مجله مواد نوین
سال دهم شماره 1 (پیاپی 37، پاییز 1398)

The welding of Hastelloy B-2 Ni-Mo base superalloy was carried out with the aim of investigating the effect of current intensity and frequency on the microstructural evolutions of the welded areas. For this purpose, the PC-GTAW process and the ERNiCrMo-2 filler metal was used. While other parameters were kept constant in this process, the frequency was changed at 10, 30 and 50 Hz, and pulse current was changed in the form of 80:20 and 60:40. The effect of current intensity and frequency on the microstructure of the weld metal was investigated by optical microscopy, scanning electron microscopy and EDS accessory was employed for analyzing the precipitates formed. The results showed that a decrease in the maximum current intensity led to a reduction in the heat input applied to the workpiece, resulting in a severe decrease in the size of the dendrites in the weld metal along with a reduction in the grain size of the HAZ and eventually, a significant decrease in the amount of molybdenum carbides in interdendritic regions of the weld metal will be resulted. By increasing the frequency led to increasing the width and the depth of weld. With increasing the frequency, the overlapping of the pulses increases and the heat is applied the molten pool at a shorter time interval and thus, the weld dimensions increase. The microstructural studies showed that increasing the frequency led to an increase in dendrites of the weld metal along with an increase in the extent of the HAZ.

Neutron absorbers materials must have the ability to slow down the fast neutrons and absorb the thermal neutrons. In recent years, geopolymers have used as a replacement for cements and used to store nuclear waste. In this study, neutron absorption properties of geopolymer samples were investigated. In this regard, Taftan Mountain volcanic ash was used for synthesis of the samples. Sodium hydroxide and sodium silicate were used as alkaline activators. The samples were cured at 65°C for 24 hours and then kept at ambient temperature for seven days in order to geopolymerisation reactions were completely done. The fast neutrons were retarded by light elements such as hydrogen or carbon and boron absorb thermal neutrons In order to improve the absorption properties of the samples boron oxide and boron carbide (by 5-10 wt%) were added to them. The x-ray diffraction (XRD) and Fourier transformation spectroscopy (FTIR) were performed to investigate the phase and chemical bond, properties of the samples respectively. The compression strength and absorption properties of the samples were studied by compression and absorption tests. The XRD results confirmed the existence of geopolymer phases and the FTIR results showed the existence of a geopolymer network. The sample without any additive had maximum compressive strength 37.5 MPa. The absorption test results showed that the samples containing boron carbide had higher neutron absorption than the samples containing boron oxide due to existence of carbon in them.

In this study, bone tissue engineering scaffolds were synthesized by radical crosslinking reaction of poly(ethyleneglycol) (PEG) diacrylates in the presence of polycaprolactone (PCL) and hydroxyapatite (HA) particles and particulate technique with sodium chloride. The prepared samples were characterized using techniques such as scanning electron microscopy (SEM) and fourier transform infrared spectroscopy (FTIR). Percent of porosity, pore size, swelling, gel fraction and mechanical properties were investigated. FTIR indicated that crosslinking between PEG chains were successfully done. The highly interconnected porous morphologies with macropores larger than 100 micron are observed in all scaffolds. The porosities of the scaffolds ranged from 69% to 74%. Results showed that increasing the ratio of PEGDA to polycaprolactone led to increase of swelling ratio and decrease of compressive modulus of the network, respectively. It was found that the incorporation of HA paticles with the polymer matrices resulted a decrease swelling ratio, and also an increased compressive modulus of the networks (from 0.96 Mpa to 1.122 MPa). Moreover, all networks had a gel fraction more than 94%. Thus, the results indicated that the PCL/PEG/HA scaffolds have the potential of being used as promising substrates in tissue engineering.

Magnesium hybrid surface composites are widely used in automotive and aerospace industries because they have low weight, high specific strength, and suitable wear properties. In this research, a mixture of silica and graphite nanoparticles was used to produce hybrid surface composites on AZ31B magnesium alloy by the use of friction stir processing (FSP). The effect of FSP passes on microstructure, microhardness, and wear resistance of composites was investigated. According to microstructural investigations, after 4 passes, the particles dispersed well and properly prevented grain growth so that the mean grain size of AZ31B–SiO2–graphite composite decreased about 80% compared with as-received AZ31B and above 50% compared with FSPed as-received AZ31B. Furthermore, in comparison with as-received AZ31B, microhardness of the composite increased more than 17% and its wear resistance increased more than 33% after 4 passes. The results indicated that grain size decrease plays a more significant role in composite hardness than dispersion hardening and other strengthening mechanisms. Furthermore, by increasing FSP passes, grain size variance and hardness variance decreased in the stir zone, which indicates that particles are dispersed and the microstructure becomes homogenized.

