Fracture toughness of microsphere Al2O3–Al particulate metal matrix composites

The fracture toughness and behaviour of COMRAL-85^TM, a 6061 aluminium–magnesium–silicon alloy reinforced with 20 vol% Al₂O₃-based polycrystalline ceramic microspheres, and manufactured by a liquid metallurgy route, have been investigated. Fracture toughness tests were performed using short rod and short bar (chevron-notch) specimens machined from extruded 19 mm diameter rod, heat treated to the T6 condition. The fracture toughness in the R–L orientation was found to be lower than in the C–R or L–R orientations owing to the presence of particle-free bands in the extrusion direction. Short rod tests were also conducted for the R–L orientation on six powder metallurgy composites with particle volume fractions ranging between 5% and 30%. It was found that the fracture toughness decreased progressively with particle volume fraction, but at a decreasing rate. A detailed examination of the fracture behaviour was made for both the liquid metallurgy and powder metallurgy processed composites.     

Mechanical properties of Friction Stir Processed 2618/Al2O3/20p metal matrix composite

The effect of Friction Stir Processing (FSP) on the mechanical properties of 2618 aluminium alloy reinforced with 20% of alumina particles aluminium alloy has been studied in the present paper. The material was processed into the form of sheets of 7 mm thickness after T6 treatment and was tested in tension and fatigue at room temperature.

Tensile tests were also performed at higher temperatures and different strain rates in the nugget zone, in order to analyse the superplastic properties of the recrystallized material and to observe the differences with the parent materials as a function of the strong grain refinement due to the Friction Stir Process. The high temperature behaviour of the material was studied, in longitudinal direction, by means of tensile tests in the temperature and strain rate ranges of 400–500 °C and 10⁻³–10⁻¹ s⁻¹, respectively.

Fracture surfaces of the deformed fatigue test specimens were comprehensively examined in a scanning electron microscope equipped with field emission gun to determine the macroscopic fracture mode and characterize the fine-scale topography and microscopic mechanisms governing fatigue fracture.

The mechanisms governing fatigue life, cyclic deformation and fracture characteristics are analysed in function of magnitude of applied stress, intrinsic micro structural evolution and material deformation behaviour.     

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.     

Investigation on diffusion bonding characteristics of SiC particulate reinforced aluminium metal matrix composites (Al/SiCp-MMC)

Joining characteristics of SiC particulate reinforced aluminium metal matrix composites (Al/SiC_p-MMC) were investigated by vacuum diffusion bonding process. The joining performances of the similar and dissimilar composites were studied, and the influences of SiC_p volume percentage and the insert alloy layer on bonding quality and properties of the bonded joints were also estimated. The experimental results indicate that the strength of vacuum diffusion bonded joints decreases with increasing SiC_p volume percentage, and obtaining satisfactory bonding quality in the diffusion bonded joints of the dissimilar Al/SiC_p-MMC is much more difficult than that of the similar Al/SiC_p-MMC. Moreover, the results still manifest that the diffusion bonding either for the similar or for the dissimilar Al/SiC_p-MMC, the suitable insert alloy layer can improve evidently the joining quality of joints, and the strength of diffusion bonded joints corresponding to using the insert alloy layer is apparently higher than that of no insert layer.     

Investigation of mechanical and tribological behaviors of multilayer graphene reinforced Ni3Al matrix composites

Multilayer graphene (MLG) shows an attractive prospect for the demanding engineering applications. This paper reports the mechanical and tribological properties of MLG reinforced Ni₃Al matrix composites (NMCs) under dry sliding at varying sliding speed. The hardness and elastic modulus of the NMCs are significantly influenced with MLG content. It is found that the hardness and elastic modulus of the NMCs are found to be increased by increasing MLG content up to 1.0 wt.%, while decreased when MLG content is above 1.0 wt.%. Tribological experiments suggest that MLG can dramatically improve the wear resistance and decrease the friction coefficient of the NMCs. Such marked improvement of wear resistance is attributed to the reinforcing mechanisms of MLG, such as crack deflection and pull-out, and reduction of friction coefficient is related to the formation of a tribofilm on the sliding contact surface.     

