A method for improving work hardening capacity of nano-enhanced metal matrix composites

Abstract

This invention discloses a method for improving the work hardening capability of nano-reinforced metal matrix composites. The composite matrix and reinforcement have the following characteristics: the matrix has fine grains, the reinforcement has good thermal stability, and the reinforcement is a nano-one-dimensional or two-dimensional reinforcement. The dislocation density and grain size in the matrix are adjusted by vacuum heat treatment of the composite bulk. The distribution and recrystallization content of the composite are further optimized by hot extrusion. The work hardening behavior of the material is characterized by uniaxial tensile test. The initially extruded CNTs / Al composite material has no obvious work hardening process. After processing through the designed process, the work hardening capability of the material is significantly improved. While ensuring that the strength of the material does not decrease, the uniform elongation is increased by 161% compared with the initial extruded state, and the work hardening index is greatly improved, thus obtaining an aluminum matrix composite material with excellent comprehensive mechanical properties.

Description

Technical Field

[0001] This invention belongs to the technical field of powder metallurgy preparation of carbon nanotube aluminum matrix composites, and particularly relates to a method for improving the work hardening ability of nano-reinforced metal matrix composites. Background Technology

[0002] Due to their combination of the excellent strength, plasticity, electrical and thermal conductivity of the metal matrix, and the superior properties of the reinforcement, such as ultra-high strength, modulus, high-temperature stability, and corrosion resistance, metal matrix composites (MMCs) hold a significant position in important national fields such as aerospace, transportation, and weaponry. Powder metallurgy is one of the important methods for preparing MMCs. This is because, on the one hand, the low forming temperature effectively controls the interfacial reaction between the reinforcement and the matrix; on the other hand, the forming temperature below the melting point helps maintain the microstructure of the powder. Therefore, the uniform dispersion of the reinforcement in the powder determines the uniform dispersion of the reinforcement within the final bulk, resulting in composite materials with excellent macroscopic properties. In the powder metallurgy process for preparing composite powders, the most commonly used, simplest, and most effective method for dispersing the reinforcement is high-energy ball milling. The high-energy impact of the milling balls refines the matrix grains, while the reinforcement gradually disperses and embeds itself into the matrix on the surface of the powder and during plastic deformation, achieving a tight interfacial bond with the matrix. These advantages significantly improve the yield strength and tensile strength of MMCs. Long-duration high-energy ball milling is commonly used to achieve more uniform dispersion of reinforcements and better material properties. However, under prolonged large plastic deformation, the grain size becomes very small, even reaching submicron or nanometer scale. During plastic deformation, the multiplied dislocations tend to annihilate at grain boundaries rather than be preserved within the grains. This results in metal matrix composites having high yield strength, but with minimal work hardening after yielding, and even exhibiting typical work softening. This situation is detrimental to the application of materials in engineering fields and is currently a key challenge in improving the comprehensive mechanical properties of fine-grained metal matrix composites.

[0003] Aluminum (Al), the most abundant metallic element in the Earth's crust, possesses advantages such as low density, good electrical and thermal conductivity, and high specific strength. Aerospace is one of the most challenging and widely influential high-tech fields in the world today, representing a concentrated manifestation and important indicator of a nation's comprehensive national strength. This field places stringent performance requirements on structural materials. Aluminum and its alloys, due to their lightweight, corrosion resistance, and excellent thermal and electrical conductivity, have become an important component of aerospace materials, widely used in key components such as the main structure, skin, and structure of aircraft. However, with increasingly demanding service conditions, there is a growing expectation for lightweight, high-strength materials with even better performance. The key to obtaining high-performance aluminum-based composite materials is, first and foremost, the selection of high-performance reinforcements. Carbon nanotubes (CNTs) possess low density, ultra-high strength and modulus, and excellent electrical and thermal conductivity, making them highly effective reinforcements.

[0004] Current research on improving the work hardening ability of CNTs / Al composites is limited. The obtained CNTs / Al materials exhibit fine matrix grains, making it difficult to store a high dislocation content. To enhance the strength of CNTs / Al composites, it is urgent to address their weak work hardening ability, thereby further improving their application value.

[0005] Planetary ball milling, as a representative of high-energy ball milling, is suitable for the uniform dispersion of high-content reinforcements, and the refinement of matrix grains can produce a significant strengthening effect. However, the characteristics of nanoscale grains result in weak work hardening ability of the material. This invention involves vacuum heat treatment of the sintered composite material block, controlling the grains to undergo a recrystallization process, while significantly reducing the dislocation density in the matrix. Subsequently, during hot extrusion, the properties of CNTs are utilized to achieve a dynamic recrystallization process in the CNT-rich regions. By optimizing the size of the matrix grains and controlling the recrystallization fraction, the dislocation density storage capacity within the grains is improved, ultimately yielding a nanocrystalline metal matrix composite material that achieves almost no decrease in strength and plasticity while significantly enhancing work hardening ability.

[0006] Specifically, this invention provides a method for improving the work hardening ability of nano-reinforced metal matrix composites, the method specifically including the following steps:

[0007] Step 101: Prepare nano-reinforced metal matrix composite powder, wherein the reinforcement is a one-dimensional or two-dimensional nano-reinforcement, and the composite powder is a uniformly distributed aluminum matrix composite powder with high carbon nanotube content;

[0008] Step 102: Perform plasma sintering on the composite material powder obtained in step 101 to obtain a composite material block;

[0009] Step 103: The sintered composite material block is subjected to vacuum heat treatment at a temperature of 550°C for 24 hours, followed by furnace cooling.

