Papers by Author: Won Yong Kim

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Authors: Taek Kyun Jung, T.J. Sung, Mok Soon Kim, Won Yong Kim
Abstract: Al-8Fe-2Mo-2V-1Zr alloy powders were prepared by gas atomization and melt spinning method. In melt spinning technique, melt spun ribbons were pulverized by a speed rotor mill to make a powder shape. In order to produce a bulk form, powders were canned and hot extruded in the extrusion ratio of 25 to 1 at 693K. For the gas atomization and hot extrusion processed bulk material, equiaxed grains with the average size of 400 nm and finely distributed dispersoids with their particle sizes ranging from 50nm to 200nm were observed to display a characteristic nano-structured feature over the entire region. For the melt spun and hot extrusion processed alloy, a refined microstructural feature consisting of equiaxed grains with the average size of 200 nm and fine dispersoids with their particle sizes under 50 nm appeared to exhibit a difference in microstructure. Yield strength of the latter alloy was higher than that for the former alloy up to elevated temperatures. The maximum yield strength was measured to about 800 MPa at room temperature for the latter alloy.
Authors: Gyu-Sam Shim, Mok Soon Kim, Won Yong Kim, Hiroshi Yamagata
Authors: Shae K. Kim, Hoon Cho, Jin Kyu Lee, Won Yong Kim, Hyung Ho Jo
Abstract: The aim of this study is to report results of initial trials involving the strip casting process for ETPC (Electrolytic Tough Pitch Copper) and to investigate the effect of process parameters on the chemistry, microstructure evolution and mechanical properties of ETPC. A vertical twin roll strip caster used in this research was designed for manufacturing high-purity and/or reactive metal strips with a system for controlled atmosphere ranging from 103 to 10-3torr. It is expected that oxygen content can be reduced as low as 0.001wt% in as-cast strip.
Authors: Sun Ki Kim, Han Sol Kim, Won Yong Kim
Abstract: This study is to examine wear properties of Ti-Nb-Si alloys under dry condition and to investigate its wear mechanism. A ball-on-disc type wear testing machine was used to evaluate the wear factor. Optical microstructure observation revealed that the microstructure appeared to mixture appearance consisting of β phase and small amount of α″ martensite phase. Yield strength increased with increasing Nb and Si content. Wear resistance of the present alloy are strongly dependent upon yield strength and elastic modulus. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis confirmed the tribo-chemical reaction between the alumina ball and the present alloy occurs due to decomposition of alumina to aluminum.
Authors: Won Yong Kim, Han Sol Kim, Eun Kyun Chung
Abstract: In this study, we have investigated a texture and microstructure of gold bonding wire by transmission electron microscope, X-ray diffraction and electron back scatter diffraction according to process parameters, such as casting, drawing and annealing processes. The gold was cast into 7mm diameter rods by vertical continuous casting technique, the cast rods were drawn to a final wire size of 25μm, and then the fine wires were annealed. A unidirectional solidification structure having <100> fiber texture of cast rod was strongly developed depending on casting route and speed. In the sample with unidirectional microstructure developed in partial it is found that dislocation substructure with nano-size appeared to display a characteristic feature, while no dislocation substructure is seen in the sample with complete unidirectional microstructure. Interface stability between solid and liquid may be responsible for this difference in dislocation substructure. With decreasing casting speed <100> fiber component became weak. Initial texture plays important role on development of <111> fiber texture at drawing and final annealing stages. On the basis of results obtained it is suggested that both the initial texture are important in controlling the texture of gold bonding wire.
Authors: Dong Suk Lee, Taek Kyun Jung, Mok Soon Kim, Won Yong Kim
Abstract: Two atomized alloy powders, those chemical compositions are Al-10Si-5Fe-1Zr and Al- 10Si-5Fe-4Cu-2Mg-1Zr, were pre-compacted by cold pressing with 350MPa and subsequently hot forged at temperatures ranging from 653K to 845K and at an initial strain rate of 10-2/s in order to produce bulk cylindrical type alloys with the diameter of 10 mm. The addition of Cu and Mg into the present alloy causes a decrease in the eutectic reaction temperature of Al-10Si-5Fe-1Zr alloy from 841K to 786K and results in a decrease of flow stress at the given forging temperature. TEM observation revealed that in addition to Al-Fe based intermetallics, Al2Cu and Al2CuMg intermetallics appeared to display the alloying effect additionally. The volume fraction of intermetallic dispersiods increased by the addition of Cu and Mg. Compressive strength of the present alloys was closely related to the volume fraction of intermetallic dispersoids.
