Defect and Diffusion Forum Vols. 312-315

Abstract: Bronze-processed Nb3Sn-based multifilamentary composites with external diameter of 0.8 and 0.5 mm and coupled Nb filaments have been studied by transmission (TEM) and scanning (SEM) electron microscopy. After the two-staged annealing, 575°С, 150 h + 650°С, 200 h, commonly used for ITER conductors, a nanocrystalline layer of superconducting Nb3Sn compound is formed in every Nb filament as a result of solid-state reactive diffusion of Sn from the bronze matrix. It is demonstrated that in the wires of smaller external diameter the Nb filaments transformation into the Nb3Sn compound is more pronounced, that is the amount of the residual Nb is smaller. Besides, the nanocrystalline structure of the Nb3Sn diffusion layers is more perfect in 0.5 mm diameter wires, namely, the Nb3Sn grains are finer (their average size being 60 nm compared to 70 nm in 0.8 mm diameter wires) and are more uniform in sizes (the root mean square deviation of grain size distribution is correspondingly 15 and 17 nm).

Abstract: The diffusion coefficients of hydrogen (H) and deuterium (D) in Pd were determined in the frame of a quantum mechanical consideration. The three dimensional (3D) wave functions and eigenenergies of H and D at a stable octahedral (o) site and a metastable tetrahedral (t) site were determined by solving the Schrödinger equation with 3D potentials obtained by the first principles calculation. The states of H and D along the diffusion path were also determined by using transient potentials which were obtained with an aid of the nudged elastic band method. The magnitudes of tunneling matrix elements (J) were evaluated from the eigenenergy curves along the diffusion path. In the Pd-H system, thermally activated tunneling transitions are dominant jump processes. On the other hand, in the Pd-D system, it is revealed that a transition via an extended state where the wave function spreads both to o- and t-sites also has a considerable contribution to the diffusion. The calculated diffusion coefficients for H and D quantitatively agreed with experimental values and the so-called inverse isotope effect was reproduced.

Abstract: Gallium-based diffusive-hardening solders do not contain lead. These alloys have specific rheological properties. Their synthesis includes, as a rule, mechanical mixing of such initial components as gallium, copper and tin powders [1]. Then the metallic paste undergoes irreversible phase transformations forming a solid alloy with a specific structure. Here, we investigated microstructures of diffusive-hardened Cu-Ga-Sn alloys by SEM and EDX methods. Thermal diffusivity and heat conductivity of alloys mentioned above were studied by the laser flash method. Also differential scanning calorimetry (DSC) experiments were performed to obtain heat properties of the samples.

Abstract: The methods of thermal, thermogravimetric, mass spectrometric and high-temperature X-ray analysis have been used to determine the chemism of the processes involving the oxidation of a sulfide-metal copper-nickel alloy during continuous heating to 1190 K in an air flow. It has been found that the fineness of particles, the size of the sulfide phases in these particles, and the phase composition influence the phase transformation temperatures, as well as the kinetics and the sequence of the oxidation processes.

Abstract: A homogeneous precipitation process was employed to prepare nanosized W-10%wtCu-10%wtAg powders using ammonium meta tungstate, copper nitrate and silver nitrate as precursors. The initial precipitates were obtained by reacting ammonium meta tungstate, copper nitrate and silver nitrate solutions under certain PH and temperature. In order to synthesis W-Cu-Ag composite powders, the initial precipitates washed, dried, and then calcined in air in order to prepare CuWO4-x, Ag2W4O13 and WO3 oxide powders for the next step reduction. The reduction was carried out in a hydrogen atmosphere to form the final W-Cu-Ag nanocomposite powders. The powders were characterized by X-ray diffraction (XRD) technique. The morphologies of the powders were observed by scanning electron microscopy (SEM).