[0010] Step 104: Perform hot extrusion treatment on the composite material after vacuum heat treatment to obtain an extruded composite material with a smooth surface and no cracks.

[0011] Specifically, the method for preparing nano-reinforced metal matrix composite powder in step 101 includes:

[0012] Step 201: Powder proportioning and processing:

[0013] Each ball mill jar was filled with CNT powder, pure Al powder, and stearic acid. The CNT powder with a diameter of 20 nm and a length of 2 μm accounted for 4 wt.%, the pure Al powder with a particle size of 15-53 μm accounted for 96 wt.%, and the total amount of stearic acid added was 1 wt.%.

[0014] The grinding balls used are 10mm diameter zirconia balls with a ball-to-material ratio of 5:1.

[0015] Initially, 0.8 wt.% of stearic acid was added to the powder, and after ball milling for 8 hours, 0.2 wt.% of stearic acid was added to the powder.

[0016] Step 202: Ball milling process and parameter design:

[0017] Argon gas is introduced into the sealed ball mill jar to remove the air inside.

[0018] The ball milling parameters are: rotation speed 180-230 rpm / min, ball milling method is: forward rotation for 10 min, stop for 10 min, reverse rotation for 10 min, and repeat in this cycle. When the ball milling time is 2 hours, stop for 1 hour. The total ball milling time is 24 hours.

[0019] Specifically, in step 201, the order in which the materials are added to the ball mill jar is as follows: first, add the grinding balls, then add pure Al powder, CNTs powder, and stearic acid particles simultaneously and evenly, and stir evenly.

[0020] Specifically, the plasma sintering process in step 102 includes loading the composite material powder into a graphite mold, pre-cooling and pressing it for a period of time, and then loading the graphite mold into a plasma sintering device for sintering.

[0021] Specifically, the sintering temperature is 590℃, the holding time is 30 min, the forming pressure is 30 MPa, and the vacuum degree is 1.5 × 10⁻⁶. -1 Pa, to obtain a dense cylindrical block.

[0022] Specifically, the holding pressure during the pre-cooling and pressing process is 0.5T, and the holding time is not less than 10 minutes.

[0023] Specifically, in the vacuum heat treatment process described in step 103, sponge titanium is placed inside the vacuum tube furnace for heat treatment.

[0024] Specifically, the hot extrusion process includes placing the vacuum heat-treated composite material into a box furnace, holding it at 500°C for 20 minutes, and then placing it into an extrusion device preheated to 370°C, with an extrusion ratio of 18:1 and an extrusion speed of 4 mm / s.

[0025] Specifically, the composite material that undergoes the vacuum heat treatment followed by the hot extrusion treatment exhibits improved uniform elongation and work hardening index compared to the composite material that undergoes the hot extrusion treatment without vacuum heat treatment.

[0026] Compared with the prior art, the present invention has the following beneficial technical effects:

[0027] (1) The high-content reinforced composite material powder prepared by the present invention is simple and easy to operate, the obtained reinforced material is evenly dispersed and has good bonding with the matrix. The high-content CNTs / Al composite material block prepared by powder metallurgy has the characteristics of high yield strength and fracture elongation.

[0028] (2) In this invention, a vacuum heat treatment process is introduced into the sample after sintering. The combination of vacuum heat treatment and the dual effect of sponge titanium can effectively prevent the oxidation of the composite material block, so that the material can be heated and cooled more uniformly, reducing the possibility of deformation and cracking during the cooling process. At the same time, the oxide layer on the sample surface can be thinner, which significantly improves the cleanliness and smoothness of the material. It can be used directly in the subsequent hot extrusion process without grinding. The parameters of vacuum heat treatment are selected to be higher than the recrystallization temperature of the composite matrix and lower than the reaction temperature between the matrix and the reinforcement, and the temperature is increased as much as possible within a suitable range, so that the recrystallization and grain growth driving force of the grains are greater. Long-term vacuum heat treatment can effectively improve the microstructure characteristics. That is, in the region where the reinforcement is rich, the grain boundary is fixed and the grain growth is not obvious, but the dislocations and grain boundaries are restored under the action of long-term heat treatment, and the defect content is reduced. In the region where the reinforcement content is low, the grains can grow fully, which is conducive to the storage of dislocations during tensile deformation, and can eventually form a heterostructure. Subsequently, the hot extrusion process further densifies the structure, and the dynamic recrystallization and recrystallization after heat treatment give the material more space to accommodate dislocations, ultimately improving the material's work hardening ability.

[0029] (3) The high-content CNTs / Al composite material prepared by the present invention has good strength and plasticity matching. The work hardening ability of the material is significantly improved by adding the designed vacuum heat treatment process. While ensuring that the strength of the material does not decrease, the uniform elongation is increased by 161% compared with the initial extrusion state, and the work hardening index is greatly improved. It has good application prospects in aerospace, weaponry, transportation and other fields. Attached Figure Description

[0030] Figure 1 The powder morphology of high CNTs / Al composite material with 4 wt.% content prepared at different times in Example 1 of the present invention is shown in (a) and (b) the powder morphology and (c) the powder morphology and (d ...

