فصلنامه مواد پیشرفته در مهندسی
سال سی و سوم شماره 2 (پاییز 1393)

In this study، Al/Steel multilayer composite was produced by accumulative roll bonding (ARB) process using Al-1100 and St-12 strips. Microstructure، mechanical properties and corrosion behavior of the composite were studied by scanning electron microscopy (SEM)، tensile test، Vickers microhardness tests، cyclic polarization and electrochemical impedance spectroscopy (EIS) measurement in 3. 5 wt% NaCl solution. After one ARB cycle (2 roll-bonding cycles)، the multilayer composite of 4 layers of Al and 2 layers of steel was produced. The tensile strength of the Al/steel multilayer composite reached 390. 57 MPa after the first ARB cycle، which was 1. 29 times larger than that of the starting steel while composite density was almost half the density of the steel. Corrosion behavior of the composite revealed a considerable improvement in the main electrochemical parameters، as a result of enhancing influence of cold rolling. The results indicated that strength and corrosion resistance of Al/steel composite generally decreases and elongation increases after annealing.

In this study، mixed metal oxides Al2O3/MgO/TiO2 coatings with Al/Mg/Ti ratios of 5:1: 3 and 2. 5:3: 4 were coated on AA1100 aluminum by sol-gel method. The surface morphology، phase analysis and the corrosion behavior of the Al2O3/MgO/TiO2 coatings were characterized by scanning electron microscope (SEM)، X-ray diffraction (XRD)، and electrochemical impedance spectroscopy measurements (EIS) in 3. 5 wt. % NaCl solution. The thermal behaviors، the bonds configuration، and functional groups of the coated samples were studied by thermo-gravimetric and differential thermal analysis (TG-DTA) and Fourier transform infrared spectroscopy (FTIR)، respectively. The results demonstrated that heat treatment at 450 °C caused an increase in porosity and coating cracking، finally leading to the decrease of corrosion resistance. The best corrosion resistance was achieved for the sample with Al/Mg/Ti molar ratio of 5:1: 3 without any heat treatment. The structure of this sample was amorphous، and heat treatment resulted in crystallization and decrease of the corrosion resistance.

This research aimed at producing microcellular foams (with cell size of 1-10 μm and cell density above 10 9 cell/cm3) from PC/EPDM in order to use in medical devices. Due to the weak nucleation behavior of microcellular polycarbonate foams، it is difficult to prepare them. This research provides valuable information regarding the possibility of making microcellular foams from this polymer by using multiwalled carbon nanotubes (MWNT،s) as nucleating agents (the value of 1-3 phr). The nanocomposite samples were prepared in an internal mixer and foamed via a batch processing method using supercritical carbon dioxide as the foaming agent. The results showed that the addition of nanoparticles up to 3 phr improves the foamability of PC/EPDM blend. Furthermore، as nanoparticle content increased a decrease in cell size and hence an increase in cell density were observed. Another finding showed that cell size distribution is directly related to uniform dispersion of carbon nanotubes.

Mechanical property of porous nickel-titanium alloy produced by volumetric combustion synthesis (VCS) for bone surgery applications is reported in this paper. Stress-strain behavior of the alloy is determined by uniaxial tension test. Superelastic diagram of the porous alloy is compared with that of the solid material cooled from austenite stability temperature. Due to movements of the dislocations، growth of the nucleation sites and thinning of the martensite plates during cooling، plastic deformation and necking behavior of these materials are principally different from that of the ordinary materials. Elastic modulus and yield stress of the material have nonlinear relationship with porosity percentage and obey the following correlations  and The stress-strain curves of the alloy show more than 6 percent elongation before rupture، even with 30 percent porosity. A comparison of the cleavage surfaces of the combustion synthesis samples with those of the powder metallurgical ones indicate great influence of production process on fracture mechanism.

In recent years، much research has been performed in the field of nanomaterials synthesis using mechanochemical process. In this research، TixAly/Al2O3 ceramic matrix nanocomposite was produced by the mechanochemical method. Aluminum and inexpensive titanium oxide powders were used as raw materials، and milling was performed under N2 atmosphere. The results showed that reduction of TiO2 by Al is the first step of synthesis process، and then Ti reacts with residual Al. The synthesis after 10 hours of milling resulted in titanium aluminide and aluminium oxide. With the increase of milling time to 80 hours، titanium aluminide quantity was increased. Also، the results showed that the heating of samples containing titanium aluminide in the argon and nitrogen atmospheres does not lead to complete decomposition of aluminides.

