Iranian Journal of Materials Science and Engineering

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Effects of temperature on properties and behavior of a 20 vol % particulate SiC reinforced 6061 aluminum alloy and 6061 unreinforced Al alloy were investigated. Yield strength and elongation to failure were measured as a function of test temperatures up to 180^oC. In addition, the effects of holding time at 180^ oC on tensile properties and fracture mechanisms of the materials at this temperature were studied. The behaviors of the materials were characterized by using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDS), X-ray diffraction (XRD), atomic absorption (AA), hardness measurement and image analyzing (IA). The results show that an increase in temperature leads to a decrease in the yield strength and increase in the elongation to failure of the materials. On the other hand, while increasing holding time at 180^oC produces an increase in the elongation to failure of the unreinforced alloy, it reduces the elongation to failure of the composite. It was also observed that reduction in yield strength with increasing holding time at 180^oC was faster for the composite material compared to the unreinforced alloy. The results from SEM, XRD, EDS, IA and hardness tests indicated that some chemical reactions had taken place at the interface between the reinforcement and the matrix alloy during holding the specimens at elevated temperature. Therefore, different trend in elongation to failure of the unreinforced alloy and the composite material with holding time at elevated temperature could be attributed to development of chemical reactions between the reinforcement and the matrix alloy at the interface.

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In this study different volume fractions of SiC particles of various sizes were introduced into the semisolid A356 aluminum alloy by a mechanical stirrer. Then the slurry was poured into a permanent die of certain dimensions either when the metal alloy was partially solid (semisolid-semisolid or SS route) or after reheating to above the liquidus temperature of the alloy (semisolid-liquid or SL route). Both the SS and SL composite samples were solution-treated at 520°C for 8 hours followed by quenching in water at room temperature. Microstructural characterization studies have been conducted on both the SL and SS samples to quantify the effects of the size andcontent of the SiC particles as well as the solutionizing treatment on the morphology and size ofthe eutectic silicon particles and the matrix grain (globule) size. The results were rationalized in terms of the different nucleation, fragmentation, spherodization and coarsening events, which had taken place during the processing of these composites. Finally the impact of these microstructural features in improving the wear properties of the composites has been discussed.

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The new in situ method for AI-TiC composite fabrication has been carried out. In this method, fabrication of AI-TiC composite by simultaneous introduction of titanium oxide and carbon into aluminum melt was investigated.. Under the process conditions, titanium and carbon reaction results in titanium carbide whiskers. The salt containing keriolite (Na3AIF6), titanium oxide (TiO2) and graphite used for this purpose. Using Scanning Electron Microscopy (SEM) and X-Ray Diffraction analysis (XRD) the resulted composite was characterized. It was shown that it contains Al as matrix and TiC as the reinforcement. Then, mechanical properties of fabricated composite were examined.

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An AI-7wt%Si-5vol%TiCp was worn against a cast iron disc in a tri-pin-on-disc machine, under dry sliding conditions at the sliding speed of 0.24 m/s and applied loads of 6, 20 and 40 N/pin. Stress-strain (σ-ε) curves were constructed by measuring the microhardness and the equivalent strain gradients in near surface regions on the cross-sectional surface prepared parallel to sliding direction.It was shown that, both the magnitude of plastic strains and the depth of plastic deformed zones increased with the applied load. The material exhibited considerable work softening in addition to work hardening at the highest applied load. The softened layer placed just beneath the mechanical mixed layer (MML), was mostly covered with the fine fractured eutectic Si and TiC fragments most of which were associated with microcracks at Al/Si and Al/TiC interfaces. The results were discussed in terms of some of the current work hardening models.

