발간논문

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The tensile fracture behavior of an Al-bearing TWIP steel was investigated by conducting a series of tensile tests on smooth and notched specimens with different notch geometries, focusing on the effects of evolution of the stress triaxiality and the effective strain during deformation. The flow curve and digital image correlation (DIC) analysis evidenced suppression of dynamic strain aging due to Al addition, and therefore, the effects of local inhomogeneous deformation associated with Portevin-Le Chatelier (PLC) band on fracture could be excluded. The smooth specimen fractured with negligible necking despite the absence of PLC bands. As a result, the effective strain was uniform through the gage section and the stress triaxiality (η) of ~0.33 was nearly unchanged over the entire cross-section up to the maximum load. This led to the fracture surface of the smooth specimen being entirely covered with fine equiaxed dimples. For notched specimens, the fracture strain was drastically reduced with decreasing notch radius, indicating the high notch susceptibility of the steel. The effective strain of the notched specimens was the highest at the edge of the notch root, regardless of the notch radius, so cracks first developed at the surface of the notch root. Although the η at the center of the notched specimens (0.40~0.48 depending on the notch radius) was higher than that of the smooth one, the center of the fracture surface of all notched specimens exhibited dimple features that were very similar to the smooth one, even in size. In contrast, in spite of the same η of ~0.33, fractography at the edge of the notched specimens revealed a fracture mode transition from dimple fracture to void sheet fracture to quasi-cleavage fracture as the notch radius decreased. The present results were rationalized in terms of the local evolution of stress triaxiality and effective strain during deformation, which were analyzed using the finite elemental method and DIC technique. It can be said that the fracture mode of TWIP steel, showing limited necking, was more influenced by the distribution and/or gradient of stress traiaxiality and effective strain rather than their local absolute values - that is, the severer their gradient is, the easier the quasi-cleavage fracture occurs. (Received 18 January, 2024; Accepted 26 January, 2024)

keyword : Twinning Induced Plasticity Steel, Notch Tensile Fracture, Fractography, Effective Strain, Stress Triaxiality

The effect of powder processing on the microstructure and sinterability of the heavy alloy W-Ni- Cu was investigated. The heavy alloy powders were prepared by the ball milling and hydrogen reduction of metal oxide powders. As the milling time increased, the size of the powder mixture decreased and at 5 h of milling was found to be about 2.5 μm. Microstructural analysis revealed that the powder mixture was changed to W and NiCu alloys with an average particle size of about 200 nm after hydrogen reduction at 800℃ for 2 h. The reduction kinetics of the oxide powder mixture was evaluated by the amount of peak shift with heating rates using TGA in a N₂-10% H₂ atmosphere. The activation energy of the reduction reaction, calculated from the slope of the Kissinger plot, was measured to be 42.8 kJ/mol for CuO, 57.9 kJ/mol for NiO, and 50.1~112.6 for kJ/mol WO₃. The relative densities of the heavy alloy sintered at 1100℃ and 1200℃ using oxide powder were 81.4% and 96.0%, while the specimen using metal powder as a raw material showed a relatively low value of 67% and an inhomogeneous microstructure. It was explained that the changes in sintered microstructure with different powder synthesis methods are mainly due to the powder characteristics, such as the size of the particles of the initial mixed powder. (Received 10 January, 2024; Accepted 29 January, 2024)

keyword : W-Ni-Cu heavy alloy, ball milling, hydrogen reduction, sintering, microstructure

A ceramic-based plasma etcher window (Lid) requires robust resistance to plasma, especially when exposed to harsh fluorine-based plasma conditions. In this study, a Y₂O₃ film was deposited using e-beam evaporation with ion beam-assisted deposition (IBAD), and the physical properties of the IBAD-based Y₂O₃ coating film were thoroughly examined to enhance the mechanical and chemical resistance of the ceramic part, including the Y₂O₃ film, against etching plasma. The hardness and surface morphology of the IBADbased Y₂O₃ could be precisely controlled by various deposition processing parameters, such as beam voltage, beam current, and Ar/O₂ gas ratio. Following the IBAD deposition of the Y₂O₃ film, a plasma etching process (Ar/CF4 mixture gases with 150 W RF power for 60 minutes) was applied to evaluate the plasma resistance of the deposited Y₂O₃ coating film. The surface morphology characteristics of the Y₂O₃ films were compared using atomic force microscopy, and their grain size was studied through scanning electron microscopy image analysis. Furthermore, a nanoindenter was used to determine the hardness of the Y₂2O₃ film. These results suggest that optimizing the IBAD coating process requires an in-depth study that fully considers the correlation between deposition processing parameters and physical properties. This optimization can be instrumental for enhancing the durability of the ceramic part. (Received 6 November, 2023; Accepted 27 January, 2024)

