Study on the Mechanism of CNTs Regulating the Microstructures and Properties of Al–Cu–Mg Alloy

Abstract

The purpose of this study was to investigate the effect of carbon nanotube addition on the microstructure and mechanical properties of Al–Cu–Mg alloy composites. By analyzing the XRD results, it was deduced that the extruded and heat-treated composites, after the addition of CNTs, were preferentially grown on the (220) crystal plane. In addition, the distribution of carbon nanotubes at α-Al grain boundaries was observed by SEM and TEM. The incorporation of carbon nanotubes leads to an increase in the degree of recrystallization in the composite. It is worth noting that according to the study of the four strengthening mechanisms of dislocation strengthening, grain refinement strengthening, load transfer strengthening, and second stage strengthening, when the carbon nanotube content is 1.5 wt.%, the tensile strength (480.4 MPa) and yield strength (456.68 MPa) are significantly improved.

1. Introduction

The density of the Al–Cu–Mg (2024Al) alloy is low. This alloy has high fracture toughness and fatigue strength and has become a popular material in many fields such as aerospace and civil manufacturing [1,2]. With the instant development of the space field, the requirements for materials are becoming higher and higher; not only is there strong demand for high-strength and lightweight structural materials, but materials with improved working performance when used under long-term high temperatures is required. The service temperature of Al–Cu–Mg alloy is generally below 200 °C. The micro-yield strength of the alloy is low, and its high thermal expansion coefficient may cause structural instability in harsh environments such as those featuring high temperatures and stress alternation [3,4,5]. Therefore, improving the strength of aluminum alloys has become a research hotspot in recent years.

Nowadays, the synthesis of aluminum matrix composites (AMCs) by adding reinforcing materials to aluminum alloys is an effective method to improve the strength and toughness of materials [6]. Ceramic-particle-reinforced AMCs are the most widely used because of their low production cost. The experimental results of Wang et al. [7] showed that TiC particles pinned grain boundaries in TiC/Al–Cu–Mg composites, which hindered the migration of dislocations, strengthened precipitated phases, and improved the strength and toughness of composite materials. Gao et al. [8] successfully prepared (TiCp-TiB₂p)/Al–Cu–Mg–Si composites by combining combustion synthesis with hot pressing and hot extrusion. They showed that TiC and TiB₂ ceramic particles could effectively refine α-Al grains. However, the significant difference in thermal expansion coefficients between the reinforced particles and the matrix can lead to the formation of cracks and other defects during the manufacturing process, which can adversely affect the ultimate mechanical properties. Therefore, nanophases represented by carbon nanotubes (CNTs) used as the reinforcing phase of Al matrix composites have also been widely studied at home and abroad [9,10,11]. In the past decade, CNTs have been considered one of the most attractive reinforcement materials for AMCs due to their higher modulus (about 1 TPa), high strength (about 30 GPa), and low density [12,13,14,15,16].

In order to obtain high efficiency CNT/AMCs, it is significant to study the microstructures of CNT/AMC composites to reveal their influencing mechanism on the properties of the materials. Jiang et al. [17] prepared CNT/Al nanolaminate composites using the flake powder metallurgy method. Compared with conventional nanocomposites with the same composition, the tensile strength and strain capacity of the CNT/Al nanolaminate composites reached 375 MPa and 12%, respectively. Yuan [18] studied the transformation of CNTs during the preparation process and their dispersion in the Al and discussed the impact of CNT content on the resultant mechanical characteristics. It was considered that load transfer strengthening was the main mechanism responsible for the strength enhancement after the addition of CNTs. Therefore, in order to explore the practical application of CNT/aluminum composites, it is significant to study the influence of adding CNTs to the composite materials.