[0031] Figure 2 This is a process route diagram for vacuum heat treatment of sintered CNTs / Al composite materials in Example 1 of the present invention;

[0032] Figure 3 The results show the CNTs structure and phase composition of the 4 wt.% high-content CNTs / Al composite material in extruded state and heat-treated state prepared in Example 1 of this invention. (a) is the Raman spectrum and (b) is the XRD spectrum.

[0033] Figure 4 The recrystallization results of the 4 wt.% high CNTs / Al extruded bulk material prepared in Example 1 of this invention are shown in (a) as the change of large-angle grain boundaries with heat treatment temperature, (b) as the change of small-angle grain boundaries with heat treatment temperature, and (c) as the change of recrystallization fraction with heat treatment temperature.

[0034] Figure 5 The following diagrams are used to analyze the uniaxial tensile and work hardening behavior of the 4 wt.% high CNTs / Al extruded bulk material prepared in Example 1 of this invention: (a) is the engineering stress-strain curve, (b) is the true stress-strain curve, (c) is the work hardening rate curve, and (d) is the fitted work hardening index.

[0035] Figure 6 The transmission electron microstructure of the 4 wt.% high-content CNTs / Al extruded bulk material prepared in Example 1 of the present invention is shown in the following images: (a) is the bright field image of CNTs in region 1 and inside the grain; (b) is the dark field image corresponding to region (a); (c) is the bright field image of CNTs in region 2 and inside the grain; and (d) is the dark field image corresponding to region (c).

[0036] Figure 7 This invention provides a comparative example 1 showing the effect of short-time heat treatment on the properties of composite materials.

[0037] Figure 8 The present invention provides a process flow diagram for a method to improve the work hardening ability of nano-reinforced metal matrix composites. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this disclosure. All other embodiments obtained by those skilled in the art based on the described embodiments of this disclosure without creative effort are also within the scope of protection of this disclosure.

[0039] Unless otherwise defined, all terms used herein (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this subject matter pertains. It will be further understood that terms such as those defined in commonly used dictionaries shall be interpreted as having the meaning consistent with their meaning in the context of the specification and in the relevant art, and shall not be interpreted in an idealized or overly formal form unless otherwise explicitly defined herein.

[0040] The term "embodiment" as used herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of the phrase "embodiment" in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0041] This invention provides a method for improving the work hardening ability of nano-reinforced metal matrix composites, specifically including the following steps:

[0042] Step 101: Prepare nano-reinforced metal matrix composite powder, wherein the reinforcement is a one-dimensional or two-dimensional nano-reinforcement, and the composite powder is a uniformly distributed aluminum matrix composite powder with high carbon nanotube content;

[0043] Step 102: Perform plasma sintering on the composite material powder obtained in step 101 to obtain a composite material block;

[0044] Step 103: The sintered composite material block is subjected to vacuum heat treatment at a temperature of 550°C for 24 hours, followed by furnace cooling.

[0045] Step 104: Perform hot extrusion treatment on the composite material after vacuum heat treatment to obtain an extruded composite material with a smooth surface and no cracks.

[0046] In this invention, for metal matrix powders with a certain plastic deformation capacity during high-energy ball milling, the metal matrix powder is ensured to be deformed into ellipsoidal or flake-shaped powders through the ball milling process. Examples include copper and its alloys, magnesium and its alloys, titanium and its alloys, aluminum alloys, and alloy steel powders. The reinforcement is selected from one-dimensional and two-dimensional nanoparticles such as carbon nanotubes, graphene, and silicon carbide, which possess high strength, modulus, excellent mechanical properties, and good high-temperature thermal stability. The principle of preparing high-content reinforcements through powder metallurgy to improve the work hardening ability of composite materials is as follows: by subjecting the sintered composite material block to vacuum heat treatment at a specific temperature, dislocations from the high-energy ball milling process are recovered, and grains recrystallize. Furthermore, hot extrusion further refines the grains, while dynamic recrystallization occurs in areas with high reinforcement content. Ultimately, the dislocation density inside the composite material block is low, and grains grow in areas with low CNT content, improving the dislocation storage capacity during plastic deformation and ultimately achieving the goal of improving the work hardening ability of the composite material.

[0047] Furthermore, the method for preparing nano-reinforced metal matrix composite powder in step 101 includes:

[0048] Step 201: Powder proportioning and processing:

[0049] Each ball mill jar was filled with CNT powder, pure Al powder, and stearic acid. The CNT powder with a diameter of 20 nm and a length of 2 μm accounted for 4 wt.%, the pure Al powder with a particle size of 15-53 μm accounted for 96 wt.%, and the total amount of stearic acid added was 1 wt.%.

[0050] The grinding balls used are 10mm diameter zirconia balls with a ball-to-material ratio of 5:1.

[0051] Initially, 0.8 wt.% of stearic acid was added to the powder, and after ball milling for 8 hours, 0.2 wt.% of stearic acid was added to the powder.

[0052] Step 202: Ball milling process and parameter design:

[0053] Argon gas is introduced into the sealed ball mill jar to remove the air inside.

[0054] The ball milling parameters are: rotation speed 180-230 rpm / min, ball milling method is: forward rotation for 10 min, stop for 10 min, reverse rotation for 10 min, and repeat in this cycle. When the ball milling time is 2 hours, stop for 1 hour. The total ball milling time is 24 hours.

[0055] In this invention, a zirconia ball mill jar containing metal powder, reinforcement, process control agent, and milling balls is sealed. Air is expelled by filling the jar with argon gas, thus preventing powder oxidation during the milling process. Care must be taken to ensure the argon flow rate is not too high to prevent the low-density reinforcement and metal powder from flying out. After filling, the jar is secured to a planetary ball mill using clamps.