Authors: Jun Kyung Sung, Mok Soon Kim, Won Yong Kim, Akihiko Chiba
Abstract: A recrystallized Co-Ni-Cr-Mo based superalloy was produced by cold working of 72% and subsequent recrystallization heat treatment. Microstructural observation revealed that a full recrystallization of the cold-worked alloy occured when heat treatment was performed at and above 1273K for 1h. So that, recrystallization heat treatment was carried out in a temperature range from 1273K to 1473K for 1h~24h, by which the average grain size was controlled to 28µm~238µm. Tensile tests were carried out from room temperature (RT) to 1073K in order to understand the effect of grain size on the mechanical properties of the Co-Ni-Cr-Mo based superalloy. At RT and 943K, yield strength, tensile strength and elongation of the recrystallized alloy were improved with decreasing grain size. The alloy having a grain size less than 42µm exhibited a steady-state flow behavior in the true stress-true strain curve at 943K. However, the alloy having a grain size of 28µm showed lower yield strength than that of 42µm at 1073K. It was found that the steady state flow is closely related to the occurrence of {111}<112> deformation twinning in the Co-Ni-Cr-Mo based superalloy.
Authors: Won Yong Kim, Han Sol Kim
Abstract: The effect of Ge and oxygen content on microstructural formation and mechanical properties of Ti-Nb alloys were investigated in order to design a desirable Ti based alloy through casting process. Three phase mixtures consisting of bcc-structured β phase, orthorhombic structured α" phase and intermediate ω phase were found depending on Nb, Ge, oxygen content in the present alloy system. The volume fraction of α" phase and ω phase decreased with increasing Ge or oxygen content. This microstructural information may indicate that both Ge and oxygen act to increase the stability of β phase rather than α" phase in metastable β-Ti based alloys prepared by water quenching. Elastic modulus values were sensitive to phase stability of constituent phases.
Authors: Won Yong Kim, Han Sol Kim
Abstract: Elastic modulus and mechanical property of Ti-Nb-O alloys prepared by arc melting and subsequent water quenching were investigated in order to correlate the result of microstructural observations and phase stability. In quenched state, it was evidently observed that the volume fraction of bcc-structured β phase increased with increasing content of oxygen, and the occurrence of intermediate ω phase was suppressed in the present Ti-Nb based alloys. This microstructural result was supported by changes in martensite transformation temperature, which decreased with increasing content of oxygen within the chemical composition range investigated. Therefore, it is suggested that oxygen plays a role as a β phase stabilizer rather than α stabilizer in metastable β Ti-Nb alloys. Yield strength increased with increasing content of oxygen without a large consumption of ductility in Ti-Nb based alloys with Nb content of 24% while elongation value decreased with increasing content of oxygen in Ti-Nb based alloys with Nb content of 28%. The variation of mechanical property was explained by the phase stability, phase formation and microstructure in correlation with oxygen and Nb content.
Authors: Won Yong Kim, Han Sol Kim, Shae K. Kim, Tae Yeub Ra, Mok Soon Kim
Abstract: Microstructure and mechanical property at room temperature and at 1773 K of Nb-Si based refractory intermetallic alloys were investigated in terms of compression and fracture toughness test. Mo and V were chosen as ternary alloying elements because of their high melting points, atomic sizes smaller than Nb. Both ternary alloying elements were found to have a significant role in modifying the microstructure from dispersed structure to eutectic-like structure in Nb solid solution/Nb5Si3 intermetallic composites. The 0.2% offset yield strength at room temperature increased with increasing content of ternary elements in Nb solid solution and volume fraction of Nb5Si3. At 1773 K, Mo addition has a positive role in increasing the yield strength. On the other hand, V addition has a role in decreasing the yield strength. The fracture toughness of ternary alloys was superior to binary alloys. Details will be discussed in correlation with ternary alloying, volume fraction of constituent phase, and the microstructure.
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