Heat treatment method for improving grain boundary strength of alloy

Abstract

The invention relates to the technical field of metal material modification, and discloses a heat treatment method for improving alloy grain boundary strength, which comprises the following steps: laying a solid gas phase generation source formed by filling gaps of framework particles with reaction precursor powder at the bottom of a graphite box, homogeneous metal or alloy chippings which are consistent with the chemical components of the workpiece are accumulated on the sacrificial filter layer as a sacrificial filter layer; placing the workpiece and the graphite box in a vacuum furnace for degassing in a non-contact manner at a specific horizontal distance; and performing periodic pressure gating treatment, and alternately performing an atmosphere generation and filtering stage for maintaining high vacuum and a blocking and diffusion stage for filling inert gas to establish back pressure. The dynamic matching of the gas phase generation rate and the solid diffusion rate is accurately regulated and controlled through breathing type pressure pulse circulation, and associated carbon and oxygen impurities are intercepted by using a homogeneous filter layer. And the deep strengthening of the grain boundary is realized on the premise of keeping the original smooth finish of the surface of the workpiece.

Description

Technical Field

[0001] This invention relates to the field of metal material modification technology, specifically a heat treatment method for improving the grain boundary strength of alloys. Background Technology

[0002] Refractory metals such as tantalum, niobium, and tungsten, and their alloys, possess excellent high-temperature mechanical properties, making them core materials for manufacturing critical heat-resistant components in the aerospace and nuclear industries. To overcome the problem of insufficient grain boundary strength in these materials under high-temperature service conditions, microalloying by introducing active elements such as yttrium and hafnium into the matrix is ​​a major technical approach to improve their creep resistance. Existing technologies commonly employ vacuum high-temperature vapor phase doping technology, which involves reducing metal oxides with a reducing agent in a vacuum environment to generate metal vapor containing strengthening elements. The concentration gradient then drives the diffusion of active atoms from the workpiece surface to the internal grain boundaries, thereby achieving solid solution strengthening.

[0003] In existing vacuum continuous gas-phase doping processes, there is a significant kinetic mismatch between the generation rate of the gas phase source and the solid-state diffusion rate of active atoms within the matrix. Under constant high-temperature vacuum conditions, the generation and transport rates of the gas phase precursor are typically higher than the diffusion rate of atoms migrating into the metal lattice. This rate difference results in the adsorption rate of active atoms on the workpiece surface being much higher than their diffusion flux into the matrix. The resulting surface supersaturation phenomenon leads to the accumulation of strengthening elements on the surface in the form of deposited layers or compound crusts. This not only worsens the surface roughness and alters the dimensional accuracy of the workpiece, but the dense surface deposit also blocks diffusion channels, hindering the effective penetration of subsequent strengthening atoms into the deeper layers of the matrix.

[0004] Furthermore, reaction systems using carbon powder or carbides as reducing agents inevitably generate carbon- and oxygen-containing gaseous products such as carbon monoxide along with metal vapor. In process environments lacking effective selective filtration mechanisms, these associated impurity gases are directly transported to the workpiece surface with the main gas flow. Due to the extremely high chemical affinity of refractory metals for carbon and oxygen, competitive oxidation or carburizing reactions occur on the workpiece surface while strengthening atoms are being adsorbed. This leads to oxidation discoloration or the formation of brittle interstitial solid solution layers, making it difficult to maintain the original metallic luster and surface cleanliness of the workpiece while simultaneously achieving grain boundary strengthening. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a heat treatment method for improving the grain boundary strength of alloys, which solves the problems of workpiece surface crusting, coarsening, and contamination by associated carbon and oxygen impurities caused by the kinetic mismatch between gas phase transport rate and solid-state diffusion rate in existing gas phase doping processes.

[0006] To achieve the above objectives, the present invention provides the following technical solution, comprising the following steps: Step S1: Filling the diffusion medium and filter layer: A solid gas phase generation source composition is laid at the bottom of the graphite box as a diffusion medium layer, and a sacrificial filter material is stacked on top of the medium layer as a filter layer. The sacrificial filter material is a metal or alloy fragment of the same chemical composition as the substrate of the workpiece to be treated. Step S2, Furnace layout: Place the filled graphite box and the workpiece to be processed in the vacuum furnace, so that the workpiece and the graphite box are not in contact and the horizontal distance between them is 100mm to 300mm. Step S3, Vacuum Degassing: Vacuum the furnace and heat it to remove gas; Step S4, Periodic Pressure Swing Diffusion Treatment: Raise the furnace temperature to the treatment temperature and execute multiple consecutive pressure pulse cycles, each of which alternately includes an atmosphere generation and filtration stage and a blocking and diffusion stage; During the atmosphere generation and filtration stage, a high vacuum state is maintained, a gas phase containing strengthening elements is generated using a dielectric layer, and carbon and oxygen impurities are intercepted by the filtration layer. During the blocking and diffusion stage, an inert gas is introduced to establish back pressure, suppressing the generation of the gas phase and promoting the diffusion of strengthening elements into the workpiece.

[0007] By employing the above technical solution, this invention utilizes a periodic pressure swing diffusion mechanism and homogeneous filtration technology to achieve precise control over gas-solid reaction kinetics and solid-state diffusion equilibrium. Its specific mechanism and effects are described below: First, regarding the periodic pressure-switching diffusion mechanism: This invention regulates the thermodynamic and kinetic states of the carbothermic reduction reaction through periodic changes in furnace pressure. During the atmosphere generation and filtration stage, the high vacuum environment within the furnace reduces the partial pressure of the gaseous products, allowing the reduction reaction in the solid gas phase source to meet the thermodynamic conditions for generation, producing atomic vapor containing strengthening metals (such as yttrium). Active atoms are transported with the gas phase and adsorbed onto the workpiece surface. During the blocking and diffusion stage, inert gas is introduced, raising the ambient pressure to several hundred Pascals or higher. This back pressure increases the collision frequency of gas phase molecules, shortens the mean free path, and inhibits the forward propagation of the carbothermic reduction reaction by increasing the total ambient pressure, thus significantly reducing or stopping the release of strengthening elements at the source. During this stage, the strengthening atoms adsorbed on the workpiece surface diffuse along grain boundaries into the matrix under high temperature. This alternating "gas-phase supply-solid-state diffusion" mode ensures that the concentration of strengthening elements on the workpiece surface is always lower than their solid solution limit in the matrix, avoiding the accumulation of enriched layers and liquid phase sintering caused by surface supersaturation. Deep grain boundary strengthening is achieved without changing the workpiece dimensional accuracy and surface roughness.

[0008] Secondly, regarding the homogeneous sacrificial filter layer: Metal scraps with the same chemical composition as the workpiece matrix are used as the filter medium. Because the metal scraps have a large specific surface area and are in a high-temperature, high-activity state, when a mixed airflow containing reinforcing metal atoms and associated carbon and oxygen impurities (such as CO gas or oxide particles) passes through this filter layer, the metal scraps preferentially react chemically or physically with the carbon and oxygen impurities, trapping the impurities inside the graphite cartridge. This design utilizes the affinity of homogeneous materials for impurity elements, acting as a sacrificial layer to protect the downstream workpiece, effectively preventing carburization or oxidation on the workpiece surface, and ensuring that the treated workpiece maintains its original metallic luster.

[0009] Preferably, in the step of filling the diffusion medium and the filter layer, the solid gas phase generator composition is prepared from raw materials comprising the following components: reaction precursor powder and framework dispersion particles. The reaction precursor powder is a mixture of yttrium oxide powder and carbon source powder, with a molar ratio of yttrium oxide to carbon source of 1:3.5 to 1:4.0; the framework dispersion particles are stabilized fused zirconia particles; and the mass ratio of reaction precursor powder to framework dispersion particles is 1:4.0 to 1:5.0.

