Interfacial analysis between Mg matrix and carbon nanotubes in Mg-6 wt.% Al alloy matrix composites reinforced with carbon nanotubes

Magnesium containing 6 wt.% aluminum alloy composites reinforced with carbon nanotubes were fabricated with powder metallurgy based wet-processing. Yield stress and tensile strength were successfully improved by the addition of carbon nanotubes. Field emission-transmission electron microscopy microstructural analysis clarified that needle-like ternary carbides of Al₂MgC₂ were synthesized at some interfaces between magnesium matrix and carbon nanotubes, and the other interfaces were clean without any other materials or defects. Tensile loading transfer from magnesium matrix to carbon nanotubes was effectively strengthened by both the production of Al₂MgC₂ compounds and the clean interface between magnesium matrix and carbon nanotubes.     

Enhancing tensile and compressive strength of magnesium using ball milled Al+CNT reinforcement

In this work, Mg/Al–CNT nano-composites were fabricated using powder metallurgy route involving microwave assisted rapid sintering and hot extrusion. Ball milled Al–CNT particles comprising different contents of CNTs coated with fixed amount of Al were used for strengthening. Microstructural characterization of these Mg/Al–CNT nano-composites reveal reasonably uniform distribution of Al–CNT particles up to CNT content of 0.30% by weight, significant grain refinement and the presence of minimal porosity compared to monolithic Mg. Importantly, for the nominally identical processing conditions, the textures of as-extruded nano-composite specimens is significantly influenced by the presence of Al–CNT particles. Nano-composite configurations exhibit different tensile and compressive response as a function of CNT content. Among the different Mg/Al–CNT formulations synthesized, the Mg/Al–CNT configuration with Al–CNT particles composition of 1.00% Al and 0.30% CNT by weight (Mg/1.00Al–0.30CNT) exhibit higher tensile yield strength (0.2% YS), ultimate tensile strength (UTS) and failure strain (FS) (up to +72%, +48%, +9%, respectively) compared to monolithic Mg.     

Potential and challenges of metal-matrix-composites reinforced with carbon nanofibers and carbon nanotubes

With a continuous improvement of the production techniques for carbon nanofibers and carbon nanotubes along with an improvement of the available qualities of the materials, these reinforcements have been introduced into polymers, ceramics and metals. While in the field of polymers first success stories have been published on carbon nanofiller reinforcements, up to now metals containing these types of nanofillers are still a topic of intensive research. Basically a similar situation were found in those days, when micron sized carbon fibers came on the market. Today many applications of carbon fiber reinforced composites are existing, while metals reinforced with conventional carbon fibers are still only found in niche applications.     

A magnesium alloy matrix composite reinforced with metallic glass

Novel light-weight materials of advanced performance are now experiencing global interest due to the strong need to reduce energy consumption in land and air transportation sectors. Here we report on a novel magnesium alloy matrix composite material. The reinforcing phase in the magnesium alloy is a fine dispersion of metallic glass particles. The composite is sintered from the powder mixture of the alloy and metallic glass at a temperature slightly above the glass transition T_g of the metallic glass particles that is close to the Mg alloy’s solidus temperature. At the compaction temperature, the metallic glass acts as a soft liquid-like binder but upon cooling it becomes the hard reinforcement component of the composite. Processing, microstructure and mechanical properties of the composite are discussed.     

Effect of strain rate and ball milling of reinforcement on the compressive response of magnesium composites

Effect of strain rate change and reinforcement ball milling on the compressive response of Mg composites is investigated in this work. Quasi-static response was determined using a servo hydraulic MTS machine while dynamic response was assessed by Split Hopkinson Pressure Bar. The presence of either as-received or ball milled Al particles significantly assisted in improving compressive response of Mg in both regimes, compared to monolithic Mg. In the quasi-static regime, the Mg/1.626Al composite containing ball milled Al particles exhibits significantly higher compressive yield strength, ultimate compressive strength and work of fracture of (+76, +87% and +58%) compared to monolithic Mg. However, with a fixed amount of Al, composites containing ball milled particles show a higher strength compared to composites containing as-received particles. Results also revealed that the tremendous increase in strain rate led to an increase in flow stress of all synthesized material while the failure strain was marginally compromised.     

