ADVANCED MATERIALS IN ENGINEERING (ESTEGHLAL)
Year:2014 | Volume:33 | Issue:1|Papers Count:11

An important parameter in composite materials is mechanical behavior and matrix-reinforcement interface interaction under applied stresses. In this investigation, bending strength of carbon-carbon composites synthesized from wood was analyzed as a measure of the composite mechanical properties. Also, densification efficiency of the products was determined by measuring the bulk density and open porosity percentage. Using scanning electron microscopy, optical microscopy, X-Ray diffraction, and Raman spectroscopy, the final product was examined to evaluate and interpret the morphology and internal texture and results of mechanical test. The results showed that we are able to use an ordinary material such as wood for production of a unique product with great properties called two-dimensional carbon-carbon composites.

In this article, sintering activation methods of Mo powder (chemical, mechanical and surface activation) were studied. For this purpose, the milled/reduced Mo nanopowder was sintered at 900, 1100 and 1400oC for 1 hr. For comparison of sintering activation methods (mechanical and chemical activation) and their effects on microstructural characteristics, commercial micropowder Mo as well as Ni additive was used. The samples were compacted under a pressure of 400 MPa and then sintered at 1400oC for 1 hr. The microstructure of sintered samples was studied by scanning electron microscope (SEM) along with EDS. Phase analysis was performed using X-ray diffraction (XRD) technique. The sintered densities of samples were measured by Archimedes method. Relative densities of specimens obtained from micro, nano, micro+20% nano and micro+1.5% Ni additive powders were attained as 80%, 93%, 86% and 95%, respectively. It was found that the  d-NiMo intermetallic layer may be formed at the grains' boundaries due to Ni additive, leading to grain boundary microcracks as well as loss of mechanical properties of samples.

In this study, pure powders such as molybdenum, silicon, aluminum and titanium carbide were utilized to produce MoSi2 compound, MoSi2/20 Vol % TiC composite, MoSi2-x Al alloyed compound and MoSi2-x Al/20 Vol % TiC alloyed composite. The initial powders were mixed in specified ratios, and then, were activated by mechanical milling. Milled powders were compacted, synthesized and sintered in the temperature range of 1100-1400oC. SEM was used to investigate the microstructural change and XRD for identification of phases. Effect of aluminum addition on phase formation was investigated. Addition of aluminum by over 9 atomic percent resulted in the formation of Mo (Si, Al)2 in alloyed matrix.

In some applications such as morphing technology, high strain and anisotropic behavior are considered a good advantage. The corrugated composite sheets due to their special geometries have a potential of high deflection under axial loading. In this research, to investigate the strain and anisotropic behavior of corrugated composite sheets some glass/epoxy samples with Quasi-sinusoidal, trapezoidal, rectangular and triangular geometries were manufactured and put to tension flexural tests in the longitudinal and transverse directions of corrugation. Then, in order to determine anisotropic behavior of corrugated sheets two concepts were introduced: tensile anisotropic and flexural anisotropic criteria based on which anisotropic magnitude was investigated theoretically and experimentally. This research used Yokozeki’s theoretical model for quasi-sinusoidal geometry and his model for trapezoidal, rectangular and triangular geometries. Experimental results showed that corrugated sheets have a strain more than 90%. In the corrugated samples, the strain magnitude was dependent on amplitude and pitch of elements; in other words, it was dependent on the number of elements per length unit. Generally, the Quasi-sinusoidal corrugated sheets have a high strain (more than 50%). Experimental results of trapezoidal sheets showed that amplitude of the elements is one of the most important parameters in the ultimate strain. Generally, increasing the amplitude leads to the growth of the ultimate strain.

In this study, the effect of different amounts of Y2O3 on the properties of mullite-zirconia composites was investigated. For this purpose, these composites were fabricated by reaction-sintering of alumina and zircon as raw materials.Besides, the slip casting method was used for forming these composites, and sintering process was carried out at 1600oC. Then, the physical and mechanical properties, phase composition and the microstructure of these composites were investigated. The results showed that yittria addition up to 0.5 wt.% has no effect on the properties of these composites. Besides, addition of more than 0.5 wt.% yittria formed solid solution with zirconia grains and led to stabilization of tetragonal zirconia phase and increasing of its amount. Hence, yittria addition increases the hardness and bending strength of composite by stabilizing tetragonal zirconia phase and then, decreasing the micro-crack formation during zirconia phase transformation. As results show, addition of 0.75 wt.% yittria leads to a considerable increase in the bending strength.