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In this research, the mechanism of joining three sheets of metals, i.e. brass-steel-brass, by cold roll welding process has been studied. For this purpose, the two surfaces of steel sheets were roughened with stainless steel wire brush by different amounts, then the brass sheets were put on both sides of the steel sheets, before they were subjected to cold roll process. During rolling, peaks of the asperities on the surfaces of the steel sheet were pressurized, i.e. deformed, much more than that of trough. Hence, more hardening due to formation of higher dislocation density in the peaks regions were detected in comparison to the trough regions. Therefore, due to the differences in the amounts of work hardening occurred during cold rolling in the peaks & trough of the scratches and also due to the nature of the rough surfaces of the steel sheets, which causes the smooth surface of soft brass sheets laied over the rough surface of the steel sheet to be shappend according to the profile of the steel sheet scratches during cold rolling, mechanical locking occurred at the interface of brass & steel sheets. In addition, while the extrusion of brass took place through cracks within the surface of hardend peaks and metal bonding occurred on the contact points of the brass sheet & the vergin steel. Therefore, it seems two mechanisms were in operation is making a suitable joining between the sheets. One was a locking mechanism due to the roughness of the steel sheets & the other was bonding mechanism due to the bonding between the peak points of the scratches &soft brass surface. The strength of the bonded points in the interface were later increased by annealing the composite, so that by annealing the samples within the 500-900°C range for aperiod of 1 1/2 hr the interface strength increase substantially. The results of peeling test indicated that the interface strength of the samples annealed at 700°C or more increased so much that the brass sheet toms during peeling & the fracture did not pass through the interface.

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Copper coated SiC powders having three different amounts of copper, in the range of 20-60 wt%, were prepared via electroless coating process. The produced composite powders were uniaxially cold compressed and sintered at different temperatures and times under protective atmosphere. It was found that composite Cu/SiC powders and a relatively dense copper matrix composite with a uniform distribution of SiC reinforcing particles imbedded in copper matrix can be fabricated via electroless coating method followed by conventional cold pressing and sintering process. The results also show that SiC particles have a poor wettability with copper and so liquid phase sintering of the Cu/SiC composite powders did not enhance densification of the samples. Regarding this fact, optimum sintering temperatures, which depends on copper content, was determined to be in the range of 1050-1080?C.

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Fracture behavior of a 7075 aluminium alloy reinforced with 15 Vol%. SiC particles was studied after T6 and annealing heat treatments under uniaxial tensile loading at room temperature. The scanning electron microscopy of fractured surfaces and EDS analysis showed:, that fracture mechanism changed from due mainly to fractured particle in T6 condition to interface decohesion in samples in annealed state. Different fracture mechanisms in annealed and T6 conditions can be ascribed mainly to the significant difference in the stress concentration levels around the particles. In T6 condition, very high local stress sufficient to cause fracture of particle can be generated during loading, while the presence of large precipitates at the particle/matrix interface produced interface decohesion leading to final fracture in the annealed state.

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Enhancing the properties of dental resin composites is of interest to researchers. The objective of the present investigation was to improve the strength and fracture toughness of dental composites via addition of silicon carbide whiskers and substitution of commonly used filler materials with stabilized zirconia ceramic powder. It was also intended to study the effect of powder- to- whisker ratio on mechanical properties of the resultant composites. The flexural strength and fracture toughness of composite samples with different whiskers loadings were measured. It was found that addition of whiskers to the composites enhances the mechanical properties of the composites. The strength and fracture toughness increased by increasing the amount of whiskers. The flexural strength of a composite having 60wt% whisker and 10wt% zirconia powder was about 210 MPa while that of the composite having only 60wt% ceramic powder was about 110 MPa. The microstructural examinations revealed that reinforcing mechanism was whiskers pull-out as well as crack deflection.

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Abstract: Static recrystallization (SRX) behavior of a composite based 7075 Aluminum alloy reinforced with SiC particles was studied during annealing the deformed samples at high temperatures. The results showed an absence of SRX in the samples annealed after hot working at the same deformation temperature, however, a rise in annealing temperature of 100-1500 􀁱C above that the deformation temperature led to full recrystallization. This can be ascribed to the relatively moderate dynamic recovery and the presence of dispersions which stabilize the substructure. Particle stimulated nucleation (PSN) had a significant effect on the grain size in deformed samples at low temperature, but no PSN was observed in samples strained at high temperatures. The possible cause might be that at high temperature the dislocations can be annihilated by climb process around the particles together with the absence of deformation zone for nucleating the recrystallization.

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Abstract:

materials can be fabricated by sintering of W-Cu composite powders. In this research W-20%wt Cu composite powders

was synthesized via a co-precipitation method. Precipitate obtained from a mixture of copper nitrate and ammonium

paratungstate (APT) in distilled water contained W-Cu compounds. This precipitate was washed, dried and calcined

at 550

of dried precipitate powder was determined by thermogravimetry (TG), differential thermal analysis (DTA) and X-ray

diffraction (XRD). The sintering of the reduced powders was investigated as a function of temperature. Relative density

of more than 98% obtained for the powders sintered at 1200

close to theoretical calculations. The hardness of the sintered powders was 320 Vickers.