keyword : ceramic chuck, plasma resistance, Y₂O₃ film, ion beam-assisted deposition, deposition parameters

Ti6Al4V is commonly used in implants because of its excellent mechanical properties, corrosion resistance, and biocompatibility. While Hydroxyapatite (HAp) is typically used for strong biological bonding between Ti6Al4V implants and bone tissue, this study takes a different approach by incorporating Equine Bone (EB), which has a chemical structure similar to human bone tissue, as a substitute for HAp. In this study, to develop implant materials with a low elastic modulus, high strength, and excellent wear resistance, Ti6Al4V used in biomedical applications was combined with natural EB. Subsequently, a Ti6Al4V-0.05EB composite was fabricated using ball milling followed by Selective Laser Melting (SLM). SLM can reproduce even the interior of a 3D structure, so various studies are being conducted to apply it as a biomaterial. However, Ti6Al4V alloys produced by SLM are known to have low ductility due to localized heat gradients, rapid solidification, and cooling rates. This reduced ductility can result in decreased formability of biomaterials, and the high elastic modulus may lead to stress shielding phenomena, potentially reducing the lifespan of the biomaterial. To minimize this, a post-heat treatment was applied to the Ti6Al4V-0.05EB composite material manufactured by SLM. Afterwards, the microstructure, mechanical properties, and wear resistance, which are important in biomaterials, were evaluated. (Received 15 January, 2024; Accepted 7 February, 2024)

keyword : Powder metallurgy, selective laser melting, Microstructures, Wear, Biomaterials

A Cu-based paste containing Cu formate and Cu particles was prepared for the compressionassisted sinter-bonding of Cu-finished wide-bandgap power devices onto a Cu-finished substrate at a relatively low bonding temperature of 250℃ in air. A mixture of Cu formate and Cu particles was designed to mitigate the tremendous volume shrinkage during reduction of Cu formate, which approaches approximately 90%, and could be a significant obstacle in the formation of a high-density bond-line. The mixture was spontaneously formed during the 15-min reduction of the initial Cu₂O particles by a simple wet process using formic acid. In the bonding, pure Cu generated in situ from the Cu formate at a temperature exceeding 200℃ exhibited significant sinterability, and the generated hydrogen reduced oxide layers on the Cu finishes. Furthermore, the mixed particles resulted in low volume shrinkage in the bond-line during bonding, compared to the use of Cu formate particles alone. Consequently, a robust die shear strength of 22.2 MPa was achieved by sinterbonding for even 10 min at low temperature and the compression of 10 MPa, even though Cu oxide shells were formed in the bond-line because of the long sintering in air. The simple wet process provided an efficient preparation of an effective filler system before the paste formulation for the sinter-bonding. (Received 12 December, 2023; Accepted 15 February, 2024)

keyword : Sinter-bonding, die bonding, Cu complex particle, Cu formate, in situ reduction, shear strength

AlN-MgO composites with different compositions were prepared by spark plasma sintering, and the effects of their composition on their microstructure, thermal properties, and mechanical properties were systemically investigated. MgO compositions in the AlN-MgO composites were controlled from 20 to 80 wt%. The results indicated that a phase transition did not occur during the sintering process, and different solid solutions were formed within the MgO and AlN lattices. The AlN-MgO composites exhibited finer-grain microstructures than those of the sintered pure AlN and MgO samples. Transmission electron microscopy analysis showed that both oxygen-rich, low-density grain boundaries and clean boundaries with spinel phases were present in the composites. The sintered pure AlN sample exhibited the highest thermal conductivity (53.2 W/mK) and lowest coefficient of thermal expansion (4.47 × 10^-6 /K) at 100℃. And, the sintered pure MgO sample exhibited moderate thermal conductivity (39.7 W/mK) and a high coefficient of thermal expansion (13.05 × 10^-6 /K). With increasing MgO contents in the AlN-MgO composites, however, the thermal conductivity of the AlN-MgO composites decreased, from 33.3 to 14.9 W/mK, while their coefficient of thermal expansion generally increased, from 6.49×10^-6 to 10.73×10^-6 /K with increasing MgO content. The composite with an MgO content of 60 wt% exhibited the best mechanical properties overall. Thus, the composition and microstructure of AlN-MgO composites have a determining effect on their thermal and mechanical properties. (Received 19 December, 2023; Accepted 4 January, 2024)

