Effect of carbon nanotubes on mechanical properties of aluminum matrix composites: A review

2.1 Structure of CNTs

CNTs are composed of curved carbon monolayers, typically categorized into two types based on their structures: single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). The CNTs first observed in 1991 were actually MWCNTs. A few years later, SWCNTs were successfully synthesized [39,40,41]. Depending on their rolling directions, three distinct types of SWCNT structures can be achieved, including armchair structure, Z-shaped structure, and chiral structure, as illustrated in Figure 1.

Figure 1

Structures of different types of CNTs. The chiral vector denotes different types of nanotubes. Reproduced by Sanginario et al. [41] with permission of The Royal Society of Chemistry.

2.2 Properties of CNTs

The SP2 hybridization of the carbon–carbon (C–C) bonds in CNTs imparts exceptional characteristics to these materials. With a density of only 1.3 g/cm³ [42], CNTs are considered an ideal reinforcing material for aluminum matrices. A comparison of certain properties of CNTs with those of common engineering materials is presented in Table 1 [43].

3.1 Tensile properties

Previous studies [24,49–51] have shown that, compared to pure aluminum, the optimal toughness and tensile strength of the composite are achieved when the CNT content is around 2 wt%. This represents the peak in mechanical enhancement, with the highest levels of toughness and tensile strength observed. When the CNT content exceeds this 2 wt%, the strength of the composites begins to decrease. This trend is attributed to the role of CNTs in strengthening the composite. The addition of CNTs beyond the optimal threshold likely leads to issues such as agglomeration or poor dispersion within the matrix, which negatively impacts the overall strength of the material.

The Young’s modulus obtained from tensile tests of CNT-reinforced AMMCs is illustrated in Figure 2. When CNT content increases from 1 to 5%, the elastic modulus of the composites rises from 70 to 110 GPa. However, it has also been reported that adding CNTs could reduce the elastic modulus of the composite [24]. This reduction is attributed to poor dispersion in the powder and the formation of large CNT clusters in the final microstructure. The increase in Young’s modulus is more pronounced for pure aluminum matrices than for aluminum alloy matrices. As the CNT content increases, the rate of stiffness increases, primarily due to the formation of CNT clusters and reduced load transfer efficiency to individual CNTs at higher CNT loads. Some studies have reported Young’s moduli higher than these predicted values for two reasons: first, the elastic modulus of CNTs is greater than 1,000 GPa, and second, the formation of alumina during processing can further increase stiffness. Compared to pure aluminum, the addition of CNTs increases the modulus of the composites. Deng et al. [52] demonstrated that the modulus of a 2024Al substrate was significantly improved by adding a certain amount of CNTs. At 2 wt% CNT content, the estimated elastic modulus increases by 23%. However, at 5 wt%, the elastic modulus decreases, yet it remains 20% higher than that of pure aluminum. The stiffness of CNTs/AMMCs produced by plasma sputtering is increased by 40% [53]. In the elastic region, CNTs enhance the flexibility of nanocomposites by maintaining the flexibility energy. The increase in modulus is largely due to the load transfer from the AMMCs to the CNTs [54].

Figure 2

Relationship between CNT content and elastic modulus of CNTs/AMMCs. The experimental data in the figure are sourced from previous studies [24,52–57].

To strengthen the interface bonding between the aluminum matrix and CNTs, several effective methods have been developed to control the interface structure. These include the in situ formation of Al₄C₃ between the aluminum matrix and CNTs [55], CNT coating [56], and CNT etching [57]. Zhang et al. [56] confirmed that the tensile strength of composites reinforced with 1 vol% CNTs could reach 50 MPa by coating the SiC layer formed in situ on the CNTs.

Despite considerable efforts, the service performance of CNTs/AMMCs still falls short of some industrially used aging aluminum alloys, such as the AA7075-T6 alloy. However, the addition of CNTs can enhance certain aspects of the aluminum matrix. For instance, the incorporation of CNTs promotes the formation of stacking faults within the aluminum matrix, which can improve the strain-hardening capability and modulus of the material, delay crack formation, growth, and fracture, and ultimately enhance its mechanical properties.

Density functional theory (DFT) simulations indicate that as the volume percentage of CNTs increases from 0.5 to 1.0 vol%, the stacking fault energy of the aluminum matrix decreases, and the density of stacking faults increases [58]. When CNTs are uniformly dispersed and exhibit good interface bonding with Al–1 vol% CNTs, the peak tensile strength (378 ± 8 MPa) and good ductility (17.1 ± 1.5%) are achieved. The primary mechanisms for strengthening in CNTs/AMMCs are attributed to Orowan looping and stacking fault strengthening.

