https://doi.org/10.3365/KJMM.2026.64.8.691

강규병(Gyu Byeong Kang) ; 조영찬(Young Chan Cho) ; 백민아(Min Ah Baek) ; 하태준(Taejun Ha) ; 김동회(Dong Hoe Kim) ; 나태욱(Taewook Na)

Temperature-dependent phase-selective nitriding was applied to a face-centered cubic (FCC) dual-phase Fe-Mn-Cu-Al alloy to investigate the correlation between microstructural evolution, diffusion kinetics, and surface mechanical performance. The alloy exhibited distinct phase separation into Fe-rich and Cu-rich regions caused by metastable liquid phase separation during solidification, resulting in a heterogeneous chemical distribution and mechanical response. Gas nitriding treatments were conducted at 700, 800, and 900 °C for 1-2 h under an NH3 (9.85%) + N2 balance atmosphere, with controlled H2 addition to regulate the nitrogen potential and suppress excessive surface film formation. Microstructural characterization using optical microscopy, SEM/EDS, XRD, and GDS demonstrated that the nitriding behavior depended strongly on both temperature and phase constitution. Nitrogen preferentially diffused into the Fe-rich domains to form internal nitride precipitates, whereas the Cu-rich regions exhibited limited nitriding due to their lower nitrogen solubility and slower diffusion rates. The nitrided layer thickness increased with temperature and holding time, reaching a maximum depth of approximately 60 μm at 800 °C for 2 h. Micro-Vickers hardness measurements revealed a significant increase in surface hardness from 250 HV in the as-cast condition to 909 HV under the optimal nitriding condition. Pin-on-disc wear testing showed a pronounced reduction in weight loss and wear track width, confirming improved wear resistance. These findings indicate that diffusion-controlled phase-selective nitriding provides an effective strategy for enhancing surface durability while maintaining bulk structural integrity in dual-phase Fe-Mn-Cu-Al alloys.

https://doi.org/10.3365/KJMM.2026.64.8.702

(Won Seon Seo) ; (Sangwoo Kim) ; (Beom Zoo Lee) ; (Ji Hye Jeon) ; (Minzhen Li) ; (Min Young Kim) ; (Jeong Yun Hwang) ; (Young-Min Byoun) ; (Gi Hyeon Han) ; (Changhyun Jin)

The interfacial properties of sub-micrometer spherical silica particles surface-modified with gamma-glycidoxypropyltrimethoxysilane (GPTMS), octadecyltriethoxysilane (OTES), and a mixture of GPTMS and OTES at various ratios were systematically investigated and compared. To suppress the self-aggregation of the silane-based surface modifiers, promote monolayer formation, and enhance particle dispersibility, all samples were dispersed in methyl ethyl ketone (MEK) at a concentration of 1 wt.% and physically agitated using ZrO2 balls for 3 h. Under these processing conditions, neither the GPTMS- nor the OTES-modified silica exhibited noticeable aggregation or dispersion instability. The hydrodynamic particle sizes of the pristine spherical silica, GPTMS-modified silica, and OTES-modified silica were approximately 632, 634, and 821 nm, respectively. This difference can be ascribed to the lower molecular weight of GPTMS compared to OTES, as well as the more efficient bonding behavior of GPTMS on the spherical silica surface. However, among the mixed GPTMS/OTES systems, only the 1:1 ratio (792 nm) exhibited interfacial characteristics intermediate between those of the individually modified samples. Conversely, the 3:1 and 1:3 ratios displayed relatively broader size-distribution profiles. These results suggest that 1:1 hybrid silanization provides a promising strategy for balancing and potentially overcoming the limitations associated with individual surface modifiers.

https://doi.org/10.3365/KJMM.2026.64.8.710

김민기(Minki Kim) ; 박지영(Jiyoung Park) ; 조해주(Haeju Jo) ; (Tomasz Choma) ; (Julia Chmielewska) ; (Christian Leinenbach) ; 이욱진(Wookjin Lee)

