Papers by Author: Won Yong Kim

Abstract: Texture and cyclic tensile behavior of Ti-26Nb-0.5Si (denoted as atomic percent) alloys in which the microstructures were varied by quenching, cold rolling and recrystallization heat treatment were investigated in order to understand the relationship between pseudoelastic behavior and texture formation. Three phase mixtures consisting of bcc-structured β phase, orthorhombic structured α" phase and hcp-structured intermediate ω phase were characterized to display the constituent phases. The volume fraction of constituent phases was found to be insensitive to the given materials processing. Two-stage yielding, one at low stress with low strain hardening rate and the other one at high stress with high strain hardening rate, appeared to exhibit a characteristic flow behavior in the present alloys. It is revealed that stress-induced martensite transformation resulting in two-stage yielding was closely associated with pseudoelasticity. On the basis of texture analyses, we have suggested that pseudoelasticity of the present alloys is hindered by the development of {001}<110> rotated cube component.

Abstract: Microstructure and texture formation behavior of a gold rod and fine wire which were produced by continuous casting process and drawing, respectively were investigated by means of optical micrographs, TEM images and X-ray pole figure. Well-developed unidirectional structure aligned to casting direction was found in 7 mm gold rod. Higher casting speed was found to be favor in developing the directional microstructure. 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. <100> fiber component was observed to be well developed over the whole microstructure in unidirectional sample. With decreasing casting speed <100> fiber component became weak. Two texture components consisting of <111> fiber and <100> fiber were seen in as-drawn samples. For the development of <111> fiber texture component initial texture component plays a more significant role than subsequent annealing process. Annealing heat treatment would be effective way to control the formation of <100> fiber component. On the basis of results obtained it is suggested that both the initial texture and final annealing are important in controlling the texture of gold bonding wire.

Abstract: Microstructure and texture of Ti-Nb-Si based alloys, prepared by water quenching from β-phase field, cold rolling and recrystallization heat treatment followed by water quenching, were investigated in terms of optical microstructure and analysis of X-ray pole figure result. Optical microstructure observation and X-ray diffraction analysis revealed that the microstructure of as-quenched sample appeared to mixture appearance consisting of mostly bcc-structured β phase and small amount of orthorhombic-structured α″ phase. After cold rolling elongated structure parallel to the rolling direction was observed, and equiaxed structure with the average grain size of about 20~30μm was developed for the sample after recrystallization heat treatment. In as-cold rolled sample we have found well-developed α-fiber texture components which are frequently observed in bcc-structured metals and alloys. In recrystallized sample, rotated cube texture component was weakly detected. The variation of elastic modulus values was interpreted in terms of changes in texture components depending on thermomechanical processing.

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.

Abstract: Mechanical properties and elastic modulus were examined in order to clarify the influence on microstructures in Ti-26Nb-xSi, where x= 0.5, 1 in atomic percent, prepared by arc melting, cold rolling and recrystallization heat treatment. On the basis of microstructural observations and phase analyses, it is evidently revealed that the microstructure of as-quenched sample appeared to mixture appearance consisting of mostly bcc-structured β phase and small amount of orthorhombic-structured α″ phase. Elongated structure parallel to the rolling direction was observed in cold rolled samples, and equiaxed structure with the average grain size of about 20μm was developed for the sample after recrystallization heat treatment. Randomly distributed feature of pole figure was characterized without showing a specific texture component in asquenched sample. Rotated cube, α-fiber and γ-fiber texture components were detected in cold-rolled sample. After recrystallization heat treatment the intensity of α-fiber texture component was markedly decreased, while the rotated cube component becomes sharpened and γ-fiber component remains relatively unchanged. From both elastic modulus and strength point of view recrystallization treatment would be desirable to meet the required mechanical properties of the present alloys for biomedical applications.

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.