CNT reinforced light metal composites produced by melt stirring and by high pressure die casting

Light metal matrix composites are of great interest due to their potential for reducing CO₂ emission through lightweight design e.g. in the automotive sector. Carbon nanotubes can be considered as ideal reinforcements, due to their high strength, high aspect ratio and thermo-mechanic properties. In this research, CNT reinforced light metal composites were produced by melt stirring and by high pressure die casting, which can be both easily scaled up. The light metal composites showed significantly improved mechanical properties already at small CNT contents. The influence of CNT concentration on the composites was also studied.     

Mechanical and tribological properties of self-lubricating metal matrix nanocomposites reinforced by carbon nanotubes (CNTs) and graphene – A review

Rapid innovation in nanotechnology in recent years enabled development of advanced metal matrix nanocomposites for structural engineering and functional devices. Carbonous materials, such as graphite, carbon nanotubes (CNT's), and graphene possess unique electrical, mechanical, and thermal properties. Owe to their lubricious nature, these carbonous materials have attracted researchers to synthesize lightweight self-lubricating metal matrix nanocomposites with superior mechanical and tribological properties for several applications in automotive and aerospace industries. This review focuses on the recent development in mechanical and tribological behavior of self-lubricating metallic nanocomposites reinforced by carbonous nanomaterials such as CNT and graphene. The review includes development of self-lubricating nanocomposites, related issues in their processing, their characterization, and investigation of their tribological behavior. The results reveal that adding CNT and graphene to metals decreases both coefficient of friction and wear rate as well as increases the tensile strength. The mechanisms involved for the improved mechanical and tribological behavior is discussed.     

Enhanced strength and excellent transport properties of a superaligned carbon nanotubes reinforced copper matrix laminar composite

Superaligned carbon nanotube (SACNT) reinforced copper matrix laminar composites have been fabricated by means of the traditional copper sulfate electroplating process. The mechanical properties and transport properties of the Cu/SACNT composites with different SACNT content have been studied systematically, and the experimental results show that the as-prepared composites possess a better comprehensive performance than pure copper. The simple rule of mixtures (ROM) has been used to estimate the potential maximum properties of the Cu/SACNT composites. The Cu/SACNT composite is considered to be a promising material for electronics and communications applications.     

Investigation of carbon nanotube reinforced aluminum matrix composite materials

We have increased the tensile strength without compromising the elongation of aluminum (Al)–carbon nanotube (CNT) composite by a combination of spark plasma sintering followed by hot-extrusion processes. From the microstructural viewpoint, the average thickness of the boundary layer with relatively low CNT incorporation has been observed by optical, field-emission scanning electron, and high-resolution transmission electron microscopies. Significantly, the Al–CNT composite showed no decrease in elongation despite highly enhanced tensile strength compared to that of pure Al. We believe that the presence of CNTs in the boundary layer affects the mechanical properties, which leads to well-aligned CNTs in the extrusion direction as well as effective stress transfer between the Al matrix and the CNTs due to the generation of aluminum carbide.     

Grain size effect on the strengthening behavior of aluminum-based composites containing multi-walled carbon nanotubes

Strengthening efficiency of multi-walled carbon nanotubes (MWCNTs) is investigated for aluminum-based composites with grain sizes ranging from ∼250 to ∼65 nm. The strength of composites is significantly enhanced proportional to an increase of the MWCNT volume. However, the increment differs depending on deformation mode of the matrix. The strengthening efficiency of MWCNTs in ultrafine-grained composites is comparable with that predicted by the discontinuous fiber model, whereas the efficiency becomes half of the theoretical prediction as grain size is reduced below ∼70 nm. For nano-grained aluminum, activities of forest dislocations diminish and dislocations emitted from grain boundaries are dynamically annihilated during the recovery process, providing a weak plastic strain field around MWCNTs. The observation may provide a basic understanding of the strengthening behavior of nano-grained metal matrix composites.     