[0056] In this invention, the process of purging the sealed ball mill jar with argon gas to remove air from the jar specifically includes the following steps: First, open the outlet valve to ensure unobstructed gas flow, adjust the gas flow rate to 1.5 L / min, and introduce argon gas through the inlet, ensuring a purging time of at least 5 minutes; when stopping the gas intake, first close the inlet valve, and then close the outlet valve. The relatively low gas flow rate in the above operation facilitates the slow removal of air by the argon protective gas and does not cause a reduction in the mass of the aluminum powder and reinforcement.

[0057] In addition, to avoid chemical reactions caused by excessively high temperatures inside the ball mill jar, a 1-hour stop time is added after the effective ball milling time reaches 2 hours to allow the jar to cool down sufficiently. Furthermore, the ambient temperature is controlled below 20°C to ensure consistent temperature during each ball milling start-up. When the ball milling reaches the intermediate stage and the powder reaches a flake-like state, a certain amount of stearic acid is added again to slow down the powder cold welding process. Prolonging the plastic deformation process is beneficial for the dispersion of high-content reinforcements. At this stage and in the final ball-milled state, 1 g of powder is taken for morphological characterization analysis. Note that direct contact between the powder and air should be avoided during powder collection; the operation must be performed in a glove box.

[0058] Furthermore, in step 201, the order in which the materials are added to the ball mill jar is as follows: first, the grinding balls are added, followed by pure Al powder, CNTs powder, and stearic acid particles, which are then added simultaneously and uniformly, and stirred evenly.

[0059] Furthermore, the plasma sintering process in step 102 includes loading the composite material powder into a graphite mold, holding it under pressure for a period of time, and then loading the graphite mold into a plasma sintering device for sintering.

[0060] Furthermore, the sintering temperature is 590℃, the holding time is 30 min, the forming pressure is 30 MPa, and the vacuum degree is 1.5 × 10⁻⁶. -1 Pa, to obtain a dense cylindrical block.

[0061] In this invention, the prepared composite material powder is subjected to plasma sintering. A graphite mold of a specific size is selected. First, graphite paper is placed on the inner wall of the graphite mold to prevent the sintered sample from sticking to the graphite mold and causing demolding difficulties. Then, the powder is placed into the graphite mold and pre-cooled and pressed for a period of time. The holding pressure during the pre-cooling and pressing process is 0.5T, and the holding time is not less than 10 minutes. Subsequently, high-temperature resistant fiber felt is wrapped around the outside of the graphite mold to keep the mold warm during the sintering process. The mold is then placed in a plasma sintering device for sintering to obtain a dense cylindrical block. K-type thermocouple contact temperature measurement is used during the sintering process to ensure the sensitivity and accuracy of temperature testing. The sintering temperature is controlled according to the material system to avoid obvious interfacial reactions between the metal matrix and the reinforcement. At the same time, a certain pressure is maintained during the sintering process. After sintering, the block sample is removed, and the sample surface is polished in preparation for the subsequent vacuum heat treatment process.

[0062] Furthermore, during the vacuum heat treatment process described in step 103, sponge titanium is placed inside the vacuum tube furnace for heat treatment.

[0063] In this invention, taking CNTs / Al composite material as an example, the surface of the sintered sample is polished and then placed in a vacuum heat treatment furnace for heat treatment. Vacuum heat treatment can effectively prevent the oxidation of the bulk material. The vacuum degree is not higher than 5 Pa. At the same time, 20g of sponge titanium is placed in the vacuum tube furnace for heat treatment to further ensure that the sample is not oxidized.

[0064] In this invention, the sintered composite material block is subjected to long-term vacuum heat treatment. To compare the differences in work hardening ability of different processes, different temperatures are used. The temperature is selected above the recrystallization temperature of the material. If there is an interfacial reaction between the metal matrix and the reinforcement, the temperature should be controlled below the reaction temperature and as close to the reaction temperature as possible. This is to maximize the driving force for grain recrystallization and growth and reduce defects in the microstructure. Through vacuum heat treatment, the microstructure of the composite material is improved. In the reinforcement-rich region, the grain boundaries are fixed, dislocations and grain boundaries are restored, and the defect content is reduced. In the region with low reinforcement content, the grains grow sufficiently to form a heterogeneous structure.

[0065] Furthermore, the hot extrusion process includes placing the vacuum-heat-treated composite material into a box furnace, holding it at 500°C for 20 minutes, and then placing it into an extrusion device preheated to 370°C, with an extrusion ratio of 18:1 and an extrusion speed of 4 mm / s.

[0066] In this invention, before placing the composite material into the box furnace, the sample surface is polished with sandpaper to remove oxide scale, and high-temperature grease is applied to the extrusion die and the inner wall of the sleeve. The sample is then kept at 500°C for 20 minutes in the box furnace, and then immediately placed into the extrusion equipment preheated to 370°C to finally obtain a composite material extrusion with a diameter of 7mm and a smooth, crack-free surface.

[0067] The following examples use uniformly distributed high-content carbon nanotube reinforced aluminum matrix composite (CNTs / Al) powder as an example for illustration.

[0068] The following examples use the following raw materials, planetary ball milling, plasma sintering, hot extrusion, and performance testing equipment:

[0069] Intragranular and grain boundary composite powders were prepared using a German Pulverisette 5 planetary ball mill.

