Enhancing Strength and Toughness of Aluminum Laminated Composites through Hybrid Reinforcement Using Dispersion Engineering

: In this work, we propose a hybrid approach to solve the challenge of balancing strength and ductility in aluminum (Al) matrix composites. While some elements of our approach have been used in previous studies, such as in situ synthesis and ex situ augmentation, our work is innovative as it combines these techniques with specialized equipment to achieve success. We synthesized nanoscale Al 3 BC particles in situ using ultra-ﬁne particles by incorporating carbon nanotubes (CNTs) into elemental powder mixtures, followed by mechanical activation and annealing, to obtain granular (UFG) Al. The resulting in situ nanoscale Al 3 BC particles are uniformly dispersed within the UFG Al particles, resulting in improved strength and strain hardening. By innovating the unique combination of nanoscale Al 3 BC particles synthesized in situ in UFG Al, we enabled better integration with the matrix and a strong interface. This combination provides a balance of strength and ﬂexibility, which represents a major breakthrough in the study of composites. (Al 3 BC, CNT)/UFG Al composites exhibit simultaneous increases in strength (394 MPa) and total elongation (19.7%), indicating increased strength and suggesting that there are promising strengthening effects of in situ/ex situ reinforcement that beneﬁt from the uniform dispersion and the strong interface with the matrix. Potential applications include lightweight and high-strength components for use in aerospace and automotive industries, as well as structural materials for use in advanced mechanical systems that require both high strength and toughness.


Introduction
Hybrid reinforcement is a novel and significant strategy, the core idea of which is the use of hybrid reinforcements to exert respective advantages and fabricate advanced MMCs that are currently motivated by the purpose of overcoming the strength-ductility trade-off [1][2][3][4][5].Due to the higher efficiency of nano-sized reinforcements compared to the counterpart micron-sized reinforcements [6], a step forward has been taken in the pursuit of simultaneous improvement of strength and ductility.Although the use of ex situ non-reinforcement always brings the dilemma of uniform dispersion in MMCs [7,8].Along with all of the advantages of using ex situ reinforcements, deleterious aspects, such as the formation of incoherent interfaces between the reinforcement-matrix, problems such as stress concentration and deformation discontinuity at the microscale scale during the deformation process emerge [9,10].Therefore, it seems that the simultaneous use of the benefits of in situ and ex situ reinforcements as part of a hybrid reinforcement strategy would be one of the optimal ways to realize the simultaneous improvement of strength and ductility in MMCs.
Recently, the laminated structures that imitate the structure of nacre were noticeably succussed to provide materials that were both strong and damage tolerant [11,12].Furthermore, a reinforcement strategy based on the core idea of using hybrid reinforcements to exert their respective advantages to achieve synergistic effects, leading to excellent overall performance, has been demonstrated [4,5].In addition, the grain refinement of the matrix grains, which is apparently one of the best ways of simultaneously improving the strengthductility relationship, could not overcome the dislocation annihilation at grain boundaries (GBs), which leads to fast exhaustion of the strain-hardening capacity [13].To cope with this issue, by employing a graphene (GNS)-copper (Cu) hybrid material, exceptional tensile strength and ductility has been achieved using a heterogenous structure [14].Creating a laminated structure accompanied by hybrid reinforcement significantly contributes to hetero-deformation-induced (HDI) stress strengthening and sustained strain hardening, generating the key mechanical properties of GNS-Cu/Al.Moreover, high-performance reinforcement and a tailored architecture has been gained using a reduced graphene oxide (RGO)-CNT hybrid/Al composite prepared through a composite flake assembly process due to the formation of a planar network of RGO and CNT, which improves the load transfer efficiency between the matrix and the reinforcement in composites [5].Recently, a strong and tough AMC reinforcement with graphene oxide (GO)-CNT hybrid was prepared via powder metallurgy [4].The strength of the composites was improved via the synergistic effect of carbon nanotubes using in situ Al 4 C 3 and GO.More recently, superior tensile properties of AMC reinforced using both in situ Al 2 O 3 nanoparticles and ex situ GNSs, which were developed by manipulating the PM route [15].The composite possesses a high tensile strength of 464 MPa and appreciable amount of ductility (8.9%), which result from the combination effect of grain refinement, in situ Al 2 O 3 nanoparticles, and ex situ GNSs.It was inferred that the presence of a synergetic strategy