[0010] By employing the above technical solution, fused zirconia particles are used as an inert physical framework at high temperatures, and the precursor powder is dispersed and filled in the gaps between them. This dispersion structure prevents the sintering and agglomeration of the reaction powder at high temperatures, maintains the porosity of the gas phase release channels, and thus ensures the stability of the gas phase source release rate during multiple pressure pulse cycles. Simultaneously, limiting the carbon source ratio to slightly higher than the stoichiometric ratio ensures the complete reduction of yttrium oxide.

[0011] Preferably, the solid gas phase generator composition is prepared by: firstly, dry mixing yttrium oxide powder and carbon source powder to obtain a reaction precursor powder, and then mechanically mixing the reaction precursor powder with the skeleton dispersion particles in a drum, so that the precursor powder fills the gaps between the skeleton dispersion particles.

[0012] By adopting the above technical solution, the stepwise mixing process ensures that the micron-sized reaction powder can be uniformly filled in the gaps constructed by the millimeter-sized skeleton particles, forming a uniform reaction bed and avoiding the phenomenon of intense local reactions or stagnant local reactions.

[0013] Preferably, in the step of filling the diffusion medium and the filter layer, the physical form of the sacrificial filter material is spiral, sheet, filament or granular debris, and its thickness or equivalent diameter is 0.1 mm to 0.5 mm; the stacking thickness of the filter layer is 30 mm to 50 mm, and the stacking thickness of the filter layer is greater than 1.5 times the laying thickness of the medium layer.

[0014] By adopting the above technical solutions, the debris of the aforementioned physical forms can all provide a large specific surface area. Controlling the size and shape of the debris can achieve a suitable porosity, avoiding both excessively fine debris that leads to excessive flow resistance affecting gas phase transport and excessively coarse debris that results in insufficient specific surface area and reduced filtration efficiency. Setting the filter layer thickness to be 1.5 times greater than the media layer thickness is to ensure that there is sufficient sacrificial material to cope with impurities generated throughout the entire treatment cycle, preventing the filter layer from failing due to saturation.

[0015] Preferably, the workpiece to be treated is selected from tantalum-based alloy, niobium-based alloy, tungsten-based alloy, pure tantalum, pure niobium or pure tungsten plate; in the periodic pressure-switching diffusion treatment step, the treatment temperature is 1450°C to 1550°C, and the number of pressure pulse cycles is 5 to 10.

[0016] By employing the above technical solution, this temperature range provides the necessary activation energy for the diffusion of refractory metal grain boundaries. Combined with 5 to 10 cycles, it ensures sufficient infiltration of strengthening elements while avoiding abnormal grain growth in the matrix caused by prolonged high-temperature holding. The broadened material range indicates that this invention is universally applicable to various refractory metal systems.

[0017] Preferably, in the vacuum degassing step, the process parameters for vacuum degassing are: a vacuum degree better than 5.0 × 10⁻⁶. -3 Under the condition of Pa, the temperature is raised to 950℃ to 1050℃ and held for 30min to 60min.

[0018] By adopting the above technical solution, deep degassing can be performed before reaching the reaction initiation temperature, which can remove the moisture and physically adsorbed gases adsorbed in the reaction source and filter layer in advance, preventing these impurities from being released and participating in the reaction in the subsequent higher temperature stage, thereby reducing the risk of workpiece contamination.

[0019] Preferably, in the periodic pressure swing diffusion treatment step, the control parameters for the atmosphere generation and filtration stages are: stopping the gas supply and opening the high vacuum valve to reduce the furnace pressure to 2.0 × 10⁻⁶. -2 Keep below Pa for 5 to 15 minutes.

[0020] By adopting the above technical solution, the pressure was reduced to 2.0 × 10⁻⁶. -2 Below Pa, the carbothermic reduction reaction is ensured to have a sufficiently large Gibbs free energy driving force, causing the reaction to proceed in the direction of generating metal vapor, and completing a quantitative gas phase supply within a limited time.

[0021] Preferably, in the periodic pressure-switching diffusion process, the control parameters for the blocking and diffusion stages are: high-purity argon gas is introduced, and the pressure inside the furnace is maintained at 600 Pa to 1000 Pa for 35 min to 60 min.

[0022] By employing the above technical solution, an inert gas back pressure of 600 Pa to 1000 Pa is sufficient to kinetically suppress the mean free path of gas phase molecules and the reaction formation rate. The longer retention time allows surface-adsorbed atoms ample time to migrate deeper into the grain boundaries, reducing the surface concentration gradient and freeing up surface active sites for the next cycle of adsorption.

[0023] Preferably, in each pressure pulse cycle, the holding time of the blocking and diffusion phase is 3.0 to 7.0 times that of the holding time of the atmosphere generation and filtration phase.

[0024] By employing the above technical solution, limiting the diffusion time to be significantly longer than the gas phase supply time is a key parameter for controlling surface deposition behavior. This time ratio ensures that the diffusion consumption rate of surface atoms is greater than or equal to the gas phase adsorption rate, fundamentally eliminating the thermodynamic tendency for surface crust formation.

[0025] Preferably, the method further includes a cooling step: after all pressure pulse cycles have ended, the furnace is kept filled with inert gas, the furnace pressure is adjusted to 50 kPa to 80 kPa, and the furnace is cooled to below 800°C before the blower is turned on for forced cooling.

[0026] By adopting the above technical solution, maintaining a high positive pressure of inert gas during the cooling stage can prevent high-temperature oxidation caused by the backflow of external air, while using gas convection to assist uniform cooling and reduce thermal stress on the workpiece.

[0027] This invention provides a heat treatment method for improving the grain boundary strength of alloys. It has the following beneficial effects: 1. This invention employs a periodic pressure-switching diffusion process, which alternates between a high-vacuum stage and a back-pressure diffusion stage to control the generation of the gas phase and the infiltration of atoms, respectively. This method regulates the kinetic balance between the gas phase supply rate and the solid-state diffusion rate, avoiding the supersaturation accumulation of strengthening elements on the workpiece surface, and achieving deep strengthening of grain boundaries without changing the dimensional accuracy and surface roughness of the workpiece.

[0028] 2. This invention utilizes homogeneous metal scraps to construct a sacrificial filter layer. Based on the chemical affinity of homogeneous materials for carbon and oxygen, it preferentially adsorbs associated impurity gases during gas-phase transport. This design blocks direct contact between impurities and the workpiece surface, preventing carburization or oxidation and ensuring the surface quality of the workpiece after treatment.

