Journal of Metallurgical and Materials Engineering
Year:2018 | Volume:30 | Issue:1 |Papers Count:14

The electrochemical exfoliation of graphite is a method to obtain graphene. Despite the important role of electrical power in this process, it hasn’ t been investigated extensively. In this research, the effect of electrical power in an extensive range on the amount of exfoliated graphene and oxygen functional groups was studied. Transmission electron microscopy, weighing, UV-vis spectroscopy and electrical conductivity were used for characterization of the products. According to the results, by decreasing the power, the amount of exfoliated graphene and its functional groups increased.

The goal of this study was to develop nano/ultrafine bainitic microstructure in AISI 52100 steel. For this purpose, the specimens were austenitized, followed by austempering treatment at different temperatures and times. Austempered samples were characterized using field emission scanning electron microscopy, X-ray diffraction and tension and wear tests. The results showed, at austempering temperature up to 250oC, the strength and ductility reached 2000 MPa and 7%, respectively. By further increasing the austempering temperature, the strength and ductility were simultaneously reduced (to about 1808MPa and 3%).

In this paper, bismuth titanate (Bi4Ti3O12, Bi12TiO20) nanostructured films were successfully fabricated via sol-gel method. The structure and dielectric properties of the prepared thin films as a function of annealing temperature and applied frequency were investigated. In this order, two different solutions with optimal ratio of raw materials were prepared. The solutions were deposited on the substrates by dip coating, and then heat treated at different temperatures ranging from 300 to 700 oC for 1 h. The prepared films were characterized by means of Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and LCR meter. It was found that the single phases were formed when Bi4Ti3O12 and Bi12TiO20 samples were annealed at 600 oC, therefore, optimal condition could be obtained at this temperature. Dielectric studies showed that the dielectric constant and loss factor were increased with increased annealing temperatures. Also, the values of dielectric constant were decreased and dielectric loss increased with increasing frequency.

The thermodynamic and kinetic behavior of nickel aluminide IC221M alloy in molten eutectic of lithium/sodium carbonate was investigated. The nickel aluminide IC221M alloy was produced by vacuum arc re-melting (VAR) method. Surface layer products formed after immersion in molten carbonate at 600, 650 and 700 oC were examined by SEM and XRD. The results along with weight difference calculations showed that IC221M alloy doesn’ t have a good resistance at 650 and 700 oC. It was evidenced that the destruction of the alloy is a diffusion-controlled process. The activation energy of the process was calculated to be 423 kJ/mol and the process kinetics were controlled by aluminum diffusion in γ ´ phase.

Dual and triple phase steels have unique properties such as continuous yielding behavior, low ratio of yield strength to tensile strength, high work hardening rate and high homogenous elongation percentage; thus, they are used widely in automotive industries. The aim of this study is to develop a triple-phase microstructure of ferrite-bainite-martensite with high strength and desirable ductility. In this research, the samples of 4140 steel were austenitized at 850° C for 1 hour. Then, the samples were kept at temperature of 720° C (the ferrite-austenite region) for 3 minutes, and transferred to a salt bath at different temperatures of 380, 400, 420 and 450° C for 4 minutes to obtain various morphologies of bainite, finally quenched in water to form martensite. The observation of the microstructure by a scanning electron microscope showed that with increasing austempering temperature from 380 to 450° C, the morphology changes from lower bainite to upper bainite. Hardness and tensile tests revealed that by increasing the austempering temperature the hardness, yield strength, ultimate tensile strength, elongation and the amounts of “ n” and “ k” in the Holloman equation, decrease. Fractography of the tensile tested specimens by a stereo-microscope showed that with increasing the austempering temperature, the fracture surface changes from ductile to brittle.

In this study 38wt%TiO2-36wt%NiO-26wt%C powder mixture was ball milled for 5, 10, and 20 hours and then heat treated at 300, 600, 900 and 1200 oC for one hour. Results showed that chemical reaction between initial components of powder did not occur during milling. Increasing milling time from 5 to 20h and heat treatment temperature from 300 to 900oC led to reduction of NiO to Ni by graphite and transformation of anatase to rutile. Heat treatment at 1200oC led to appearance of NiTiO3 phase. In the next step 10 wt% of 20h milled samples which heat treated at various temperatures were added to MgH2 as catalyst and then ball milled for 20 hours. The effect of this process on the hydrogen desorption from MgH2 was investigated. Results showed that adding catalyst produced at 1200oC to MgH2 and ball milling for 20 hours led to 6. 45wt% hydrogen desorption and 170oC decrease in desorption temperature. The reason of this improvement in hydrogen desorption properties was related to the catalytic effect of NiTiO3 phase.

In the present research, FSW of thin sheets of the 7075-T6 aluminum alloy were investigated. Welding speed was held at 50 mm/min. The tool rotational speeds were 600, 1000 and 1600 rpm. The microstructure was investigated by both transmission electron microscope (TEM) and electron backscattered diffraction scans (EBSD). The results indicated that several phenomena occur instantly during FSW in the stir zone, such as precipitation, continuous dynamic recrystallization (CDRX), grain growth and precipitates dissolution which have a severe effect on the mechanical properties. It could also be concluded that for thin sheet of 7075-T6 aluminum alloy, plastic strain is the dominating factor over heat input of the FSW in determination of the microstructure of the weld zone.

