Key Engineering Materials Vols. 345-346

Abstract: In the present paper, lead zirconate titanate (PZT) and lead titanate (PT) piezoelectric ceramics were subjected to both high electric field (which is higher than the coercive electric field) with low frequency and low electric field with high frequency (which is the resonance frequency). After applying certain electric field systematically, resonance and anti-resonance frequencies and an electrostatic capacity were measured by means of an impedance analyzer, and an electromechanical coupling coefficient, a dielectric constant, an elastic coefficient and a piezoelectric constant were evaluated from the frequencies and capacity measured. Then variation of the material properties in process of time was investigated experimentally, and the dependence of the variation of the properties due to mainly domain switching on conditions of applied electric field was elucidated.

Abstract: Aging behavior of Al-2.1Li-2.9Cu-0.12Zr(wt%) alloy has been studied as functions of aging time and temperature by using a differential scanning calorimetry(DSC) and transmission electron microscopy(TEM). The aged specimens at 130, 160, 190°Cwere compared with the as-quenched specimen in view of the aging behavior by observing the reaction enthalpy during the heating period of DSC experiments. DSC peak associated with the formation of GP zone is not observed at 130 and 160°C, but it does at 190°C, so it is found that the thermal stability of GP zone is changed at between 160 and 190°C. At the aging temperature of 130°C, the heat absorption corresponds to the formation of δ´ increases, while the heat evolution related with the formation of T1 reveals an opposite trend. In aging temperature of 160 and 190°C, aging time representing the drastic decrease of heat absorption of δ´ is coincident with the transition time showing the decreasing of heat evolution of T1. The micro-Vickers hardness of the specimens aged at 160°C and 190°C shows maximum values about 182 and 165 at aging times of about 72 hr and 25hr, respectively. From comparing reaction enthalpies for the dissolution of δ´ and the formation of T1 phases with the aging time showing the maximum hardness, it is found that δ´ phase rather than T1 phase markedly contributes to the hardening at an aging temperature of 160°C. In contrast, T1 phase plays an important role in hardening at an aging temperature of 190°C.

Abstract: This paper deals with the damage resistance of glass/epoxy laminates with embedded shape memory alloy (SMA) subjected to low-velocity impact at various temperatures. For this goal, the impact tests were performed by using an instrumented impact-testing machine at three temperatures: +20°C, -10°C and -40°C. And the resultant damages were inspected through the scanning acoustic microscope (SAM). Also, based on the impact force history and the damage configuration, the impact resistance parameters were employed to evaluate damage resistance of laminates with embedded SMA wires. As results, it was found that the damage resistance of glass/epoxy laminates with embedded SMA wires is dependent on the service temperature.

Abstract: In Ni-SiC composite coating, the SiC content is dependent on the surface properties of SiC particles. As sulfuric acid has a strong dehydration force, addition of sulfuric acid in the Ni sulfamate bath changed the surface properties of SiC particles, affecting the codeposition behavior of SiC particles. Also the additives such as SDS affect the electrodeposition behavior of the Ni-SiC composite coating. In this study, effects of the HSO4 ‾ and the current density on the electrodeposition behavior of the Ni-SiC composite coating have been investigated. The Ni-SiC composite coatings were electrodeposited at current densities of 50~200mA/cm2. The surface and cross-sectional morphologies of the Ni-SiC composite coatings were observed using SEM, and their mechanical properties were characterized with micro-Vikers hardness.

Abstract: Crash simulations were performed for automotive sheets. To understand the influence of crystal structures in sheet materials on crashworthiness, the effect of the yield function shape was studied by adopting the recently developed non-quadratic anisotropic yield surface, Yld2004-18p. The effect of the back-stress was also investigated by comparing simulation results obtained for the isotropic, kinematic and combined isotropic-kinematic hardening laws based on the modified Chaboche model. In addition, the effects of anisotropy and sheet thickness on crashworthiness were evaluated.

