Papers by Author: J.S. Park

Abstract: The cold workability of Ti-based bulk metallic glasses (BMGs) have been investigated. Ti45Zr16Be20Cu10Ni9 BMG with a large compressive plastic strain of 4.7 % shows a high cold workability, i.e. total reduction ratio of 50 % by cold rolling at room temperature. The multiple shear bands formed during rolling are effective in enhancing the plasticity. The cold rolled Ti45Zr16Be20Cu10Ni9 BMG (reduction ratio: 30 %) exhibits a large plastic strain of ~14 %.

Abstract: Metallic glass particle reinforced Al-6.5Si-0.25Mg (wt%) alloy matrix composites have been fabricated using infiltration casting process. The Al-Si-Mg alloy melt infiltrates into the porous Ni-Nb-Ta metallic glass pre-form by applying the pressure. The Ni60Nb20Ta20 metallic glass particles are homogeneously distributed in the matrix. The composite exhibits higher yield strength than the monolithic Al-6.5Si-0.25Mg (wt%) alloy.

Abstract: Three kinds of atmospheres (air, highly-pure Ar and ultra highly-pure Ar gas) with different oxygen partial pressures were applied to investigate the tensile properties and creep behavior of SiC fibers such as Hi-NicalonTM and TyrannoTM-SA. These fibers were annealed and crept at elevated temperatures ranging from1273-1773 K in such environments. After annealing at 1773 K, the room temperature tensile strengths of SiC-based fibers decreased with decreasing the oxygen partial pressure and the near stoichiometric fiber TyrannoTM-SA shows excellent strength retention. At temperatures above the 1573 K, the creep resistance of SiC fibers evaluated by bending stress relaxation (BSR) method under high oxygen partial pressure was lower than that of in low oxygen partial pressure. The microstructural features on these fibers were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD).

Abstract: This study was to aimed to investigate on the sliding wear behaviors at both ambient and elevated temperatures in Al/SiCp composites fabricated by the newly developed duplex process. Wear test using wear tester of pin-on-disc type was carried out under the conditions of load 100N , velocity 0.64m/s, distance 4000m. The sliding wear behaviors under the conditions of ambient temperature, 200°C and 200°C-Ar gas and microstructures were studied using OM and SEM. The results obtained from this study are as follows; Wear amount decreased with the decrease in particle size. The wear resistance at 200°C-Ar on composites was greater than those at 200°C and room temperature in the as-composites.

Abstract: The effects of grain-size in La0.7Ca0.3MnO3-δ (LCMO) system have been studied in connection with the magnetic and electronic properties. LCMO system prepared by the solid-state reaction was annealed in air at 1200, 1300 and 1400°C. The grain sizes of LCMO samples become larger with increasing of annealing temperature (TA ). The magnetization in LCMO samples increased while the coercive field decreased with increasing TA. The conductivity increased and the metal-insulator transition temperature TM-I decreased with increasing of TA. These physical properties are due to the oxygen deficiency caused by the increase of grain size. Finally, it was found that the grain size and the mechanical connection between grains play an important role in determining the electronic and magnetic properties.

Abstract: This study dealt with the characterization of MoSi2 based composites containing three types of additive materials such as SiC, NbSi2 and ZrO2 particles have been investigated, based on the detailed examination of their microstructures and fracture surfaces. The effects of reinforcing materials on the high temperature strength of MoSi2 based composites have been also examined. MoSi2 based composites were fabricated by the hot press process under the vacuum atmosphere. The volume fraction of reinforcing materials in the composite system was fixed as 20 %. The microstructures and the mechanical properties of MoSi2 based composites were investigated by means of SEM, EDS, XRD and three point bending test.