최신논문

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This study investigated the effect of gas pressure and alloying elements on the oxidation behavior of copper. Copper alloys containing 1 wt.% of zinc, nickel, or manganese were cast using vacuum induction melting, followed by solution treatment at 400℃ for 60 minutes. Oxidation were conducted at 700℃ for 60 minutes under pressures of 101325, 20, and 10 Pa. The oxide layers formed at atmospheric pressure on both the pure copper and copper alloys naturally delaminated. In contrast, the oxide layers formed under lowpressure conditions remained intact without delamination. This improved bonding strength between the copper matrix and Cu₂O was attributed to the formation of large grains and dense oxide layer, which increased the contact area with the copper substrate. Additionally, the oxide layers formed under low-pressure exhibited higher dislocation density, which reduced lattice mismatch and interface energy, thereby contributing to the formation of stable interface. In copper alloys, thicker oxide layers were observed compared to pure copper, with voids detected between the oxide layer and the copper substrate. These voids were presumed to be Kirkendall voids, resulting from the difference in diffusion rates between copper and the alloying elements. However, adhesion evaluations indicated that these voids did not significantly impact interfacial adhesion. Through interface control, dense and stable oxide layers were successfully formed on both pure copper and copper alloys. (Received 12 November, 2024; Accepted 11 December, 2024)

keyword : Copper, Copper oxide, Interface between Cu and oxide, Copper alloy oxidation, Low gas pressure oxidation

This study investigated the manufacturing conditions and properties of Ti₄₇Cu₃₈Zr_7.5Fe_2.5Sn₂Si₁Ag₂ amorphous bulk metallic glass (BMG) to examine its applicability for scalpels for medical use. Ti-base BMG alloys have excellent mechanical properties and biocompatibility, making them a suitable scalpel material. In this study, a vacuum suction casting method was used to manufacture a BMG alloy scalpel. After examining various manufacturing conditions, to successfully suppress the occurrence of pores and manufacture a BMG with excellent shape the optimal conditions were determined to be 236 A of arc current, 20 s of melting time, 2 s of suction time, 10°C of coolant temperature, and -0.1 MPa of suction pressure. XRD analysis confirmed that the Ti-base BMG had a completely amorphous structure. Vickers hardness measurements showed that the Ti₄₇Cu₃₈Zr_7.5Fe_2.5Sn₂Si₁Ag₂ amorphous bulk metal material (BMG) scalpel had a hardness of 781 Hv, and was superior to a commercial stainless steel (SUS) scalpel in terms of durability. In the pork skin cutting tests, the Ti-base BMG scalpel maintained its edge without chipping, while the SUS scalpel showed chipping and an uneven cutting surface. These research results suggest that Ti-based BMG can provide better high-strength performance and durability than commercial SUS scalpels, and demonstrate its potential for application as a medical tool. However, bulk amorphous alloys with excellent glass formability (GFA) still require a detailed study of the optimal fabrication conditions to avoid pores and defects. (Received 18 November, 2024; Accepted 2 December, 2024)

keyword : Bulk metallic glass, Scalpel, Fabrication method, Shape optimization, XRD

To evaluate the deformation mechanisms of Ti-5Mo-1Fe alloy, compression tests were performed at strain rates ranging from 10-3/sec to 3×103/sec at room temperature. The results showed that Ti-5Mo-1Fe alloy exhibited different deformation mechanisms depending on the strain rate. Under quasi-static strain rates, the Stress-Induced Martensitic (SIM) transformation from the metastable β phase (BCC) to the α" phase was observed through the double yield phenomenon in the stress-strain curve. This contributed to enhanced compressive strength, work hardening, and ductility by absorbing and dispersing deformation energy. Shear bands (SB) were observed near the fracture zone under quasi-static conditions, since the SIM transformation acts as an obstacle to dislocation movement. In contrast, a dynamic strain rate generated adiabatic shear bands (ASB) due to localized heating, leading to coarsened grains near the fracture zone. With increasing strain rates, significant temperature rises were detected in the specimens, leading to increased β phase stability and a reduced chemical driving force for the α" transformation, thereby suppressing the SIM transformation. Consequently, dislocation slip became the dominant deformation mechanism at high strain rates. This study provides insights into the strain rate sensitivity of metastable β-titanium alloys, offering fundamental information for the development of advanced Ti alloys with high strength, good ductility, and impact resistance. (Received 9 September 2024; Accepted 20 December 2024)