[0029] 3. This invention employs a composite medium structure with fused zirconia particles as the physical framework. The high-temperature stability of zirconia isolates the reaction precursor powder, preventing powder sintering and agglomeration at high temperatures. This structure maintains the porous characteristics of the diffusion medium layer and gas release channels, ensuring the stability of the gas phase supply during multi-cycle heat treatment. Attached Figure Description

[0030] Figure 1 Figure (a) shows the interstitial element content distribution of the waste sacrificial filter material along the stacking thickness direction in an embodiment of the present invention, where Figure (a) is the carbon content distribution at each sampling location and Figure (b) is the oxygen content distribution at each sampling location. Figure 2 Figure 1 is a comparative statistical chart of the deposition behavior of workpiece surface under different pressure control processes in the embodiments of the present invention. Figure 2(a) is a comparative chart of the weight gain per unit area of ​​each group of samples, and Figure 3(b) is a comparative chart of the surface roughness of each group of samples. Figure 3 The following are comparison charts of the chemical composition and impurity content of the workpieces in the embodiments of the present invention, wherein Figure (a) is a comparison chart of the content of interstitial impurity elements, and Figure (b) is a comparison chart of the content of alloying elements and metallic impurities. Figure 4 Figure (a) is a comparison of the yield strength of the three matrix materials at room temperature (25°C), and Figure (b) is a comparison of the yield strength of the three matrix materials at high temperatures (1200°C or 1400°C). Figure 5 Figure (a) shows the distribution of microhardness along the thickness direction of the sample in an embodiment of the present invention. Figure (b) is a distribution curve of the absolute value of microVickers hardness at different depths from the surface, and Figure (c) is a distribution curve of the microhardness increment compared to the matrix material. Figure 6 Figure (a) shows the influence of the horizontal distance between the workpiece and the graphite box on the processing effect in an embodiment of the present invention. Figure (b) is a biaxial comparison of the surface deposition layer thickness and surface roughness as a function of the distance. Figure (c) is a trend diagram of the effective strengthening depth as a function of the distance. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Raw materials not specifically mentioned (such as high-purity graphite powder, high-purity argon gas, etc.) are all commercially available analytical grade or higher grade products.

[0033] Yttrium oxide powder, chemical formula Y2O3, CAS number 1314-36-9, purity ≥99.9% based on total rare earth oxides, central particle size D50 of 2-5 μm, and cubic crystal system.

[0034] Fused zirconia particles, chemical formula ZrO2, CAS number 1314-23-4, purity ≥99.0%, are stabilized with yttrium oxide to prevent crystal transformation and pulverization. The particle size range is 0.5~1.0mm, and the physical form is hollow spheres or irregular fused sand.

[0035] Tantalum and tantalum alloy turning chips, used as sacrificial filter materials, have the same chemical composition as the tantalum-based workpiece matrix to be treated. They are in the form of spiral turning chips or flake cutting chips with a thickness of 0.1-0.5 mm and a width of 2-5 mm. Before use, they are ultrasonically cleaned with acetone to remove oil and vacuum dried at 100°C for 2 hours to remove residual cutting fluid from the surface.

[0036] Niobium and niobium alloy turning chips, as sacrificial filter materials, have the same chemical composition as the niobium-based workpiece matrix to be treated. The physical form is spiral turning chips or flake cutting chips with a thickness of 0.1-0.5 mm and a width of 2-5 mm. Before use, they are ultrasonically cleaned with acetone to remove oil and vacuum dried at 100°C for 2 hours to remove residual cutting fluid from the surface.

[0037] The workpiece to be processed is a rolled plate sample of industrial grade Ta-10W tantalum-tungsten alloy (tungsten content 9.0-11.0 wt%, tantalum balance) or C-103 niobium-hafnium alloy (hafnium content 9.0-11.0 wt%, titanium content 0.7-1.3 wt%, niobium balance).

[0038] Preparation Example 1: This preparation example provides a method for preparing a solid vapor generation source composition for improving the grain boundary strength of alloys, comprising the following steps: The first step is to accurately weigh yttrium oxide powder and high-purity graphite powder, control the molar ratio of the two to be 1 (Y2O3): 3.8, and place the mixed powder in a three-dimensional mixer for dry mixing for 3 hours to obtain the reaction precursor powder. The second step involves weighing the reaction precursor powder and fused zirconia particles, controlling the mass ratio m (reaction precursor powder): m (fused zirconia particles) to be 1:4.5, and mixing them for 45 minutes using a mechanical drum mixer to ensure that the powder is evenly dispersed in the gaps between the skeleton particles, thus obtaining reaction source composition A.

[0039] Preparation Example 2: This preparation example provides a method for preparing a solid vapor generation source composition for improving the grain boundary strength of alloys, comprising the following steps: The first step is to accurately weigh yttrium oxide powder and high-purity graphite powder, control the molar ratio of the two, n(Y2O3):n(C), to be 1:3.5, and place the mixed powder in a three-dimensional mixer for dry mixing for 2 hours to obtain the reaction precursor powder. The second step involves weighing the reaction precursor powder and fused zirconia particles, controlling the mass ratio m (reaction precursor powder): m (fused zirconia particles) to be 1:4.0, and mixing them for 30 minutes using a mechanical drum mixer to ensure that the powder is evenly dispersed in the gaps between the skeleton particles, thus obtaining the reaction source composition B.

[0040] Preparation Example 3: This preparation example provides a method for preparing a solid vapor generation source composition for improving the grain boundary strength of alloys, comprising the following steps: The first step is to accurately weigh yttrium oxide powder and high-purity graphite powder, control the molar ratio of the two to be 1 (Y2O3): 4.0, and place the mixed powder in a three-dimensional mixer for 4 hours to dry mix and obtain the reaction precursor powder. The second step involves weighing the reaction precursor powder and fused zirconia particles, controlling the mass ratio m (reaction precursor powder): m (fused zirconia particles) to be 1:5.0, and mixing them for 60 minutes using a mechanical drum mixer to ensure that the powder is evenly dispersed in the gaps between the skeleton particles, thus obtaining the reaction source composition C.

[0041] Preparation Example 4: This preparation example provides a method for preparing a solid vapor generation source composition for improving the grain boundary strength of alloys, comprising the following steps: The first step is to accurately weigh yttrium oxide powder and high-purity graphite powder, control the molar ratio of the two to be 1 (Y2O3): 3.8, and place the mixed powder in a three-dimensional mixer for dry mixing for 3 hours to obtain the reaction precursor powder. The second step involves weighing the reaction precursor powder and fused zirconia particles, controlling the mass ratio m (reaction precursor powder): m (fused zirconia particles) to be 1:4.0, and mixing them for 45 minutes using a mechanical drum mixer to ensure that the powder is evenly dispersed in the gaps between the skeletal particles, thus obtaining the reaction source composition D.

[0042] Example 1: This embodiment provides a heat treatment method for improving the grain boundary strength of an alloy, specifically for processing Ta-10W tantalum-tungsten alloy plates, including the following steps: Step 1, filling the reaction source: A molybdenum-made material box was selected, and the reaction source composition A prepared in Preparation Example 1 was spread flat on the bottom of the graphite box as a medium layer with a thickness of 15 mm; tantalum alloy turning chips that have been cleaned and dried were evenly piled on top of the medium layer as a sacrificial filter layer with a thickness of 40 mm, and it was ensured that the filter layer was in close contact with the inner wall of the graphite box.

[0043] Step 2, furnace layout: Place the filled graphite box on the furnace bed on the inlet side of the vacuum annealing furnace, and place the Ta-10W alloy plate to be processed on the molybdenum material rack in the center of the furnace uniform temperature zone. Adjust the position so that the horizontal distance between the workpiece and the edge of the graphite box is 200mm, and there is no physical contact between the two.

[0044] Step 3, Low-temperature degassing: Start the vacuum system and pump the furnace pressure to 5.0 × 10⁻⁶. -3 Below Pa, the furnace temperature is raised to 1000℃ at a rate of 8℃ / min, and held at this temperature and vacuum for 45 minutes to remove adsorbed gases.

[0045] Step 4, Periodic Pressure Gating: The furnace temperature is further increased to 1500℃ at a rate of 12℃ / min, maintained at this temperature, and 8 consecutive pressure pulse cycles are executed. Each cycle is as follows: First, the gas charging is stopped and the high vacuum valve is fully opened, allowing the furnace pressure to drop to 2.0 × 10⁻⁶. -2 Keep the pressure below 800 Pa for 10 minutes; this stage is the reaction and filtration stage. Then close the high vacuum valve and fill with high-purity argon gas, control the pressure inside the furnace to maintain at 800 Pa, and keep it for 45 minutes; this stage is the reaction blocking and diffusion stage.

