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Abstract: The effects of oxygen content on the β to α phase transformation of the Ti-5Al-2.5Fe alloy during continuous cooling were investigated by varying the oxygen content between 0.08 - 0.41 wt%. Dilatometer tests were performed at cooling rates of 0.05 - 100 ℃/s combined with microstructure observation to examine phase transformation mechanisms and resulting microstructure. Our results showed that oxygen acted as a strong α phase stabilizer by increasing β-transus temperature, and promoted diffuse-controlled α phase transformation with hindering martensitic transformation. Depending on the oxygen content, the different transformation mechanism led to a distinct difference in microstructure evolution. As an oxygen content increased, α phase was developed in thicker lath forms and was globularized at a slow cooling rate, whereas the formation of acicular-shaped α phase was significantly reduced. Hardness behavior after cooling was discussed in the light of microstructure morphology and possible precipitation formation. Based on the analysis results, continuous cooling transformation diagrams of the investigated alloys were established. (Received March 2, 2017; Accepted May 8, 2017)

keyword : metals, phase transformation, microstructure, dilatometry, continuous cooling transformation diagram

Abstract: A new Ni based superalloy with high fatigue resistance and fine grain was considered as a material for high-pressure buckets in fossil fuel power plants. Since such buckets are usually exposed to extreme operating conditions, low cycle fatigue tests were conducted to understand the microstructure evolution of the material during cyclic fatigue under total strains of 0.8, 0.9, 1.0 and 1.2%, and temperatures of 566, 593, 621, 649 and 700 ℃. Microstructural analysis of fractured specimens was conducted using TEM, SEM and OM to observe changes in the γ″ morphology, carbides, and fracture mode. Based on the observations of microstructural changes and low cycle fatigue properties, it can be concluded that grain size had the dominant effect on low cycle fatigue properties, regardless of the size of the transformed carbides and precipitates in these alloy systems. (Received March 7, 2017; Accepted June 7, 2017)

keyword : alloys, forging, fatigue, microstructure, carbides, transmission electron microscopy

Abstract: Mill-annealed Ti-6Al-4V (Ti64) sheets were formed into rectangular shaped pans with two square recesses by the superplastic forming (SPF) technique under high pressure using commercial grade argon gas. The SPF process induced a significant decrease in the tensile ductility and high cycle fatigue (HCF) resistance of the Ti64 alloy. The fractographic analysis of as-fabricated Ti64 sheets showed a hydrogen-affected layer on the surface, possibly incurred by a small amount of water vapor in the commercial grade argon gas. This hydrogen-affected layer with a depth of approximately 60 μm tended to decrease tensile ductility and the resistance to HCF, by encouraging easy crack initiation on the surface. The effect of hydrogen on the mechanical properties of the SPF processed Ti64 sheets was discussed based on fractographic and micrographic observations. (Received May 8, 2017; Accepted May 19, 2017)

keyword : alloys, hot working, hydrogen, fatigue, titanium alloy, superplastic forming

Abstract: The wetting behavior of liquids on solid surfaces is important in the study of surfaces and interfaces. In this study, the wetting behavior of deionized (DI) water and hexadecane on fluorinated and micro-patterned polymer surfaces was investigated. The sample surfaces were processed by spray coating and thermal imprinting, respectively. The apparent contact angle and sliding angle of each sample were measured. Hexadecane, which has a low surface tension, spread easily over the micro-patterned surface, indicating oleophilicity. However, the apparent contact angle of DI water on the micro-patterned surface was approximately 134.2°, indicating that the surface was hydrophobic. Although the apparent contact angle of DI water on the micro-patterned surface indicated hydrophobicity, the sliding angle was very high at 60º. These results indicated that DI water droplets on the micro-patterned surfaces have an intermediate wetting state between the Wenzel and Cassie-Baxter states. Based on the experimental results, it was concluded that the wetting behavior of DI water droplets on the micro-patterned surfaces is determined by the force balance between the capillary force and the air counterforce. (Received April 3, 2017; Accepted May 14, 2017)

keyword : apparent contact angle, intermediate wetting state, micro-patterned surface, sliding angle, wetting behavior

