مجله مواد نوین
سال هفتم شماره 2 (پیاپی 26، زمستان 1395)

In the present research, the finite element analysis of the surface transformation hardening in the vicinity of workpiece discontinuity was performed both thermally and metallurgically using ABAQUS finite element software. In order to predict the volume fraction of the obtained phases and the hardness profile during continuous cooling stage, the appropriate user-defined subroutine based on the Kirkaldy kinetic model was employed in the data flow of the software. Besides the effects of different controlling strategies and the application of a quenching medium at the discontinuity surface was investigated. Based on the obtained results, these strategies which are based on controlling the input power and travelling speed of the heat source as a function of the maximum temperature at the surface, allows for gaining a more uniform thermal history at the surface. However, as these strategies could not control the cooling rate in the vicinity of discontinuities, applying a quenching medium with an optimum cooling power is essential in order to achieve a uniform hardened layer at the workpiece depth.

In this research,the effects of heat treatment on microstructure and mechanical properties of explosively bonded 304 stainless steel-Ck45 Carbon steel with constant explosive ratio and different standoff were investigated. Samples were heat treated in 2500C and 3500C for 2 and 4 hours in the furnace, and optical microscopy, electron microscopy, micro hardness test and tensile shear test were carried out on samples. Diffusion of iron, chromium, manganese elements and chemical composition of local melted zone were investigated by EDS analysis. The results showed that by increasing standoff from 4 to 5 mm, impact kinetic energy was increased and interface was deformed under severe plastic deformation. Metallography study showed that wavy-vortex interface was created by increasing the standoff. Heat treatment at 3500C for 2 h, increased the thickness of interface intermettallics compounds. Hardness in samples with 4- 5mm standoff was decreased by heat treating 3500C , 2 hr from 270.11 to 171.00 Hv and 279.00 to 195.00 Hv respectively. Strength of samples also decreased from 449.21 MPa to 370.81 MPa and 510.57 to 433.83 MPa respectively. The hardness and strength changes are due to a changes in intermetallics thickness and grain growth phenomenon.

Reactive dyes are used extensively in the textile industry. These dyes are highly water soluble and as a result, their removal from the textile effluent by conventional methods is very difficult. Therefore, many attentions have to be focused on techniques that can lead to the complete destruction of these pollutants. This had led to the study of other methods such as an advanced oxidation process based on photocatalysis. In this research, TiO2–activated carbon nano-composites with different concentrations of the activated carbon (0, 5, 10, 15 weight %) were prepared by a sol–gel method. The structure and morphology of the nanocomposites were evaluated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) techniques. The photocatalytic activity of the nano-composites were evaluated through degradation of reactive red 198 under ultraviolet light irradiations. The results indicated that the TiO2 nano-composite contained only anatase phase and average particle size was 35 nm. The photocatalytic results indicated that the photo-degradation of the reactive red 198 is increased by increasing the activated carbon content to 10 wt% and then decreased. TiO2-5 wt% activated carbon showed the best photocatalytic properties. The photocatalytic results indicated that the reactive red 198 degraded around 42% and 35% during a 210 min irradiation period in the presence of TiO2-5 wt% activated carbon nano-composite and TiO2, respectively.

Composite of Al-Al2O3 have been fabricated by spark plasma sintering (SPS) that is known as one of advanced methods of sintering for metal powders. The microstructural studies and elemental qualitative analyses of composites were carried out by Optical Microscopy, Scanning Electron Microscope (SEM) and EDS analysis.Also, In order to study of mechanical properties, the compressive strength, hardness tests and relative density of samples with zero to 15 Wt% Al2O3 were investigated. The results show increases of hardness up to 95% by increasing of Al2O3 amount, sensitively increasing of strength up to 40% in samples with 5 Wt% Al2O3. Furthermore, in these samples Al2O3 particles are homogenously dispersed in Al matrix. In following, strength amounts are decreased by increasing of Al2O3 Wt% up to 10 and 15%.According to microstructural studies and reducing the relative density of 0.98 to 0.88 is related to unsuitable compressed and the creation of porosity in samples.

Ni-P electroless composite coatings are produced by hard or soft particles in Ni-P matrices. In the co-deposition particles, nanodiamonds have a very high hardness, biological compatibility, high chemical stability and good resistance to abrasive and harsh environments,. In this study Ni–P nanodiamond composite coatings were deposited in various nanodiamond concentration on steel substrates. The effect of adding nanodiamonds and heat treatment on morphology, crystalline structure, hardness and wear resistance were investigated by FESEM, XRD, micro Vickers and pin on disk. The results demonstrated that coating produced with 0.6 g/l nanodiamond concentration had highest particles incorporation and hardness. The crystalline structure composite coating was finer and surface morphology changed. Also the heat treated composite coating had the highest wear resistance at 4000C for an hr. The mechanisms of improvement of the tribological properties of the electroless composite coating are also discussed.

