Iranian Journal of Materials Science and Engineering

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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.

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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.

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The effects of gating system and pattern geometry on the metal flow in the lost foam casting (LFC) process have been investigated using glass covered mold and video recording system. Unlike convectional casting process, the type of the gating system showed little effect on fillability in lost foam, but pattern thickness had large effect on mold filling. The mold filling behavior seems to be controlled by the combined influences of heat and mass transfer. The flow rate increased with increasing pattern thickness.

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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.

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Thin sheets of hydroxyapatite were fabricated by tape casting process. The non-aqueoustape casting formulation was optimized by adjusting the concentration of organic additivesspecially those of binder and plasticizer in the slurry. The optimized slurry was cast on a glasscarrier using a designed laboratory scale doctor blade. Following a binder removal stage, thetapes were sintered at different temperatures in air atmosphere. Heat treatment at 1250 °C led tothe formation of a dense microstructure as was evidenced by the scanning electron microscopy.

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During mold filling, molten metal can only advance as quickly as the air inside thecavity is expelled. In this work an analytical model describing air flow is developed based on aincompressible flow theory. Air pressure has serious effects upon the filling behaviour such assurface profile and filling time. In this work a new mathematical model is proposed for calculationthe air pressure during the mold filling. A single phase computational fluid dynamic code based onthe SOLA-VOF algorithm used for prediction the fluid flow. Air discharged through the vents ismodelled by ideal gas assumption, conservation of mass equation and Bernoulli law. A newalgorithm was developed to interpolates the air pressure on the surface cell. The creation of airback pressure was correlated with sizes of vents and pouring basin height. In order to verify thecomputational results a series of experimental test was conducted. Comparison between theexperimental data and simulation results has shown a good agreement.

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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.

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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.

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The mold-filling behavior in the casting of aluminum alloy (A413) using lost foam casting (LFC) was explored. The effects of gate numbers, type of gating and casting thickness on the filling behavior were evaluated. Although, unlike convectional casting process, the gating system showed little effect onfilling ability, casting thickness created a greater effect on the mold filling. In contrast with convectional casting process, the mold filling seems to be controlled by castinggeometry as a consequence of combined influence of heat and mass transfer. The melt used to enter from the first gate instead of last gate which is in contrast with convectional casting process.

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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.

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In this research pitting Corrosion of a sensitized 316 stainless steel was investigated employing potentiodynamic, potentiostatic techniques. Sensitization process was carried out on as-received alloy by submitting the specimen in electric furnace set at 650°Cfor five hours and then the specimen was quenched 25°C water. Potentiodynamic polarization of as received and sensitized specimens in 1M H2SO4 solution at room temperature and 70°C clearly revealed that the sensitization process has caused a magnificent change on electrochemical behavior of the specimen by changing critical current density for passivation, passivation potential and passive current density. Optical microscopy examination of the specimen surface after oxalic acid electrochemical etching also showed the deterioration of grain boundary of sensitized specimen due to chromium carbide precipitation in compared to as-received one. Several anodic potentiodynamic polarization on rode shaped working electrodes prepared from as-received and sensitized specimen in 3.5% NaCl test solution proved an average ~220 mV drop in pitting potential due to sensitization. Anodic potentiostatic polarization at 400 and 200 mV above corrosion potential also demonstrate the deterioration of pitting resistance of alloy as a result of sensitization. Scanning electron microscopy examination of anodically polarized of sensitized specimen at 700mVprior and after oxalic acid etching revealed large stable pits with lacy cover and also openpits with deep crevice for etched specimens.

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Life assessment on the base of grain boundary creep cavitation of 1%Cr - 0.5%Mo low alloy steel has been discussed in this paper. Since microstructural degradation is one of the most important mechanisms that affects creep life, it is necessary to assess microstructural damage in order to estimate the life. Microstructural damage within the grain boundaries is a continuous phenomena starting from about the beginning of secondary stage of creep process. In this research, the amounts of damage accumulation in the form of grain boundary cavitations for various creep times up to the ends of secondary creep stage for each creep condition was found by using quantitative metallography technique, i.e. image analyser. Then from the data obtained for grain boundary area cavitated and number of cavities per unit area, which was about linear as a function of time for each of creep conditions, the amount of damage in the tertiary stage was estimated for various times. Then a creep damage parameter was proposed for the creep process. Finally, having this damage parameter (?) and using continuum damage mechanics (CDM), a new version of Rabotnov-Kachanov equation for tertiary creep rate was established.

