نشریه فرآیندهای نوین در مهندسی مواد
پیاپی 57 (تابستان 1400)

In this paper, first the crystal plasticity notions developed by Taylor are combined with the mathematical form of Hill’s yield criterion for the anisotropic materials and a novel model is developed for description of mechanical response of grains in a specimen, based on their orientation. The advantage of the proposed model compared to other crystal plasticity finite element techniques is that in the conventional crystal plasticity codes, the deformation taken to be consisted of slip on all slip systems which is not valid assumption, yet here, the deformation taken to be consisted of slip on 5 slip systems. Using the proposed model which is called combined Hill-Taylor model, compression test of specimens with different number of grains are simulated and the state of strain in each grain and the condition for elimination of size effect in the final geometry of specimen is studied. The results suggest that the state of strain in each grain is individual and depends on the orientation of that gain which changes abruptly by passing through the grain boundaries. It is also observed that as the number of grains increases, the final geometry approaches to the expected ideal geometry. This trend is studied in statistical point of view and it became clear that as the number of grains increases the average of the state of strain approaches the ideal condition while the scatter in the state of strain in grains continue to maintain.

Joining of Inconel 738 and Nimonic 75 nickel base superalloys using Ni-Cr-B-containing interlayer, MBF-80, performed by transient liquid phase process (TLP) at 1080, 1120, 1150 and 1180°C and different times. Bonding microstructure was studied using a scanning electron microscope (SEM) and a light microscope. Microstructural studies showed that in short bonding time, the microstructure consists of continuous eutectic intermetallic phases in the center line of the joint and with increasing bonding time at constant temperature, eutectic intermetallic phases gradually decrease from the bonding and finally completely disappear. In order to predict the time required to isotherm solidification completion, Fick’s diffusion equations were used and it was observed that there is a good agreement between the predicted time of isotherm solidification completion and experimental results.

Traditional photovoltaic devices for replacing fossil fuels have problems, including high construction and installation costs. Hence, there is now more focus on the new generation of solar cells, including solar tiles with higher efficiency and lower const. Due to the emergence of nanolayers and the extensive advances in the selection of raw materials and devices for the application of this type of layers, in the present study, the preparation and achievement of optimal conditions for the main layer of solar tile has been considered. The best sample containing adsorbent layer film ink was obtained from CZTS and was made by solothermal method at 550 ºC. Using X-ray diffraction (XRD) and Raman spectroscopy analysis, phase studies of the synthesized samples as well as the identification of functional groups in the compounds were performed. Field emission scanning electron microscopy (FESEM) was used to study the surface morphology and the microstructure of the prepared inks. A UV-Vis spectrophotometer was used to analyze the ultraviolet-visible absorption spectrum. XRD analysis showed the formation of pure cassiterite as well as the presence of secondary phases of CZTS in some samples. and the results were confirmed by Raman spectroscopy. In the studies, the sample synthesized at 550 ºC with crystalline structure of cassiterite with suitable peak intensities was selected as the most suitable sample. FESEM microscopy showed that all samples of CZTS nanoparticles had a petal-like shape and with increasing temperature the petals bended. Finally the best conditions for the homogeneity of the morphological particles of the sample were observed at 550 ºC. Also, based on the results of the EDS, the sample with the highest weight percentage of copper and the lowest weight percentage of sulfur had priority in terms of application in solar cell structures, such characteristics were observed in the sample synthesized at 550 ºC. UV-Vis results showed that the optical cleavage band of CZTS nanoparticles in the best sample was 1.49 eV. In general, the results of the studies in this work showed the appropriateness of the solothermal synthesis method and also the effect of temperature on the final characteristics of the thin film, including the type of structure, morphology, transmition and energy bandgap.