Due to their superior properties such as high specific strength, high creep resistance and high strength at elevated temperatures, aluminum composites reinforced with alumina nano particles are widely used for advanced purposes such as aerospace and auto industries. Lack of an appropriate welding process limits their applications. Transient liquid phase (TLP) bonding is one of the state-of-the-art joining processes. It is used for welding composites and advanced materials. Microstructure and mechanical properties of TLP bonding depend on the bonding time and temperature. In the current study, the effect of bonding time on the microstructure and bonding strength of the TLP diffusion bonded of Al2O3 p/Al nanocomposite was investigated. A thin layer of copper deposited by electroplating was used as an interlayer. The bonding times of 20 and 40 min were not sufficient for completing the isothermal solidification, and the bonding strengths were not satisfactory. By increasing the bonding time to 60 min at constant bonding temperature of 580oC, the isothermal solidification was completed and the final joint microstructure consisted of softα-Al phase with dispersed CuAl2 precipitated particles. Decreasing the amount of brittle eutectic structures in the joint seam by increasing the bonding time was the main reason for improvement of the joint shear strength. The maximum joint shear strength was achieved at 580oC for 60 min which was about 85% of the shear strength of the base material.

Employing direct and alternative electric currents at the time of casting and solidification modified grains of Al and Si. The highest wear resistance was obtained in the direct current, and for alternative current the wear resistance corresponded to the electric current. The change of polarity in the pure Al did not influence the wear resistance, but for the Al-Si alloy the highest wear resistance was obtained when the mold was connected to the positive and the molten metal to the negative pole. Direct current used in the Al-Si alloy brought about three different microstructures including the stretched clusters of Si in the electrons' direction near the negative pole, fine clusters of Si in the intermediate zones that surrounded the oval shape ofα-Al, and broken Si clusters near the positive pole.

In this paper, the effect of MgCl2 addition on the kinetics of MA spinel formation was investigated. For this purpose, the stoichiometric mixture of MgCO3 and calcined aluminum was calcined at 1100oC for 1 hr. Then, the calcined composition was wet-milled and after addition of 6% MgCl2 the compositions were pressed and fired at 1300 and 1500oC for different times. Spinel phase content was determined using semi-quantitative phase analysis. With regard to Jander's equation, the rate constant was calculated, and the activation energy was obtained from Arrhenius equation. The results showed that the addition of MgCl2 leads to the acceleration of the spinel formation reaction. Besides, 55.71 Kcal/mol as the activation energy was calculated for the composition containing 6 wt.% MgCl2 compared with 93.06 Kcal/mol for the composition without MgCl2.

Fe-Co alloys have unique magnetic applications. Fe50Co50 alloy has the highest saturation magnetization value among Fe-Co alloys. Moreover, the introduction of Si into Fe can result in a decrease of magnetic anisotropy. In this study, in order to utilize combined advantages of Si and Co, the effect of adding 10 and 20 at.% Si on the microstructural and magnetic properties of Fe65Co35 alloy was investigated. For this purpose, initial powder mixtures with specific compositions were milled by means of planetary ball mill for different milling times. Microstructural properties and morphology of the obtained powders were analyzed by X-ray diffraction analysis (XRD) and scanning electron microscope (SEM). Also, magnetic properties of the samples were determined by means of vibration sample magnetometer (VSM). The results showed that the crystallite size was finer and more uniform and lattice strain was decreased slightly for longer milling times. Observations indicated that the addition of Si to the alloys leads to finer particles. The results also showed that increasing the Si content increases the reduction rate of lattice parameter and coercivity.

In this research according to unique properties of fused silica and Its numerous applications fused silica parts with 77% by weight solid loading formed by gel casting. Rheological behavior of the slurry and sintering conditions were optimized. Sample sintered at optimum conditions has bulk density of 1.71 g/cm3, open porosity of 18.13%, water absorption of 10.60%, linear shrinkage after firing of 3.5%, closed porosity of 1.09% and relative density of 78.80%. Its thermal expansion coefficient in range of room temperature to 1000oC has been measured 0.4432×10-6 1/oC. Results showed that With increasing temperature and time In addition to the increased vscous flow, Crystallization also extend. Finally sintered at high temperature and short time for an instance with the lowest overall high crystallinity and density, were found suitable.