Papers by Author: Chong Soo Lee

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Authors: B.J. Duggan, Chong Soo Lee, R.E. Smallman
Authors: Young Gun Ko, Yong Nam Kwon, Jung Hwan Lee, Dong Hyuk Shin, Chong Soo Lee
Abstract: Cavitation behavior during superplastic flow of ultra-fine grained (UFG) Ti-6Al-4V alloy was established with the variation of grain size and misorientation. After imposing an effective strainup to 8 via equal-channel angular pressing (ECAP) at 873 K, alpha-phase grains were markedly refined from 11 μm to ≈ 0.3 μm, and misorientation angle was increased. Uniaxial-tension tests were conducted for initial coarse grained (CG) and two UFG alloys (ε = 4 and 8) at temperature of 973 K and strain rate of 10-4 s-1. Quantitative measurements of cavitation evidenced that both the average size and the area fraction of cavities significantly decreased with decreasing grain size and/or increasing misorientation. It was also found that, when compared to CG alloy, cavitation as well as diffused necking was less prevalent in UFG alloys, which was presumably due to the higher value of strain-rate sensitivity. Based on the several theoretical models describing the cavity growth behavior, the cavity growth mechanism in UFG alloys was suggested.
Authors: Jong Taek Yeom, Jeoung Han Kim, Nam Yong Kim, Nho Kwang Park, Chong Soo Lee
Abstract: The dynamic globularization behavior during hot working of Ti-6Al-4V alloy was investigated by high temperature torsion tests. The torsion tests were carried out to investigate microstructure evolution occurring during dynamic globularization in Ti-6Al-4V alloy. The torsion tests were performed under a wide range of temperatures and strain rates with true strain up to 2. The flow curves revealed that the amount of flow softening for the fine alpha-lamellae structure was higher than that for the coarse alpha-lamellae structure under the temperature of 900oC. The effects of hot deformation parameters and initial microstructures on the dynamic globularization were analyzed.
Authors: Jeoung Han Kim, Jong Taek Yeom, Nho Kwang Park, Chong Soo Lee
Abstract: The high-temperature deformation behavior of the single-phase α (Ti-7.0Al-1.5V) and α + β (Ti-6Al-4V) alloy were determined and compared within the framework of self-consistent scheme at various temperature ranges. For this purpose, isothermal hot compression tests were conducted at temperatures between 650°C ~ 950°C to determine the effect of α/β phase volume fraction on average flow stress under hot-working condition. The flow behavior of α phase was estimated from the compression test results of single-phase α alloy whose chemical composition is close to that of α phase of Ti-6Al-4V alloy. On the other hand, the flow stress of β phase in Ti-6Al-4V was predicted by using self-consistent method. The flow stress of α phase was higher than that of β phase above 750°C, while the β phase revealed higher flow stress than α phase at 650°C. Also, at temperature above 750°C, the predicted strain rate of β phase was higher than that of α phase. It was found that the relative strength between α and β phase significantly varied with temperature.
Authors: Jong Taek Yeom, Chong Soo Lee, Jeoung Han Kim, Dong Geun Lee, Nho Kwang Park
Abstract: A methodology for evaluating and predicting component lives in creep-fatigue interaction region was investigated for Waspaloy. A unified viscoplasticity constitutive equation including multi-back stresses was used to describe cyclic material behaviors. Also, a continuum damage model coupling with the creep-fatigue damage rules was established based on the analysis of creep and low cycle fatigue behavior. Multi-axial fatigue and creep equivalent stress concepts were employed to predict three dimensional component lives. Notched cyclic tests under various stress conditions in the creep-fatigue interaction region were carried out to validate the life prediction methodology with FEM simulation based on the continuum damage model. The comparison of experimental data and prediction results indicates that the continuum damage model is a powerful approach for the prediction of component lives.
Authors: B.J. Duggan, Chong Soo Lee, R.E. Smallman
Authors: Jeoung Han Kim, S. Lee Semiatin, Chong Soo Lee
Abstract: The high temperature deformation mechanisms of two phase a+b alloy and a near-a alloy were investigated, and compared within the framework of inelastic-deformation theory. For this purpose, load-relaxation tests were conducted on the two alloys at temperatures of 750~900°C. The flow stress-vs.-strain rate curves for both alloys were well fit with inelastic deformation equations describing dislocation glide and grain boundary sliding. The amount of grain boundary sliding resistance was higher in the near-a alloy rather than the two phase a+b alloy due to difficulty in stress relaxation at triple junction region.
Authors: Young Won Chang, Chong Soo Lee, Chan Gyung Park
Authors: Chan Gyung Park, Chong Soo Lee, Young Won Chang
Authors: Dirk Ponge, D.S. Grummon, Chong Soo Lee, Günter Gottstein
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