발간논문

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A three-point bending test was performed on roll-bonded Mg/Al/STS clad-metal plates under two different testing conditions (Mg layer in tension, or STS in tension) and their mechanical response and fracture behavior were investigated. Bending strength was found to be greater under the condition of Mg layer in tension. Heat treatment at 200℃ increased the bending formability, suggesting the interfacial strength increased at 200℃. Under the condition of Mg in tension, the clad heat-treated at 300℃ and 400℃ fractured in two steps, with the first step associated with the interfacial fracture between Mg and Al, and the second the fracture of the Mg layer. STS/Al layers were found to be bent without complete fracture. Under the condition of STS in tension, the clad heat-treated at 300℃ and 400℃ exhibited a very small load drop at the displacement, which is similar to that of the first load drop associated with the interfacial fracture under the condition of Mg in tension. In this case, no interfacial cracks were found and the complete cut-through fracture of clad was observed at low temperature heat treatment conditions, suggesting excellent interfacial strength. When the heat treatment temperature was higher than 300℃, interfacial cracks were observed. The local stress condition and the position of the interface with respect to the surface were found to have a great influence on the fracture behaviors of clad metals.

keyword : bending strength, interfacial fracture, stainless steel, aluminum, magnesium, clad metal

The corrosion product of archaeological iron objects is supposed to be removed because it causes re-corrosion. So far it is removed by scapel and sand blaster but they depend on the skill and experience of a conservator and the glass-dust of the sand blaster is harmful to humans. Therefore this study applies a laser cleaning system which is used in various industrial cleaning processes, to remove corrosion product from archaeological iron objects. In addition, this work studies the alternation of corrosion product after laser irradiation, which evaluates the reliability of the laser cleaning system. Optical microscopy, SEM-EDS, XRD, Raman have been used to observe and analyse the surface of the objects. The results show the capacity of laser cleaning some corrosion product, but blackening appears with increasing pulses and laser energy, and some corrosion products, goethite and hematite, are partially altered to magnetite. These problems, blackening and alternation of corrosion product, should be solved by further studies which find the optimal laser irradiation condition and use a wetting agent.

keyword : metals, surface modification, corrosion, SEM, laser cleaning

Fe-(10, 20, 30, 40) wt%Cr alloys were corroded in Na2SO4 salts (m.p.=884℃) at 800-900℃ for 3- 300 hrs. Their corrosion resistance increased with an increase in Cr content owing to the formation of slowly growing Cr2O3. During corrosion, Na2SO4 dissociated and reacted with the alloys to form Cr2O3 and Fe2O3. Since Fe2O3 dissolved fast into the salts, most of the scales consisted primarily of Cr2O3. Inside the scale, a small amount of sulfides also existed. The oxidation, dissolution and detachment of the formed scales occurred significantly.

keyword : metals, casting, oxidation, scanning electron microscopy (SEM), hot corrosion

Nanopowders of Mo, Ta and Si were made by high-energy ball milling. A dense nanostructured MoSi2-TaSi2 composite was sintered by the high-frequency induction heated combustion method within 2 minutes from mechanically activated powder of Mo, Ta and Si. A highly dense MoSi2-TaSi2 composite was produced under simultaneous application of a 80 MPa pressure and the induced current. Mechanical properties and microstucture were investigated. The hardness and fracture toughness of the MoSi2-TaSi2 composite were 1200 kg/mm2 and 3.5 MPa.m1/2, respectively. The mechanical properties were higher than those of monolithic MoSi2.

keyword : compounds, nanostructured materials, sintering, mechanical properties

Magnesium prepared by mechanical grinding under H2 (reactive mechanical grinding) with transition elements or oxides showed relatively high hydriding and dehydriding rates when the content of additives was about 20 wt%. Ni was chosen as a transition element to be added. Fe2O3 was selected as an oxide to be added. Ti was also selected since it was considered to increase the hydriding and dehydriding rates by forming Ti hydride. A sample Mg-14Ni-3Fe2O3-3Ti was prepared by reactive mechanical grinding, and its hydrogen storage properties were examined. This sample absorbs 4.02 wt% H for 5 min, and 4.15 wt% H for 10 min, and 4.42 wt% H for 60 min at n=2. It desorbs 2.46 wt% H for 10 min, 3.98 wt% H for 30 min, and 4.20 wt% H for 60 min at n=2. The effects of the Ni, 3Fe2O3, and Ti addition, and hydriding-dehydriding cycling were discussed.

keyword : hydrogen absorbing materials, mechanical alloying/milling, scanning electron microscopy (SEM), X-ray diffraction, magnesium