In the preparation of CNT/Al composites, the dispersion of CNT in the parent alloy is particularly important. While achieving uniform dispersion of CNT, it is also necessary to maintain good structural integrity in the material. Among the known preparation routes, the preparation of composite powders by high-energy ball milling has become the most commonly used method, which can disperse CNT into Al powder and play a greater role for CNTs. Xu et al. [10] prepared a high-strength 1.5 wt.% CNT/Al composite by combining low-speed and high-speed ball milling through the use of variable-speed ball milling powder metallurgy technology. By optimizing the balance between the uniform dispersion, interfacial bonding, and structural integrity of carbon nanotubes, the enhancement of CNT to the material was realized, and the tensile strength of the prepared 1.5 wt.% CNT/Al composite reached 376 MPa. Liu et al. [19] prepared carbon-nanotube-reinforced aluminum composites by ball milling and powder metallurgy. The results show that ball milling is beneficial to the dispersion of carbon nanotubes in the aluminum matrix and the strengthening of the aluminum matrix. The 6 h ball milling process yields a good interfacial bonding between the CNT/Al composite and the Al matrix, and its yield strength is 42.3% higher than that of the Al matrix with the same processing technology. Therefore, the preparation of CNT/Al composite powder by high-energy ball milling is a relatively mature process that has great advantages in improving the dispersibility of CNT.

In this paper, the recrystallization behavior of CNT/Al–Cu–Mg composites was studied. The composites were prepared by adding single-walled carbon nanotubes (SWCNTs). The microstructure, test properties, and strengthening mechanism of CNT/Al–Cu–Mg composites were observed and analyzed, and the effect of carbon nanotubes on aluminum composites was revealed.

2. Experimental Materials and Approach

2.1. Preparation and Process Parameters of the Composite Materials

In this paper, a 1.5 wt.% CNT/Al–Cu–Mg composite was prepared by mixing CNTs with Al powder, which was then pressed into an aluminum alloy solution by high-energy ball milling (Figure 1).

A total of 1.5 wt.% CNT/Al–Cu–Mg composite was prepared in a high-temperature melting device using industrial pure Al (99.5 wt.%), Al-50 wt.% Cu, Al-10 wt.% Mg, and Al-10 wt.% Ti master alloys as raw materials. The CNTs used were purchased from Suzhou Tianfeng GRAPHENE Technology Co., Ltd., Suzhou, China, with a specific size of 5~50 nm. The 2024 Al alloy melt was prepared at 800 °C. In order to prevent the CNTs from transforming into CO₂ at high temperature, we used the thermodynamic formula:

$$∆\mathrm{G}=∆{\mathrm{G}}^0+\mathrm{RT}\ln \frac{{\alpha }_{{\mathrm{Al}}_4{\mathrm{C}}_3}}{{{\alpha }^3}_{\mathrm{C}}{{\alpha }^4}_{\mathrm{Al}}}$$

(1)

Based on our calculations using this formula, when the Al alloy melt was in the semi-solid state, namely 500 ± 20 °C, the mixed powder after ball milling was wrapped and pressed with Al foil and severe mechanical stirring was applied. After the stirring, the melt was cast into ingots, and the prepared 1.5 wt.% CNT/Al–Cu–Mg ingots were extruded into 1 cm rods at 520 °C, so that the CNTs were well-dispersed in the composites. The prepared rods underwent solid solution treatment at 520 °C for 10 h and aged treatment at 170 °C for 12 h after water quenching.

2.2. Characterization

The size of the tensile part is shown in Figure 2. The metallographic samples (10 mm × 10 mm × 10 mm) prepared by wire cutting were etched with Killer reagent for 15 s after rough grinding, fine grinding, and polishing and then observed by a metallographic microscope (OM; ECLIPSE MA200 Nikon, Nikon Precision (Shanghai) Co., Ltd., Shanghai, China). X-ray diffraction (XRD) patterns of the 1.5 wt.% CNT/Al–Cu–Mg composite in different states were obtained with an X-ray diffractometer (Bruker D8 Advance, Bruker AXS, Beijing, China). The microstructure was characterized by scanning electron microscopy (SEM; Nova Nano SEM 450, Shanghai Uzong Industrial Co., Ltd., Shanghai, China) and high-resolution transmission electron microscopy (TEM; Tecnai G2 TF30, Beijing Shishi Technology Development Co., Ltd., Beijing, China). After the transmission sample was cut by wire cutting, it was ground to a thickness of 80 μm. A small disc of 3 mm was perforated in the center of the sample, and a pit of about 2 mm was concaved on the pit machine. Finally, the sample was placed on an ion thinning instrument and thinned by an electron beam. The tensile test was carried out at a tensile rate of 1 mm/min, and three samples were selected to ensure the accuracy of the experiment.