[0046] Step 5, Cooling and Removing from the Furnace: After 8 cycles, keep the furnace filled with argon gas, adjust the pressure to 60 kPa, cool the furnace to 800°C, then turn on the circulating fan to force cooling to room temperature, and remove the workpiece.

[0047] Example 2: This embodiment provides a heat treatment method for improving the grain boundary strength of an alloy, specifically for treating C-103 niobium-hafnium alloy plates, including the following steps: Step 1, filling the reaction source: A tungsten-made material box was selected, and the reaction source composition B prepared in Preparation Example 2 was spread flat on the bottom of the graphite box as a medium layer with a thickness of 10 mm; niobium alloy turning chips that have been cleaned and dried were evenly piled on top of the medium layer as a sacrificial filter layer with a thickness of 30 mm, and it was ensured that the filter layer was in close contact with the inner wall of the graphite box.

[0048] Step 2, furnace layout: Place the filled graphite box on the furnace bed on the inlet side of the vacuum annealing furnace, place the C-103 alloy plate to be processed on the tungsten material rack in the center of the furnace homogenization zone, and adjust the position so that the horizontal distance between the workpiece and the edge of the graphite box is maintained at 100mm, and there is no physical contact between the two.

[0049] Step 3, Low-temperature degassing: Start the vacuum system and pump the furnace pressure to 5.0 × 10⁻⁶. -3 Below Pa, the furnace temperature is raised to 950°C at a rate of 5°C / min, and held at this temperature and vacuum for 30 minutes.

[0050] Step 4, Periodic Pressure Gating: Continue to raise the furnace temperature to 1450℃ at a rate of 10℃ / min, maintain this temperature, and execute 5 consecutive pressure pulse cycles. Each cycle operates as follows: First, stop the gas charging and fully open the high vacuum valve to reduce the furnace pressure to 1.5 × 10⁻⁶. -2 Keep the pressure below 600 Pa for 15 minutes; this stage is the reaction and filtration stage. Then close the high vacuum valve and fill with high-purity argon gas, controlling the pressure inside the furnace to be maintained at 600 Pa for 60 minutes; this stage is the reaction blocking and diffusion stage.

[0051] Step 5, Cooling and Removing from the Furnace: After 5 cycles, keep the furnace filled with argon gas, adjust the pressure to 50 kPa, cool the furnace to 800°C, then turn on the circulating fan to force cooling to room temperature, and remove the workpiece.

[0052] Example 3: This embodiment provides a heat treatment method for improving the grain boundary strength of an alloy, specifically for processing pure tantalum plates, including the following steps: Step 1, filling the reaction source: A molybdenum-made material box was selected, and the reaction source composition C prepared in Preparation Example 3 was spread flat on the bottom of the graphite box as a medium layer with a thickness of 20 mm. Pure tantalum turning chips that have been cleaned and dried were evenly piled on top of the medium layer as a sacrificial filter layer with a thickness of 50 mm, and it was ensured that the filter layer was in close contact with the inner wall of the graphite box.

[0053] Step 2, Furnace layout: Place the filled graphite box on the furnace bed on the inlet side of the vacuum annealing furnace, and place the pure tantalum plate to be processed on the molybdenum material rack in the center of the furnace homogenization zone. Adjust the position so that the horizontal distance between the workpiece and the edge of the graphite box is maintained at 300mm, and there is no physical contact between the two.

[0054] Step 3, Low-temperature degassing: Start the vacuum system and pump the furnace pressure to 5.0 × 10⁻⁶. -3 Below Pa, the furnace temperature is raised to 1050℃ at a rate of 10℃ / min, and held at this temperature and vacuum for 60 minutes.

[0055] Step 4, Periodic Pressure Gating: Continue to raise the furnace temperature to 1550℃ at a rate of 15℃ / min, maintain this temperature, and execute 10 consecutive pressure pulse cycles. Each cycle operates as follows: First, stop the gas charging and fully open the high vacuum valve to reduce the furnace pressure to 0.8 × 10⁻⁶. -2 Keep the pressure below 1000 Pa for 5 minutes; this stage is the reaction and filtration stage. Then close the high vacuum valve and fill with high-purity argon gas, control the pressure inside the furnace to maintain at 1000 Pa, and keep it for 35 minutes; this stage is the reaction blocking and diffusion stage.

[0056] Step 5, Cooling and Removing from the Furnace: After 10 cycles, keep the furnace filled with argon gas, adjust the pressure to 80 kPa, cool the furnace to 800°C, then turn on the circulating fan to force cooling to room temperature, and remove the workpiece.

[0057] Example 4: This embodiment provides a heat treatment method for improving the grain boundary strength of an alloy, specifically for processing Ta-10W tantalum-tungsten alloy plates, including the following steps: Step 1, filling the reaction source: A molybdenum-made material box was selected, and the reaction source composition D prepared in Preparation Example 4 was spread flat on the bottom of the graphite box as a medium layer with a thickness of 12 mm. Tantalum alloy turning chips that have been cleaned and dried were evenly piled on top of the medium layer as a sacrificial filter layer with a thickness of 35 mm, and it was ensured that the filter layer was in close contact with the inner wall of the graphite box.

[0058] Step 2, furnace layout: Place the filled graphite box on the furnace bed on the inlet side of the vacuum annealing furnace, and place the Ta-10W alloy plate to be processed on the molybdenum material rack in the center of the furnace homogenization zone. Adjust the position so that the horizontal distance between the workpiece and the edge of the graphite box is maintained at 150mm, and there is no physical contact between the two.

[0059] Step 3, Low-temperature degassing: Start the vacuum system and pump the furnace pressure to 5.0 × 10⁻⁶. -3 Below Pa, the furnace temperature is raised to 1000℃ at a rate of 8℃ / min, and held at this temperature and vacuum for 45 minutes.

[0060] Step 4, Periodic Pressure Gating: The furnace temperature is further increased to 1520℃ at a rate of 12℃ / min, maintained at this temperature, and six consecutive pressure pulse cycles are executed. Each cycle is as follows: First, the gas charging is stopped and the high vacuum valve is fully opened, allowing the furnace pressure to drop to 1.8 × 10⁻⁶. -2 Keep the pressure below 900 Pa for 12 minutes; this stage is the reaction and filtration stage. Then close the high vacuum valve and fill with high-purity argon gas, control the pressure inside the furnace to maintain at 900 Pa, and keep it for 40 minutes; this stage is the reaction blocking and diffusion stage.

[0061] Step 5, Cooling and Removing from the Furnace: After 6 cycles, keep the furnace filled with argon gas, adjust the pressure to 60 kPa, cool the furnace to 800°C, then turn on the circulating fan to force cooling to room temperature, and remove the workpiece.

[0062] Example 5: This embodiment provides a heat treatment method for improving the grain boundary strength of an alloy, specifically for processing Ta-10W tantalum-tungsten alloy plates, including the following steps: Step 1, loading the reaction source: A molybdenum-made material box was selected, and the reaction source composition A prepared in Example 1 was spread flat on the bottom of the graphite box as a medium layer with a thickness of 18 mm. Tantalum alloy turning chips that have been cleaned and dried were evenly piled on top of the medium layer as a sacrificial filter layer with a thickness of 30 mm (to verify the lower limit of the filter layer thickness), and the filter layer was ensured to be in close contact with the inner wall of the graphite box.

