Method for improving obdurability of aluminum-based composite material through matrix constraint shape memory alloy reinforcement body phase change

Abstract

The invention relates to a method for improving the obdurability of an aluminum-based composite material, in particular to a method for improving the obdurability of the aluminum-based composite material through matrix constraint shape memory alloy reinforcement body phase change. The invention aims to solve the problems that the existing shape memory alloy reinforcement cannot realize phase change activation and regulation, so that the strength of the shape memory alloy passively reinforced aluminum-based composite material is reduced and the toughness is insufficient. The method comprises the following steps: 1, preparing pre-deformation powder; 2, introducing a reinforcing phase; 3, sintering; 4, hot extrusion; and 5, solid solution and aging treatment. The method is used for improving the obdurability of the aluminum-based composite material through matrix constraint shape memory alloy reinforcement body phase change.

Description

Technical Field

[0001] This invention relates to a method for improving the strength and toughness of aluminum-based composite materials. Background Technology

[0002] Aluminum matrix composites are highly favored in aerospace, transportation, and other fields due to their excellent specific strength and specific stiffness. Traditionally, their strength has been improved by introducing ceramic reinforcements. However, this method often leads to a significant loss of toughness and plasticity, resulting in an irreconcilable "strength-toughness" inversion. The root cause lies in the modulus mismatch and brittle interfacial reaction between hard ceramic particles and the aluminum matrix, which easily induces stress concentration and becomes a crack initiation point, severely restricting the development and application of high-performance aluminum matrix composites.

[0003] To overcome this bottleneck, shape memory alloys (MMEs) have been proposed as a novel type of reinforcement possessing both high strength and unique phase transformation capabilities. Theoretically, their stress-induced martensitic phase transformation can absorb energy, potentially achieving "phase transformation toughening." However, early research and practice have not fully met expectations, with the strength improvement of composite materials falling short of expectations. This exposes the limitations of traditional design concepts: treating MMEs merely as passive reinforcing phases while neglecting the effective activation of their phase transformation potential. The core issue is that if the interface between the reinforcement and the matrix is ​​weak, or if the matrix itself is too soft to provide effective constraint, the phase transformation of the MME cannot generate beneficial recovery stress to strengthen the matrix, and its toughening effect cannot be fully realized.

[0004] The authorized patent CN119220839B discloses a NiTi / Al-based composite material with a dual heterogeneous structure and its preparation method. The core of this method involves high-energy ball milling of NiTi with pure aluminum powder and aluminum alloy powder to obtain two composite powders of different sizes. These powders are then layered and stacked with unmilled aluminum powder in a specific ratio. After SPS sintering and hot extrusion, a spatial heterogeneous structure consisting of a "high-density particle hard region" and a "particle-free tough region" is formed within the material. This structure mainly relies on the strain incompatibility between the soft and hard regions to induce geometrically necessary dislocations (GNDs), resulting in heterogeneous deformation-induced (HDI) strengthening, aiming to simultaneously improve strength and plasticity. However, this technical solution relies on macroscopic "structural heterogeneity" to coordinate deformation, failing to actively stimulate and utilize the "phase transformation behavior" of the shape memory alloy (SMA) reinforcement itself.

[0005] Furthermore, in the general powder metallurgy approach to improving the strength and toughness of aluminum-based composites, existing technologies, such as the published patent CN117512406A, provide a wear-resistant, high-strength, and tough aluminum-based composite material and its preparation method. This method ball-mills spherical aluminum alloy powder into flakes, then mixes it with the original spherical powder and hard ceramic reinforcements (such as SiC, TiC, etc.). Through medium-temperature sintering and large deformation treatment, it aims to obtain a matrix with a bimodal grain structure and utilizes the broken alumina film to achieve dispersion strengthening, thereby improving wear resistance and toughness. The key technical point of this patent lies in controlling the microstructure (bimodal grains) of the matrix itself through the mixing of powder morphology (flakes and spheres) and subsequent thermomechanical treatment. Its reinforcement is traditional hard, brittle ceramic particles. The "strength-plasticity" trade-off it solves is achieved through the conventional mechanism of matrix microstructure refinement and hard particle dispersion strengthening. However, it does not realize how to utilize the mechanical properties of the matrix to "constrain" and "excite" the phase transformation behavior of the reinforcement.

[0006] While existing technologies have reported methods for preparing flake aluminum powder through ball milling to improve reinforcement distribution, these methods generally target hard, brittle reinforcing phases (such as ceramic particles), aiming to improve distribution or reduce porosity. They do not address, and cannot resolve, the phase transformation activation and regulation issues of "intelligent" reinforcements like shape memory alloys. Therefore, there is an urgent need in this field for a systematic method that can actively design and precisely control the interaction between the matrix and shape memory alloy to fully unleash its phase transformation toughening and strengthening potential. Summary of the Invention

[0007] This invention aims to address the problem that existing shape memory alloy reinforcements cannot achieve phase transformation activation and regulation, resulting in reduced strength and insufficient toughness of aluminum matrix composites "passively reinforced" by shape memory alloys. It provides a method to improve the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements through the matrix.