3. Results and Discussion

3.1. Alloy Phase Composition

Figure 3 shows XRD patterns of the composites with 1.5 wt.% CNTs after extrusion and heat treatment. In the XRD pattern, black and red represent the extruded and heat-treated states of 1.5 wt.% CNT/Al–Cu–Mg composites, respectively. The results showed that after adding CNTs, the composite contained α-Al and Al₁₂Mg₁₇ phases, and there are obvious C peaks and Al₄C₃ phases, indicating that the CNTs were partially decomposed. In addition, there were diffraction peaks with the same crystal plane index in the extruded heat-treated states, with composites showing three typical Al peaks at (111), (200), and (220), which indicates that the appropriate number of CNTs can stabilize the structure of the composites, among which the (220) diffraction peak was stronger and shows good orientation in the (220) crystal plane.

3.2. Microstructures of the Alloy

Figure 4 shows the microstructure of the 1.5 wt.% CNT/Al–Cu–Mg composite, as observed under the OM. The results show that the microstructure in the composite material is mainly streamlined with the extrusion direction of the bar, and the obvious granular black phase is evenly distributed in the diagram. The formation is due to the addition of carbon nanotubes. Combined with the XRD (Figure 3) and energy spectrum (EDS; Figure 5) results of the microstructure, it was concluded that the black phase in the microstructure consisted of CNT agglomerations. After heat treatment, the agglomeration of CNTs was further refined, where a coarse grain zone and a fine crystal zone appeared. The grain boundary of the rough crystal zone appeared after T6 heat treatment, and the grain size was between 8 μm and 12 μm. In the fine-grained region where CNTs were densely distributed, the grains were more obvious, and a large number of fine near-equiaxed crystals with an equivalent diameter of 5 μm were distributed. In addition, the microstructure of the composite material was observed to have obvious holes, indicating that after high-energy ball milling and extrusion, the molding had good compactness, and the extrusion process could effectively consolidate the composite material to improve its strength.

Figure 5 shows the EDS element distribution of the composite. The figure shows that the Cu element was dispersed more evenly and was mostly distributed in the white-dotted particles. The C element distribution was consistent with the black phase distribution and tended to aggregate during the extrusion process. It was found that the black particles are intermetallic compounds composed of Al and C as the main elements, and the results also show that CNT and Al were evenly dispersed. Hot extrusion improved the uniform distribution of elements and precipitates, leading to a reduction in the size of the second-phase particles.

Figure 6 shows the TEM images of the composite with 1.5 wt.% CNTs. From the diagram, it can be seen that the dispersed second phase was mainly distributed along grain boundaries and in the intracrystalline region, and a large quantity of dislocations was wrapped around the second phase in the crystal (Figure 6a). From the energy spectrum, it was determined that it contained Al, Cu, and Ti; that is, a (Al,Cu)₃Ti phase. It has been previously found that after mechanical ball milling of an Al–Cu–Ti alloy, the Cu element promoted the formation of the nanocrystalline L1₂ (Al,Cu)₃Ti structure, which had higher structural symmetry and more slip systems, and the high-temperature stability and brittleness of the Al₃M phase was improved [20]. The formation of the (Al, Cu)₃Ti phase indicated that the pinning effect of the second-phase substructure and dislocation stabilized the state of the deformed structure.

In addition, a large number of rod-like phases were found (Figure 6b), and the presence of CNTs can be seen in the TEM image (shown by the red arrow). It is observed that the distribution of carbon nanotubes in aluminum is uniform, indicating that the ball milling process improves the dispersion of carbon nanotubes, and the carbon nanotubes are well embedded in the aluminum matrix. At the same time, there were also some short, rod-like phases in the image; these were the Al₄C₃ phase, as determined by the calibrated diffraction pattern (Figure 6c,d). Its elastic modulus was 130 Gpa [21], and it maintained a good relationship with the matrix. Al₄C₃ phase is formed at the interface between the Al matrix and CNTs, which plays an important role in the load transfer between them [22].