Laser directed energy deposition (L-DED) of high-manganese steel effectively suppresses manganese (Mn) macro-segregation due to its rapid solidification characteristics. However, this process inherently induces a severe thermal gradient along the building direction (BD), resulting in coarse, BD-aligned columnar grains and highly anisotropic mechanical properties. To mitigate these microstructural limitations, minor amounts of hafnium (Hf) or cerium (Ce) were micro-alloyed into a base Fe-24Mn-4Cr-0.4C matrix, and their subsequent effects on the microstructure and tensile behavior along both the BD and transverse direction (TD) were systematically investigated. Both Hf and Ce acted as oxygen getters, forming HfO2 and CeO2 precipitates that reversed the Marangoni flow and produced wider, shallower melt pools, thereby weakening the BD thermal gradient. This structural alteration shortened the BD solidification-cell length, increased the fraction of high-angle grain boundaries, and reduced the (001)//BD cube-like texture. Furthermore, CeO2 promoted heterogeneous nucleation more effectively than HfO2 due to its superior lattice matching with gamma-Fe. Sub-boundary refinement consequently increased the BD yield strength while preserving the TD strength, thereby reducing the anisotropy. Although the BD elongation decreased slightly with Ce addition, it dropped significantly with Hf addition, because the low coherency of the HfO2 interface promoted interfacial decohesion and void nucleation. These findings indicate that Hf and Ce micro-alloying refines the solidification structure and balances strength and ductility, with Ce being the more favorable additive.

https://doi.org/10.3365/KJMM.2026.64.8.724

안다영(Dayoung An) ; 박상하(Sangha Park) ; 신세은(Se-Eun Shin)

This study investigated the effects of hot isostatic pressing (HIP) and quenching-tempering (QT) treatments on the microstructure and hardness of gas-atomized D2 tool steel. Increasing the HIP temperature improved densification behavior, resulting in near-full density at 1265 °C. Subsequent QT treatment refined the carbide network structure, reducing the average network thickness from 11.17 μm to 6.46 μm. X-ray diffraction (XRD) analysis revealed a decrease in retained austenite alongside the formation of a martensitic matrix with Cr-rich carbides after QT treatment. Consequently, significant secondary hardening was achieved, yielding the highest Vickers hardness of 778.38 HV for the specimen processed at 1265 °C. These results demonstrate that combining HIP and QT treatments is an effective approach for refining carbide morphology and enhancing the hardness of D2 tool steel.

https://doi.org/10.3365/KJMM.2026.64.8.734

한도현(Dohyeon Han) ; 이주헌(Juheon Lee) ; 한수철(Sucheol Han) ; 이헌묵(Hunmook Yi)

Pyrometallurgical recycling of spent LiNixCoyMn1-x-yO2 (NCM) lithium-ion batteries (LIBs) has attracted considerable attention owing to its high feedstock flexibility and suitability for large-scale processing. Recent studies have reported that carbothermic reduction-based pyrometallurgical processes can induce chemical phase separation through the control of oxygen partial pressure, thereby enabling the selective separation and recovery of valuable components based on differences in their physicochemical properties. However, previous studies have mainly focused on Li recovery, whereas the purification of a Ni-Co from pyrometallurgically recycled NCM, which primarily consists of a Ni-Co alloy, MnO, and LiAlO2, remains insufficiently investigated. In this study, a citric acid-based selective leaching process was proposed for the selective removal of MnO and LiAlO2 to recover high-purity Ni-Co from pyrometallurgically recycled NCM. Citric acid concentration (5, 10, and 20 wt%), leaching time (0.5, 1, and 2 h), and reaction temperature (20-80 °C) were selected as the major process variables. Thermodynamic calculations indicated that MnO and LiAlO2 exhibited a higher driving force for dissolution than Ni, Co, confirming the thermodynamic feasibility of selective leaching. Experimental results showed that MnO achieved a leaching efficiency higher than 95%, while LiAlO2 reached a maximum leaching efficiency of 89.5%. In contrast, the leaching efficiency of the Ni-Co remained below 5% under all experimental conditions. These results indicate that the selective leaching of MnO and LiAlO2 was successfully achieved, leading to the purification of the Ni-Co phase. Under the optimal conditions, a Ni-Co purity of 90.14% and a recovery ratio of 96.75% were achieved. The proposed process demonstrates an environmentally friendly purification route for improving the efficiency and economic viability of spent battery recycling.

https://doi.org/10.3365/KJMM.2026.64.8.744

김윤재(Yunjae Kim) ; (Vasudevan Rathinam) ; 이관형(Gwan Hyeong Lee) ; 박재우(Jaewoo Park) ; 김우재(Woojae Kim) ; 김상일(Sang-il Kim)