keyword : AlN-MgO composites, spark plasma sintering, microstructure, thermal properties, mechanical properties

This study explores Mg-composition dependent cycle stability in a Zn_1-xMg_xO Li-ion battery, where battery cycles transition from an electronic transport-limited to an ionic transport limited regime. We investigated the impact of Mg doping in Zn_1-xMg_xO nanocrystals on Li-ion battery performance, focusing on Mg compositions between x=0.05 and x=0.15. Mg composition dependent structural and electrical properties were explored using field effect transistors (FETs) and various microscopic/spectroscopic methods. The electronic conductivity was found to be sensitive to changes in Mg composition. Consistently, the initial capacity decreased with an increase in Mg composition, aligning with the reduction in electronic conductivity due to Mg doping. However, with successive cycles, the capacity became independent of the electronic conductivity, an outcome attributed to the formation of a solid-electrolyte interphase (SEI) and the conversion reactions. Initially, Mg doping reduces electronic conductivity due to increased carrier trapping, leading to lower discharge capacity. However, as cycling progresses, the impact of Mg doping diminishes. The formation of the SEI layer becomes more influential, significantly affecting Li-ion transport. Over time, factors like SEI formation, conversion reaction dynamics, and structural changes within the electrode start to dominate the battery's capacity, rather than the initial electronic conductivity influenced by Mg doping. This understanding can guide the development of materials with lower resistance, facilitating faster charging and discharging rates. More importantly, this study indicates that the initial capacity is closely tied to the conductivity of the Zn1-xMgxO material. (Received 28 November, 2023; Accepted 23 January, 2024)

keyword : ZnMgO, Li ion battery, FET, specific capacity, Mg doping

Because n-type Bi₂Te₃-based materials exhibit lower thermoelectric performance than p-type materials and because their thermoelectric properties are sensitively changed by the composition and carrier concentration, optimizing these aspects in n-type materials is necessary to improve the thermoelectric figure of merit (ZT). In this study, the thermoelectric performance of n-type Bi₂Te₃-based materials was improved by reducing thermal conductivity through the formation of a Bi₂Te₃-Bi₂Se₃ solid solution, Bi₂Te_3-ySe_y and optimizing the carrier concentration through doping. As the amount of Se increased in Bi₂Te_3-ySe_y, the carrier concentration decreased, resulting in decreased electrical and thermal conductivities and increased Seebeck coefficients. As a result, Bi₂Te_2.85Se_0.15 exhibited ZT = 0.56 at 323 K, and Bi₂Te_2.4Se_0.6 exhibited ZT = 0.60 at 423 K. Among the Bi₂Te_3-ySe_y solid solutions, the doping effect was investigated for Bi₂Te_2.85Se_0.15 and Bi₂Te_2.7Se_0.3, which recorded excellent thermoelectric performance at low temperatures. When halogen element (I) was doped, the power factor improved owing to the increase in carrier concentration, and the thermal conductivity decreased. As a result, the ZT values were greatly enhanced to ZT = 0.90 at 423 K for Bi₂Te_2.85Se_0.15:I_0.005 and ZT = 1.13 at 423 K for Bi₂Te_2.7Se_0.3:I_0.0075. When the transition elements (Cu and Ag) were doped, the power factor was improved by the increase in Seebeck coefficient, and thereby Bi₂Te_2.85Se_0.15:Cu_0.01 and Bi₂Te_2.85Se_0.15:Ag_0.01 exhibited ZT = 0.76 and ZT = 0.75 at 323 K, respectively, and Bi₂Te_2.7Se_0.3:Cu_0.01 exhibited ZT = 0.73 at 423 K. Conversely, doping with other transition elements (Ni and Zn), as well as group-III (Al and In) and group-IV (Ge and Sn) elements, resulted in power factors and thermal conductivities that were similar to or slightly less than those of undoped Bi₂Te_2.85Se_0.15, leading to minimal or no improvement in ZT. Next, n-type Bi₂Te_2.85Se_0.15:I_0.005 and Bi₂Te_2.7Se_0.3:I_0.0075, which exhibited the best thermoelectric performances, were fabricated as bulky compacts, and the uniformity of their thermoelectric properties were evaluated. (Received 2 January, 2024; Accepted 6 February, 2024)