3.3. Mechanical Properties and Strengthening Mechanism of the Alloy

The strength of 1.5 wt.% CNT/Al–Cu–Mg composites in T6 state is shown in Figure 7 and

Table 1. Tensile properties of composites.

Materials	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation (%)
Sample 1	450.28	479.6	11.36
Sample 2	456.68	480.4	11.91
Sample 3	453.56	478.9	11.71

. The tensile curve strength of the three samples is similar, indicating that the performance of the composite material is stable. The tensile curve shows that the strength of the composites is obviously improved after adding carbon nanotubes, which is due to the fact that carbon nanotubes hinder the movement of dislocations, indicating that CNTs had a significant strengthening effect on the Al–Cu–Mg alloy. On the one hand, CNTs can share a part of the load for the matrix during the tensile process of the composite, and the load can be transferred through the bridging mechanism. On the other hand, CNTs generally accumulate at the grain boundary, and then the boundary blocking phenomenon occurs, which plays a role in grain refinement. And CNTs pin dislocations at grain boundaries to prevent dislocation movement, which is conducive to obtaining high-density, uniformly dispersed dislocations, thereby improving the strength of the composite. The tensile strength and elongation remained at a stable value, with the highest values reaching 480.4 MPa and 11.91%, respectively.

Combined with the tensile curve and microstructure observation, the uniform dispersion of CNTs, the strength of the matrix, and the interfacial reaction and bonding improve the properties of the composites. The results show that the ball milling process helps to achieve a uniform distribution of CNTs in the composites, resulting in significant strength enhancement. At the same time, the fibrous texture formed during hot extrusion also contributes to the strengthening of the Al matrix. On the interface bonding, on the one hand, the uniform distribution of CNTs is a prerequisite for obtaining high-performance composite materials. On the other hand, the Al₄C₃ generated in situ also plays a key role in transforming the bonding mode of CNTs from mechanical bonding to chemical bonding, which is conducive to the transfer of load.

In order to further understand the effect of microstructure on composite properties, we quantitatively calculated the contribution of the relevant strengthening of mechanisms to the yield tensile strength (YTS). The tensile strength of CNT/Al composites is determined by the interaction of matrix and reinforcement. The addition of CNTs will greatly refine the grain size of the matrix due to the pinning effect, resulting in the enhancement of fine grain strengthening. According to the results given in Figure 7, the addition of CNTs significantly improved the strength and elongation of the composite. Due to the addition of CNTs, several strengthening mechanisms can work synergistically to improve the properties of the Al matrix. This was attributed to fine grain strengthening (∆σ_GR) [23], load transfer strengthening (∆σ_CNTs) [24], dislocation strengthening (∆σ_CET) [25], and second-phase strengthening (∆σ_Orowan) [26]. The strengthening effects of different mechanisms are different. Therefore, in this paper, the strengthening effects of various mechanisms are calculated by formulas to clarify the enhancement contribution of each mechanism and explore the enhancement mechanism of CNTs added to composites. That is, the yield strength (∆σ_0.2) can be expressed as:

$$∆{\sigma }_{0.2}=∆{\sigma }_{GR}+∆{\sigma }_{CNTs}+∆{\sigma }_{CET}+∆{\sigma }_{Orowan}$$

(2)

3.3.1. Grain Refinement Strengthening

Fine grain strengthening is an important strengthening mechanism in metal material strengthening. CNTs mainly exist at grain boundaries and play a significant pinning role on grain boundaries, which eventually leads to a decrease in grain size in CNT/Al composites. Moreover, when the composites containing CNTs are subjected to temperature changes during subsequent processing, due to the pinning and rotation of CNTs, the grain boundary movement is affected to form microstructures with different grain sizes. Previous studies have demonstrated that carbon nanotubes effectively reduce the grain size of the α-Al phase. When the grains are smaller, the plastic deformation becomes dispersed across more grains under external forces, resulting in reduced stress concentration and more-uniform deformation. Furthermore, the decreasing grain size leads to increasingly tortuous grain boundaries, hindering dislocation movement and promoting dislocation accumulation at the grain boundaries. This phenomenon ultimately enhances the material’s strength and plasticity. According to the Hall–Petch equation:

$$∆{\sigma }_{GR}=k_y\left(D_c^{-\frac{1}{2}}-D_a^{-\frac{1}{2}}\right),$$

(3)

where k_y is a constant, determined by the nature of the material: that is, 40 MPa/$\sqrt{\mathsf{\mu }\mathrm{m}}$ [27]. The equivalent diameters of the grains in the master alloy and the composite with 1.5 wt.% CNTs were calculated to be 387.3 μm (D_a) and 4.38 μm (D_c), respectively. The grain refinement strengthening contributed by adding 1.5% CNTs to the Al–Cu–Mg alloy was 17.08 MPa.

3.3.2. Load Transfer Strengthening

CNTs have excellent physical properties and are used as reinforcing materials. The strengthening of the composites is due to the fact that CNTs are superior to the Al matrix in terms of strength and hardness. In the process of ball milling and extrusion, the length–diameter ratio of CNTs is large, and the load transfer is closely related to this factor. Adding CNTs to prepare composite materials, the load is transferred to the CNT and bears part of the stress, which is one of the reasons for the improvement in the strength of the composite material.

$$∆{\delta }_{CNTs}=\frac{{\delta }_m{V}_{CNTs}(S-4)}{4},$$

(4)

Here, ${\delta }_m$ represents the YTS of the substrate, and S denotes the length-to-diameter ratio of carbon nanotubes. A significant Orowan strengthening effect was observed in the composite when the length-to-diameter ratio of CNTs was small [28,29]. S is 7 in this paper.

V_CNTs represents the volume fraction of CNTs, given as:

$$V_{CNTs}=\frac{m_{CNTs}/{\rho }_{CNTs}}{\left(m_{CNTs}/{\rho }_{CNTs}\right)+(100-m_{CNTs})/{\rho }_{Al}}.$$

(5)

Among them, m_CNTs represents the mass percentage of CNTs, and $\rho$ represents density (${\rho }_{CNTs}$ = 2.1 g/cm³, ${\rho }_{Al}$ = 2.71 g/cm³). When adding 1.5% CNTs, this enhanced contribution is 7.024 MPa. Xie et al. [30] studied the effect of the aspect ratio (R) of CNTs in the composite on the mechanical properties of CNT/Al with 0~2.0 wt.% carbon nanotubes. It was found that CNTs were uniformly dispersed in the Al matrix when the content of CNTs was 1.0 wt.%. The mechanical properties and fracture observation showed that a strengthening mechanism after adding carbon nanotubes may be load transfer strengthening.

3.3.3. Dislocation Strengthening

Dislocation strengthening is also one of the most effective strengthening methods in metal materials. The effect of CNT addition on the strength enhancement of CNT/Al–Cu–Mg composites is mainly attributed to its dislocation hindrance. The strength depends on the dislocation density. The higher the density, the more significant the strengthening effect in the composite.

$$∆{\sigma }_{CET}=k{G}_{m}b{\rho }^{1/2},$$

(6)

where k represents the coefficient determined by the matrix material, and ρ denotes the density of dislocation. The dislocation density and half peak width of the composites are shown in

Table 2. Dislocation density and half peak width of composite materials.

Materials	ρ	FWHM (°)
1.5 wt.%CNT/Al–Cu–Mg	0.0704	0.216

. The equation that relates the density of dislocation and coefficient of thermal expansion (CTE) is given below:

$$\rho =\frac{4∆C∆T}{b{d}_p\left(1-V_p\right)},$$

(7)

where ΔC is the difference between the CTE of the CNTs and the aluminum matrix (that is, ΔC = 23.5 × 10^–6 − 11.5 × 10^–6 = 12 × 10^–6 K⁻¹), and ΔT is the difference between room temperature (25 °C) and the experiment temperature (800 °C). When 1.5% CNTs were added, the maximum contribution of this strengthening was 249.57 MPa.