[0063] Step 2, furnace layout: Place the filled graphite box on the furnace bed on the inlet side of the vacuum annealing furnace, and place the Ta-10W alloy plate to be processed on the molybdenum material rack in the center of the furnace homogenization zone. Adjust the position so that the horizontal distance between the workpiece and the edge of the graphite box is maintained at 250mm, and there is no physical contact between the two.

[0064] Step 3, Low-temperature degassing: Start the vacuum system and pump the furnace pressure to 5.0 × 10⁻⁶. -3 Below Pa, the furnace temperature is raised to 1020°C at a rate of 8°C / min, and held at this temperature and vacuum for 45 minutes.

[0065] Step 4, Periodic Pressure Gating: The furnace temperature is further increased to 1480℃ at a rate of 12℃ / min, maintained at this temperature, and seven consecutive pressure pulse cycles are executed. Each cycle is as follows: First, the gas charging is stopped and the high vacuum valve is fully opened, allowing the furnace pressure to drop to 1.2 × 10⁻⁶. -2 Keep the pressure below 700 Pa for 8 minutes; this stage is the reaction and filtration stage. Then close the high vacuum valve and fill with high-purity argon gas, control the pressure inside the furnace to maintain at 700 Pa, and keep it for 50 minutes; this stage is the reaction blocking and diffusion stage.

[0066] Step 5, Cooling and Removing from the Furnace: After 7 cycles, keep the furnace filled with argon gas, adjust the pressure to 70 kPa, cool the furnace to 800°C, then turn on the circulating fan to force cooling to room temperature, and remove the workpiece.

[0067] Comparative Example 1: Compared with Example 1, the difference is that the upper sacrificial filter layer is not provided in the graphite box, and the reaction source composition A is directly exposed to the furnace space, otherwise they are the same.

[0068] Comparative Example 2: Compared to Example 1, the difference lies in that the heat treatment process in step four does not perform periodic pressure-gated cycles, but maintains a high vacuum state throughout, that is, the furnace pressure is always maintained at 2.0 × 10⁻⁶. -2 Below Pa, no argon gas pressurization operation is performed; all other procedures are the same.

[0069] Comparative Example 3: Compared to Example 1, the difference is that the sacrificial filter material is replaced with a porous alumina ceramic plate, which has a similar thickness and porosity to the tantalum alloy turning chip deposit, while all other aspects are the same.

[0070] Comparative Example 4: Compared with Example 1, the difference is that the sacrificial filter material is replaced with titanium sponge particles, otherwise they are the same.

[0071] Comparative Example 5: Compared with Example 1, the difference is that no reaction source composition or filter material is filled in the graphite box, and only conventional vacuum annealing is performed; all other aspects are the same.

[0072] Test Example 1: Verification of Carbon Capture Efficiency of Sacrificial Filter Materials Experimental description: Sampling: After the heat treatment process of Example 1 was completely completed and the furnace temperature cooled to room temperature, the furnace chamber was opened. Based on the stacking height of the filter layer within the graphite box, it was divided into a lower layer (0-10 mm from the reaction medium layer), a middle layer (15-25 mm from the reaction medium layer), and an upper layer (30-40 mm from the reaction medium layer, i.e., closer to the workpiece). Approximately 10 g of discolored tantalum alloy turning chips were randomly collected from each of the three areas as test samples. Unused, cleaned tantalum alloy turning chips from the same batch were used as a blank control sample.

[0073] Pretreatment: The four groups of samples were placed in anhydrous ethanol and ultrasonically cleaned for 5 minutes to remove any physical dust that may be adsorbed on the surface. After that, they were dried in a vacuum oven at 80°C to constant weight.

[0074] Shearing sample preparation: The dried spiral turning chips are sheared into fine particles with a length of less than 2 mm to meet the sample injection requirements of the analytical instrument. 0.5 g of each sample is weighed, and three copies of each sample are weighed in parallel for repeatability testing.

[0075] Instrumental analysis: The carbon (C) content of each group of samples was determined using a LECO CS-844 high-frequency infrared carbon-sulfur analyzer. The samples were burned in a pure oxygen stream using a high-frequency induction furnace, and the carbon content was calculated by measuring the CO2 absorbance using an infrared detection cell. The oxygen (O) content of each group of samples was determined using a LECO ONH-836 oxygen-nitrogen-hydrogen analyzer, and the inert gas melting-infrared thermal conductivity method was used for detection.

[0076] Data processing: Record the values ​​of each individual measurement, remove outliers, and calculate the arithmetic mean of each group of samples.

[0077] Experimental data: Table 1: Interstitial element analysis results of waste filter layer material at different locations in Example 1

[0078] Conclusion Analysis: Reference Appendix Figure 1 According to the data in Table 1, the distribution of interstitial elements in the waste filter layer exhibits a significant vertical concentration gradient.

[0079] First, the carbon content in the bottommost sample (L-Pos group) surged from an initial average of 26 ppm to over 4200 ppm, with a simultaneous and significant increase in oxygen content. This data confirms a violent chemical reaction occurring in the region near the reaction source. Due to the carbothermic reduction reaction (Y₂O₃ + 3C → 2Y + 3CO) in the reaction source releasing CO-containing gaseous byproducts, the highly reactive tantalum matrix acts as a chemical getter as it passes through the high-temperature tantalum shavings layer. The reaction 7Ta + 5CO → 5TaC + Ta₂O₅ (and subsequent oxidation reactions) indicates that oxygen is absorbed and fixed by the tantalum matrix in the form of oxides. The two-order-of-magnitude increase in content suggests that the vast majority of the CO gas was effectively intercepted by the first line of defense as soon as it left the reaction source.

[0080] Secondly, as the gas path extends upwards, the carbon and oxygen content of the middle layer samples (M-Pos group) drops significantly, to about 35% of that of the lower layer, while the average carbon content of the top layer samples (U-Pos group) is only 143 ppm. This decreasing concentration distribution from bottom to top (4235 ppm → 1552 ppm → 143 ppm) confirms the effective operation of the sacrificial filter layer's hierarchical filtration mechanism. The lower and middle layers, as the main impurity containment areas, saturate the oxidizing atmosphere in the gas flow by consuming their own chemical reactivity, thereby ensuring that the gas flow reaching the upper layer and ultimately escaping to the workpiece area has extremely high purity.

[0081] The low carbon level of the upper sample (only slightly above the background value) indicates that the filter layer was not penetrated throughout the entire pulse heat treatment cycle and still retains residual gas-gathering capacity. This provides a safe, carbon-poor, low-oxygen processing environment for the workpiece. These test results strongly support the in-situ chemical filtration mechanism proposed in this invention, demonstrating that the technical solution of using homologous metal debris to eliminate the risk of carbon contamination in the carbothermal reduction process is feasible on an industrial scale, and that the filtration efficiency is sufficient to ensure the pure diffusion of yttrium into the workpiece afterwards.

[0082] Test Example 2: Verification of Pressure Gating Control over Surface Deposition Behavior Experimental description: Based on the matrix material types (Ta-10W tantalum-tungsten alloy, C-103 niobium-hafnium alloy, or pure tantalum) used in Examples 1-5 and Comparative Examples 1-5, corresponding rolled plate samples from the same batch were selected and divided into 9 experimental groups, each containing 3 parallel samples. The sample dimensions were all 100mm × 100mm × 2mm. All samples were ultrasonically cleaned in acetone to remove surface oil. After drying, the length and width dimensions of each sample were measured to calculate the total surface area, and the initial mass before processing was weighed using an electronic analytical balance with an accuracy of 0.1mg.

