Nanotube Reinforced Aluminum Matrix Composites Research Articles

Re-dissolution behavior of secondary phases and mechanical response of CNT/2024Al composites

Heat treatment is one of the most effective strategies for regulating microstructure and enhancing the mechanical properties of aluminum matrix composites. In this work, the effects of solution treatment on the static recrystallization behavior, characteristics of secondary phases dissolving, and mechanical response of carbon nanotubes reinforced aluminum matrix composites (CNT/2024Al) were systematically investigated using transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM). Since plenty of deformation stored energy in as-extruded composites is consumed, the residual energy is insufficient to induce static recrystallization during solution treatment. Besides, various interfacial phases aggregated around the Al2Cu phases, which reduced the effective contact area between the Al2Cu phases and matrix, and severely hindered the dissolution process of the Al2Cu phases. The yield strength, tensile strength, and elongation of as-solution treated composites were 23.2%, 44.9%, and 23.0% higher than those of as-extruded composites, respectively. The enhanced yield strength is attributed to solid solution strengthening induced by the dissolution of secondary phases, thermal mismatch strengthening, and the strengthening originating from the formation of Al4C3 phases.

Chemical and mechanical treatments for enhancement of carbon nanotube reinforced aluminum matrix composites

A widespread application of carbon nanotube (CNT) and its metal matrix composites (MMCs) has not been achieved due to difficulty to disperse CNT in the matrix to efficiently obtain the excellent properties of CNT. In this study, CNT/Al composites were fabricated by uniformly dispersing CNT in Al matrix using chemical and mechanical treatments, which include untangling CNT in an organic solvent (DMAc) with a certain dispersant such as potassium carbonate (K2CO3) or ammonium carbonate ((NH4)2CO3)), dry ball milling and post-sintering repeated hot rolling. The composites were consolidated by spark plasma sintering (SPS). It was found that chemical treatments with ammonium carbonate were more effective to untangle CNT with less damage, while CNT/Al composites with CNT treated by potassium carbonate showed higher dispersibility of CNT in Al matrix. It was also seen that the post-SPS repeated hot rolling can highly improve dispersibility of CNT as well as mechanical properties such as hardness, tensile strength and deformability of the composites.

Structural modeling and mechanical research of carbon nanotube reinforced aluminum matrix composites using strain gradient theory

Carbon nanotube (CNT)-reinforced aluminum alloys (Al) are considered to be the ideal metal-matrix composites owing to their high strength and elastic moduli. However, they have low ductility. In addition, the inhomogeneity of the structure makes a strong strain gradient during deformation. In this research, a series of simulations under uniaxial tension and shear loading paths were conducted to investigate the mechanical properties of CNT/Al6061 composite. The novel multiple mechanism-based strain gradient models (MMSG) were proposed to study and analyze the correlation between microstructure evolution and macro mechanical responses. The interfacial bonding, grain size, and dislocation density were mainly considered during the simulation. The results of this study can be used to reveal the synergetic strength and ductility of all the structural microstructure, which also affects the strain partition and failure of different phases in CNT/Al6061 composites.

Effects of solid-state interfacial reaction on the mechanical properties of carbon nanotubes reinforced aluminum matrix composites with heterogeneous structure

To overcome strength-ductility trade-off in carbon nanotubes (CNTs) reinforced Al matrix composites, we developed a novel and controllable strategy via a combination of different length of CNTs and fine/coarse-grained matrices. Microstructure analysis showed that Al4C3 nanorods about 100 nm were distributed in the fine grain zone (FGZ), while CNTs of micron length were distributed in the coarse grain zone (CGZ). Different sizes of reinforcements play different strengthening roles, and the bimodal grains can effectively coordinate deformation, thereby achieving a good combination of strength and ductility. Heat treatment does not affect the microstructure of the FGZ, but significantly affect the CNT/Al interface bonding in the CGZ. Heat treatment at an appropriate temperature can further improve the strength of the composite by improving the interface bonding. When the heat treatment temperature is too high, the coarsening of Al4C3 in CGZ will deteriorate the mechanical properties of the composite.

Effects of Solid-State Interfacial Reaction on the Mechanical Properties of Carbon Nanotubes Reinforced Aluminum Matrix Composites with Heterogeneous Structure

Effects of Solid-State Interfacial Reaction on the Mechanical Properties of Carbon Nanotubes Reinforced Aluminum Matrix Composites with Heterogeneous Structure

Improving the mechanical properties of carbon nanotubes reinforced aluminum matrix composites by heterogeneous structural design

A novel aluminum matrix composites with different lengths of CNTs and heterogeneous structural were prepared in this paper. The synergistic effect of reinforcements with different lengths improves the mechanical properties of composites. Meanwhile, the heterogeneous structure design introduced fiber-like reinforcement to enhance the strength and toughness of composite.