At elevated temperatures, specifically at 400°C, the strength of CNTs/AMMCs increases with the CNT content up to 5 vol%. Notably, CNTs/AMMCs demonstrate a more pronounced reinforcing effect at high temperatures compared to room temperature [59]. This implies that CNTs/AMMCs might have specific applications where high-temperature strength is a critical factor.

3.2 Yield strength

Choi and Bae [60] found that the addition of CNTs increased the yield strength of CNTs/Al composites compared to pure aluminum. The study by Bustamante et al. showed that due to the dispersion strengthening mechanism of CNTs, the yield strength of CNTs/AMMCs was significantly improved when the content of CNTs increased to 0.75 wt%. Compared to unreinforced aluminum, an Al-2 wt% CNT composite shows a 26% increase in yield quality. However, the expansion of CNTs in the composite leads to a reduction of about 46% in fracture strain compared to pure aluminum. It is noteworthy that the yield strength values reported in various studies show considerable variation. This disparity is largely attributed to differences in processing routes. In the literature, the diameter of CNTs ranges from 20 to 140 nm. Due to the small size of CNTs, dislocations cannot penetrate through them, making the Orowan ring mechanism a feasible method for strengthening.

The aspect ratio of CNTs significantly affects the mechanical properties of CNTs/AMMCs. The addition of just 0.5 wt% CNTs can notably enhance the strength of Al composites without any loss of plasticity in hot extrusion. With a CNT content of 1.0 wt%, the CNTs are uniformly distributed within the aluminum matrix, significantly improving both strength and hardness. The primary strengthening mechanism in CNTs/AMMCs is believed to be dispersion strengthening [61]. CNTs of different lengths can effectively utilize both load transfer and Orowan strengthening mechanisms. Long CNTs primarily contribute to load transfer strengthening, while short CNTs predominantly facilitate the Orowan strengthening mechanism [62].

3.3 Compressive strength

Experimental studies [36,38,63–72] have demonstrated that adding different contents of CNTs to AMMCs can achieve higher compressive strength. When the CNT content is 1.6 vol%, the compressive strength of the CNTs/AMMCs increases by 350% [65]. Compression tests conducted at various temperatures for different strain percentages help analyze the material’s resistance to deformation. Table 2 provides a summary of the influence of CNTs on the strength of AMMCs. The patent CN110669956A [66] introduces a method for preparing a CNT-reinforced aluminum matrix composite coated with alumina. The alumina coating on the aluminum matrix endows the material with superior comprehensive mechanical properties. The yield strength at room temperature exceeds 320 MPa, the tensile strength surpasses 440 MPa, elongation is greater than 20%, and electrical conductivity is significantly enhanced. Compared to pure aluminum, the addition of CNTs greatly improves the compressive strength of AMMCs, yet the elongation of the composites remains unchanged, indicating excellent comprehensive mechanical properties.

The reinforcing effectiveness of CNTs in AMMCs diminishes with an increase in temperature and a decrease in the loading rate. This trend suggests that the contribution of CNTs to the overall strength of the composite is temperature and strain rate dependent. CNTs play a significant role in impeding dislocation movement and stabilizing the microstructure of the composite. Consequently, CNTs/AMMCs can exhibit enhanced strength at temperatures up to 603 K (330°C), regardless of the loading rate applied. When compared to standard AMMCs, those reinforced with CNTs demonstrate a higher sensitivity to strain rate and a lower activation volume across all tested temperatures [68]. This indicates that CNTs/Al composites respond more distinctly to changes in strain rate, suggesting a more complex interplay of mechanical behaviors at the nanoscale. These properties highlight the potential of CNTs/Al composites in applications where materials are subjected to varying temperatures and loading conditions, as they provide improved performance characteristics under these conditions. Figure 3(a) and (b) show the typical true stress–true strain compressive curves for 7055Al alloy and CNTs/7055Al composite at 400°C with a 0.01 1/s strain rate, prepared using high-energy ball milling and powder metallurgy methods. It is observed that the flow stress of both materials decreases with increasing temperature or decreasing strain rate, and the stress in the CNTs/7055Al composite is higher than that in the 7055Al alloy matrix, indicating that CNTs still have a strengthening effect at high temperature [72].

