Iranian Journal of Materials Science and Engineering

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Compositions of Al2O3+Si, SiO2+Al and Al+Si systems were prepared to study the effect of reaction bonding process on the mullite formation. The composition of each system was adopted according to mullite stoichiometery and sintered in 700-1600°C range. Results showed that the formation of reaction bonded mullite starting from Al2O3+Si mixtures, proceeded in two partially overlapping steps, the oxidation of Si to SiO2, and the reaction of SiO2 and Al2O3 to form mullite. In this system, up to 1400°C, conversion of Si to SiO2 was taken place and cristobalite formed, but mullite formation was not observed. Mullite phase started to form at 1450°C. Results indicated that complete reaction was not occurred up to 1600°C and 2 hours soaking time. XRD patterns of samples in Al+ SiO2 system showed that the reaction through sequences: (a) reduction of SiO2 by Al, (b) formation of a- Al2O3 and SiO2-rderived Si oxidation, and (c) mullite formation. X-ray diffraction patterns of heat-treated Al+Si system showed that reaction between Al and oxygen at 900°C was occurred with the reaction product being a- Al2O3 Oxidation of Si and formation of mullite were not detected in this system. SEM micrographs showed that both Al2O3+Si and SiO2+Al systems have similar microstructures, which consisted of a- Al2O3, mullite and free Si. The microstructures of the samples in Al+Si system consisted of a- Al2O3 free Al and Si with intermetallic Al-Si compound.

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In wire arc spraying, the atomizing air pressure and applied nozzle system are important factors influencing particles characteristics and coating quality. The aim of this paper is to study how the characteristics of particles such as size, velocity and temperature are influenced by the operating conditions in wire arc spray. For that, three types of wires are tested: solid wire of stainless steel 316L, cored wires 97MXC and 98MXC. Arc spray gun is an Arc Jet 9000 manufactured by TAFA. For each condition, the particles temperature and particles velocity are measured using an imaging CCD camera, Spray Watch (Oseir Ltd). Particles size distributions are determined with a laser grain meter. The morphology and composition of particles were evaluated with SEM, EDX and X-Ray diffraction. Results induce important modifications in the particles size distributions and particles velocity. A small diameter nozzle and high atomizing air pressure resulted in a reduction in particles size distributions and an increase in particles velocity.

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Ultra-fine particles of barium hexaferrite have been synthesized by co-precipitation method using barium nitride and iron chloride precursors with a Fe/Ba molar ratio of 11. Co-precipitation was carried out at 25 and 80°C using NaOH as a precipitant. Effect of coprecipitation and annealing temperatures on the phase composition and morphology of the products have been investigated using XRD and SEM, respectively. XRD results indicated existence of BaFeO3-x as a major phase in co-precipitated samples. Analysis of the XRD results also revealed that barium hexaferrite starts to form at a relatively low temperature of 700°C for sample synthesized at 80°C. SEM micrographs exhibit plate-like hexagonal particles of barium hexaferrite for calcined samples. The SEM results showed that the mean particle size of co-precipitated sample at 25°C is smaller than that of 80°C after calcining.

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plasma-sprayed silicon layers have been used to produce silicon nitride layers with fibrous microstructure which optimizes fracture toughness and strength. SEM examination of the specimens shows that the surface is covered by fine needles and whiskers of Si3N4.In order to study the oxygen contamination effect as well as other contaminants introduced during spraying and nitridation processes, surface sensitive analysis techniques like AES and XPS have been used to determine concentration of these contaminants.

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Ultrafine hydroxyapatite (HAp) powders with crystallite size in the range of 10-90 nm were synthesized by chemical precipitation process using Ca(OH)2 and H3PO4 solutions as starting materials. Molar ratio of Ca/P=1.68 was kept constant throughout the process and alkaline condition for the reaction was maintained using ammonium hydroxide. The role of raw material concentration on HAp crystallite size and morphology were investigated using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. The results revealed that variations in crystallite size and morphology of synthesized HAp are strongly affected by the concentration of acid solution. To study the sintering behavior of HAp particles, the powders were pressed at 200 MPa using a uniaxial press. Sintering experiments were carried out at temperatures of 1100, 1250 and 1300°C with various soaking times at maximum temperatures. XRD was also used in determining thephases present after sintering process. The results indicated the decomposition of HAp into a-tricalcium phosphate (TCP) and b-TCP phases at 1300°C. The microstructure of the sintered HAp ceramics was characterized by SEM.

