발간논문

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Tensile, low cycle fatigue, and fatigue crack growth rate tests were conducted at RT and 300℃ for type 304 stainless steel. Tensile was tested under displacement control and low cycle fatigue was tested under strain control. Fatigue crack growth rate test was conducted under load control and crack was measured by DCPD method. Yield strength and elongation decreased at 300℃. Dynamic strain aging was not detected at 300℃. Low cycle fatigue life increased but fatigue strength decreased at 300℃. Fatigue crack growth rate increased at 300℃. Dislocation structures were mixed with cell and planar and did not change with temperature. Grain size did not change but plastic strain increased at 300℃. Strain induced martensite after low cycle fatigue test increased at RT but decreased at 300℃. It was concluded that the increase of low cycle fatigue life at 300℃ was due to the decrease of strain induced martensite at which crack was initiated.

keyword : 304, stainless steel, fatigue, strain induced martensite, temperature

The high strain rate deformation behavior of ultra-fine grained 5083 aluminum alloys prepared via equal channel angular (ECA) extrusion was investigated in this study. The microstructure of ECA extruded specimens consisted of ultra-fine grains, and contained a considerable amount of second phase particles, which were fragmented and distributed homogeneously in the matrix. According to the dynamic torsion test results, the maximum shear stress and fracture shear strain of the route A (no rotation) specimen were lower than those of route C (180° rotation) specimen since that adiabatic shear bands of 100 μm in width were formed in the route A specimen. The formation of adiabatic shear bands was addressed by concepts of critical shear strain, deformation energy required for void initiation, and microstructural homogeneity associated with ECA operations.

keyword : aluminum alloy, ultra-fine grain, equal channel angular extrusion, dynamic torsion test, adiabatic shear band

A study was conducted on the effects of carbon, tungsten, and vanadium on the wear properties and surface roughness of four High Speed Steel (HSS) rolls manufactured by the centrifugal casting method. Hot-rolling simulation tests were carried out using a high-temperature wear tester capable of controlling speed, load, and temperature. HSS rolls contained a large amount (up to 25 vol.%) of carbides such as MC, M2C, M7C3, and M6C carbides formed in the tempered martensite matrix. The matrix consisted mainly of lath tempered martensite when the carbon content in the matrix was small, and contained a considerable amount of plate tempered martensite when the carbon content increased. The high-temperature wear test results indicated that the wear properties and surface roughness of the rolls improved when the amount of hard MC carbides formed inside solidification cells increased. The rolls distribution was also homogeneous. The best wear properties and surface roughness were obtained from a roll where a large amount of MC carbides was homogeneously distributed in the lath tempered martensite matrix. The proper contents of carbon equivalent, tungsten equivalent, and vanadium were 2.0~2.3%, 9~10%, and 5~6%, respectively.

keyword : high speed steel roll, carbide, solidification cell, tempered martensite matrix

Approaches for analyzing indentation hardness are still controversial, although the instrumented indentation technique has been generalized as one powerful method that can record surface deformation behaviors. Material pile-ups around the indenter/surface contact region make the conventional Oliver and Pharr`s analysis on the instrumented indentation curve inaccurate. Thus, in order to prove the validity of the hardness analyses, five approaches were applied to the experimental data obtained from fused quartz and (100) monocrystalline tungsten specimens; an elastic recovery analysis on instrumented indentation curves, three indentation work analyses on the unit plastic volume, and a differentiation analysis on remnant indentation morphologies were tried. Five kinds of indentation hardness overlapped on one result plot showed the validity of each analysis. The modified indentation work approach based on a new definition of plastic volume showed consistent results with those from the Oliver-Pharr`s and image differentiation methods. In the case of pile-up accompanying deformation, the Oliver-Pharr`s and image differentiation methods showed the upper and lower limits of indentation hardness, respectively.

keyword : indentation hardness, surface pile-up, Oliver-Pharr`s analysis, modified Stilwell-Tabor`s analysis, image differentiation analysis

In the galvanizing coating process, the effects of the silicon content on the coatability and wettability of molten zinc were investigated on Dual-Phase High Strength Steels (DP-HSS) with various Si contents using the galvanizing simulator and dynamic reactive wetting systems. DP-HSS showed good coatability and a well-developed inhibition layer in the range of Si content below 0.5 wt%. Good coatability was the results of the mixed oxide Mn2SiO4, being formed by the selective oxidation on the surface, with a low contact angle in molten zinc and a large fraction of oxide free surface that provided a sufficient site for the molten zinc to wet and react with the substrate. On the other hand, with more than 0.5 wt%, DP-HSS exhibited poor coatability and an irregularly developed inhibition layer. The poor coatability was due to the poor wettability that resulted from the development of network-type layers of amorphous SiO2, leading to a high contact angle in molten zinc, on the surface.