Abstract: Silicon carbide (SiC) materials have been extensively studied for high-temperature components in fusion blanket system and gas turbines, because they have excellent a hightemperature mechanical properties, high thermal conductivity and wear resistance. However, the brittle characteristics of SiC such as low strain-to fracture still impose a severe limitation on the practical application of SiC materials. Therefore, a study of the sliding wear characteristics and fabrication of SiC ceramic by sintering temperature and additives are need. As the result of abrasion, the friction coefficient of the monolithic SiC sintered at 1800°C was the lowest, and the friction coefficient of that sintered at 1760°C was the highest. The monolithic SiC manufactured at 1800°C showed the highest hardness and the lowest friction coefficient. The friction coefficient of the monolithic SiC sintered by the SiO2 contents of 2wt% was the lowest, and the friction coefficient that sintered by the SiO2 contents of 5wt% was the highest. 1800°C of sintering temperature and 2wt% of SiO2 contents ware shown high hardness, low friction coefficient and wear loss compare with other temperatures and SiO2 contents.

Abstract: Since the early 1960s hot isostatic pressing (HIP) was used to improve the quality of castings made of various alloys. The closure of pores, originating from the casting process, is considered as the main source of these improvements. For the aluminium alloy Al-9Si-3Cu specimens for tensile testing were machined from castings either squeeze casted and heat treated to T4 and T6 conditions or investment casted. The effect of HIP on density, roughness and mechanical properties was investigated. The density and roughness of the squeeze casted specimens did not change remarkably and their tensile strength became worse, because the initial heat-treatment-state was destroyed by HIP. The investment casted specimens became denser and roughness became worse due to closure of pores near the surface. Although roughness after HIP was higher than in the initial state, tensile strength was improved significantly by HIP, because of closure of the pores in the specimen.

Abstract: As the design rule goes down sub-70 nm for the ULSI devices, the total thermal budget that the device can take during the fabrication is also reduced very much. Hence, in this work, we propose a novel low-temperature LPCVD process for formation of thin dielectric oxide film which does not need SiH2Cl2 gas. We have also evaluated the electrical reliability of the film by making the capacitors with oxide-nitride-oxide (ONO) structure. The leakage current of the new oxide was similar to that of the high-temperature wet oxide until the electric field is lower than 5 MV/cm. When the film was annealed by N2 gas, however, it has shown much better characteristics over the entire range.

Abstract: The precipitation behavior of MX nitrides during aging was investigated for Cr-Mo-N-X (X=V,Nb,Ti) ferritic steels. MX nitrides, which are finely dispersed in the matrix of Cr-Mo ferritic heat resistance steel, increase the creep strength of the steels by the precipitation strengthening mechanism. However, MX particles usually coarsen during aging, and this results in a decrease of creep strength. In order to clarify the coarsening process of MX particles, TEM samples were prepared from the aged for up to 3000hours specimens. During the aging, V, Nb and Ti containing nitrides (called MX nitrides) were precipitated out. From TEM observation, particle size distribution was confirmed and size distribution follows a typical log-normal distribution. The observed coarsening behavior well agreed with the calculated coarsening behavior of precipitates by Oswald ripening equation.

Abstract: The process design of profile ring rolling for a large-scale turbine diaphragm was made using the calculation method and three dimensional finite element method (FEM). The design criteria are to achieve uniform distributions of strain and temperature, and defect-free profiled ring products. Based on the compression test results of the low alloy steel, deformation processing map was generated using the combination of the dynamic materials model (DMM) and stable or unstable forming criteria. The processing map was used to determine the optimum ring rolling temperature and feed rate of the mandrel. FEM analysis was simulated to predict the formation of rolling defects and deformed shape in the profile ring rolled diaphragm product. Finally, optimum process design to obtain a sound large-scale turbine diaphragm without forming defects was suggested and its validation was made by the comparison between the experimental data and FE analysis results.