Fatigue properties and fracture analysis of a SiC fiber-reinforced titanium matrix composite

Tension–tension fatigue properties of SiC fiber reinforced Ti–6Al–4V matrix composite (SiC_f/Ti–6Al–4V) at room temperature were investigated. Fatigue tests were conducted under a load-controlled mode with a stress ratio 0.1 and a frequency 10 Hz under a maximum applied stress ranging from 600 to 1200 MPa. The relationship between the applied stress and fatigue life was determined and fracture surfaces were examined to study the fatigue damage and fracture failure mechanisms using SEM. The results show that, the fatigue life of the SiC_f/Ti–6Al–4V composite decreases substantially in proportion to the increase in maximum applied stress. Moreover, in the medium and high life range, the relationship between the maximum applied stress and cycles to failure in the semi-logarithmic system could be fitted as a linear equation: S_max/μ = 1.381 − 0.152 × lgN_f. Fractographic analysis revealed that fatigue fracture surfaces consist of a fatigued region and a fast fracture region. The fraction of the fatigued region with respect to the total fracture surface decreases with the increase of the applied maximum stresses.     

Deformation and damage in Al/Al2O3

Al/Al₂O₃-composites as an example of light weight materials are very interesting for many industrial applications because of a favourable combination of low density and improved mechanical properties. The prediction of the macroscopic mechanical behaviour of these materials related to their microstructure requires the knowledge of damage initiation and crack development under external loading conditions and, if present, residual stresses have to be taken into consideration. Different types of material degradation like particle/matrix-debonding, particle failure and matrix cracking can be observed in these types of metal matrix composites. The aim of the present work is to introduce damage criteria into a FE-microstructural model in order to foresee the degradation process in an Al₂O₃-particle reinforced Al(6061) composite during mechanical loading. Presently, the conventional fabrication route of the Al/Al₂O₃-composite is a metallurgical method with extrusion for homogenisation of the microstructure and final heat treatment to achieve a defined precipitation state. The influence of thermal residual stresses due to cooling down from annealing temperature on the deformation and damage initiation of Al(6061)/10vol%Al₂O₃-composites is investigated through finite element analyses using the experimentally mapped real microstructure as binary data. Especially, the stresses in the ceramic particles which are responsible for particle cracking and the hydrostatic stresses in the Al-matrix making the particle–matrix interface prone to debonding were analysed. The results show the importance of the thermal residual stresses with respect to damage criteria as obtained by micromechanical FE-calculations.     

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.     

Thermal stability and tensile properties of sol-enhanced nanostructured Ni–TiO2 composites

A highly-dispersed TiO₂ nano-particles reinforced Ni–TiO₂ composite was prepared by sol-enhanced composite electroplating. The microstructure, thermal stability and tensile properties of the sol-enhanced and traditional Ni–TiO₂ composites were explicitly compared. TiO₂ nano-particles agglomerated to large clusters of ∼400 nm in the traditional Ni–TiO₂ composite. In contrast, nano-sized TiO₂ particles (∼15 nm) were distributed at grain boundaries in the sol-enhanced composite. The grain size, higher micro-strain (∼0.31%) and higher microhardness (∼407 HV₅₀) of the sol-enhanced Ni–TiO₂ composite were stabilized up to 250 °C compared to 150 °C of the traditional composite. The sol-enhanced Ni–TiO₂ composite showed a much higher tensile strength of ∼1050 MPa compared to ∼610 MPa of the traditional composite. The lattice diffusion dominated at high temperatures during grain growth for the sol-enhanced composite. The distribution and location of TiO₂ nano-particles played a significant role in determining the thermal stability and tensile behaviors.     