Abstract: The effects of oxygen content on microstructures, elastic modulus and tensile properties of quenched Ti-Nb alloys were investigated in order to design a desirable Ti based alloy through casting process. From the microstructural and phase analysis, it is evidently revealed that the volume fraction of β phase increased with increasing content of oxygen, and the occurrence of intermediate ω phase was suppressed in metastable β Ti-Nb based alloys. Martensite transformation temperature decreased with increasing content of oxygen. Therefore, it is suggested that oxygen acts to stabilize β phase rather than α stabilizer in quenched state. Yield strength increased with increasing content of oxygen without a large consumption of ductility in metastable β Ti-Nb based alloys. The variation of mechanical property was explained by the phase stability, phase formation and microstructure in correlation with oxygen and Nb content.

Abstract: We have newly designed a metastable β Ti-Nb-Si based alloy with biocompatible alloying elements without containing toxic V or Al for orthodontic applications. Microstructures and pseudoelastic behavior of β Ti-Nb-Si alloys were investigated in order to correlate the pseudoelasticity and microstructure together with martensite transformation. Nb and Si alloying to the present alloy make β phase to be stable. It is found that metastable β phase is favorable to display pseudoelastic behavior than stable or unstable β phase. Optical microscope (OM) revealed that stress-induced martensitic transformation takes place during room temperature deformation in the present alloys. Within the alloys having β (bcc) phase studied the alloy with low content of Si appeared to exhibit a dominant behavior for stress-induced martensitic transformation than that with high content of Si. After recrystallization heat treatment pseudoelasticity of the present alloy appeared to be prominent. The pseudoelastic behavior of this alloy was correlated to the stress-induced martensite transformation. Pseudoelasticity of the present alloys is hindered by the development of {001}<110> rotated cube component.

Abstract: Mechanical Property and elastic modulus of Ti-Nb based alloys with various Si content, prepared by water quenching from high temperature β phase field, cold rolling and recrystallization heat treatment followed by water quenching, were investigated in terms of tensile test and resonance vibration method. TEM observations revealed that in addition to orthorhombic structured α" phase and bcc structured β phase, an intermediate ω phase is characterized in the microstructure. The volume fraction ratio of constituent phases was dependent upon Nb and Si contents. In as-quenched samples yield strength increased with increasing Si content. This propensity was similar to the cold rolled alloys. In recrystallized samples however, yield strength decreased slightly with increasing Si content even though the alloy containing higher Si content showed smaller grain size. From these results, it is found that strength of the present alloy influences largely on solid solution hardening and phase stability but does weakly on grain size. Yield strength of cold rolled sample was higher than as-quenched and recrystallized samples. Elastic modulus values were measured to 55GPa, 63GPa and 44GPa for as-quenched, cold rolled and recrystallized samples, respectively. The variations of yield strength and elastic modulus values were interpreted in terms of changes in microstructure as well as grain size and phase stability.

Abstract: This work describes the effect of microstructures on elastic modulus in Ti-26Nb-xSi alloy (x=0.5~1.5at.%) prepared by arc melting, cold rolling and recrystallization heat treatment. OM observation and x-ray diffraction analysis revealed that the microstructure of as-quenched sample appeared to mixture appearance consisting of mostly bcc-structured β phase and small amount of orthorhombic-structured α″ phase. After cold rolling, elongated structure parallel to the rolling direction was observed, and equiaxed structure with the average grain size of about 20~30μm was developed for the sample after recrystallization heat treatment. In as-quenched sample randomly distributed feature of pole figure was characterized without showing a specific texture component. In cold-rolled sample α-fiber, γ-fiber and rotated cube texture components were detected. After recrystallization heat treatment the intensity of α-fiber texture component was markedly decreased, while the rotated cube component becomes sharpened and γ-fiber component remains relatively unchanged. The elastic modulus increased by cold rolling and then decreased by recrystallization over the entire chemical composition range investigated. The variation of elastic modulus values was interpreted in terms of changes in texture components depending on thermomechanical processing.