Advanced Materials and Processing

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Authors: Young Seok Song, M.R. Lee, Jeong Tae Kim
Abstract: To check long term degradation behavior of hydraulic forged superalloy 718 during exposure at high temperature, an Alloy 718 ingot with a diameter of 400mm was manufactured by the vacuum melting process, VIM followed by VAR. The ingot was broken down for uniform microstructure and mechanical properties by a controlled cogging process using a hydraulic press. To investigate long term degradation behavior of impact properties and hardness, the specimens were exposed to 600oC, 650oC and 700oC for holding times up to 12,112 hours. Impact energy absorption tests were performed at room temperature. The fractured area and the microstructure of the impact specimens were observed by OEM and SEM and Brinell hardness tests were also performed. The changes of impact energy and hardness are remarkably different for each temperature condition. The results suggest that the impact properties and hardness of Ni based superalloy 718 is strongly related to temperature and time during high temperature exposure.
Authors: Ji Fang Li, Rui Yang, Jian Yu
Abstract: The natural photo-oxidation of PP bars containing nucleating agent (TMB-5) were studied by using infrared microscope (IM), differential scanning calorimetry (DSC), scanning electron microscope (SEM) and polarizing optical microscope (POM). The results indicated that nucleating agent accelerated the oxidation of PP significantly. The oxidation began from the surface and developed into the interior with time. There is a transition region near the surface, caused by the fast cooling during the injection moulding. This region is prior to be oxidized because of the low density. Therefore, it corresponded to the surface oxidation layer in PP and PP/TMB-5 bars. The nucleating agent helped to reduce the spherulite size of PP and broaden the transition region greatly – from about 25 μm to more than 200μm, covering the morphology of amorphous, fine crystals to relatively perfect crystals. More crystal boundaries, i.e. low density, and broad transition region led to higher oxidation degree and further oxidation along the depth. However, the thickness of the surface oxidation layer maintained nearly constant till the fragments dropped from the seriously damaged surface.
Authors: Chung W. Bark, Sang Woo Ryu, Yang Mo Koo, Hyun M. Jang
Abstract: This time, we would like to report our recent study on Pb(ZrTi)O3(PZT)-based ferroelectrics, currently one of interesting topics in condensed matter science. In this study, a new method, called synchrotron X-ray microdiffraction (XRMD) in situ, was introduced to examine the electric field-induced structural modulation of the epitaxially grown tetragonal PZT thin film. To evaluate the d-spacing (d001) from the measured intensity contour in the two theta-chi space, the peak position in each diffraction profile was determined by applying the two-dimensional Lorentzian fitting. By tracing the change of d-spacing as a function of the applied electric field and by examining the Landau free energy function for P4mm symmetry, we were able to estimate the two important parameters that characterize the field-induced structural modulation. Further theoretical analysis shows that the compressive epitaxial in-plane stress dominantly contributes to the elongation of the c-axis lattice constant in the c-axis oriented epitaxial PZT film.
Authors: Jun Liu, Xiao Meng Lü, Ji Min Xie, Hui Zhang, Zhan Jun Gu
Authors: Dal Woo Jung, Il Bum Kwon, Nak Sam Choi
Abstract: A temperature-compensating fiber Bragg grating(FBG) sensor having two different FBGs in one fiber line was applied to the real time measurement of mechanical normal strain in structures. Measurement of mechanical strains of the aluminum beam surface by the double FBG sensor was performed under various thermal conditions, which was compared with results of electrical resistance strain gage. The FBG sensor fabricated in this study could detect accurately values of mechanical strains without containing any thermal strain component.
Authors: Deuck Seung Bae, Woo Sang Jung
Abstract: The relationship between precipitation behavior of MX(M=V, Nb) nitrides and mechanical property during aging in 7Cr-1.5Mo ferritic heat resistant steels were investigated in this study. Artificial aging was carried out at 700°C for different time from 100 to 10000 hours to simulate the environment of fossil power plant. The Ostwald ripening equation is related to the containing elements can explain the accelerated coarsening of MX in the model alloys. The Ostwald ripening equation was modified by introducing the effective diffusion coefficient as a sum of the lattice diffusion coefficient and dislocation diffusion coefficient, based on an assumption that V, Nb and Ti might diffuse not only by the lattice diffusion mechanism but also by the dislocation diffusion mechanism.