Abstract: Since their development in the 1970s ZrO₂ ceramics have been shown to be promising alternative materials for total hip arthroplasty (THA) and total knee arthroplasty (TKA). However, ZrO₂ is so brittle that in vivo failure can result from fracture of the ceramic femoral head and fracture of the ceramic acetabular liner due to the low toughness of the ceramics. To improve its fracture toughness, the approach commonly utilized has been the addition of a second phase to form a composite and produce nanostructured materials. The purpose of this study was to produce a mechanical synthesis of CoZr and ZrO₂ nanopowders from CoO and Zr powders, and fabricate dense nanocrystalline 2CoZr-ZrO₂ composites within two minutes using this pulsed current activated sintering method. Their mechanical properties, biocompatibility and microstructure were then evaluated. Nanopowders of CoZr and ZrO₂ were synthesized mechanochemically according to the reaction (2 CoO + 3 Zr → 2CoZr + ZrO₂) from CoO and Zr powders by high-energy ball milling. The synthesized powders were consolidated by the pulsed current activated sintering method within two minutes under 80Mpa pressure. CoZr and ZrO₂ composite was sintered by the rapid heating of the nanophase. The hardness and fracture toughness of the 2 CoZr-ZrO₂ composite in this study were better than the hardness of monolithic CoZr and the fracture toughness of a monolithic ZrO₂. Additionally, the 2 CoZr-ZrO₂ composite exhibited good biocompatibility. (Received January 18, 2017; Accepted February 18, 2017)

keyword : sintering, composite, synthesis, mechanical properties, nanomaterials

Abstract: In the photovoltaic industries, the main technique of metallization is screen printing with silver pastes, due to its simple and quick process. However, the high price of silver paste is one of the barriers to the producing of low cost solar cells. Consequently, a primary target in photovoltaic research is to reduce consumption of silver paste or substitute silver with other lower cost materials. As a proper candidate, copper has been extensively investigated for that purpose by many institutes and companies, since it is less expensive than silver with similar conductivity. The plating technique has been actively researched to apply copper as a contact for solar cells. However, copper paste, which was originally developed for integrated circuit applications, has been recently attracted attention. Initially, copper paste was applied for a low temperature annealing process since copper tends to oxidize easily. Recently, a firing type copper paste was also developed which involves coating of the copper particles with a barrier layer. This paper reviews recent developments of copper pastes for the application to solar cells, and its appropriate annealing conditions for better electrical properties. Also, the light I-V characteristics of copper paste on the solar cells reported in the literature are summarized. The 20.7% of the conversion efficiency of a PERT (passivated emitter and rear totally diffused) structure solar cell demonstrates the potential of copper paste as a promising future metallization material. (Received May 11, 2017; Accepted May 23, 2017)

keyword : copper paste, curing atmosphere, passivated busbar, HIT solar cells

Abstract: In this paper, the soft magnetic properties and compaction density of Fe-Si-B-Nb-Cu nanocrystalline powder cores, mixed with two kinds of permalloy powders (Ni₇₀Fe₃₀ or Ni₅₀Fe₅₀), have been closely studied. Permalloy powders having an average diameter of 4 μm were added in the range of 5 to 15 wt%, and the powder composite was mixed with 1~2 wt% of water glass diluted with DI water. The powder cores were compacted in toroidal form at a pressure of about 35 tons, and then heat treated at 510 ~ 570 ℃ for 1 h. The magnetic properties of the soft magnetic composites (SMCs) were measured with a B-H analyzer and the microstructure was observed with FE-SEM. As a result, it was determined that the compaction density of the powder core increased with the increasing content of the mixed permalloy powder, which distinctly improved the composite`s magnetic properties, including initial permeability and core loss. (Received December 30, 2016; Accepted May 12, 2017)

keyword : nanocrystalline, powder core, compaction density, permalloy, soft magnetic properties