[0070] SPS-state composite material bulks were prepared using Shanghai Chenhua SPS-20T-10-ⅠⅠⅠ plasma sintering equipment;

[0071] The sintered bulk samples were subjected to vacuum heat treatment using a 1200℃ vacuum heat treatment tube furnace from Henan Chengyi Equipment.

[0072] The Ningbo Pavor vertical hot extrusion equipment YP61-315 was used for the hot extrusion process of sintered samples.

[0073] The powder morphology and internal morphology were characterized using a German ZEISS Sigma 300.

[0074] Powder and bulk materials were characterized by TEM analysis using a FEI Talos F200X high-resolution transmission equipment from the United States.

[0075] The degree of structural damage to CNTs in composite powder was characterized using a German WITec Alpha30R microconfocal Raman spectrometer.

[0076] The powder was characterized and analyzed by XRD using a D8 Discover X-ray diffractometer.

[0077] The tensile properties of extruded CNTs / Al composites with intragranular and grain boundary distributions were tested using an INSTRON 3382 electronic universal testing machine (USA).

[0078] The powder used is 15-53 micron pure Al powder prepared by Xi'an Ouzhong Technology, the CNT powder is multi-walled carbon nanotubes (Baytubes C150P), and the stearic acid is produced by Macklin Corporation with the chemical formula C150P. 18 H 36 O2. Example 1

[0079] The preparation method of the high-content CNTs / Al composite material in this embodiment includes high-energy ball milling to prepare CNTs / Al composite powder and sintering process, and hot extrusion of the sintered block to obtain aluminum-based composite material with high strength and good plasticity. On the other hand, the sintered block is subjected to vacuum heat treatment and then hot extrusion process to improve the work hardening ability of the composite material.

[0080] The specific method for preparing high-content CNTs / Al composite powder includes the following steps:

[0081] 1. Powder formulation and ball milling process:

[0082] A total of 120g of powder was placed in the ball mill jar, of which 4.8g (4 wt.%) contained CNTs.

[0083] CNTs / Al powder ratio: 96 wt.% pure Al powder with a particle size of 15-53μm, 1 wt.% total stearic acid, 0.8 wt.% stearic acid initially added, 10 mm diameter zirconia balls used for milling, and a ball-to-material ratio of 5:1.

[0084] In the above preparation process, in step 1, the order of materials loaded into the container is as follows: first, add the milling ball, then add the pure aluminum powder, CNT powder, and stearic acid particles simultaneously and evenly, and stir evenly with a spatula to make the initial CNTs, Al powder, and stearic acid particles premixed evenly.

[0085] II. Powder Proportioning and Processing:

[0086] Argon gas is introduced into the sealed grinding jar. First, the outlet valve is opened to ensure unobstructed gas flow. The gas flow rate is adjusted to 1.5 L / min. The outlet valve of the grinding jar is opened, and then the gas tube is inserted into the inlet valve of the grinding jar. The gas introduction time is 5 minutes. The inlet gas tube is pulled out first, and then the outlet valve is closed to avoid excessive gas pressure inside the jar. This completes the argon gas filling process. After the argon gas filling is completed, the grinding jar is immediately installed into the ball mill and the fixing device is tightened.

[0087] The ball milling parameters were set to a rotation speed of 200 rpm / min, and the milling method was: forward rotation for 10 minutes, stop for 10 minutes, reverse rotation for 10 minutes, stop for 10 minutes, and repeat this cycle. To avoid excessive temperature inside the jar, a 1-hour cooling period was allowed after the effective milling time reached 2 hours. The final milling time was 24 hours. During this period, composite powder was collected from the glove box at 8 hours and 24 hours of milling (for experimental analysis and characterization). Note that after 8 hours of milling, 0.2 wt.% stearic acid was added again to the ball milling jar containing the CNTs / Al composite powder to suppress excessive cold welding of the powder.

[0088] 3. Powder retrieval and addition of stearic acid in the glove box:

[0089] During the time periods (8h, 24h) described in step 102, powder collection is performed in a glove box to avoid rapid oxidation of the powder. The ball mill jar is moved into the glove box, the lid is opened, and 0.5g of powder is taken with a spatula and placed into a centrifuge tube. The lid is then closed. After 8 hours of effective ball milling, the pre-weighed stearic acid and the ball mill jar are placed into the glove box, and 0.2 wt.% stearic acid is added to the jar. The lid is then closed, the jar is removed, and placed into the ball mill. Ball milling continues for 24 hours.

[0090] Morphology analysis was performed on the high CNTs / Al composite powder prepared by ball milling in this embodiment:

[0091] The morphology of the composite powders obtained at different ball milling times was characterized by SEM scanning. Figure 1 The powder morphology of high CNTs / Al composite materials is shown, among which Figure 1 (a) and (b) represent the powder morphology after ball milling for 8 hours and 24 hours, respectively. It can be seen that as the ball milling time increases, the powder morphology becomes flake-like after 8 hours of ball milling, and finally becomes granular after 24 hours of ball milling, but the size is small, indicating that the cold welding phenomenon is reasonably controlled.