[0083] Each group of samples was heat-treated according to the specific process parameters set in the corresponding examples (Examples 1-5) or comparative examples (Comparative Examples 1-5). This included the loading and configuration of the reaction source and filter layer, the heat treatment temperature profile, and specific pressure control strategies (such as periodic gating, constant high vacuum, or conventional annealing).

[0084] After heat treatment and cooling to room temperature, the samples were removed and their surfaces were gently purged with dry compressed nitrogen to remove electrostatically adsorbed dust. The final mass after treatment was weighed again using the same electronic analytical balance. The weight gain per unit area of ​​each sample was calculated by dividing the mass difference before and after treatment by the total surface area of ​​the sample.

[0085] The surface roughness of the specimens was measured using a contact surface profilometer. Five test points were selected in the center and edge regions of each specimen to measure the arithmetic mean deviation Ra of the profile. The measurement data at each point were recorded, and the average roughness of a single specimen was calculated.

[0086] Experimental data: Table 2: Comparison of workpiece surface deposition states under different pressure control processes.

[0087] Conclusion Analysis: refer to Figure 2 According to the data in Table 2, the surface conditions of the workpieces treated with different pressure control strategies show orders of magnitude differences.

[0088] Comparative Example 2, under conditions of maintaining a high vacuum (less than 2.0 E⁻² Pa) throughout, showed an average weight gain per unit area of ​​18.52 mg / cm². 2 Furthermore, the surface roughness Ra deteriorated sharply to approximately 3.8 μm, macroscopically manifesting as a dark gray granular deposition layer. This indicates that under continuous high vacuum conditions, the carbothermic reduction reaction remained in a region of intense thermodynamic activity, with a continuous supply of gaseous yttrium to the workpiece surface. Since the gas-phase transport rate was much greater than the grain boundary diffusion rate of yttrium atoms into the matrix, excess yttrium atoms accumulated on the workpiece surface and underwent liquid-phase sintering at high temperatures, forming a rough, yttrium-rich deposition shell. This deposition layer not only altered the dimensional accuracy of the workpiece but may also become a crack initiation point during subsequent service, failing to achieve the initial goal of grain boundary strengthening.

[0089] In contrast, Example 1 employed a periodic pressure-gated process, controlling the weight gain per unit area of ​​the sample to 0.82 mg / cm². 2 The yttrium content was only about 4.4% of that in Comparative Example 2, and the surface roughness Ra remained around 0.4 μm, maintaining good metallic luster and smoothness. The low weight gain indicates that the yttrium was introduced at a trace doping level, rather than through macroscopic deposition. During the high-pressure (800 Pa) stage in the off-state, the argon backpressure effectively suppressed the generation of the gas phase source, cutting off the external supply and allowing sufficient time for the limited yttrium atoms adsorbed on the surface to diffuse inward along the grain boundaries. This pulsed feed-diffusion cycle mechanism avoided surface supersaturation, ensuring that the yttrium was primarily used for pinning and strengthening the internal grain boundaries, rather than for ineffective surface accumulation. The test results confirm the necessity of pressure gating technology in controlling gas-solid reaction kinetics and solid-state diffusion equilibrium.

[0090] Test Example 3: Analysis of the Effectiveness of Chemical Purity and Impurity Control in Workpieces Experimental description: Three parallel samples with dimensions of 5mm × 5mm × 2mm were cut from the center of the plates treated in Examples 1-5 and Comparative Examples 1-5 using a wire cutting machine.

[0091] Use metallographic sandpaper ranging from 400 to 2000 grit to grind the sample surface step by step to remove the cutting oxide layer and any atypical deposits that may exist in the shallow layer, ensuring that the test results reflect the internal composition of the matrix. Then, place the sample in a mixture of acetone and anhydrous ethanol for ultrasonic cleaning for 15 minutes, and dry it for later use.

[0092] The carbon (C) content was determined using a LECO CS-844 high-frequency infrared carbon-sulfur analyzer. Approximately 0.5g of sample was weighed and placed in a ceramic crucible. Pure iron and tungsten-tin flux were added, and the sample was burned at high temperature in an oxygen stream. The amount of CO2 gas released was detected by an infrared absorption cell.

[0093] The oxygen (O) and nitrogen (N) content was determined using a LECO ONH-836 inert gas melting analyzer. The sample was placed in a high-purity graphite box and pulsed heated to above 2500℃ under helium protection. The released gas was quantitatively analyzed by an infrared detector (for O) and a thermal conductivity detector (for N).

[0094] The contents of metallic elements such as yttrium (Y) and titanium (Ti) were determined using an Agilent 5110 inductively coupled plasma optical emission spectrometer (ICP-OES). The sample was dissolved in a mixture of hydrofluoric acid and nitric acid, and after being brought to a final volume, it was nebulized and introduced into the plasma torch flame. The mass fraction of the elements was calculated based on the characteristic spectral intensities.

[0095] Experimental data: Table 3: Analysis results of chemical composition and impurity content of each group of samples

[0096] Conclusion Analysis: Reference Appendix Figure 3 According to the chemical composition analysis data in Table 3, the heat treatment scheme proposed in this invention demonstrates excellent impurity control capabilities while achieving yttrium alloying.

[0097] Regarding the anti-carbonization capability, the carbon content of the samples in Comparative Example 1 (without filter layer) and Comparative Example 3 (physical filtration with alumina ceramic) reached as high as 1245 ppm and 1102 ppm, respectively, far exceeding the matrix standard. This indicates that if the carbon-containing atmosphere generated by the reaction Y₂O₃ + 3C → 2Y + 3CO is not chemically intercepted, it will penetrate into the workpiece matrix or pass through inert physical pores without hindrance, leading to severe carbonization of the workpiece. In contrast, the carbon content of Examples 1-5 was controlled within the range of 45-71 ppm, which is at the same level as the 55 ppm of Comparative Example 5 (conventional vacuum annealing, without carbon source). This result confirms the sacrificial chemical filtration mechanism of the Ta / Nb metal debris layer: utilizing the extremely high chemical affinity of metal for carbon, CO in the gas flow is forcibly converted into solid carbides, thereby creating a clean microenvironment similar to high-vacuum annealing in the carbothermic reduction system.

[0098] Regarding the deoxidation and alloying effects, the oxygen content of Example 1 (145 ppm) was significantly lower than that of the conventionally annealed sample of Comparative Example 5 (210 ppm), while a yttrium content of 0.28 wt% was detected. This indicates that the yttrium atoms penetrating into the workpiece acted as a strong in-situ getter, capturing interstitial oxygen atoms in the matrix and generating thermodynamically stable yttrium oxide particles, thus achieving deep purification. In contrast, although the yttrium content in Comparative Example 2 (constant high vacuum) was as high as 1.85 wt%, its oxygen content actually increased to 450 ppm. Combined with the results of Example 2, this is because excessive yttrium oxidized on the surface, forming a loose oxygen-rich layer, failing to achieve effective internal strengthening and instead introducing contamination.

[0099] Regarding the purity of the composition, 0.42 wt% titanium impurities were detected in Comparative Example 4 (titanium sponge filter), proving that non-homogeneous filter materials can contaminate the workpiece due to interdiffusion or vapor transport at high temperatures. In contrast, no foreign metal impurities were detected in any of the example groups, verifying that using homogeneous matrix materials (Ta to Ta, Nb to Nb) as the filter medium is a key technical feature for ensuring the purity of the alloy composition.