Authors: Wei Zhang, Se Ahn Song, Hong Sik Jeong, Jin Gyu Kim, Youn Joong Kim
Abstract: Characteristic 60° dislocations occurred in hexagonal phase of Ge2Sb2Te5 thin foil cooled from 500°C to room temperature in a high voltage transmission electron microscope. The Burgers vector of dislocation was identified as 1/ 24 < 9902 > which is the edge component of 1 3 < 2110 > projected on the (1120) lattice plane. The dislocation resulted from the cooling-induced stress/strain in the Ge2Sb2Te5 alloy.
Authors: Tae Won Park
Abstract: Explosion Bulge Test in air has been carried out in order to evaluate base metal and weldment of pre-heating free steel, PFS-700 which was developed for using in submarine, destroyer and aircraft carrier and specified as Korean military specification, KDS-9515-3001. PFS-700 steel has good weldability so that it can be welded without pre-heating before welding. 1.4wt.% of Copper is added to compensate for the decrease of strength due to the decrease of Carbon in PFS-700. Usually, PFS-700 is aged at the temperature of about 650°C for the combination of strength and toughness. PFS-700 of the thickness of 25mm aged at 650°C was used for explosion bulge test. Underwater weapon system demands to conduct this test for the security of weld structures, and test method and procedure are specified in the military specification, MIL-STD-4139(SH). PFS-700 was explosion bulge tested as specified in this spec. Mechanical tests such as tensile test, CVN impact test, dynamic tear test and bend test are conducted from the specimens taken from prolonged EBT specimen and compared with the properties specified in KDS-9515-3001. EBT results show that PFS-700 welded with the welding material of spoorlarc-120 has over the thickness reduction of 14% without propagation of crack to the hole-down area.
Authors: Shao Jun Qiu, Hui Qing Fan, Xiao Dong Zheng, Jin Chen, Chen Yang, Pin Yang Fang
Abstract: To adjust the morphology and crystallinity of the chemically prepared Zr-riched` lead zirconate titanate (Pb(Zr0.95Ti0.05)O3) powders, polyethylene glycol (PEG 20000) was used in aqueous solution (modified Pechini method). Thermal analysis of the as-synthesized gel showed two distinct stages, which indicates the volatilization of water and the decomposition of polymers, respectively. When the gel were calcined at 400 oC, the powders consisted mainly of perovskite PZT accompanied with small amounts of secondary phases. The higher the calcination temperature applied, the stronger the intensity of XRD, which means the crystallinity were improved. Scanning electron microscopy of as-prepared powders suggested that the powders were soft agglomerate particles when calcined at 600 to 800 oC. Compared with the normal Pechini method, PEG could help the formation of crystalline phase and soft agglomerate. When the powders compacts were sintered at 975 to 1125 oC, high pure (>99.5%) of tetragonal phase ceramics were got having relative density of 85% to 94%.
Authors: Shinji Fujisaki, Md. Hasan Zahir, Yumi H. Ikuhara, Yuji Iwamoto, Kotaro Kuroda
Abstract: La2O3 and/or Ga2O3 doped, and the double dopant γ-Al2O3 composite powder samples were prepared by the sol-gel method. Nanostructural characterizations of the powder samples were performed by high resolution transmission electron microscopy (HRTEM) in order to clarify the role of the metal oxide dopants for the improved hydrothermal stability of the La2O3-doped Ga2O3-Al2O3 composite material. The results of HRTEM study and selected area electron diffraction (SAED) analysis revealed the formation of the uniform solid solution of Ga2O3-Al2O3. In addition, the results of X-ray elemental mapping analysis revealed the existence of well-dispersed La2O3 on the grain surface of Ga2O3-Al2O3 solid solution. These structural features could act an important role for the improved hydrothermal stability of the La2O3-doped Ga2O3-Al2O3 composite material.

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