Hwi Jun Kim

Structural disordering process of an amorphous alloy driven by the elastostatic compression at room temperature

This study demonstrates that permanent deformation results when an amorphous alloy is subjected to a stress below yield at room temperature. The resultant deformation was observed to be homogeneous and is thought to occur as a result of the structural disordering, in which densely packed short-range ordered clusters break down to form new, loosely packed ones. It was found that this disordering process is irreversible even if the applied stress is removed, resulting in the creation of excess free volume. These findings were analyzed using the molecular dynamics simulations in the light of atomic-scale structural changes.

New Cu-Based Bulk Metallic Glasses with High Strength of 2000 MPa

New Cu-based bulk amorphous alloys exhibiting a large supercooled liquid region and good mechanical properties were formed in a quaternary Cu-Ni-Zr-Ti systems consisting of only metallic elements. The compositional range for the formation of the amorphous alloys that have high glass forming ability (GFA) (> 3 mm diameter) and large supercooled liquid region (> 50 K) is defined in the pseudo-ternary phase diagram Cu-Ni-(Zr, Ti). A bulk amorphous Cu54Ni6Zr22Ti18 alloy with the diameter of 6 mm can be prepared by copper mold casting. The Cu54Ni6Zr22Ti18 alloy shows glass transition temperature (Tg) of 712 K, crystallization temperature (Tx) of 769 K and supercooled liquid region (ΔTx) of 57 K. The Cu54Ni6Zr22Ti18 alloy exhibits high compressive fracture strength of about 2130 MPa with a plastic strain of about 1.5 %. The new Cu-based bulk amorphous alloy with high GFA and good mechanical properties allows us to expect the extension of application fields as a new engineering material.

Synthesis of Cu-Based Bulk Metallic Glass Matrix Composites by Warm Processing of Gas Atomized Powders

The bulk metallic glass matrix composite comprising Cu54Ni6Zr22Ti18 metallic glass powder and ductile brass powder was fabricated by the warm process. The warm process was carried out by spark plasma sintering, which led to the homogeneous distribution of both phases of brass and metallic glass without pores. The metallic glass matrix composite material exhibits the same crystallization behavior of the metallic glass powder. A compressive strength of 1.0 GPa with a plastic strain of 3 % was obtained in the present metallic glass composite. The composite with enhanced strength and ductility was successfully achieved by introducing a ductile phase in the hard bulk metallic glass.

Effect of Electric Current and Sliding Speed on Wear Rate of Metallized Carbon Strip Sliding against Copper Trolley

Pure copper and copper alloy infiltrated carbon materials were newly developed for the application of the contact strip on the pantograph system of high-speed electric railway. After the compaction and following baking process at an elevated temperature, the shape and size of pores in carbon materials were controlled by the processes developed in this work. The infiltration of pure copper or copper silicon alloy was performed in a specially designed chamber equipped with an evacuation system by a diffusion pump and also a pressurizing system by a high pressure gas. In order to evaluate the performance of metallized carbon strip samples, the wear tests were performed in a wear testing machine which was designed for an actual operating condition for the high-speed electric railway. Wear properties of samples were discussed by tacking various wear testing data.

Characteristics of ZnO Thin Film by Atomic Layer Deposition for Film Bulk Acoustic Resonator

Polycrystalline ZnO thin films were for the first time deposited on SiO2/Si (100) substrate using 2-step deposition; atomic layer deposition (ALD) and RF magnetron sputtering, for Film Bulk Acoustic Resonator (FBAR) applications. The film deposition performed in this study was composed of following two procedures; the 1st deposition was using ALD method and 2nd deposition was using RF magnetron sputtering. The ZnO buffer layer ALD films were deposited using alternating diethylzinc (DEZn)/H2O exposures and ultrahigh purity argon gas for purging. Exposure time of 1 sec and purge time of 23 sec yielded an ALD cycle time. Two-step deposited ZnO films revealed stronger c-axis preferred-orientation than one-step deposited. Therefore, this method could be applied to the FBAR applications, since FBAR devices require high quality of thin films.

Planar Hall Effect of GaMnAs

Anisotropy magneto-resistance and planar Hall-effect of ferromagnetic GaMnAs epitaxial films were investigated. The films were grown on 2 o off-cut GaAs (001) substrate in an optimized growth condition via low temperature molecular beam epitaxy. The GaMnAs layer revealed an easy axis along the (2x4) reconstruction direction of the substrate or along the off-cut direction. The large value of the anisotropy magneto-resistance ratio of ~7 % was realized by a well-alignment of the easy axis of the homogeneous ferromagnetic GaMnAs layer with the current. It also gives a very high planar Hall resistance ratio of ~500 %.