between in situ and ex situ reinforcements, accompanied by a tailored architecture, could provide a significant contribution to both strengthening and toughening mechanisms to achieve the best combination of strength and ductility.Promisingly, the configuration of a microstructure in mesoscale and uniform spatial distribution of hybrid reinforcements offers great potential regarding the tuning of the mechanical properties of advanced AMCs [16].Motivated by the above considerations, an easy and innovative route on the basis of elemental powder strategy via in situ synthesis was employed to introduce nano-Al 3 BC into Al matrix composite reinforced with CNTs to attain a significant synergy of high strength and toughness.This method differs from those used by other researchers in several ways: 1-High-speed cutting and coating: High-speed cutting is used to break the CNT group, and B is used as a new step in manufacturing.This step allows carbon nanotubes to be dispersed within the Al matrix, which can improve the mechanical properties.2-Micro-micro rolling (MMR) process: The MMR process involves micro-scale rolling of composite materials and appears to be a unique and innovative process used in the production of laminated composites.This process will introduce microstructures and textures that can help improve the properties of (Al 3 BC, CNT)/Al composites.3-In Situ Formation of Al 3 BC: The in situ formation of Al 3 BC particles in composites through the reaction of Al particles with B and carbon elements and subsequent annealing distinguishes this method from another method based on the addition of preformed particles.This in situ synthesis provides better control of the size, distribution, and coupling between the material and the matrix.4-Low growth of Al 3 BC: This method relies on using a solid-state reaction to produce low-growth Al 3 BC particles, resulting in nanoscale Al 3 BC particles with unique properties being produced.The presence of Al 4 C 3 can also assist in the formation of submicron-sized heterogeneous core layers, such as Al 3 BC particles.5-Minimal damage to CNTs: Experiments show that CNTs suffer little or no major damage during pre-dispersion, extrusion, and MMR, which suggests that a careful and well-optimized approach is used to preserve the integrity of CNTs in composites.We demonstrated that a synergetic strengthening of intragranular and intergranular occurs in (Al 3 BC, CNT)/Al laminated composites via in situ nano-Al 3 BC, CNTs, Al 4 C 3 , and Al 2 O 3 , respectively.Specifically, strong bonding between Al 3 BC and Al inside of the Al grain's interior with its uniform dispersion, as well as a significant decrease in the dislocation annihilation at GBs due to the CNTs, leads to the significant storage capability of dislocations with increased strain hardening and, consequently, uniform tensile ductility at high flow stresses.
The fabrication of (Al 3 BC,CNT)/Al composites includes the high-speed shearing process for breaking CNTs clusters and B agglomerations using a molar ratio of Al/B/CNT = 3/1/1, which uniformly coats the surface of the Al flakes (Figure 1a) and the micro-micro rolling (MMR) process (Figure 1b) [17,18] produces the laminated composites (details of the experimental process are given in the Supplementary Materials).Diffraction peaks of Al 3 BC and Al were detected using (Al 3 BC,CNT)/Al composites (Figure 2b), while only Al peaks were found in (B,CNT)/Al composites (Figure 2a).The formation of Al 3 BC was mostly caused by the reaction Al particles with the B element and the carbon formed due to the decomposition of stearic acid during the fabrication process, as well as subsequent annealing [19].Indeed, due to the appearance of some areas in which boron and carbon atoms are supersaturated, Al 3 BC particles are in situ homogeneously nucleated in a solid state with a limited growth rate.In fact, due to the very low solubility of boron and carbon atoms in Al, especially during solid-state reactions [20,21], the growth processes of Al 3 BC particles are soon compromised by the deficiency in carbon and boron atoms; consequently, nanoscale Al 3 BC particle are formed.However, the presence of Al 4 C 3 effectively contributes to heterogeneously nucleated platelet Al 3 BC particles of submicron size via the liquid-solid reaction method [22,23].For this concept, it should be noted that the characteristic peaks of the Al 4 C 3 phase have not been observed in (CNT,B)/Al composite powder due to either very low relative content or a lack of formation.
strengthening of intragranular and intergranular occurs in (Al3BC, CNT)/Al laminated composites via in situ nano-Al3BC, CNTs, Al4C3, and Al2O3, respectively.Specifically, strong bonding between Al3BC and Al inside of the Al grain's interior with its uniform dispersion, as well as a significant decrease in the dislocation annihilation at GBs due to the CNTs, leads to the significant storage capability of dislocations with increased strain hardening and, consequently, uniform tensile ductility at high flow stresses.
