Ji-Woon Lee

Evaluation of dynamic recrystallization behaviors in hot-extruded AA5083 through hot torsion tests

Hot torsion tests were carried out to evaluate the dynamic recrystallization (DRX) behaviors of hot-extruded AA5083 at various deformation conditions. Flow curves showed the peak followed by the flow softening to the steady-state or to the failure strain, indicating that the DRX occurred during deformation. The peak stress increased as the temperature decreased and the strain rate increased. Constitutive relationship and Zener-Hollomon (Z) parameter were used to evaluate the DRX characteristics. Peak and steady-state stresses were generalized by the dimensionless parameter, Z/A, to reveal the DRX mechanism. The empirical relationship of the DRXed grain size with the deformation conditions was established, and decreased with increasing Z parameter. The relationship for the fraction of DRXed grains was established as a function of the effective strain at given deformation conditions from the experimental data. The Avrami relationship based on micro-hardness measurement was used to describe the DRX kinetics, and was fitted well with the observed DRX fraction.

Fabrication of Bi-doped In2O3-ITO nanocomposites and their photoluminescence properties

For the first time, Bi-doped In2O3-indium tin oxide (ITO) nanocomposites were prepared on Si substrates with the assistance of a Au catalyst through the simple gas-phase transport of a mixture of Bi, In, and Sn powders. The square-shaped Bi-doped In2O3-ITO nanostructures were straight, a few hundreds of nanometres in width, and below a few tens of micrometres in length. Electron microscopy, x-ray diffraction, and energy-dispersive x-ray spectroscopy analyses indicated that the Bi-doped In2O3-ITO nanorods were single crystals with a basis of cubic In2O3 structures. The photoluminescence spectra revealed that the Bi-doped In2O3-ITO nanorods had a strong orange emission band centred at approximately 626 nm without any shoulder bands. The enhancement of orange emission might be due to the oxygen deficiencies of structural defects in the nanorods.

Hot Deformation Behavior of Hot-Extruded AA7175 Through Hot Torsion Tests

The hot deformation behavior of hot-extruded AA7175 was investigated with flow curves and processing maps through hot torsion tests. The flow curves and the deformed microstructures revealed that dynamic recrystallization (DRX) occurred in the hot-extruded AA7175 during hot working. The failure strain was highest at medium temperature. This was mainly influenced by the dynamic precipitation of fine rod-shaped MgZn2. The processing map determined the optimal deformation condition for the alloy during hot working.

Effect of Zn on Pore Characteristics in Lotus-Type Porous Cu

The effect of Zn on pore characteristics in lotus-type porous Cu alloy was investigated. The lotustype porous Cu-Zn alloys were fabricated with Zn content from 0.01 to 0.1 at% by the centrifugal casting method. The results demonstrated that the porosity was rarely affected by Zn content. However, the average pore diameter and pore number density of the lotus type porous Cu-Zn alloys were significantly affected by the Zn content. The average pore diameter decreased as the Zn content increased up to 0.01 at%, and then increased as the Zn content increased up to 0.1 at%. In contrast, the variations in the pore number density of the lotus-type porous Cu-Zn alloys showed the reversed tendency with respect to that of the average pore diameter. The increase in heterogeneous nucleation sites for pores attributed to the decreased average pore diameter and the increased pore number density.

Pore Characteristics of Lotus-Type Porous Cu–Fe and Cu–Cr Alloys Fabricated by Unidirectional Solidification

Lotus-type porous Cu-Fe and Cu-Cr with long cylindrical pores was fabricated by centrifugal casting under hydrogen atmosphere and the effect of alloying elements on pore characteristics of lotus-type porous Cu was investigated. For the lotus type porous Cu-Fe alloy, the porosity slightly decreased and the average pore diameter slightly increased with increasing Fe content. For the lotus-type porous Cu-Cr alloy, the porosity sharply decreased and the average pore diameter drastically increased with an increase in the Cr content. From these results, it was found that the pore evolution and growth are affected by alloying element and this leads to the change in the pore characteristics of lotus-type porous Cu-Fe and Cu-Cr alloys.

Effect of the Spray Distance on the Properties of High Velocity Oxygen-Fuel (HVOF) Sprayed WC-12Co Coatings

In this study influence of spray distance on the properties of WC-12Co coatings deposited by HVOF was investigated. WC-12Co coating was sprayed at spray distance of 300, 385 and 450 mm. From microstructure observation, it is confirmed that the porosity of coatings increases with increasing the spray distance. The X-ray diffraction patterns indicate that the coatings consist of pure WC, W, and Co as well as W2C and Co6W6C phases. The increase of the spray distance accelerated the decarburization of coatings. From micro hardness tests, it was found that the hardness and the fracture toughness decreased with increasing spray distance. These mechanical properties would be related with not only porosity but also the degree of decarburization.

Microstructure and Density of Sintered ZnO Ceramics Prepared by Magnetic Pulsed Compaction

Three different sintered ZnO ceramics were prepared by magnetic pulsed compaction (MPC) and other conventional methods. The microstructures of the sintered ZnO ceramics prepared by MPC at sintering temperatures ranging from 900 to 1300°C showed a homogeneous grain growth compared to those of the samples prepared using other methods under the same sintering conditions. This implies that interpowders and/or intergrains can induce the minimization of wall friction effect because of the application of a high compaction pressure for a short process time in the MPC method. In addition, the microstructure of the sample obtained using the cold isostatic pressing method showed the presence of heterogeneous regions, indicating its low quality even though the densities of the three different samples were almost similar in the range of 97–99% at sintering temperatures of 900, 1100, and 1300°C. Therefore, different methods used for the compaction of ZnO ceramics may result in different microstructural and physical properties of the product.

Emission of SiOx Nanowires Resulting from Pressure Changes in a Closed System Comparable to a Tungsten Electron Gun at a High Voltage

SiOx nanowire emission with changes in the vapor pressure of Sn metal results in a screw motion of SiOx nanowires in a closed system consisting of Sn and SiOx. A convoluted flouncing-off process is elucidated here, in which the elementary particles are not electrons, but nanowires. In order to understand the unprecedented synthetic phenomena, the system that emits SiOx nanowires is compared with tungsten electron gun by comparing the component, functionalization, and theoretical background of both the techniques. The results so obtained, which are the first of its kind, open the way not only to produce three-dimensional micro- and nano-structures from one-dimensional nanostructures without requiring any additional artificial manipulation process but also to focus them onto a desired small region for elaborate deposition.