Diffusion in Solids and Liquids V

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Authors: Y. Lahmar-Mebdoua, A. Vardelle, Pierre Fauchais, D. Gobin
Abstract: A plasma-sprayed coating is built up by the layering of individual splats. The latter are formed by spreading and solidification of molten particles sprayed onto a solid substrate. The coating properties depend on its microstructure and the quality of contact between the splats and the underlying layer and between the piled-up splats. This work deals with a 1D model of heat transfer between plasma-sprayed alumina splat and smooth substrate. The model is based on heat diffusion in the solidifying splat and substrate and includes undercooling phenomenon, heterogeneous nucleation and crystal growth kinetics. It assumes that splat spreading and solidification are two independent processes. The model predicts splat cooling and solidification taking into account, as far as possible, the in-flight particle properties drawn from the literature in order to study their effect on splat thermal history. The effect of the quality of contact between the splats as well as the already-deposited and solidified layer thickness on the grain size distribution and front solidification velocity is investigated.
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Authors: E. Çakır, Celaletdin Ergun, Filiz Çinar Şahin, İ. Erden
Abstract: In the present study, a method based on sulfuric acid dehydration of sugar was developed to synthesis a precursor material, which can yield B4C/ TiB2 composites at much lower temperatures compared to traditional carbothermal methods. The precursor material for pure B4C and B4C / TiB2 composites were heat treated at 1650oC under Ar and Ar+H2 atmosphere. Then the samples were characterized by X-ray diffraction (XRD) and crystallized B4C and B4C / TiB2 composites can be obtained at 1650oC
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Authors: Arash Rezaei, Shahram Ahmadi, Ali Shokuhfar
Abstract: In this investigation, the dynamic recrystallization phenomenon (DRP) of an Al-Li-Cu-Mg-Zr alloy was studied at two temperatures, 350°C and 400°C. Wedge samples were subjected to hot rolling deformation in both temperature and one passes. For wedge specimens, reduction up to 70% was considered. Results showed that the grain size of the specimens after hot rolling decreases from 100 μm to 30 μm because of DRP. Furthermore, it is observed that critical reduction for starting DR at 400°C is 40% and at 350°C reduction must increase to 50%.
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Authors: R. Bakhtiari, A. Ekrami
Abstract: 4340 steel bars were austenitized at 850oC for 1 hour followed by heating at 700oC (ferrite and austenite region) for 90 min and quenching into a salt bath with different temperatures of 300, 350, 400 and 450oC. The steel bars were held for 1 hour at these temperatures before air cooling to room temperature. Various ferrite-bainite microstructures with 34% volume fraction of ferrite and different bainite morphologies were obtained. The results of SEM studies showed that by increasing the austempering temperature, the morphology of bainite varies from lower to upper bainite. According to the T-T-T diagram of the studied steel, the bainite transformation will not complete for the holding time of 1 hour at 400 and 450oC austempering temperatures and there can be 9 and 23Vol% of martensite at these temperatures respectively. Microstructural studies of specimens austempered at these temperatures showed complex microstructures of ferrite, bainite and martensite. Mechanical testing results showed reduction in yield and ultimate tensile strengths, hardness, uniform and total elongations and impact energy with increase of the austempering temperature from 300 to 400oC. But for dual phase steel austempered at 450oC, the yield and tensile strengths and hardness increased and a severe reduction in total elongation and impact energy was obtained. This brittle behaviour was related to martensite formation during cooling from this austempering temperature.
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Authors: T. Abid, A. Boubertakh, S. Hamamda
Abstract: The effect of pre-aging and maturing at room temperature on the hardening response of an Al-Mg-Si alloy is investigated using differential scanning calorimetry (DSC), hardness measurements (Hv) and scanning electron microscopy (SEM).Two experimental conditions are examined. First, natural aging for different times (3 weeks and 1 month) followed by artificial aging at 180°C as a function of time. Second, pre-aging at temperatures in the range 75-100°C followed by artificial aging at 180°C after natural aging for the same periods. The present results indicate that the effect of the pre-aging just after the heating and quenching is used in order to correct the undesirable effect of aging at room temperature. However, during the artificial aging, the alloy hardening becomes faster. Aged samples which have already undergone pre-aging and maturing reveal the precipitation of ''phase. This later phase has the highest hardness value.