[0100] Test Example 4: Mechanical Properties Test at Room Temperature and High Temperature Experimental Description: Standard tensile specimens were cut along the rolling direction from the plates treated in Examples 1-5 and Comparative Example 5 using a slow wire electrical discharge machining (WEDM) machine. Room temperature tensile specimens were processed according to GB / T 228.1 standard, with a gauge length of 25mm × 6mm × 2mm. High temperature tensile specimens were processed according to GB / T 228.2 standard, with pin-hole connections at both ends to accommodate high-temperature fixtures. Six specimens were cut from each process, three for room temperature testing and three for high-temperature testing.

[0101] Mechanically grind the sides and surface of the cut sample using 800-1200 grit SiC sandpaper to remove the recast layer and micro-cracks, preventing premature fracture caused by processing defects.

[0102] Room temperature tensile tests were conducted on a Zwick / Roell Z100 universal testing machine with a loading rate set to 1 mm / min. The engineering stress-strain curves were recorded, and the specified plastic extension strength (Rp0.2), tensile strength (Rm), and elongation after fracture (A) were automatically calculated.

[0103] High-temperature tensile testing was conducted on a Gleeble-3500 thermal simulation testing machine equipped with a high-temperature vacuum furnace. The specimens were mounted in a molybdenum-based high-temperature fixture, and a vacuum of 1.0 × 10⁻⁶ was applied. -3 Below Pa, heat to 1200℃ (for Ta-10W and C-103) or 1400℃ (for pure Ta) at a rate of 10℃ / s, hold for 5 minutes to equilibrate the overall temperature of the sample, then heat at 1.0×10 Pa.-3 s -1 The strain rate is applied until fracture.

[0104] Experimental data: Table 4: Results of room temperature and high temperature mechanical property tests on the examples and comparative samples

[0105] Conclusion Analysis: Reference Appendix Figure 4 According to the mechanical property test data in Table 4, the material treated by the process of this invention shows a significant strengthening effect at both room temperature and high temperature.

[0106] Taking the Ta-10W alloy as an example, comparing the data of Example 1 and Comparative Example 5 (conventional vacuum annealed state), it can be seen that under room temperature (25℃) conditions, the yield strength (Rp0.2) of Example 1 increased from 358.1 MPa in Comparative Example 5 to 445.2 MPa, an increase of 24.3%; at the same time, its elongation after fracture remained at 26.5%. Similarly, compared with reference A, the room temperature yield strength of Example 2 (C-103) increased by about 17.9% (312.4 MPa vs 265.0 MPa); and compared with reference B, the room temperature yield strength of Example 3 (pure tantalum) increased by about 23.1% (215.6 MPa vs 175.2 MPa), proving the universal strengthening effect of this method on different refractory metal matrices. This excellent strength-toughness matching verifies the grain refinement strengthening mechanism generated by the distribution of yttrium along grain boundaries. Typically, high-temperature annealing (above 1500°C) leads to drastic grain growth in tantalum-tungsten alloys, resulting in softening (i.e., Hall-Petch relationship failure). The low yield strength of Comparative Example 5 reflects the coarse-grained microstructure. In contrast, Example 1 maintained high strength after undergoing the same thermal process, indicating that the incorporated yttrium element formed effective zener pinning points, inhibiting grain boundary migration and grain growth.

[0107] The strengthening advantage is even more pronounced under a high-temperature testing environment of 1200℃. The high-temperature yield strength of Example 1 remains at 142.8 MPa, while that of Comparative Example 5 is only 82.6 MPa, representing an improvement of 72.8%. Deformation at high temperatures is mainly dominated by diffusion-controlled dislocation climb and grain boundary sliding. The dispersed yttrium microphase and solute atoms segregated at grain boundaries in the example samples effectively hinder dislocation movement and increase the resistance to grain boundary sliding. This data proves that the present invention not only maintains the surface quality of the material, but also achieves heat-resistant modification of the internal microstructure of the material through bulk diffusion, effectively overcoming the problem of significant degradation of mechanical properties after high-temperature annealing of traditional refractory metals.

[0108] Test Example 5: Microhardness Distribution Uniformity Test Experimental description: From the treated plates of Example 1 (preferred process parameters: 1500℃ / 8 cycles) and Example 5 (process for verifying the lower limit of filter layer thickness: 1480℃ / 7 cycles) and the untreated Ta-10W raw material, 10mm×5mm cross-sectional samples were cut using a wire EDM machine in a direction perpendicular to the rolling plane.

[0109] The sample was embedded in conductive resin and ground with 240-grit to 2000-grit wet sandpaper. Then, it was mechanically polished on a rotary polisher with diamond polishing paste with grit sizes of 2.5μm and 0.5μm until the sample cross-section was a scratch-free mirror finish with no work-hardened layer residue.

[0110] The test was conducted using a fully automated Micro-Vickers Hardness Tester. The load was set to 1.96 N (HV0.2), and the holding time was 15 seconds. Starting from one surface along the thickness direction of the sample, test points were selected every 0.1 mm towards the center until the geometric center of the plate (1.0 mm from the surface) was reached. The diagonal length of each point was recorded and automatically converted into a Vickers hardness value.

[0111] Experimental data: Table 5: Distribution data of micro Vickers hardness (HV0.2) along the thickness direction of the cross section

[0112] Conclusion Analysis: Reference Appendix Figure 5 According to the microhardness distribution data in Table 5, different process parameters have a decisive influence on the diffusion depth and strengthening uniformity of yttrium in the tantalum-tungsten alloy matrix.

[0113] The hardness of the matrix material remained uniform across the cross-section, with an average value of approximately 244 HV, reflecting the mechanical characteristics of a single solid solution. The hardness distribution curve of Example 1 (1500℃ / 8 cycles) exhibited a high-level flattening characteristic, with a near-surface hardness reaching 342 HV. In the core region, 1.0 mm from the surface, the hardness remained around 312 HV, an increase of approximately 28% compared to the matrix. This indicates that under the influence of higher diffusion temperatures and multiple breathing-type pressure gating, yttrium atoms gained sufficient driving force to cross grain boundary barriers and migrate inward, forming solute gas clusters or nano-precipitates that permeate the entire thickness of the plate, achieving uniform strengthening of the material as a whole.

[0114] In contrast, the hardness distribution of Example 5 (1480°C / 7 cycles) exhibited a significant gradient decrease. Although its near-surface hardness (318 HV) was improved, the hardness value rapidly decreased to 258 HV after a depth exceeding 0.45 mm, and essentially returned to the matrix level (approximately 245 HV) after 0.6 mm. This distribution, characterized by a hard surface but a soft interior, indicates that under the constraints of slightly lower temperatures, reduced pressure opening cycles, and a thinner filter layer, the gas-solid reaction was mainly confined to the shallow surface, with the atomic diffusion rate lagging behind the surface adsorption rate, failing to establish an effective inward concentration gradient. These test results confirm that the pressure-gating and temperature synergy mechanism optimized in this invention is key to achieving fully transparent grain boundary strengthening in thick refractory metal materials.

[0115] Test Example 6: Investigation of the Influence of the Horizontal Distance Between the Workpiece and the Graphite Box on the Diffusion Effect Experimental description: Select tantalum-tungsten alloy (Ta-10W) rolled plates from the same batch, cut them into samples of uniform size (20mm×20mm×2mm), and process them with standard grinding, cleaning and drying procedures before use.

[0116] The samples were divided into 5 groups, each containing 3 parallel samples. The reaction source composition A prepared in Preparation Example 1 was used. During the furnace loading process, the horizontal straight-line distance between the edge of the workpiece and the edge of the graphite box was precisely adjusted using a high-temperature resistant ceramic spacing fixture. The set spacings for the 5 groups of samples were 50 mm, 100 mm, 200 mm, 300 mm, and 400 mm, respectively.

