발간논문

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The evolutions of microstructure and mechanical properties during warm multiple deformation (WMD) at 673 K and a strain rate of 10^(-1) sec were investigated for Al-3 wt%Mg (A3M) alloy and Al-3 wt%Mg-0.2 wt%Sc-0.l wt%Zr (A3MSZ) alloy. When the WMD process was repeated up to 8 cycles at 673 K for the A3MSZ alloy, ultra-fine grained structure having mean grain size less than 3 μm was obtained and most grain boundaries consisted of high angle grain boundaries. However, A3M alloy showed grain growth behavior rather than grain refinement during the WMD process, which was originated from the lack of fine particles to prohibit the grain growth. In the ultra-fine grained A3MSZ alloy fabricated by WMD process, the maximum elongation to failure was 510% and the corresponding strain rate sensitivity, m, was about 0.4754 when tensile test was performed at 673K and a strain rate of 10^(-3) / sec.

keyword : Warm multiple deformation, Dynamic recrystallization, Superplasticity

The height difference △hb between the ideally sharp Berkovich indenter tip and a rounded tip was measured by direct observation using atomic force microscopy (AFM). The accuracy of the indirect area function method for measuring △hb was confirmed. The indentation size effects (ISE) in (100) single crystal copper, (100) single crystal tungsten, and fused quartz were characterized by applying the ISE model considering the rounded tip effect. The model fit the data well for these materials even though fused quartz does not deform by dislocations. However, a very small value of the ISE characteristic length h` was obtained for fused quartz. The present h` value for (100) copper is 32% larger than a previously-measured value for polycrystalline copper. This may indicate that grain boundaries suppress the dislocation activity envisioned in the ISE model.

keyword : Indentation size effect, Nanoindentation, Rounded tip effect

A study was made to investigate microstructural evolution and mechanical properties of ultra-fine grained (UFG) pure-Ti produced by equal channel angular (ECA) pressings. The deformed structures were analyzed by finite element method and transmission electron microscopy with the increment of straining. After 4 isothermal ECA pressings, initial coarse grains (30 μm) were significantly refined to ~0.3 μm with homogeneous distribution of microstructure which was resulted from 180˚ rotation of the sample between pressings. UFG pure-Ti exhibited the considerable improvement in yield strength while losing strain hardening capacity as compared to coarse grained microstructure at ambient temperature, which was mainly attributed to ultra-fine grain microstructure with non-equilibrium grain boundaries. Such high strength and unusual strain hardening behavior of UFG pure-Ti were discussed in relation with two dislocation models based on dislocation bow-out and dynamic recovery associated with trapped lattice dislocations.

keyword : Pure-Ti, Ultra-fine grain, Dislocation bow-out, Strain hardening, Dynamic recovery

The particle size distribution and phase transformation of nano-titanium carbide (TiC) powders synthesized by mechanical alloying (high energy shaker mill) from activation carbon and TiH2 powders were observed. TiC powders have been synthesized by mechanical alloying (MA) using TiH2-50 at%C mixture powders. X-ray diffractions (XRD) and field emission scanning electron microscopy (FESEM) were used to analyze the structural evolution, particle size and morphologies of powder during milling. The changes of particle size and internal strain for different phase were determined as a function of milling time. TiC and Ti2O3 were formed when milling for 3 hours and TiC phase amount increased with milling time. After 10 hours milling, TiC was formed completely with a nano particle of 300 nm average particle size, were observed with the FESEM and particle size analyzer.

keyword : TiC, TiH2, Metal carbide, Activated carbon, Nano-powder

The metallic coatings for electromagnetic interference (EMI) shielding must have a low electric resistance for effective electromagnetic wave shielding as well as good corrosion resistance to guarantee reliability of the electronic devices. Normally, the metallic coatings for EMI shielding consist of conductive layer, corrosion resistance layer and buffer layer. As the corrosion induced the delamination of coating layer, it could not function of EMI shielding. Therefore, in this paper, the effect of the stainless steel layer on the corrosion resistance was investigated. Several metals were deposited onto polycarbonate by RF magnetron sputtering. The structural characteristics of the film were investigated by means of X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The electrochemical properties were examined by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The results obtained from potentiodynamic polarization and EIS showed that 2205 stainless steel of upper layer had the better protective efficiency than 310S stainless steel.

keyword : EMI shielding, EIS, SEM, Corrosion resistance, Multilayer, RF magnetron sputtering

Self-assembled InAs/GaAs quantum dots (QDs) were grown by atomic layer epitaxy technique and their structure and thermal stability have been studied by using high resolution electron microscopy with in-situ heating experiment capability. The QDs were found to form a hemispherical structure with side facet in the early stage of growth. The average height and diameter of the QDs were found to be ~5 nm and ~23 nm, respectively. Upon capping by GaAs layer, however, the apex of QDs changed to a flat one. In situ heating experiment within TEM revealed that the uncapped QDs remained stable until 580℃. However, at temperature above 600℃, the QDs collapsed due to the diffusion and the evaporation of In and As from the QDs. The density of the QDs decreased abruptly by this collapse and most QDs disappeared at above 600℃. During the observation in a high-voltage electron microscope at high temperature, electron radiation defects, such as twin, stacking faults, amorphization and void, were also generated near surface and propagated into the substrate.

keyword : InAs/GaAs quantum dot, Atomic Layer epitaxy, Thermal stability, Transmission electron microscopy, TEM

Atomic arrangement of layer structures in n-type Bi2(Te0.95Se0.05)3 thermoelectric compound fabricated by gas atomizing and hot extrusion was investigated by high resolution electron microscope (HREM). Two kinds of grains having five and seven atomic layered packs are observed in hot extruded material. The sequence of five atomic layers, i.e. Te-Bi-Te-Bi-Te corresponded to one quintet along c-axis of the Bi2Te3 crystal structure. On the other hand, the seven atomic layers occur in the sequence Te-Bi-Te-Bi-Te-Bi-Bi corresponding to one septet of Bi4Te3 crystal structure and can be described by insertion of two Bi layers between quintets of Bi2Te3 structure. The occurrence Bi3Te4 layered structure can be interpreted resulting in local composition change by rapid solidification process.

keyword : n-type Bi2(Te(0.95)Se(0.05))3 thermoelectric compound, Layered structure, High resolution electron microscope, Image simulation

The passivation behavior of copper anode containing impurities in copper sulfate solution was studied as a function of cooling rate in casting process. The passivaton behavior was investigated by electrochemical technique (galvanostatic and potentiostatic tests) and surface analysis (optical microscopy, electron probe micro analysis, scanning electron microscopy). The results obtained were that a anode with larger dendrite arm spacing (DAS) exhibited superior passivation properties such as longer time to passivation and higher critical current density. The optimum range of DAS was 0.2 to 0.25 mm. The improved passivation characteristics could be explained by the increase in (grain boundary)/(grain interior) concentration ratio of impurities such as As, Sb, Bi and O due to slow cooling of anode during casting process. The SEM image revealed that the higher impurities concentration ratios are, the more porous passivation film on the copper anode is.

keyword : Copper, Anode, Passivation, Critical current density, Dendrite arm spacing, Impurity concentration ratio