The response surface methodology is a statistical approach to design the experiments, modeling and analysis of the effective factors as well as to help optimizing the process. In this study, we use the central composite design technique to select the optimum scaffold composite of the Hydroxyapatite and Diopside. This method suggested twenty different scaffold specimens by optimizing the suitable percentage of porosity via determining the percentage weight concentrations of the three effective parameters. After making the scaffold and determining their porosity, the optimum case for composite scaffolds was 77.57 wt% nHA (22.43wt% Di), 0.64wt% lubricant (STPP) and heat treatment temperature 1200. Also, the results of SEM, FTIR, and XRD confirm that the scaffold specimen made with the response method is an ideal specimen for use in bone tissue engineering. In general, according to the results of this research, the response surface methodology can be a useful tool for optimizing composite scaffolds in tissue engineering.

Platelet-rich fibrin (PRF) is a natural fibrin matrix containing platelets and growth factors in the blood that increases the bone tissue repair. In this study, polycaprolactone/chitosan (scaffold A) and polycaprolactone/chitosan core-shell scaffold containing PRF (scaffold B) were fabricated by uniaxial electrospinning and coaxial electrospinning methods, respectively and were characterized. Surface morphology, fiber diameter, porosity, mechanical properties, and functional groups on the scaffolds surface were evaluated by scanning electron microscope (SEM) and transition electron microscopy (TEM), displacement liquid method, tensile strength test, and fourier transform infrared (FTIR) spectroscopy, respectively. The average fibers diameter of the scaffold B decreased to 160 nm as compared to 179 nm for the scaffold A. Also, the presence of chitosan containing PRF in the core with the formation of hydrogen bonding with polycaprolactone in the shell of the scaffold B caused a scaffold with excellent mechanical properties and elastic modulus 40 MPa. Cell viability and adherence of bone cells on the surface scaffolds were evaluated via MTT assay. Due to the present of PRF in the scaffold B, the bone cells growth and cells adhesion on the surface of scaffold B increased compared to the scaffold A. Therefore, according to the results of this study, the core-shell scaffold containing PRF can be a good suggestion for use in biomedical applications.

In this research, NiCrAlY powder was applied on steel, and Hastelloy X substrates with solid shielding shrouded plasma spray (SSPS) process and compared with atmospheric plasma spraying (APS). The high-temperature oxidation test was also performed on the coatings, and the microstructure of coatings was studied by optical microscopy (OM) and scanning electron microscopy (SEM). To investigate the influence of the SSPS process on the properties of metallic coatings, variable parameters; such as type of shroud gas (Ar, H2), the gas injection method (internal, external or simultaneous) and the flow rate of that, were examined. During the use of shroud gas, the temperature of the plasma jet has increased significantly. The oxidation test results showed the proper performance of NiCrAlY coating under the protection of argon internal shroud gas with a flow rate of 75SLPM, which was able to perform the best plasma flame protection during spraying. It can lead to a reduction in oxide and porosity of coating up to 8%. Also, the lowest thermally grown oxide (TGO) thickness was obtained for this sample after 200 hours of oxidation, indicating its excellent performance in maintaining the Al for the formation of the continuous α-Al2O3 layer during high-temperature oxidation.

In this paper, the effect of tungsten element on microstructure and mechanical properties of Fe-C-Ni hard coating was investigated. Two hard coating electrodes were made with 10 and 30 gr of tungsten powder. The microstructure of the welding metals included fine carbides in the area of ​​needle martensite and residual austenite. Electron microscopy studies showed that there were very fine cracks in the weld metal martensitic phase with 10 gr of tungsten but these microscopic cracks were not found in weld metal with 30 gr of tungsten. The results of the EDS analysis showed that the amount of soluble tungsten element in the austenite phase of both welding metals is high. This amount in weld metal with 30 gr of tungsten was about 3.66% higher than the weld metal with 10 gr of tungsten. The results of the XRD analysis showed that the phases present in the weld metal with 10 gr of tungsten included martensite, austenite and W2C carbide, but in the weld metal with 30 gr of tungsten in addition to these phases also iron oxides were observed. The results of hardness test showed that the average hardness of weld metal with 10 gr of tungsten is 42.5 RC and the average hardness of welding metal with 30 gr of tungsten is 49.6 RC.