[0008] A method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements in the matrix is ​​carried out according to the following steps:

[0009] I. Preparation of pre-deformed powder:

[0010] Aluminum-based powder and stearic acid particles are ball-milled to obtain pre-deformed powder; the aspect ratio of the pre-deformed powder is >5.

[0011] II. Introduction of Reinforcing Phase:

[0012] NiTi powder particles, pre-deformed powder and stearic acid particles were ball-milled to obtain a ball-milled mixed powder.

[0013] III. Sintering:

[0014] The ball-milled mixed powder was encapsulated in a graphite mold, then placed in a discharge plasma sintering furnace, sintered under vacuum, and finally cooled with the furnace to obtain an Al-based composite material containing a reinforcing phase.

[0015] IV. Hot Extrusion:

[0016] Al-based composite materials containing reinforcing phases are placed in a high-strength mold, heated and kept at a constant temperature, and then hot-extruded to obtain the extruded composite material.

[0017] V. Solution treatment and aging:

[0018] The extruded composite material is subjected to solution treatment, water quenching, aging treatment and air cooling in sequence, which completes the method of improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcement through the matrix.

[0019] The beneficial effects of this invention are:

[0020] This invention utilizes matrix-constrained shape memory alloy phase transformation to comprehensively enhance the strength and toughness of composite materials. Simply obtaining flake-like powder or the absence of interfacial reactions is insufficient for achieving strengthening and toughening. Only through the synergistic effect of "pre-deformed matrix providing high-strength constraint" and "optimized process to obtain a strong and tough interface" can the phase transformation behavior of shape memory alloys be successfully activated, thereby achieving a simultaneous leap in the strength and toughness of the composite material. First, aluminum-based powder is pre-milled at high energy to form large and thin aluminum-based powder, which is then mixed with NiTip at high energy to form a mixed powder of large and thin aluminum-based powder and ellipsoidal NiTip. Subsequently, a dense NiTip / d-2024Al composite material is prepared using SPS+hot extrusion, and further enhanced by solution treatment and aging treatment to strengthen the interfacial bonding. By activating the phase transformation behavior of shape memory alloys through high-strength matrix constraints and robust interfacial bonding, a synergistic effect of residual stress strengthening (strength enhancement) and phase transformation energy absorption, crack shielding, and bridging (toughening) is simultaneously achieved on a macroscopic scale. This results in a composite material with a tensile strength >565MPa and an elongation >10%, demonstrating an improvement in the strength and ductility of aluminum-based composites. This design method successfully transforms the innovative concept of "matrix-constrained shape memory alloy phase transformation" into an achievable material microstructure and superior macroscopic properties. Attached Figure Description

[0021] Figure 1 The image shows the morphology of the pre-deformed powder prepared in step one of Example 1.

[0022] Figure 2 This is a morphology image of the ball-milled mixed powder prepared in step two of Example 1;

[0023] Figure 3 For comparison, the morphology of the ball-milled mixed powder prepared in step two of experiment one;

[0024] Figure 4 SEM image of the NiTip / d-2024Al composite material prepared in Example 1;

[0025] Figure 5 SEM images of the NiTip / 2024Al composite material prepared in Experiment 1 were used for comparison.

[0026] Figure 6 SEM images of the NiTip / 2024Al (no interfacial reaction) composite material prepared in Experiment 2 were used for comparison.

[0027] Figure 7 STEM-EDS image of the interface of the NiTip / d-2024Al composite material prepared in Example 1;

[0028] Figure 8 DSC images of the NiTip / d-2024Al composite material prepared in Example 1, the NiTip / 2024Al composite material prepared in Comparative Experiment 1, and the NiTip / 2024Al (no interfacial reaction) composite material prepared in Comparative Experiment 2.

[0029] Figure 9 The in-situ room temperature tensile XRD test of the NiTip / d-2024Al composite material prepared in Example 1 is shown in (a) XRD diffraction patterns before and after deformation, (b) phase content calculation after XRD fitting of the composite material before deformation, and (c) phase content calculation after XRD fitting of the composite material after deformation.

[0030] Figure 10 The residual stress in the longitudinal and transverse directions of the NiTip / d-2024Al composite material prepared in Example 1, the NiTip / 2024Al composite material prepared in Comparative Experiment 1, and the NiTip / 2024Al (no interface reaction) composite material prepared in Comparative Experiment 2 were tested, and the stress response mechanism of NiTip and aluminum matrix before and after deformation was investigated. (a) Residual stress test in the longitudinal and transverse directions, (b) Stress response mechanism.

[0031] Figure 11 The images show room temperature tensile test results of the NiTip / d-2024Al composite material prepared in Example 1, the NiTip / 2024Al composite material prepared in Comparative Experiment 1, and the NiTip / 2024Al (no interfacial reaction) composite material prepared in Comparative Experiment 2.

