https://doi.org/10.3365/KJMM.2026.64.6.481

(Kyu-Sik Kim) ; (Ui-Jong Lee) ; (Jung-Yeul Yun) ; (Kee-Ahn Lee)

High temperature compressive deformation and energy-absorption behaviors of Ni-Fe-Cr-Al superalloy foams fabricated by powder alloying method were investigated in this study. Moreover, the effect of pore sizes (~580 μm and ~800 μm) and temperatures on the mechanical properties of Ni-based superalloy foams were also discussed. The alloying powders were sprayed to the commercial pure Ni foams, and then a sintering process was applied to fabricate homogeneous Ni-22.4%Fe-22.0%Cr-6.0%Al (in wt.%) alloy foams. The precision control of chemical composition was possible using the amount of sprayed powders. The microstructure and phase analyses of Ni-based superalloy foams were conducted by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersion spectroscopy (EDS). The results showed that superalloy foams consisted of common γ, γ’, and β-NiAl intermetallic phases with regardless of initial average pore sizes. Compressive deformation behavior of Ni-Fe-Cr-Al foams was showed typical compressive flow curves of plastically deformable metallic foams. Plateau strengths of Ni-based alloy foams with different average pore sizes were 2.98 MPa for the 800 foam and 4.38 MPa for the 580 foam at room temperature. Plateau strengths were maintained up to 873 K, however, continuously decreased with increasing temperature. Ni-Fe-Cr-Al superalloy foams showed superior energy absorption properties at room temperature compared with other Ni-based alloy foams. It is also noteworthy that quantities of the absorbed energy were almost maintained up to 873 K because of the existence of thermally stable β-NiAl and abnormal strengthening phenomena of γ’ phases. And thus, the combination of plastically deformable γ phase as matrix and the proper amount of strengthening phases (γ’, β-NiAl) in the Ni-Fe-Cr-Al superalloy foams exhibits superior energy absorption properties from room to high temperature.

https://doi.org/10.3365/KJMM.2026.64.6.493

나세빈(Se-Bin Na) ; 이상화(Sang-Hwa Lee) ; 정재길(Jae-Gil Jung) ; 장희진(HeeJin Jang)

The effect of ball milling time on the microstructure and corrosion behavior of the high-entropy FeCrMnNiCo alloy was investigated in a 0.01 M HCl aqueous solution. Alloy powders were prepared by varying the milling durations for 6, 12, 24, and 48 h, followed by spark plasma sintering at 1,000 °C and 80 MPa. Xray diffraction analysis of the sintered specimens confirmed the presence of both FCC and Cr-rich phases in all samples, with negligible phase variation observed as a function of milling time. SEM-EDS analysis revealed that increasing the milling time led to a reduction in the size and Cr concentration of the Cr-rich secondary phases. Conversely, the Cr content within the alloy matrix exhibited a corresponding increase. Potentiodynamic polarization tests demonstrated that both the corrosion potential and pitting potential increased with longer milling times. Pitting corrosion was generally observed to initiate at the phase boundaries between the Cr-rich secondary phase and the matrix. The 6 h specimen exhibited the lowest resistance to pitting, which was attributed to the significant Cr concentration gradient between the Cr-rich phase and the matrix, inducing a local galvanic effect. Consequently, these findings suggest that the FeCrMnNiCo alloy can attain a stable microstructure and superior corrosion resistance when subjected to milling for 12 h or longer.

https://doi.org/10.3365/KJMM.2026.64.6.504

이준섭(Jun-Seob Lee) ; 주티앤유앤(Tian-Yuan Zhu) ; 이정미(Jeongmi Lee) ; 김성윤(Seong-Yoon Kim)

In this study, the localised corrosion behaviour of the type 316L stainless steel manufactured by powder bed fusion (PBF) and heat-treated at 900°C for 2 h was examined in 3.5 wt% NaCl solution by cyclic potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). Three-dimensional defects were observed on both the building-direction (BD) and transverse-direction (TD) planes. Microstructural observations revealed melt-pool features and boundaries, whereas sub-cellular solidification features were not clearly distinguished. Electron backscatter diffraction (EBSD) indicated orientation partitioning by melt-pool boundaries, with local misorientation concentrated near boundaries and defect sites. Energy-dispersive X-ray spectroscopy (EDS) mapping results confirmed the absence of macroscopic Cr?Mo segregation, and the matrix was identified as single-phase face-centred cubic without detectable σ-phase or δ-ferrite phase. The polarisation response was characterised by a gradual and irregular rise in anodic current during anodic scanning, rather than a stable passive-current region. SEM observations after polarisation confirmed localised corrosion around and inside defects. The Bode plot contained both capacitive and low-frequency inductive components, and the impedance behaviour was similar irrespective of the specimen build direction. These results indicate that occluded reactions associated with three-dimensional defects can dominate the measured electrochemical response in PBF components and can govern the apparent localised corrosion resistance. The key contribution of this study is the direct linkage between exposed three-dimensional defects and localised corrosion features identified by electrochemical testing and post-test surface observation.

