Iranian Journal of Materials Science and Engineering

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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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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.

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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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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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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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Abstract: The PZT-based ceramics with a composition of Pb1.1-xLax (Zr0.53Ti0.47)O3, were prepared by conventional mixed oxide followed by mechanical alloying and sol-gel methods in which x was chosen in the range of 0.02–0.06. The samples were calcined in the range of 450 °C - 750 °C for 4h. The physical and electrical properties of the samples were determined as a function of the calcination temperature. The obtained data from two methods were compared with conventional mixed oxide method. Microstructural and compositional analyses of the samples were carried out using XRD and SEM. Dielectric properties of the samples were measured with an impedance analyzer. The ferroelectric properties of the PZT and PLZT samples were measured using the frequencies applying equipment and d33 tester. The results indicated a complete tetragonal phase prepared from both methods. It was shown that the addition of La and reduction in calcination temperature improved both the dielectric and piezoelectric properties. The dielectric constant tended to increase with doping content, giving the maximum value of about 2000 at 3 mol% La3+. In addition, the mechanical coupling factor (Qm) of the doped samples showed a significant decrease. Finally, the value of planar coupling factor (kp) reached the maximum value of 0.47 at 1 mol% La3+.

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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.

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Abstract: Horizontal continuous casting is widely used to produce semi-finished and finished metallic products. Homogeneity in metallurgical characteristics and mechanical properties in such products is of importance. In the present work microstructure and mechanical properties of a horizontal continuous cast pipe have been studied. Microstructural features were investigated by an optical microscope equipped with image analyzer and SEM was used to characterize precipitates. Tensile behavior, impact strength and hardness variations were the mechanical properties which were studied. Results showed that microstructure and mechanical properties had diversities in different parts of the pipe and distinct differences were observed between upper and lower parts of the pipe. A meaningful correlation was found in microstructure and mechanical properties in different parts of the component.

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geopolymer cement (inorganic polymeric binder) based on natural pozzolan are investigated. SiO

of activator and total molar ratios of Nacomposition exhibiting the highest 28-day compressive strength. Mixtures exhibiting the highest compressive strength were studied for their molecular structure using laboratory technique of Fourier transform infrared spectroscopy.Results obtained confirm that changes in chemical composition and curing condition can result in variations in degree of silicon substitution by aluminum in the second coordination sphere. Hydrothermal curing affects the molecular structure so that by increasing the hydrothermal curing temperatures, a lower degree of silicon substitution by aluminum in the second coordination sphere is observed. The molecular structure of the studied inorganic polymeric binde is composed of Si-O-Si chains bonded to Al-O and Si-O units creating two and three dimensional networks.

In this paper, the effects of chemical composition and curing conditions on molecular structure of2/Na2O molar ratio2O/Al2O3, and H2O/Al2O3 were changed to determine the optimum chemical

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Abstract: In this paper, a DRIE process for fabricating MEMS silicon trenches with a depth of more than 250 m is described. The DRIE was produced in oxygen-added sulfur hexafluoride (SF6) plasma, with sample cooling to cryogenic temperature using a Plasmalab System 100 ICP 180 at different RF powers. A series of experiments were performed to determine the etch rate and selectivity of the some masking materials such as resists, and metal (Al). Experiments show that different materials have different etch rates, but for the Al mask, an etch rate of 5.44x10-3 nm/min was achieved, that exhibits very stronger resistance against RIE than resists. By controlling the major parameters for plasma etch, an etch rate of 2.85 microns per minute for silicon and a high selectivity of 5.24x105 to the Al etch mask have been obtained. A 90 min etching experiments using etching gas SF6 of 60 standard cubic centimeters per minutes (sccm) with oxygen (13 sccm) were performed by supplying RF power of 5 W to an ICP of 600 watts, and silicon etching process with a depth of 257 m was demonstrated. Our experiments show that Al is the best mask material for very deep trenches in silicon.