The fabrication of (Al3BC,CNT)/Al composites includes the high-speed shearing process for breaking CNTs clusters and B agglomerations using a molar ratio of Al/B/CNT = 3/1/1, which uniformly coats the surface of the Al flakes (Figure 1a) and the micro-micro rolling (MMR) process (Figure 1b) [17,18] produces the laminated composites (details of the experimental process are given in the Supplementary Materials).Diffraction peaks of Al3BC and Al were detected using (Al3BC,CNT)/Al composites (Figure 2b), while only Al peaks were found in (B,CNT)/Al composites (Figure 2a).The formation of Al3BC was mostly caused by the reaction Al particles with the B element and the carbon formed due to the decomposition of stearic acid during the fabrication process, as well as subsequent annealing [19].Indeed, due to the appearance of some areas in which boron and carbon atoms are supersaturated, Al3BC particles are in situ homogeneously nucleated in a solid state with a limited growth rate.In fact, due to the very low solubility of boron and carbon atoms in Al, especially during solid-state reactions [20,21], the growth processes of Al3BC particles are soon compromised by the deficiency in carbon and boron atoms; consequently, nanoscale Al3BC particle are formed.However, the presence of Al4C3 effectively contributes to heterogeneously nucleated platelet Al3BC particles of submicron size via the liquid-solid reaction method [22,23].For this concept, it should be noted that the characteristic peaks of the Al4C3 phase have not been observed in (CNT,B)/Al composite powder due to either very low relative content or a lack of formation.In addition, no specific peak of Al4C3 was detected in either the (CNT,B)/Al comp or the (Al3BC, CNT)/Al laminated composite.This observation is likely due to there either no reaction between broken CNTs or a low relative content.Comparing the i sities of the peaks, it can be observed that the ratio of {111} intensity to {200} inte increased after the MMR process, indicating the development of a strong <111> fibe (e) In addition, no specific peak of Al 4 C 3 was detected in either the (CNT,B)/Al composite or the (Al 3 BC, CNT)/Al laminated composite.This observation is likely due to there being either no reaction between broken CNTs or a low relative content.Comparing the intensities of the peaks, it can be observed that the ratio of {111} intensity to {200} intensity increased after the MMR process, indicating the development of a strong <111> fiber texture.This belief was verified using a typical (001) pole figure, as shown in Figure 2d.As shown in Figure 2c, the relative intensity ratio of the I D /I G of the (Al 3 BC,CNT)/Al composite increased to 0.72, while the ratio was 0.7, 0.69, and 0.68 for (CNT,B)/Al composite, (CNT,B)/Al composite powder, and raw CNTs, respectively.These results imply that there was almost no serious damage to the CNTs during the pre-dispersion, extrusion, and MMR processes.Additionally, only a peak shift of G-band from ∼1580 cm −1 to ∼1594 cm −1 was semi-quantitatively detected in the (Al 3 BC, CNT)/Al composite, indicating that it might have originated from the infiltration of Al atoms in CNTs, causing a slight distortion of the sp 2 bonding structures of CNTs.The characteristic peaks of Al 4 C 3 [24,25] were not detected in either the (CNT,B)/Al composite or the (Al 3 BC,CNT)/Al composite.However, a few sluggish humps appeared in the composites, indicating that slight interface reaction may still exist.Indeed, the formation of Al 4 C 3 can result from CNTs that have undergone partial damage, as well as atomic carbon originating from stearic acid when subjected to elevated sintering temperatures.It seems that thanks to the flaky-shaped Al building blocks and the increased available effective surface, the deeper the CNTs are embedded into the Al particles, the smaller the growth value of the I D /I G ratio, indicating better protection of the CNTs in both the (CNT,B)/Al composite and the (Al 3 BC,CNT)/Al composite [8,26].Additionally, the TEM image in Figure 2e is provided to demonstrate the distribution of Al 3 BC particles within the UFG Al.The criteria used to evaluate uniformity encompassed the absence of localized clustering or agglomeration of Al 3 BC particles, ensuring that their distribution was not sporadic but homogeneously spread across the UFG Al.Uniformity was assessed through various characterization techniques, including the TEM image shown in Figure 2e, which, despite its limitations in representing the entire sample, provides a visual indication of the general distribution trend.Furthermore, the uniform distribution of Al 3 BC particles contributed to consistent mechanical reinforcement across the composite material.This result occurred because the interactions between dislocations and Al 3 BC particles were optimized when particles were evenly distributed, leading to enhanced strengthening effects.