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In this research, rotating bending fatigue test at minimum to maximum stress ratio of R=-1 was used for investigating the fatigue behavior of Ti-6Al-4V alloy. Both smooth and notched specimens, with elastic concentration factor, kt, of approximately 3.6 and 4.1 were used for this purpose.In addition, the effect of variation in ultimate tensile strength, UTS, on the fatigue behavior of this alloy was studied. S-N curves were drawn and the value of notch sensitivity was obtained or each case.The results showed that the presence of notch in Ti-6Al-4V alloy has a different amount of sensitivity when the notched specimens were subjected to high cycle fatigue (HCF) and low cycle fatigue (LCF) tests. However, the notch sensitivity of this alloy was shown generally to be much lower than steel alloys with similar UTS values. Thus, considering the high compatibility of this alloy with the body environment and its low sensitivity to notch, one can strongly recommend this alloy for use in biomedical application.

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The hot cracking susceptibility can be determined by establishing the transition temperature between brittle and ductile fracture at high temperature tensile testing of in situ solidified samples. High temperature tensile properties were determined for commercial cathodic pure Cu and Cu- 30%Zn alloy. The transition temperatures for pure Cu and Cu-30%Zn were evaluated from ultimate tensile stress, true strain and area reduction at different testing temperatures. The results show that hot cracking in pure Cu also occurred below and near to its melting temperature. It can be proposed that in this case excess vacancies and vacancy diffusion and condensation are the dominating mechanisms for hot crack formation. The transition temperature for Cu- 30%Zn was much lower than its solidus temperature and this alloy has more susceptibility to hot cracking as compared to pure Cu. The effect of two different cooling rates (15 °C/min and 60 °C/min) on the transition temperature was investigated. The results show that by increasing cooling rate, the transition temperature will increase. The morphology of fracture surfaces for both ductile and brittle modes were evaluated by SEM Two different morphologies, i.e. interdendiritic and intergranual fracture, was found.

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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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A film of osteoconductive and biocompatible material on biomedical metallic implants can create bioactivity of the implant and shorten healing time. Hydroxyapatite, that is the most important mineral part of human bone, was coated on Ti6Al4V using cathodic electrode position process. Pulse electrode position technique was used and the effects of different parameters such as potential, duty cycle (on time/ (on time+ off time)), temperature and current density on the morphology of the deposits were examined. Nano size deposits were formed under controlled temperature and optimization of voltage and current density.

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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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In this study the hot deformation behaviour of a precipitation hardened (PH) stainless steel at high strain rates has been predicted through hot compression testing. Stress-strain curves were obtained for a range of strain rates from 10-3 to 10+1 S-1 and temperatures from 850 to 1150°C. Results obtained by microstructure and stress-strain curves show that at low temperatures and high strain rates, where the Zener-Holman parameter (Z) is high, work hardening and dynamic recovery occure. By increasing temperature and decreasing strain rate, the Z parameter is decreased, so that dynamic recrystallization is the dominant softening mechanism. The results were fitted using a Log Z versus Log (sinh (a sp) diagram allowing an assessment of the behavior of the stresses measured at strain rates closer to those related to the industrial hot rolling schedules. It is clearly shown that the data collected from low strain rate testing can be fairly reasonably extrapolated to higher orders of magnitude of strain rate.

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In this study, commercially pure titanium/304L stainless steel explosion bonded clads have been annealed under argon atmosphere over the temperature range of 700-900°C for 1h.Microstructure of the clads have been investigated before and after anealing. X-ray diffraction studies revealed that the annealing products in the form of intermetallic phases were gradually formed at the interface of the annealed clads. It was also found that, the bonding zone width increased with temperature according to an Arrhenius type equation. On the base of this equation, the activation energy of bonding zone growth was found equal to about 66.5 kJ/mol. The bond strength of the diffusion annealed clads were evaluated stress relieved. The maximum average tensile strength of ~350MPa was obtained for the as-welded clad. It was found that the bond strength decreased with annealing at 700°C due to an increase in the width of brittle intermetallic layer.

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TRIP (TRansformation- lnduced- Plasticity) behavior is a powerful mechanism to improve mechanical properties. The basis of TRIP phenomena is the transformation of retained austenite with optimum characteristics (volume fraction, stability, size and morphology) to martensite during deformation at room temperature. Accordingly, the first requirement to obtain desired TRIP effects is to produce an appropriate microstructure. Thermo mechanical processing is an effective method to control the microstructural evolution thereby mechanical properties in TRIP steels. This work deals with a TRIP steel containing 0.2% C, evaluating the effects of straining before and during ?®a atransformation on its final characteristics, using hot compression tests. The results revealed that straining in the two phase region (dynamic transformation) not only reduces the ferrite grain size more significantly, but also increases the retained austenite volumefraction. Accordingly the final mechanical properties were also improved.

