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Fabrication of Mo-Si-B Intermetallic Compound Powders Under Different Heat Treatment Conditions

Park J. H., Lee S., Kim D., Kim Y., Yang S. H., Lee S. H.

Archives of Metallurgy and Materials

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2018

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Vol. 63, iss. 3

1509--1512

EN

In this research, we investigated the effects of reduction atmospheres on the creation of the Mo-Si-B intermetallic compounds (IMC) during the heat treatments. For outstanding anti-oxidation and elevated mechanical strength at the ultrahigh temperature, we fabricated the uniformly dispersed IMC powders such as Mo5SiB2 (T2) and Mo3Si (A15) phases using the two steps of chemical reactions. Especially, in the second procedure, we studied the influence of the atmospheres (e.g. vacuum, argon, and hydrogen) on the synthesis of IMCs during the reduction. Furthermore, the newly produced IMCs were observed by SEM, XRD, and EDS to identify the phase of the compounds. We also calculated an amount of IMCs in the reduced powders depending on the atmosphere using the Reitveld refinement method. Consequently, it is found that hydrogen atmosphere was suitable for fabrication of IMC without other IMC phases.

2

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.

3

Characteristics of TlBr single crystals grown using the vertical Bridgman-Stockbarger method for semiconductor-based radiation detector applications

Kim D. J., Oh J.-H, Kim H. S., Kim Y. S., Jeong M., Kang C. G., Jo W. J., Choi H., Kim J. G., Lee S. H., Ha J. H

Materials Science Poland

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2016

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Vol. 34, No. 2

297--301

EN

TlBr single crystals grown using the vertical Bridgman-Stockbarger method were characterized for semiconductor based radiation detector applications. It has been shown that the vertical Bridgman-Stockbarger method is effective to grow high-quality single crystalline ingots of TlBr. The TlBr single crystalline sample, which was located 6 cm from the tip of the ingot, exhibited lower impurity concentration, higher crystalline quality, high enough bandgap (>2.7 eV), and higher resistivity (2.5 × 1011 Ω·cm) which enables using the fabricated samples from the middle part of the TlBr ingot for fabricating high performance semiconductor radiation detectors.

4

Fabrication And Mechanical Properties Of A Nanostructured Complex Aluminum Alloy By Three-Layer Stack Accumulative Roll-Bonding

Lee S. H., Lee S. R.

Archives of Metallurgy and Materials

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2015

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Vol. 60, iss. 2B

1195--1198

EN

A multi-layered complex aluminum alloy was successfully fabricated by three-layer stack accumulative roll bonding(ARB) process. The ARB using AA1050 and AA5052 alloy sheets was performed up to 7 cycles at ambient temperature without lubrication. The specimen processed by the ARB showed a multi-layer aluminum alloy sheet in which two aluminum alloys are alternately stacked. The grain size of the specimen decreased with the number of ARB cycles, became about 350nm in diameter after 7cycles. The tensile strength increased with the number of ARB cycles, after 6c it reached 281MPa which is about twice higher than that of the starting material. The microstructures and mechanical properties of a three-layer AA1050/AA5052 alloy fabricated by the ARB were compared to those of the conventional ARB-processed material.

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Effect Of Compaction Pressure And Sintering Temperature On The Liquid Phase Sintering Behavior Of Al-Cu-Zn Alloy

Lee S. H., Ahn B.

Archives of Metallurgy and Materials

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2015

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Vol. 60, iss. 2B

1485--1489

EN

The liquid phase sintering characteristics of Al-Cu-Zn alloy were investigated with respect to various powder metallurgy processing conditions. Powders of each alloying elements were blended to form Al-6Cu-5Zn composition and compacted with pressures of 200, 400, and 600 MPa. The sintering process was performed at various temperatures of 410, 560, and 615°C in N2 gas atmosphere. Density and micro-Vickers hardness measurements were conducted at different processing stages, and transverse rupture strength of sintered materials was examined for each condition, respectively. The microstructure was characterized using optical microscope and scanning electron microscopy. The effect of Zn addition on the liquid phase sintering behavior during P/M process of the Al-Cu-Zn alloy was also discussed in detail.

6

Search for Variable Stars in Two Old Open Clusters: NGC 2506 and NGC 2420

Kim S.-L., Chum M.-Y., Park B.-G., Kim S. C., Lee S. H., Lee M. G., Ann H. B., Sung H., Jeon Y.-B., Yuk I.-S.

Acta Astronomica

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2001

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Vol. 51, No. 1

49--63

EN

We present results of photometric observations of two old open clusters: NGC 2506 and NGC 2420. V-band time-series and UBVI absolute CCD photometric observations were carried out to search for variable stars and to investigate their physical properties. From the UBVI photometry of NGC 2506, we obtain interstellar reddening of E(B-V)=0.04±0.03 mag, distance modulus (V-M_V)0=12.5±0.1 mag, and age log(t/yr)=9.25. From monitoring of 590 stars with 304 CCD frames taken for ten nights, three δ Sct stars and one eclipsing binary star are discovered in NGC 2506. For the other cluster, NGC 2420, we have examined light variations of 505 stars using 347 time-series data obtained for five nights, finding no variable star. It should be noted that we have found no γ Dor star among main-sequence stars near F0 in these two old open clusters, which is consistent with the suggestion that γ Dor-type phenomenon occurs in stars younger than log(t/yr)=8.4. On the other hand, it has been known that color-magnitude diagrams of these two clusters show well-established binary sequences, implying high incidence of binary systems: ≥20% for NGC 2506 and ≈50% for NGC 2420. However, only one eclipsing binary star was found in these two clusters.

7

Surface magnetic anisotropy of Co Hf thin films

Lim W. Y., Baek J. S., Kim Y. Y., Lee S. J., Yu S. C., Lee S. H.

Electron Technology

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1998

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Vol. 31, No. 1

60-64

EN

In order to investigate the effect of Hf content and the thickness of Co₁₀₀₋xHfx (x=16,24, 32 at.%) films we observed the absorption lines of spin wave resonance through the ferromagnetic resonance (FMR) measurement. One volume spin wave mode and surface spin wave models were observed for all the samples because the surface anisotropy constants of both sides of the films areless than zero. When the annealing temperature for Co84Hf16 film increased up to 225°C the surface magnetic anisotropy constant Ks2 of the film-substrate interface decreased from -0,07 erg/cm² to -0.32 erg/cm² and the Ks1 constant Ks2 of the film-air interface varied from 0.18 erg/cm² to -47 erg/cm². In Co76Hf24 film Ks2 decreased a little from -0.31 erg/cm² to -0.41 erg/cm² and Ks1 decreased rapidly from -0.19 erg/cm² to-0.60 erg/cm². In genearal it is shown that the surface anisotropy at the film-air interface is very sensitive to Hf content and annealing temperature. This result is due to the increase of Co content caused by oxidation of Hf atoms near the film-air interface during low temperature annealing (150~175°C) and the diffusion of Co atoms during high temperature annealing (200~220°C).