Figure 3a demonstrates an obvious laminated structure with measured grain sizes of approximately 190 and 545 nm.Such an observation is more likely to be associated with the reduced mobility of GB and the subsequent restriction of grain growth caused by the presence of CNTs at the GBs.Furthermore, a significant number of elongated grains exhibited a strong <111> texture, with only a few grains being oriented in the <001> direction.This finding indicates a pronounced restriction of slip, both within the Al grains and at the GBs.The microstructure reveals the presence of a laminated structure with approximately 40% LAGB content.Among these LAGBs, approximately 12% had an angle of θ < 3 • , while 28% had an angle of 3 • < θ < 15 • .These LAGBs played a crucial role in strengthening the material through dislocations and the specific strengthening mechanism associated with LAGBs.This outcome is illustrated in Figure 3b.Additionally, approximately 60% of HAGBs were present, most of which were decorated by CNTs, along with some γ-Al 2 O 3 nanoparticles.These HAGBs were highly effective in impeding the movement of dislocations, forcing them to tangle and accumulate within the grain interior and near the boundaries.This process resulted in a very high dislocation density of about 1.5 × 10 16 m −2 , as indicated by the dashed ovals shown in Figure 3c.Upon closer examination of Figure 3c, numerous spherical gray particles in the size range of 15-25 nm can be observed.These particles have been identified as Al3BC based on the STEM image provided in Figure 3d,j.In thermodynamic terms, it is important to note that Al3BC has a negative formation enthalpy of approximately −56.34 KJ/mol [27], which is lower than that of Al4C3 (~−36 KJ/mol [27]).This observation, coupled with the availability of boron atoms that can form bonds with carbon atoms at damaged CNT tips and the presence of amorphous carbon coating on CNTs, suggests that annealing heat treatment at 800 °C for 3 h in a flowing argon environment would be ideal for the in situ synthesis of Al3BC nanoparticles.It is expected that the formation of Al3BC is primarily influenced by the diffusion rate of carbon and boron in the Al matrix.This diffusion rate typically increases in line with temperature in the solid state.Therefore, it is more likely that no Al3BC would form during hot extrusion, as the high temperature would enhance the diffusion and potentially deplete the available boron atoms.In such a scenario, it is possible that a small number of nanorod-shaped Al4C3 particles may form due to exposure to high temperatures when boron atoms become scarce.In other words, the content of Al4C3 in this study is not only influenced by temperature, as previously reported by [21], but also strongly depends on the availability of free boron and carbon atoms.The lattice fringes observed in individual Al3BC, CNT, Al4C3, and Al2O3 particles in the composites had approximate spacing values of 0.175 nm, 0.34 nm, 0.89 nm, and 0.24 nm, respectively.These spacing values roughly correspond to the (110) plane of Al3BC, the (1120) plane of graphite, the (003) plane of Al4C3, and the (311) plane of γ-Al2O3, as shown in Figure 3dg.In conclusion, nanoscale Al3BC particles with sizes ranging from 6 to 25 nm have been successfully synthesized in the composites.The size range of the Al3BC nanoparticles is comparable to those previously synthesized via liquid-solid reaction [23] and self-propagating high-temperature synthesis [28].In contrast to the complexity and costliness of other proposed routes, the synthesis route proposed in this study is feasible and simple.It involves the direct formation of nanoscale Al3BC through the reaction between dissolved boron and carbon atoms and the Al matrix, which is achieved through the ball milling, rolling, and annealing processes.The resulting nanorods of in situ Al4C3 exhibit Upon closer examination of Figure 3c, numerous spherical gray particles in the size range of 15-25 nm can be observed.These particles have been identified as Al 3 BC based on the STEM image provided in Figure 3d,j.In thermodynamic terms, it is important to note that Al 3 BC has a negative formation enthalpy of approximately −56.34 KJ/mol [27], which is lower than that of Al 4 C 3 (~−36 KJ/mol [27]).This observation, coupled with the availability of boron atoms that can form bonds with carbon atoms at damaged CNT tips and the presence of amorphous carbon coating on CNTs, suggests that annealing heat treatment at 800 • C for 3 h in a flowing argon environment would be ideal for the in situ synthesis of Al 3 BC nanoparticles.It is expected that the formation of Al 3 BC is primarily influenced by the diffusion rate of carbon and boron in the Al matrix.This diffusion rate typically increases in line with temperature in the solid state.Therefore, it is more likely that no Al 3 BC would form during hot extrusion, as