It is very important to control the process of production of floor tiles to achieve the proper properties. In this study, the effect of sintering temperature on densification behavior and mechanical properties of the floor tile was investigated. For this purpose, specimens prepared with the conventional dry pressing method and sintering was performed in the range of 900 to 1200 °C for 2 hours. According to X-ray analysis, quartz and mullite phases were identified as the main phases in the floor tile bodies. Maximum density, linear shrinkage and compressive strength were obtained for the specimen sintered at 1150 ºC, and this sample had the lowest porosity (0.5%) and water absorption (0.3%). The results showed a sharp decrease in the density and compressive strength of samples sintered at 1200 °C. The existence of porosity and cracks in the microstructure of this sample could be attributed to the oversintering phenomenon. According to the results of this study, a linear relationship between the compressive strength and density of samples can be established, which indicates a strong dependence of compressive strength on the density. The high abrasion resistance of the sample sintered at 1200 °C could be due to its glassy surface.

Chromium is one of the most important pollutants in aquatic environments that, at high concentrations, cause different ecological threats and health risks for human society. Thus, it is very necessary to remove it from aquatic Solutions. Therefore, the aim of this study was to investigate the efficiency of removal hexavalent chromium by biological absorbers (oak, pomegranate, and egg shell), and the study of isotherm and kinetics of their absorption. In order to achieve to the optimal removal of chromium, the variables affecting on adsorption process such as pH, coagulation speed, reaction time and initial concentration of adsorbent material were investigated. Also, for adsorption isotherms and kinetics study the first order and second order pseudo kinematic models and Langmuir and Freundlich isotherms were used. The results showed removal efficiency of Cr increased with increasing the dose of adsorbent, contact time, and coagulation speed. But by changing the pH, depending on the absorbent type, the absorption efficiency was changed and the highest absorption efficiency for egg, oak and pomegranate shell was obtained at pH 10, 4 and 2, respectively. The kinetics was second-order for all three adsorbents and absorption isotherms from the Freundlich model for egg and pomegranate and for oak shell was more consistent with Langmuir model. According to the obtained results can be concluded that egg, oak and Pomegranate shell respectively, have high more efficiency in removing hexavalent chromium from aqueous solutions.

The destructive corrosion phenomenon has been identified as one of the major problems in the Iranian gas refineries, which is often found in the amine regenerator tower and the steam condensate regions of carbon dioxide. One of the most important causes of this problem is the lowering of the amine temperature below the safe operating temperature range. According to the simulation of the process performed using ProMax software, in the event of a drop in the temperature of the amine entering to the regenerator tower, mainly at the bottom of the tower, as well as the reboiler, which according to the body (carbon steel 516), the corrosion is likely to occur. Examining the corrosion coupon installed in this gas sweetening indicates an increase in the corrosion rate, which can increase up to 1.27 mm/year if the process conditions in the amine cycle are not controlled. Stereo microscope images and identification corrosion product by x-ray diffraction used in this study confirm the above item. Finally, different strategies for controlling the temperature of the amine to the tower in the operational safe area are presented in accordance with the strategies of each refinery, as well as considering the cost of the life cycle. The above solution may include designing and adding a new heat exchanger or periodic cleaning to identify the sediment formed in the amine-amine plate converter.

In this research perovskite phase of strontium titanate was produced from celestite concentrate (strontium sulphate). At first stage, the precursor of strontium carbonate was prepared from celestite-sodium carbonate mixture using mechanochemical process. Strontium titanate was synthesized using strontium carbonate-TiO2 mixture in the second stage. The results indicated that high energy ball milling plays a significant role in the chemical reaction and results in decreasing the temperature of post-heat treatment for producing of strontium titanate. Although some traces of rutile phase were overlapped with strontium carbonate, however no sign of chemical reaction observed in the10 hours milled mixtures of strontium carbonate-TiO2. TGA results showed that milling results in decreasing the formation temperature of perovskite strontium titanate phase in the milled mixtures. Although the traces of strontium titanate phase were revealed in the 5 hours milled mixture after isothermal heating at temperature of 600℃, the signs of un-reacted raw materials (TiO2 and SrCO3) were observed in un-milled mixture at this temperature. The traces of SrTiO3 were observed in un-milled and milled mixtures after heating at temperature of 900℃. XRD and FT-IR results indicated that strontium titanate can be synthesized form celestite concentrate (SrSO4) at lower temperatures using mechanochemical process. SEM micrographs showed that the ultrafine particles of strontium titanate (SrTiO3) can be formed in the milled samples.

