μm. After drying at 80 °C for 2 h, it was sealed and stored in an inert atmosphere.

[0073] Ultrasonic cavitation pretreatment Weigh 100 g of dried iron-silicon soft magnetic powder and add it to 2000 g of liquid medium, which is a mixed solution of 50% deionized water and 50% anhydrous ethanol (volume fraction), i.e., the mass ratio of metal powder to liquid medium is 1:20. Add 0.5 g of a nonionic surfactant suitable for water-alcohol systems to the mixed solution to improve the wettability and dispersion stability of the powder.

[0074] The powder-containing system was placed in an ultrasonic treatment tank equipped with a mechanical stirrer. The mechanical stirrer was turned on at a speed of 300 r / min to ensure that the powder was uniformly suspended in the solution. Ultrasonic treatment was performed using an immersion ultrasonic probe at a frequency of 25 kHz and a power density of approximately 400 W / L. The treatment temperature was controlled at 30±2 ℃ using a cooling circulation system, and the continuous treatment time was 30 min.

[0075] Under ultrasonic cavitation, the powder surface undergoes intense local plastic deformation and strain accumulation, forming a high-defect nanoscale layer with high-density dislocations, subgrain boundaries, and nanocrystals.

[0076] After ultrasonic treatment, ultrasonication and stirring were stopped, and the suspension was filtered to collect the powder. The powder was then washed twice with anhydrous ethanol to remove residual surfactants and solution components. Subsequently, the powder was placed in a vacuum drying oven and dried at 60 °C for 4 h to obtain iron-silicon soft magnetic powder pretreated by ultrasonic cavitation.

[0077] Post-processing and performance testing under annealing conditions without external assistance The dried powder was not subjected to external field-assisted annealing and was used directly for sample preparation and performance testing. To ensure comparability, both the ultrasonically pretreated powder and the original powder were subjected to the same pressing and conventional stress-relief heat treatment conditions (e.g., short-time annealing at a lower temperature without external field) to eliminate the influence of differences in sample preparation processes.

[0078] Electron backscatter diffraction was used to characterize the surface grain boundary structure of the comparative sample powder cross section. The results showed that the grain size within a certain depth range of the powder surface was finer than that of the original powder, and the proportion of high-angle grain boundaries was increased to a certain extent. However, the grain boundary orientation was relatively random, the grain boundary network connectivity was generally poor, and no nano-grain boundary network with obvious geometric designable features was formed.

[0079] The comparative example powder, the untreated powder, and the powder from Example 1 were pressed into ring-shaped samples, and their soft magnetic properties were tested under the same pressing density and subsequent conventional heat treatment conditions. The test results showed that the coercivity and total loss of the comparative example powder sample were improved compared to the original powder, and the initial permeability was slightly increased. However, the improvement was significantly lower than that of the powder sample obtained in Example 1. This indicates that although ultrasonic cavitation pretreatment can introduce a high-defect nanoscale layer on the powder surface, it is difficult to obtain a high proportion of high-angle grain boundaries and an orientation-tunable nanograin boundary network structure without external field-assisted annealing, resulting in limited overall performance improvement.

[0080] The test results of Examples 1-5 and Comparative Examples 1-2 are shown in Table 1 below: Table 1. Test results of Examples 1-5 and Comparative Examples 1-2

[0081] A comparison of Examples 1-5 with Comparative Examples 1-2 shows that, when only magnetic field annealing is used (Comparative Example 1), although the surface grain boundary structure of the powder can be improved to some extent, the increase in the proportion of high-angle grain boundaries is limited, the connectivity and orientation concentration of the grain boundary network are not obvious, and the improvement in soft magnetic properties is small. When only ultrasonic cavitation pretreatment is used without external field-assisted annealing (Comparative Example 2), although a high-defect nanoscale layer can be introduced into the powder surface and some grain refinement and performance improvement can be achieved, it is still difficult to form a geometrically designable nanoscale grain boundary network structure. In contrast, the method of this invention organically combines ultrasonic cavitation pretreatment with external field-assisted annealing (Examples 1-5), using the high-defect nanoscale layer constructed by ultrasound as an easily evolving structure, and achieving preferential orientation growth of grains and reconstruction of the grain boundary network during external field annealing, a high proportion of high-angle grain boundaries, good connectivity, and tunable orientation nanoscale grain boundary network can be obtained on the powder surface, resulting in a comprehensive improvement in soft magnetic properties that is significantly better than that of the comparative examples.

[0082] The above description is merely a preferred embodiment of the present invention. However, the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method of constructing a designed nanometer grain boundary network in a surface layer of a metal powder, characterized by, Includes the following steps: Step (1): Ultrasonic cavitation pretreatment to construct a high-defect nano-layer on the surface. Metal powder is dispersed in a liquid medium and subjected to ultrasonic cavitation pretreatment under ultrasonic action. The mass ratio of metal powder to liquid medium is 1:(2~50), the ultrasonic frequency is 15~80kHz, the ultrasonic power density is 50~1000W / L, the treatment time is 1~180min, and the treatment temperature is 0~80℃. A high-defect nano-layer is constructed on the powder surface. Step (2): External field-assisted annealing to control the surface grain boundary geometry After separating and drying the metal powder that has undergone ultrasonic cavitation pretreatment in step (1) from the liquid medium, it is placed in an annealing equipment for external field assisted annealing. The annealing process temperature is 200-1000 ℃, the heating rate is 1-30 ℃ / min, and the holding time is 1 min-10 h. A geometrically designable nanocrystalline boundary network is constructed on the powder surface. Step (3), Cooling and Post-processing The annealed metal powder was subjected to slow cooling and post-treatment under a controlled atmosphere.

2. The method of claim 1, wherein the method is characterized by: In step (1), the ultrasonic frequency is 20-40kHz, the ultrasonic power density is 100-600W / L, the treatment time is 5-60min, and the treatment temperature is 20-50℃.

3. The method of claim 1, wherein the method is characterized by: In step (1), during the ultrasonic cavitation pretreatment process, physical dispersion or chemical dispersion is also used to improve the dispersion uniformity of the powder in the liquid medium.

4. The method of claim 3, wherein the metal powder is a powder of a material selected from the group consisting of titanium, nickel, iron, cobalt, copper, aluminum, magnesium, and alloys thereof. The physical dispersion method is mechanical stirring or circulation.

5. The method of claim 3, wherein the metal powder is a powder of a metal selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, molybdenum, tungsten, and alloys thereof. Chemical dispersion methods involve adding dispersants or surfactants.

6. The method of claim 1, wherein the method is characterized by: The annealing process is carried out at a temperature of 500–800 ℃, a heating rate of 5–10 ℃ / min, and a holding time of 30 min–2 h.

7. The method of claim 1, wherein the method is characterized by: When the metal powder is a metal powder with magnetic response characteristics, in step (2), an external directional magnetic field is used for magnetic annealing, and the magnetic field strength is 0.01 to 3 T.

8. The method of claim 1, wherein the method is characterized by: In step (2), the annealing atmosphere is an inert gas, a reducing gas, or a vacuum.

9. The method of claim 1, wherein the method is characterized by: In step (3), the cooling method is slow cooling under a controlled atmosphere to control the stability of the surface grain boundary network structure.

10. The method of claim 1-9, wherein the method is characterized by, In step (1), the liquid medium is an alcohol.

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