the high temperature would enhance the diffusion and potentially deplete the available boron atoms.In such a scenario, it is possible that a small number of nanorod-shaped Al 4 C 3 particles may form due to exposure to high temperatures when boron atoms become scarce.In other words, the content of Al 4 C 3 in this study is not only influenced by temperature, as previously reported by [21], but also strongly depends on the availability of free boron and carbon atoms.The lattice fringes observed in individual Al 3 BC, CNT, Al 4 C 3 , and Al 2 O 3 particles in the composites had approximate spacing values of 0.175 nm, 0.34 nm, 0.89 nm, and 0.24 nm, respectively.These spacing values roughly correspond to the (110) plane of Al 3 BC, the (1120) plane of graphite, the (003) plane of Al 4 C 3 , and the (311) plane of γ-Al 2 O 3 , as shown in Figure 3d-g.In conclusion, nanoscale Al 3 BC particles with sizes ranging from 6 to 25 nm have been successfully synthesized in the composites.The size range of the Al 3 BC nanoparticles is comparable to those previously synthesized via liquid-solid reaction [23] and self-propagating high-temperature synthesis [28].In contrast to the complexity and costliness of other proposed routes, the synthesis route proposed in this study is feasible and simple.It involves the direct formation of nanoscale Al 3 BC through the reaction between dissolved boron and carbon atoms and the Al matrix, which is achieved through the ball milling, rolling, and annealing processes.The resulting nanorods of in situ Al 4 C 3 exhibit strong interfacial bonding, good dispersion within the aluminum grain interior, and a complete single-crystal structure.In the (Al 3 BC, CNT)/Al composite, the Al 3 BC particles are in situ synthesized and embedded within the Al matrix.The orientation relationship between the Al 3 BC particles and the Al matrix is represented by (0111) Al 3 BC/(111) Al, which means that the crystallographic planes of Al 3 BC align with those of the Al matrix.This alignment allows a better fit between the two materials, resulting in fewer mismatch dislocations at the interface.Thus, the in situ-Al 3 BC/Al interface demonstrates a strong bonding effect, which effectively minimizes the presence of mismatch dislocations at the interface (Figure 3h), corroborating the findings reported in previous studies [29,30].The strong bonding strength, along with the preferred orientation of the elongated Al grains, significantly contributes to the intragranular strengthening effects of Al 3 BC on the Al matrix.The majority of CNTs are embedded in the Al matrix and aligned in the RD, indicating a strong and firm bond between the CNTs and the Al matrix.The presence of a clean interface between the CNTs and Al, which lacks any voids or interfacial products (Figure 3e), suggests that both the preservation of the CNT structure and a strong bonding between Al and CNTs occur, which contribute to load transfer and strengthening.However, a small amount of nanoscale rod-like Al 4 C 3 is formed in situ through the reaction of partially damaged CNTs with the Al matrix (indicated via the red arrows shown in Figure 3c).The preferential nucleation of interfacial Al 4 C 3 at the open edges of the CNTs suggests that these open edges are more reactive than other parts of the CNTs, and these results are consistent with previous studies' findings [31,32].It appears that the formation of Al 4 C 3 could be related to the shortening and breaking of CNTs, which, in turn, increases the number of carbon atoms available to react with Al and B, leading to the production of in situ Al 3 BC nanoparticles.The presence of Al 3 C 4 in the composite has an effect on the strength and ductility of the product.Al 3 C 4 is a special phase with its own mechanical properties that contribute to all composite materials.The presence of Al 3 C 4 nanorods can act as an additional reinforcement in the composite.These nanorods may impede dislocation motion and contribute to forest hardening, thus enhancing the strength of the composite.In addition, strong interfacial bonding can be achieved via the formation of an appropriate amount of Al 4 C 3 , which can promote load transfer and enhance the fracture elongation of the CNT/Al composite.The size of Al 4 C 3 is the most important factor affecting the mechanical properties of the composite.In this study, the interfacial bond was improved, while the mean diameters of the short Al 4 C 3 rods in composite were as small as ~30-35 nm; therefore, it causes more outstanding elongation (19.7%).As it is a strong combination, it is not easy for microcracks to form and propagate at the interface during plastic deformation.The effect of Al 3 C 4 on curing is more complex and needs further analysis and research.However, since Al 4 C 3 is a brittle phase, excess Al 4 C 3 negatively affects the mechanical properties of the composite.
