Microstructure evolution during fabrication and microstructure–property relationships in vapour-grown carbon nanofibre-reinforced aluminium matrix composites fabricated via powder metallurgy

Microstructure evolution of vapour-grown carbon nanofibre (VGCF)-reinforced aluminium matrix composites during fabrication and microstructure–property relationships of these materials were studied. Composites were fabricated using powder metallurgy, i.e. by mixing VGCFs and aluminium powder via ball-milling followed by sintering or hot extrusion. The mixing condition was selected to achieve small powder particle size and homogeneously dispersed VGCFs. Aluminium grains and VGCFs were elongated along the longitudinal direction of aluminium particles in the mixed powder. Detailed observation of the aluminium grains in the composites found grain size and morphology dominated by recrystallization. Apparently, grain growth was inhibited by VGCFs. Theoretical models considering strength increment due to grain refinement resulting from VGCF addition, load bearing of VGCFs, thermal mismatch of VGCFs and aluminium and Orowan effect were developed. Theoretical values coincided well with hardness, yield strength, and ultimate tensile strength of the composites, and thus the models could precisely explain the microstructure–property relationships.     

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.     

Microwave assisted-in situ synthesis of porous titanium/calcium phosphate composites and their in vitro apatite-forming capability

Microwave irradiation has been proven to be an effective heating source in synthetic chemistry, and can accelerate the reaction rate, provide more uniform heating and help in developing better synthetic routes for the fabrication of bone-grafting implant materials. In this study, a new technique, which comprises microwave heating and powder metallurgy for in situ synthesis of Ti/CaP composites by using Ti powders, calcium carbonate (CaCO₃) powders and dicalcium phosphate dihydrate (CaHPO₄·2H₂O) powders, has been developed. Three different compositions of Ti:CaCO₃:CaHPO₄·2H₂O powdered mixture were employed to investigate the effect of the starting atomic ratio of the CaCO₃ to CaHPO₄·2H₂O on the phase, microstructural formation and compressive properties of the microwave synthesized composites. When the starting atomic ratio reaches 1.67, composites containing mainly alpha-titanium (α-Ti), hydroxyapatite (HA), beta-tricalcium phosphate (β-TCP) and calcium titanate (CaTiO₃) with porosity of 26%, pore size up to 152 μm, compressive strength of 212 MPa and compressive modulus of 12 GPa were formed. The in vitro apatite-forming capability of the composite was evaluated by immersing the composite into a simulated body fluid (SBF) for up to 14 days. The results showed that biodissolution occurred, followed by apatite precipitation after immersion in the SBF, suggesting that the composites are suitable for bone implant applications as apatite is an essential intermediate layer for bone cells attachment. The quantity and size of the apatite globules increased over the immersion time. After 14 days of immersion, the composite surface was fully covered by an apatite layer with a Ca/P atomic ratio approximately of 1.68, which is similar to the bone-like apatite appearing in human hard tissue. The results suggested that the microwave assisted-in situ synthesis technique can be used as an alternative to traditional powder metallurgy for the fabrication of Ti/CaP biocomposites.     

Studies on wear rate and micro-hardness of the Al/Al2O3/Gr hybrid composites produced via powder metallurgy

Micro-structural characterization of the composites has revealed fairly uniform distribution and some amount of grain refinement in the specimens. Further, it was observed that the micro-hardness improve when increasing the milling time and the reinforcement content due to presence of hard Al₂O₃ particles. Was also observed a low wear rate exhibited by the Al/Al₂O₃/Gr hybrid composites due to presence of Al₂O₃ and Gr which they acted as load bearing elements and solid lubricant respectively. The observed wear rate and micro-hardness have been correlated with microstructural analyses.     

Load-bearing contribution of multi-walled carbon nanotubes on tensile response of aluminum

We fabricated a uniformly dispersed and aligned multi-walled carbon nanotube reinforced aluminum matrix (Al–MWCNT) composite with minimal work hardening and without interfacial chemical compounds. In this paper, the direct load-bearing contribution of MWCNTs on the Al–MWCNT composite was investigated in detail for various volume fractions of MWCNTs. For up to 0.6 vol% of MWCNTs, the ultimate tensile strength (UTS) of the Al–MWCNT composite increased with the conservation of the remarkable failure elongation of Al. These UTS values are consistent with shear lag model. We also observed an uncommon multi-wall-type failure of MWCNTs during the hot extrusion process. However, owing to the agglomeration of MWCNTs in the Al matrix, the UTS deviated significantly from the shear lag model and the remarkable failure elongation of Al decreased. The possibility of strengthening, without degrading ductility, was demonstrated by exploiting directly the load-bearing ability of individually and uniformly dispersed aligned MWCNTs.     