W-Cu composites are widely used as contacts, heat sinks and electro discharge electrodes. These kinds of°C in air and then reduced in H2 atmosphere in order to convert to W-Cu powders. The calcination temperature°C . The corresponding electrical conductivity was too

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Abstract: In this paper‚ the physical and mechanical properties of Al nanocomposite reinforced with CNTs wereinvestigated. High purity Al powder and Carbon Nanotubes (CNTs) with different percentage were mixed by ballmilling method and the composite was fabricated by cold pressing followed by sintering technique. The variation ofdensity and hardness of composite with CNTcontent was investigated. The microstructure of composite was evaluatedby SEM (Scanning Electron Microscope) and XRD (X -Ray Diffraction). The results show that the density and hardnessincrease with CNTpercentage.

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Abstract: Equal channel angular extrusion (ECAE) is a promising technique for production of ultra fine-grain (UFG) materials of few hundred nanometers size. In this research, the grain refinement of aluminium strip is accelerated by sandwiching it between two copper strips and then subjecting the three strips to ECAE process simultaneously. The loosely packed copper-aluminium-copper laminated billet was passed through ECAE die up to 8 passes using the B_c route. Then, tensile properties and some microstructural characteristics of the aluminium layer were evaluated. The scanning and transmission electron microscopes, and X-ray diffraction were used to characterize the microstructure. The results show that the yield stress of middle layer (Al) is increased significantly by about four times after application of ECAE throughout the four consecutive passes and then it is slightly decreased when more ECAE passes are applied. An ultra fine grain within the range of 500 to 600 nm was obtained in the Al layer by increasing the thickness of the copper layers. It was observed that the reduction of grain size in the aluminium layer is nearly 55% more than that of a ECA-extruded single layer aluminium billet, i.e. extruding a single aluminium strip or a billet without any clad for the same amount of deformation. This behaviour was attributed to the higher rates of dislocations interaction and cell formation and texture development during the ECAE of the laminated composite compared to those of a single billet

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In this research an ultra-fine grained composite structure consisting of an intermetallic matrix together with dispersed nano-sized Al2O3 obtained via mechanical activation of TiO2 and Al in a high energy ball mill and sintering of consolidated samples. Phase composition and morphology of the milled and sintered samples were evaluated by XRD and SEM techniques Thermal behavior of the powder sample milled for 8 hours was evaluated by DTA technique. DTA results showed that, the reaction happens in two steps. The first step is the aluminothermic reduction of TiO2 with Al. XRD observations reveals that minor amount of Ti3Al phase formed during reduction reaction together with TiAl and Al2O3 major phases. This intermetallic phase disappeared when sintering temperature was increased to 850 ºC. The second step in DTA is related to a reaction between residual Al in the system (partly dissolved in TiAl lattice) and the Ti3Al phase produced earlier at lower temperatures. SEM micrographs reveal that by completion of the reduction reaction more homogeneous and finer microstructure is observable in sintered samples.

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Abstract: Electroless Nickel (EN) composite coatings embedded with Cr2O3 and/or MoS2 particles were deposited to combine the characters of both Cr2O3 and MoS2 into one coating in this study. The effects of the co-deposited particles on corrosion behavior of the coating in 3.5% NaCl media were investigated. The results showed that both Ni-P and Ni-P composite coatings had significant improvement on corrosion resistance in comparison to the substrate. Codeposition of Cr2O3 in coating improved corrosion characteristic but co-deposition of MoS2 decreased corrosion resistance of the coating.

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Abstract: Zirconia solid electrolytes with nonequilibrium composite structure were prepared by impregnation of a porous 8YSZ matrix with a solution of Zirconia. Microstructures were characterized by XRD and SEM. The electrical properties were studied by impedance spectroscopy as a function of temperature. Biaxial flexural strength and fracture toughness of composite samples were measured by ring on ring and Vickers microhardness indentation methods respectively. The microstructures of the composite electrolytes were composed of cubic grains surrounded by tetragonal second phase grains. It was shown that the electrical and mechanical properties of the prepared electrolyte can be adjusted by controlling the amount of doped zirconia. Increasing the amount of doped zirconia increases the tetragonal phase content which improves fracture toughness and fracture strength. In addition, increasing tetragonal phase content of the composite electrolytes decreases the conductivity at high temperatures while the situation is reversed at low temperatures.