Abstract: We report the effects of Sb content on the charge transport and power factor of Bi_2-xSb_xTe₃(1.56 ≤ □ ≤ 1.74) compounds prepared by hot pressing. From the temperature-dependence of the electrical conductivity, it was found that the charge transport in the compounds was predominantly governed by acoustic phonon scattering, and additionally affected by ionized impurity scattering. Hole concentration increased with increasing Sb content due to the generation of antisite defects, and its process-dependence is discussed in comparison with our previous work. The room-temperature mobility of the compound gradually decreased with increasing Sb content due to the intensification of ionized impurity scattering by the antisite defects, however, the effect of grain boundary scattering on the mobility could not be observed. The Seebeck coefficient decreased with the increase in Sb content, and this effect was interpreted in terms of the carrier concentration and the density-of-state (DOS) effective mass. The DOS effective mass was estimated by Pisaernko relation, and it increased from 0.75 (□ = 1.56) to 0.97 (□ = 1.74) with increasing Sb content due to the non-parabolicity in the valence band of Bi_2-xSb_xTe₃. Based on these results, the effects of Sb content on the power factor were discussed, and the highest power factor of 37.2 × 10^-4 Wm^-1K^-2 was obtained in the Bi_0.3Sb_1.7Te₃ compound at room temperature. (Received February 16, 2017; Accepted April 16, 2017)

keyword : thermoelectric, bismuth telluride, hot pressing, charge transport, power factor

Abstract: In this work, Zn(BH₄)₂ was added to improve the hydrogen-storage properties of MgH₂. A 99 wt% MgH₂+1 wt% Zn(BH₄)₂ sample was prepared by milling in a planetary ball mill in a hydrogen atmosphere. The proportion of the added Zn(BH₄)₂ was small (1 wt%) in order to increase hydriding and dehydriding rates without reducing the hydrogen-storage capacity too much. Changes in the released hydrogen quantity, Hd, with temperature, T, for as-milled 99MgH₂+1Zn(BH₄)₂ was obtained by heating the sample from room temperature to 683 K with a heating rate of 5 K/min under 1.0 bar of gas. Activation of the sample was not required. 99MgH₂+1Zn(BH₄)₂ had an effective hydrogen-storage capacity (the quantity of hydrogen absorbed in 60 min) of 4.83 wt% in the first cycle. The as-purchased MgH₂ absorbed hydrogen slowly, absorbing 0.04 wt% H in 60 min. On the other hand, in the first cycle, 99MgH₂+ 1Zn(BH₄)₂ absorbed 3.97 wt% H in 5 min, 4.49 wt% H in10 min, and 4.83 wt% H in 60 min at 593 K under 12 bar H₂. (Received February 20, 2017; Accepted May 29, 2017)

keyword : hydrogen absorbing materials, mechanical milling, hydrogen, thermal analysis, MgH₂-based alloy

Abstract: Austenitic stainless steels have excellent mechanical property and corrosion resistance, and are for these reasons used in demanding applications such as power plant, nuclear reactor and off-shore plant. Especially, austenitic stainless steel 304 (STS304) is mainly used as a pipe material in nuclear power plants. Submerged arc welding (SAW) is a commonly used method of welded steel pipes. This method has some problems of microstructure and mechanical properties such as heterogeneous microstructure, ductility decreases, brittleness of a weld region after SAW process. Especially, delta ferrite and dendrite structure is happened in the weld metal after welding process, which affects the decrease of toughness and ductility. Therefore, it requires the post weld heat treatment (PWHT) in order to solve these problems. In this study, welded STS304 was investigate the microstructure and mechanical properties after SAW process. In order to improve the toughness and ductility of weld region, PWHT was performed by controlling the delta-ferrite and dendrite structure. It was found that the mechanical property of the weld region were increased due to the decrease of the delta- ferrite and the recrystallization of the austenite structure after PWHT. (Received March 30, 2017; Accepted May 31, 2017)

keyword : austenitic stainless steel, submerged arc welding, post weld heat treatment, delta-ferrite, recrystallization