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This study was performed to compare the resistance spot weldability of Al 5052-H32 alloy and Al 6014-T4 alloy, and the cause of the differences in the weldability was investigated. In general, the surface of the aluminum alloy has an oxide film several nm thick, and local heat input is generated at the electrode-sheet interface and the sheet-sheet interface during resistance spot welding. XPS analysis proved that the Al 5052-H32 alloy has a relatively thick magnesium oxide film on the surface and contains a large amount of magnesium solute element. As a result, Al 5052-H32 has a higher resistance, in both the contact resistance of the electrode-sheet interface and sheet-sheet interface, compared with the Al 6014-T4 alloy. Therefore, the Al 5052-H32 alloy has a larger nugget diameter at the same welding current as compared to the Al 6014-T4 alloy, but the surface contamination of the electrode is aggravated due to local heat input. The results indicated that a difference in oxide film type and thickness can significantly influence resistance heat generation and electrode cooling effects, as well as produce welds with different weld morphology and microstructure. In addition, the Al 5052-H32 alloy had more pores and shrinkage in the weld than Al 6014-T4 alloy, and longitudinal cracks were observed in the center of the nugget, but had relatively excellent mechanical properties. (Received 23 August, 2023; Accepted 7 November, 2023)

keyword : Resistance spot weldability, Aluminum alloy, Oxide film, Heat generation, Contact resistance, Pores and shrinkage

To ensure the high-temperature stability of a bondline under next-generation power devices such as SiC semiconductors, a die bonding test was performed by transient liquid-phase (TLP) sinter-bonding using a Sn-coated Cu (Cu@Sn) particle-based preform. Compared to the existing 20 min-bonding result using a 30 μm Cu@Sn particle-based preform, a 5 μm Cu@Sn particle-based preform was used to significantly reduce the bonding time to 5 min, and the optimal levels of the amount of Sn in the Cu@Sn particles, the thicknesses of Sn surface finish layers on the chip and substrate, and compression pressure during the bonding were investigated. The Sn content in the Cu@Sn particles significantly changed the microstructure, including the porosity of the prepared preform. The preform porosity of 0.01% was confirmed after the formation of sufficient Sn shells with an average thickness of about 602 nm at Sn 30 wt%. In addition, in the preform with Sn 30 wt% content, the Sn phase was almost depleted after 3 min after annealing at 250 °C. The Sn finish layer was evaluated in the thickness range of 0.63-4.12 μm, and it was observed that the shear strength of the formed bondline tended to increase with increasing pressure for all Sn layer thicknesses. In particular, when the bonding was carried out at a pressure of 2 MPa using a dummy Cu chip and substrate coated with a 1.53 μm thick Sn layer, the best shear strength value of 36.89 MPa was achieved. In this case, all the Sn phases transformed into intermetallic compound phases of Cu6Sn5 and Cu3Sn, and all the phases formed within the bondline, including Cu, exhibited high melting-point characteristics. Therefore, it was determined that there would be no remelting of the bondline or a drastic decrease in mechanical properties in a high-temperature environment below 300 ℃, as initially intended. By increasing the content of the Sn shell up to 30 wt%, it was possible to achieve a nearly full density (porosity: 0.3%) bondline structure, due to the rearrangement behavior of particles, by maintaining liquid Sn for a long time during the bonding process. In conclusion, the optimal Sn finish thickness was determined to be at the level of 1.5 μm, and the optimal pressure was at the level of 2 MPa. The short bonding time of 5 min represents a significant advance in TLP bonding processes, and it is expected to contribute to a substantial improvement in the die bonding of future SiC power devices. (Received 31 August, 2023; Accepted 24 October, 2023)

keyword : Sn-coated Cu, transient liquid-phase sintering, preform, die bonding, intermetallic compound

Single-crystal superalloys have been popularly employed in high-temperature parts of gas turbines, such as blades. However, the welds of such alloys are highly susceptible to solidification cracking, which limits their applicability to high-temperature turbine blades. In this study, the effects of characteristics of weld solidification on solidification cracking susceptibilities (solidification brittle temperature range, BTR) were fundamentally investigated for the CMSX-4 single-crystal superalloy. We applied a transverse-Varestraint test procedure for both the linear and oscillated arc welds by changing the weld solidification characteristics, such as the degree of single crystal growth and formation of solidification grain boundaries. The BTR for the CMSX-4 alloy is 336 K for linear welding condition, whereas the values are 434 K and 342 K for 0.6 and 1.5 Hz oscillated welds. Interestingly, the BTR continuously increases with the weld oscillation frequency. By contrast, almost no changes in the weld mushy-zone temperature range are theoretically calculated for each welding condition via the diffusion-controlled Scheil model. The mechanism underlying the increase in BTR under oscillation welding is clarified based on the relationship between the achievement ratio of the weld single crystal growth and fraction of high-angle (>15°) solidification boundaries, which affect severe dendrite coalescence undercooling. The lower fraction of the high-angle weld solidification grain boundaries attributed to the superior achievement ratio of weld single crystal growth, which reduces the dendrite coalescence undercooling and BTR. Consequently, it enhances the solidification crack propagation resistance. (Received 26 September, 2023; Accepted 7 November, 2023)

keyword : CMSX-4 superalloy, welding, solidification crack, epitaxial growth, solidification grain boundary