Processing,microstructure and properties of ultrasonically processed in situ MgO–Al2O3–MgAl2O4 dispersed magnesium alloy composites

AZ91 alloy matrix composites are synthesized by in situ reactive formation of hard MgO and Al₂O₃ particles from the addition of magnesium nitrate to the molten alloy. The evolved oxygen from decomposition of magnesium nitrate reacts with molten magnesium to form magnesium oxide and with aluminium to form aluminium oxide. Additionally, these newly formed oxides react with each other to form MgAl₂O₄ spinel. Application of ultrasonic vibrations to the melt increased the uniformity of particle distribution, avoided agglomeration, and decreased porosity in the castings. Ultrasound induced physical phenomena such as cavitation and melt streaming promoted the in situ chemical reactions. Well dispersed, reactively formed hard oxides increased the hardness, ultimate strength, and strain-hardening exponent of the composites. Presence of well-dispersed hard oxide particles and stronger interface resulting from cavitation-enhanced wetting of reactively formed particles in the AZ91 alloy matrix improved the sliding wear resistance of the composites.     

Friction stir welding of thick plates of aluminum alloy matrix composite with a high volume fraction of ceramic reinforcement

The microstructure and mechanical properties of joints conducted by friction stir welding, FSW, at different rotational speeds in thick plates of a composite material with a high volume fraction of reinforcement, namely 2124Al/25vol%SiC_p, are studied. Original particle-free regions vanish during the stirring process, leading to a homogeneous particle distribution. Occasional breakage of some large particles occurs. Tunnel defects appear at low rpm, and disappear at high rotational speeds. The size of the thermo mechanically affected zone, TMAZ, increases with increasing rpm. Ductility of the welds in the range of 10–15% is achieved in compression tests whereas a rather brittle behavior is obtained in tension. A strength difference, SD, effect between compression and tensile test is obtained. This accounts for the little detrimental effect of the FSW process on the matrix–reinforcement interface. The SD effect is attributed to the presence of a microscopic residual stress.     

Mechanical properties of hot pressed SiCp and B4Cp/Alumix 123 composites alloyed with minor Zr

In the present study, effect of Zr addition on the microstructure and wear behavior of aluminum alloy composites (AMCs) reinforced with B4Cp and SiCp particles fabricated via hot pressing were investigated. The samples for the study composed of unreinforced aluminum alloy (Alumix 123) and the composites reinforced with 10% B4Cp and % SiCp were prepared by hot isostatic pressing (HIP) method. Similarly, all the samples alloyed with 0.2% Zr were also produced in order to make a comparison. The produced samples were evaluated for microstructural properties and mechanical tests for hardness, tensile and bending strength were performed. Wear test was carried out at 5 mm/s sliding speed under 3.0 N load for the all kind of hot pressed produced samples. The hot pressed composite microstructures have a more uniform distribution of the reinforcements. After HIP process, the composites were successfully produced with high density (>99%). The addition of Zr increased the yield and tensile strength of the samples. The highest strength value was found for the sample Al 123 matrix alloy with Zr. Evaluation of microstructures showed that copper and zirconium dispersed equally within the matrix microstructure without agglomeration. For the composite samples, Al3Zr, appeared as white precipitate, were inspected around B4C and SiC particles. The composite containing SiC particles and Zr had wear resistance value superior to those of the other counterparts.     

Investigation of microstructure and mechanical properties of nano MgO reinforced Al composites manufactured by stir casting and powder metallurgy methods: A comparative study

In the present work, Al–nano MgO composites using A356 aluminum alloy and MgO nanoparticles (1.5, 2.5, and 5 vol.%) have been fabricated via stir casting and powder metallurgy (PM) methods. Different processing temperatures of 800, 850, and 950 °C for stir casting and 575, 600, and 625 °C for powder metallurgy were considered. Powder metallurgy samples showed more porosity portions compare to the casting samples which results in higher density values of casting composites (close to the theoretical density) compare to the sintering samples. Introduction of MgO nanoparticles to the Al matrix caused increasing of the hardness values which was more considerable in casting samples. The highest hardness value for casting and sintering samples have been obtained at 850 and 625 °C respectively, in 5 vol.% of MgO. Compressive strength values of casting composites were higher than sintered samples which were majorly due to the more homogeneity of Al matrix, less porosity portions, and better wettability of MgO nanoparticles in casting method. The highest compressive strength values for casting and sintered composites have been obtained at 850 and 625 °C, respectively. Scanning electron microscopy images showed higher porosity portions in sintered composites and more agglomeration and aggregation of MgO nanoparticles in casting samples which was due to the fundamental difference of two methods. Generally, the optimum processing temperatures to achieve better mechanical properties were 625 and 850 °C for powder metallurgy and stir-casting, respectively. Moreover, casting method represented more homogeneous data and higher values of mechanical properties compare to the powder metallurgy method.     

