Taek Soo Kim

Microstructures and mechanical properties of Mg-Zn-Y alloy consolidated from gas-atomized powders using high-pressure torsion

In this paper, rapid solidified Mg95Zn4.3Y0.7 (at.%) alloy powders produced by an inert gas atomizer were consolidated using a severe plastic deformation technique of high pressure torsion (HPT) at room temperature and 373xa0K. The behavior of powder consolidation, matrix microstructural evolution, and mechanical properties of the powders and compacts were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, microhardness, and tensile testing. As the HPT processing temperature increases, the powders are more plastically deformed due to decreased deformation resistance, grain boundaries are more in equilibrium, powder bonding is enhanced due to increased interparticle diffusion, hence, tensile ductility and strength increases. On the other hand, hardness decreases with the increased processing temperature, due to less dislocation density.

Effect of Powder Size of Mg-Zn-Y Alloy on the Consolidation

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.

Microstructure and corrosion behavior of rapidly solidified Mg-Zn-Y alloys

The effect of a minor change in alloy composition on the microstructure and corrosion properties of melt spun Mg98.3−xZnxY1.7 ribbons with x=9–12 is studied by X-ray diffractometry, differential scanning calorimetry, transmission electron microscopy and a dynamic polarization test. The ribbon specimens with x=9–10 revealed an in-situ composite microstructure consisting of icosahedral quasicrystalline phase (I-phase) particles distributed in an α-Mg matrix. The ribbon specimens with x=11 and 12 contained a minor MgZn2 phase together with an α-Mg phase and I-phase. With increasing Zn content, the corrosion potential increased because of a mixed potential effect, but the formation of a MgZn2 phase deteriorated the corrosion property through preferential attack, causing an irregular boundary between the corrosion product and the substrate. These results indicate that it is important to control alloy chemistry not to form the MgZn2 phase in developing an I-phase strengthened Mg-Zn-Y alloy for structural applications.

Microstructures and mechanical properties of consolidated Mg-Zn-Y alloy

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.

Plasticity in Bulk Metallic Glass Composites Containing Dual Amorphous Phases

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.

FABRICATION OF POROUS Ti- AND W-COMPACTS BY ELECTRO-DISCHARGE-SINTERING PROCESS

A single pulse of 0.57–1.1 kJ/0.3 g-spherical Ti-powder in size range of 30–70 μm, using a constant 450 μF capacitor, was applied to produce fully porous Ti- and W-compacts by electro-discharge sintering. Microstructure examination and hardness measurement of the porous Ti-compacts revealed that more stable necks formed with increasing electrical input energy up to 1.1 kJ. On the contrary, W-compacts using rectangular W-powders in size range of 5–15 μm are composed of solid region with micropore and porous layer with weak particle bonding indicating heterogeneous structure. These results suggest that the powder morphology plays a crucial role to control the porous microstructure.

Thermoelectric Properties of n-Type 90% Bi2Te3+10%Bi2Se3 Thermoelectric Materials Produced by Melt Spinning Method and Sintering

N-type Bi2Te3-Sb2Te3 solid solutions doped with CdCl2 was prepared by melt spinning, crushing and vacuum sintering processes. Microstructure, bending strength and thermoelectric property were investigated as a function of the doping quantity from 0.03wt.% to 0.10wt.% and sintering temperature from 400oC to 500oC, and finally compared with those of conventionally fabricated alloys. The alloy showed a good structural homogeneity as well as bending strength of 3.88Kgf/mm2. The highest thermoelectric figure of merit was obtained by doping 0.03wt.% and sintering at 500oC.

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.

Thermal Properties of Diamond Aligned Electroless Ni Plating Layer/Oxygen Free Cu Substrates

Abstract The monolayer engineering diamond particles are aligned on the oxygen free Cu plates with electroless Niplating layer. The mean diamond particle sizes of 15, 23 and 50 µm are used as thermal conductivity pathway for fab-ricating metal/carbon multi-layer composite material systems. Interconnected void structure of irregular shaped diamondparticles allow dense electroless Ni plating layer on Cu plate and fixing them with 37-43% Ni thickness of their meandiameter. The thermal conductivity decrease with increasing measurement temperature up to 150 o C in all diamond sizeconditions. When the diamond particle size is increased from 15 µm to 50 µm (Max. 304 W/mK at room temperature)tended to increase thermal conductivity, because the volume fraction of diamond is increased inside plating layer. Keywords: Thermal conductivity, Diamond, Multi-layer materials, Electroless plating ······························································································································· ·································································································

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.