Papers by Author: Kyung Hyun Kim

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Authors: Jong Kwan Hwang, Keun Yong Sohn, Kyung Hyun Kim, Dae Min Kang
Authors: Su Gui Tian, Keun Yong Sohn, Kyung Hyun Kim
Abstract: The effects of antimony addition on the microstructures and creep behavior of AZ31 have been studied. Constant load creep tests were carried out at 200°C and an initial stress of 50MPa for AZ31 alloys containing antimony up to 0.84% by weight. Results show that Small additions of antimony to AZ31 effectively decreased the creep extension and steady state creep rates. The steady state creep rate of AZ31 was reduced 2.5 times by the addition of 0.84% of antimony by weight. The main reason for the higher creep resistance is due to the presence of high volume fraction (~20%) of second phases including Mg3Sb2, which effectively hindered the movement of dislocations during the elevated temperature creep.
Authors: In Bae Kim, Yong Su Park, Kyung Hyun Kim, In Gon Kim
Abstract: Effects of Si and Cr additions on the fatigue properties of Al-Zn-Mg-Cu cast alloy were investigated by low and high cycle fatigue tests. It was found that in the low cycle fatigue test, fatigue life of base alloy showed the maximum value of 3,075 cycles, whereas in Si and Cr containing alloys, it was 2,993 and 1,413 cycles, respectively. The same trend was obtained in high cycle fatigue test, i.e., the fatigue strength in base alloy showed the highest value of 104MPa and decreased to 100MPa for Cr containing alloy and 81MPa for Si containing alloy. The fatigue ratio was about 0.20 for all three alloys. The tensile strength of base alloy also showed the maximum value of 513MPa, and decreased with the addition of Si and Cr to 400 and 500MPa, respectively. Metallographic observation revealed that the fatigue crack initiated at the surface and propagated along the grain boundary.
Authors: In Bae Kim, Kwang Nyeon Kim, Kyung Hyun Kim, In Gon Kim
Abstract: Effects of Si and Cr additions and aging treatment on the stress corrosion cracking(SCC) resistance of Al-Zn-Mg-Cu cast alloys were investigated by C-ring test and electrical conductivity measurement. The SCC resistance of Al-Zn-Mg-Cu cast alloys decreased in the order of Cr containing alloy, Si containing alloy and base alloy. The lowest electrical conductivity of Cr containing alloy exhibited the best SCC resistance and this is probably due to an improved corrosion resistance by forming a passive film on the surface. The over-aged alloys showed the better SCC resistance. This is due to the increased distance between coarse particles at grain boundary. The fracture mode of the alloy was confirmed as intergranular type and showed brittle fracture surface. The SCC mechanism of Al-Zn-Mg-Cu cast alloys was found to be the anodic dissolution model.
Authors: Ling Wang, Su Gui Tian, Keun Yong Sohn, Kyung Hyun Kim
Abstract: The mechanical properties and deformation features of AZ31-x%Sb alloys have been studied by means of the measurement of the ultimate tensile properties (UTS) and TEM observation. Results show that the UTS of AZ31 alloy is effectively enhanced to 297 MPa from 222 MPa, by additions of 0.84% Sb element, at room temperature, and the ultimate tensile strength of the alloy is still maintained up to 189MPa as temperature elevated to 200°C. Contrast analysis shows that the deformation mechanisms of AZ31-0.84%Sb alloy are twins and dislocations activated on basal and non-basal planes. The alloy displays the different deformation features at different deformation conditions.
Authors: Su Gui Tian, Keun Yong Sohn, Kyung Hyun Kim
Abstract: Mechanical properties and high temperature deformation behavior of AZ31-0.84%Sb alloy have been studied. The results show that small additions of antimony to AZ31 alloy effectively enhanced the creep resistance and the ultimate tensile stress of the alloy at elevated temperature. One of the reasons for the improved high temperature properties is the high volume fraction (~20%) of the precipitates, as Mg3Sb2, which effectively hindered the movement of dislocations during the elevated temperature deformation. Considerable amounts of twins were formed during creep failure of AZ31-0.84%Sb alloy. Within the twins, many dislocation pile-ups were present on both basal and non-basal plane of magnesium matrix.
Authors: Su Gui Tian, Keun Yong Sohn, Hyun Gap Cho, Kyung Hyun Kim
Abstract: Creep behavior of AM50-0.4% Sb-0.9%Gd alloy has been studied at temperatures ranging from 150 to 200°C and at stresses ranging from 40 to 90 MPa. Results show that the creep rate of AM50-0.4%Sb-0.9%Gd alloy was mainly controlled by dislocation climb at low stresses under 50 MPa. The activation energy for the creep was 131.2 ± 10 kJ/mol and the stress exponent was in the range from 4 to 9 depending on the applied stress. More than one deformation-mechanism were involved during the creep of this alloy. Microstructures of the alloy consist of a–Mg matrix and fine particles, distinguished as Mg17Al12, Sb2Mg3, and Mg2Gd or Al7GdMn5 that were homogeneously distributed in the matrix of the alloy, which effectively reduced the movement of dislocations, enhancing the creep resistance. Many dislocations were identified to be present on non-basal planes after creep deformation.
Authors: In Bae Kim, Jin Young Hong, Bong Gil Hong, Kyung Hyun Kim
Abstract: The precipitation behavior of Mg-Y-Nd-Zr alloy was investigated by using transmission electron microscopy. The precipitation sequence of Mg-Y-Nd-Zr alloy was confirmed as follows ; Supersaturated solid solution → β''→ β'→ β.The β'' phase was identified Mg3Nd having DO19 structure with the lattice constant of a=2aMg=0.64nm and c=cMg=0.52nm. The β' precipitate was identified as Mg12NdY having base centered orthorhombic structure with the lattice constants a, b and c of 0.64, 2.23 and 0.52 nm, respectively. The β phase was confirmed as equilibrium phase having the fcc structure with the lattice constant of a=2.22 nm.
Authors: Zuzanka Trojanová, Pavel Lukáč, Kyung Hyun Kim
Abstract: The stress relaxation phenomena in magnesium alloys, containing Al, Sr and Ca solutes have been investigated. The samples were deformed at a constant initial strain rate at various temperatures between room temperature and 300 °C. Stress relaxation, i.e. a decrease in the stress with time, was measured at various stresses and at various temperatures. The stress relaxation curves were analysed as a function of the internal stress at the beginning of the relaxation and the test temperature.
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