Figure 3

Stress–strain curves of 3 vol% CNTs/7055Al (a) at 400°C and (b) at a strain rate of 0.01 1/s [72].

3.4 Stress–strain curve of CNTs/AMMCs

The adoption of a bimodal structure design, which involves uneven reinforcement or grain size distribution, has been identified as a potentially effective method to enhance both strength and ductility in CNTs/AMMCs and other ultrafine-grained composites.

The mechanical properties of bimodal materials vary significantly between different regions. This variation can lead to uneven flow during thermal deformation, potentially deteriorating the material’s deformation capacity. However, by optimizing deformation temperature and strain rate, the compatibility of deformation across different regions can be effectively improved [73]. The stress–strain curve of CNT-reinforced metal matrix composites typically consists of three stages, as depicted in Figure 4. The first stage involves elastic deformation up to the yield point; in the second stage, the composites continue to stretch elastically due to the high yield strength of CNTs. The third stage is marked by plastic deformation occurring at the second yield point. Due to the presence of crack bridging, the stress–strain behavior includes three parts: the matrix elastic deformation zone, the CNTs pull-out zone, and the CNTs fracture zone. Although the aggregation of CNTs can lead to fracture, they also act as fracture inhibitors [34].

Figure 4

Double-yield phenomenon of CNTs/AMMCs.

It is inferred that the dual deformation behavior in CNTs/AMMCs results from different microstructures promoting different load transfers between regions. Figure 5 illustrates the true σ–ε (stress–strain) curve for unreinforced aluminum and composite specimens [34]. With an increase in CNT content, the yield resistance increases, but the work-hardening effect is negligible. Results indicate that the yield strength (σ _y) and tensile strength (σ _T) are 456 and 571 MPa, respectively. When the CNT content exceeds 2 wt%, additional CNTs do not positively affect the composite [61]. Tensile test results reveal that the strength of CNTs/Al composites significantly increases while elongation markedly decreases [69]. Evidently, increasing the volume fraction of CNTs in pure aluminum reduces the elongation of the resulting CNTs/Al composites [74].

Figure 5

True stress–true strain behavior of CNTs/AMMCs. The experimental data in the figure are sourced from the study of Yoo et al. [34].

With the addition of CNTs, the tensile strength of the CNTs/Al-Mg-Sc-Zr composites increased from 382.2 ± 8.1 to 409.4 ± 5.4 MPa. This enhancement is attributed to the significant promotion of precipitation secondary Al₃(Sc,Zr) nanoparticles by heat treatment and the complete conversion of the CNTs into Al₄C₃ nanorods. The formation of a strong and stable Al₄C₃/Al interface to replace the relatively weak CNTs/Al matrix interface provides higher load transfer efficiency during deformation [75]. Additionally, the ability of MWCNTs to effectively refine the grain size of the aluminum matrix also contributes to the improved strength of the nanocomposites through grain refinement. The patent CN104532085A describes AMMCs reinforced with 0.05–5.0 wt% CNTs, which AMMCs containing 0.5–5.0 wt% Zn, with a mass ratio of Cr to Zn of 1:4–8. The elongation of these CNTs/AMMCs ranges from 19 to 22% [76]. Patent CN106555093A discloses the composition of CNTs/AMMCs containing 0.5–2% CNTs, 40–60% SiC particles, and the remainder being aluminum powder. The plasticity of these composites has increased by 5–10% [77].

These studies demonstrate that the mechanical properties of aluminum-based alloys, such as tensile strength and plasticity, can be significantly enhanced through the appropriate incorporation of nanomaterials like MWCNTs and CNTs, combined with heat treatment processes. These advancements are crucial for industries requiring lightweight and high-strength materials, such as aerospace and automotive manufacturing.

3.5 Hardness

Srivastava et al. [78] reported a significant increase in the hardness of CNTs/AMMCs with the addition of CNTs. Kwon and Leparoux [79] found that adding just 1 vol% of CNTs to aluminum can significantly improve its hardness, up to three times higher than that of pure aluminum. Observations indicate that the hardness of CNTs/AMMCs increases with the addition of less than 1.5 wt% CNTs, but generally decreases with further increases in CNT content. The hardness improvement is mainly due to the strengthening effect of CNTs [80,81].

