مجله مواد نوین
پیاپی 4 (تابستان 1390)

Aluminum and its alloys duo to their low density, good accessibility and ease of converting to foam, become the most used materials in manufacturing of metallic foams. In this study Accumulative Roll-Bonding process has been utilized for distributing TiH2 powders, as foaming agent, in the structure of an aluminum sheet. Then, via a suitable heat treatment the powders were decomposed and the preform was converted to foam. Also, using the uniaxial compression test, compressive strength of the fabricated foam in different loading directions has been investigated and a correlation between compressive strength and porosity and loading direction has been observed. Maximum compressive strength of 21.08 MPa for foam of porosity 30% in the rolling direction (RD) was measured and increasing the porosity, a decrease in compressive strength was observed. The compressive behavior of the fabricated foam was different in three loading directions.

The synthesis of AlN-SiC composites was investigated through the combustion reaction between aluminum, Si3N4 (the solid source of nitrogen) and carbon powders. The combustion reaction was initiated through the ignition of the powder mixture by microwave heating in a domestic oven and the desired ceramics were produced. To achieve higher degree of conversion, some parameters were altered during the synthesis. These included the time of mixing, the amount of carbon in the reactant mixtures and the weight of compacts. The productivity of the reaction wasn’t affected dramatically by alteration in the first two parameters, but increasing the weight of samples made it possible to achieve more complete combustion and higher degree of conversion to products. The combustion temperature of the system was studied theoretically and experimentally. The microstructure and phases of products were analyzed by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM), respectively.

TWinning Induced Plasticity (TWIP) steels, with high manganese percentage (17-35%), are used in automotive bodies. Their microstructure at room temperature is austenitic and due to low stacking fault energy, major deformation mechanism in them is twinning inside austenite grains which on the other hand, leads to enhanced mechanical strength in steel. Due to important effect of heat treatment process on mechanical properties, full annealing heat treatment and effect of Molybdenum were investigated in this study. To this aim, the steel was casted and hot rolled and then heat treated at different temperatures and the obtained microstructures were analyzed using optical, SEM and TEM microscopes. The results showed that the full annealing temperature for steel without Mo was determined to be 1100 C. and addition Mo to the steel annealing Temperature by 50 C reduced.

In the present study, in-situ synthesis of carbon nanotube/hydroxyapatite nano composite powder with stable homogeneous dispersions of carbon nanotubes (CNTs) have been prepared using surfactant as dispersing agent. By applying sol-gel method, dispersion in the hydroxyapatite matrix and its effects on the microstructure were investigated. The chemical and phase composition, structure and morphological and size analyses were performed using XRD, FT-IR, SEM, TEM/SAED/EDX, Raman, UV-Vis spectroscopy and differential scanning calorimetry (DSC). The influences of different dispersing agents (sodium dodecyl sulfate, SDS) as a benchmark for future dispersion experiments and excitation wavelength are discussed and the results are compared to the commonly used UV-Visible spectroscopic analysis. The results indicated that synthesis of hydroxyapatite particles in the presence of the carbon nanotubes had the best result in homogenization of the carbon nanotube dispersion and faster crystallization of hydroxyapatite, and usage of SDS for dispersion carbon nanotubes at hydroxyapatite matrix, the render formation hydroxyapatite coating on CNTs surface. The average crystallite size of heat-treated (at 600°C) samples, estimated by Scherrer,s equation was found to be ~50-60 nm that confirmed by TEM.

In this paper, failure stress of tubular samples with fiber reinforced composite material is reported. Investigations showed that during the loading process, maximum tension always occur at the tip of the crack and then falls away from the tip of the crack, Also when in the tested samples, applied load is more than 405N, Plastic area near the tip of crack is abserved which shows the fracture of the pipes occure in this area. The results of simulation with finite elements technique coincides with experimental data. The results of this study can be used to analyze the behavior of composite materials used in pipes.

In this study, the formation of calcium-cupper-titanate (CCTO) perovskite powder by mechanical alloying of the constituent oxides under air atmosphere at ambient temperature was investigated. By applying a relatively low-energy ball milling, the reaction sequence was identified. The chemical composition of the as-milled powders was evaluated by X-ray florescence analysis. The qualitative and quantitative structural analysis was performed by X-ray method using the Rietveld refinement. Moreover, the particle size evolution was studied by transmission electron microscopy. The mechano-synthesis of CaO, CuO, and TiO2 led to the formation of perovskite CCTO, where negligible contamination due to milling was detected. In the early stages of milling, the particle size reduction, the nanostructure formation, and a high degree of amorphization were observed. It was also found that the rate of amorphization is different in the constituent oxides. In the middle stage of milling a fully amorphous structure was formed and since the amorphous composition reached the CCTO composition, the CCTO phase was nucleated from the amorphous phase in the final stage of milling. The TEM micrograph shows the CCTO particle size is in range of 10-35nm. influences of different dispersing agents (sodium dodecyl sulfate, SDS) as a benchmark for future dispersion experiments and excitation wavelength are discussed and the results are compared to the commonly used UV-Visible spectroscopic analysis. The results indicated that synthesis of hydroxyapatite particles in the presence of the carbon nanotubes had the best result in homogenization of the carbon nanotube dispersion and faster crystallization of hydroxyapatite, and usage of SDS for dispersion carbon nanotubes at hydroxyapatite matrix, the render formation hydroxyapatite coating on CNTs surface. The average crystallite size of heat-treated (at 600°C) samples, estimated by Scherrer,s equation was found to be ~50-60 nm that confirmed by TEM.

In this investigation effect of welding dilution on chemical composition,microstructure and abrasive wear resistance on mild steel fabricated by flux-cored wire were studied.For this purpose flux cored wire containing ferro manganese was used and fabricated hardface layer in one,two and three layer. The OM, SEM, chemical analysis, hardness and wear tests were carried out on the each layer. Metallographic examination results shows that the microstructure of one and two deposite layer consisted of martensite and austenite, but with different in volume fraction of martensite and austenite and the microstructure of third deposit layer consisted of austenite. Chemical analysis test results showed by increasing layers numbers, degree of dilution was changed, so chemical composition of each layer was different. The hardness and abrasive wear tests results showed that by increasing the layers number, hardness and wear resistance were decreased. Hardness after the wear tests indicated that minimum 10% increase can be seen in the value of hardness due of workhardening under wear test. The maximum hardness and good abrasive wear resistance were related to one layer sample.SEM study of the worn surfaces showed that the wear mechanisms of all samples are ploughing, but with the difference in the depth of wear scratches. One layer sample had minimum depth of wear scratches than that others.

In the present work, the effect of addition of 2 wt% CuO on the sintering behavior, microstructure and proton conductivity of the yttrium doped barium zirconate has been studied. Yttrium doped barium zirconate (BZY20) including 2 wt% CuO, as sintering aid, was produced via solid state synthesis method using carbonate and oxides as raw materials. To characterize the crystal structure of the synthesized compounds, XRD analysis was used. The green samples, produced by dry pressing, were sintered in the range of 1300 to 1500 oC for 5 to 24 h. Microstructure of the sintered samples was studied by the means of SEM to determine the amount of remained porosities and the grain sizes. Proton conductivity of the samples was determined using impedance analyzer in a wet and dry reducing atmosphere. The results showed that using 2 wt% CuO is so effective in reducing the sintering temperature and also in increasing the size of the grains. So that the sintering temperature decreased to 1400 oC and grain size increased to more than 5 times. These results also showed that in the presence of 2 wt% CuO the conductivity of the BZY20 compounds can be improved.