Showing 373 results for Co
Hadian Fard M.j.,
Volume 1, Issue 1 (3-2004)
Abstract
Effects of temperature on properties and behavior of a 20 vol % particulate SiC reinforced 6061 aluminum alloy and 6061 unreinforced Al alloy were investigated. Yield strength and elongation to failure were measured as a function of test temperatures up to 180^oC. In addition, the effects of holding time at 180^ oC on tensile properties and fracture mechanisms of the materials at this temperature were studied. The behaviors of the materials were characterized by using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDS), X-ray diffraction (XRD), atomic absorption (AA), hardness measurement and image analyzing (IA). The results show that an increase in temperature leads to a decrease in the yield strength and increase in the elongation to failure of the materials. On the other hand, while increasing holding time at 180^oC produces an increase in the elongation to failure of the unreinforced alloy, it reduces the elongation to failure of the composite. It was also observed that reduction in yield strength with increasing holding time at 180^oC was faster for the composite material compared to the unreinforced alloy. The results from SEM, XRD, EDS, IA and hardness tests indicated that some chemical reactions had taken place at the interface between the reinforcement and the matrix alloy during holding the specimens at elevated temperature. Therefore, different trend in elongation to failure of the unreinforced alloy and the composite material with holding time at elevated temperature could be attributed to development of chemical reactions between the reinforcement and the matrix alloy at the interface.
Choo R.t.c., Toguri J.m.,
Volume 1, Issue 1 (3-2004)
Abstract
A two dimensional mathematical model has been developed for describing the temperature, flow, and electric fields in the are column of the Gas Metal Arc Welding (GMAW) of aluminum in argon shielding gas using axisymmetric Navier-Stokes, Maxwell, and differential thermal energy equations. The predicted results are most sensitive to the cathode spot radius and an optimum cathode spot radius exists on the basis of the minimum arc power consumption. The consumable electrode shape change due to droplet detachment is simulated in a quasi-steady manner using different electrode diameters. The change in electric field profile with different electrodes gives rise to large changes in flow conditions. This flow perturbation plus the non-uniform and transient J*B force field are suspected to enhance the recoil experience by the droplet on detachment, thus leading to violent and chaotic metal transfer in GMAW.
Mir Habibi A.r., Mir Habibi A.r., Mir Habibi A.r.,
Volume 1, Issue 1 (3-2004)
Abstract
The continuity and thickness of the coating layer, are the most important factors in wetting properties and strength of carbon fibers. These factors are crucial in the quality of metal matrix composites made with carbon fibers. In this research the Polyacrylonitrail base carbon fibers have been nickel coated with 0.2, 0.5, 0.8 and 11 ,u in thickness, by the electroless method. The effect of the thickness of nickel coating on surface condition and also the tensile strength of the carbon fibers has been investigated. The study of surface condition of the coated carbon fibers by SEM showed that the nickel coating at the thickness of about 0.5 pin has the best continuity oil the carbon fibers. The results of tensile tests of carbon fibers coated with different thickness of nickel showed that increasing the thickness of coating layer decreases the overall strength of fibers.
Arabi H., Rastegari S., Mohammad-Sadeghi B.,
Volume 1, Issue 1 (3-2004)
Abstract
In this research, effects of changes in aluminizing conditions on microstructure of Pt - aluminide coating applied oil a Ni - base superalloy GTD -111, has been studied. A thin layer (i.e.68,#mm ) of Pt was electroplated onto the surface of the .samples, and then they were aluminized by pack cementation technique under various conditions of time, temperature, rate of heating and pack powder compositions In addition, by application of a thin Ni layer on the substrate before applying Pt, the source of nickel available for diffusing into Pt layer during aluminizing process enhanced and the need for an extra heat treatment cycle before aluminizing process was practically omitted. Addition of a nickel layer, also prevented scaling of Pt layer during its electroplating and aluminizing processes that helps enhancement of cohesiveness of the coated layers.The general microstructure of the coating consisted of four layers, which are PtA12layer internal diffusion layer external diffusion layer and interdiffusion zone. The structure of each layer has been studied by optical and scanning electron microscopes as well as XRD technique.The results show the presence of the original Pt - electroplated layer had no effect on the thicknesses of the coating layers, but higher aluminizing time and temperature had increased the thicknesses of interdiffusion and internally diffused layers. In addition, at high temperature, aluminizing with a lower heating rate caused an increase in the thickness of internal diffusion layer. Aluminizing with a lower heating rate at high temperature (more than 900°C) had increased the thickness of interdiffusion laver Attempt has been made to justify , the changes occurred in microstructures and thick nesses of various coating layers as they exposed to different aluminizing conditions.