MWCNTs were prepared using chemical catalytic vapor deposition, and catalysts containing cobalt and magnesium were prepared using the sol–gel method. Christopher and his team [82] found that the maximum hardness of 6 wt% MWCNTs/AMMCs was 151 MPa. Experimental results [83] show that the hardness of CNTs/Al composites, prepared by pressureless infiltration, increases with the addition of CNTs but decreases with a further increase in CNT content. It has also been shown that CNTs/Al composites with a 4.5% concentration of CNTs exhibit the highest tensile strength and hardness [36].

When the weight percentage of CNTs in Al6061 alloy increases from 1 to 2 wt%, the hardness escalates from 19.58 to 32.88 HV, marking a 68% increase, as shown in Figure 6. This increase is attributed to the presence of a hard phase in the Al6061 composites, with hard CNT particles inducing density dislocation during compaction, leading to further strain hardening [36]. After adding 2% CNTs, the microhardness of CNTs/Al with a nanostructure was substantially improved, reaching 800 MPa, approximately five times that of pure Al (170 MPa). This remarkable enhancement in mechanical properties is mainly due to the fine-grain strengthening effect [84,85].

Figure 6

Hardness of with and without CNTs. Experimental data are sourced from the study of Yang et al. [36].

These findings highlight the significant role of CNTs in enhancing the hardness and overall mechanical properties of AMMCs, making them highly suitable for various high-strength applications.

3.6 Creep properties

Creep is considered a time-dependent deformation mechanism, and before creep fracture, the normal creep process usually goes through three stages: primary creep, steady-state creep, and creep fracture. The development of material creep is related to parameters such as temperature, time, stress, alloy microstructure, and composition. Strengthening non-shear second-phase CNTs into AMMCs can improve its creep resistance by reducing load transfer and dislocation activity at the matrix reinforcement interface. Choi and Bae studied the creep properties of CNT/AMMC composites at 523 K and observed improved creep properties at a pressure of 200 MPa, as reported by Choi and Bae [86]. They found that in the medium pressure region (σ  < 110 MPa), the strain rate showed little dependence on the stress. Figure 7(a) in their study presents a typical step load creep curve for ultra-fine grain (UFG) MWCNTs/Al composites, illustrating an evident secondary creep stage under various stresses. However, the extent of this stage in the composites is limited due to the debonding of the CNT reinforcement. Figure 7(b), as shown in their research [86], displayed a typical step strain rate curve for UFG aluminum and composites. The reduction in grain size from micron to UFG resulted in a significant increase in the step height of the strain rate, defined as strain rate sensitivity (where σ is the flow stress and the strain rate). This increase may originate from low-density dislocations in the lattice of small grains. Moreover, Figure 7(c) in the same study [86] showed the double logarithm plots of stress versus creep rate (obtained from step load creep test by power law equation) or strain rate (obtained from step strain rate test) for UFG aluminum and UFG MWCNTs/Al composites, as well as Al–6Mg–1Sc–1Zr–10 vol% SiC composites. When the volume fraction of CNTs is at 4.5%, the yield stress of the composite increases by approximately 70 MPa, surpassing that of Al–6Mg–1Sc–1Zr–10 vol% SiCp composites. This enhancement in flow stress may be due to the superior stiffness and yield stress of MWCNTs.

Figure 7

(a) The strain–time curve during a step load creep test, which is critical for understanding the creep behavior of these materials under a constant load over time. (b) Stress–strain curves during step strain rate tests, offering insights into how the materials deform under varying stress levels. (c) Creep properties with double logarithmic plots of stress as a function of minimum creep rate (obtained from step load creep tests by the power-law equation) or strain rate (obtained from step strain rate tests). This plot is particularly informative in comparing the creep behavior of UFG aluminum, the UFC MWCNTs/Al composite, and 10 vol% SiC/Al6Mg1Sc1Zr composite [86].

In regions of high-stress concentration (σ  > 200 MPa), the creep elongation of the composite shows a high-stress index, as highlighted by Choi and Bae [86]. With a decrease in the load, the contribution of line defects to creep elongation decreases, leading to increased dispersion and a reduction in the stress exponent. The addition of 1 vol% CNTs effectively improves the tensile ductility of aluminum matrix composites at high temperatures by increasing the strain-hardening ability of CNTs/AMMCs and delaying the recovery rate [87]. The tensile ductility of composite materials increases with the increase in strain rate due to a decrease in the spacing between CNTs, an increase in creep threshold stress, and an increase in strain rate sensitivity at high strain rates.