[0032] Figure 12 The room temperature tensile fracture morphology of the NiTip / d-2024Al composite material prepared in Example 1;

[0033] Figure 13The image shows a room temperature tensile side fracture surface SEM image of the NiTip / d-2024Al composite material prepared in Example 1. Detailed Implementation

[0034] Specific Implementation Method 1: This implementation method describes a method for improving the strength and toughness of aluminum matrix composites through matrix-constrained shape memory alloy reinforcement phase transformation, which is carried out according to the following steps:

[0035] I. Preparation of pre-deformed powder:

[0036] Aluminum-based powder and stearic acid particles are ball-milled to obtain pre-deformed powder; the aspect ratio of the pre-deformed powder is >5.

[0037] II. Introduction of Reinforcing Phase:

[0038] NiTi powder particles, pre-deformed powder and stearic acid particles were ball-milled to obtain a ball-milled mixed powder.

[0039] III. Sintering:

[0040] The ball-milled mixed powder was encapsulated in a graphite mold, then placed in a discharge plasma sintering furnace, sintered under vacuum, and finally cooled with the furnace to obtain an Al-based composite material containing a reinforcing phase.

[0041] IV. Hot Extrusion:

[0042] Al-based composite materials containing reinforcing phases are placed in a high-strength mold, heated and kept at a constant temperature, and then hot-extruded to obtain the extruded composite material.

[0043] V. Solution treatment and aging:

[0044] The extruded composite material is subjected to solution treatment, water quenching, aging treatment and air cooling in sequence, which completes the method of improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcement through the matrix.

[0045] This specific implementation aims to achieve a paradigm shift from "passive reinforcement" to "active design." It's not simply about introducing shape memory alloys (SMAs), but rather about using a systematic approach to precisely control interfacial bonding strength and matrix properties to construct an ideal confinement environment. The core of SMA reinforcement in enhancing the strength and toughness of aluminum matrix composites lies in its stress-induced martensitic transformation behavior, and the effective activation and utilization of this behavior strictly depends on "matrix confinement." This "constraint" means that the aluminum matrix must possess sufficiently high yield strength and strain hardening capacity, and be able to effectively transfer the load to the reinforcing phase through good interfacial bonding, thereby providing the necessary and continuous mechanical environment for the SMA phase transformation. Under this constraint, the SMA phase transformation can simultaneously achieve strengthening and toughening: the recovery stress generated by the phase transformation acts as an active stress source, introducing a beneficial residual compressive stress field into the matrix, directly improving the material strength; simultaneously, the phase transformation process absorbs energy, induces crack deflection, and bridges cracks through recovery stress, synergistically dissipating fracture energy and significantly improving toughness. Therefore, the essence of "matrix-constrained shape memory alloy phase transformation" is to transform the SMA from a passive load-bearing phase into an active stress regulation and energy dissipation unit by constructing a "strong matrix-strong interface" system. This breaks through the traditional passive strengthening mode that relies solely on load transfer and realizes a paradigm shift from "geometric composite" to "functional synergy", providing a novel and effective path to solve this long-standing material science problem.

[0046] The core of the refined design in this specific implementation lies in shifting from the traditional "simple material composite" to an active design of "synergistic stress field and phase transformation." The approach treats the aluminum matrix as a "constraint field" and the shape memory alloy reinforcement as a "smart stress source." By precisely controlling the interaction between the two, beneficial residual compressive stress is introduced into the composite material, activating the toughening mechanism and thus achieving a simultaneous improvement in strength and toughness. Firstly, the aluminum matrix powder undergoes pre-deformation treatment to significantly enhance its subsequent work hardening capacity and yield strength, laying the microstructural foundation for forming a high-strength constrained matrix. This pre-deformation treatment is achieved through high-energy ball milling, transforming spherical aluminum-based powder into a large aspect ratio sheet structure (aspect ratio > 5). Its significant work hardening effect provides the necessary conditions for the subsequent formation of a high-strength constrained matrix. Next, the large aspect ratio aluminum sheets are ball-milled with NiTip to ultimately form a mixed powder of large and thin aluminum-based powder and ellipsoidal NiTip. Subsequently, a dense NiTip / d-2024Al composite material is prepared using SPS+hot extrusion. Further solid solution and aging treatments are then used to strengthen the interfacial bonding. By activating the phase transformation behavior through high-strength matrix constraint and a robust interface, the synergistic effects of residual stress strengthening (strength enhancement) and phase transformation energy absorption, crack shielding, and bridging (toughening) are simultaneously achieved macroscopically.

[0047] This specific implementation involves the selection of the matrix and reinforcement, and the design of multi-scale structures. By optimizing the size and distribution of the reinforcement and the heat treatment state of the matrix, performance matching is achieved. Then, the focus is on the controllable preparation of the "constrained-phase transformation" core structure. By precisely controlling the discharge plasma sintering parameters (temperature, pressure, time) and the subsequent solution-aging heat treatment regime, densification is achieved while constructing a strong and tough interfacial diffusion layer and controlling the yield strength of the matrix. This method successfully transforms the innovative concept of "matrix-constrained shape memory alloy phase transformation" into an achievable material microstructure and excellent macroscopic properties.