Akhlaghi F., Zahedi H., Sharifi M.,
Volume 1, Issue 2 (6-2004)
Abstract
In this study different volume fractions of SiC particles of various sizes were introduced into the semisolid A356 aluminum alloy by a mechanical stirrer. Then the slurry was poured into a permanent die of certain dimensions either when the metal alloy was partially solid (semisolid-semisolid or SS route) or after reheating to above the liquidus temperature of the alloy (semisolid-liquid or SL route). Both the SS and SL composite samples were solution-treated at 520°C for 8 hours followed by quenching in water at room temperature. Microstructural characterization studies have been conducted on both the SL and SS samples to quantify the effects of the size andcontent of the SiC particles as well as the solutionizing treatment on the morphology and size ofthe eutectic silicon particles and the matrix grain (globule) size. The results were rationalized in terms of the different nucleation, fragmentation, spherodization and coarsening events, which had taken place during the processing of these composites. Finally the impact of these microstructural features in improving the wear properties of the composites has been discussed.
Fazel Najafabadi M., Golazar M.a.,
Volume 1, Issue 2 (6-2004)
Abstract
The new in situ method for AI-TiC composite fabrication has been carried out. In this method, fabrication of AI-TiC composite by simultaneous introduction of titanium oxide and carbon into aluminum melt was investigated.. Under the process conditions, titanium and carbon reaction results in titanium carbide whiskers. The salt containing keriolite (Na3AIF6), titanium oxide (TiO2) and graphite used for this purpose. Using Scanning Electron Microscopy (SEM) and X-Ray Diffraction analysis (XRD) the resulted composite was characterized. It was shown that it contains Al as matrix and TiC as the reinforcement. Then, mechanical properties of fabricated composite were examined.
Barzegar Alamdarit B., Ashrafi Zadeh S.n., Ashrafi Zadeh F.,
Volume 1, Issue 2 (6-2004)
Abstract
In the current research, the optimum conditions for the electrolytic deposition of TiO2 coatings on titanium pieces were experimentally investigated. Flat pieces of commercially available titanium with dimensions of 50 x20 x3 mm were used as the anode and cathode electrodes. The coatings were applied on the cathode in an electrolyte solution essentially from water and methanol, containing different amounts of TiCI4, and H202. Coatings of sufficient thickness and adequate adhesion to the substrate were obtained at the optimum conditions of theELD process. The latter conditions were electrode gap distance of 3 cm, TiCl4 concentration of 0.005M, H202 concentration of 0.1 M, current density of 35 mA/cm2, methanol/water volume ratio of 9, and pH of the electrolyte in the vicinity of 1.40. Results of XRD analysis revealed the presence of anatase crystals of titanium oxide in the coated layers, where the deposited coating was treated at some temperatures in the range of 400 to 600°C for a period of at least 2 hours. Scanning electron microscopy (SEM) pictures also confirmed the formation of a uniform coating layer with cracked suiface area. At the optimum conditions of the process coatings with thicknesses of up to 10 flm were easily obtained through the application of one to three deposited layers.