Additionally, spherical nanoparticles of γ-Al 2 O 3 can be observed within both the interior of the Al grains and at the grain boundaries (indicated via yellow arrows, as shown in Figure 3c).These nanoparticles are the result of the transformation of the native amorphous (am)-Al 2 O 3 skin that is broken during processing at temperatures above 450 • C [33].Therefore, the oxygen introduced during the fabrication process predominantly exists in the form of oxides, specifically γ-Al 2 O 3 .Considering that the native Al 2 O 3 skin on the surface of the nanoflakes is approximately 4 nm thick, the volume fraction of Al 2 O 3 is estimated to be about 2.6% (8/300 × 100%) of the total volume of the 300-nanometer nanoflake Al powder, which aligns with the value reported in [33].By assuming that all oxygen present in am-Al 2 O 3 is completely transformed into γ-Al 2 O 3 after annealing at 800 • C, it is found that the oxygen content also remains virtually unchanged after annealing.The nano-Al 3 BC and Al 4 C 3 play important roles in generating high effective stress via forest dislocation cutting and Orowan strengthening, while the CNTs and nano-Al 2 O 3 effectively provide high back stress via the accumulation and hindering of dislocation annihilation at the interfaces and GBs.Enhanced strength in composites can be achieved through promoted movable dislocation-reinforcement interactions, leading to improved performance based on the Orowan strengthening phenomenon [34].The Orowan strengthening occurs a result of the force exerted on a dislocation to bypass particles within the matrix [35].In this context, Al 3 BC nanoparticles are uniformly dispersed throughout the laminated UFG Al grain, with the interparticle spacing estimated to be approximately 170-190 nm, which is close to the particle diameter of about 190 nm.The intragranular Al 3 BC nanoparticles and Al 4 C 3 in nanorod form could store mobile dislocations in the form of Orowan loops in the internal regions of Al grains during straining, which more effectively provides HDI stress hardening and forest dislocation hardening.This result is consistent with those reported by Liu et al. [35].This outcome can reduce the ratio of Orowan strengthening to total strengthening.Considering the overall ultimate tensile strength, other strengthening mechanisms come into play.These mechanisms include forest hardening, which arises from dislocation-dislocation interactions that occur due to the relatively higher dislocation density present, and GB strengthening, which results from the presence of UFG Al grains.Both of these factors significantly contribute to the overall enhancement of the strength of material.The presence of nanoparticles (Al 3 BC, Al 2 O 3 ) or even Al 4 C 3 nanorods causes mechanical incompatibility and promotes dislocation formation (mostly misorientation at less than 3 • , which is taken into account as dislocations [36,37]), resulting in stronger dislocation-dislocation interactions and forest dislocation strengthening.Also, during deformation, GBs prevent dislocation movement due to the misorientation of adjacent grains.Higher misorientation causes resistance to movement [38,39].In addition, UFG Al has a large GB relative to the grain size, resulting in greater resistance to movement and higher stress.Dislocations from LAGBs (approximately 28%) result in a combination of easy sliding and depositing in the Al lamella, resulting in a combination of high elongation while maintaining high tensile strength.This finding is consistent with the literature mentioned above.Indeed, the presence of a high dislocation density, which is estimated to be approximately 8.13 × 10 15 m −2 , is primarily attributed to the inhomogeneous deformation between the matrix and the reinforcements.This high dislocation density plays a significant role in generating non-directional short-range local stresses, which facilitate dislocation movement.Moreover, it leads to long-range interactions with mobile dislocations, thereby exerting a directional long-range back stress that opposes the applied stress, especially during forward loading (as illustrated in Figure 4).Figure 5a presents the tensile stress-strain curves of the (Al3BC,CNT)/Al laminated composites and CNT/Al composites.It is evident that the (Al3BC,CNT)/Al laminated composites exhibit significantly higher strength than the CNT/Al composites.This result can be attributed to the combined intra-and inter-granular effects of the reinforcements, as well as the laminated structure of the composites.Furthermore, Figure 5b illustrates the variation in the strain-hardening rate as a function of true strain, demonstrating the superior maintenance of a high strain-hardening rate in the (Al3BC,CNT)/Al laminated com- Figure 5a presents the tensile stress-strain curves of the (Al 3 BC,CNT)/Al laminated composites and CNT/Al composites.It is evident that the (Al 3 BC,CNT)/Al laminated composites exhibit significantly higher strength than the CNT/Al composites.This result can be attributed to the combined intra-and