3.7 Fatigue properties

The decline in the safety performance of materials used in vehicle and aviation foundations is often attributed to fatigue performance. CNTs/AMMCs, while exhibiting high strength, are susceptible to fracture or complete failure under alternating loads after a certain number of cycles. However, the addition of CNTs has been shown to significantly improve the fatigue properties of AMMCs, as indicated by Coble [88]. Carvalho et al. [89] conducted tensile fatigue tests with a stress ratio of R = 0.1 and a frequency of 16 Hz. The results of these tests are presented in Figure 8. This figure illustrates the fatigue test outcomes for a 2 wt% CNTs/AlSi composite. Notably, the number of stress cycles sustained by this composite was higher than that of unreinforced AlSi, indicating enhanced durability. When compared to the unreinforced AlSi alloy, the 2 wt% CNTs/AlSi composite demonstrated superior performance, as shown in Figure 8. Specifically, the fatigue strength of the composite increased by 270% [89].

Figure 8

Maximum stresses versus cycles to failure for AlSi with and without CNTs. The experimental data is sourced from the study of Carvalho et al. [89].

This remarkable increase in the fatigue strength can be attributed to two key mechanisms: crack bridging and CNTs pull-out. Crack bridging helps to distribute the stress across the crack, reducing the stress intensity at the crack tip and hence slowing down the crack propagation. On the other hand, the CNT pull-out mechanism provides energy dissipation during the fatigue process, contributing to the increased fatigue life of the composite. These mechanisms make CNT-reinforced composites particularly valuable in applications where materials are subjected to cyclic loading and high-stress conditions.

Liao et al. [90] performed fatigue tests under tensile load with a stress ratio of R = 0.1 and a frequency of 5 Hz. Their research highlighted that the addition of CNTs enhances the material’s resistance to damage under cyclic loading. It was observed that as stress fluctuates, the service life of the material decreases with increasing stress levels. However, the incorporation of CNTs into the material increases the number of loading cycles it can withstand. Similarly, Shin and Bae [7] studied the fatigue properties of Al2024 alloy reinforced with MWCNTs. They conducted fatigue experiments with a stress ratio of R = −0.5 and a frequency of 5 Hz. Their findings revealed that the number of cycles to failure for MWCNTs/2024Al composites increases with the addition of more CNTs. Notably, there was a significant increase in fatigue strength to 600 MPa at 2.5 × 10⁶ cycles, indicating that CNT-reinforced composites possess higher fatigue strength. Figure 9, referenced in previous studies [91,92], illustrates the relationship between the maximum stress of MWCNTs/2024Al composite and the number of failure cycles. At any given number of cycles, the maximum stress that can be withstood due to the addition of CNTs is much higher than that of the pure 2024Al alloy. Additionally, the maximum stress increases with the increase in CNT content. While the fatigue life of MWCNT-reinforced composites decreases with the increase of applied stress, it increases significantly under high-stress levels. When the composite is subjected to fluctuating loads, the inconsistency in deformation between the matrix lattice and the reinforcing fibers causes drawing and acts as a crack bridge joint. This mechanism contributes to the increase in the number of fatigue cycles as the CNT content in the composite increases, as discussed in previous studies [93,94]. Thus, CNTs effectively strengthen AMMCs, enhancing their durability and suitability for high-stress applications.

Figure 9

Fatigue life of MWCNTs/2024Al composites. The experimental data is sourced from previous studies [79,80].

The CNTs/7055Al composite exhibits a distinctive bimodal structure, characterized by ultra-fine crystalline regions that are rich in CNTs and coarse crystalline bands where CNTs are absent. After enduring 10⁷ fatigue cycles, the CNTs/7055Al composites displayed the formation of dislocation cells, entanglement, and subgrain structures. The load transfer effect promoted by the CNTs significantly improved the fatigue strength of CNTs/7055Al, increasing from 350 to 400 MPa, and the number of cycles reached 10⁷. The predominant damage mechanism observed in the CNTs/7055Al composites was mainly strain localization, as discussed in the study of Bi et al. [95]. Similarly, the bimodal 2009Al-based alloy and CNTs/2009Al composites demonstrated basic cyclic stability during cyclic deformation. The CNTs/2009Al nanocomposites were capable of maintaining high cyclic stress levels across all applied strain amplitudes. Additionally, these nanocomposites exhibited excellent fatigue life at low strain amplitudes, attributed to their high monotonic strength, as reported in the study of Mohammed et al. [96].