[0048] The beneficial effects of this embodiment are:

[0049] This invention utilizes matrix-constrained shape memory alloy phase transformation to comprehensively enhance the strength and toughness of composite materials. Simply obtaining flake-like powder or the absence of interfacial reactions is insufficient for achieving strengthening and toughening. Only through the synergistic effect of "pre-deformed matrix providing high-strength constraint" and "optimized process to obtain a strong and tough interface" can the phase transformation behavior of shape memory alloys be successfully activated, thereby achieving a simultaneous leap in the strength and toughness of the composite material. First, aluminum-based powder is pre-milled at high energy to form large and thin aluminum-based powder, which is then mixed with NiTip at high energy to form a mixed powder of large and thin aluminum-based powder and ellipsoidal NiTip. Subsequently, a dense NiTip / d-2024Al composite material is prepared using SPS+hot extrusion, and further enhanced by solution treatment and aging treatment to strengthen the interfacial bonding. By activating the phase transformation behavior of shape memory alloys through high-strength matrix constraints and robust interfacial bonding, a synergistic effect of residual stress strengthening (strength enhancement) and phase transformation energy absorption, crack shielding, and bridging (toughening) is simultaneously achieved on a macroscopic scale. This results in a composite material with a tensile strength >565MPa and an elongation >10%, demonstrating an improvement in the strength and ductility of aluminum-based composites. This design method successfully transforms the innovative concept of "matrix-constrained shape memory alloy phase transformation" into an achievable material microstructure and superior macroscopic properties.

[0050] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the aluminum-based powder mentioned in step one is atomized spherical 2024 aluminum alloy powder with a diameter of 53μm~150μm. Everything else is the same as in Specific Implementation Method One.

[0051] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the mass ratio of aluminum-based powder to stearic acid particles in step one is 1:(0.003~0.006). Everything else is the same as in Specific Implementation Method One or Two.

[0052] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the ball milling described in step one is specifically carried out at a rotation speed of 300 rpm to 500 rpm and a ball-to-material mass ratio of (10 to 15):1, for 8 to 15 hours. Everything else is the same as in Specific Implementation Methods One to Three.

[0053] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that: the atomic ratio of Ni to Ti in the NiTi powder particles described in step two is 50.82:49.18; and the diameter of the NiTi powder particles described in step two is 15μm~53μm. Everything else is the same as in Specific Implementation Methods One to Four.

[0054] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that: the mass ratio of the pre-deformed powder to NiTi powder particles in step two is 1:(0.11~0.5); the mass ratio of the pre-deformed powder to stearic acid particles in step two is 1:(0.003~0.006). Everything else is the same as in Specific Implementation Methods One to Five.

[0055] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that the ball milling described in step two is specifically carried out at a rotation speed of 300 rpm to 500 rpm and a ball-to-material mass ratio of (10 to 15):1, for 8 to 15 hours. Everything else is the same as in Specific Implementation Methods One to Six.

[0056] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that the sintering described in step three is specifically carried out at a sintering temperature of 500℃~530℃ and a uniaxial pressure of 40MPa~60MPa, with a holding time of 5min~15min. Everything else is the same as in Specific Implementation Methods One to Seven.

[0057] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that: in step four, the high-strength mold consists of, from top to bottom, an extrusion cup, a pure aluminum block, an Al-based composite material containing a reinforcing phase, and a graphite gasket; in step four, the material is first held at a temperature of 400℃~430℃ for 1h~3h, and then hot-extruded at a temperature of 400℃~430℃, an extrusion ratio of (16~25):1, and an extrusion rate of 0.1mm / s~0.5mm / s. Everything else is the same as in Specific Implementation Methods One to Eight.

[0058] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods One to Nine in that: in step five, the solution is first applied at a temperature of 490℃~510℃ for 10min~30min, followed by water quenching, then aging at a temperature of 160℃~190℃ for 3h~6h, and finally air cooling. The rest is the same as Specific Implementation Methods One to Nine.

[0059] The beneficial effects of the present invention are verified using the following embodiments:

[0060] Example 1:

[0061] A method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements in the matrix is ​​carried out according to the following steps:

[0062] I. Preparation of pre-deformed powder:

[0063] Aluminum-based powder and stearic acid particles were ball-milled for 10 hours at a rotation speed of 400 rpm and a ball-to-material mass ratio of 15:1 to obtain pre-deformed powder; the aspect ratio of the pre-deformed powder was 5~9.

[0064] The aluminum-based powder is a pure gas-atomized spherical 2024 aluminum alloy powder with a purity of 99.9% and an average diameter of approximately 72.48 μm.

[0065] The mass ratio of the aluminum-based powder to the stearic acid particles is 1:0.005;

[0066] II. Introduction of Reinforcing Phase:

[0067] NiTi powder particles, pre-deformed powder and stearic acid particles were ball-milled for 10 hours at a rotation speed of 300 rpm and a ball-to-material mass ratio of 15:1 to obtain a ball-milled mixed powder.

