발간논문

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We fabricated the IN 713C alloy by a metal injection molding (MIM) process and investigated the high temperature tensile properties. The microstructure of the MIMed IN 713C alloy mainly consisted of γ, γ′ and carbides. 0.98% (area fraction) of the pores was observed near the surface of the specimen, whereas 0.27% of the pores were located in the interior of the specimens. Tensile tests were conducted from room temperature to 900℃. The result of the tensile tests showed that this alloy had similar or somewhat higher strengths (YS: 734 MPa, UTS: 968 MPa, elongation: 7.16% at room temperature) from RT to 700℃ than those of cast IN 713C alloys. Above 700℃, however, strengths decreased rapidly with an increasing temperature (lower than those of cast IN 713C). The observation results of fractography revealed that the tensile fracture of the MIMed IN 713C alloy started near the surface defects and then the initial crack pro-pagated along the inter-particle region where oxides were observed regardless of deformation temperatures.

keyword : Alloys, Powder processing, High temperature, Mechanical properties, Tensile test

Nano-sized yttria (Y2O3) dispersed copper (Cu) powders were prepared at a low temperature of 400℃ by a PVA solution route. Gel precursors consisting of metal cations and a PVA polymer were heated in an Ar-4%H2 atmosphere. The PVA was burned out below 300℃ and the fully crystallized Cu powder exhibiteda porous morphology and fine particles with a size of 10-15 nm with uniformly dispersed nano-sized Y2O3. The development of a crystalline phase and the final powder morphology were strongly dependent on the PVA content. The synthesized powder that did not employ PVA had severely agglomerated, larger particles that were not uniform in size. This paper introduces a PVA solution technique for fabricating nano-sized oxide ceramic dispersed metal powders. We also studied the effects of PVA content and heating temperature on the powder properties. Characterization of the synthesized powders was conducted using XRD, SEM, TEM, and EDS.

keyword : Copper-Yttria, Nano powder, PVA, Chemical solution method, Powder synthesis

Solid-solution nanocrystalline powder, (Ti,V)C, was prepared via high-energy milling of Ti-V alloys with graphite. The synthesis process was investigated in terms of the phase evolution by analyzing XRD data. The rapid sintering of nanostuctured (Ti,V)C hard materials was performed by a pulsed current activated sintering process. This process allows quick densification to near theoretical density and inhibits grain growth. A dense, nanostructured (Ti,V)C hard material with a relative density of up to 100% was produced by simultaneous application of 80 MPa and a pulsed current for 2 min. The microstructure and mechanical properties of the resulting binderless (Ti,V)C were investigated.

keyword : Sintering, Mechanical properties, Nanostructured material, Fracture toughness, Powder metal-lurgy

Creep rupture properties and microstructures of an as-cast Mg-4Al-2Sn alloy with and without Sr contents (1 wt% and 2 wt%) were investigated. Creep rupture tests were performed at 150℃ and 200℃ at 50 MPa and 70 MPa. The microstructure of the Mg-4Al-2Sn alloy initially had Mg17Al12 and Mg2Sn phases. The addition of Sr resulted in the formation of a ternary MgSnSr phase. Creep strength increased with an increasing Sr due to thermally stable MgSnSr precipitates, which suppressed coarsening of the Mg17Al12precipitates during creep. Scanning electron microscopy showed that Mg17Al12 phases at the grain bounda-ries are the starting point of void initiation, the cracks propagated along the grain boundaries. However, the MgSnSr phase interrupted crack propagation.

keyword : magnesium alloys, casting, creep test

Die soldering is one of common defect generating during the die casting process of aluminum alloys. Until now, the various researchers have been usually studied about die soldering phenomenon in relation to the aluminum components. However, the influence of the mold materials was rarely studied. So, in this study, we have carried out the die-soldering tests using both STD61 and S45C steel to confirm the effect of die materials on die-soldering. Each Aluminum alloys were melted and held at 680℃ and then STD61and S45C steel were dipped for 2hr in the aluminum melt. After dipping, the substrate was cooled in the air and analyzed using a SEM and EPMA. The result of soldering test, reaction layer of STD61 substrate has increased according to Si contents of aluminum, however, reaction layer thickness was deceased in the S45C substrate.

keyword : soldering, intermetallics, defect, image analysis, aluminizing

This study investigated the effect of a deformed microstructure in Cu-added bake-hardenable (BH) steel on primary recrystallization during the annealing process. To analyze primary recrystallization behavior in cold-rolled BH steel during annealing, deformed grains were classified by four types. If the average orientation of a unique grain (UG) was made up of a single α-fiber component, the deformed grain was classified as Type I. If the average orientation of the UG was made up of a single γ-fiber component, the deformed grain was classified as Type II. If the average orientations of the UG were made up of multiple γ- fiber components, the deformed grain was classified as Type III. If the average orientation of the UG could not be assigned to α-fiber or γ-fiber components, the deformed grain was classified as Type IV. No recry-stallization in the deformed grains of Type I was observed because the stored energy and misorientation in this type of grain is relatively low compared to the deformed grains of other types. The recrystallization nuclei in deformed grains of Type II were characterized as being composed of γ-fiber (ND//<111>) and random texture components. The recrystallization nuclei in deformed grains of Type III and Type IV mainly consisted of γ-fiber (ND//<111>) and random texture components, respectively. It was found that recry-stallization kinetics in the deformed grain of Type III was similar with those in the deformed grains of Type IV. The relatively low recrystallization kinetics of Type II was due to a low average misorientation in the deformed grains.