Exploiting the synergic strengthening effects of stacking faults in carbon nanotubes reinforced aluminum matrix composites for enhanced mechanical properties

Carbon nanotubes (CNTs) reinforced Al matrix composites containing high-density stacking faults were successfully fabricated through spark plasma sintering (SPS) and subsequently hot rolling with high strain rate. It is found that the inclusion of CNTs facilitate the formation of stacking fault in Al matrix. With increasing the CNTs from 0.5 vol% to 1.0 vol%, the density of stacking fault increases, which is mainly because of the reduced stacking fault energy of Al matrix in Al/CNTs composites, as revealed by the density functional theory simulation. Combined with the effective exploitation of the strengthening effectiveness of CNTs due to uniform dispersion of CNTs and well bonded Al-CNTs interface, the highest tensile strength (378 ± 8 MPa) together with good ductility (17.1 ± 1.5%) can be achieved for 1 vol% CNTs reinforced Al matrix composite. Moreover, the enhanced strength deriving from each strengthening mechanism relies on the CNTs content. When the CNTs content is 1.0 vol%, Orowan looping and stacking fault strengthening are the dominant strengthening mechanisms. The reported microstructure design and control can be informative for improving the mechanical properties of CNTs reinforced metal matrix composites.

Microscopic stresses in carbon nanotube reinforced aluminum matrix composites determined by in-situ neutron diffraction

One of the most desired strengthening mechanisms in the carbon nanotube reinforced aluminum matrix composites (CNT/Al) composites is the load transfer strengthening mechanism (LTSM). However, a fundamental issue concerning the LTSM is that quantitative measurements of load partitioning in these composites during loading are very limited. In this study, in-situ neutron diffraction study on the tensile deformation of the 3 vol.% CNT/2009Al composite and the unreinforced 2009Al alloy was conducted. The {311} and {220} diffraction elastic constants (DECs) of the 2009Al alloy were determined. Using those DECs the average stress in the 2009Al matrix of the composite was calculated. Then the average stress in the CNTs was separated by using the stress equilibrium condition. Computational homogenization models were also applied to explain the stress evolution in each phase. Predicted results agree with experimental data. In the present case, the average stress in the CNTs reaches 1630 MPa at the yield strength of the composite based on linear regression of the measured data, which leads to an increment of yield strength by about 37 MPa. As the result of this work, an approach to quantify load partitioning in the CNTs is developed for the CNT/Al composites, which can be applied to optimize the mechanical properties of the composites.

Carbon nanotubes reinforced aluminum matrix composites with high elongation prepared by flake powder metallurgy

High elongation aluminum matrix composites reinforced with carbon nanotubes (CNTs) were prepared by flake powder metallurgy, and densified by hot extrusion with a high extrusion ratio. After hot extrusion, fine and equiaxed grains were obtained in 2 wt% CNT/Al composite, and interfacial product (Al4C3) as well as bridge-connected CNTs aligned along the extrusion direction were found both in 1 wt% CNT/Al and 2 wt% CNT/Al composites. Both composites obtained high elongation while their strength was enhanced. Especially the elongation of 2 wt% CNT/Al composite is comparable to that of pure Al prepared by the same method, which reached about 30%. The good ductility mainly attributes to firm interfacial combination between CNTs and matrix, fine and equiaxed grains formed by dynamic recrystallization during hot extrusion, and bridge-connected CNTs aligned along the extrusion direction.

Effect of Milling Time on Dry Sliding Wear Behaviors of Carbon Nanotubes Reinforced Al Matrix Composites.

In this study, the effect of mechanical milling time on microstructure, hardness and dry sliding wear behaviors of carbon nanotube reinforced aluminum matrix composites was investigated.Multi-walled carbon nanotubes (amount of 1%) were added to gas atomized AA7075 alloy as a reinforcement material and it was mechanical milled five different times. Milled Al-CNTs composite powders were added in a steel mold and pre-formed by cold pressing and hot pressing. Pre-formed composites samples were sintered. Microstructures of produced samples were examined by Scanning electron microscope. Density and hardness values were measured 1 ms-1 sliding speed, 30 N loads and five different sliding distances were used in dry sliding wear tests on pin on disc type wear apparatus. As a result of the studies, it was found that the particle size increased as the milling time increased. Powders milled for 100 min had the largest particle size, whereas powders milled for 120 min had a smaller particle size. The results indicated that composites mechanically milled for 120 min had both the highest hardness value and the lowest weight loss.