inter-granular effects of the reinforcements, as well as the laminated structure of the composites.Furthermore, Figure 5b illustrates the variation in the strain-hardening rate as a function of true strain, demonstrating the superior maintenance of a high strain-hardening rate in the (Al 3 BC,CNT)/Al laminated composites.The improved dislocation storage capability of ultra-fine-grained (UFG) Al in the (Al 3 BC,CNT)/Al laminated composites is attributed to the laminated structure and the in situ formation of intragranular Al 3 BC nanoparticles.Specifically, the (Al 3 BC,CNT)/Al laminated composites exhibit a strength of 394 MPa, a uniform elongation of 13.2%, and a total elongation of 19.7%, which is approximately 29% higher in strength and shows an increase in the total elongation from 17.7 to 19.5% compared to the CNT/Al composites.Moreover, the uniform elongation of the (Al 3 BC,CNT)/Al laminated composites is 13.2%, whereas for PM (powder metallurgy) 16 vol.%B4C/UFG Al with an average grain size of 500 nm, it is only 3.5% [40].Additionally, for PM (0.5 wt.% CNT, 2 vol.% γ-Al 2 O 3 )/UFG Al with a unique intragranular dispersion of ultra-short CNTs, the uniform elongation is approximately 5.9% [41].Considering the similarity of the in situ reinforcements found in the (Al 3 BC,CNT)/UFG Al and the 15 vol.%Al 3 BC/UFG Al composites [29], the superior strength and ductility of the (Al 3 BC,CNT)/UFG Al laminated composite can be attributed to the synergistic contribution of intra-(e.g., in situ formed Al 3 BC, partially formed Al 4 C 3 ) and inter-granular strengthening (e.g., CNT, γ-Al 2 O 3 ).The presence of in situ formed nanodispersoids, particularly the spherical nanoparticles of Al 3 BC (Figure 5c), contributes to the synergetic intra-and inter-granular reinforcements within the elongated Al grains.This finding highlights the promising strengthening effects of Al 3 BC, which benefits from the strong interface with the matrix.The GBs decorated by CNTs and some γ-Al 2 O 3 (~2 wt.%) effectively lose their efficiency through dislocation annihilation, as dislocations nucleate in the neighboring matrix grain near the GBs.This process reduces the magnitude of stress concentration [42] and results in an increase in the critical resolved shear stress due to harder dislocation motion.This finding supports the notion that dislocation multiplication and accumulation, rather than annihilation through recovery, are the governing mechanisms.This result can be attributed to the profound role of in situ reinforcements, which mostly formed a strong interface in the Al grain interior.Recently, a significant high combination of ultimate tensile strength and ductility as a consequence of the presence of CNTs and Al 4 C 3 , as well as a high dislocation density (∼8.13 × 10 15 m −2 ), was demonstrated [43].Most importantly, these dislocations were mostly caused by inhomogeneous deformation between the matrix and the reinforcements; significantly, both produce the non-directional short-range local stress required for a dislocation to move and dramatically provide longrange interactions via mobile dislocations, exerting a directional long-range back stress opposed to the stress applied in the case of forward loading.
Incorporating both Al 3 BC and Al 2 O 3 improves the mechanical properties of the composite material.This improvement is mostly achieved through Zener drag, which occurs when dispersed particles are randomly distributed within the material matrix.The effectiveness of Zener drag depends on various factors, such as the nature, geometry, size, spacing, and volume fraction of the particles [44].In our composite, the presence of these nanoparticles effectively prevents the migration of both HAGBs and LAGBs.As a result, dynamic and static recrystallization is hindered, and grain growth is restrained.This characteristic allows the composite to retain its mechanical properties in advanced Al-based MMCs.The introduction of CNTs and some γ-Al 2 O 3 to GBs significantly reduces the dislocation annihilation efficiency, thereby contributing to the occurrence of back stress hardening.On the other hand, reinforcements embedded within the grain interior of Al enhance dislocation blocking and forest hardening, leading to the creation of an advanced approach to achieving ultra-high strengthening and toughening efficiency through intragranlar/intergranular dispersion engineering.
Additionally, the integration of CNTs within the GBs significantly hinders the movement of dislocations across the interface between Al and CNTs [45].This obstruction of GB motion is more effective when using CNTs due to their elongated nanofiber-shaped structure compared to nearly spherical nature of nanoparticles [46].The unique morphology of CNTs provides a strong impediment to dislocation movement within the composite.By reinforcing the material, CNTs synergistically enhance its strength through related mechanisms, such as load distribution and dislocation inhibition, while preserving the inherent ductility of the Al matrix.