Abstract: Friction stir processing (FSP) was applied to modify the microstructure of pure copper and Cu/SiC nanocomposite layers. Optical and scanning electron microscopy (SEM) was employed to investigate the microstructure on the modified surface. Also, the wear resistance and friction coefficient behavior of specimens were investigated. FSP homogenizes and refines the copper structure and creates a microstructure with nano-sized SiC particles (30 nm) distributed in the pure copper matrix. Also, it is found that the traversal speed of tool significantly influence the microstructure of developed zone in pure copper. Generally, higher tool traverse speed leads to a more homogeneous microstructure and SiC particles dispersion. This means that higher traverse speeds result in agglomeration of SiC particles which reduces the microhardness values.

Abstract: Zn and Zn/Al 85/15 coatings are produced at optimum conditions by twin wire electric arc (TWEA) spraying technique and are used in various industrial applications to improve corrosion protection. As an alternative for Zn coating, Zn/Al 85/15 alloys coating show very good corrosion resistances and can protect substrate materials against harsh corrosive environments. The weight ratio of 85 Zn and 15 Al is regarded as optimum value. The forms of coating material are wires with different diameters. The aim of this study is comparison of anticorrosion performance of Zn and Zn/Al 85/15 coatings on ductile iron pipe surfaces as protective coatings against corrosion. Test samples were prepared from Zn and Zn/Al 85/15 coated ductile iron pipes. Coatings were investigated by optical microscopy, scanning electron microscopy (SEM) and have been analyzed by energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). They were compared according to protection performance against corrosion in salt spraying test according to ASTM B117. As a result of this study, Zn/Al 85/15 coating showed clear anticorrosion protection capacity against Zn coatings.

Abstract: Ultrahigh molecular weight polyethylene (UHMWPE) is a unique polymer with outstanding physical and mechanical properties that makes it particularly attractive to fabricate the bearing surface for artificial joints. Despite the requirement of visco-elastic properties of the UHWMPE and its composites, the characterization of them has received relatively little attention. The objective of this work is concerned with the studies on visco-elastic behaviour of UHMWPE and nanocomposites, which were prepared at optimized ball milling time with different cooling techniques. It is observed that stiffness of the materials increases appreciably at 0.2wt.% CNTs with an increase of frequency till 30Hz which confirms the reinforcing effect of CNTs in composites. The loss modulus of the sample is observed to be converged at higher temperature irrespective of frequency. The damping effect of the sample could be kept within the limit of polymer at any frequency range when the temperature is low and it is also possible at any temperatures at higher frequencies except LN2 cooled sample. The relaxation fraction increases with an increase of temperature and decreases with an increase of frequency. It is concluded that air cooled sample could be used wherever modulus is the main criteria irrespective of temperature and frequency, LN2 cooled sample can be used where more damping is required and water cooled samples may be used where more strength and toughness are required.

Abstract: In this article the last findings of composites with polymer, metal and ceramic matrices containing carbon fibers and their applications in internal combustion engines in the world beside their advantages and superiorities in comparison with common industrial materials are investigated. Afterwards, the position of these materials in automotive engines in Iran is analyzed. Researches show that carbon fibers-reinforced composites due to unique properties (including high specific strength and specific modulus, low thermal expansion coefficient, high fatigue strength, and high thermal stability) can replace common structural materials in different engine parts (such as, casing, different components, cylinder lining, and etc.). Applying these composites will result in weight reduction and consequently fuel consumption reduction, less pollution, better function and efficiency, and more lifetimes. Of course one should consider that there is a vast potential for application and development of composites in automotive engine, which will become in practice by complementary researches through time. Due to novelty of this technology in the world and also the lack of carbon fibers production and insufficient knowledge of production and application of carbon fiber composites in Iran, this issue has not been seriously investigated.

Abstract: The production of silicon carbide whiskers (SiCw) by using rice husks has attracted a considerable attention due to a lower production cost as compared to the other processing routs. In the present investigation, the effect of pyrolysis furnace type (vertical tube, horizontal tube, and graphite chamber) on the yield of the resultant SiCw was investigated. It was concluded that the maximum yield was achieved by using a horizontal tube furnace whereas the minimum yield was obtained in a vertical tube furnace. These results were rationalized in terms of the different conditions for the evacuation of the produced gases from the different pyrolysis furnaces.