[0068] The atomic ratio of Ni to Ti in the NiTi powder particles is 50.82:49.18; the average diameter of the NiTi powder particles is approximately 16.10 μm.

[0069] The mass ratio of the pre-deformed powder to NiTi powder particles is 1:0.22; the mass ratio of the pre-deformed powder to stearic acid particles is 1:0.005.

[0070] III. Sintering:

[0071] The ball-milled mixed powder was encapsulated in a graphite mold and then placed in a discharge plasma sintering furnace. After vacuuming, it was held at a sintering temperature of 530℃ and a uniaxial pressure of 50MPa for 10 minutes and finally cooled with the furnace to obtain an Al-based composite material containing a reinforcing phase.

[0072] IV. Hot Extrusion:

[0073] The Al-based composite material containing the reinforcing phase was placed in a high-strength mold and kept at 430℃ for 1 hour. Then, it was hot-extruded at 430℃, with an extrusion ratio of 25:1 and an extrusion rate of 0.5 mm / s to obtain the extruded composite material.

[0074] The high-strength mold (H13 steel) consists of, from top to bottom, an extrusion cup, a pure aluminum block, an Al-based composite material containing reinforcing phases, and a graphite gasket;

[0075] V. Solution treatment and aging:

[0076] First, the extruded composite material was solution treated at 510℃ for 10 min, then water quenched, then aged at 160℃ for 3.5 h, and finally air cooled to obtain NiTip / d-2024Al composite material.

[0077] In the NiTip / d-2024Al composite material, NiTip represents NiTi particles, and d-2024Al represents pre-deformed 2024Al powder.

[0078] Comparative Experiment 1: This comparative experiment differs from Example 1 in that step one is omitted, and in step two, the pre-deformed powder is replaced with aluminum-based powder; the aluminum-based powder is pure gas-atomized spherical 2024 aluminum alloy powder with a purity of 99.9% and an average diameter of approximately 72.48 μm; in step five, the NiTip / 2024Al composite material is prepared. Everything else is the same as in Example 1.

[0079] Comparative Experiment 2: This comparative experiment differs from Example 1 in that: Step 1 is omitted; in Step 2, the pre-deformed powder is replaced with aluminum-based powder; the aluminum-based powder is pure gas-atomized spherical 2024 aluminum alloy powder with a purity of 99.9% and an average diameter of approximately 72.48 μm; in Step 3, the sintering temperature is 500℃ and the uniaxial pressure is 50 MPa, and the mixture is held at this temperature for 10 min; in Step 4, the mixture is first held at 410℃ for 1 h, and then hot-extruded at 410℃, an extrusion ratio of 25:1, and an extrusion rate of 0.5 mm / s; in Step 5, the extruded composite material is first solution-treated at 500℃ for 10 min, then water-quenched, then aged at 160℃ for 3.5 h, and finally air-cooled to obtain the NiTip / 2024Al (no interfacial reaction) composite material. All other steps are the same as in Example 1.

[0080] In the NiTip / d-2024Al composite material prepared in Example 1, NiTip exhibits significant deformation and good interfacial bonding with the matrix alloy. The composite material shows obvious phase transformation characteristics and a large enthalpy change; its tensile strength is 569 MPa and its elongation is 10.8%. This demonstrates an improvement in the strength and plasticity of aluminum-based composite materials.

[0081] Compared with the NiTip / 2024Al composite material prepared in Experiment 1, the NiTip deformation was smaller, the reaction layer at the interface with the matrix alloy was obvious, the content of intermetallic compounds at the interface was higher, the phase transformation characteristics of the composite material disappeared, the tensile strength was 367 MPa, and the elongation was 3.4%.

[0082] In contrast, the NiTip / 2024Al (no interfacial reaction) composite material prepared in Experiment 2 retains its near-spherical shape with minimal deformation. It exhibits no interfacial reaction with the matrix alloy, resulting in weak interfacial bonding strength and no matrix constraint on the NiTip. The phase transformation characteristics of the composite material disappear, with a tensile strength of 354 MPa and an elongation of 7.0%.

[0083] Figure 1 The image shows the morphology of the pre-deformed powder prepared in step one of Example 1. As can be seen from the image, the 2024 aluminum alloy powder underwent significant deformation during high-energy ball milling, changing from a spherical shape to a flat sheet shape. Due to the addition of stearic acid, the 2024 aluminum alloy sheets were uniformly dispersed without significant stacking, representing the ideal powder morphology of the pre-ball-milled 2024 aluminum alloy powder. The 2024 aluminum alloy powder was fully deformed into a large and thin sheet structure, and its significant work hardening effect provided the necessary conditions for the subsequent formation of a high-strength constrained matrix.