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Abstract:In this study, an effort has been made to determine the influence of rotational speed of tool on themicrostructure and hardness values of friction stir welded 2024-T851 aluminum alloy. The microstructure of stir zonein the joints has been investigated. It was found that the particles such as Al6(CuFeMn) particles are broken up duringfriction stir welding, and the degree of break up of these particles in the stir zone increases with increasing rotationalspeed. Since the break up of these particles and the recrystallization of new grains happen simultaneously, the brokenparticles would be placed in the grain boundaries. Moreover, the hardness value in the stir zone increases withincreasing rotational speed

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Abstract:

powerful method for the characterization of microstructures of crystalline materials in terms of crystallite size and

dislocation structures. In this paper the effect of the sliding on the microstructure of A356 in the as-cast and heat

treated conditions are studied, The X-ray phase analysis shows that with increasing applied load, the dislocation

density is increased, whereas the crystallite size is decreased. It was found that heat treatment raised dislocation

density during wear. The screw or edge character of dislocations in worn specimens were determined by analyzing the

dislocation contrast factors, it was demonstrated that the character of the prevailing dislocations in high loads is

nearly pure screw.

Diffraction peak profile analysis has recently been developed to such an extent that it can be applied as a

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The effects of lower bainite volume fraction on tensile and impact properties of D6AC ultrahigh strength steel were studied in the current work. To obtain mixed microstructures containing martensite and different volume fractions of the lower bainite, specimens were austenitized at 910° C, then quenched in a salt bath of 330°C for different holding times, finally quenched in oil. In order to obtain fully martensitic and bainitic microstructures, direct oil quenching and isothermal transformation heat treatment for 24 hours were used respectively. All specimens were double tempered at 200°C for 2 hours per tempered. Microstructures were examined by optical and scanning electron microscopes. Fracture morphologies were studied by scanning electron microscopy (SEM). Results showed that both yield and ultimate tensile strength generally decreased with an increase in volume fraction of lower bainite. However, a few exceptions were observed in the mixed microstructures containing 12% lower bainite, showing a higher strength than the fully martensitic microstructure. This can be explained on the basis of two factors. The first is an increase in the strength of martensite due to the partitioning of the prior austenite grains by lower bainite resulting in the refinement of martensite substructures. The second is a plastic constraint effect leading to an enhanced strength of lower bainite by the surrounding relatively rigid martensite. Charpy V-notch impact energy and ductility is improved with increasing the volume fraction of lower bainite.

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Abstract: In this study Al-5 (Vol) % SiCp nanocomposite powder has been successfully synthesized by high-energy planetary milling of Al and SiC powders for a period of 25 h at a ball-to-powder ratio of 15:1. The changes of the lattice strain, the crystallite size of the matrix phase, and the nanocomposite powder microstructure with time have been investigated by X-ray diffraction (XRD), X-ray mapping, and scanning electron microscopy (SEM) analyses. The morphologies of the nanocomposite powders obtained after 25 h of milling have also been studied by transmission electron microscopy (TEM). The results showed that nanocomposite powders were composed of near-spherical particles and, moreover, the SiC particles were uniformly distributed in the aluminum matrix.

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Abstract: SiC nano particles with mono dispersed distribution were synthesized by using of silicon alkoxides and phenolic resin as starting materials. After synthesis of sample, characterizations of the obtained powder were investigated via Fourier Transform Infrared Spectroscopy (FTIR) with 400-4000 cm-1, X-ray Diffractometry (XRD), Laser Particle Size Analyzing (LPSA), Si29 NMR analysis, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). FTIR and Si29 NMR results of the gel powder indicated that Si-O-C bonds were formed due to hydrolysis and condensation reactions . FTIR results showed a very strong peak for heat treated powder at 1500°C after carbon removal which is corresponded to Si-C bond. Obtained pattern from X-ray diffractometry showed that the final products contain -SiC phase with poly crystalline planes and little amounts of residual carbon. PSA results showed that the average particles size were 50.6 nm with monosized distribution. Also microstructural studies showed that the SiC nano powders have semi spherical morphology with mean particles size of 30-50 nm and also there are some agglomerates with irregular shape.