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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: The theoretical calculation of dislocation density in different regions of a deformed workpiece of 99.99% pure copper has been carried out using different procedures consisting of Finite Element Method (FEM) and hardness measurement. To assess the validity of the results pertaining to these procedures, the dislocation density is experimentally measured utilizing the Differential Scanning Calorimetry (DSC). Comparing the predicted and experimental results, it was found that the average error in prediction of the dislocation density by the hardness measurement and FEM is 12% and 2.5%, respectively. Also, for further confirmation of the evaluated dislocation density of each region of the deformed workpiece, the annealing process was carried out and in the region of higher dislocation density, a finer grain size was observed.

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Abstract: Strength at elevated temperatures and thermal shock resistance of austempered ductile irons at high temperatures has been less intentioned, because of instability of ausferrite phase. In this research the tensile properties of this iron and pearlitic ductile cast iron have been investigated by short time high temperature tensile tests. Also thermal shock tests were done at the molten lead bath at 1000 􀁱C . In these experiments, at first samples were immersed partially in the molten lead bath for 25 seconds and then either cooled in air or quenched in water. Results of short time high temperature tensile and thermal shock tests showed that ADI samples have higher strength and shock resistance than the pearlitic ductile samples.

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Abstract: Static recrystallization (SRX) behavior of a composite based 7075 Aluminum alloy reinforced with SiC particles was studied during annealing the deformed samples at high temperatures. The results showed an absence of SRX in the samples annealed after hot working at the same deformation temperature, however, a rise in annealing temperature of 100-1500 􀁱C above that the deformation temperature led to full recrystallization. This can be ascribed to the relatively moderate dynamic recovery and the presence of dispersions which stabilize the substructure. Particle stimulated nucleation (PSN) had a significant effect on the grain size in deformed samples at low temperature, but no PSN was observed in samples strained at high temperatures. The possible cause might be that at high temperature the dislocations can be annihilated by climb process around the particles together with the absence of deformation zone for nucleating the recrystallization.

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Abstract: Ball mills are used in the last stage of ore processing for grinding raw materials. Forged 70Cr2 alloy steel and Austempered Ductile Iron (ADI) balls are materials from which grinding balls are made for Sarcheshmeh Copper Plant (SCP) ball mills. In the present study wear and impact properties of these two kinds of balls have been investigated. Some balls randomly were selected as samples. They were cut to investigate the cross section under optical and scanning electron microscopes. The microstructure of the sample balls was studied and quantitative measurements of microstructural features were performed. The hardness of different parts of cross sections of balls was measured. The wear resistance of the balls was measured by Pin on Disc method. Repeated dropt test was employed to evaluate impact resistance of the balls. The microstructure of ADI balls consisted of bianitic matrix with graphite nodules and some retained austenite and martensite. Micro cracks and porosities in the cast structure were frequently observed. In the case of forged steel balls the microstructure composed of tempered martensite in outer area and bianitic structure with some tempered martensite in central areas. The wear and impact resistance of forged steel balls were markedly higher than those of ADI balls. The difference was consistent with the differences between the microstructures of the two kinds of balls. Cast structure with microcracks and shrinkage porosities in ADI balls gives rise to lower impact resistance.

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Abstract: Problems such as the difficulty of the selection of processing parameters and the large quantity of experimental work exist in the morphological evolutions of Semisolid Metal (SSM) processing. In order to deal with these existing problems, and to identify the effect of the processing parameters, (i.e. shearing rate-time-temperature) combinations on particle size and shape factor, based on experimental investigation, the Artificial Neural Network (ANN) was applied to predict particle size and shape factor SSM processed Aluminum A.356.0 alloy. The results clearly demonstrated that, the ANN with 2 hidden layers and topology (4, 2) can predict the shape factor and the particle size with high accuracy of 94%.The sensivity analysis also revealed that shear rate and solid fraction had the largest effect on shape factor and particle size, respectively. The shear rate had a reverse effect on particle size.

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Abstract: The performance of an Imidazoline based commercial corrosion inhibitor in CO2 corrosion of a gas-well tubing steel was studied by employing Electrochemical Impedance Spectroscopy (EIS) technique. Inhibitor performance was investigated by means of its efficiency at various concentration and also its behavior at various exposure time. EIS results showed that inhibitor interaction to the electrode surface obeys Lungmuir adsorption isotherm. Interpretation of some parameters such as Rct, Rpf, Cdl, and Cpf associated to the equivalent circuit fitted to the experimental rsults showed that not only inhibitor efficiency and surface coverage improve by increasing in inhibitor concentration in the solution but also at constant inhibitor concentration both surface coverage and efficiency improve with exposure time and reach to their highest value after 4 hours.