3.3.4. Second-Phase Strengthening

Due to the thermal decomposition of carbon nanotubes, they will react with the Al matrix to form a new phase (such as Al₄C₃). The formation of the second phase can effectively improve the interface between CNTs and the Al matrix, which is also affected by the preparation process in the preparation of composite materials (CNT dispersion and structure). At the same time, the formation of the second phase will increase the shear strength of the interface. Although the decomposition of CNTs and the interaction between carbides and dislocations formed by the matrix will lead to an increase in density, these particles can also increase the strength of the composite. The addition of carbon nanotubes (CNTs) disperses within the aluminum matrix, forming a dispersed phase, which pins the dislocations to hinder their movement and improve the overall performance of the material. This strengthening follows the Orowan mechanism, and the formula is:

$$∆{\sigma }_{Orowan}=\frac{0.13b{G}_m}{\lambda }ln\frac{d_p}{2b}.$$

(8)

In the formula, G_m is the shear modulus of the matrix (that is, G_m = 26 Gpa), b is the Burgers vector (0.286 nm), and d_p is the average size of the CNTs (i.e., 102 nm; see Figure 7). λ is the center distance of the CNTs, which was calculated as follows:

$$\lambda \approx d_p\left[{\left(\frac{1}{{2V}_p}\right)}^{\frac{1}{3}}-1\right]$$

(9)

where V_p is 1.92 vol. % (volume percent). The second-phase strengthening contribution was 3.102 MPa.

Under the action of four strengthening mechanisms, the YTS of the composites with CNTs is improved. The significant improvement in the tensile strength of 1.5 wt.% CNT/Al–Cu–Mg composites is mainly attributed to the aspect ratio and dislocation strengthening of CNTs. In addition, the improvement in the strength of the composites can be attributed to the formation of the Al₄C₃ phase at the interface, which prolongs the dislocation slip and fracture time and increases the fracture ductility to a certain extent. The specific contributions of each strengthening mechanism are shown in

Table 3. Experimental values of yield strength.

Fine Grain Strengthening ∆σGR (MPa)	Load Transfer Strengthening ∆σCNTs (MPa)	Dislocation Strengthening ∆σCET (MPa)	Second Phase Strengthening ∆σOrowan (MPa)	Theoretical Value ∆σ0.2 (MPa)
17.08	7.024	249.57	3.102	276.776

. The data show that the addition of CNTs can not only play an important role in hindering dislocation movement, but also the second-phase Al₄C₃ generated by its decomposition plays an important role in transforming load and shear slip deformation to improve the comprehensive properties of CNT/Al composites. The data showed that the CNTs and the aluminum substrate formed a good bonding surface, which effectively hindered dislocation movement, transferred the load to the CNTs, and reduced the stress concentration. In addition, the presence of dislocations also greatly affected the strength and toughness of the composite [31].

4. Conclusions

In summary, a CNT/Al–Cu–Mg composite is prepared by molten CNTs being added to a reinforced Al–Cu–Mg alloy, followed by high-energy ball milling and extrusion molding. The microstructure and phase composition were studied, and the mechanical properties were analyzed to explore the strengthening mechanism of CNTs on an aluminum alloy. The conclusions were as follows.

• Because the presence of carbon nanotubes (CNTs) hinders the growth of grains, the α-Al phase was refined. Moreover, the presence of CNTs effectively suppressed the preferred orientation of α-Al along the (220) crystal direction, thereby enhancing the anisotropic of the Al–Cu–Mg alloy bar.
• The CNT/Al–Cu–Mg composites had obvious grain boundaries, and the microstructure was distributed in a parallel extrusion direction. The microstructure formed the (Al, Cu)₃Ti phase with an L1₂ structure and a short, rod-like Al₄C₃ phase. Both phases had a coherent relationship with the matrix, pinning dislocations, stabilizing the modified microstructure, and improving the properties of the composites.
• The strength improvement of the CNT/Al–Cu–Mg composites can be attributed to several factors, including fine grain strengthening, load transfer strengthening, dislocation strengthening, and second-phase strengthening. The tensile and yield tensile strengths of the composites after the addition of 1.5 wt.% CNTs reached 480.4 MPa and 456.68 MPa, respectively, and the elongation was 11.91%. The results showed that the choice of molten addition of CNTs as a metal matrix composite reinforcement material had high potential.