Sheikh Abdolhossein A., Nili Ahmad Abadi M.,
Volume 1, Issue 3 (9-2004)
Abstract
In the present research influences of different combinations of five types of inoculants and four types of nodularizers on eutectic nucleation in ductile cast iron were studied. The alloys were kept at 1320°C for 0, 5, 10, 15 and 20 min under nitrogen atmosphere. Cooling curves and image analysis were used to characterize the influence of holding time and different combinations of inoculants and nodularizers on the graphite morphologies and eutectic nucleation. When dominant morphology of graphite is spheroidal or flake, by decreasing the different combinations of nodularizers and inoculants fading resistance, temperature of eutectic undercooling decreases. Nevertheless, when there is combination of spheroidal, vermicular and flake graphites temperature of eutectic undercooling increases by reduction of material fading resistance.
Kaviani S., Marashi S. P. H.,
Volume 1, Issue 3 (9-2004)
Abstract
The surface condition and microstructure of near stoichiometric (Fe823Ndll.8B5.9) Nd-Fe-B alloy ribbons and the effect of melt spinning parameters were investigated using optical, scanning and transmission electron microscopes (SEM, TEM). The formation of gas pockets on the roll surface of the ribbons during melt spinning can prevent heat transform and result in local coarse grains. The local thickness would also be less in these places and thus perforates preferentially during ion beam milling. Therefore different areas of the sample should be carefully observed in the TEM. Reducing inert gas pressure in the chamber will eliminate the gas pockets. As a general trend, decrease in the ribbon thickness and mean Nd2 Fe14B grain size were observed on increasing the roll speed. By careful adjustment of the melt spinning parameters, the nanostructure will develop. An orientation relationship was found between Nd2 Fe14B and α-Fe precipitates for coarse grain samples melt spun at low roll speed. Dark field image of such grain also shows that some of these α-Fe precipitates have the same orientation. X-ray diffraction evident the development of texture by decreasing the roll speed.
Saghafian Larijani H., Rainforth W. M.,
Volume 1, Issue 3 (9-2004)
Abstract
An AI-7wt%Si-5vol%TiCp was worn against a cast iron disc in a tri-pin-on-disc machine, under dry sliding conditions at the sliding speed of 0.24 m/s and applied loads of 6, 20 and 40 N/pin. Stress-strain (σ-ε) curves were constructed by measuring the microhardness and the equivalent strain gradients in near surface regions on the cross-sectional surface prepared parallel to sliding direction.It was shown that, both the magnitude of plastic strains and the depth of plastic deformed zones increased with the applied load. The material exhibited considerable work softening in addition to work hardening at the highest applied load. The softened layer placed just beneath the mechanical mixed layer (MML), was mostly covered with the fine fractured eutectic Si and TiC fragments most of which were associated with microcracks at Al/Si and Al/TiC interfaces. The results were discussed in terms of some of the current work hardening models.
Mir Habibi A.r., Rabiei M., Agha Baba Zadeh R., Moztar Zadeh F., Hesaraki S.,
Volume 1, Issue 3 (9-2004)
Abstract
ZnS : Cu phosphors were prepared by using laboratory grade chemicals through coprecipitating Cu along with ZnS using H2S and thiourea. Photo- and electroluminescence studies indicate that these phosphors have better emission characteristics compared to the phosphors in which activator is externally added. Phosphors with luminescence at ~530nrn were prepared. The difference between the characteristic properties of the samples seems to be due to formation of nanoparticles during the preparation of the samples by different methods.
Karaminezhaad M., Maghsoudi A.a., Nozhati R., Sakhaei A.,
Volume 1, Issue 4 (12-2004)
Abstract
A large number of reinforced concrete structures subjected to chloride ions. Two basicapproaches for preventing corrosion of reinforcing steel embedded in concrete are: Increasing theconsolidation of concrete and using different coating on rebars. In present research steel rebarsare coated in different ways: a) 40 µm of zinc electroplated on steel rebar b) Zinc powder withepoxy paste (zinc rich). The rebars were placed in a macrocell design according to ASTM G109-92. Concrete operations were done with mixture designs of high and normal strength concrete.The results show corrosion decrease of zinc coated rebars.
