نشریه مهندسی متالورژی و مواد
سال بیست و ششم شماره 1 (پاییز و زمستان 1393)

In this study, plasma electrolytic oxidation of 2024-T3 aluminum alloy was investigated. The electrolyte used in this investigation was an alkaline solution containing additives of silicate, phosphate, potassium hydroxide and glycerin. A conventional DC power source with a low applied voltage was employed. Corrosion resistance of the oxide layer was investigated by means of Tafel plots and electrochemical impedance spectrometry. Furthermore, the wear resistance was investigated by pin on disc method. The surface and cross-sectional morphology of the oxide layer was also investigated by scanning electron microscopy. The composition of the oxide layer was studied by XRD. The results showed that after plasma electrolytic oxidation, the corrosion and wear resistance of the alloy are improved.

In this research, aluminizing packs containing NaF and NH4Cl activators were used for aluminizing of pure titanium samples at 1100 ºC. The results showed that the layers of TiAl3, TiAl2, γ-TiAl, α2-Ti3Al, and α-solid solution are formed on titanium samples in both cases. However, the weight gain and the thickness of TiAl3 layer in treated samples with NH4Cl activator are greater than those when NaF activator is used. Using a thermodynamic model showed that the partial pressures of aluminum-containing gases when NH4Cl is used are much greater and accordingly, the experimental results were justified.

Inthis investigation, the slope casting method was used and its combined effect with melt overheating and melt treatmenton microstructural modification and hardness of hypereutectic Al–Si alloy was studied. The results showed that the combined method leads to the refinement of primary silicon and a change in its morphology from plate-like to quasi-spherical shape. In addition, the modification and refinement of primary silicon increased the hardness of hypereutectic Al-Si alloy.

Comparison of the effects of two substrates and additives on pulse current electrodeposition of Au nanoparticles showed that using indium-tin oxide instead of Cu and cysteine instead of potassium iodide can improve morphology, current efficiency, photocatalytic effect and increase the number of nanocrystals. The examinations using the field-emission scanning-electron-microscope (FE-SEM) showed that Au nanoclusters appear on the Cu surface when pulse frequency increases from 50 to 142.8 Hz and potassium iodide additive is added, whereas both the coating thickness and the current efficiency increased with the frequency of the pulse current. As current density decreased, the deposition thickness and the cathodic current efficiency decreased from 10 to 2.5 mA/cm2.

In this research, kinetics of carbothermic reduction of molybdenite in the presence of different amounts of sodium carbonate has been studied. Mixed powders consisting of molybdenite, graphite and sodium carbonate with three mole-ratios of 1:4:2,1:4:3 and 1:4:4 were mechanically activated for 40 hours. The results of X-ray diffraction on activated samples showed that no reaction occurrs in different samples during milling. The effect of mechanical activation on reduction of molybdenite particle size in three mixtures having different amounts of sodium carbonate was investigated by the Williamson-Hall method. The results indicated that the grain size of molybdenite increases with increasing the amount of sodium carbonate in different samples. To study the kinetics of carbothermic reduction of molybdenite in the presence of different amounts of sodium carbonate, simultaneous thermal analysis (STA) with heating rates of 10, 15 and 20 0C/min was carried out on activated samples. The results were evaluated using the Friedman, Kissinger and Ozawa approaches and it was found that the temperature and activation energy of molybdenite carbothermic reduction in the presence of sodium carbonate increase with an increase in the amount of sodium carbonate in different samples.

For the purposes of this study, Hydroxyapatite (HA)-Al2O3-TiO2 nanobiocomposites with significant mechanical properties, biocompatibility and capability to form surface apatite were fabricated by cold pressing and sintering. Samples were examined for their compressive strengths. The results of compression experiments showed that sample A (50 wt.% TiO2-30 wt.% HA- 20 wt.% Al2O3) was superior compared with sample B (30 wt.% TiO2-50 wt.% HA- 20 wt.% Al2O3). In addition, the examination of porosity in sample's surfaces showed that sample A has less prosity than sample B. In vitro bioactivity of the nanobiocomposites in a simulated body fluid (Simulated Body Fluid (SBF)) was also investigated. After immersing the samples in the SBF solution for 7 days, sample B exhibited greater ability to form calcium phosphate compounds on the surface. In vitro studies showed that MG-67 osteoblast-like cells were attached and spread on the sample's surfaces. The results showed that cells proliferated in greater numbers on the B sample compared to the A sample. Finally, X-Ray diffraction (XRD) and scanning electron microscopic examinations, energy-dispersive X-ray Analysis (EDX), and Fourier transform infrared spectroscopy (FTIR) were performed in order to identify different phases, to study the microstructures, to determine concentration of different elements, and to identify the bonds formed in samples, respectively. To prevent the formation of microcracks and secondary phases, sintering operation was conducted at 1000 °C. Based on the results obtained and considering desirable properties of samples, both nanobiocomposites can be used in dental implants and orthopedic applications.

In the present work, attempts have been made to find appropriate coating by testing different compounds. Results showed that the optimum enamel is a compound with 1 wt.% cobalt oxide, 3 wt.% chromium oxide (in addition to hundred percent) with the firing temperature of 840 ˚C. Constants of VFT equation were determined by examination of melting behavior of the optimum enamel and measurement of the transition and softening temperatures (Tg and Ts). Results also showed that in the firing temperature, the viscosity of coating is 103.8 Pa.s, the contact angle (wetting of melt with the ceramic substrate) is 135° and the surface tension of the melt is 245.1 mN/m.

Structural sandwich beams are widely used because of their high specific stiffness and strength, noise reduction, thermal insulation and impact energy absorption characteristics. Conventional manufacturing method of composite sandwich structures is completed by a separate adhesive joining stage by which the composite faces are joined to the core. In this investigation, liquid phase sintering in the Al-Zn alloy system was used in order to form a diffusion bond between metallic foam and plate. Joining treatment was conducted at 610 oC for 30 minutes in an inert atmosphere. The shear strength of the joint was measured to be about 4.8 kPa.

The quenching and partitioning (Q&P) process is a new heat treatment cycle to produce the third generation of advanced high strength steels based on diffusion of carbon from martensite to retained austenite phase. The application of this process for various steels leads to a remarkable combination of mechanical properties including high strength and good ductility. Using this process in automotive industry causes better mechanical performance, greater car safety, weight saving and cost reduction. In this study, a low alloy high carbon steel has been subjected to Q&P process and its microstructure and mechanical properties have been investigated. The results showed that the microstructure of the quenched and partitioned steel contains carbon-enriched stabilized austenite and carbon-depleted martensite resulting in increasing the strength of steel with an acceptable ductility. Conducting Q&P heat treatment caused the retained austenite content and the hardness of steel to increase. Fracture in specimens treated by Q&P process was of brittle type and their fracture surface was considered to be quasi-cleavage.