https://doi.org/10.3365/KJMM.2026.64.10.859

정승문(Seungmun Jung) ; 서주원(Joowon Suh) ; 정원종(Wonjong Jeong) ; 이창규(Chang-Kyu Rhee) ; 류호진(Hojin Ryu) ; 강석훈(Suk Hoon Kang)

In this study, friction welding was successfully applied to fabricate dissimilar joints between 12Cr ferritic-martensitic oxide dispersion strengthened (12Cr FM-ODS) steel and commercial 9Cr ferritic-martensitic steel (9Cr FMS). The resulting microstructural evolution, crystallographic texture, and mechanical properties were systematically investigated in both the as-welded state and after post-weld heat treatment (PWHT) at 750 °C for up to 100 h. Microstructural observations revealed that the joint region was clearly divided into a dynamic recrystallization zone (DRZ), a thermo-mechanically affected zone (TMAZ), and the respective base metals. The DRZ exhibited a refined microstructure composed of ultrafine equiaxed grains with a high fraction of high-angle grain boundaries (>15°), induced by severe plastic deformation and dynamic recrystallization during welding. Electron backscatter diffraction (EBSD) analysis demonstrated that a characteristic shear deformation texture (<111>//TD and <110>//SPN) developed in the weld zone, overriding the initial extrusion texture of the ODS base metal. During prolonged PWHT, the 12Cr FM-ODS steel side exhibited outstanding microstructural stability with negligible grain growth, owing to the effective Zener pinning effect of finely dispersed Y2O3 nanoparticles. Conversely, the heat-affected zone on the 9Cr FMS side experienced significant grain coarsening and a phase transformation from martensite to ferrite, which caused a marked reduction in local hardness by approximately 70% after 100 h. Despite this localized softening, uniaxial tensile tests performed at room temperature, 400 °C, and 500 °C demonstrated that all specimens consistently fractured in the 9Cr FMS base metal, well away from the weld interface. These findings demonstrate the structural integrity and excellent thermal stability of the dissimilar joint, confirming that friction welding is a highly promising solid-state joining technique for ODS steels in advanced nuclear structural applications.

https://doi.org/10.3365/KJMM.2026.64.10.867

김현(Hyun Kim) ; 최효남(Hyo-Nam Choi) ; 신재혁(Jae-Hyuck Shin) ; 최정원(Jeong-Won Choi) ; 이승준(Seung-Joon Lee)

In the present study, we aimed to analyze the microstructural characteristics and joint efficiency of an AA7068-T6 alloy after friction-stir welding (FSW) at welding speeds from 150 to 350 mm/min at a tool rotation speed of 600 rpm under an Ar gas atmosphere. In the stir zone during FSW, the peak temperature decreased, whereas both the effective strain rate and the Zener-Hollomon parameter increased with increasing welding speed. Continuous dynamic recrystallization occurred actively regardless of the welding speed, leading to fine, equiaxed grains. Regarding the texture evolution of the stir zone, the welds exhibited the development of a recrystallization texture (e.g., Goss). Consequently, the weld produced at 250 mm/min exhibited superior weldability, achieving a joint efficiency of ~80%.

https://doi.org/10.3365/KJMM.2026.64.10.882

최광수(Kwangsu Choi) ; 양현석(Hyunseok Yang) ; 정우철(Woo-Chul Jung) ; 김성웅(Seong-Woong Kim) ; 김채원(Chae-won Kim) ; 공만식(Man-Sik Kong)