In this paper، manufacturing and evaluation of ethanol gas sensors based on thin films of nanostructure tin oxide have been investigated. SnO2 thin films were prepared by both thermal evaporation (type I) and sputtering (type II) methods and heat treated on silicon wafer substrates. Scanning electron microscope (SEM)، atomic force microscope (AFM)، X-ray diffraction (XRD) and energy dispersive spectra (EDS) are employed to study the morphology and chemical composition of the semiconductor samples. Nano-scale grain size with homogenous distribution showed in SEM images of both types. Sensors resistance in air and its variation-transient response toward ethanol vapours (3000 ppm) was determinated. The response of the stable sensors was obtained 3 and 1. 18 for type I and type II، respectively. That showes thin film of nanostructure tin oxide by thermal evaporation (type I) has better sensitivity than the other. More effective surface، adsorption sites and base-line resistance due to the more fine grain size in type I nanostructure، are its important reasons. however، it is slow due extensive rise time and fall time.

Hercynite، FeAl2O4، was synthesized via molten salt synthesis method in the coke bed at 800°C with 3h of holding time. It was synthesized by reacting stoichiometric compositions of Al2O3 and FeCl2. 4H2O in eutectic compositions of alkaline chlorides NaCl-KCl-LiCl. The reactant to salt ratio was 1 to 3. The phase formation، and morphology of these synthesized powders after washing and filtration were characterized via X-ray diffraction (XRD)، and scanning electron microscopy (SEM). Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were performed at temperatures up to 1000 °C at a heating rate of 10 °C/min in argon atmosphere، to elucidate the different reaction mechanisms in the synthesis of Hercynite by the molten-salt method. The effects of processing parameters including the temperature and holding time on the formation of FeAl2O4 were investigated. The results demonstrated that the formation of FeAl2O4 spinel could be initiated at 700°C. By increasing the temperature to 900 °C and holding time، the amounts of FeAl2O4 particles in the resulting powder increased at the expense of Hematite and Al2O3. Morphology of the synthesized powder was cubic and tetragonal، increased by increasing the holding time and temperature.

In this study، ZrB2-HfB2 composite was produced by pressureless sintering method. MoSi2 B4C and SiC particles were used as reinforcement. ZrB2 powder was milled in planetary ball mill apparatus and then reinforcement particles were added to the milled powder. The composite powders were then CIPed and sintered at 2100oC and 2150oC. Scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS)، flexural test، and resonance frequency method were used to compare the added particle effects on mechanical properties and pressureless sintering behavior of ZrB2-HfB2 composite. The analysis showed that the ZrB2-HfB2-MoSi2-SiCnano composite displays the largest gain in flexural strength. Furthermore، increasing the sintering temperature leads to an increase in flexural strength of samples.

In the present study، pure Aluminum powder with 5%wt Titanium Dioxide was mechanically milled at different times. Using phase analysis through X-ray diffraction (XRD)، it was found that increasing of the milling times over 10 hours causes the reduction of Titanium by Aluminum and formation of Al2O3 in the structure. Also، it was shown that if the process persists، Aluminum reacts with Titanium and causes the formation of Al3Ti in the composition. The reactions were studied through the thermodynamic relations. Furthermore، after distribution of reinforcement particles in the matrix، using X-ray diffraction peak broadening، according to Williamson-Hall equation، the mean crystallite size and lattice strain were determined، and by scanning electron microscopy (SEM)، the structure and morphology of the powder particles were studied.

in this research، the kinetics of carbothermic reduction of molybdenite in the presence of sodium carbonate was studied. For this purpose، mixed powder of molybdenite، graphite، and sodium carbonate with 1:4: 2 mole ratio was investigated using simultaneous thermal analysis (STA) at the heating rates of 10، 15 and 20 0C /min. The results of thermal analysis were evaluated through Friedman، Kissinger، Ozawa and Coats-Redfern methods. The activation energy of reduction reaction was determined 220 kj/mole، and it was found that the reaction was chemically controlled. To study the reaction mechanism، the mixed powder was heated to 400، 800 and 1100 0C in argon atmosphere at the heating rate of 10 0C/min. X- Ray diffraction of the reaction products and thermodynamic analysis at these temperatures indicated that carbothermic reduction of molybdenite in the presence of sodium carbonate would advance through the formation of intermediate phases، Na2MoO4 and MoO2.

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 1600 °C. 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.