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1

Multistep Heat-Treatment Effects on Electrospun Nd-Fe-B-O Nanofibers

Jeon E. J., Eom N. S. A., Lee J., Lee B., Cho H. M., On J. S., Choa Y.-H, Kim B. S.

Archives of Metallurgy and Materials

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2018

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Vol. 63, iss. 3

1433--1437

EN

Neodymium-Iron-Boron (Nd-Fe-B) magnets are considered to have the highest energy density, and their applications include electric motors, generators, hard disc drives, and MRI. It is well known that a fiber structure with a high aspect ratio and the large specific surface area has the potential to overcome the limitations, such as inhomogeneous structures and the difficulty in alignment of easy axis, associated with such magnets obtained by conventional methods. I n this work, a suitable heat-treatment procedure based on single-step and multistep treatments to synthesize sound electrospun Nd-Fe-B-O nanofibers of Φ572 nm was investigated. The single-step heat-treated (directly heat-treated at 800°C for 2 h in air) samples disintegrated along with the residual organic compounds, whereas the multistep heat-treated (sequential three-step heat-treated including three steps;: dehydration (250°C for 30 min in an inert atmosphere), debinding (650°C for 30 min in air), and calcination (800°C for 1 h in air)) fibers maintained sound fibrous morphology without any organic impurities. They could maintain such fibrous morphologies during the dehydration and debinding steps because of the relatively low internal pressures of water vapor and polymer, respectively. In addition, the NdFeO3 alloying phase was dominant in the multistep heat-treated fibers due to the removal of barriers to mass transfer in the interparticles.

2

Microstructure Development of Spark Plasma Sintered Ti-Nb Alloy by Heat Treatment

Haq M. A., Abbas S. F., Eom N. S. A., Kim T. S., Lee B., Park K.-T., Kim B. S.

Archives of Metallurgy and Materials

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2018

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Vol. 63, iss. 3

1429--1432

EN

The β-phase Titanium (β-Ti) alloys have been under the spotlight in the recent past for their use as biomedical prosthetic materials owing to their excellent properties such as low elastic modulus, high corrosion resistance and tensile strength. Recently, Niobium (Nb) has gained a lot of attention as a β-phase stabilizing element in Ti alloys to replace Vanadium (V) due to its excellent solubility in Ti, low elastic modulus and biocompatibility. In this work, low cost Ti-20Nb binary alloy has been fabricated via powder metallurgy procedures. The blended powder mixtures of Ti and Nb were sintered at 900°C for 20 mins by the Spark Plasma Sintering (SPS) with an applied uniaxial pressure of 40 MPa. The heating rate was fixed at 50°C/min. The sintered alloy was subject to heat treatments at 1200°C in vacuum condition for various time durations. The characterizations of microstructure obtained during this process were done using FE-SEM, EDS and XRD. By increasing heat treatment time, as understood, the volume of residual Nb particles was decreased resulting in accelerated diffusion of Nb into Ti. Micro hardness of the alloy increased from 340 to 355 HV with the increase in β phase content from 30 to 45%. The resultant alloys had relatively high densities and homogenized microstructures of dispersed lamellar β grains in α matrix.

3

Fabrication and Characterization of CoCrFeNiMn High Entropy Alloy Powder Processed by Gas Atomization

Park T. G., Lee S. H., Lee B., Cho H. M., Choi W. J., Kim B. S., Shin K. S., Kim T.-S.

Archives of Metallurgy and Materials

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2018

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Vol. 63, iss. 2

1055--1059

EN

In this study, precisely controlled large scale gas atomization process was applied to produce spherical and uniform shaped high entropy alloy powder. The gas atomization process was carried out to fabricate CoCrFeNiMn alloy, which was studied for high ductility and mechanical properties at low temperatures. It was confirmed that the mass scale, single phase, equiatomic, and high purity spherical high entropy alloy powder was produced by gas atomization process. The powder was sintered by spark plasma sintering process with various sintering conditions, and mechanical properties were characterized. Through this research, we have developed a mass production process of high quality and spherical high entropy alloy powder, and it is expected to expand applications of this high entropy alloy into fields such as powder injection molding and 3D printing for complex shaped components.

4

Preparation Of Fine Grained SiC At Reduced Temperature By Two-Step Sintering

Kim K.-W., Oh K.-S., Lee H., Kim B. S., Chung T.-J.

Archives of Metallurgy and Materials

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2015

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Vol. 60, iss. 2B

1539--1542

EN

Two-step sintering route was applied for the densification SiC by promoting the role of liquid phase. The specimen contained 8 vol% of liquid phase composed of Al2O3 and Y2O3. The heating schedule consisted with initial rapid heating to 2000°C and immediate quenching to 1700 or 1750°C. By heating at elevated temperature, even distribution of the liquid phase was intended. The heat treatment at reduced temperature was to suppress the evaporative loss of the liquid and to secure the time for densification. The two-step sintering effectively suppressed loss of mass and coarsening. The resultant SiC was thus dense and was composed of fine grains exhibiting hardness of 2321 kgf/mm2.

5

CO Oxidation Properties Of Selective Dissoluted Metallic Glass Composites

Kim S. Y., Lee M. H., Kim T. S., Kim B. S.

Archives of Metallurgy and Materials

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2015

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Vol. 60, iss. 2B

1227--1229

EN

Porous metallic materials have been widely used in many fields including aerospace, atomic energy, electro chemistry and environmental protection. Their unique structures make them very useful as lightweight structural materials, fluid filters, porous electrodes and catalyst supports. In this study, we fabricated Ni-based porous metallic glasses having uniformly dispersed micro meter pores by the sequential processes of ball-milling and chemical dissolution method. We investigated the application of our porous metal supported for Pt catalyst. The oxidation test was performed in an atmosphere of 1% CO and 3% O2. Microstructure observation was performed by using a scanning electron microscope. Oxidation properties and BET (Brunauer, Emmett, and Teller) were analyzed to understand porous structure developments. The results indicated that CO Oxidation reaction was dependent on the specific surface area.

6

Numerical heat transfer in a rectangular duct for a non-Newtonian fluid with shear rate-dependent thermal conductivity and temperature dependent viscosity

Shin S., Kim B. S.

Applied Mechanics and Engineering

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1998

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Vol. 3, no 2

323-337

EN

The present numerical study investigates the effect of the shear rate-dependent thermal conductivity (SRDC) fluids on the heat transfer enhancement in a 2:1 rectangular duct flow. An axially and peripherally constant heat flux boundary conditions (H2) was adopted for a top-wall-heated configuration. The present numerical results of Nusselt numbers for SRDC fluid show the heat transfer enhancement over those of a shear rate-independent thermal conductivity fluid. The heat transfer enhancement is due to the effect of the increased thermal conductivity near the wall, which is attributed to the thigh wall shear rates. The present study proposes a correlation between the Nusselt number and variable thermal conductivity as follows; ___ . Both the temperature and shear rate-dependencies of the properties play important roles on the heat transfer enhancement.