keyword : galvanizing steel, GI, coatability, wettability, inhibition Layer, selective oxidation, dual phase high strength steel

Ti and Ti based hydrogen storage alloys have been thought to be the third generation of alloys with a high hydrogen capacity, which makes it difficult to handle because of high reactivity. In order to solve the problem, the activation of a wide range of hysteresis of hydriding/dehydriding and without degradation of hydrogen capacity due to the hydriding/dehydriding cycle have to be improved in order to be aplied. Ti-Cr alloys have a high capacity about 0.8 wt.% in an ambient atmosphere. When the Ti-Cr alloys are added to Nb and Ta elements, they formed a laves phase in the alloy system. The Nb element was expected to make easy diffuse hydrogen in the Ti-Cr storage alloy, which was a catalytic element. In this study, the Ti-Nb-Cr ternary alloy was prepared by melt spinning. As-received specimens were characterized using XRD (X-ray Diffraction), SEM (Scanning Electron Microscopy) with EDX (Energy Dispersive X-ray) and TG/DSC (Thermo Gravimetric Analysis/Differential Scanning Calorimetry). In order to examine hydrogenation behavior, the PCI (Pressure-Composition-Isotherm) was performed at 293, 323, 373 and 423 K.

keyword : hydrogen storage alloy, melt spinning, Ti-Cr alloy, Laves phase

Co base superalloys have been widely used for the parts of gas turbine due to their excellent strength, thermal fatigue, oxidation resistance and weldability at high temperature. In this study, directional solidifications were carried out at various solidification rates, including 0.5~300 μm/s in the Co base superalloy FSX-414. The cellular interface were formed at a low solidification rate, 1 μm/s, and the dendritic interface was found at higher solidification rates, 5~300 μm/s. As the spacing of dendrite structure decreased, the size and spacing of eutectics decreased. Dendrite arm spacing decreased with increasing solidification rates and temperature gradient. It was interesting to find the M23C6 eutectic microstructure formed between γ dendrites. Composition analysis showed that Cr and W were segregated severely between the dendrites, which resulted in the formation of Cr-rich M23C6 and W-rich MC carbides.

keyword : directional solidification, thermal gradient, solidification rate, Co-base superalloy, segregation

Isotropic conductive adhesives (ICAs) have been used or considered as an interconnect material for radio frequency identification (RFID) inlays or other flip chip assemblies due to the advantages of having a low temperature and high-speed bonding. In this work, the curing properties of commercial ICAs for the RFID tag application and the signal transmission in conductive lines that contained the ICA joints were evaluated as a function of the degree of cure at 900 MHz frequency range. The ICAs showed adequate signal transmission only after the curing process passed over the critical time. It was also found that the insertion loss of signal was more dependent on the contact states of Ag fillers in the bondline in preference to the electrical resistance of the ICA itself.

keyword : RFID inlay, isotropic conductive adhesive, snap cure, chip bonding, signal transmission, S-parameter

In order to commercialize Polymer Electrolyte Fuel Cell (PEFC), the cathode catalyst such as Platinum supported Carbon (Pt/C) need to have a high activity of Oxygen Reduction Reaction (ORR). In this study, the 20% Pt/C was synthesized using the chemical reduction method while the crystallinity of Platinum (Pt) particles were controlled under heat treatment conditions. The activity of synthesized Pt catalysts was evaluated using electrochemical measurement. Compared with the i(ORR) at 0.8 V of 20% Pt/C heat-treated at 500℃ and the 20% Pt/C that were not heated and commercial 20% Pt/C, the i(ORR) at 0.8 V of 20% Pt/C heat-treated at 500℃ was 9.5 and 1.7 times higher than those of the 20% Pt/C and commercial 20% Pt/C that were not heated. It was considered that the crystallinity and particle size affect the ORR activity of the Pt/C catalysts.

keyword : Polymer electrolyte fuel cell, PEFC, cathode catalyst, oxygen reduction reaction activity, Platinum, crystallinity