[0092] IV. High-content CNTs / Al composite powder discharge plasma sintering process:

[0093] The composite material powder obtained after ball milling was loaded into graphite molds with a diameter of 30 mm and pre-pressed at a pressure of 0.5 T for at least 10 minutes. The molds were then placed in a plasma sintering furnace for sintering. The sintering temperature was 590℃, the pressure was 30 MPa, and the vacuum degree was 1.5 × 10⁻⁶. -1 The sample was held at a constant temperature (Pa) for 30 minutes to densify it. Temperature was measured using a K-type thermocouple contact method to ensure accuracy.

[0094] V. Vacuum heat treatment process:

[0095] By subjecting the sintered composite material block to long-term heat treatment at different temperatures, the temperature is selected above the recrystallization temperature of the material. If there is an interfacial reaction between the metal matrix and the reinforcement, the temperature should also be controlled below the reaction temperature.

[0096] The high-content CNTs / Al composite material obtained by plasma sintering in step 111 was subjected to vacuum heat treatment. After polishing the surface of the sintered sample, it was placed in a vacuum heat treatment furnace. Vacuum heat treatment effectively prevents oxidation of the bulk material. The vacuum level was not higher than 5 Pa. Simultaneously, 20g of titanium sponge was placed inside the vacuum tube furnace to further ensure a low oxygen content environment during the heat treatment process. The heat treatment temperatures were set at 400 / 500 / 550 / 600℃, held for 24 hours, and then cooled with the furnace. The vacuum heat treatment process is as follows: Figure 2 As shown. Furthermore, for samples produced using traditional powder metallurgy methods, the heat treatment step is not introduced; the sintered samples are directly hot-extruded, facilitating subsequent comparison of differences in work hardening behavior.

[0097] 6. Hot extrusion process:

[0098] The block composite material sample that underwent vacuum heat treatment in step 112 and the control group (composite material that did not undergo vacuum heat treatment) were subjected to hot extrusion. First, the sample was kept at 500°C in a box furnace for 20 minutes, and then placed in an extrusion device preheated to 370°C. The extrusion ratio was 18:1 and the extrusion speed was 4 mm / s. Finally, composite material extruded rods with a diameter of 7 mm and a smooth surface without cracks were obtained.

[0099] In the above-mentioned forming process, in step 113, the extruded sample of the high-content CNTs / Al composite material without heat treatment has an average elastic modulus of 85.5 GPa, an average yield strength of 447.7 MPa, an average tensile strength of 459 MPa, and an average elongation at break of 9.8%. For the CNTs / Al composite material after optimal vacuum heat treatment followed by hot extrusion, the average elastic modulus is 92.9 GPa, the average yield strength is 409.5 MPa, the average tensile strength is 465.5 MPa, and the elongation at break is 11.3%. Specific data are shown in Table 1. It can be seen that the heat treatment process did not reduce the elastic modulus, tensile strength, and elongation at break of the high-content CNTs / Al composite material; on the contrary, it improved them to some extent. Furthermore, analysis of the uniform elongation and the change in the work hardening index show that the work hardening ability of the CNTs / Al composite material is significantly improved.

[0100] Table 1 Mechanical properties of high CNTs / Al composites

[0101]

[0102] The high-CNTs / Al composite material samples prepared in this embodiment through plasma sintering, vacuum heat treatment, and hot extrusion were subjected to the following tests and analyses:

[0103] (1) Phase composition analysis of high CNTs / Al composite materials

[0104] The structural damage and phase composition of CNTs in extruded composite material blocks were analyzed by Raman spectroscopy combined with XRD. The results are as follows: Figure 3 As shown, the composite material still exhibits obvious CNT structural characteristic peaks after heat treatment, namely 1351 cm⁻¹. -1 Nearby D peak and 1577cm -1 The nearby G peak indicates that CNTs still exist in the matrix during the heat treatment process. The area ratio of the D peak to the G peak can analyze the degree of structural damage to the CNTs. As the heat treatment temperature increases, the ratio decreases, proving that an interfacial reaction occurred between the damaged CNTs and the matrix, forming aluminum carbide. The detected CNTs are all in a relatively intact state, hence the low ratio. When the temperature exceeds the reaction temperature of CNTs with Al (600℃), no characteristic peaks of CNTs are found in the microstructure, indicating that the CNTs have completely reacted with the matrix. Furthermore, aluminum carbide peaks are present in both the extruded state and the heat-treated and then extruded samples, proving that there is a certain degree of interfacial reaction during sample preparation. XRD analysis shows that the main phase is pure aluminum peaks, while the aluminum carbide peaks are relatively low, indicating a low degree of interfacial reaction.

[0105] (2) Recrystallization analysis of high CNTs / Al composite material

[0106] EBSD analysis was performed on high-CNT / Al composites with and without heat treatment processes to determine the recrystallization of the microstructure. The results are as follows: Figure 4 As shown, combined with Figure 4 As can be seen from the large and small angle grain boundary conditions in (a) and (b), with increasing heat treatment temperature, the content of large angle grain boundaries decreases while the content of small angle grain boundaries increases in the final bulk microstructure. This is because, during hot extrusion, dislocations in the metal crystals continuously multiply and become entangled, leading to work hardening. When the dislocation density increases to a certain level, dynamic recrystallization begins to occur in the metal crystals, which is one of the main mechanisms of plastic deformation in materials. During dynamic recrystallization, dislocations within the grains are rearranged and eliminated, forming new grains with smaller orientation differences, i.e., small angle grain boundaries. Therefore, with the progress of dynamic recrystallization, small angle grain boundaries gradually increase, while large angle grain boundaries decrease accordingly. Through analysis... Figure 4 (c) It can also be seen that the recrystallization fraction first decreases and then increases with the increase of heat treatment process temperature. The higher the recrystallization content, the more beneficial it is to the work hardening behavior of the material. This also indicates that the sample after vacuum heat treatment and extrusion has good work hardening ability.