[0117] Keep other process parameters constant: After vacuum degassing, heat to 1500℃ and perform 8 pressure pulse cycles (high vacuum for 10 min, 600 Pa argon back pressure for 50 min), then cool to room temperature with the furnace.

[0118] After the sample was removed, the surface roughness (Ra) of the central region of the sample was measured using a laser confocal microscope; the cross-sectional morphology was observed and the thickness of the surface deposit (shell) was measured using a scanning electron microscope (SEM); the microhardness distribution was tested along the depth direction of the cross-section using a Vickers hardness tester, and the depth where the hardness value was 20 HV higher than that of the matrix was defined as the effective strengthening depth.

[0119] Experimental data: Table 6: Test data of surface quality and reinforcement depth at different horizontal spacings

[0120] Conclusion Analysis: Reference Appendix Figure 6According to the test data in Table 6, the horizontal distance between the workpiece and the graphite box is a key geometric parameter that affects the gas phase transport efficiency and solid-state diffusion balance. Its value changes lead to drastically different physical metallurgical results.

[0121] When the horizontal spacing is 50 mm (Group 6-1), due to the excessively short transport path, the flux density of active yttrium atoms reaching the workpiece surface is too high, exceeding the limiting flux for atomic diffusion from the surface to the grain boundaries within the matrix. This supply-demand imbalance causes a large amount of strengthening elements to remain on the workpiece surface, generating a yttrium-rich deposition layer with a thickness of 28.4 μm through reaction. This directly leads to a deterioration of the surface roughness to Ra 3.12 μm, compromising the dimensional accuracy and surface finish of the workpiece.

[0122] When the horizontal spacing is between 100 mm and 300 mm (groups 6-2 to 6-4), the transport rate of gas molecules and the diffusion rate of solid-state molecules reach a thermodynamic matching state. As the spacing increases, the gas concentration gradient decreases moderately, avoiding surface supersaturation precipitation. The thickness of the surface deposited layer is always controlled below 1.0 μm (close to no deposition), and the effective strengthening depth is maintained within the ideal range of 110 μm to 150 μm, achieving a balance between deep strengthening and surface quality.

[0123] When the horizontal spacing increases to 400 mm (group 6-5), the effective gas phase partial pressure reaching the workpiece surface is significantly reduced due to the molecular mean free path and scattering effect under high vacuum conditions. At this point, the surface cannot establish a sufficient chemical potential gradient to drive deep diffusion, resulting in a sharp drop in the effective strengthening depth to 48.7 μm, which cannot meet the strengthening requirements of the material for high-temperature service.

[0124] In summary, limiting the horizontal spacing to the range of 100mm to 300mm is an inevitable choice based on the characteristics of gas phase transport dynamics, and the critical value of this range has a clear significance in defining the technical effect.

[0125] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A heat treatment method for improving the grain boundary strength of an alloy, characterized in that, Includes the following steps: Step S1: Filling the diffusion medium and filter layer: A solid gas phase generation source composition is laid at the bottom of the graphite box as a diffusion medium layer, and a sacrificial filter material is stacked on top of the medium layer as a filter layer. The sacrificial filter material is a homogeneous metal or alloy shavings with the same chemical composition as the substrate of the workpiece to be treated. Step S2, Furnace layout: Place the filled graphite box and the workpiece to be processed in the vacuum furnace, so that the workpiece and the graphite box are not in contact and the horizontal distance between them is 100mm to 300mm. Step S3, Vacuum Degassing: Vacuum the furnace and heat it to remove gas; Step S4, Periodic Pressure Swing Diffusion Treatment: Raise the furnace temperature to the treatment temperature and execute multiple consecutive pressure pulse cycles, each of which alternately includes an atmosphere generation and filtration stage and a blocking and diffusion stage; During the atmosphere generation and filtration stage, a high vacuum state is maintained, a gas phase containing strengthening elements is generated using the medium layer, and carbon and oxygen impurities are intercepted by the filter layer. During the blocking and diffusion stage, an inert gas is introduced to establish back pressure, suppressing the generation of the gas phase and promoting the diffusion of strengthening elements into the workpiece.

2. The heat treatment method for improving the grain boundary strength of an alloy according to claim 1, characterized in that, In the step of filling the percolation medium and the filter layer, the solid gas phase generation source composition is prepared from raw materials comprising the following components: The reaction precursor powder is composed of yttrium oxide powder and carbon source powder, wherein the molar ratio of yttrium oxide to carbon source is 1:3.5 to 1:4.0; The skeletal dispersed particles are stabilized fused zirconia particles; The mass ratio of the reaction precursor powder to the skeleton dispersion particles is 1:4.0 to 1:5.

0.

3. The heat treatment method for improving the grain boundary strength of an alloy according to claim 2, characterized in that, The solid gas phase generator composition is prepared by the following method: First, yttrium oxide powder and carbon source powder are dry-mixed to obtain a reaction precursor powder. Then, the reaction precursor powder is mechanically mixed with the framework dispersion particles in a drum, so that the precursor powder fills the gaps between the framework dispersion particles.

4. The heat treatment method for improving the grain boundary strength of an alloy according to claim 1, characterized in that, In the step of filling the diffusion medium and the filter layer, the physical form of the sacrificial filter material is spiral, sheet, filament or granular debris, and its thickness is 0.1 mm to 0.5 mm. The thickness of the filter layer is 30mm to 50mm, and the thickness of the filter layer is greater than 1.5 times the thickness of the medium layer.

5. The heat treatment method for improving the grain boundary strength of an alloy according to claim 1, characterized in that, The workpiece to be processed is selected from tantalum-based alloys, niobium-based alloys, tungsten-based alloys, pure tantalum, pure niobium, or pure tungsten plates; In the periodic pressure-switching diffusion treatment step, the treatment temperature is 1450°C to 1550°C, and the number of pressure pulse cycles is 5 to 10.

6. The heat treatment method for improving the grain boundary strength of an alloy according to claim 1, characterized in that, In the vacuum degassing step, the process parameters for vacuum degassing are as follows: With a vacuum degree better than 5.0×10 -3 Under the condition of Pa, the temperature is raised to 950℃ to 1050℃ and held for 30min to 60min.

7. The heat treatment method for improving the grain boundary strength of an alloy according to claim 1, characterized in that, In the periodic pressure swing diffusion process, the control parameters for the atmosphere generation and filtration stages are as follows: Stop the gas filling and open the high vacuum valve to reduce the pressure inside the furnace to 2.0 × 10⁻⁶. -2 Keep below Pa for 5 to 15 minutes.

8. The heat treatment method for improving the grain boundary strength of an alloy according to claim 1, characterized in that, In the periodic pressure-switching diffusion process, the control parameters for the blocking and diffusion stages are as follows: High-purity argon gas is introduced, and the pressure inside the furnace is controlled to be maintained at 600 Pa to 1000 Pa for 35 min to 60 min.

9. The heat treatment method for improving the grain boundary strength of an alloy according to claim 8, characterized in that, In each pressure pulse cycle, the holding time of the blocking and diffusion phase is 3.0 to 7.0 times that of the holding time of the atmosphere generation and filtration phase.

10. The heat treatment method for improving the grain boundary strength of an alloy according to claim 1, characterized in that, The method also includes a cooling and unloading step: After all pressure pulse cycles have ended, maintain the furnace in an inert gas state, adjust the furnace pressure to 50 kPa to 80 kPa, and then turn on the fan for forced cooling after the furnace has cooled to below 800°C.

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