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The process of mullitization of kyanite concentrate was studied at different conditions of heat treatment (1400
– 1600 °C and 0.5 – 3.5 hours) and particle size of raw materials (38-300 ?m). Kyanite concentrate was obtained from
ore-dressing of kyanite deposits of Mishidowan-Bafgh region at 100 km northeastern part of Yazd. The results of
microstructure (shape, distribution and size of the grains) and phase evolution studies by SEM and XRD showed that
total transformation of kyanite to mullite takes place by heat treatment between 1500 –1550 °C during 2.5 hours.. At
temperatures below 1500 °C need-like mullite grains are always produced. At higher temperatures the mullite grains
reveal rounded and platelet morphology. At 1550 °C, the rate of mullitization and densification were improved by
increasing soaking time from 1h to 3h and decreasing particle size of materials from 300 to 38 m

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The wide-gap aluminothermic rail welds with root opening of 50-70 mm were produced using plain carbon steel rail and non-alloy aluminothermic charge. Mechanical properties and micro-structure of the weld metal and HAZ as well as the impact energy and the fracture toughness of the welds were investigated. The yield and tensile strength of wide-gap welds were about 98% and 95% of the base metal, respectively. Both minimum and maximum hardnesses of the joint were seen in HAZ which were related to the grain coarsening and normalizing, respectively. The mean value of wide-gap weld fracture toughness is more than narrow-gap weld. Moreover, trans-granular cleavage indicated the brittle fracture mode of the weld metal.

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Abstract: A thermodynamic model was used to find out the optimum temperature for the growth of ZnS single crystals in closed ampoules by chemical vapor transport technique. Based on this model 1002 °C was found to be optimum temperature for 2 mg/cm3 concentration of transporting agent (iodine). ZnS Crystals were grown in optimum (1002 °C) and non-optimum (902 °C and 1102 °C) temperatures. The composition structure and microstructure of the grown crystals were studied by Atomic absorption spectroscopy, X-ray diffraction and Scanning electron microscopy measurements. Properties of the grown crystals were correlated to the growth conditions especially a stability in mass transport along the closed tube length.

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In this study the effect of Al–5Ti–1B grain refiner on the structural characteristics and wear properties of Al–12Zn–3Mg–2.5Cu alloy was investigated. The optimum amount for Ti containing grain refiners was selected as 2 wt.%. T6 heat treatment, (i.e. heating at 460 °C for 1 h before water quenching to room temperature and then aging at 120 °C for 24 h) was applied for all specimens before wear testing. Dry sliding wear resistant of the alloy was performed under normal atmospheric conditions. The experimental results showed that the T6 heat treatment considerably improved the resistance of Al–12Zn–3Mg–2.5Cu alloy to dry sliding wear.

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Abstract:Nanostructured titania was synthesized by colloidal and polymeric sol-gel routes. Stable colloidal and polymeric titania sols were prepared by adjusting the proper values of the acid/alkoxide and the water/alkoxide molar ratios. The properties of sols were determined by dynamic light scattering technique and synthesized titania was characterized by thermogravimetry and differential thermal analysis, X-ray diffraction, Fourier transform infrared spectroscopy, optical microscopy and field emission scanning electron microscopy. The results showed particle size distribution of colloidal sol 10-50 nm compared to polymeric one which was 0.5-2 nm. Phase analysis of the colloidal sample revealed anatase as the major phase up to 550 °C, while the polymeric route resulted only anatase phase up to 750 °C. On the basis of results, titania prepared by the polymeric route showed better thermal stability against phase transformation than the sample prepared by the colloidal route. Also, microstructural studies showed that titania nanopowder can be produced by both sol-gel routes