Myung Hyun Lee

Development of Ni-Nb-Ta Metallic Glass Particle Reinforced Al Based Matrix Composites

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.

Spectroscopic Analysis of Plasma Induced in Laser Welding of Aluminum Alloys

This paper describes the features and characteristics of plasma induced in the pulsed YAG laser welding of Al-Mg alloys in air and argon atmospheres. In the air environment, the identified spectra are atomic lines of Al, Mg, Cr, Mn, Fe and Zn, and singly ionized Mg line, as well as strong molecular spectrum of AlO, MgO and AlH. It has been confirmed that the resonant lines of Al and Mg were strongly self-absorbed. These facts have shown that the laser-induced plasma is relatively a low temperature and high density metallic vapor. The intensities of molecular spectra of AlO and MgO are different each other depending on the power density of laser beam. Under the low power density irradiation condition, the MgO band spectrum is predominant in intensity, while the AlO spectrum became much stronger with the increase in high power density. This was attributed by the great difference in boiling point and vaporization energy of Al and Mg. In argon atmosphere the band spectra of MgO and AlO completely vanished, but AlH molecular spectra is detected clearly. The hydrogen source is presumably the hydrogen solved in the base metal, absorbed water on the surface oxide layer, or H2 and H2O in the shielding gas.

Ti-Based Bulk Metallic Glass with High Cold Workability at Room Temperature

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 %.

Deformation Behavior of Ni-Based Metallic Glass Matrix Composites Reinforced by Brass Short-Fibers Synthesized by Warm Extrusion of Powders

Deformation behavior of Ni-based metallic glass matrix composites reinforced by short brass-fibers, synthesized by warm extrusion of gas atomized powders, has been investigated under the uniaxial compression condition at room temperature. The brass-fibers are well distributed in the metallic glass matrix and enhanced macroscopic plasticity is observed due to the formation of multiple shear bands, initiated from the interface between brass-fiber and metallic glass matrix, as well as their confinement between the brass-fibers, stemming from the constrained plastic deformation of the reinforcing brass phase.

Modification of Crucible Shape in Top Seeded Solution Growth of SiC Crystal

The modified crucible in the top seeded solution growth (TSSG) method was proposed to get the stable growth condition and the enhanced growth rate. The temperature gradient and the distribution of dissolved carbon in the solvent were obtained by numerical approaches. Compared simulation results and experiment, we investigated the region where is the most of dissolved carbon and it is supplied to the seed through what path. The cross-sectional samples of crucibles and grown SiC crystals were systematically analyzed to investigate the effect of the crucible modification on SiC crystal growth.

The Effect of Stepped Wall of the Graphite Crucible in Top Seeded Solution Growth of SiC Crystal

The crucible design having a stepped wall was introduced for increasing the growth rate of SiC crystal without metal addition in top-seeded solution growth (TSSG) method. The numerical simulation confirmed that new crucible design to increase the solvent-crucible interface could definitely change the temperature distribution and increase the carbon concentration. The simulation result, SiC single crystals were grown with Si solvent at 1900°C using a normal crucible and a stepped crucible to investigate the effect of crucible design having a stepped wall. Grown SiC layers were analyzed using Optical microscopy and Raman spectra. The growth rate in the stepped crucible was finally 66um/h observed.

A New Control Algorithm for Impressed Current Cathodic Protection on the Underwater Hull

Cathodic protection is a system of preventing corrosion by forcing all surfaces of a hull to be cathode by providing external anodes. A metal can be made cathodic by electrically connecting it to a more anodic metal within the electrolyte. Anodes of these metals corrodepreferentially, the corrosion current of the anode achieving cathodic protection of the underwater hull to which they are connected. This paper presents a new current control algorithm for ICCP (Impressed Current Cathodic Protection) system. The anode of ICCP system is controlled by an external DC source with converter. The function of anode is to conduct the protective current into seawater. The DC source is generally obtained from main power system that contains a transformer, converter and etc. The proposed algorithm can operate AC-DC converter with current link. This algorithm includes the harmonic suppression control strategy and the optimum protection strategy and has tried to test the requirement current density for protection, the influence of voltage, the protection potential. Also, the properties of ICCP protected hull of ship in seawater are evaluated.