발간논문

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Zr-based amorphous alloy matrix composites reinforced with stainless steel (STS) and tantalum continuous fibers were fabricated without pores or defects by a liquid pressing process, and their quasi-static and dynamic deformation behaviors were investigated by using a universal testing machine and a Split Hopkinson pressure bar, respectively. The quasi-static compressive test results indicated that the fiberreinforced composites showed amaximum strength of about 1050~1300 MPa, and its strength maintained over 700 MPa until reaching astrain of 40%. Under dynamic loading, the maximum stresses of the composites were considerably higher than those under quasi-static loading because of the strain-rate hardening effect, whereas the fracture strains were considerably lower than those under quasi-static loading because of the decreased resistance to fracture. The STS-fiber-reinforced composite showed a greater compressive strength and ductility under dynamic loading than the tantalum-fiber-reinforced composite because of the excellent resistance to fracture of STS fibers.

keyword : Composites, infiltration, strain rate, impact test, Liquid pressing process

The remaining life estimation of the level luffing crane component, which has operated for about 20 years is examined carefully, especially on the crane structures. To analyse the crane sructures, the basic load and load combination needed to be considered. We modeled various parts of the level luffing crane to analyse fatigue. Fatigue analysis results showed that the level luffing crane is in the fatigue life so that the crane is in the safe state in fatigue cumulative damage. Analysis results show that the remaining life of a jib upper beam would be about 10 years therefore, the level luffing crane should be stable for fatigue for that period.

keyword : metals, machining, mechanical properties, impact test, fatigue analysis

Wrought magnesium alloys show a low formability at room temperature, and a remarkable anisotropy of mechanical properties make it difficult to use them in a deformation process in industry. The microstructure and crystallographic texture of metals are developed during thermo-mechanical processes, and they are significant to the understanding of the mechanical properties of metals. This work studies the microstructure, texture development and tensile properties of the extruded AZ31 Mg alloy after rolling at 100 and 300℃. After 40% rolling at 100℃, many deformed twins were observed and a relatively weak texture developed. The basal poles were split and rotated towards the rolling direction about 20°. During 60% rolling at 300℃, the dynamic recrystallization (DRX) took place and developed a strong <0001>∥ND fiber texture, which influenced the poor formability at room temperature.

keyword : alloys, rolling, texture, tensile test, AZ31

The composite coatings of (ZrO2-8Y2O3)/(Ni-22Cr-10Al-1Y) were prepared by the air plasma spraying method. They consisted of (Ni,Cr)-rich regions,(ZrO2-Y2O3)-rich regions, and Al2O3-rich regions that were formed by oxidation of Al from (Ni-22Cr-10Al-1Y) during spraying. The coatings corroded at 800 and 900℃ in NaCl-Na2SO4 molten salts up to 50 hr. Ni, Cr and Al oxidized to NiO, Cr2O3 and α-Al2O3, respectively. These oxides and (ZrO2-Y2O3) were dissolved off into the molten salts during hot corrosion, which resulted in the ever-lasting corrosion of the composite coatings. Chromium diffused out from the (Ni,Cr)-rich regions and oxidized to Cr2O3, which was most frequently found as surface scales. Aluminum retained in the (Ni,Cr)-rich regions were similarly diffused out.

keyword : composites, plasma spray, corrosion, scanning electron microscopy, SEM, NiCrAlY

Graphite epoxy composite material (GECM) shows high specific strength and its application in the aerospace industry is gradually increasing. However, its application would induce galvanic corrosion between GECM and metallic materials. This work focused on the effects of anodizing and thermal oxidation on galvanic corrosion in a 3.5% NaCl solution between GECM and aluminium and titanium. In the case of anodized aluminium, galvanic corrosion resistance to the GECM was greatly improved by the anodizing treatment regardless of area ratio. In the case of anodized titanium, the anodizing by a formation voltage of 50V increased corrosion resistance of titanium in galvanic tests. Thermal oxidation of titanium also improved corrosion resistance of Ti to GECM.