Pakshir M., Amini R.,
Volume 2, Issue 1 (3-2005)
Abstract
Anodes are critical component of cathodic protection systems. As part of this effort, three different anodes were tested in a cathodic protection system that was designed and constructed to prevent further corrosion of reinforced concrete. This anodic system includes an electrically conductive coating composition applied in fluid form over an outer surface of the concrete mix. The composition further includes a predetermined amount of electrically conductive carbon material (coke, carbon black, graphite) uniformly distributed in the epoxy resin (as a binder) whereby the coating composition has a predetermined value of resistively. This investigation attempts to find the best type and optimum content of conductive carbon filler in poxy coating, to ensure optimal anode working parameters for marine environments (basically marine and sewer environments) and if any of the coating systems tested in this study excel over the other. In this study, electric and electrochemical parameters of three layer (with average coating thickness of 300µm) coke-epoxy, carbon black-epoxy and graphite-epoxy conducting paints (with different amount of filler) have been determined during long-term anodic polarization (70 days) in a seawater solution. During this test, on the basis of impedance measurements, the electrical resistances of these coatings have been calculated every 14 days. if conductive paints exhibit good electric and electrochemical stability, they will be attractive for cathodic protection of reinforced concrete.
Dehghanian C., Saremi M., Mohammadi Sabet M.,
Volume 2, Issue 1 (3-2005)
Abstract
The synergistic behavior of molybdate and phosphate ions in mitigating the corrosion of mild steel in simulated cooling water was evaluated performing potentiodynamic polarization and impedance spectroscopy tests. Phosphate and molybdate showed a synergistic effect on corrosion inhibition of steel in simulated cooling water. The observed reduction in anodic and cathodic current densities could be the consequence of incorporation of both phosphate and molybdate ions in forming a protective layer on the surface. The charge transfer resistance of the protective layer formed on steel surface was much greater in presence of both ions in solution than that when each inhibitor used alone.
Mehryab A., Arabi H., Tamizifar M., Seyedein S.h., Razazi M.a.,
Volume 2, Issue 1 (3-2005)
Abstract
In this research, the mechanism of joining three sheets of metals, i.e. brass-steel-brass, by cold roll welding process has been studied. For this purpose, the two surfaces of steel sheets were roughened with stainless steel wire brush by different amounts, then the brass sheets were put on both sides of the steel sheets, before they were subjected to cold roll process. During rolling, peaks of the asperities on the surfaces of the steel sheet were pressurized, i.e. deformed, much more than that of trough. Hence, more hardening due to formation of higher dislocation density in the peaks regions were detected in comparison to the trough regions. Therefore, due to the differences in the amounts of work hardening occurred during cold rolling in the peaks & trough of the scratches and also due to the nature of the rough surfaces of the steel sheets, which causes the smooth surface of soft brass sheets laied over the rough surface of the steel sheet to be shappend according to the profile of the steel sheet scratches during cold rolling, mechanical locking occurred at the interface of brass & steel sheets. In addition, while the extrusion of brass took place through cracks within the surface of hardend peaks and metal bonding occurred on the contact points of the brass sheet & the vergin steel. Therefore, it seems two mechanisms were in operation is making a suitable joining between the sheets. One was a locking mechanism due to the roughness of the steel sheets & the other was bonding mechanism due to the bonding between the peak points of the scratches &soft brass surface. The strength of the bonded points in the interface were later increased by annealing the composite, so that by annealing the samples within the 500-900°C range for aperiod of 1 1/2 hr the interface strength increase substantially. The results of peeling test indicated that the interface strength of the samples annealed at 700°C or more increased so much that the brass sheet toms during peeling & the fracture did not pass through the interface.