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Abstract: In this study Al-5 (Vol) % SiCp nanocomposite powder has been successfully synthesized by high-energy planetary milling of Al and SiC powders for a period of 25 h at a ball-to-powder ratio of 15:1. The changes of the lattice strain, the crystallite size of the matrix phase, and the nanocomposite powder microstructure with time have been investigated by X-ray diffraction (XRD), X-ray mapping, and scanning electron microscopy (SEM) analyses. The morphologies of the nanocomposite powders obtained after 25 h of milling have also been studied by transmission electron microscopy (TEM). The results showed that nanocomposite powders were composed of near-spherical particles and, moreover, the SiC particles were uniformly distributed in the aluminum matrix.

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Abstract: Nano-structural synthesized materials can be fabricated utilizing intensive milling after combustion synthesis. The Al2O3-TiB2 ceramic composite has been synthesized by aluminothermic reactions between Al, Ti (TiO2), and B (B2O3 or H3BO3). Boric acid (H3BO3) is less expensive than boron oxide, and after being dehydrated at 200°C, boron oxide will be obtained. In this study, Al, TiO2, and boric acid were used as the starting materials to fabricate an Al2O3-TiB2 ceramic composite. After mechanical activation and thermal explosion processes, intensive milling was performed for 5, 10, and 20h to assess the formation of a nano-structural composite. The X-ray phase analysis of the as-synthesized sample showed that considerable amounts of the remained reactants incorporated with the TiO phase were present in the XRD pattern. The results showed that the average crystallite size for alumina as a matrix were 150, 55 and 33 nm, after 5h, 10h, and 20h of intensive milling, respectively. The SEM microstructure of the as-milled samples indicated that increasing the milling duration after combustion synthesis causes a significant reduction in the particle size of the products, which leads to an increase in the homogeneity of particles size. A significant increase in the microhardness values of the composite powders was revealed after intensive milling process.

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Abstract: Composite specimens were prepared by dispersion of various amounts of nano-sized Al2O3 fillers in a monomer system containing 60% Bis-GMA and 40% TEGDMA. For comparative purposes, composite samples containing micrometer size Al2O3 fillers were also prepared following the same procedure. The mechanical properties of the light- cured samples were assessed by three-point flexural strength, diametral tensile strength, and microhardness tests. The results indicated a more than hundred percent increase in the flexural strength and nearly an eighty percent increase in the diametral tensile strength values in the samples containing nano-size Al2O3 filler particles. It is interesting to note that, this improvement was observed at a much lower nano-size filler content. Fracture surfaces analyzed by scanning electron microscopy, indicated a brittle type of fracture in both sets of specimens.

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Abstract: Microwave processing is one of the novel methods for combustion synthesis of intermetallic compounds and
composites. This method brings about a lot of opportunities for processing of uniquely characterized materials. In this
study, the combustion synthesis of TiAl/Al2O3 composite via microwave heating has been investigated by the
development of a heat transfer model including a microwave heating source term. The model was tested and verified
by experiments available in the literature. Parametric studies were carried out by the model to evaluate the effects of
such parameters as input power, sample aspect ratio, and porosity on the rate of process. The results showed that
higher input powers and sample volumes, as well as the use of bigger susceptors made the reaction enhanced. It was
also shown that a decrease in the porosity and aspect ratio of sample leads to the enhancement of the process.

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TiO2/SiO2 nanocomposite with molar ratio 1:1 was synthesized by a free calcination sol-gel method using titanium tetra chloride and tetraethylorthosilicate as raw materials. In the composite, TiO2 nanocrystals are highly dispersed in the amorphous SiO2 matrix and the mater showed size quantization effect arising from the presence of extremely small titanium oxide species having a low coordination number. Thermal phase transformation studies of the as-prepared composite were carried out by means of X-ray diffraction (XRD) patterns and thermogravimetry–differential scanning calorimetry (TG–DSC) analyses. The studies showed existence of anatase phase in all the tested temperatures. When temperature exceeds 400°C, brookite phase was formed beside anatase phase. At 950°C amorphous silica matrix was transformed to crystobalite and brookite phase disappeared. Finally, small peaks of rutile phase were detectable at 1100°C.