Chalcopyrite CuInTe2 has emerged as a promising p-type thermoelectric material due to its intrinsically low thermal conductivity. However, its practical application is fundamentally hindered by an inherently low carrier concentration, leading to a restricted power factor. Herein, we present a synergistic optimization strategy to enhance the low-to-intermediate temperature thermoelectric performance of the Ag pre-doped CuInTe2, Cu0.9Ag0.1InTe2 system by CdTe alloying. A series of (Cu0.9Ag0.1InTe2)1-x(CdTe)2x solid solutions (x = 0.00, 0.05, 0.10, 0.15, and 0.20) were successfully synthesized using a conventional solid-state reaction combined with spark plasma sintering. X-ray diffraction confirms the successful formation of the single-phase (Cu0.9Ag0.1InTe2)1-x(CdTe)2x solid solutions, also reveals the systematic lattice expansion upon alloying. The incorporation of Ag at the Cu sites profoundly optimizes the hole carrier concentration, drastically increasing it to optimal levels (~10^19 cm-3) and thus substantially enhancing the electrical conductivity and power factor within the low-to-intermediate temperature regime (300-500 K). Concurrently, the solid solution alloying introduces severe mass and strain field fluctuations, which dominate point-defect scattering and effectively suppress the lattice thermal conductivity across the entire temperature range. Consequently, the synergistic effect of carrier activation and phonon suppression yields improved thermoelectric figure of merit (zT) at low-to-intermediate temperatures.

https://doi.org/10.3365/KJMM.2026.64.8.753

이승훈(Seung Hun Lee) ; 이성준(Sung Jun Lee) ; 이상훈(Sanghun Lee) ; 이우재(Woo Jae Lee) ; 채문석(Munseok S Chae) ; 김양도(Yangdo Kim) ; 박유세(Yoo Sei Park)

Sn-incorporated Co3O4 electrocatalysts were successfully synthesized through a coprecipitation method followed by thermal annealing for application in the oxygen evolution reaction (OER) under alkaline conditions. The Sn content in the catalysts was systematically tuned by varying the molar ratio of Co and Sn precursors during the synthesis process. Structural characterization using X-ray diffraction analysis confirmed that the intrinsic spinel Co3O4 crystal structure was well preserved after Sn incorporation, without the formation of undesirable secondary phases or impurities. Morphological and compositional analyses further verified the uniform distribution of Sn species within the Co3O4. Among the synthesized catalysts, Co3O4@Sn-3 demonstrated the highest OER performance, exhibiting an overpotential of 380 mV at a current density of 10 mA/cm2 which was lower than that of pristine Co3O4@Sn-0 (400 mV). Arrhenius analysis revealed that the activation energy for the OER decreased after Sn incorporation, indicating that Sn doping effectively reduced the electrochemical reaction barrier. The improved OER activity is attributed to the modulation of the surface electronic structure and the facilitated charge-transfer process induced by Sn incorporation into Co3O4. Furthermore, the enhanced catalytic activity and reduced activation energy barrier highlight the potential of Sn-modified Co3O4 electrocatalysts as efficient and cost-effective OER catalysts for alkaline water electrolysis applications.

https://doi.org/10.3365/KJMM.2026.64.8.760

박다경(Da Kyeong Park) ; 강지원(Ji-Won Kang) ; 박종혁(Jong-Hyeok Park) ; 김지현(Jihyeon Kim) ; 박다희(Dahee Park) ; 이주영(Jooyoung Lee) ; 진송(Song Jin) ; 김양도(Yangdo Kim) ; 김치호(Chiho Kim) ; 최승목(Sung Mook Choi)

Achieving highly efficient anion exchange membrane water electrolysis (AEMWE) requires not only intrinsically active oxygen evolution reaction (OER) catalysts but also electrode architectures that enable efficient charge transport and the full utilization of active sites. Herein, we report a NiCo electrode featuring an Fe-modulated oxyhydroxide layer, fabricated via electrodeposition on Ni foam followed by a simple post-treatment process. This binder-free integrated configuration ensures strong interfacial coupling and efficient electron transport without the need for polymeric binders. Structural analysis reveal a hierarchical architecture composed of a conductive electrodeposited NiCo electrode on Ni foam and a surface-reconstructed oxyhydroxide layer. Fe is predominantly localized at the surface, where it induces an electronic redistribution between the Ni and Co centers, thereby increasing their oxidation states and forming a hydroxyl-rich environment that facilitates the adsorption and transformation of OER intermediates. As a result, the electrode exhibits a low overpotential of 287 mV at 50 mA cm-2 and a Tafel slope of 44.28 mV dec-1, along with a reduced charge transfer resistance and an enhanced electrochemically active surface area. In an AEMWE single cell, it delivers a current density of 2.6 A cm-2 at 2.0 V and maintains stability with only ~1.8% degradation over 100 h at 1 A cm-2. This work demonstrates an effective strategy for coupling catalytic activity with interfacial charge transport in practical AEMWE systems.