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Authors: Alexander Feher, S.B. Feodosyev, I.A. Gospodarev, V.I. Grishaev, K.V. Kravchenko, E.V. Manzhelii, Eugenyi Syrkin
Abstract: The calculation of the local density of electronic states of graphene with vacancies, using the method of Jacobi matrix, was performed. It was shown that for atoms in the sublattice with a vacancy the local density of electronic states conserves the Dirac singularity, similarly as in an ideal graphene. A quasi-Dirac singularity was observed also in the phonon spectra of graphite for the atom displacements in the direction perpendicular to layers. Changes of phonon spectra of graphite intercalated with various metals were analyzed. On the basis of our results and using the BCS theory and Eliashberg equation we proposed what dynamic properties an intercalated graphite system should show to obtain an increased Tc.
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Authors: Beril Kaya, Suat Yılmaz, Celaletdin Ergun
Abstract: Medical grade cast CoCrMo alloy (F75) was ion nitrided under different process parameters including time (2, 9 hr) and temperature (600, 800 0C) at a gas mixture of 80%N2–20%H2. The nitrided surfaces were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). MicroVickers hardness tests were done on the cross sections of the nitrided specimens to investigate the hardness profile. The significant effects of nitriding temperature and time on the microstructure and hardness value of nitrided surface layers were displayed.
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Authors: R. Gheriani, Rachid Halimi
Abstract: Titanium carbides are well known materials with great scientific and technological interest. The applications of these materials take advantage of the fact that they are very hard, refractory and that they have metallic properties. In this work, we have studied the influence of the heat treatment temperatures (400-1000°C) on the interaction between the titanium thin films and steel substrates. Steel substrates, 100C6 type (AFNOR norms) containing approximately 1 wt % of carbon were coated at 200°C with titanium thin films by magnetron sputtering. The samples were characterized by X-ray diffraction (XRD) and Auger electron spectroscopy (AES). Vikers micro-hardness measurements carried out on the annealed samples showed that the micro-hardness increases with annealing temperature, reaches a maximum (3500 kg/mm2), then decreases progressively. The growth of micro-hardness is due to the diffusion of the carbon, and to the formation of titanium carbide. However, the decrease of micro-hardness is associated with the diffusion of iron and the formation of iron oxide (Fe2O3). At higher temperatures, we note the formation of titanium dioxide (TiO2).
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Authors: R. Gheriani, R. Bensaha, Rachid Halimi
Abstract: X-ray microanalysis was used in the qualitative and quantitative study of titanium carbides obtained by deposition of thin titanium films on steel substrates using reactive r. f. sputtering of a pure titanium target. The samples are subjected to high vacuum annealing at a temperatures between 200 and 1000°C for 60 min. The morphological analysis by SEM shows that all films exhibited a dense microstructure. The EDS pattern of X-ray microanalysis shows that the non treated samples mainly consist of titanium. The concentration of Ti decreases progressively with the annealing temperature therefore the carbon and iron concentrations increases, this is as result of atomic interdiffusion between the substrate and the thin film. In the highest temperatures of annealing we note the diffusion of the elements of substrate towards outside layers even for those having weak concentrations. The relationship between mechanical properties and the reaction is carried out by Vickers micro-hardness measurements.
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Authors: Farid Mechighel, Bernard Pateyron, Mahfoud Kadja, Mohammed El Ganaoui, S. Dost
Abstract: A numerical simulation study has been carried out to examine the effect of a static magnetic field on the solidification process of an alloy. A mathematical model, based on the continuum model, was developed for the computation of a transient double-diffusive fluid flow under Lorentz body force. The model includes conservation of mass and momentum, heat, species and electrical charge balance equations. The simulation domain was selected as a cavity filled with a metallic alloy and differentially heated, which may be taken as a Bridgman model domain used in the crystal growth process. The solution is carried out by using a Finite Volume Method. Study of the direction and the intensity of the applied magnetic field effects on stabilizing the double diffusive flow field were also carried out. Simulation results indicate that the use of a static, magnetic field in this growth setup is effective in suppressing natural convection in the solution.
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