https://doi.org/10.3365/KJMM.2026.64.6.512

(Arun Lalachan) ; (Savyasachi Nellikode) ; (Jae-Deuk Kim) ; (Jeong-Hwan Jo) ; (Doo-Young Kim) ; (Ho-Young Kong) ; (Woo-Young Chung) ; (Chang-Wook Ji) ; (Joo-Yong Cheon) ; (Yeong-Do Park)

Advanced high-strength steels (AHSS) are extensively employed in automotive structural components due to their superior mechanical properties, enabling weight reduction and enhanced safety. Among joining techniques, arc-plug welding (APW), a variant of gas metal arc welding (GMAW), presents a viable single-sided welding method suitable for complex assemblies where conventional methods such as resistance spot welding (RSW) face accessibility limitations. However, systematic investigations on the influence of welding consumables on the microstructural evolution and mechanical performance of APWs in AHSS remain limited. This study evaluates the effect of two welding consumables?ER70S-6 and ER110S-G?on the microstructure and mechanical behavior of APWs performed on 1.0 mm thick, 1180 MPa grade AHSS sheets. The APW process involved a 6 mm diameter plug hole with controlled welding parameters optimized individually for each consumable. Microstructural characterization of the weld zone (WZ) and coarse-grain heat-affected zone (CGHAZ) was conducted using optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Mechanical performance was assessed through Vickers microhardness profiling, tensile shear testing (TST), and cross-tension testing (CTT). Results reveal that ER70S-6 welds exhibit a microstructure dominated by grain boundary ferrite (GBF) and acicular ferrite (AF), which impart higher ductility but lower hardness (~300 HV) in the WZ. Conversely, ER110S-G welds display a bainitic microstructure characterized by lath and granular bainite (LB and GB), resulting in increased hardness (~450 HV) and higher strength, albeit with reduced toughness. Mechanical testing demonstrated that ER110S-G welds achieved marginally higher tensile shear strengths compared to ER70S-6 welds. Failure analysis indicated a shift in fracture mode from weld zone pullout in ER70S-6 welds to CGHAZ fracture in ER110S-G welds, reflecting the microstructural and hardness variations. Cross-tension tests further corroborated these observations, showing ductile failure behavior for ER70S-6 and brittle fracture tendencies for ER110S-G.

https://doi.org/10.3365/KJMM.2026.64.6.531

(Junseo Gu) ; (Jeonghoon Oh) ; (Kwanlae Kim)

Polyvinylidene fluoride (PVDF)-based piezoelectric nanogenerators are promising for mechanical energy harvesting but are often limited by their low output performance and complex fabrication processes. In this study, a novel hybrid nanogenerator (HNG) was fabricated from a monolithic volumetric PVDF nanofiber (NF) laminate that synergistically integrated piezoelectric and dual-mode triboelectric effects. Volumetric structures of large- and small-diameter NFs were created by alternately electrospinning PVDF solutions of varying concentrations. This unique architecture simultaneously harnesses three mechanisms: (i) the intrinsic piezoelectricity of PVDF, (ii) structural contact electrification driven by strain-induced bond scission at the fiber interfaces owing to topographical asymmetry, and (iii) phase-induced contact electrification originating from a disparity in the crystalline phase of the PVDF. The resulting HNG demonstrated dramatically enhanced output performance under mechanical preload conditions, far exceeding that of single-component PVDF NF mats. The performance scaled proportionally with the number of layers, as confirmed by fast Fourier transform analysis, which revealed a progressively dominant triboelectric contribution. Furthermore, the laminate structure significantly outperformed its single-component counterparts in the contact-separation mode, validating the versatility of this design for energy-harvesting applications. By harnessing the synergistic effects, a substantial enhancement in electrical output is achieved in both mechanical preload and contact?separation modes, eliminating the need for dissimilar materials and fillers.

https://doi.org/10.3365/KJMM.2026.64.6.543

(Hwanhui Lee) ; (Kunok Chang)