Here, we investigated the influence of δ-precipitate (orthorhombic D0a Ni₃Nb-ordered phase) on the room- and high-temperature tensile properties in wrought nickel-based Inconel 625 superalloys subjected to solution and aging heat treatment. Typically, solution heat-treatment temperatures in these alloys affect the solid-state precipitation of δ-phase, which governs high-temperature tensile properties. While precipitation of fine D0_a δ-phase is known to have beneficial effects on the mechanical properties owing to the retardation of grain coarsening, Widmanstatten δ precipitation plays a deleterious influence on the fracture toughness, tensile ductility, and fatigue resistance. Therefore, to enhance the mechanical properties of this alloy series, it is key to generate a high number density of fine D0_a δ precipitate by adjusting solid solution treatment temperatures. In this study, solution heat treatments were conducted above and below δ-phase solvus temperatures. By applying solution heat treatment at 900℃ and 970℃, this alloy was confirmed to have a Widmanstatten δ phase and is composed similarly to the annealed microstructure. This Widmanstatten δ precipitate was densely distributed at both intergranular and intragranular grains. On the other hand, when solution treatment was applied at 1040 and 1100℃, more coarse particles (approximately 30 μm) with a significant reduction of Widmanstatten type δ phase were obtained. We found that grain size and Widmanstatten δ-phases have an important role in the high-temperature tensile properties of Inconel 625 superalloy series. (Received 11 July, 2023; Accepted 9 October, 2023)

keyword : Inconel 625 superalloys, solid solution heat treatment, precipitates, grain size

In this paper, we investigated a transparent conductive electrode (TCE) that satisfies electrical, optical, and mechanical properties, formed by depositing ultra-thin Ag metal in the form of a random grain boundary with an indium zinc oxide (IZO) layer on a PET substrate. Commonly used ITO electrodes are brittle and difficult to apply to flexible devices. In contrast, IZO-based electrodes are mechanically flexible and can be used as flexible TCE, and have high electrical and optical properties. A 90 nm thick IZO electrode has a transmittance of 90.2% at a wavelength of 460 nm and a sheet resistance of 29.5 ohm/sq. In particular, Ag metal was deposited in the form of an atypical metal island using an RF magnetron sputtering system. At 3 nm there were few metal clusters in the form of islands, and many void channels were formed, resulting in high sheet resistance as well as a decrease in optical transmittance. However, about 5 nm thickness, the number of void channels decreased and the optical path changed, improving the electrical and optical properties. Results showed that the sheet resistance was reduced to 19.8 ohm/sq, and the transmittance was also increased to 91.1%. The mechanical properties were also found to be the same for conventional IZO and Ag/IZO TCE. (Received 6 October, 2023; Accepted 8 November, 2023)

keyword : Flexible, OLED, Transparent electrode, Grain boundary

Mg₃Sb₂-based n-type materials are consisted of earth-abundant elements and possess comparable thermoelectric properties with n-type Bi2Te3 at low temperatures, which make them promising candidates for cooling and power generation applications in terms of cost and performance. Substitution of Sb atom with chalcogen elements (Te, Se S) is a conventional method for n-type doping, but doping cations such as rare-earth elements and transition metals is also widely studied for its unique advantages. In this study, La and Mn were selected for co-doping of Mg₃SbBi, and the thermoelectric performances of the doped materials were investigated. Mg₃La_0.005Mn_xSbBi (0 ≤ x ≤ 0.015) polycrystalline samples were made by sintering the fine powders of the mother alloy after arc melting, in which elemental Mn and LaSb compound were included for n-type dual doping. Considering the loss of Mg at elevated temperatures by vaporization, the molar ratio of Mg, Sb, and Bi in the mixture for arc melting was set to 4 : 1 : 1 with excess Mg. Analysis shows that all the samples are n-type, and the electrical conductivity of Mg₃La_0.005Mn_0.015SbBi increased by 62% from the Mn-free Mg₃La_0.005SbBi at 298 K. In addition, the lattice thermal conductivity (k_lat) decreased with increasing Mn content in the measured temperature range of 298-623 K. The minimum value of k_lat was about 0.60 W m^-1K^-1 in Mg₃La_0.005Mn_0.015SbBi at 523 K, which is about 19% smaller than that of the Mn-free sample. As a result of these enhancements in thermoelectric performance, the maximum figure of merit (zT_max) of 1.12 was obtained in Mg₃La_0.005Mn_0.01SbBi and Mg₃La_0.005Mn_0.015SbBi at 573 K, and the zT at 298 K increased by 73% to 0.35 in Mg₃La_0.005Mn_0.015SbBi compared to Mn-free Mg₃La_0.005SbBi, which is beneficial to room-temperature applications. (Received 10 October, 2023; Accepted 1 November, 2023)