keyword : BH Steel, Cold-Rolled, Annealing, Stored Energy, Recrystallization

In this study, Ni-YSZ anode catalysts with a core-shell structure were synthesized by an elec-troless nickel plating method for solid oxide fuel cells (SOFCs). We investigated the microstructure and composition of the Ni-YSZ powder that were synthesized according to the plating conditions, which were the pretreatment cycles, temperature and time. The surface of the synthesized Ni-YSZ powders was homogenously plated with Ni particles and the powders had a core-shell structure. The catalyst nuclei of Sn and Pd were uniformly distributed on the surface of YSZ powders fabricated by the pretreatment of three cycles. As the plating time and temperature increased, the nickel particles increased and the plating layer was dense. However, when the plating temperature was over 90℃, the decomposition of the solution occurred. Therefore, the plating conditions were adjusted to control the shape of the Ni-YSZ core-shell powder. Finally, the Ni-YSZ particles with a size of 0.5 μm and a core-shell structure were fabricated at 80℃ for 10 minutes.

keyword : composites, plating, surface, scanning electron microscopy, SEM, core-shell

The doping effect of (Na0.5K0.5)NbO3(NKN) on the crystal structure, microstructure and electrical properties of BaTiO3 (BT) ceramics was investigated as alternatives for rare-earth elements. x(Na0.5K0.5) NbO3-(1-x)BaTiO3 ceramics with 0.00 ≤ x ≤ 0.10 were synthesized by a solid-state reaction method. The dielectric constant and the insulation resistance (IR) of the sample with x=0.05 were about 43% and 45% higher, respectively, than those of the undoped one, and the temperature coefficient of capacitance (TCC) met the X7R specification. The sample with x=0.05 had the lowest tetragonality among the four compositionsand had the finest grain size of < 2 um. The sample doped over the solubility limit (x=0.10) showed the lowest dielectric properties by the presence of a secondary phase with a needle shape. Although the addition of NKN decreased the tetragonality, the electrical properties were enhanced by the formation of the defect dipole and conduction electron, which are caused by the compensation of oxygen vacancy and electron. The addition of x=0.05 NKN to BaTiO3 improved the electrical properties due to the substitution behavior that resulted in making the defect dipole and conduction electron.

keyword : dielectrics, powder processing, electrical properties, X-ray diffraction, substitution behavior

Undoped and Li-doped ZnO(LZO) thin films were grown on quartz substrates by a sol-gel spin- coating method. The effects of Li contents on the structural and optical properties of the ZnO thin flims were investigated by X-ray diffraction (XRD), photoluminescence (PL), and ultraviolet-visible spectrophotometer. In the XRD investigation, undoped and LZO thin films had a wurtzite structure and high c-axis orientation. The intensity and the full width at half maximum (FWHM) of the (002) peak for LZO was the largest and smallest at 2.5 at%, which indicate that the LZO thin film is the most crystalline at 2.5 at%. In the PL investigation, the near band edge emissions of the LZO thin films were blue-shifted and the FWHM of the LZO thin films decreased with increasing Li doping concentrations.

keyword : Zinc oxide, Sol-gel, Lithium, Thin films, Optical properties, X-ray diffraction

In this study, MgH2 was used as a starting material instead of Mg. A sample with a composition of 86 wt% MgH2-10 wt% Ni-2 wt% NaAlH4-2 wt% Ti (designated MgH2-10Ni-2NaAlH4-2Ti) was prepared by reactive mechanical grinding. Its Pressure-Composition-Temperature (PCT) curve measurement, hydriding-dehydriding cycling performance measurement, and microstructure observation were then performed. The desorption PCT curve at 593 K for MgH2-10Ni-2NaAlH4-2Ti at the first cycle exhibited a long plateau at around 2.6 bar, and a short plateau at about 3.6 bar, which correspond to equilibrium plateau pressures of Mg-H and Mg2Ni-H systems, respectively. The hydriding rate at 593 K under 12 bar H2 increased as the number of cycles, n, increased from 1 to 4, and it decreased from n=4 to n=15. At n=14, MgH2-10Ni-2NaAlH4-2Ti absorbed 3.02 wt% H for 2.5 min, 3.36 wt% H for 5 min, 3.51 wt% H for 10 min, and 3.58 wt% H for 30 min, which is 93.0% of the hydrogen absorbed for 30 min at n=4.

keyword : hydrogen absorbing materials, mechanical alloying/milling, microstructure, scanning electron microscopy, SEM, NaAlH4, Ni and Ti addition