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Partially stabilized zirconia (PSZ) has been proven to be an excellent candidate as a thermal barrier coating (TBe) for hot sections in, for instance, heat or internal combustion engines and gas turbine parts. The main functions of these coatings are reducing heat losses, reducing fuel consumption, increasing efficiency, and extending durability and life. One of the main problems involved is wear behavior in the development of such coatings for these applications. Using the air plasma spraying (APS) technique, conventional and nanostructured 8 wt % yttria partially stabilized zirconia (Y-PSZ) coatings were deposited on austenitic stainless steel (AISI304) disc-shaped substrates. The coated substrates were subjected to pin-on-disc wear tests, using 10 mm silicon nitride and zirconia balls as the pin. The coefficient of friction was recorded in real time. The weight loss of coated substrates was measured. Coatings were characterized before and after being subjected to wear testing by various techniques including optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and X-ray diffraction (XRD). Effects of various experimental parameters such as wear distance, test temperature, and counter face material were also investigated. Results obtained revealed that, regardless of experimental conditions applied, the nanostructured zirconia coating shows better wear and tribological properties than that of the conventional one.

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In this research the influence of Ti contents on the amounts of inclusions formation and mechanical properties of a high alloy high strength steel, C300, has been investigated. For this purpose several bars were casted under the same solidification conditions, but different amounts of Ti element. All the seven casted bars were homogenized at 1200°C for a period of 2 hours. Then, they were immediately hot rolled after homogenization so that the out rolling temperature was kept in the range of 1000-1200° C. The specimens were then solution annealed at 820°C for hour and finally they were aged for a period of 3 hours at 500°C. The samples were subjected to tensile, impact and hardness tests in order to relate the variation in volume percent of inclusions due to different amount of Ti, to mechanical properties. The results showed that by increasing the amount of Ti a serious decline in toughness properties of the alloy due to increase in inclusion population occurred. So this research provides a very useful information about the relation between volume fraction of inclusions and mechanical properties of a C300 high strength steel.

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Although the flash smelting technologies use different furnace designs, the refractory linings are exposed to very similar aggressive environments and, as a result, the corrosion analysis results on one type of furnace could be generally applied to other furnaces of similar high temperature processes. Particularities regarding the different chemistries of the pyrometallurgical process and operating parameters of these furnaces could also bring particular aspects to be considered when analyzing the refractory ware and final failure in use. This paper presents a review of the existent experimental. data of corrosion analyses on refractory linings used in two particular flash furnaces for zinc-lead and respectively nickel-copper smelting. Although various modern water cooling systems are generally used to protect the refractory wall linings against corrosion by molten slag and matte, the performance of the refractory roof lining, usually used without water cooling, represents a permanent concern and the object of research studies to extend their life in service. The failure mechanisms analysed in this study are based on postmortem analyses and laboratory corrosion experiments with magnesite-chrome bricks of different chemical and mineralogical compositions. The gaseous atmosphere, usually rich in SO2 and/or CO and various metal fumes, produces irreversible microstructural changes which could shorten the life in service of the refractory lining. The experimental data proved that thermal cycling in SO2/SO3 atmospheres could bring more damage than a continuous use at relatively constant temperature, due to the magnesium and calcium sulphate formation. The laboratory experiments and postmortem analyses showed that that metal fumes at various partial pressure of oxygen would condense as oxides and react inside the pores and at the grain boundaries, contributing to the continuous deterioration of the ceramic matrix of the refractory brick lining. The mechanisms of corrosion, discussed based on laboratory experiments, were confirmed by the postmortem analyses on brick samples used in the industrial flash smelting furnaces.

