نشریه فرآیندهای نوین در مهندسی مواد
پیاپی 49 (تابستان 1398)

In this research, the fracture behavior and ductile to brittle transition (DBT) phenomenon, as well as the correlation between fracture surface morphologies and ductility/toughness in a Zr-based bulk metallic glass (BMG) is investigated. The amorphous alloy was produced by arc melting pure elements and suction casting into a water-cooled copper mold. Then, the three point bending test was used at two temperatures of 77 and 298 K and displacement rate of 0.2 mm/min. Fracture surfaces were observed through scanning electron microscopy after bending tests. The fracture toughness of samples is determined by measuring the size of fracture surface morphologies, and the brittle and ductile fracture mechanisms were theoretically studied by using the fluid meniscus instability model. Although the Zr-based BMG is nearly ductile at room temperature, at very low temperature (77 K) it becomes more brittle. Results show that the mean fracture toughness changes from ~16 MPa.m1/2 at 298 K to ~3.5 MPa.m1/2 at 77 K. Furthermore, the critical wavelength of meniscus instability (λc) is calculated to be 127 nm for the present alloy. According to the results, if the initial wavelength of meniscus instability (λI) is smaller than the λc, periodic nano-corrugation morphologies can be observed on the fracture surface. On the contrary, if λI is larger than λc, the dimples or vein-like patterns are more likely to be form on the fracture surface.

This paper investigates the applicability of nickel-nickel oxide metallic foams as a current collector for supercapacitor. A simple galvanic displacement reaction was employed to fabricate dendritic Cu dealloyed nanoporous Ni-NiO foam. A comprehensive characterization of foams are presented and includes the analysis of their structural, chemical, and electrochemical properties. The process is studied under well-defined experimental conditions using XRD, SEM, electrochemical impedance spectroscopy (EIS) and galvanostatic charge and discharge (GCD). XRD results confirm the presence of nickel and nickel oxide phases. Also, in the SEM test, porosities were observed in the range of micro and dendrites in the nanoscale. The outcome of these experiments demonstrates that the Ni-NiO foam has a higher specific capacitance. The best specific capacitance for Ni-NiO foam was calculated 924 F/g at 1A/g. Ni-NiO foam maintains 81.8% of its specific capacitance at a current density of 20 A/g and after 3000 cycles. The created foam electrode can be used as a current collector for the deposition of subsequent layers and is a candidate for use in supercapacitors.

.Nano-particles used in metal matrix composites show a various range of mechanical, chemical and physical features, causing significant improvements in mechanical strength, hardness and thermal characteristics. They can also change the capability of machining. Electrical discharge machining is considered as an integrate part of hard metal machining. In this paper, the parameters of electrical discharge machining for aluminum composite material improved by Nano-particles of titanium dioxide have been studied. The purpose of this study was to evaluate the impacts of electrical current and voltage and pulse on and off-time on the material removal rate, tool wear rate and surface roughness. Kerosene as a dielectric and copper electrode were used to carry out the experiment. In addition, Analysis of variance was utilized to authenticate the experimental results. The result shows that Nano-particles titanium dioxide has trivial effects on machining parameters due to being insulators. They also do not melt in the process of electrical discharge machining. Moreover, the electrical current and the pulse on time have the most influence on the material removal rate, tool wear rate and surface roughness. By increasing the electrical current and pulse on time, tool wear rate and surface roughness have grown, while by increasing the pulse off time tool wear rate has decreased. The average wear rate of the electrode in the aluminum alloy 2024 reinforced with 5% titanium oxide nanoparticles is 46.3%, equivalent to 0.346 gr, more than the weight loss of the aluminum 2024 specimen.

In order to investigation of dynamic and static restoration of SP-700 alloy, in this study continuous and interrupted hot torsion tests carried out at 850 and 1000ºC at different pass-strains and inter-pass times. The dominant mechanism in hot deformation at 1000°C is dynamic recrystallization (DRX) and consequently the entire microstructure comprises equiaxed grains, whereas at 850°C serration and tanglement of the grain boundaries were observed. Nevertheless, the microstructure of sample twisted at 850°C, indicates the occurrence of DRX and the formation of very fine grains. The mechanism of the formation of recrystallized grains in the vicinity of grain boundaries and triple points is bulging. With an increase in pass-strain (ε=0.5) at 1000°C, due to the increase in driving force for nucleation and growth of new grains, the kinetics of static restoration increases. In fact, at 850°C, in addition to static restoration there is another factor contributing in fractional softening which is β to α phase transformation.