[0107] (3) Mechanical property testing and work hardening behavior analysis of high CNTs / Al composite materials

[0108] The tensile properties of the prepared high-CNT / Al composite material were tested, and the results are as follows: Figure 5 As shown, Figure 5 (a) is the engineering stress-strain curve. Figure 5 (b) shows the true stress-strain curve, which, in contrast to the initial tensile curve of the composite material, begins to decline shortly after yielding, indicating a significant work softening process. This phenomenon is significantly improved by introducing vacuum heat treatment. Furthermore, the work hardening capacity of the material gradually increases with increasing heat treatment temperature. However, excessively high temperatures (600℃) lead to complete reaction between CNTs and the matrix, causing the reinforcing effect of CNTs to disappear. Although the work hardening behavior is better, the overall strength of the material is significantly reduced. Therefore, to balance the material's strength and work hardening capacity, samples subjected to vacuum heat treatment at 550℃ followed by hot extrusion exhibit good comprehensive mechanical properties. The partial interfacial reaction of CNTs also improves the interfacial bonding of the material, resulting in a certain degree of increase in strength and plasticity, while significantly improving the work hardening capacity. Figure 5 (c) and Figure 5 (d) The analysis further confirmed that the work hardening ability of the composite material was significantly improved, and the sample treated with vacuum heat at 550℃ had the best strength, plasticity and work hardening ability.

[0109] TEM transmission analysis was performed on CNTs / Al composite samples that had undergone vacuum heat treatment at 550℃. The results are as follows: Figure 6 As shown, a large number of dislocations exist inside the grains of the structure and interact with CNTs at the same time. CNTs can act as pile-up dislocations, and their presence is also conducive to the stability and further improvement of the dislocation density within the grains. This is one of the reasons why CNTs / Al composites have good work hardening ability.

[0110] Comparative Example 1

[0111] The preparation method of high-content CNTs / Al composite powder includes the following steps:

[0112] 1. Powder formulation and ball milling process:

[0113] A total of 120g of powder was placed in the ball mill jar, of which 4.8g (4 wt.%) contained CNTs.

[0114] CNTs / Al powder ratio: 96 wt.% pure Al powder with a particle size of 15-53μm, 1 wt.% total stearic acid, 0.8 wt.% stearic acid initially added, 10 mm diameter zirconia balls used for milling, and a ball-to-material ratio of 5:1.

[0115] In the above preparation process, in step 1, the order of materials loaded into the container is as follows: first, add the milling ball, then add the pure aluminum powder, CNT powder, and stearic acid particles simultaneously and evenly, and stir evenly with a spatula to make the initial CNTs, Al powder, and stearic acid particles premixed evenly.

[0116] II. Ball milling process and parameter design:

[0117] Argon gas is introduced into the sealed grinding jar. First, the outlet valve is opened to ensure unobstructed gas flow. The gas flow rate is adjusted to 1.5 L / min. The outlet valve of the grinding jar is opened, and then the gas tube is inserted into the inlet valve of the grinding jar. The gas introduction time is 5 minutes. The inlet gas tube is pulled out first, and then the outlet valve is closed to avoid excessive gas pressure inside the jar. This completes the argon gas filling process. After the argon gas filling is completed, the grinding jar is immediately installed into the ball mill and the fixing device is tightened.

[0118] The ball milling parameters were set to a rotation speed of 200 rpm / min, and the milling pattern was 10 min forward, 10 min stop, 10 min reverse, 10 min stop, and so on, in a cyclical manner. To avoid excessive temperature inside the jar, after 2 hours of effective milling, a 1-hour cooling period was allowed. The final milling time was 24 hours. Note that after 8 hours of milling, 0.2 wt.% stearic acid was added again to the ball milling jar of the CNTs / Al composite powder to suppress excessive cold welding of the powder.

[0119] 3. Powder retrieval and addition of stearic acid in the glove box:

[0120] To prevent rapid oxidation of the powder, the can opening operation was performed in a glove box. After the effective ball milling time reached 8 hours, the pre-weighed stearic acid and the ball mill jar were placed into the glove box, and 0.2 wt.% stearic acid was added to the ball mill jar. The can lid was then closed, the ball mill jar was removed and placed into the ball mill, and ball milling continued for 24 hours. Finally, the powder was collected from the glove box and then vacuum-sealed for storage.

[0121] IV. High-content CNTs / Al composite powder discharge plasma sintering process:

[0122] The composite material powder obtained after ball milling was loaded into graphite molds with a diameter of 30 mm and pre-pressed at a pressure of 0.5 T for at least 10 minutes. The molds were then placed in a plasma sintering furnace for sintering. The sintering temperature was 590℃, the pressure was 30 MPa, and the vacuum degree was 1.5 × 10⁻⁶. -1The sample was held at a constant temperature (Pa) for 30 minutes to densify it. Temperature was measured using a K-type thermocouple contact method to ensure accuracy.

[0123] 5. Hot extrusion process:

[0124] The composite material block sample that has undergone sintering in step 204 is subjected to hot extrusion. First, the sample is kept at 500°C in a box furnace for 20 minutes, and then placed in an extrusion device preheated to 370°C. The extrusion ratio is 18:1 and the extrusion speed is 4 mm / s. Finally, a composite material extruded bar with a diameter of 7 mm and a smooth surface without cracks is obtained.