keyword : composites, anodization, corrosion, Al, Ti, galvanic

Co-application of organic coating and cathodic protection has not provided enough durability to low-alloyed steels inseawater ballast tank (SBT) environments. An attempt has made to study the effect of alloy elements (Al, Cr, Cu, Mo, Ni, Si, W) on general and localized corrosion resistance of steels as basic research to develop new low-allowed steels resistive to corrosion in SBT environments. For this study, we measured the corrosion rate by the weigh loss method after periodic immersion in synthetic seawater at 60 ℃, evaluated the localized corrosion resistance by an immersion test in concentrated chloride solution with the critical pH depending on the alloy element (Fe, Cr, Al, Ni), determined the permeability of chloride ion across the rust layer by measuring the membrane potential, and finally, we analyzed the rust layer by EPMA mapping and compared the result with the E-pH diagram calculated in the study. The immersion test of up to 55 days in the synthetic seawater showed that chromium, aluminium, and nickel are beneficial but the other elements are detrimental to corrosion resistance. Among the beneficial elements, chromium and aluminium effectively decreased the corrosion rate of the steels during the initial immersion, while nickel effectively decreased the corrosion rate in a longer than 30-day immersion. The low corrosion rate of Cr- or Al-alloyed steel inthe initial period was due to the formation of Cr2FeO4 or Al2FeO4, respectively -the predicted oxide in the E-pH diagram- which is known as a more protective oxide than Fe3O4. The increased corrosion rate of Cr-alloyed steels with alonger than 30-day exposure was due to low localized corrosion resistance, which is explained bythe effect of the alloying element on a critical pH. In the meantime, the low corrosion rate of Ni-alloyed steel with a longer than 30-day exposure wasdue to an Ni enriched layer containing Fe2NiO4, the predicted oxide in the E-pH diagram. Finally, the measurement of the membrane potential depending on the alloying element showed that a lower permeability of chloride ion does not always result in higher corrosion resistance in seawater.

keyword : alloys, thermomechanical processing, corrosion, scanning electron spectroscopy, SEM, E-pH diagram

The specific motivation for joining an Al alloy and Zn-coated steel arises from the need to save fuel consumption by weight reduction and to enhance the durability of vehicle structures in the automobile industry. In this study, the lap joining A6K31 Al alloy (top) and SGARC340 Zn-coated steel (bottom) sheets with a thickness of 1.0 mm and 0.8 mm, respectively, was carried out using the friction stir weld (FSW) technique. The probe of a tool did not contact the surface of the lower Zn-coated steel sheet. The friction stir welding was carried out at rotation speeds of 1500 rpm and travel speeds of 80~200 mm/min. The effects of tool geometry and welding speed on the mechanical properties and the structure of a joint were investigated. The tensile properties for the joints welded with a larger tool were better than those for the joints done with a smaller tool. A good correlation between the tensile load and area of the welded region were observed. The bond strength using a larger tool (M4 and M3) decreased with an increase in welding speed. Most fractures occurred along the interface between the Zn-coated steel and the Al alloy. However, in certain conditions with a lower welding speed, fractures occurred at the A6K31 Al alloy.

keyword : alloy, welding, mechanical properties, focused ion beam, FIB, friction stir welding, FSW

This paper investigated the synthesis conditions of nano-sized TiO2 powder in a hydrothermal process at a temperature range of 100~180℃ considering the precipitation agent, precipitation pH, reaction temperature and time. Titanium hydroxide formed by NH4OH exhibited a lower crystallization temperature than that by NaOH and formed less aggregated TiO2 particles. As the precipitation pH increased above 8, the shape of the particles changed from spherical to needle form, which appeared to be caused by dissolution and re-precipitation of the titanium hydroxide in an alkali environment.

keyword : nanostructured materials, chemical synthesis, crystallization, X-ray diffraction, TiO2 hydrothermal synthesis

In this report, we describe mold materials that were used to produce movable metal type by the investment casting method during the Joseon dynasty period in Korea. Samples were obtained from the Wibuinja collection, which is held by the National Museum of Korea. Most of the mold material remnants were found in the depressed areas of the movable type specimens, and we therefore performed non-destructive analyses including XRF, EDS, and XRD. Through these analyses, we were able to identify the mold remnants as hydrocerussite [trilead dihydroxide dicarbonate, Pb3(CO3)2(OH)2] formed in platy hexagonal crystallites. Hydrocerussite was first used to make white pigments and cosmetics in ancient Greece, but this is the first report of hydrocerussite used as mold material applied around a disposable pattern for investment casting. The results of this study will further the understanding of the production process for early movable metal type and ancient casting technologies.

keyword : alloys, casting, defects, X-ray diffraction, movable metal type

Formation of Sn through-silicon-via (TSV) and its interconnection processes were studied in order to form a three-dimensional interconnection structure of chip-stack packages. Different from the conventional formation of Cu TSVs, which require a complicated Cu electroplating process, Sn TSVs can be formed easily by Sn electroplating and reflow. Sn via-filling behavior did not depend on the shape of the Sn electroplated layer, allowing a much wider process window for the formation of Sn TSVs compared to the conventional Cu TSV process. Interlocking joints were processed by intercalation of Cu bumps into Sn vias to form interconnections between chips with Sn TSVs, and the mechanical integrity of the interlocking joints was evaluated with a die shear test.

keyword : electronic materials, joining, strength, scanning electron microscopy, microelectronic packaging