Papers by Author: Joong Hwan Jun

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Authors: Jeong Min Kim, Gun Ha Lee, Joong Hwan Jun, Ki Tae Kim, Woon Jae Jung
Abstract: Mg-8%Al-1%Ca was selected as a base alloy composition and small amounts of minor alloying elements, RE or Sr, were added. Microstructure of as-cast Mg-8%Al-1%Ca base alloys consists of dendritic primary Mg, Mg-Al-Ca and Mg17Al12 phases. Mg-Al-RE-(Ca) phase in the RE-added alloys and Mg-Al-Sr-(Ca) phase in the Sr-added alloys were additionally found. The creep resistance of Mg-8%Al based alloy was significantly improved by the minor alloying elements additions with 1%Ca. Comparatively globular primary phase could be obtained after just 30min. through an isothermal heating of as-cast Mg-Al-Ca alloys.
Authors: Jeong Min Kim, Bong Koo Park, Kee Sam Shin, Joong Hwan Jun, Ki Tae Kim, Woon Jae Jung
Abstract: Effort has been devoted to develop new heat resistant diecasting alloys based on Mg-Al- Zn system in this research. Small amounts of cerium-rich misch metal and antimony additions to AZ91 alloy could enhance the tensile strength at an elevated temperature while keeping the good castability. The increase of Zn content in Mg-8(wt%)Al-xZn-0.5RE-0.5Sb alloys, was observed to significantly increase the yield strength at 175oC although the castability such as fluidity and hot cracking resistance was slightly decreased. Intensive microstructural investigation on the new Mg- Al-Zn diecasting alloys was also carried out.
Authors: Jeong Min Kim, Bong Koo Park, Joong Hwan Jun, Ki Tae Kim, Woon Jae Jung
Abstract: Various amounts of Ca were added to AZ91D magnesium alloy, and their effects on the die-casting abilities were investigated. It was observed that fluidity as die filling ability tends to decrease by Ca additions except for about 2%Ca. This reduction of fluidity by Ca was more significant at high superheats probably due to the high affinity between Ca and oxygen. Contrary to expectation, hot cracking resistance was found to increase by Ca additions. High Ca alloys showed some die-sticking tendency. However, the tendency was not observed below 2%Ca.
Authors: Joong Hwan Jun, Young Kook Lee, Jeong Min Kim, Ki Tae Kim, Woon Jae Jung
Abstract: We reports the damping properties of an Fe-23%Mn alloy with various amounts of thermal or deformation-induced ε martensite. By controlling cooling temperatures and cold rolling degrees, the volume fractions of thermal and deformation-induced ε martensites are changed from 33 to 50% and from 33 to 75%, respectively. The damping capacity of the Fe-23%Mn alloy increases with an increase in thermal ε martensite content, whereas the damping capacity associated with deformation-induced ε martensite shows a peak value at 57% of ε martensite. Transmission electron micrographs on deformed samples reveal that the decay of damping over 57% of deformation-induced ε martensite is caused by an introduction of perfect dislocations, which play a role in suppressing the movement of damping sources. For the same amount of ε martensite, deformation-induced ε martensite exhibits higher level of damping capacity than thermal ε martensite. This may well be owing to relatively greater length of γ/ε interfaces in response to higher number density of ε martensite plates.
Authors: Joong Hwan Jun, Geon Ha Lee, Jeong Min Kim, Ki Tae Kim, Woon Jae Jung
Abstract: Changes in microstructure and damping capacity with aging time for solutionized AZ91 (Mg-9%Al-1%Zn-0.2%Mn) alloy have been investigated based on experimental results from optical micrography, X-ray diffractometry, hardness test and damping capacity measurement vibrating in a flexural mode. Discontinuous β (Mg17Al12) precipitates form along the primary grain boundaries, the amount of which increases as the aging time increases. The hardness of α matrix with respect to aging shows a typical “S” shape, indicating a generation of fine continuous precipitation in the matrix during aging. The peak level of damping capacity for the AZ91 alloy is obtained after 1 hour of aging, over which the damping capacity becomes deteriorated continuously. The optimum density of continuous β precipitates with fine morphology, which may well act as pinning points for dislocation lines, might be responsible for the improvement of damping capacity.