[0084] Figure 2 The image shows the morphology of the mixed powder after ball milling prepared in step two of Example 1. As can be seen from the image, due to the prior deformation of the 2024 aluminum alloy powder, the 2024 aluminum alloy powder deforms more than the NiTi powder during high-energy ball milling, resulting in large and thin flakes. After ball milling, the NiTi powder gradually changes from a spherical shape to an ellipsoidal shape. Figure 1 Compared to pre-deformed powder, the 2024 aluminum alloy powder in the ball-milled mixed powder has thinner flakes and a larger radial length. The purpose is to use the pre-deformed, high-hardness flake-shaped 2024 aluminum alloy powder to roll and coat NiTi particles, transforming them from spherical to ellipsoidal shapes, while avoiding excessive cold welding or contamination of the powder caused by over-ball milling. This morphology is beneficial for obtaining a tight bonding interface after sintering.

[0085] Figure 3To compare the morphology of the ball-milled mixed powder prepared in step two of experiment one; as shown in the figure, due to the higher hardness of 2024 aluminum alloy powder, when 2024 aluminum alloy powder and NiTi powder are directly ball-milled, the 2024 aluminum alloy powder deforms less than the NiTi powder, and a significant amount of 2024 aluminum alloy powder still retains a spherical or ellipsoidal shape. Figure 2 In comparison, the deformation of 2024 aluminum alloy powder and NiTi powder is relatively small overall.

[0086] Figure 4 The image shows a SEM image of the NiTip / d-2024Al composite material prepared in Example 1. As can be seen from the image, firstly, the preparation method of SPS sintering + low-temperature hot extrusion achieved a uniform distribution of NiTip in the 2024 aluminum alloy. Secondly, due to the low sintering and hot extrusion temperatures and the carefully designed heat treatment regime (510℃ / 10min + 160℃ / 3.5h), no intermetallic compounds were found at the NiTip / 2024Al interface in the SEM, or the interface reaction was too weak to be distinguishable under SEM. A higher resolution will be used for further observation of the interface. Figure 7 Finally, because NiTip particles undergo significant deformation during ball milling with the pre-deformed 2024Al matrix, the large deformation of NiTip particles can be clearly seen in the composite material, resulting in flat or long strip shapes.

[0087] Figure 5 SEM images of the NiTip / 2024Al composite material prepared in Experiment 1 were used for comparison. Figure 6 SEM images of the NiTip / 2024Al (no interfacial reaction) composite material prepared in Experiment 2 were used for comparison. The images show that the NiTip in the NiTip / 2024Al composite material exhibits minimal deformation and maintains a spherical particle morphology. However, a significant interfacial reaction is clearly observed in the NiTip / 2024Al composite material, resulting in a thicker interfacial reaction layer. In contrast, after lowering the sintering temperature, the NiTip / 2024Al composite material (no interfacial reaction) exhibits a clear interface, no interfacial reaction, and less deformation and more uniform distribution of the NiTip.

[0088] Figure 7 The image shows a STEM-EDS image of the interface of the NiTip / d-2024Al composite material prepared in Example 1. As can be seen from the image, the NiTip / 2024Al interfacial reaction is very weak, with an interfacial reaction layer thickness of approximately 200 nm. This indicates that during the composite material preparation process (such as sintering, hot extrusion, and heat treatment), the diffusion or chemical reaction between NiTip and 2024Al is limited, and no obvious brittle intermetallic compound layer is formed. The weak interfacial reaction helps maintain the bonding strength of the interface, avoiding interfacial embrittlement caused by excessive reaction, thereby improving the mechanical properties of the composite material.

[0089] Figure 8 The figures show the DSC spectra of the NiTip / d-2024Al composite material prepared in Example 1, the NiTip / 2024Al composite material prepared in Comparative Experiment 1, and the NiTip / 2024Al (no interfacial reaction) composite material prepared in Comparative Experiment 2. As can be seen from the figures, the degree of interfacial reaction in the NiTip / d-2024Al and NiTip / 2024Al (no interfacial reaction) composite materials is controllable, and the NiTi composition changes are small. Therefore, the NiTi phase transition characteristics are obvious. Regardless of cooling or heating, a one-step phase transition occurs within the composite material, and the two peaks are identified as corresponding to the transformations B2→B19' and B19'→B2, respectively. However, due to the severe interfacial reaction and large changes in NiTi composition, no phase transition peak was detected in the DSC test of the NiTip / 2024Al composite material, indicating that no phase transition occurs in the NiTip / 2024Al composite material.

[0090] Figure 9 The in-situ room temperature tensile XRD test of the NiTip / d-2024Al composite material prepared in Example 1 is shown. (a) shows the XRD diffraction patterns before and after deformation, (b) shows the calculated phase content after XRD fitting of the composite material before deformation, and (c) shows the calculated phase content after XRD fitting of the composite material after deformation. As can be seen from (a), the positions of the main diffraction peaks are basically the same, indicating that the main phase composition of the material is Al and NiTi shape memory alloy. However, after deformation, the intensity and half-width of the NiTi diffraction peak in the red spectrum changed significantly, indicating that the NiTi phase underwent a reversible phase transformation from austenite (A phase) to martensite (M phase) under external force. This phase transformation leads to the release of strain energy in NiTip, thereby forming internal residual compressive stress, which further improves the mechanical properties of the composite material. Figures (b) and (c) visually illustrate this process: before deformation, NiTi mainly exists as a stable austenitic phase (5.2%) and a small amount of martensite (2.8%), forming an interface bond with the aluminum matrix; after deformation, under the influence of external load, partial martensitic phase transformation occurs inside NiTi, with the A phase (3.3%) and M phase (4.7%) coexisting, generating a stress field induced by the phase transformation. This stress field can couple with the plastic deformation of the aluminum matrix, effectively hindering crack propagation and improving the strength and toughness of the material. Overall analysis shows that the phase transformation behavior of NiTi, under the constraint of the matrix, can achieve energy absorption and stress regulation, which is a key mechanism for strengthening and toughening composite materials.