keyword : thermoelectric, Mg_{3La0, 005MnxSbBi, dual doping, La, Mn}

Immiscible Au-Ni alloy thin films undergo phase separation and dewetting because of thermodynamic and morphological instability at elevated temperatures below the miscibility gap. We report the formation and assembly of bimetallic nanoparticles (BNPs) on topographic Si templates. An ordered array of inverted pyramidal pits were produced via solid-state and liquid-state dewetting of a 12-nm-thick Au-Ni thin film by respectively using thermal annealing and laser irradiation. Upon direct thermal annealing at 600 and 800 ℃, the thin film on the templates self-assembled into an ordered array of BNPs composed of Au-rich and Ni-rich sub-clusters in pits. But the relative proportions of the two sub-clusters varied with annealing temperature due to the additional formation of smaller Ni-rich NPs that were scattered around the BNPs. Laser irradiation of the film, in contrast, formed an ordered array of fully mixed alloy NPs on the template and left no other residues on the surface. Subsequent thermal annealing induced the elements within the NPs to segregate, resulting in Au-rich and Ni-rich sub-clusters. In brief, the combination of solid-state and liquidstate dewetting processes on a topographic template not only enabled the 2-dimesional self-assembly of BNPs but also allowed control of the mixing of alloying elements within the BNPs. These results offer insights into the tailored fabrication of BNPs, which have potential applications in bio-functional catalysts, and plasmonic and chemical sensors. (Received 6 October, 2023; Accepted 19 November, 2023)

keyword : thin films, alloys, annealing, dewetting, self-assembly, nanoparticles

keyword : Carbon fiber reinforced polymer, CFRP, has been extensively used in civil engineering for applications such as reinforcing and retrofitting various architectural materials, Therefore, understanding the degradation of CFRP under high temperatures is important, This study aims to investigate the thermo-mechanical and microstructural properties of CFRP plates at elevated temperatures up to 350 ℃, The plate-type CFRP composites were subjected to temperatures of 50, 100, 150, 200, 250, 300, and 350 ℃, and then compared with pristine CFRP samples, X-ray diffraction analysis was conducted to examine the crystal structures of the carbon fibers and epoxy resin matrices in the CFRP, At temperatures higher than 150 ℃, the FWHM increased due to the degradation and softening of the resin matrix, Delamination and debonding between the matrix and fibers were observed in samples exposed to temperatures above 200 ℃, The maximum tensile strength of the CFRP plates exposed at 350 ℃ significantly decreased to 0, 605 GPa, a reduction of approximately 40% compared to the pristine sample, On the other hand, Young, s modulus remained relatively unchanged across the different temperatures, This suggests that the polymer matrix degradation plays a crucial role in the mechanical properties of CFRP, as the matrix layers contribute significantly to the distribution of forces, , Received 21 September, 2023, Accepted 25 October, 2023

To select materials suitable for products, material perception, which is the feeling consumers have about materials, has been studied. Material perception data were obtained through surveys using digital logic for bipolar adjective pairs. The material perception data were analyzed through unsupervised learning of data mining. Prior to data analysis, to increase the reliability of the data, the homogeneity of the data between surveys was tested using clustering analysis, correlation analysis and chi-squared test. After checking the homogeneity of the data between surveys, the data were merged. The merged material perception data were analyzed using relative frequencies, hierarchical clustering, and association rules. The relative frequencies obtained from survey participants' selections were used to determine the prevailing perceptions of each material and as basic data for other analyses. In the hierarchical clustering analysis, hierarchy was identified using distances within clusters and distances between clusters. Through association rule analysis, the consumer's simultaneous perceptions of the material can be known, so not only the individual characteristics of the material but also the relational characteristics can be considered when selecting materials based on consumer's perception. The analyzed characteristics were designed into a material perception map, and this material perception map will be a powerful tool to help product designers make better choices that match consumers' perception and experience when selecting materials. (Received 6 October, 2023; Accepted 9 November, 2023)

keyword : Association Rule, Clustering, Material Perception, Material Selection, Unsupervised Learning