A low cycle fatigue model of a short-fibre reinforced 6061 aluminium alloy metal matrix composite

A low cycle fatigue model has been developed to predict the fatigue life of both the unreinforced aluminium alloy and the short-fibre reinforced aluminium alloy metal-matrix composites based solely on crack propagation from microstructural features. In this approach a crack is assumed to initiate and grow from a microstructural feature on the first cycle. The model assumes that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the material takes place. The low-cycle fatigue crack growth and the condition for failure are controlled by the amount of cyclic plasticity generated within the fatigue-damaged zone ahead of the crack tip and by the ability of the short fibres to constrain this cyclic plasticity. The fatigue crack growth rate is directly correlated to the range of crack-tip opening displacement. The empirical Coffin–Manson and Basquin laws have been derived theoretically and applied to compare with total-strain controlled low-cycle fatigue life data obtained on the unreinforced 6061 aluminium alloy at 25 °C and on the aluminium alloy AA6061 matrix reinforced with Al₂O₃ Saffil short-fibres of a volume fraction of 20 vol.% and test temperatures from −100 to 150 °C. The proposed model can give predicted fatigue lives in good agreement with the experimental total-strain controlled fatigue data at both high strain low-cycle fatigue and low strain high-cycle fatigue regime. It is remarkable that the addition of high-strength Al₂O₃ fibres in the 6061 aluminium alloy matrix will not only strengthen the microstructure of the 6061 aluminium alloy, but also channel deformation at the tip of a crack into the matrix regions between the fibres and therefore constrain the plastic deformation in the matrix. The overall expected effect is therefore the reduction of the fatigue ductility.     

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.     

Thermal conductivity and thermal contact conductance studies on Al2O3/Al–AlN metal matrix composite

Al₂O₃/Al–AlN is a metal matrix composite (MMC) used for making heat sink of electronic devices. This paper presents the detailed investigations carried out on thermal contact resistance across this MMC contact in vacuum at different contact pressures. The experimental results are compared with the theoretical models available in the literature for metallic contacts and they are found to be in good agreement with each other.     

On the impact toughness of Al-15 vol.% B4C metal matrix composites

The present work was performed on ten metal matrix composites (MMCs) produced using the new powder injection technique. These MMCs were divided into two series in which pure aluminum was the matrix for one series, while an experimental 6063 alloy was the matrix for the second series. Small amounts of Ti, Zr and Sc were added to those composites, either individually or combined. In all cases the volume fraction of the reinforced B₄C particles was in the range 12–15 vol. %. The molten metal was cast in an L-shaped metallic mold preheated at 350°C. Unnotched rectangular impact samples (1 cm × 1 cm × 5 cm) were prepared from these castings and heat treated. Samples were tested using instrumental impact testing machine. Microstructure and fracture surface were examined using Hitachi SU-8000 FESEM. The results show that the presence of Ti improves the wettability of the B₄C particles and their adherence to the matrix. Repeated remelting at 730 °C applying vigorous mechanical stirring could lead to fragmentation of some of the B₄C particles. Aluminum based composites exhibited better toughness compared to those obtained from 6063 based composites in all the studied conditions. The composite impact toughness was controlled by the precipitation and coarsening of hardening phase particles namely Mg₂Si, Al₃Zr and/or Al₃Sc. Cracks in the fracture surface were observed to be initiated at the particle/matrix interfaces and propagate either through the B₄C particles or through the protective layers. No complete debonding was reported due the presence of Zr/Ti/Sc rich layers which improved the particle/matrix adhesion.     

Size dependent strengthening mechanisms in carbon nanotube reinforced metal matrix composites

The matrix grain size plays a dual role in metal matrix composites (MMCs). Contrary to enhance the strength of matrix, grain refinement can weaken the thermal expansion mismatch strengthening induced by the reinforcement. In this article, a dislocation density based model is developed to describe the factors affecting the strengthening mechanisms in Carbon nanotube (CNT)-reinforced MMCs with different matrix grain sizes. Two kinds of thermal expansion mismatch strengthening mechanisms are considered, i.e., geometrically necessary dislocations (GNDs) are distributed in entire matrix and GNDs are limited in dislocation punched zones (DPZs). In addition, comparisons between the predictions and some available experimental results are also performed.     

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.     

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.     

Thermal conductivity of Cu/diamond composites prepared by a new pretreatment of diamond powder

Cu/diamond composites were fabricated by spark plasma sintering (SPS) after the surface pretreatment of the diamond powders, in which the diamond particles were mixed with copper powder and tungsten powder (carbide forming element W). The effects of the pretreatment temperature and the diamond particle size on the thermal conductivity of diamond/copper composites were investigated. It was found that when 300 μm diamond particles and Cu–5 wt.% W were mixed and preheated at 1313 K, the composites has a relatively higher density and its thermal conductivity approaches 672 W (m K)⁻¹.     

