Papers by Author: J.K. Lee

Abstract: 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.

Abstract: Bulk metallic glass (BMG) composites with dual amorphous phases were fabricated by spark plasma sintering of a mixture of Cu-based and Zr-based amorphous powders in their overlapped supercooled liquid region. The Zr-based amorphous phases are well distributed homogeneously in the Cu-based metallic glass matrix after consolidation. The BMG composite still remains as an amorphous structure after consolidation. The BMG composite with dual amorphous phases shows macroscopic plasticity after yielding, and the plastic strain increased to around 3.4% in the BMG composite containing 30 vol% Zr-based amorphous phase. The successful consolidation of BMG composite with enhanced plasticity was achieved by introducing a second amorphous phase in the metallic glass matrix.

Abstract: MgZn4.3Y0.7 alloy powders were prepared using an industrial scale gas atomizer, followed by warm extrusion. The powders were almost spherical in shape. The microstructure of powders as atomized and bars as extruded was examined as a function of initial powder size distribution using Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscope (EDS) and X-ray Diffractometer (XRD). The grain sizes were decreased with extruding as well as decreasing the initial powder sizes. Both the ultimate strength and elongation were enhanced as the initial powder sizes were decreased.

Abstract: The microstructure and mechanical properties of the Mg97Zn1Y2 alloy prepared by spark plasma sintering of gas atomized powders have been investigated. After consolidation, precipitates were observed to form in the α-Mg solid solution matrix of the Mg97Zn1Y2 alloy. These precipitates consisted of Mg12YZn and Mg24Y5 phases. The density of the consolidated bulk Mg-Zn-Y alloy was 1.86 g/cm3. The ultimate tensile strength and elongation were dependent on the consolidation temperature, which were in the ranges of 280 to 293 MPa and 8.5 to 20.8 %, respectively.

Abstract: 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.

Abstract: Silicon containing self setting bone cement has been prepared by adding silicon containing hydroxyapatite whisker to obtain a biomaterial having an improved resorption properties. Silicon containing calcium phosphate bone cement was composed of a-TCP: TeCP: DCPD: Si-HA whisker with a NH4H2PO4 as a setting liquid. From the XRD analysis, it was confirmed that calcium deficient hydroxyapatite phase appeared when it immersed in PBS solution. The dissolution rate of silicon containing calcium phosphate cement was measured in PBS solution and showed high dissolution rate. Based on in-vivo test, silicon containing self setting bone cement can be considered a useful material for bone bonding materials.

Abstract: 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.