Papers by Author: Sung Hak Lee

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Authors: Jeong Hyeon Do, Chang Woo Jeon, Duk Hyun Nam, Choong Nyun Paul Kim, Young Bum Song, Sung Hak Lee
Abstract: This study aimed at investigating the ballistic performance of Zr-based bulk metallic glass/Ti surface composites fabricated by high-energy electron-beam irradiation. The ballistic impact test was conducted on surface composite plates to evaluate the ballistic performance. Since the surface composite layers were observed to block effectively a fast traveling projectile, while many cracks were formed in the composite layers, the surface composite plates were not perforated. The surface composite layer containing ductile β dendritic phases showed the better ballistic performance than the one without containing dendrites because dendritic phases could hinder the propagation of shear bands or cracks.
Authors: Y.G. Ko, Y.G. Kim, S. Namgung, Dong Hyuk Shin, Sung Hak Lee
Abstract: In this study, dynamic deformation behavior of submicrocrystalline aluminum alloy was established with respect to equal-channel angular (ECA) pressing routes such as A, B, and C. After 8-pass ECA pressings, the deformed samples, regardless of the routes applied, were consisted of ultrafine grains together with high dislocation density near the boundaries. Microstructural observation revealed that the sample deformed via route B showed more diffused diffraction pattern than those deformed via route A and C, suggesting the fact that route B was most effective for a rapid evolution in the grain boundary orientation from low-angle to high-angle characteristics. In the torsion tests, the shear stress decreased once reaching the maximum point. This maximum was the highest in the sample deformed via route B, and decreased in the order of the route C and route A. The dynamic deformation was explained based on microstructural uniformity associated with ECA pressing routes.
Authors: Byoung Chul Hwang, Chang Gil Lee, Sung Hak Lee
Abstract: High deformability has been considered as a critical factor of ultra-high strength steel plates subjected to compressive, tensile, and bending deformation induced by large ground movements. In this paper, various dual phase microstructures consisting of soft ferrite and strong low-temperature transformation phases without deformation in the (austenite + ferrite) two-phase temperature region after controlled rolling were introduced and then the mechanical properties were discussed with emphasis on deformability such as yield ratio and uniform elongation. Ultra-high strength steel plates fabricated by a modified thermo-mechanical control process showed lower yield ratio of under 0.75 and higher uniform elongation of 5% as a minimum, as compared to commercial API X100 and X120 grade pipeline steels, without much sacrifice of Charpy impact properties because of an appropriate formation of soft ferrite and strong low-temperature transformation phases.
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