These findings underscore the significant impact of CNTs on enhancing the mechanical properties of aluminum composites, particularly in terms of fatigue strength and cyclic stability. The presence of CNTs not only contributes to an increase in fatigue strength but also aids in maintaining stability under cyclic loading conditions, making these materials highly suitable for applications where endurance and reliability are critical.

3.8 Fracture strain and toughness

Previous studies [97,98] indicate that AMMCs with more than 3 vol%. CNTs exhibit a failure strain more than 60% lower than that of pure aluminum. This significant reduction in the failure strain of the aluminum alloy matrix is attributed to the challenges associated with dispersing CNTs within the aluminum alloy. For both pure aluminum and aluminum alloy matrices, toughness decreases as the CNT content increases. In CNTs/Al composites, lower CNT contents (<2 vol%) are associated with higher toughness. However, when the CNT content exceeds 3 vol%, the toughness of both pure aluminum and aluminum alloys diminishes. This trend suggests that regardless of the dispersion quality of CNTs in the aluminum matrix, an increase in CNT content leads to increased brittleness of the material. While the addition of CNTs enhances strength and stiffness, it also reduces the plasticity and toughness of the materials.

In CNT-reinforced composites, the bridging connections of CNTs play a crucial role in resisting fracture by disrupting the propagation of cracks. As the fracture progresses, CNTs are pulled out from the lattice due to friction, which helps to inhibit further fracturing [16]. This implies that the friction force on CNTs can drive some CNTs to bridge and pull out during stress fluctuations. Small pores are often found in the damaged areas, and the carbon concentration near the fracture is usually high due to CNTs contacting the fracture surface. In matrices with high CNT content, wide cracks are visible near the fracture zone. In regions where CNTs contact the crack surface (length more than 20 mm), some CNT-containing areas act as joints on the fracture interface. The fracture in 2024Al-CNTs is often influenced by the matrix elongation, with the fracture path crossing the direction of the applied force. The primary driver of fatigue fracture in metal matrix composites is the initiation from both large-scale and small-scale cracks. These full-size and tiny voids are a result of weak interfaces between the reticular structure and the reinforcement. Consequently, mode I crack is a predominant fracture mode in many composites, where the axial stress perpendicular to the fracture zone drives the fracture along a conventional external load path. However, particle pull-out can negatively impact mechanical properties due to poor maintenance of the interface between particles and the matrix [99–101]. On the other hand, particle pull-out can significantly enhance fracture diffusion and crack-bridging performance [102–104].

Patent CN110724842 (A) [105] reveals a high toughness CNT-reinforced AMMCs with a non-uniform structure and its preparation method. The toughness of this composite is notably higher than that of other composite materials with a uniform structure. In this composite, CNTs adhere to the substrate, resulting in additional lattice elongation and enhancing fatigue protection by controlling fracture elongation. The superior quality and flexibility of CNTs make the fatigue mass of the strengthened 2024Al lattice larger than that of unreinforced 2024Al [17]. It is hypothesized that particle-reinforced metal matrix composites benefit from the uniform dispersion of particles and strong adhesion. When these composites are subjected to permanent tension, the particles bear the stress within the mesh until they break without pulling out. However, under alternating loads, any aluminum matrix composite will undergo particle stretching. The vibration can cause CNTs to hinder fracture and lead to crack propagation. As the crack propagates, the pulling out and sliding of CNTs consume energy, thereby reducing the propagation rate of microcracks. Thus, CNTs act as a connector of the crack under vibration, increasing strain intensity [17].

Crack deflection is a common phenomenon in aluminum matrix composites. In regions with uniformly dispersed CNTs, intergranular cracks appear in the aluminum matrix and propagate along the crystal structure of the material, leading to intergranular fracture. Therefore, the fracture modes of CNTs/AMMCs include intragranular fracture and intergranular fracture. Intergranular fracture primarily results from defects within the aluminum grains, where the fracture direction passes through the aluminum grain, accompanied by significant elongation. This detailed understanding of fracture mechanisms in CNT-reinforced aluminum matrix composites provides valuable insights for enhancing the durability and toughness of these advanced materials.