The effects of rapid induction heating on the thermal stability of a Ni3Al base alloy containing Cr, Mo, Zr and B was investigated. The alloy was produced with vacuum-arc remelting technique. Microstructural changes in the dendritic and interdendritic region during rapid heating process are clarified using scanning electron microscopy and image analyzing technique. The results of this type of heat treatment were compared with normal heat treatment. Results showed that rapid heating can accelerate the elimination speed of high zirconium phase and also the growth kinetics of γ ' precipitates but has no significant effect on the growth mechanism.

TiO2– MWCNT nanocomposite films containing various percentages of multi-wall carbon nanotubes were coated on fluorinated tin oxide (FTO) conductive glass substrates by sol– gel dip coating technique. Dye-sensitized solar cells were assembled by using anodes made of these nanocomposite films, iodide/triiodide redox couple electrolyte and platinum coated FTO cathodes. Cells having TiO2– 3wt. %MWCNT nanocomposite anode showed maximum solar-to-electric energy conversion efficiency of 5. 31%, which was 69% higher than conventional TiO2 solar cells. The enhancement of conversion efficiency was attributed to the increased electrical conductivity of the anode, decreased charge carrier recombination in the anode and decreased charge transport resistance at the TiO2/dye/electrolyte interface.

Titania is one of the best-known photo-catalysts. Its photocatalytic properties can be improved by coating on silica spheres and using an additive. In this study, titania sol was separately synthesized by colloidal and polymeric methods. Titanium tetra-isopropoxide, isopropanol, cerium nitrate, deionized water; nitric acid and chloride acid were used as precursors for the preparation of titania sols. Cerium-doped titania sols were prepared with Ce/Ti molar ratios of 0. 05, 0. 1 and 0. 15 using two different methods and then their properties were investigated. The properties of the samples were evaluated using X-ray diffraction and scanning electron microscopy. The photocatalytic activity of titania-ceria composites was evaluated using methylene blue photo-degradation by ultraviolet waves. Removal ability of methylene blue from aqueous solution was also determined by ultraviolet spectroscopy. According to the results, the titania-ceria composites synthesized through the polymeric method showed better photocatalytic activity than the pure titania. Photocatalytic efficiency increased from 67. 39% for pure polymeric titania to 81. 39% for cerium-doped polymeric titania with a molar ratio of Ce/Ti equal to 0. 05. The photocatalytic efficiency of samples with Ce/Ti molar ratio of 0. 1 and 0. 15 was calculated to be 80. 25% and 79. 34%, respectively. Also, the colloidal titania-ceria composites exhibited more photocatalytic activity than pure titania in the early stages of radiation.

The aim of this study was to propose a facile route for the fabrication of TiB2-TiC composite powder by microwave-assisted self-propagating high-temperature synthesis process. For this purpose, TiO2-B2O3-Mg, TiO2-Mg-C and TiO2-B2O3-Mg-C mixtures were heat treated in a domestic microwave oven. The products were characterized by XRD and SEM. Type of reactions in all investigated mixtures was SHS. Result of 2TiO2: B2O3: 9Mg: C mixture showed successful formation of TiB2-TiC-MgO composite. Unwanted MgO compound was leached out by an HCl acid, yielding a pure TiB2-TiC powder. In this work, the possibility of chemical reactions was evaluated by thermodynamic calculations.

(Zr55Cu30Al10Ni5)100-xNb(x= 0, 2) bulk metallic glass matrix/tungsten wire composites were fabricated by infiltration process at 950 ◦ C and 15 minute processing time. Structural studies and mechanical behaviors of the materials were investigated by scanning electron microscopy and compressive tests, respectively. Based on the results, the fracture surface in the composite sample with X=2 was vein like pattern and with increase of the vein's intensity in the matrix, the ductility reached 28%. In the niobium-free composite sample the secondary intermetallic phases of (W2Zr, W5Zr3) (as the crack nucleation sites) act as the stress concentration sites and the fracture surface changed to smooth mode. In addition, the composite ductility was reduced because of the rapid convection of shear bands to cracks.

In this research, the simulation of wear behavior of 4130 steel has been carried out by ABAQUS software. The wear behavior of 4130 steel was simulated at three normal loads of 1, 3 and 5 N under pin-on-disk test. It was found that the predominant wear mechanism was the abrasion and with increasing normal load the contact stress between pin and disk, width and depth of wear track increase during the wear test. Also, it was concluded that the results of simulation, had a good agreement with experimental results that shows the validity of simulation to predict the wear process.

Having investigated the different reports and various fluxes, ammonium chloride was eventually selected for pyrometallurgical recovery of metallic zinc from the galvanizing flue dust in the present work. This flue dust constitutes 10 to 15 wt. % of galvanizing wastes. The mechanism of ammonium chloride in the extraction of the metallic zinc from the galvanizing dust was explained. Furthermore, the efficiencies of zinc recoveries in the temperature range of 500 to 700 º C, treating time of 0 to 40 minutes and mixing percentages of 10 to 30wt% fluxes were investigated. The optimum efficiency of zinc recovery was calculated equal to 85% which was observed in the temperature 600º C, flux mixing ratio of 20wt% and treating time of 20 to 30 minutes.