نشریه مهندسی متالورژی و مواد
سال سی و دوم شماره 1 (پاییز و زمستان 1399)

In this research, leaching kinetic of activated molybdenite was studied. Mechanical activation of molybdenite concentrate was carried out at different conditions of milling and presence of alumina. Leaching experiments were performed in nitric acid media. The classic shrinking core model was modified, in order to study the leaching kinetic of activated molybdenite. The model was developed step by step. Finally, a comprehensive model was obtained which contains the effect of important parameters on the leaching rate of activated molybdenite. The calculated values of activation energy revealed that the activated molybdenite without additive is more reactive with the minimum of 20% and 56% reduction in activation energy value after 4 and 24 hours of milling, respectively.  The minimum of 3% and 6% reduction in activation energy value was obtained for the activated molybdenite in presence of alumina after 2 and 12 hours of milling, respectively.

In this research, the process of dissolution of γ' precipitates in GTD-111 casting superalloy was investigated. For this purpose, the dissolution operation was carried out at a temperature of 1100°C to 1230°C in 4 different times on samples prepared from the ingot with the aim of determining the effect of temperature and time on the behavior of the dissolution of the γ′ precipitates in the dendrite core and inter-dendritic regions. The microstructure of the samples was studied using the SEM and FESEM electron microscopes. Quantitative studies of microscopic images showed that complete dissolution of the initial γ′ precipitates occurred at 1230°C/40min for dendrite core regions and 1180°C/120min in the inter-dendritic regions. Investigating the micro-hardness of samples in the dendrite core and inter-dendritic regions of the specimens showed that the microhardness in these regions does not change with increasing temperature. Also the highest and lowest hardness was obtained at 1200℃ and 1120℃ in both regions respectively.

Effects of silicon carbide as the reinforcement on densification and mechanical properties of zirconium diboride-based ultra-high temperature composites were studied. A monolithic ZrB2 ceramic (as the baseline) and three ZrB2-based composites reinforced with 25 vol% SiC in different morphologies (particulate, whisker and particulate/whisker mixture) were fabricated by spark plasma sintering route. The sintering process was conducted at 1900 °C for 7 min under 40 MPa. A relative density of ~96% was obtained for the ZrB2 ceramic but the other composites approached their theoretical densities. The hardness and fracture toughness values of SiC reinforced samples were higher than those for the monolithic one. The simultaneous addition of SiC particulates/whiskers showed a synergistic effect on the enhancement of mechanical performance of composite.

Metallic foams and cellular metals are a type of new-advanced engineering materials which can be successfully used in various industrial applications due to their unique behavior and properties. In this work, steel foams were produced through powder metallurgy route using urea granules as a space holder, and porosity percentage, microstructure, and compressive behavior of them were investigated. In addition, the compressive behavior of manufactured foams was simulated using finite element method by the Gurson–Tvergaard–Needleman model and the effects of operational parameters in this model were investigated due to better prediction of mechanical behavior of steel foams. The results indicated that the average of porosity in the steel foam is 79.3 percent, which consists of cells formed by the dissolution of urea granules and remained pores between the sintered iron particles. Stress vs. strain curves of the manufactured steel foams showed the conventional behavior of metal foams, with elastic deformation region and a relatively longitudinal plateau region and, a fracture point, finally. Mesh sizes, fn, q1 and q2 have the significant effect on stress vs. strain curves, but q1 and fn have the most and the least effects, respectively.

In this article, by the regression analysis, suitable patterns for the hardness and the microstructure of the cylinder-head aluminum alloy have been presented. Objective functions were the hardness, the phase size and the spherical degree and variables were selected as the temperature and the time for the solution and ageing, in experiments. Results showed that the highest hardness (164 VHN) obtained when the sample was heated at 510°C for 300 minutes; then, aged for 360 minutes at 175°C. The optimal heat treatment was determined to achieve the smallest value of the phase area, as well as a higher amount of spherical phases. Results of the regression analysis demonstrated that the ageing time was more effective than the temperature in hardness variations, and the solution temperature was more important than the ageing temperature.