keyword : Titanium, Mechanical properties, Compressive behaviors, Strain rate

Machining processes are crucial in industrial applications for shaping steel into specific structural products, and represent a significant portion of the product cost. However, the machinability of lightweight steel is poor, and it exhibits the characteristics of difficult-to-machine materials. This study investigated the effect of adding sulfur on the machinability of austenitic lightweight steel. Additionally, research was also conducted on aging heat treatment, to determine its effect on the steel’s mechanical and machinability properties, to enhance its usability as a structural material. S was added in the range of 0.0048 to 0.0494 wt.%, and the microstructure was observed, and mechanical and machinability properties were evaluated. After aging at 550℃ for 6 hours, changes in the steel microstructure, including precipitates, were analyzed, and the same tests were performed. The added S was found to combine with the abundant Mn in the matrix, forming MnS inclusions. These inclusions did not affect the tensile properties but reduced impact energy while significantly improving machinability. However, after aging, κ-carbides formed within the grains and along grain boundaries, resulting in increased yield and tensile strength but decreased impact energy. Notably, machinability considerably deteriorated after aging, likely due to the interference of κ-carbides with dislocation movement. Consequently, the machinability of austenitic lightweight steel can be improved by the addition of S, and it is recommended that the machining process be conducted before aging during product manufacturing, considering the significant reduction in machinability after aging. (Received 5 July, 2024; Accepted 5 August, 2024)

keyword : Machinability, Lightweight steel, Heat-treatment, Microstructure, Inclusions

The mechanical properties and microstructural behaviors of multiple-repaired welds were investigated in the heat affected zone (HAZ) of HY100 steels. Test coupons were prepared from flux cored arc welds at the same location of weldments that had been repaired one, three and five times. The multiple- repaired welding caused continuous high temperature reheating of the HAZ. The effects of multiple-repaired welding to the HAZ on the fine-grained heat affected zone were investigated with respect to hardness, tensile strength and toughness. The peak temperature and cooling rate were obtained with quasi-stationary temperature distribution according to the modified Rosenthal equation. The prior austenite grains (PAG) that had been repeatedly reheated above A_C3 temperature were observed to have recrystallization and grain growth, and the distributed precipitates were identified as M₃C and M₂₃C₆. Martensite-austenite constituents were identified through EBSD and TEM analyses to have a measured fraction of less than 2%. The PAG size (PAGS) of the original weld and the weld repaired one time were ~3 μm, while those that had been repaired three and five times were ~6 μm. The grain size strengthening and precipitation strengthening were calculated to be 294 MPa, 265 MPa, respectively, for the one time repaired and 220 MPa, 297 MPa for the five times repaired, respectively, for a total difference of 42 MPa. The welds repaired five times exhibited reasonable strength and impact toughness with insignificant variation in their microstructural behaviors. (Received 15 October, 2024; Accepted 4 December, 2024)

keyword : Repair welding, FCAW, Heat input, Heat Affected Zone, Prior austenite grain size, Precipitate

IN792, a Ni-based superalloy, exhibits excellent mechanical properties at medium and high temperatures (600~900℃). However, it is known to be very susceptible to casting defects such as hot cracking and hot tearing during casting. This study investigated the effect of alloy elements such as Hf, Zr, and B on the solidification behavior and castability of IN792. Four types of ingots were manufactured by adjusting the contents of Hf (0~1.0wt%), Zr and B, and directional solidification and castability tests were performed. As a result of directional solidification, as the Hf content increased, there was little change in dendritic spacing, which is a measure of segregation behavior, but the mushy zone increased. The volume fraction of residual liquid in the mushy zone was also measured to determine the change rate during solidification. The measurement results showed the 0.8wt%Hf alloy had the largest change rate. It is predicted that rapid changes in the residual liquid during solidification will lead to a higher strain rate. The castability test was quantified using the length and frequency of cracks after casting, to determine the effects and causes of grain boundary strengthening elements and process variables on casting cracks. The results indicated the specimen containing 0.8wt% Hf had the worst castability. Mapping results showed that Zr content around cracks was highest in the 0.8wt% Hf sample. It was concluded that castability is determined by the complex effects of not only Hf but also Zr and B. (Received 28 October, 2024; Accepted 4 December, 2024)

keyword : Superalloy, IN792, Directional solidification, Castability, Hafnium

The precipitation behavior and mechanical properties of high-nitrogen austenitic stainless steels were investigated for various chemical compositions and solution treatment temperatures. For this study, Fe- 22.1Cr-12.2Ni-5.1Mn-2.1Mo alloys with three different sets of Nb, V, and N contents were prepared, and two distinct solution treatment temperatures were applied. An increase in Nb or N content resulted in finer austenite grain size, attributed to the formation of additional precipitates, such as Z-phase and (Nb,V)(C,N), at the grain boundaries. Notably, the effect of a 0.07 wt% increase in Nb content was found to be comparable to more than a 100℃ reduction in solution treatment temperature. The yield strength was observed to increase with decreasing grain size, consistent with the Hall-Petch relationship. When the grain size effect was excluded, and in the absence of Z-phase, a higher N content led to an increase in strength due to solid solution strengthening. On the other hand, an increase in Nb content resulted in reduced strength, even though there was little change in the austenite N content. Thermodynamic analysis suggested that Z-phase precipitation may replace MX particles, which could account for the observed decrease in strength. While Zphase precipitation promotes a finer grain size, coarse Z-phase particles did not contribute effectively to precipitation hardening. Therefore, to achieve higher strength in high-nitrogen austenitic stainless steels, alloy design must be optimized, particularly when Nb addition is considered. (Received 10 October, 2024; Accepted 12 December, 2024)

keyword : High nitrogen austenitic steel, Precipitation, Tensile properties, Niobium, Z-phase