Forming Mechanism and Mechanical Properties of Carbon Nanotube Reinforced Aluminum Matrix Composites by Selective Laser Melting

Forming Mechanism and Mechanical Properties of Carbon Nanotube Reinforced Aluminum Matrix Composites by Selective Laser Melting

High-speed cryogenic machining of the carbon nanotube reinforced nanocomposites: Finite element analysis and simulation

In this paper, a new approach for evaluating the cryogenic machining process of the carbon nanotube reinforced aluminum matrix composites is developed based on finite element method. Finite element modeling in commercial code ABAQUS/Explicit was used to simulate high-speed machining of carbon nanotube reinforced composites under dry and cryogenic conditions, where different parameters (carbon nanotubes loading and the cutting speed) were investigated. The matrix phases are given a Johnson–Cook failure criterion. For considering more realistic assumptions, mechanical and thermal properties of the materials are assumed as a function of temperature. Results shown that at the cutting velocity of 60 m/s, cryogenic cooling has caused decrease of workpiece plastic strain by 12% in comparison with the dry cooling. The model can be used to study the effect of weight fraction, orientation, and length of the carbon nanotubes on the manufacturing of the nanocomposites.

Carbon nanotube reinforced aluminum matrix composites produced by spark plasma sintering

Spark plasma sintering (SPS) has been recognized, in the recent past, as a very useful method to produce metal matrix composites with enhanced mechanical and wear properties. Obviously, the materials final properties are strongly related to the reinforcement types and percentages as well as to the processing parameters employed during synthesis. The present paper analyses the effect of 0.5 and 1% of carbon nanotubes (CNTs) addition on the mechanical and microstructural behavior of Al-based metal matrix composites produced via SPS. The results show that the carbon nanotubes addition results in an increase in porosity and an increase in strength with respect to pure SPSed aluminum.

Microstructure and mechanical properties of carbon nanotubes reinforced aluminum matrix composites synthesized via equal-channel angular pressing

In this work, 2vol% carbon nanotubes (CNTs) reinforced aluminum (Al) matrix composites of superior microstructural homogeneity are successfully synthesized using Bc equal-channel angular extrusion (ECAP) route. The key step in arriving at high level of homogeneous distribution of CNTs within Al was preparation of the powder using simultaneous attrition milling and ultra-sonication processes. Microstructure as revealed by electron microscopy and absence of Vickers hardness gradients across the material demonstrate that the material reached the homogeneous state in terms of CNT distribution, porosity distribution, and grain structure after eight ECAP passes. To facilitate comparison of microstructure and hardness, samples of Al were processed under the same ECAP conditions. Significantly, the composite containing only 2vol% exhibits 20% increase in hardness relative to the Al samples.

In-situ characterization of interfacial shear strength in multi-walled carbon nanotube reinforced aluminum matrix composites

Quantitative characterization of interfacial shear strength (IFSS), in multi-walled carbon nanotube (MWCNT)-reinforced Al matrix composites, has been carried out using an in-situ pull-out technique. The end of an MWCNT protruding out of the tensile fracture surface of the composite was bonded to the tip of an atomic force microscopy (AFM) cantilever using an electron-beam-induced deposition method. The MWCNT was pulled out from the Al matrix using a nanomanipulator system installed inside a scanning electron microscope (SEM). The effective embedded length of the MWCNT incorporated in the Al matrix was evaluated by observing the pulled out MWCNT under high-resolution transmission electron microscopy (HRTEM). Valid results were obtained for six MWCNTs with straight, visibly damage-free embedded parts nearly perpendicular to the fracture surface. The IFSS between the MWCNT and the Al matrix was 24.8 ± 3.2 MPa. The ultimate tensile strength (UTS) of the MWCNT/Al composites is in good agreement with that estimated by the shear lag model using these IFSS values, resulting in the realization of 60% load transfer efficiency at the directly contacted MWCNT-Al interface in the MWCNT/Al composites. No MWCNT failure was observed during the pullout in this study.