[0125] VI. Vacuum heat treatment process:

[0126] The extruded composite material block was heat-treated for a different time than in Example 1. The temperature was selected above the recrystallization temperature of the material. If there was an interfacial reaction between the metal matrix and the reinforcement, the temperature should also be controlled below the reaction temperature. The final designed heat treatment temperature was 500°C and the holding time was 12 hours.

[0127] The specific process involves polishing the surface of the extruded sample and then placing it in a vacuum heat treatment furnace. Vacuum heat treatment effectively prevents oxidation of the bulk material, with a vacuum level not exceeding 5 Pa. Simultaneously, 20g of sponge titanium is placed inside the vacuum tube furnace to further ensure a low oxygen content in the environment during heat treatment. The heat treatment temperature is set to 500℃, held for 12 hours, and then cooled in the furnace. The purpose of this comparative example is to highlight the advantages of the invention. When the temperature is the same but the holding time is different, the performance of the composite materials is compared to illustrate the advantages. The results are as follows: Figure 7 As shown in the comparison, it can be found that when the heat treatment time is 12 hours, the strength and plasticity of the resulting composite material are both lower than those of the material treated for 24 hours. This indicates that the 24-hour heat treatment time used in this invention patent is unique, and the composite material prepared using this process has significant performance advantages.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for improving the work hardening ability of nano-reinforced metal matrix composites, characterized in that, The method specifically includes the following steps: Step 101: Prepare nano-reinforced metal matrix composite powder, wherein the reinforcement is a one-dimensional or two-dimensional nano-reinforcement, and the composite powder is a uniformly distributed aluminum matrix composite powder with high carbon nanotube content; Step 102: Perform plasma sintering on the composite material powder obtained in step 101 to obtain a composite material block; Step 103: The sintered composite material block is subjected to vacuum heat treatment at a temperature of 550°C for 24 hours, followed by furnace cooling. Step 104: Perform hot extrusion treatment on the composite material after vacuum heat treatment to obtain an extruded composite material with a smooth surface and no cracks.

2. The method for improving the work hardening ability of nano-reinforced metal matrix composites according to claim 1, characterized in that, The method for preparing nano-reinforced metal matrix composite powder in step 101 includes: Step 201: Powder proportioning and processing: Each ball mill jar was filled with CNT powder, pure Al powder, and stearic acid. The CNT powder with a diameter of 20 nm and a length of 2 μm accounted for 4 wt.%, the pure Al powder with a particle size of 15-53 μm accounted for 96 wt.%, and the total amount of stearic acid added was 1 wt.%. The grinding balls used are 10mm diameter zirconia balls with a ball-to-material ratio of 5:

1. Initially, 0.8 wt.% of stearic acid was added to the powder, and after ball milling for 8 hours, 0.2 wt.% of stearic acid was added to the powder. Step 202: Ball milling process and parameter design: Argon gas is introduced into the sealed ball mill jar to remove the air inside. The ball milling parameters are: rotation speed 180-230 rpm / min, ball milling method is: forward rotation for 10 min, stop for 10 min, reverse rotation for 10 min, and repeat in this cycle. When the ball milling time is 2 hours, stop for 1 hour. The total ball milling time is 24 hours.

3. The method for improving the work hardening ability of nano-reinforced metal matrix composites according to claim 2, characterized in that, In step 201, the order in which the materials are added to the ball mill jar is as follows: first, add the grinding balls, then add pure Al powder, CNTs powder, and stearic acid particles simultaneously and evenly, and stir evenly.

4. The method for improving the work hardening ability of nano-reinforced metal matrix composites according to claim 1, characterized in that, The plasma sintering process described in step 102 includes loading the composite material powder into a graphite mold, pre-cooling and pressing it for a period of time, and then loading the graphite mold into a plasma sintering device for sintering.

5. The method for improving the work hardening ability of nano-reinforced metal matrix composites according to claim 4, characterized in that, The sintering temperature was 590℃, the holding time was 30 min, the forming pressure was 30 MPa, and the vacuum degree was 1.5 × 10⁻⁶. - 1 Pa, to obtain a dense cylindrical block.

6. The method for improving the work hardening ability of nano-reinforced metal matrix composites according to claim 4, characterized in that, The pressure holding pressure during the pre-cooling and pressing process is 0.5T, and the holding time is not less than 10 minutes.

7. The method for improving the work hardening ability of nano-reinforced metal matrix composites according to claim 1, characterized in that, In the vacuum heat treatment process described in step 103, sponge titanium is placed inside the vacuum tube furnace for heat treatment.

8. The method for improving the work hardening ability of nano-reinforced metal matrix composites according to claim 1, characterized in that, The hot extrusion process includes placing the vacuum heat-treated composite material into a box furnace, holding it at 500°C for 20 minutes, and then placing it into an extrusion device preheated to 370°C, with an extrusion ratio of 18:1 and an extrusion speed of 4 mm / s.

9. The method for improving the work hardening ability of nano-reinforced metal matrix composites according to claim 1, characterized in that, Compared with the composite material that undergoes hot extrusion treatment without vacuum heat treatment, the composite material that undergoes hot extrusion treatment after vacuum heat treatment has improved uniform elongation and work hardening index.

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