Paydar M.h., Fadaei R., Shariat M.h.,
Volume 2, Issue 2 (6-2005)
Abstract
Copper coated SiC powders having three different amounts of copper, in the range of 20-60 wt%, were prepared via electroless coating process. The produced composite powders were uniaxially cold compressed and sintered at different temperatures and times under protective atmosphere. It was found that composite Cu/SiC powders and a relatively dense copper matrix composite with a uniform distribution of SiC reinforcing particles imbedded in copper matrix can be fabricated via electroless coating method followed by conventional cold pressing and sintering process. The results also show that SiC particles have a poor wettability with copper and so liquid phase sintering of the Cu/SiC composite powders did not enhance densification of the samples. Regarding this fact, optimum sintering temperatures, which depends on copper content, was determined to be in the range of 1050-1080?C.
Morakabati M., Arabi H., Mirdamadi Sh., Abbasi S.m.,
Volume 2, Issue 2 (6-2005)
Abstract
This study was launched to investigate the effects of heating rate and aging parameters on the kinetic of precipitation reactions in a high alloy high strength steel having Ni, Co, Mo and Ti. For this purpose, as quenched specimens were subjected to three types of aging methods with different heating rates. These methods consisted of aging in Pb bath, salt bath, and furnace at different aging cycles. The kinetic of precipitation in each method was studied by hardness measurements and was described adequately by the Johnson-mehl-Avrami equation. Remarkable increase in hardness and its rate is observed when the rate of heating increases. The substantial increase in hardness of the specimens aged rapidly in salt & Pb baths, compared with those aged normally in furnace, seemed to be due to the formation of thermo elastic stresses during sudden expansion of the substance subjected to rapid heating. According to the results obtained in this research, increase in the Avrami constants, n & k, and decrease in the start time of transformation, ts, are associated with heating rate increasing. Analysis of the observed and calculated data for hardness using Arrhenius equation, shows that for the same amount of volume fraction of precipitates, the activation energy of precipitates decreased for f=25 and 50%, while at f=90 % it increased by increasing heating rate.
Ghobeiti Hasab M., Seyyed Ebrahimi S.a., Badaee A.,
Volume 2, Issue 2 (6-2005)
Abstract
In this research the sol-gel auto-combustion method was used to prepare strontium hexaferrite nanopowder. A solution of distilled water, ferric and strontium nitrates, citric acid, trimethylamine, and n-decyltrimethylammonium bromide cationic surfactant, was heated to form a viscous gel. The gel was heated and then ignited automatically. As-burnt powder was calcined at temperatures from 700 to 900?C in air to obtain SrO.6Fe2O3 phase. The influence of the calcination temperature on the phase composition of the products has been investigated. X-ray diffraction confirmed the formation of single-phase strontium hexaferrite nanopowder at temperature of 800?C.
Razaghian A., Yu D., Chandra T.,
Volume 2, Issue 3 (9-2005)
Abstract
Fracture behavior of a 7075 aluminium alloy reinforced with 15 Vol%. SiC particles was studied after T6 and annealing heat treatments under uniaxial tensile loading at room temperature. The scanning electron microscopy of fractured surfaces and EDS analysis showed:, that fracture mechanism changed from due mainly to fractured particle in T6 condition to interface decohesion in samples in annealed state. Different fracture mechanisms in annealed and T6 conditions can be ascribed mainly to the significant difference in the stress concentration levels around the particles. In T6 condition, very high local stress sufficient to cause fracture of particle can be generated during loading, while the presence of large precipitates at the particle/matrix interface produced interface decohesion leading to final fracture in the annealed state.
Mirbagheri S.m.h., Chirazi A.,
Volume 2, Issue 3 (9-2005)
Abstract
A numerical model has been developed for the determination of liquid flow permeability through columnar dendrite during growth. The model is inclusive two stages, first numerical evolution of the dendrite shape during growth, and second numerical determination of the interdendritic liquid permeability. Simulation results shown which solute concentration by evolution of dendrite shape could result to reduction of the permeability during solidification time. Comparison between the experimental data from other authors and the present numerical model data, for the low and high solid fractions, has shown a good agreement rather than current numerical models. Therefore present permeability model, in this investigation, could be used for all of the micro solidification codes by coupling on the segregation and the Fick's equations in domain of the inter-dendritic liquid for mushy alloys.