Therefore, the combined effects of Al 3 BC, Al 2 O 3 , and CNTs contribute significantly to the overall improvement in the strength and ductility of the composite.These materials work together through various mechanisms, including Zener drag, dislocation inhibition, and load distribution, to enhance the mechanical properties of the composite while maintaining the malleability of the Al matrix.This multifaceted enhancement strategy offers possibilities to create advanced materials with superior mechanical performances that make them suitable for a wide range of applications.This advancement represents a progression beyond the work of Jiang et al. [10].The presence of a well-arranged array of in situformed hybrid nanoparticle-rich zones, surrounded by CNT-decorated GBs, synergistically enhances both the strength and ductility of the composite material.Several future directions and potential applications can be explored.Composite materials have a good combination of high strength and ductility, as well as applications in many industries.They are used in lightweight, aerospace, and automotive performance applications to improve fuel efficiency and reduce emissions.In addition, the mechanical properties of composite materials in the medical field facilitate the production of implants and medical devices.To further advance the practical applications of the (Al3BC, CNT)/Al composite, more research can focus on improving the manufacturing process to develop Several future directions and potential applications can be explored.Composite materials have a good combination of high strength and ductility, as well as applications in many industries.They are used in lightweight, aerospace, and automotive performance applications to improve fuel efficiency and reduce emissions.In addition, the mechanical properties of composite materials in the medical field facilitate the production of implants and medical devices.To further advance the practical applications of the (Al 3 BC, CNT)/Al composite, more research can focus on improving the manufacturing process to develop products using better technology.Exploring the differences and combinations between Al 3 BC and CNTs can provide composite materials with unique properties for different applications.Additionally, studying the behavior of composites in different environments, such as high-temperature or corrosive environments, will provide insights into specific applications.

Conclusions
In summary, our work presented a new (Al 3 BC, CNT)/UFG Al composite sheet that exhibits good support-length properties, having a strength of about 394 MPa and an elongation of about 19.7%.The composite achieved a uniform interparticle/interparticle distribution using a dispersion engineering approach that distinguishes it from most AMCs.The success of this composite could be attributed to the good relationship between the in situ nano-Al 3 BC particles in the UFG Al particles and the Al 4 C 3 formed at the end of the GB.These materials provided more significant support than conventional CNT/UFG Al composites.Through a careful combination of in situ and ex situ reinforcement techniques, we provided support distribution, leading to the development of dislocation blocks, stress recovery, and forest hardening.This process leads to an increase in the strength and ductility of the composite.The success of our combination and the surprising properties of this unique combination opened up new possibilities for research into and further development of advanced materials with improved equipment and general applications.Overall, the exceptional mechanical properties of this composite make it a promising material for use in various real-world applications.For instance, its high strength and strain-hardening capacity can be advantageous for the development of lightweight and resilient structures for use in aerospace, defense, medical device, and engineering system applications.

Figure 2 .
Figure 2. (a,b) XRD spectra and (c) Raman spectra of the studied materials; (d) (001) pole fig a typical (Al3BC,CNT)/Al laminated composite, (e) TEM image of a cross section showing th formity of the Al3BC particle (yellow arrows) dispersion within the UFG Al matrix.(All of the sample planes were perpendicular to the RD).

Figure 2 .
Figure 2. (a,b) XRD spectra and (c) Raman spectra of the studied materials; (d) (001) pole figure of a typical (Al 3 BC,CNT)/Al laminated composite, (e) TEM image of a cross section showing the uniformity of the Al 3 BC particle (yellow arrows) dispersion within the UFG Al matrix.(All of the tested sample planes were perpendicular to the RD).

Figure 3 .
Figure 3. (a) EBSD image of (Al3BC, CNT)/Al laminated composite, (b) the distribution of the grain boundary misorientation angle (), (c) a TEM image showing the elongated Al grains containing different reinforcement elements, (d) a STEM image showing a nanoscale Al3BC in Al matrix, (e-h), HRTEM images of (e) the CNT-Al interface, (f) a typical structure of CNTs with small interfacial Al4C3, (g) the structure of γ-Al2O3, (h) the Al3BC-Al interface formed in the grain interior of Al, and (i) the relative fast Fourier transforms of Al3BC.The dashed ovals show dislocation entanglements.The red, black, blue, and yellow arrows show Al4C3, CNTs, Al3BC, and γ-Al2O3, respectively.

Figure 3 .
Figure 3. (a) EBSD image of (Al 3 BC, CNT)/Al laminated composite, (b) the distribution of the grain boundary misorientation angle (θ), (c) a TEM image showing the elongated Al grains containing different reinforcement elements, (d) a STEM image showing a nanoscale Al 3 BC in Al matrix, (e-h), HRTEM images of (e) the CNT-Al interface, (f) a typical structure of CNTs with small interfacial Al 4 C 3 , (g) the structure of γ-Al 2 O 3 , (h) the Al 3 BC-Al interface formed in the grain interior of Al, and (i) the relative fast Fourier transforms of Al 3 BC.The dashed ovals show dislocation entanglements.The red, black, blue, and yellow arrows show Al 4 C 3 , CNTs, Al 3 BC, and γ-Al 2 O 3 , respectively.