In this study, the correlation between the behavior of individual additive elements and spiral casting fluidity in TiAl alloy systems was systematically investigated using a gravity precision casting process. The content and type adjustments of substitutional elements (Al, Nb, W, Cr) were evaluated to elucidate their specific effects on liquidus temperature variations, the thermal interval controls of two-phase regions and compositional supercooling induced by solute rejection. These thermodynamic parameters were identified as the primary factors that collectively dictate melt fluidity flow resistance and ultimate mold filling capacity by modifying the primary solidification pathways and phase stability. Furthermore, the precipitation behavior of secondary phases and solid solution effect of interstitial and ternary elements (B, Si and C) during high-temperature solidification were quantitatively analyzed, revealing their distinct roles in altering dendritic growth kinetics and liquid-solid interfacial mobility. Based on these thermodynamic solidification pathway control mechanisms, a novel multi-component Ti-Al-Nb-W-Si-C alloy was designed, exhibiting a significantly refined solidification microstructure and superior casting fluidity compared to the commercial 4822 alloy. Additionally, the homogenized Ti-Al-Nb-W-Si-C alloy exhibited a yield strength of 313 MPa at 1000 °C, ensuring exceptional high-temperature strength retention. The present work demonstrates the feasibility of alloy design that simultaneously satisfies both the castability and high-temperature mechanical properties required for advanced high-temperature structural materials.

https://doi.org/10.3365/KJMM.2026.64.10.896

(Jiwoo Yoon) ; (Sangbaek Park)

Metal-organic frameworks (MOFs) are promising hosts for semi-solid-state electrolytes (SSSEs) because their ordered micropores can confine liquid electrolyte while providing regulated pathways for Li-ion transport. However, producing such MOFs uniformly and with high crystallinity in large quantities remains challenging, particularly through aqueous routes that avoid the solvent-handling burden associated with organic synthesis. Herein, we report that the crystal quality of zeolitic imidazolate framework-8 (ZIF-8) in 100 g-scale aqueous synthesis is governed by the nucleation rate: a long precursor feeding time slows nucleation and produces heterogeneous particles with poorly developed crystallinity, whereas a short feeding time consistently promotes rapid nucleation and uniform, well-defined polyhedral particles. Similarly, a Cu/Zn bimetallic MOF yields uniformly faceted crystals, indicating that this nucleation-growth criterion is transferable across compositions. The resulting ZIF-8 and Cu/Zn MOFs successfully retain their microporous structures with surface areas comparable to those of laboratory-scale ZIF-based MOFs, and remain intact while effectively hosting electrolyte within their pores after separator fabrication. These separators exhibit ionic conductivities typical of MOF-based SSSEs, a wider anodic stability window than a commercial separator with liquid electrolyte, stable long-term Li plating/stripping, and high capacity retention in Li || LiFePO4 (LFP) cells. These results demonstrate that the nucleation rate is key to maintaining MOF quality during large-scale aqueous synthesis without sacrificing the characteristics required for SSSE applications

https://doi.org/10.3365/KJMM.2026.64.10.906

하성훈(Sung-Hun Ha) ; 김준혁(Jun-Hyeok Kim) ; 김종만(Jong-Man Kim)

In this work, we present a highly conductive and superhydrophobic silver nanowire (AgNW)/polydimethylsiloxane (PDMS)-coated fabric mesh (APFM). The proposed APFM is readily fabricated by sequentially coating a woven fabric mesh with AgNWs followed by a thin PDMS overcoat via a simple, reproducible process. The high-density AgNW percolation network formed on the fabric mesh provides high electrical conductivity, achieving a minimum sheet resistance of 1.2 ± 0.07 Ω sq?¹, while imparting nanoscale roughness to the mesh surface. Combined with the inherent microscale texture of the woven mesh and the low surface energy of the PDMS overlayer, this hierarchical surface morphology renders the APFM superhydrophobic, as evidenced by a high static contact angle of 154 ± 2.2°, a low contact angle hysteresis of 6.1 ± 0.1°, excellent water repellency, and a low sliding angle of <10°. The APFM was successfully implemented as a high-performance electrothermal heater, exhibiting a linear relationship between the applied voltage and the resulting steady-state temperature. Notably, it achieves a maximum average steady-state temperature of ~70.1 °C at an input voltage of only 1.2 V, demonstrating efficient low-voltage operation. Owing to its low operating voltage and robust superhydrophobicity, the APFM heater offers a promising waterproof heating solution for a wide range of practical applications, particularly in humid or wet environments.

