Seung Yong Shin

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.

Diffusion Bonding of Al 6061 Alloys Using an Eutectic Reaction of Al-Ag-Cu

In this study, a diffusion bonding of aluminium alloy A6061 was preformed using an Ag-28Cu filler metal in order to conduct eutectic brazing. Melting mechanism was surveyed. Interface behaviors of the brazed joints were observed after brazing and T6 solution treatment. Also, tensile property of the brazed joints was examined. During diffusion bonding of Al6061 alloys with Ag-28Cu filler metal, eutectic melts were formed by eutectic reaction between Al6061 and Ag-28Cu filler metal. It was found that the reaction layer consist of two phases formed at the interface between AA6061 and Ag-28Cu filler metal. EPMA analysis revealed that two phases in the reaction layers consist of Ag-rich phase and Cu-rich phase. Tensile strength was 300 MPa after ageing treatment at 175°C.

Effect of Oxygen Content on Thermoelectric Properties of Sintered Bi-Te Based Compounds

The n-type thermoelectric materials of Bi2Te2.7Se0.3 doped with SbI3 were prepared by spark plasma sintering technique. The powders were ball-milled in an argon and air atmosphere. Then, powders were reduced in H2 atmosphere. Effects of oxygen content on the thermoelectric properties of Bi2Te2.7Se0.3 compounds have been investigated. Seebeck coefficient, electrical resistivity and thermal conductivity of the sintered compound were measured at room temperature. It was found that the effect of atmosphere during the powder production was remarkable and thermoelectric properties of sintered compound were remarkably improved by H2 reduction of starting powder. The obtained maximum figure of merit was 2.4 x 10-3/K.

Identification of Phases in the Wide-Gap Region Brazed with BNi-3 Filler Metal Powder Using Electron Backscatter Diffraction

The phases formed in the wide-gap region brazed with BNi-3 filler metal powder in IN738 superalloy were investigated by electron backscatter diffraction (EBSD). The morphology and chemical composition of the phase were investigated by field emission scanning electron microscopy (FESEM) and Auger electron spectroscopy (AES). The wide-gap region brazed with BNi-3 filler metal had a microstructure consisting of primary Ni3B, binary eutectic of Ni3B-Ni solid solution and ternary eutectic of Ni3B-Ni-Ni3Si structure. EBSD pattern analysis revealed that the Ni3B had orthorhombic structure with lattice parameter of a=0.439, b=0.522 and c=0.662 nm, and the Ni3Si phase had cubic structure with lattice parameter of a=0.350 nm.

Crystallization Behavior and Mechanical Property of Cu Based Metallic Glass Powders

Crystallization behavior of gas atomized and spark plasma sintered (SPS) Cu54Ni6Zr22Ti18 amorphous powders were studied using X-ray diffractometer (XRD), differential scanning calorimeter (DSC) and transmission electron microscope (TEM). By heating to the temperatures of 837K and 909K, the amorphous phases in the powders formed particles such as Cu10Zr7 and Cu51Zr14. The size of crystals devitrified is about 20nm and 50nm, respectively. In order to identify the sintering ability of SPS, the compressive strength was measured with the initial powder size and SPS pressure.

Fabrication and Thermoelectric Properties of N-Type Bi2Te3 Based Compounds by Spark Plasma Sintering

N-type Bi2Te3 based thermoelectric compound was prepared by spark plasma sintering with a temperature range of 340~460°C and powder size of ~75㎛, 76~150㎛, 151~250㎛. Thermoelectric properties of the compound were measured as a function of the sintering temperature and powder size. With increasing sintering temperature, the electrical resistivity and thermal conductivity of the compound greatly changed because of the increase in relative density. The Seebeck coefficient and electrical resistivity were varied largely with increasing powder size. Therefore, the compound sintered at 460°C, with the powder of ~75㎛, showed a figure of merit of 2.44 x 10-3/K. Also, the bending strength was 75MPa.

Fluxless Brazing of Al6061 Alloys Using Ag-28Cu Insert

A diffusion brazing of aluminium alloy A6061 was preformed using a Ag-28Cu insert to conduct eutectic brazing. Interface behaviors of the brazed joints were observed after brazing at 450-560°C. The tensile property of the brazed joints was also examined. During diffusion brazing of Al6061 alloys with Ag-28Cu insert, eutectic melts were formed by eutectic reaction between Al6061 and Ag-28Cu insert. It was found that the reaction layers composed of two phases were formed at the interface between Al6061 and Ag-28Cu insert. EPMA analysis revealed that two phases in the reaction layers consist of Ag-rich phase and Cu-rich phase. The tensile strength of the joints brazed at 560°C for 30min was 160 MPa.

Effects of Atomizing Gas on Microstructural Characteristics and Mechanical Properties of Spray Formed High Si Steels

In order to investigate the feasibility of spray forming process for production of high Si steel sheets, 4.8~6.4%Si steels were prepared by spray forming process using N2 and Ar gases and their cold workability was compared with that of conventional ingot cast samples. Atomizing gas affected significant effect on deposit features, microstructural characteristics and mechanical properties of the spray formed high Si steels.

Consolidation of bulk metallic glass composites

Bulk metallic glass (BMG) composites combining a Cu54Ni6Zr22Ti18 matrix with brass powders or Zr62Al8Ni13Cu17 metallic glass powders were fabricated by spark plasma sintering. The brass powders and Zr-based metallic glass powders added for the enhancement of plasticity are well distributed homogeneously in the Cu-based metallic glass matrix after consolidation. The matrix of the BMG composite remains as a fully amorphous structure after spark plasma sintering. The BMG composites show macroscopic plasticity after yielding, and the plastic strain increased to around 2% without a decrease in strength for the composite material containing 20 vol% Zr-based amorphous powders. The proper combination of strength and plasticity in the BMG composites was obtained by introducing a second phase in the metallic glass matrix.

Wide-Gap Brazing of IN738 and the Bonding Strength Using Ni-Based Filler Metal Powders

The microstructure and bonding strength of the wide-gap region brazed with different filler metal powder (BNi-3 and DF 4B) and various powder mixing ratios of additive powder to filler metal powder were investigated. The microstructure characterization showed that Cr borides with a blocky morphology were existed in the brazed region in both filler metal powder. The bonding strength of the wide-gap region brazed with 60 wt.% IN738 additive and 40 wt.% DF 4B powder exhibited 92% tensile strength of IN738 superalloy at room temperature. The Cracks in the wide-gap brazed region initiated at the intermetallic compound and eutectic structure, and then propagated through them.