The growing need for sustainable energy development has become a critical issue due to worsening environmental pollution and climate change. Among the various technologies, the photoelectrochemical technology has been recognized as one of the essential approaches for advancing sustainable energy production. Out of the semiconductor materials used in PEC systems, cadmium sulfide (CdS) and cadmium selenide (CdSe) have been widely studied as promising candidates due to their advantageous properties. CdS, with a bandgap of 2.4 eV and high photoactivity, and CdSe, with a narrower bandgap of 1.9 eV and excellent light absorption characteristics, offer complementary advantages. In this study, we synthesized the CdS and CdSe materials via hydrothermal and chemical bath deposition methods, respectively, to fabricate a CdS/CdSe heterojunction photoanode system. The heterojunction CdS/CdSe photoanode formed a type-II structure, which facilitated efficient charge separation and transfer. Moreover, the CdS/CdSe photoanode exhibited high light absorption properties with very low charge transfer resistance, attributed to the role of CdS particles beneath CdSe as an electron transfer layer and the porous structure of the composite material. As a result, the CdS/CdSe photoanode achieved high photocurrent density of 4.51 mA·cm-2 comparing to their individual cases, representing a 78% improvement in PEC performance compared to the only CdSe photoanode case. (Received 7 October, 2024; Accepted 24 October, 2024)

keyword : Cadmium sulfide, Cadmium selenide, Heterojunction, Photoanode, Photoelectrochemical

The development of green hydrogen through anion exchange membrane water electrolysis (AEMWE) is essential for achieving carbon neutrality. Developing non-precious-metal catalysts for the hydrogen evolution reaction (HER) is crucial for the commercialization of AEMWE. In this study, Ni-CeO₂/ C catalysts were synthesized via a co-precipitation method and a reduction heat treatment was conducted from 300 to 500 ℃ to form metallic Ni for the HER. Through this process, CeO₂ nanoparticles were uniformly dispersed around Ni metal nanoparticles. Among these catalysts, Ni-CeO₂/C 400 exhibited a prominent Ni⁰ peak according to an XPS analysis and formed smaller nanoparticles compared to Ni-CeO₂/C 500, yielding advantageous physicochemical properties for the HER. Subsequently, an electrochemical half-cell LSV analysis demonstrated the lowest HER overpotential of 164 mV at 10 mA cm^-2 and a Tafel slope of 89 mV dec-1, suggesting the formation of a trade-off point in the HER performance due to variations in the oxidation state and particle size of the Ni metal. Furthermore, a non-precious-metal-based AEMWE single cell with Ni- CeO₂/C 400 as the cathode and Co₃O₄ as the anode achieved a current density of approximately 700 mA cm^-2 at 2.0 Vcell. It also exhibited stable durability at a constant current of 500 mA cm^-2 for 100 hours, suggesting the potential for long-term hydrogen production in non-precious-metal-based AEMWE systems. (Received 14 November, 2024; Accepted 16 December, 2024)

keyword : Anion exchange membrane water electrolysis, Hydrogen evolution reaction, Hydrogen production, Ni electrocatalyst, Non-precious catalyst

Water splitting is a sustainable method of hydrogen production that is crucial for achieving carbon neutrality. However, the high cost of green hydrogen remains a major challenge. One strategy to reduce costs is the use of non-precious metal catalysts, such as Co, which is known for its excellent catalytic properties. However, the limited supply and increasing cost of Co pose challenges. In this study, we developed a method to significantly reduce Co usage in catalyst-coated substrates for anion exchange membrane water electrolysis. By incorporating a Co solution into the slurry during a printing transfer method with NiFe oxide, we achieved a 72-fold reduction in Co consumption while improving the over potential by 12.9% (380 mV at 100 mA cm^-2). In single-cell anion exchange membrane water electrolysis tests, the catalyst-coated substrate electrode delivered a current density of 900 mA cm^-2 at 1.8 V_cell, outperforming NiFe oxide electrodes (630 mA cm^-2). This strategy demonstrates a broadly applicable approach for reducing the use of expensive elements in green hydrogen production. (Received 12 November, 2024; Accepted 12 December, 2024)

keyword : Green hydrogen production, Water splitting, Catalyst-coated substrates, Anion exchange membrane water electrolysis, Co consumption