High shear dispersion technology prior to twin roll casting for high performance magnesium/SiCp metal matrix composite strip fabrication

SiC particulate (SiC_p) reinforced AZ31 magnesium alloy composite strips were produced by a novel process. In the process, a high shear technique was utilised to disperse the reinforcing particles uniformly into the matrix alloy, and AZ31/5 vol%SiC_p slurry was solidified into thin strip by a horizontal twin roll caster. The experimental results showed that the AZ31/5 vol%SiC_p strip obtained with high shear treatment exhibited a significantly refined microstructure and uniform distribution of reinforcing SiC particles. High cooling rate in the TRC process was also considered to contribute to the grain refinement of the matrix alloy, together with the possible heterogeneous nucleation effect of the reinforcing particles. The mechanical properties of the high shear treated composites strips showed enhanced modulus, yield strength and ductility by hardness and tensile tests. The experimental results were discussed in terms of the microstructural features and the macroscopic reliability, where necessary, analytical and statistical analyses were conducted.     

Kinetic analysis on Al2O3/Cu composite prepared by mechanical activation and internal oxidation

The Al₂O₃/Cu composite was prepared by mechanical activation and internal oxidation process. Kinetic factors, which influenced the internal oxidation process, were also discussed in the present paper. The results showed that the duration of the internal oxidation was highly shortened after the powders were treated by mechanical activation. The typical internal oxidation duration was only 1 h. It is unnecessary for further prolonging internal oxidation time. Besides, in order to get a complete internal oxidation heating rate and green density of the compact are another two control factors. Their parameters should be controlled from Al contents in the powders. In this research, the optimum internal oxidation duration, heating rate and green density are 1 h, 20 °C/min and 80%, respectively, for Cu–0.8 wt% Al/Cu₂O powders. After the internal oxidation, uniform spherical -Al₂O₃ particles with an average size 0.5–0.8 μm in diameter were observed in the Cu matrix.     

Assessment of tensile behaviour of an Al–Mg alloy composite reinforced with NiAl and oxidized NiAl powder particles helped by nanoindentation

AA5056 matrix composites have been reinforced with as-received and oxidized NiAl particles and their nanohardness investigated as a function of distance to reinforcement. Results indicate that a non-heat treatable aluminium matrix, as is the present case, does not require that the intermetallic particles are surrounding by a protective Al₂O₃ layer to avoid reactions at matrix-reinforcement interfaces. On the other hand, the quality of the matrix-reinforcement bonding has been quantified by the reinforcement influence distance, defined as the distance from the particle at which the nanohardness of the matrix drops to its asymptotic value.     

Size control of in situ formed reinforcement in metal melts-theoretical treatment and application to in situ (AlN + Mg2Si)/Mg composites

An approach of selecting alloying elements to control the size of in situ formed reinforcement in metal melts is proposed. This approach is based on the effect of alloying addition on nucleation of reinforcement predicted by the extended Miedema model and Wilson equation and on growth of reinforcement predicted by diffusion coefficient model. Using this approach, the effect of alloying element addition on the size of in situ formed Mg₂Si in (AlN + Mg₂Si)/Mg composites is evaluated. The results show that the addition of Ti, V, Mo, Cr, Fe and Mn can delay the nucleation and promote the growth of Mg₂Si, while the addition of Ge, Sn, Cu and Zn can promote the nucleation and hinder the growth of Mg₂Si. The microstructure of (AlN + Mg₂Si)/Mg with the addition of Ti has been investigated based on the theoretical prediction.     

Production of high strength Al-Al2O3 composite by accumulative roll bonding

Recently accumulative roll bonding has been used as a novel method to produce particle reinforced metal matrix composites. In this study, aluminum matrix composite reinforced by submicron particulate alumina was successfully produced and the effects of number of ARB cycles and the amount of alumina content on the microstructure and mechanical properties of composites were investigated. According to the results of tensile tests, it is shown that the yield and tensile strengths of the composite are increased with the number of ARB cycles. Scanning electron microscopy (SEM) reveals that particles have a random and uniform distribution in the matrix by the ARB cycles and a strong mechanical bonding takes place at the interface of particle-matrix. It is also found that the tensile strength of the composite, as a function of alumina content, has a maximum value at 2 vol.%, which is 5.1 times higher than that of the annealed aluminum.     