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Abstract: In chloride salt solutions, titanium alloys exhibit reasonably high pitting potentials as high as +10 V (vs. Ag/AgCl) at room temperatures. On the other hand, anodic pitting potentials are significantly lower in bromide solutions. In this study, pitting corrosion of commercially pure titanium in aqueous NaBr solution of 0.1 M concentration at room temperature was studied and the effect of an external magnetic field oriented both parallel and perpendicular to electrode surface was investigated. Cyclic potentiodynamic and potentiostatic polarization tests were carried out. Anodic breakdown potential of +1.45 V (vs. Ag/AgCl) obtained in the absence of magnetic field, decreasing to +1.11 V in the presence of a 0.05 T parallel magnetic field. The perpendicular magnetic field actually did not affect the breakdown potential. Applying of an external magnetic field, independent of its orientation, shifted the repassivation potential approximately 150 mV in the positive direction. SEM microscopy observations of sample surfaces indicated that applying of magnetic field results in some variations in the pit shapes and their sizes.

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Abstract: Hardfacing is one of the most useful and economical ways to increase the service life of components subjected to abrasive wear. Iron based hardfacing alloys have long been considered as candidate coatings for wear-resistant applications in industry. In the present work two layer of Fe-34Cr-4.5C%wt hardfacing alloy was deposited on ASTM A36 carbon steel plates by SMAW method. The microstructure consists of large primary and eutectic M7C3 carbides, metastable austenite and small amount of secondary carbides. The microstructure was analyzed by optical and scanning electron microscopes. In the same condition of size, shape, distribution and volume fraction of carbides the as-welded matrix changed to martensite, tempered martensite and ferrite by heat treatment processes. The wear resistance was measured by pin-on-disk method under loads of 5, 10 and 20N and for sliding distance of 1500m. The results showed that the as-welded sample with austenitic matrix has the most and the ferritic matrix specimen has the least wear resistance. The predominate mechanisms for mass losses were determined to be micro-cutting, microploughing.

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Abstract: The use of alkali-activated cementitious materials especially over the past decades has significantly been increased. The goal of this research is to investigate the effects of silica modulus and alkali concentration on alkali-activation of blast-furnace slag. In this research, the most important physical characteristics of cementitious systems, i.e. the 28-day compressive strength and final setting time, were studied by changing influencing parameters such as silica modulus, i.e. SiO2/Na2O, (0.44, 0.52, 0.60, and 0.68) and Na2O concentration (4, 6, 8 and 10% by weight of dry binder) at a constant water-to-dry binder ratio of 0.25. Final setting time of the studied systems varies in the range between 55-386 minutes. The obtained results show that systems cured at an atmosphere of more than 95% relative humidity at room temperature exhibit relatively high 28-day compressive strengths up to 107 MPa.

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Abstract: Cooling slope-casting processing is a relatively new technique to produce semisolid cast feedstock for the thixoforming process. Simple equipment, ease of operation, and low processing costs are the main advantages of this process in comparison with existing processes such as mechanical stirring, electromagnetic stirring, etc. The processing parameters of cooling slope casting are length, angle and the material of the inclined plate and their combinations, which usually affect the micro structural evolutions of the primary solid phase. In order to clarify the effect of the processing parameters on the evolution of the particle size, based on experimental investigation, Artificial Neural Network (ANN) was applied to predict the primary silicon crystals (PSCs) size of semisolid cast ingot via a cooling slope casting process of Al-20%(wt.%) Si alloy. The results demonstrated that the ANN, with 2 hidden layers and topology (4, 3), could predict the primary particle size with a high accuracy of 94%. The sensitivity analysis also revealed that material of the cooling slope had the largest effect on particle size.

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Abstract: The horizontal continuous casting process has received a significant attention for near net shape casting of non ferrous metals and alloys. Numerical Simulation has been widely used for process design and optimization of continuous casting process. In the present study, a 3-dimensional heat flow model was developed to simulate the heat transfer and solidification in a horizontal billet continuous casting system in which the air gap formation and its effect on heat extraction rate from solidifying billet was also considered. In order to test the developed model, it was run to simulate the heat transfer and solidification for an industrial billet caster. The predicted temperature distribution within the mold and billet was compared with those measured on the industrial caster in which a good agreement was obtained. Finally, parametric studies were carried out by validated model to evaluate the effects of different parameters on solidification profile and temperature distribution within the model brass billet. The microstructure of cast billet was analyzed to determine the secondary dendrite arm spacing (SDAS) under different cooling conditions. Based on measured SDAS and predicted solidification rate a correlation between SDAS and cooling rate was proposed for continuously cast brass billet.