Fatigue properties of friction stir welded particulate reinforced aluminium matrix composites

Few papers have discussed the friction stir welding (FSW) of particulate reinforced aluminium matrix composites and most of them focused on the set-up of the welding process parameters and their effect on microstructure, hardness and tensile behaviour. The aim of this study was to investigate the fatigue resistance of FSW joints on an as-cast particulate reinforced aluminium based composite (AA6061/22 vol.%/Al₂O_3p). The welding process was performed using different process parameters, also investigating their effect on joint microstructure. The mechanical properties of the FSW composites were compared with those of the base material and the results were correlated to the microstructural modifications induced by the FSW process on the aluminium alloy matrix and the ceramic reinforcement. FSW reduced the size of both particle reinforcement and aluminium grains, and also led to a significant increase in interparticle matrix microhardness, for all process parameters. The FSW specimens belonging to a different set of parameters, tested without any post-weld heat treatment, exhibited a very high joint efficiency (ranging from 90% to 99%) with respect to the ultimate tensile strength of the base material. The stress controlled fatigue test showed a high spread both for the base and FSW composites. Statistical analysis disclosed that all FSW specimens belonging to different process parameters showed apparently slightly worse fatigue behaviour than that of the base composite. Statistical processing applied to the different welding parameters revealed that all the welded specimens belonged to the same population. Therefore it can be concluded that the parameters used produced joints with similar microstructure and comparable fatigue behaviour. The slight difference in the fatigue behaviour of the FSW specimens whose process parameters differed form those of the unwelded composite was explained by the different microstructural homogeneity in the transition from the base to the FSW zone.     

Determination of Young’s modulus in polyester-Al2O3 and epoxy-Al2O3 nanocomposites using the Digital Image Correlation method

Young’s modulus of nano-composite systems composed of unsaturated polyester and epoxy resins with alumina nanoparticles of different sizes has been experimentally estimated. The nanoparticles used were spherical alpha-Al₂O₃ having 30-40 and 200 nm in diameter. Young’s modulus was estimated using an inverse problem that is solved by means of the classical Levenberg-Marquardt technique. A cantilever beam under bending was used in the experiments and the experimental procedure was performed using the Digital Image Correlation method, which is a well-established optical-numerical method for estimating full-field displacement. Experimental results indicate that Young’s modulus increases with increasing nanoparticle volume fraction. Finally, the estimated Young’s moduli were compared with classical theoretical models, showing that the experimental results are in agreement with literature data.     

Abrasive wear of Al2O3-reinforced aluminium-based MMCs

The effects of volume fraction, Al₂O₃ particle size and effects of porosity in the composites on the abrasive wear resistance of compo-casting Al alloy MMCs have been studied for different abrasive conditions. It was seen that porosity in the composites is proportional to particle content. In addition, process variables like the stirring speed, and the position and diameter of the stirrer affect of the porosity content in a way similar to that observed for particle content. In addition, the abrasive wear rates of composites decreased more rapidly with increase in Al₂O₃ volume fraction in tests performed over 80 grade SiC abrasive paper than in tests conducted over 220 grade SiC abrasive paper. Furthermore, the wear rates decreased with increase in Al₂O₃ size for the composites containing the same amount of Al₂O₃. Hence, it is deduced that aluminium alloy composites reinforced with larger Al₂O₃ particles are more effective against abrasive wear than those reinforced with smaller Al₂O₃ particles. At the same time the results show that the beneficial effects of hard Al₂O₃ particles on wear resistance far surpassed that of the sintered porosity in the compocasting metal-matrix composites (MMCs). Nevertheless, the fabrication of composites containing soft particles such as graphite favors a reduction in the friction coefficient. For this reason graphite and copper were used in the matrix in different amounts to detect their effect on wear resistance. Finally, it was seen that wear rate of the composites decreased considerably with graphite additions.