The composite material CNTs/7055Al-E prepared by extrusion exhibits a structure composed of coarse and fine particles. In contrast, the composites designated as CNTs/7055Al-ER, which were hot rolled after extrusion, developed a uniform UFG structure. Notably, the hot rolling process did not cause further structural damage or aggregation of the CNTs. Compared to CNTs/7055Al-E, the elongation of CNTs/7055Al-ER increased by two times, although there was a slight decrease in strength. The mechanism of improving mechanical properties is related to the grain refinement and improved grain orientation of the composite material after adding CNTs [106].

CNTs/Al-Mg composites with both uniform and bimodal grain structures were developed through a powder assembly process. For the bimodal CNTs/Al-Mg composites, which contained 25% coarse grain, the elongation increased from 4.2 to 5.2% compared to the homogenous structure. This improvement was a result of the activation of various dislocation mechanisms, leading to higher strain hardening ability, as explained in the study of Fu et al. [107].

Compared to pure aluminum, the addition of CNTs enhances the strength of AMMCs and improves the strain rate sensitivity. Under dynamic loads, both the strength and elongation at break of the material increase due to the heightened strain hardening rate. The long-range back stress of dislocation at the CNTs interface and the geometrically necessary dislocations accumulated along the interface strain gradient are considered the main mechanisms of strain hardening in CNTs/Al [108].

Furthermore, interface bonding and structure are critical factors that influence the mechanical properties of CNT-reinforced AMMCs. To enhance interfacial bonding, CNTs/AMMCs were prepared by modifying CNTs with copper nanoparticles. This approach significantly improved the strength and ductility of CNTs/AMMCs. The copper nanoparticles aid in repairing structural defects of CNTs, passivating the reactivity of defect sites, and inhibiting the formation of a large Al₄C₃ phase. Additionally, serving as a bridge between aluminum and CNTs, copper nanoparticles improve interfacial wettability and create an “aluminum-copper-CNTs” interlocking effect, as reported in reference [109]. These advancements highlight the ongoing innovations in the development of CNT-reinforced composites, focusing on optimizing their mechanical properties for various industrial applications.

3.9 In situ tensile and nano-mechanical properties

Boesl et al. prepared CNTs/AMMCs using SPS at 500°C and 80 MPa [110]. They observed that increasing the CNT content from 3 to 5.5% led to an increase in tensile strength from 68 to 95.5 MPa, closely aligning with the calculated values of 90–100 MPa. Tensile test results indicated that fiber reinforcement is a key mechanism in CNTs/AMMCs [111].

Chen et al. [112,113] and Zhou et al. [114] analyzed the interface phenomena, interfacial shear strength, and fracture morphology of CNTs/AMMCs during in situ tensile tests. They found that the pull-out of CNTs is a primary failure mechanism in these composites. When CNTs peel off, effective load transfer between the walls of CNTs leads to interaction between the defective structure and CNTs, resulting in crack propagation.

It was also determined that higher interfacial strength can be achieved by pulling CNTs out of AMMCs. Various mechanisms contribute to the pull-out of CNTs in AMMCs, including load transfer, grain refinement through the pinning effect of CNTs, solid solution strengthening by CNTs, strengthening by in situ formed or precipitated carbides, and thermal mismatch between CNTs and the matrix [115]. An in situ scanning electron microscope (SEM) study revealed that when a single CNT was pulled from the fracture surface, the inner core of MWCNTs was pulled away, leaving the shell fragments in the matrix under tension, resembling a “scabbard” fracture mode [115].

Nanoindentation is utilized as a rapid evaluation tool for mechanical properties. This technique efficiently calculates tensile properties, although it’s noted that the hardness and elastic modulus of CNTs/AMMCs studied by nanoindentation methods often yield values higher than those obtained from macroscopic tests without considering the effects of porosity and CNT clusters. Bakshi et al. prepared CNT-reinforced Al–12%Si eutectic alloy coating on steel pipes using plasma spraying technology [53]. The analysis of nanoindentation load-displacement curves indicated a compressive strength of 20 GPa and a tensile strength of 120 MPa. This discrepancy is attributed to the presence of CNT clusters, which lead to poor wettability and lack of penetration of the molten metal, behaving similarly to porosity. The effects of CNT clusters and grain boundary oxide particles are best determined through nanoindentation tests. In situ tensile tests showed that the yield strength of 1 vol% CNTs/Al composite is 263 MPa, significantly higher than the 78 MPa for pure Al. The mechanical properties of the composites, such as hardness and elastic modulus, obtained by nanoindentation tests were found to be similar to those obtained by tensile tests [79].