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.     

High temperature thermal stability of pure copper and copper–carbon nanotube composites consolidated by High Pressure Torsion

The thermal stability of ultrafine-grained (UFG) microstructures in pure copper samples and copper–carbon nanotube (CNT) composites processed by High Pressure Torsion (HPT) was compared. The UFG microstructure in the sample consolidated from pure Cu powder exhibited better stability than that developed in a casted Cu specimen. The addition of CNTs to the Cu powder further increased the stability of the UFG microstructure in the consolidated Cu matrix by hindering recrystallization, however it also yielded a growing porosity and cracking during annealing. It was shown that the former effect was stronger than the latter one, therefore the addition of CNTs to Cu has an overall benefit to the hardness in the temperature range between 300 and 1000 K. A good agreement between the released heat measured during annealing and the calculated stored energy was found for all samples.     

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.     

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.     

Using integrated hybrid (Al + CNT) reinforcement to simultaneously enhance strength and ductility of magnesium

Magnesium nano-composites containing hybrid aluminium-carbon nanotube (Al-CNT) reinforcement were synthesized through powder metallurgy route using microwave assisted rapid sintering technique followed by hot extrusion. Compared to monolithic Mg, microstructural characterization revealed reasonably uniform distribution of Al-CNT particles in the matrix and reduction in average matrix grain size in the case of hybrid nano-composites. Compared to monolithic Mg, the Mg/Al-CNT nano-composites exhibited higher elastic modulus, strength and failure strain up to 1.00 wt.% Al content. The CNT content was kept constant at 0.18 wt.%. Among the different nano-composite formulations, the Mg/1.00Al-0.18CNT nano-composite exhibited the best improvement in elastic modulus (E), tensile yield strength (0.2%YS), ultimate tensile strength (UTS) and failure strain (up to +3.6%, +38%, +36% and +42%, respectively) compared to pure Mg. The effect of hybrid Al-CNT reinforcement integration on the enhancement of mechanical properties of Mg is investigated in this paper.     

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.     

Influence of nano-alumina and sub-micron copper on mechanical properties of magnesium alloy AZ31

Magnesium composites of AZ31–Al₂O₃–Cu formulations were produced using the disintegrated melt deposition technique following by hot extrusion. Microstructural characterization showed reasonable distribution of secondary phases up to 1 vol.% of copper. A tendency to form clustered agglomeration and longer shape of secondary phases was observed when the amount of copper was increased to 1.5 vol.%. Mechanical tests indicated remarkable improvements in 0.2%YS, UTS and microhardness when nano-alumina and sub-micron copper were added into AZ31. The ductility was increased up to 9.3% in the case of AZ31–1.5Al₂O₃–1.0Cu sample and significantly reduced (5.5%) when the amount of copper was increased to 1.5 vol.%. Heat treated sample of AZ31–1.5Al₂O₃–1.0Cu showed overall improvement in both tensile strength and ductility. The results suggest that the judicious selection of composition and heat treatment has the capability to enhance overall tensile response of Mg–Al₂O₃–Cu nanocomposites.     

Microstructure and grain growth of the matrix of SiCf/Ti–6Al–4V composites prepared by the consolidation of matrix-coated fibers in the β phase field

The microstructural characteristics in the matrix of SiC_f/Ti–6Al–4V composites prepared by consolidation of the matrix-coated fibers in the high-temperature β single-phase field were investigated using both experimental and modelling methods. Some of the critical microstructure features, like volume fraction of component phases, composition of matrix alloys and matrix morphology were systematically studied, providing valuable insight into the microstructural characteristics in the matrix of SiC_f/Ti–6Al–4V composites. In order to assist in understanding the grain growth occurred in the matrix during consolidation processing, a theoretical model was developed. Excellent agreement between theoretical and experimental results was achieved.     

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.     

Microstructure and fatigue strength of high-strength Cu–Fe and Cu–V in-situ nanocomposite wires

The results of the quantitative analysis of the microstructure of the Cu–Fe and Cu–V in-situ nanocomposite wires with diameter of 0.44–0.80 mm by transmission electron microscopy are presented. Comparative fatigue tests of Cu–Fe and Cu–V in-situ nanocomposite wires and pure copper samples have been carried out using a dynamic mechanical analyzer (DMA). The in-situ nanocomposites have significantly higher characteristics of low-cycle fatigue failure resistance as compared to that of pure copper. The fatigue crack propagation areas for the nanocomposite conductors and pure copper are characterized by fatigue striations and secondary cracking.     

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.