[0091] Figure 10Residual stress tests were conducted in the longitudinal and transverse directions on the NiTip / d-2024Al composite material prepared in Example 1, the NiTip / 2024Al composite material prepared in Comparative Experiment 1, and the NiTip / 2024Al (no interface reaction) composite material prepared in Comparative Experiment 2. The stress response mechanisms of NiTip and the aluminum matrix before and after deformation were also examined. (a) shows the residual stress tests in the longitudinal and transverse directions, and (b) shows the stress response mechanism. The difference between the longitudinal and transverse stresses in the NiTip / 2024Al composite material (no interface reaction) was relatively small, at -19.95 MPa. The values ​​of a and -6.37 MPa indicate weak interfacial bonding and limited stress transfer, resulting in minimal constraint of the NiTip by the matrix during tensile testing. In contrast, the NiTip / 2024Al and NiTip / d-2024Al samples exhibited high residual compressive stresses in both directions, reaching -53.12 MPa and -57.96 MPa longitudinally, and -52.11 MPa and -56.78 MPa laterally. This suggests that interfacial reactions or microstructure optimization effectively enhanced the constraint between the matrix and the shape memory alloy, leading to a stronger stress-matching effect during loading. Figure (b) illustrates the stress response mechanism of the composite material between NiTip particles and the aluminum matrix before and after deformation. Before deformation, NiTip particles are uniformly distributed within the aluminum matrix, and the stress is in equilibrium. After deformation, NiTip undergoes a phase transformation that induces stress release, forming a compressive stress field centered on the particles in the phase transformation region, effectively suppressing crack propagation and stress concentration in the matrix. The stress cycling curves below indicate that the phase transformation behavior of the shape memory alloy reduces the stress amplitude (Δσ). eff This reduces fatigue life and crack resistance, thus improving the material's fatigue life and crack resistance, demonstrating the synergistic effect of matrix constraint and phase transformation coupling in strengthening and toughening.

[0092] Figure 11The figures show room temperature tensile test results for the NiTip / d-2024Al composite material prepared in Example 1, the NiTip / 2024Al composite material prepared in Comparative Experiment 1, and the NiTip / 2024Al (no interfacial reaction) composite material prepared in Comparative Experiment 2. As can be seen from the figures, the NiTip / d-2024Al composite material exhibits significantly superior comprehensive mechanical properties compared to NiTip / 2024Al and NiTip / 2024Al (no interfacial reaction) composite materials. Compared to NiTip / 2024Al and NiTip / 2024Al (no interfacial reaction) composite materials, the NiTip / d-2024Al composite material, while maintaining an elongation of 10.8%, shows significant improvements in elastic modulus, yield strength, and tensile strength. The tensile strength increases from 370 MPa to 569 MPa, and the yield strength increases from 282 MPa to 403 MPa, representing increases of 53.8% and 42.9%, respectively. By activating phase transformation behavior through high-strength matrix constraints and robust interfaces, a synergistic effect of residual stress strengthening, phase transformation energy absorption, crack shielding, and bridging (toughening) is simultaneously achieved on a macroscopic level. This design method successfully transforms the innovative concept of "matrix-constrained shape memory alloy phase transformation" into an achievable material microstructure and superior macroscopic properties.

[0093] Figure 12 The figure shows the room-temperature tensile fracture morphology of the NiTip / d-2024Al composite material prepared in Example 1. As can be seen from the figure, the room-temperature tensile fracture surface of the composite material exhibits obvious ductile fracture characteristics, with numerous plastic tear marks and dimple structures, indicating that the material underwent significant plastic deformation during tensile testing. The left figure shows numerous equiaxed dimples in the aluminum matrix region, indicating that the matrix underwent sufficient ductile fracture under load. The right figure shows multiple fractured NiTip particles and the interface morphology of the tightly bonded structure with the matrix. Some NiTip particles underwent brittle fracture along the load direction, proving that the load was effectively transferred from the aluminum matrix to the NiTip reinforcing phase during tensile testing. Due to the good interfacial bonding between NiTip and the aluminum matrix, there is little interfacial debonding, allowing the load to be continuously transferred and concentrated at the NiTip, leading to its fracture under stress. The fracture of the NiTip is accompanied by the release of phase transformation stress, which alleviates crack propagation to some extent, resulting in an overall fracture surface exhibiting a mixed ductile and brittle fracture characteristic. This demonstrates that the synergistic deformation and stress transfer mechanism between the matrix and NiTip are important reasons for the high strength and high toughness of composite materials.