Effects of CNT diameter on mechanical properties of aligned CNT sheets and composites

Drawing, winding, and pressing techniques were used to produce horizontally aligned carbon nanotube (CNT) sheets from free-standing vertically aligned CNT arrays. The aligned CNT sheets were used to develop aligned CNT/epoxy composites through hot-melt prepreg processing with a vacuum-assisted system. Effects of CNT diameter change on the mechanical properties of aligned CNT sheets and their composites were examined. The reduction of the CNT diameter considerably increased the mechanical properties of the aligned CNT sheets and their composites. The decrease of the CNT diameter along with pressing CNT sheets drastically enhanced the mechanical properties of the CNT sheets and CNT/epoxy composites. Raman spectra measurements showed improvement of the CNT alignment in the pressed CNT/epoxy composites. Research results suggest that aligned CNT/epoxy composites with high strength and stiffness are producible using aligned CNT sheets with smaller-diameter CNTs.     

In situ polymerized nanocomposites: Polystyrene/CNT and Poly(methyl methacrylate)/CNT composites

Carbon nanotubes (CNTs) were incorporated into polystyrene (PS) and poly(methyl methacrylate) (PMMA) matrices via in situ emulsion and emulsion/suspension polymerization methods. The polymerizations were carried out using various initiators, surfactants, and carbon nanotubes to determine their influence on polymerization and on the properties of the composites. The loading of CNTs in the composites varied from 0 to 15 wt.%, depending on the CNTs used. Morphology and dispersion of the CNTs were analyzed by transmission and scanning electron microscopy techniques. The dispersion of multi-walled carbon nanotubes (MWCNT) in the composites was excellent, even at high CNT loading. The mechanical properties, and electrical and thermal conductivities, of the composites were also analyzed. Both electrical and thermal conductivities were improved.     

Fabrication and characterization of Al-matrix composites reinforced with amino-functionalized carbon nanotubes

We report the fabrication of Al-matrix composites reinforced with amino-functionalized carbon nanotubes (fCNTs) using powder metallurgy process. Functionalization of the nanotubes was carried out by ball milling multiwalled carbon nanotubes (MWCNTs) in the presence of ammonium bicarbonate. It has been found that the mechanical properties of Al-fCNT composites were much superior to the composites fabricated using non-functionalized or acid functionalized carbon nanotubes. The enhancement in mechanical properties in these composites are attributed mainly to the better and homogeneous dispersion of fCNT in Al matrix as compared to non-functionalized or acid functionalized carbon nanotubes and the formation of a strong interfacial bonding between fCNT and Al matrix leading to an efficient load transfer from Al matrix to fCNT following high-resolution transmission electron microscopy.     

Fabrication of aluminum matrix composites with enhanced mechanical properties reinforced by in situ generated MgAl2O4 whiskers

Aluminum matrix composite reinforced by in situ generated single crystalline MgAl₂O₄ whiskers was fabricated by chemical synthesis method in an Al-Mg-H₃BO₃ system. A large number of MgAl₂O₄ whiskers were generated during the sintering process and distributed homogeneously in the Al matrix. The whiskers penetrate into the matrix grains to form the framework of the materials, leading to an incredible increase in mechanical properties of the composites. The generation mechanism of the MgAl₂O₄ whiskers was also discussed.     

Interface tailoring to enhance mechanical properties of carbon nanotube reinforced magnesium composites

This study highlights the use of a metallic coating of nanoscale thickness on carbon nanotube to enhance the interfacial characteristics in carbon nanotube reinforced magnesium (Mg) composites. Comparisons between two reinforcements were targeted: (a) pristine carbon nanotubes (CNTs) and (b) nickel-coated carbon nanotubes (Ni–CNTs). It is demonstrated that clustering adversely affects the bonding of pristine CNTs with Mg particles. However, the presence of nickel coating on the CNT results in the formation of Mg₂Ni intermetallics at the interface which improved the adhesion between Mg/Ni–CNT particulates. The presence of grain size refinement and improved dispersion of the Ni–CNT reinforcements in the Mg matrix were also observed. These result in simultaneous enhancements of the micro-hardness, ultimate tensile strength and 0.2% yield strength by 41%, 39% and 64% respectively for the Mg/Ni–CNT composites in comparison with that of the monolithic Mg.