In this research, cathodic plasma electrolysis, as a novel surface coating process, was studied for formation of graphene nano-sheets coating on nickel surface. In this process, the plasma is formed around the cathode by application of high voltage between two electrodes with different surface area and graphene nano-sheet produced from electrolyte contain ethanol as carbon precursor and deposited on substrate by formation of plasma at atmosphere pressure in one step and short time. The quality of coating is studied by using Raman spectroscopy and scanning electron microscopy. The intensity and the position of Raman spectrum bands confirm formation of turbostratic graphene nano-sheets with maximum 10 layers on surface. An increase of process time cause to a slightly increase in thickness at graphene layers and a reduction at structural defects of layers. The effect of laser wavelengths on Raman spectrum by using 633 and 785 nm laser were shown an increase at firs order scattering, D and G and a reduction at second order scattering, 2D and D'' bands intensity.

In this study, the effect of carbon nanotubes and single layers of graphite (graphene) on the electromagnetic properties of epoxy were investigated and compared. To do this, the epoxy-graphene nanocomposite and epoxy-carbon nanotube nanocomposite samples up to 0.5wt% were fabricated by casting method. After that, the morphology and distribution of nano-fillers in epoxy matrix was investigated by scanning electron microscopy. Then, the electromagnetic properties of samples in the X-band (8 to 12 GHz) of electromagnetic radiation were determined by a scalar network analyzer. The results showed that the electromagnetic properties and dissipation factor of epoxy were improved by graphene much more than of carbon nanotubes. So that, by adding of 0.5wt% of graphene or carbon nanotubes in epoxy matrix, the real part of permittivity was improved by 36 and 16 percent respectively. The average value of the imaginary part of permittivity of 0.5wt% of graphene nanocomposite sample was reached to 0.5 that is 3 and 12.5 times of 0.5wt% of carbon nanotubes nanocomposite sample and pure epoxy respectively. The 0.13 and 0.045 was obtained for dissipation factor of nanocomposite samples with 0.5wt% of graphene and 0.5wt% of carbon nanotube respectively. As the results, epoxy nanocomposites containing graphene are more suitable than epoxy-carbon nanotubes nanocomposites for electromagnetic interference shielding and absorbing electromagnetic waves absorption application.

To produce aluminum-matrix composite reinforced with graphite particles, mixing of pure aluminum and graphite powders by a new method, called homogenization in liquid phase, was used. The graphite content was chosen from zero to 4.5wt.% in this study. Composite samples were produced using the mixed powder by way cold-press sintering under different pressure, temperature and time. The specimen microstructures were investigated by optical microscopy and scanning electron microscopy (SEM). To evaluate the mechanical properties of samples, hardness and wear tests were performed. According to the results, homogenization in liquid phase method caused significant improvement in distribution of graphite particles in the matrix and the addition of 2.5wt.% graphite in the matrix. Improved mechanical properties to great extent.

Deep cryogenic heat treatment is a conventional supplementary treatment are used on tool steels, carburized steel and high speed steel to improve their wear resistance and hardness. In this research, the effect of deep cryogenic treatment on the hardness, structure and wear resistance of 1.2344 hot work tool steel has been investigated. To perform heat treatment, the samples were preheated at 650°C for 20 minutes. The samples were then austenized at 1050 ºC for 50 min followed by oil quenching. To investigate the effect of deep cryogenic treatment, some specimens were placed under cryogenic treatment at -196 ºC for 24 hours. It can be conducted that deep cryogenic heat treatment eliminates the retained austenite from 15% in the quench samples to a percentage lower than the detection limit of the XRD technique in the deep cryogenic samples. The results of this research showed that cryogenic treatment leads to an increase in hardness and wear resistance levels by 25 % and 49 %, respectively, in comparison with the quenching tempering treatments. The deep cryogenic heat treatment eliminated retained austenite and increased the carbide percentage. Moreover, the deep cryogenic heat treatment made a more homogeneous carbide distribution with a more uniform particle size and some newly formed nano-sized carbides. These improvements increased the hardness and microhardness of the deep cryogenically treated samples. It was also clarified that the predominant wear mechanism is a combination of adhesive and tribo-chemical wear and the collected wear derbies of the cryogenically treated samples were more brittle and smaller.