In this study the effect of orthotropic parameters on failure of the orthotropic materials under the ‎rolling contact problem is investigated. Rolling contact problem between a rigid cylinder and an ‎orthotropic half space for the specific regime of contact zone, a central stick zone accompanied ‎with two slip zones, is considered in plane strain condition regarding the Coulomb law of friction. ‎ Although the problem is analyzed using the previous methods obtained in our last paper, in this ‎paper the main emphasis is on the results and the dependence of the surface and subsurface ‎stresses on the orthotropic parameters and the coefficient of friction. ‎ The interesting results from this study suggest that the‏ ‏stresses can be reduced by controlling of ‎the orthotropic parameters and, as a result, increasing the life of the orthotropic media. Finally, it ‎can be concluded that the replacement of orthotropic and non-homogeneous materials instead of ‎isotropic and homogeneous materials will be caused in many applications to increase the life of ‎the components and reduce the cost of consumables.‎‏ ‏

The purpose of this research is to investigate microstructural features and mechanical properties of MO40 low alloy steel under dual-phase ferritic-bainitic in comparison with those of full bainitic conditions. To do so, various heat treatment cycles including Austenitizing the samples at 860°C for 50 minutes, step quenching in primary 620°C salt bath for different duration times of 1 to 10 minutes, quenching in secondary 350°C salt bath for 30 minutes and then water quenching to room temperature in order to develop dual-phase ferritic-bainitic microstructures in the samples. The isothermal heat treatment in primary 620°C salt bath for various duration times was used in order to develop different volume fractions of ferritic phase in the microstructures, then the secondary 350°C salt bath was used for subsequent transformation of remaining prior austenite to bainite. Tensile test, micro and macro hardness testers, optical metallography and FE-SEM equipped with EDS were used for investigation of variations of mechanical properties and microstructural features. The experimental results show that there is a non-linear and abnormal relationship between the mechanical properties of dual-phase ferritic-bainitic samples and variations of ferrite and bainite volume fractions. The mechanical properties of dual-phase ferritic-bainitic samples involving 8-12 vol% grain boundary ferrite in the vicinity of bainite is considerably higher than other dual phase ferritic-bainitic samples and even full bainitic ones. This improvement in mechanical properties of dual-phase ferritic-bainitic samples involving 8-12 vol% ferrite is originating from interaction of bainite crystals on grain boundary ferrite.

In this research, microstructure and tribological properties of CK45 steel coated with Fe-Al and Fe-Al-Si alloys using GTAW process were studied. In order to evaluate the microstructure and formed phases in the claddings, optical microscopy and X-ray diffractometery analysis were used. Also, tribological properties of the claddings were evaluated by pin-on-disk wear test at two different temperatures of 25 and 500 oC. Finally, the wear surfaces after the wear test were characterized by scanning electron microscopy to detect the main wear mechanism. It was found that the microstructure of Fe-Al-Si cladding was dendritic and lathy in some areas, while in Fe-Al cladding blade shape microstructure was seen. In Fe-Al-Si cladding owing to the presence of Si element, the width of cellular and planar solidification area was lower and dendritic solidification was seen. XRD analysis revealed that the main phase in Fe-Al binary and Fe-Al-Si ternary claddings were Fe3Al and (Fe,Si)3Al intermetallic compounds, respectively. Fe-Al-Si cladding indicated higher microhardness and wear resistance as compared with Fe-Al cladding. With addition of Si into the surface owing to microstructure refining and formation of (Fe,Si)3Al complex intermetallic compound the wear behavior of the surface improved. The predominant wear mechanism of both claddings was adhesion at room temperature, but at 500 oC, the predominant wear mechanism in Fe-Al cladding was micro-plowing abrasive while in Fe-Al-Si cladding was adhesive and abrasive.

A new superconductor, Y2Ba5Cu7Ox , (Y257), a member of YBCO family, was synthesized using the standard solid-state reaction method. The powder was calcinated in the air flow and then in the next step the sample was sintered in the presence of oxygen flow. The unit cell of the compound includes the subsequent block of Y123, Y124 and an excess BaO layer which are set in the c direction. The structural properties and space group of the compound were studied using the X-ray diffraction experiment which is analyzed by Rietveld method. The electric resistivity of the sample showed a transition to a superconducting state at onset superconducting transition temperature 98 K and reached to zero resistivity at offset superconducting transition temperature 92 K. In normal state, from the room temperature down to the onset superconducting transition temperature, the electric resistivity shows a metallic behavior. In the measurement of resistivity versus temperature in the presence of magnetic field, the resistivity decreases to zero with a slower rate at higher magnetic fields and shows a broadening behavior. Applying the magnetic field leads to the broadening of the superconducting transition region and decreases the offset superconducting transition temperature. The experimental resistivity data satisfies the thermally activated flux creep model.