Authors: Ki Tae Kim, Jeong Min Kim, Ki Dug Sung, Joong Hwan Jun, Woon Jae Jung
Abstract: Small amounts of various alloying elements were added to a high strength Al-Zn-Mg-Cu alloy and their effects on the microstructure, mechanical properties, and casting characteristics were investigated. Silicon additions with or without extra Mg to the Al-Zn-Mg-Cu alloy could enhance the castability such as fluidity, feedability, and hot tearing resistance significantly while maintaining a high strength. However, in these alloys containing silicon the compositional adjustment was necessary to prevent the Mg2Si phase formation from degrading the precipitation of MgZn2 phase that is responsible for the high strength. Zr addition to the base alloy was also observed to improve the feedability without deteriorating the tensile strength.
Authors: Jung Chul Kim, Seung Han Baik, Joong Hwan Jun, Young Kook Lee
Abstract: Effect of Cr addition on damping capacity, mechanical property, and corrosion resistance of Fe-18%Mn martensitic alloy has been studied. Martensite start temperature (Ms) of the alloy decreases linearly from 150 to 25 with increasing Cr content up to 15%. The damping capacity decreases gradually from 27 to 23% in SDC with increasing Cr content from zero to 10%, and decreases rapidly with further Cr content. The tensile strength of the alloy maintains a level of 60 / regardless of Cr content with an elongation of 20 to 25%. Immersion test in 5% NaCl solution leads to the result that the corrosion resistance of the alloy becomes excellent above 10% Cr. From the above results, it is concluded that the optimum Cr content to improve the mechanical property and corrosion resistance of the alloy with a lesser decrease in damping capacity is about 10%.
Authors: Joong Hwan Jun, Geon Ha Lee, Jeong Min Kim, Ki Tae Kim, Woon Jae Jung
Abstract: Microstructures, tensile properties, and damping capacity of the hot-rolled and annealed K1A (Mg-0.7%Zr) wrought alloy were investigated in comparison with those of a commercial AZ31 (Mg-3%Al-1%Zn-0.2%Mn) wrought alloy. The K1A alloy in hot-rolled state shows 116 and 183MPa of tensile yield strength (σYS) and ultimate tensile strength (σUTS), respectively, which are significantly higher than those (40 and 130MPa) of the K1A casting alloy. The hot-rolled K1A alloy exhibits 10.4% of specific damping index (SDI) and subsequent annealing treatment remarkably enhances its SDI up to 19.7% without sacrificing the tensile strength. Although the annealed AZ31 alloy has 3.9% of low SDI, its tensile properties are still higher than those of the K1A alloy.
Authors: Min Ha Lee, Joong Hwan Jun, Jürgen Eckert
Abstract: Mechanical treatments such as deep rolling are known to affect the strength and toughness of metallic glass due to the residual stress. It is well known that compressive residual stress states usually enhance the mechanical properties in conventional metallic materials. We present investigations on the change of fracture behavior related with mechanical properties of “brittle” bulk metallic glass by cold rolling at room temperature. Improvement of the intrinsic plasticity is observed not only after constrained cyclic compression but also after cold rolling. Moreover, neither nanocrystallization nor phase separation occurs during deformation. By these findings we provide a unique fundamental basis by considering the introduction of structural inhomogeneity and ductility improvement in metallic glasses. The experimental evidence clearly supports that such an inhomogeneous glassy can be produced by residual stress in well known “brittle” bulk metallic glasses, and does not depend on a specific pinpointed chemical composition.
Authors: J.H. Chung, Joo Youl Huh, Jae Ho Jun, D.H. Kim, Joong Hwan Jun
Abstract: In the attempt to develop materials for interconnects of planar-type solid oxide fuel cells (SOFC), the oxidation behavior and scale electrical properties of four, commercially available, ferritic stainless steels were investigated by oxidizing at 800°C for times up to 900 h in the dry and wet air environments. The growth kinetics of oxide scales were significantly deviated from the parabolic growth law, exhibiting the temporal growth exponent being much less than 0.5. Titanium addition in ferrite stainless steels was detrimental to oxidation resistance since its fast out-diffusion onto the scale surface enhanced the scale growth. For the alloys without Ti contents, the presence of water vapor in the oxidizing environment enhanced the scale growth rate, but it resulted in a drastic, microstructural change of oxide scale that consisted of Cr2MnO4 spinel and Cr2O3, which in turn lowered the area specific resistance (ASR) and the apparent activation energy for conduction.
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