Journal of Advanced Materials in Engineering (Esteghlal)

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Despite excellent bioactivity of bioactive ceramics such as hydroxyapatite, their clinical applications have been limited due to their poor mechanical properties. Using composite coatings with improved mechanical properties could be a solution to this problem. Therefore, the strength of metal substrate and the bioactivity of the improved composite coating combined could yield suitable results. The aim of this work was fabrication and characterization of hydroxyapatite-forsterite-bioactive glass nanocomposite coating. The sol-gel technique was used to prepare hydroxyapatite-forsterite-bioactive glass nanocomposite in order to coat on 316L stainless steel (SS) by deep coating technique. The X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and energy dispersive X-ray analysis (EDX) techniques were used to investigate the microstructure and morphology of the prepared coating. The results obtained from XRD analysis showed that the suitable temperature for calcination is 600 °C. At this temperature, the homogenous and crack-free coating could attach to the 316L SS substrate. The crystallite size of composite coatings determined via AFM was lower than 100 nm. Overall, the results obtained from this work indicate that hydroxyapatite-forsterite-bioactive glass nanocomposite coating can be a good candidate for biomedical applications.

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Fe-Co alloys have unique magnetic applications. Fe50Co50 alloy has the highest saturation magnetization value among Fe-Co alloys. Moreover, the introduction of Si into Fe can result in a decrease of magnetic anisotropy. In this study, in order to utilize combined advantages of Si and Co, the effect of adding 10 and 20 at.% Si on the microstructural and magnetic properties of Fe65Co35 alloy was investigated. For this purpose, initial powder mixtures with specific compositions were milled by means of planetary ball mill for different milling times. Microstructural properties and morphology of the obtained powders were analyzed by X-ray diffraction analysis (XRD) and scanning electron microscope (SEM). Also, magnetic properties of the samples were determined by means of vibration sample magnetometer (VSM). The results showed that the crystallite size was finer and more uniform and lattice strain was decreased slightly for longer milling times. Observations indicated that the addition of Si to the alloys leads to finer particles. The results also showed that increasing the Si content increases the reduction rate of lattice parameter and coercivity.

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Presence of nanoscale bainitic ferrites and high carbon retained austenites that are stable at ambient temperature within the microstructures of super strong bainitic steels makes it possible to achieve exceptional strengths and ductility properties in these groups of nanostructured steels. This article aims to study the effect of the dislocation density variations during tensile testing in ambient temperature on deformation behavior of nanostructured low temperature bainitic steels. Results indicate that dislocation absorption from bainitic ferrite subunits by surrounding retained austenite reduces the work hardening and therefore increases the formability of bainitic ferrite during deformation, which in turn results in a suitable combination of strength and ductility.

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Al-Zn-Mg/3 wt-% Al₂O₃ nanostructured composite powder was synthesized through Mechanical Alloying (MA). At first, the 7014 alloy matrix constituents were milled in a planetary ball mill for 20 hours. Then, 3 wt.% µ-Al₂O₃ particles were  added to the pre-milled matrix and the nanostructured composite powder was produced at different MA times to investigate the effects of MA time on the characteristics of the produced composite powders such as morphology, crystallite size, lattice strain and microhardness. The characterization results proved that synthesizing nanostructured composite powder with a low amount of micrometric reinforcements in addition to pre-milled micrometric matrix is possible. Also, synthesis of the nanostructured composite powder with the minimum crystallite size of 24 nm and the minimum mean particle size of 5 µm was confirmed. Moreover, the steady state occurred after around 20 hours milling and further milling did not affect the powder characteristics excluding crystallite size, lattice strain and microhardness. In addition, sinterability of the composite powders increased with increasing the milling time due to decreased average particle size. However, after the steady state, the sinterability did not change.
 
 

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In the present study, silica aerogel was evaluated by a two-step catalytic process at the ambient pressure drying, under different synthesis conditions. The effects of  the catalyst and water content in the hydrolysis step on the physical properties of silica aerogel, including density, porosity and shrinkage, were investigated. The results showed that increasing the water content in the hydrolysis step increased the shrinkage of gel network and density of obtained aerogel. Moreover, in the presence of insufficient water, NH₄OH as the condensation catalyst in the gel formation step was more effective on the physical properties of silica aerogel, as compared to HCl as  thehydrolysis catalyst; Moreover, the increase in the NH₄OH content led to lower density and higher porosity. On the other hand, NH₄OH effect on the physical properties of silica aerogel was not noticeable in the presence of enough water content. In the NH₄OH/HCl molar ratio of 6, the best silica aerogel sample was obtained with the density of 0.214 g/cm³, porosity of 90% and shrinkage of 23%