Bone defects and developed surgery with low injuries have conducted researches to design and produce composites containing biodegradable polymer and bioactive ceramic, introducing as a biocompatible scaffold for bone tissue engineering. In the present study, bioactive nanocomposite scaffolds of silk fibroin/titanium dioxide nanoparticles (SF/TiO2) and silk fibroin/titanium dioxide nanoparticles which contain flour ions (SF/TiO2-F) were prepared. Accordingly, flour ions were bound to TiO2 nanoparticles and both SF/TiO2 and SF/TiO2-F scaffolds were produced. Then, the bioactivity and apatite formation on the surface of prepared scaffolds were evaluated by immersing in simulated body fluid (SBF) for 28 days. As a resultant, SF/TiO2-F scaffolds showed improved bioactivity in comparison with SF/TiO2 ones, as such, after 12 days of immersion in SBF, apatite precipitations covered around 40 %  of surface of prepared scaffolds.

In the present study, a polysulfone-chitosan-polyvinyl alcohol membrane was prepared through electrospinning. For this purpose, each of the polysulfone, polyvinyl alcohol and chitosan powders were dissolved in a suitable solvent. The polyvinyl alcohol and chitosan solutions were then mixed in different proportions. Subsequently, two separate syringes were used to simultaneously electrospin polysulfone and polyvinyl alcohol/chitosan solutions two . The morphology of the prepared membrane was evaluated using SEM micrographs. From the SEM images, it was clear that  the chitosan-polyvinyl alcohol solution can only be electrospun at low chitosan content. Higher chitosan concentrations do not lead to proper fiber formation. The optimum sample with uniform fibers and adaptable chitosan content was obtained when the chitosan to polyvinyl alcohol ratio was fixed at 30:70. It was found that the feed rate has a significant impact on the morphology of the membrane. Moreover, it was concluded that for a uniform and beadless morphology of polysulfone fibers, a minimum concentration of the initial solution is required. According to the fibers morphology, the threshold concentration of polysulfone was 20 wt%.,

This study aimed to investigate the corrosion behavior of TiO2-N duplex coating formed by Plasma Electrolytic Oxidation (PEO) and gas nitriding. A TiO2 film formed on the titanium substrate by PEO in electrolyte containing sodium carbonate and sodium hydroxide in the first step. In the second coating process, the titanium substrate and the titanium oxide coated substrate was nitrided in a tube furnace at 1000 oC for 6 hours to compare the corrosion properties of the obtained coatings. X-ray diffraction, scanning electron microscopy of top-surface and cross-sectional structure of the layers, and potentiodynamic polarization along with electrochemical impedance spectroscopy was used to investigate the properties of the coatings. The XRD results showed that the titanium oxide coating consisted of a rutile phase, and the nitrified samples consisted of titanium nitride and TiO0.34N0.74 phases. The morphology of the coatings showed that the titanium oxide sample coating had micropores known as the pancake structure with a diameter of 4.1 microns on the surface. Also, the surface morphology of nitrided oxide-coated titanium indicates a slight change in the surface and a reduction in pore diameter of 2.8 microns due to the penetration of nitrogen in the titanium oxide coating. Finally, the results of impedance and polarization showed that the titanium oxide sample, due to its insulating and dense oxide structure, prevented the transfer of more corrosive ions to the metal surface, and its resistance was improved up to 10 times compared to other samples.

This study aimed to investigate the corrosion behavior of zinc nano-crystalline coating on st12 steel substrate by surface mechanical attrition treatment (SMAT) and electroplating process into chloride bath with and without of gelatin. To study the behavior of coatings, they were characterized by using X-ray diffraction, field emission scanning electron microscopy, EDS, potentiodynamic polarization and EIS tests in 3.5 wt. % NaCl solution. The results showed that SMAT process reduced the size of the crystallites and the presence of gelatin caused the induced orientation of Zn deposition so that the grain size in the presence of gelatin and SMAT reached from 55 to 29 nm and the rate of its corrosion reached from 80.8 to 20.1 µA.cm-2.

The effect of different amounts of ZnO on structural properties of SiO2-Al2O3-p < sub>2O5-CaF2-CaO-K2O-Na2O-xZnOglasses was studied. The effect of ZnO addition on glass transition and crystallization temperatures were studied by DTA. FTIR and RAMAN were used to study the characteristic bands of prepared glasses. All samples were heat treated at two different temperatures of 750 and 1100°C. XRD analysis showed that anorthite and calciumphosphate were the main crystalline phases. Based on DTA and XRD results, samples with higher amounts of ZnO were crystallized at higher temperatures. No significant band changes were detected as a result of ZnOaddition as spectroscopic results showed. Vickers microhardness numbers showed that ZnO addition up to 2gr lead to a significant (p < 0.05) improvement in microhardness values.