[0094] Figure 13 The image shows a room temperature tensile fracture surface SEM image of the NiTip / d-2024Al composite material prepared in Example 1. Figure 13The analysis clearly reveals the microstructural basis of the synergistic effect of "matrix constraint-phase transformation". As shown in the figure, the NiTip particles, after pre-deformation and composite ball milling, exhibit a significant ellipsoidal to elongated morphology in the aluminum matrix, indicating that they were subjected to high-intensity plastic deformation constraints imposed by the matrix during the preparation process. Simultaneously, the particle-aluminum matrix interface is dense and continuous, without obvious pores or reactive brittle layers, confirming the formation of a strong and tough metallurgical interface. This "strong matrix-strong interface" constraint system ensures that under external load, stress can be effectively transferred to the NiTip and induce its martensitic phase transformation. During the phase transformation, the recovery stress generated by the volume change of the NiTip acts as a stress source, forming a residual compressive stress field in the surrounding matrix, achieving active strengthening of the material. Simultaneously, the phase transformation itself absorbs a large amount of energy, and the resulting local stress field causes significant deflection and branching of cracks extending thereto (crack shielding). Furthermore, deformed NiTip particles crossing cracks can bridge the crack sides through the phase transformation recovery stress, inhibiting further crack opening. The "stress strengthening" and "energy dissipation-crack control" mechanisms activated under constrained conditions coexist and couple with each other in the microstructure, thereby achieving a significant and synchronous improvement in the strength and toughness of composite materials on a macroscopic level.

Claims

1. A method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements in a matrix, characterized in that... It is done in the following steps: I. Preparation of pre-deformed powder: Aluminum-based powder and stearic acid particles are ball-milled to obtain pre-deformed powder; the aspect ratio of the pre-deformed powder is >5. II. Introduction of Reinforcing Phase: NiTi powder particles, pre-deformed powder and stearic acid particles were ball-milled to obtain a ball-milled mixed powder. III. Sintering: The ball-milled mixed powder was encapsulated in a graphite mold, then placed in a discharge plasma sintering furnace, sintered under vacuum, and finally cooled with the furnace to obtain an Al-based composite material containing a reinforcing phase. IV. Hot Extrusion: The Al-based composite material containing the reinforcing phase is placed in a high-strength mold, heated and kept at a constant temperature, and then hot-extruded to obtain the extruded composite material. V. Solution treatment and aging: The extruded composite material is subjected to solution treatment, water quenching, aging treatment and air cooling in sequence, which completes the method of improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcement through the matrix.

2. The method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements according to claim 1, characterized in that... The aluminum-based powder mentioned in step one is an atomized spherical 2024 aluminum alloy powder with a diameter of 53μm~150μm.

3. The method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements according to claim 1, characterized in that... The mass ratio of aluminum-based powder to stearic acid particles mentioned in step one is 1:(0.003~0.006).

4. The method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements according to claim 1, characterized in that... The ball milling described in step one is specifically carried out at a rotation speed of 300 rpm to 500 rpm and a ball-to-material mass ratio of (10 to 15):1 for 8 to 15 hours.

5. The method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements according to claim 1, characterized in that... The atomic ratio of Ni to Ti in the NiTi powder particles mentioned in step two is 50.82:49.18; the diameter of the NiTi powder particles mentioned in step two is 15μm~53μm.

6. The method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements according to claim 1, characterized in that... The mass ratio of the pre-deformed powder to NiTi powder particles in step two is 1:(0.11~0.5); the mass ratio of the pre-deformed powder to stearic acid particles in step two is 1:(0.003~0.006).

7. The method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements according to claim 1, characterized in that... The ball milling described in step two is specifically carried out at a rotation speed of 300 rpm to 500 rpm and a ball-to-material mass ratio of (10 to 15):1 for 8 to 15 hours.

8. The method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements according to claim 1, characterized in that... The sintering described in step three is specifically carried out at a sintering temperature of 500℃~530℃ and a uniaxial pressure of 40MPa~60MPa, with a holding time of 5min~15min.

9. The method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements according to claim 1, characterized in that... The high-strength mold described in step four consists of, from top to bottom, an extrusion cup, a pure aluminum block, an Al-based composite material containing a reinforcing phase, and a graphite gasket. In step four, the material is first held at a temperature of 400℃~430℃ for 1h~3h, and then hot-extruded at a temperature of 400℃~430℃, an extrusion ratio of (16~25):1, and an extrusion rate of 0.1mm / s~0.5mm / s.

10. The method for improving the strength and toughness of aluminum matrix composites by constraining the phase transformation of shape memory alloy reinforcements according to claim 1, characterized in that... In step five, the solution is first applied at a temperature of 490℃~510℃ for 10min~30min, followed by water quenching, then aging at a temperature of 160℃~190℃ for 3h~6h, and finally air cooling.

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