Journal of Advanced Materials in Engineering (Esteghlal)

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In this paper, the flow-field of an incompressible viscous flow past a solid-sphere at low Reynolds numbers (up to 270) is investigated numerically. In order to extend the capabilities of the finite volume method, the boundary (body) fitted coordinates (BFC) method is used. Transformation of the partial differential equations to algebraic relations is based on the finite-volume method with collocated variables arrangement. For solving the obtained algebraic relations, the TDMA in periodic state is used. To approximate the convective fluxes, the differencing scheme of Van leer is used and SIMPLEC handles the linkage between velocities and pressures. The verification of the code is checked by the analysis of flow past a solid sphere at low Reynolds numbers of 20 to 210. A good agreement is obtained between the present results and the available experimental and numerical data. The flow-field past a sphere at low Reynolds numbers of 210 to 270 shows that the steady non-axisymmetric regime is going to build up at the Reynolds number of 211. Keywords: Solid-Sphere, Wake, Three Dimensional Analysis, Boundary Fitted Cordinates

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All algorithms for impedance calculation use an analog-to-digital converter. The high accuracy of the impedance seen by a distance relay is an important factor in the correct isolation of the faulty part of power systems. To achieve this, a novel technique based on third order interpolation is used in this paper. According to this technique, the times and the values of the obtained samples are changed to real ones. To evaluate the new technique, it is applied to six digital distance algorithms, namely, Discrete Fourier Transform (DFT), Half-cycle Discrete Fourier Transform, Least Square, Mann-Morrison, Least Square with Delete Dc, and Prodar70. The technique is found to be capable of accurately computing the impedance in the algorithms mentioned. Comparisons are made among the results to show the efficiency of the new technique for decreasing errors in all algorithms.

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The initial stages of the  precipitation in a dilute Ni-Al binary alloy, Ni-11.6 at.%Al, were studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD), electron diffraction and electron microscopy (FEG-SEM and TEM) techniques. Three samples were similarly solution treated and then cooled to room temperature under different cooling rates, 170، 25 and 0.03oCs-1. The results indicate a clearly homogenous  nucleation during rapid quenching which takes place via simultaneous ordering and phase separation. However, by decreasing the cooling rate to 25oCs-1 the nucleation mechanism changes to heterogeneous on the preferred nucleation sites. The capability of the mentioned empirical techniques for studying the initial stages of the γ′ is another subject which is studied in this article.

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Precipitation has always been one of the important methods in the preparation of ceramic nanopowders. In this study, the most important parameters, ageing time and concentration parameters, have been studied. Yttrium oxide (Yttria) nanopowder was synthesized by precipitation method. Yttria micropowder and ammonium hydrogen carbonate were used as precursor materials. The study involved aging time and concentration in four and three levels, repectively (3, 6, 12 and 24h for ageing time and 0.25, 0.5 and 0.75 mol/L for concentration). Synthesized phases, thermal behavior and particle size were studied by X-ray diffraction pattern (XRD), thermogravimetry (TG), differential thermal analysis (DTA) and field emission scanning electron microscopy (FE-SEM). Fourier transform infrared spectroscopy analysis (FTIR) was used for studying bonding before and after the heat treatment at 900, 1000 and 1100 ^°C.

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This study aims at investigation of the hydrogen damage after dissolution annealing and two-stage aging in aluminum 7075 alloy. Dissolution annealing was performed at 500 to 575 ^°C for duration of 1 to 20 hours. The first stage of two-stage aging was performed at 180, 200 and 220 ^°C for 30 minutes. The second stage was carried out at 120 and 150 ^°C for 10, 15 and 20 hours. Structural characteristics and chemical composition of precipitates was investigated using SEM and EDS methods, respectively. Reduction of the tensile strength in T6 process after hydrogenation reached to 150 MPa, although it decreased only, about 50 MPa in the two-stage process. Overall, tensile strength after hydrogen charging was significantly increased in the two-stage aging compared to the T6 process.

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In this research, the nanopowders of lanthanum and neodymium co-doped yttria were synthesized by sol-gel combustion method. Citric acid and glycine were used as the gel maker and fuel respectively. The effect of molar ratio of citric acid to glycine on the grain size and morphology was evaluated. The optimized products were characterized by X-ray diffraction (XRD), field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), UV–visible (UV–Vis), thermal gravimetric-differential thermal analysis (TG/DTA), and Fourier transform infrared spectrometer (FTIR). The optimized products which are synthesized with a molar ratio citric acid to glycine 1.06:1.06 have an average grain size of 30-40nm with spherical morphology, and without agglomeration. Also, their band gap is 3.29eV.

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Age hardening processes cause a wide range of changes in 17-4 PH stainless steel properties. Aging at 480ºC for 1 hour (A480-1) and aging at 620ºC for 4 hours (A620-4) are two most applicable heat treatment processes for this alloy. Not only the studies on fatigue behavior of this alloy in these two heat treatment conditions are few, but also the methods of fatigue test were mostly axial. In this study, after microstructure studies, hardness and tension tests, fatigue behavior at A480-1 and A620-4 conditions were evaluated by using a rotational bending fatigue test machine. Tension results showed that yield strength and ultimate tensile strength of A480-1 specimens were 40 percent more than A620-4 specimens. However, elongation of A620-4 specimens was 30 percent more than A480-1 specimens. Fatigue results revealed the endurance limit of aged specimens is 50 MPa more than overaged specimens. Overall results showed that A480-1 condition specimens are more resistant to fatigue than A620-4 condition specimens.