https://doi.org/10.3365/KJMM.2026.64.10.913

김도윤(Do-Yoon Kim) ; 최윤수(Yunsu Choi) ; 최지민(Jimin Choi) ; 권용우(Yongwoo Kwon)

The phase-field method (PFM) provides highly accurate microstructure simulations but suffers from substantial computational costs that increase rapidly with system size, dimensionality, and physical complexity. To overcome this limitation, generative artificial intelligence can be utilized as an ultra-fast surrogate model for microstructure generation. In this study, we constructed a Wasserstein generative adversarial network with gradient penalty (WGAN-GP) trained on two-dimensional (2D) PFM grain growth data. The trained generator bypasses conventional sequential time integration and directly produces microstructures at desired timesteps, achieving a computational speedup of several orders of magnitude compared with the original PFM. To improve physical consistency, a spatial residual term derived from the Allen-Cahn equation was incorporated into the generator loss as a physics-inspired regularization term. During the early stage of training, the resulting physics-informed WGAN-GP (PI-WGAN) exhibited approximately 50% fewer dangling grain boundaries than the baseline WGAN-GP, indicating more rapid establishment of continuous and physically consistent grain boundary networks. PI-WGAN also suppressed anomalous disconnected boundaries throughout training, while the difference between the two models gradually decreased as training progressed. Furthermore, PI-WGAN reproduced the mean and median grain counts of the PFM data with mean relative errors of only 0.8?1.3%. The incorporation of physics-inspired regularization required only negligible additional computational overhead during training. These results demonstrate that a simple physics-based constraint can improve the structural consistency of generated microstructures, particularly during the early stages of learning. This 2D study therefore provides a proof of concept for the use of physics-informed generative models as computationally efficient microstructure generators and establishes a basis for their future extension to large-scale, three-dimensional (3D), and more complex phase-field problems.

https://doi.org/10.3365/KJMM.2026.64.10.928

김영대(Youngdae Kim) ; 송인환(Inhwan Song) ; 정진성(Jinesung Jung) ; 김윤진(Yunjin Kim) ; 강병연(Byeongyeon Kang)

Recently, the operating pattern of industrial gas turbines has increasingly shifted from conventional base-load operation toward cyclic start-stop operation due to the growing penetration of renewable energy into the power grid. Under such flexible operating conditions, localized damage modes associated with repeated thermal and mechanical loading may become increasingly important in addition to conventional creep-dominant degradation. In this study, the degradation and damage characteristics of service-exposed first-stage blades from an F-class gas turbine, manufactured from a single-crystal Ni-based superalloy, were investigated. The blades experienced approximately 17,462 equivalent operating hours (EOH) and 851 equivalent starts (ES). Visual inspection, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), coating characterization, microstructural analysis, and stress-rupture testing were conducted to evaluate the service-induced degradation. Multiple radial and tangential cracks were observed in the fillet region, whereas coating delamination was mainly found near the trailing-edge regions at the airfoil tip and bottom. SEM-EDS mapping revealed Al-rich oxide layers near the crack surface and Cr- and Ti-rich oxides inside the cracks, indicating that internal oxidation occurred during crack development under cyclic start-stop operating conditions. In contrast, severe creep-dominant degradation such as extensive γ' rafting and creep void formation was not observed in the fillet crack region. The thermal barrier coating (TBC) system remained relatively stable at the examined locations, with TGO thicknesses below approximately 3.5 μm. The Larson-Miller parameter (LMP)-based stress-rupture evaluation indicated that the blades retained substantial creep-rupture resistance without evidence of severe creep-dominant degradation. In contrast, localized cracking accompanied by internal oxidation was prominently observed in the fillet region under ES-dominant cyclic start-stop operation. These results suggest that integrity assessment of service-exposed gas turbine blades operated under flexible operating conditions should consider localized crack-related damage in addition to conventional creep-dominant degradation.