keyword : thermoelectric, chalcogenide, bismuth telluride, uniformity

Archaeological excavations in Buyeo had revealed Bronze vessels interred within a hole in the building at the Gwanbuk-ri site. These bronze vessels are dated to the Unified silla period (9-10th centuries). To identify the bronze vessels’ manufacturing technology, metallurgical analyses were carried out using Optical Microscopy, SEM-EDS and EPMA. The results allowed a reconstruction of the manufacturing process of bronze vessels in the Unified silla. It proved that bronze vessels were manufactured with Cu-Sn alloy with varying tin contents, which may be related to the application of uncontrolled procedures in making the bronze alloys. Also a casting process was used to shape the bronze vessels and the strength and hardness of the container were improved through a quenching process. Twins were observed in one of the samples, so it was assumed that hot working had been attempted. Non-metallic inclusions in bronze vessels have circular or polygonal shapes, and Cu and S were detected. The presence of Cu-S inclusions showed the probable use of copper sulphide ores for metal production and smelting. Se and Te in the inclusions of the bronze vessels show that the copper ore is different. From the results of metallurgical analyses of bronze vessels excavated from other regions of the same era, casting and quenching treatments were confirmed, and the sequential relationship of the technological system was revealed. (Received 5 December, 2023; Accepted 20 January, 2024)

keyword : Tin bronze, Vessel, Microstructure, Technology, Unified silla

In this study, we designed and manufactured a new Fe-bearing Al-10Si-Mg casting alloy (F alloy) and investigated its microstructure, mechanical properties, and thermal conductivity. Two types of Fe-bearing Al-10Si-Mg alloys were used: the Conventional-F alloy, injected at 720℃ and cooled by water quenching, and the Superheated-F alloy, heated to 820℃ and maintained at that temperature for 1 hour. Subsequently, it underwent a degassing process at 720℃ before being cooled by water quenching. Both the Conventional-F alloy and the Superheated-F alloy exhibited dendritic microstructures and Fe-intermetallic compounds. The Secondary Dendrite Arm Spacing (SDAS) of the Conventional-F alloy measured 32.4 μm, whereas the Superheated-F alloy measured 28.6 μm. Additionally, the average eutectic Si sizes were 10.3 μm for the Conventional-F alloy and 7.7 μm for the Superheated-F alloy. Fe-rich IMCs were observed in the eutectic region, with their size decreasing due to the superheating treatment. Tensile tests at room temperature were conducted at a strain rate of 10^-3/s. The Conventional-F alloy exhibited a yield strength (YS) of 93.4 MPa, ultimate tensile strength (UTS) of 183 MPa, and an elongation (El.) of 6.4%. Conversely, the Superheated- F alloy displayed a YS of 115.4 MPa, UTS of 218.2 MPa, and an El. of 5.1%. The mechanical properties notably improved with the superheating treatment. Regarding thermal conductivity, the Conventional-F alloy exhibited 114.9 W/m·K, while the Superheated-F alloy displayed 153.7 W/m·K. This represents a roughly 14% increase compared to the thermal conductivity of the commercial Al-10Si-Mg material (Silafont36: 135.1 W/m·K). The effects of the Superheating Treatment on microstructural characteristics, deformation behavior, and thermal conductivity of the Fe-bearing Al-10Si-Mg casting alloys were discussed. (Received 12 December, 2023; Accepted 18 January, 2024)

keyword : Al-10Si-Mg, Fe addition, Superheating, Microstructure, Tensile Property, Thermal conductivity