Interfacial characterization in carbon nanotube reinforced aluminum matrix composites

In this work, the effects of the sintering parameters, such as temperature and the techniques used (HP and SPS), on CNT/Al composite interfaces are studied. The major role of the native aluminum oxide (Al2O3) layer covering the aluminum grains is highlighted. It is shown that, for a sintering temperature below 620°C, the amorphous Al2O3 layer prevents the reaction between aluminum and carbon. For greater sintering temperatures, the breaking of the oxide layer due to its crystallization leads to the formation of aluminum carbide (Al4C3) by reaction between aluminum and the CNT. The Al4C3 crystals grow perpendicularly to the matrix grain boundaries by thermally activated diffusion of the carbon atoms coming from the CNT. It is also demonstrated that, by limiting the sintering time, which is the case in SPS, it is possible to limit the growth of the Al4C3 crystals and thus to preserve the CNT.

FEA evaluation of the Al4C3 formation effect on the Young’s modulus of carbon nanotube reinforced aluminum matrix composites

The effect on Young’s modulus of Al4C3 formation in the carbon nanotube (CNT)–Al interface, for Al matrix composites, was analyzed using Finite Elements Analysis (FEA) models, incorporating different CNT volume fractions and interfacial Al4C3 layers of various thicknesses. Reinforcements were modeled with and without the hemispherical end caps that characterize CNT. FEA estimations were compared to the Rule of Mixtures predictions. Results showed that the elastic modulus of the composites significantly rises as the CNT volume fraction increases, while the increase in the Al4C3 layer thickness provokes a slight increment in the Young’s modulus. This could be attributed to a combined reinforcement effect of the CNT and the interface. The inclusion of CNT with hemispherical end caps in the FEA model drops the estimated Young’s modulus. Finally, the results presented in this work demonstrated the importance of the model selection for analyzing the mechanical behavior of the composites.

Computational structural modeling and mechanical behavior of carbon nanotube reinforced aluminum matrix composites

Due to their remarkable mechanical properties, carbon nanotube (CNT) reinforced aluminum (Al) matrix composites have attracted a wide range of research interests. This work attempts to experimentally and numerically investigate the relationship between the micro-structures and mechanical behavior of CNT/Al composites. Three-dimensional (3D) computational structural modeling of CNT/Al composites is performed, in which the size, morphology, orientation, location and volume fraction of CNTs are reproduced to be similar to those of the actual micro-structures of CNT/Al composites. The strengthening of the mechanical properties of the constituent materials of CNT/Al composites and reasonable load and boundary conditions are studied based on the models of CNT/Al composites developed. The tensile mechanical behavior of CNT/Al composites is numerically evaluated and experimentally verified. Results show that the enhanced mechanical properties of CNT/Al composites can be attributed to three factors: CNT reinforcements, matrix grain refinement and layered architectures. Through the microscopic structural modeling methods presented herein, the effects of model size, interfacial behavior, volume fraction of CNTs and layered structures on the mechanical behaviors of CNT/Al composites can be reproduced to understand the strengthening and deformation mechanisms of CNT/Al composites.

Consolidation of Carbon Nanotube Reinforced Aluminum Matrix Composites by High-Pressure Torsion

Al-3 vol pct carbon nanotube (CNT) composites are fabricated by consolidation through high-pressure torsion (HPT) at room temperature. The densification behavior, microstructural evolution, and mechanical properties of Al/CNT composites are studied. The results show that density and microstructural homogeneity increase with increasing number of revolutions under a high pressure of 6 GPa. Substantial grain refinement is achieved after 10 turns of HPT with an average grain thickness of ~38 nm perpendicular to the compression axis of HPT. The Al/CNT composite shows a considerable increase in hardness and strength compared to the Al matrix. The strengthening mechanisms of the Al/CNT composite are found to be (i) grain refinement of Al matrix and (ii) Orowan looping. Raman spectroscopy and high-resolution transmission electron microscopy reveal that the structure of most of CNTs is changed during processing through mechanical milling and HPT.

Tensile Strength and Electrical Conductivity of Carbon Nanotube Reinforced Aluminum Matrix Composites Fabricated by Powder Metallurgy Combined with Friction Stir Processing

A route combining powder metallurgy and subsequent friction stir processing was utilized to fabricate carbon nanotube (CNT) reinforced Al (CNT/Al) and 6061Al (CNT/6061Al) composites. Microstructural observations indicated that CNTs were uniformly dispersed in the matrix in both CNT/Al and CNT/6061Al composites. Mg and Si elements tended to segregate at CNT–Al interfaces in the CNT/6061Al composite during artificial aging treatment. The tensile properties of both the Al and 6061Al were increased by CNT incorporation. The electrical conductivity of CNT/Al was decreased by CNT addition, while CNT/6061Al exhibited an increase in electrical conductivity due to the Mg and Si segregation.