Mixtures of nickel oxide and activated carbon (99% carbon) with stiochiometric ratio were milled for different times in a planetary ball mill. The stiochiometric ratio of mixture was prepared in an un-milled condition. The unmilled mixture and milled samples were subjected to thermogravimetric analysis (TGA) under an argon atmosphere and their solid products of the reduction reaction were studied using XRD experiments. TGA showed that the reduction of NiO started at ~800℃ and ~720℃ in un-milled and one-hour milled samples respectively whilst after 25 h milling it decreased to about 430℃. Increasing the amount of carbon more than stiochiometric ratio did not affect on the kinetics of carbothermic reduction. Thermodynamics assessments for NiO-C system were done using HSC software. Thermodynamics assessment and the experimental results indicated that Boudouard reaction plays a significant role in the carbothermic reduction reaction of nickel oxide. The results revealed that formation and increasing of monoxide carbon gas (COg) can change the kinetic reaction mechanism into solid-gas mechanism which has the major effect on increasing the rate of reaction. The decrease in the particle size/crystallite size of the nickel oxide in the milled samples not only resulted in decreasing in the reaction temperature, but also resulted in formation of fine particles of metallic nickel. Therefore, it is possible to achieve the metallic nickel particle in the range of submicron or nanometer via carbothermic reduction reaction of NiO using mechanical activation of NiO-C mixture.

In this study, synthesis and studying sensitivity of nano particles of manganese copper ferrite with common formula of cupper manganese ferrite cadmium doped has been considered. Applied method for manufacturing this nanoparticle is co-precipitation method that is a novel method for this combination. Nano particle structure has been investigated using experiments such as X-ray diffraction which showed single–phase and Nano characteristics of this combination and scanning electron microscope which showed the size of nanoparticles and uniformity. Sensitivity of the nano particles was tested using a laboratory system equipped with sensor heater with the ability of temperature control on which nano sensor was put, a chamber for injecting related materials and a connector electronic range that transferred Nano-sensor information to computer and it was analyzed by a software. This system could control temperature. This Nano sensor was tested using Methane that sensitivity and suitable temperature for this Nano sensor was 300˚C.

In this paper, the sensing properties of SnO2-PdPt nanohybrid to methane gas and effect of reduced graphene oxide (rGO) on improving its sensing performance was investigated. For this reason, first SnO2 was synthesized by hydrothermal method and then hybridized by Pd, Pt and PdPt catalysts. For investigating the effect of rGO, by the in-situ hydrothermal method, SnO2-rGO was synthesized instead of SnO2. Results showed that the nanohybrid sensor with bimetallic alloy catalyst, had higher response t lower temperature compared with monometallic catalysts and on the other hand, adding rGO, reduced the optimum sensing temperature of SnO2-PdPt and enhanced its response to methane. The SnO2-PdPt nanosensor showed 52.22% response to 1000ppm CH4 at 200oC. The sensing response and recovery times for this hybrid were 94s and 3.5min respectively, whilst the SnO2-rGO-PdPt showed 69.5% response at 150oC to the same concentration of methane. The response and recovery times for this hybrid were 50s and 4.5min respectively.

In this work, self-fluxing NiCrBSi coatings were deposited by plasma spraying. Simultaneous effect of temperature and time of the fusing heat treatment on microstructure, surface roughness and microhardness as well as wear performance of these coatings was evaluated. Fusing process was carried out at 1000, 1050 and 1100˚C for 5, 15 and 25 min. The morphologies and microstructures of the coatings as well as the wear tracks were characterized using optical microscope and scanning electron microscope. X-Ray Diffraction was applied to determine the phase composition of coatings. Wear performance of the fused coatings was investigated by Pin-On-Disk test. In consequence of the fusing process, the thickness, porosity and surface roughness decreased, the splat boundaries were eliminated, the microhardness increased, a metallurgical bond was created between the coating and the substrate, and hard carbide and boride precipitates (CrB and Cr7C3) were formed. Exceeding the optimum parameters of the fusing caused over-fusing phenomenon and thereby, degradation of coating properties. It was found that the temperature of 1000˚C and the time of 5 min are the optimum conditions of fusing process in this study, as the lowest porosity, the highest microhardness as well as the best wear performance were obtained in coating fused at these parameters. Dominant wear mechanism in this sample was abrasive wear.

The non-isothermal crystallization kinetics of Zr56Co28Al16, Zr56Co24Ag4Al16 and Zr56Co22Cu6Al16 BMGs were studied by differential scanning calorimetry at the continuous heating rates of 10, 20, 30 and 40 K/min. The crystallization kinetics parameters, including the effective and local activation energies corresponding to the characteristic temperatures, sensitivity of the characteristic temperatures to the heating rate were investigated. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the glassy alloys structure. The activation energies of characteristic temperatures were obtained by Kissinger and Ozawa methods. Also, the heating rate sensitivity of characteristic temperatures was determined by Lasoca method. The Ag-bearing Zr-based BMG presented higher activation energies with values of Eg=402, EX1= 336 and EX2= 395 kJ/mol and lower heating rate sensitivity in regard to characteristic temperatures, indicating a higher stabilization of the supercooled liquid, which can be correlated with the existence of strong icosahedral short range order (ISRO) clusters in the structure