The activation of Mg-10 wt%Fe2O3 was completed after one hydriding-dehydriding cycle. Activated Mg-10 wt%Fe2O3 absorbed 5.54 wt% H for 60 min at 593 K under 12 bar H2, and desorbed 1.04 wt% H for 60 min at 593 K under 1.0 bar H2. The effect of the reactive grinding on the hydriding and dehydriding rates of Mg was weak. The reactive grinding of Mg with Fe2O3 is believed to increase the H2-sorption rates by facilitating nucleation (by creating defects on the surface of the Mg particles and by the additive), by making cracks on the surface of Mg particles and reducing the particle size of Mg and thus by shortening the diffusion distances of hydrogen atoms. The added Fe2O3 and the Fe2O3 pulverized during mechanical grinding are considered to help the particles of magnesium become finer. Hydriding-dehydriding cycling is also considered to increase the H2-sorption rates of Mg by creating defects and cracks and by reducing the particle size of Mg.

keyword : Hydrogen absorbing materials, mechanical alloying/milling, microstructure, X-ray diffraction, Fe2O3 addition

Improvement in microwave absorbance has been investigated by insertion of a periodic air cavity in rubber composites filled with magnetic powders. A mixture of Co2Z hexagonal ferrite and Fe powders were used as the absorbent fillers in silicone rubber matrix. The complex permeability and complex permittivity of the magnetic composites were measured by reflection/transmission technique. In the grid-type magnetic absorbers, the equivalent permeability (εeq) and permittivity (εeq) are calculated as a function of air volume rate (K) on the basis of effective medium theory. Reduction in the material parameters (especially, dielectric permittivity and magnetic loss) has been estimated with the increase of K. Plotting the εeq and εeq on the solution map of wave-impedance matching, wide bandwidth microwave absorbance has been predicted in the magnetic composites with an optimum value of K.

keyword : composites, powder processing, magnetic properties, computer simulation, microwave absorbers

In this study, we have investigated the surface morphology of hydroxyapatite (HA) coated Ti alloy surface using pulsed laser plating. The HA (tooth ash) films were grown by pulsed KrF excimer laser, film surfaces were analyzed for topology, chemical composition, crystal structure and electrochemical behavior. The Ti-6Al-4V alloy showed α and β phase, Cp-Ti showed α phase and the HA coated surface showed HA and Ti alloy peaks. The HA coating layer was formed with 1-2㎛droplets and grain-like particles, particles which were smaller than the HA target particle, and the composition of the HA coatings were composed of Ca and P. From the electrochemical test, the pitting potential (1580 mV) of HA coated Ti-6Al-4V alloy was higher than those of Cp-Ti (1060 mV) and HA coated Cp-Ti (1350 mV). The HA coated samples showed a lower current density than non-HA coated samples, whereas, the polarization resistance of HA coated samples showed a high value compared to non-HA coated samples.

keyword : alloys, coating, surface modification, corrosion, pulsed laser plating

When viewed using a magnetic resonance imaging (MRI) system, invasive materials inside the human body, in many cases, severely distort the MR image of human tissues. The degree of the MR image distortion increases in proportion not only to the difference in the susceptibility between the invasive material and the human tissue, but also to the intensity of the magnetic field induced by the MRI system. In this study, by blending paramagnetic Ti particles with diamagnetic graphite, we synthesized Ti100-xCx composites that can reduce the artifact in the MR image under the high-strength magnetic field. Of the developed composites, Ti70C30 showed the magnetic susceptibility of χ=67.6×10-6, which corresponds to 30% of those of commercially available Ti alloys, the lowest reported in the literature. The level of the MR image distortion in the vicinity of the Ti70C30 composite insert was nearly negligible even under the high magnetic field of 4.7 T. In this paper, we reported on a methodology of designing new structural materials for bio-applications, their synthesis, experimental confirmation and measurement of MR images.

keyword : biomaterials, powder processing, magnetic properties, SEM, MR image artifact

TiC added PMC (polyethylene matrix composites) have been prepared with various volume fractions of TiC. Two types of TiC powder with their different particle diameters, 1.5㎛(TiC-1.5 PMC) and 90㎛(TiC-90 PMC), were utilized in the present research. The electrical resistivities of the PMC decreased gradually with increasing TiC volume fraction (V); in the case of smaller particle size, the value of the electrical resistivities at room temperature jumped down in the relatively narrow range of V=0.40-0.42, reaching as low as 0.1 Ωcm. The temperature dependencies of electrical resistivities were also measured to examine the possible application of the PMC to PMC PTC thermistors. The resistivity showed a positive temperature dependence and changed abruptly in the tenth order of its magnitude during the melting transition of the polyethylene matrix. The electric current-cutoff characteristics of the PMC indicated excellent features in the over current response despite their low resistivity values, as compared with those of other PTC materials.

keyword : composites, extrusion, electrical properties, resistivity