Understanding the microstructural evolution of β-Nb rich precipitates is essential for controlling the corrosion resistance and mechanical strength of zirconium?niobium (Zr?Nb) alloys. In this study, the microstructural evolution of β-Nb rich precipitates in Zr?Nb alloys was simulated using the phase-field method to investigate the time-dependent evolution of β-Nb rich precipitates. Furthermore, the effect of interfacial coherency between the α-Zr matrix and β-Nb rich precipitates was examined by classifying the interface as semi-coherent, coherent, or incoherent. These interfacial coherency conditions were modeled using the phase-field method by varying the interfacial energy and their effects were quantified by tracking the area and number of β-Nb rich precipitates as a function of time. In addition to considering interfacial coherency, simulations were performed with Nb concentrations of 1.0, 1.25, and 1.5 mol% to examine compositional effects. The results show that, under the semi-coherent interfacial condition, both the number and total area of β-Nb rich precipitates were higher than the values for the coherent and incoherent interfacial conditions. The increases in both the number and total area of β- Nb rich precipitates became more pronounced as the Nb concentration increased. These results provide a robust foundation for future studies that extend the framework to larger computational domains and more sophisticated interface descriptions.

https://doi.org/10.3365/KJMM.2026.64.6.554

이민호(Min-Ho Lee) ; 손경식(Kyung-Sik Son) ; 이재승(Jae-Seung Lee) ; 김진우(Jin-Woo Kim) ; 안종필(Jong-Pil Ahn) ; 소성민(Sung-Min So) ; 이희수(Hee-soo Lee)

In this study, the effects of the chemical form of sintering additives on the sintering behavior and mechanical properties of Si3N4 ceramics were investigated by comparing an AY composition containing oxide-based sintering additives (Y2O3 and Al2O3) with an ANYN composition containing nitrate-based precursors (Al(NO3)3·9H2O and Y(NO3)3·6H2O). The nitrate-based composition was designed based on the assumption that the precursors would be converted into fine oxide phases during heat treatment, thereby promoting a more homogeneous dispersion of the sintering additives. After fabrication of the sintered specimens, phase evolution, relative density, flexural strength, Vickers hardness, and fracture toughness were evaluated. In addition, the microstructure and crack propagation behavior around the indentation impressions were examined to clarify the mechanisms responsible for the resulting mechanical properties. The results showed that the ANYN composition exhibited higher flexural strength than the AY composition, and additional flexural tests confirmed a consistent trend of strength improvement. The Vickers hardness values of the AY and ANYN compositions were comparable, at 18.3 ± 0.8 GPa and 18.8 ± 0.4 GPa, respectively. However, the fracture toughness slightly increased from 5.48 ± 0.08 MPa·m¹/² for the AY composition to 5.70 ± 0.14 MPa·m¹/² for the ANYN composition. Microstructural observations revealed that the ANYN composition exhibited a relatively finer and more homogeneous microstructure, while crack deflection and branching of indentation-induced cracks were more frequently observed. These results suggest that the use of nitrate-based precursors is beneficial for improving the dispersion uniformity of sintering additives and controlling the microstructure of Si3N4 ceramics. Consequently, the nitrate-based precursor approach may serve as an effective strategy for designing sintering additives to enhance the strength and fracture resistance of Si3N4 ceramics.

https://doi.org/10.3365/KJMM.2026.64.6.564

김경택(Kyeng-Taek Kim) ; 김성제(Seong-Je Kim) ; 강민혁(Min-Hyeok Kang) ; 이상봉(Sangbong Yi)

With the rapid advancement of information technology, the use of advanced electronics comprised of high density integrated circuits has become indispensable in various application fields, such as unmanned aerial vehicles and autonomous driving. Mutual interference among internal components can reduce electromagnetic compatibility and degrade signal quality. Thus, lightweight materials with excellent electromagnetic shielding properties and durability are required, particularly, for application into electronic devices. In this study, we have investigated the improvement of electromagnetic shielding effectiveness of lightweight magnesium alloys via aging treatments. Formation and growth of secondary phases, were analyzed during the aging treatment of AZ61 magnesium alloy sheets at 200 °C for 1 h, 35 h, 184 h, 250 h, and 300 h. The correlation among microstructural evolution, electrical conductivity, and electromagnetic shielding effectiveness was examined. EBSD, XRD, and EDX analyses revealed that the amount of Mg17Al12 precipitates gradually increased with aging time. In the range of low-frequency, the as-rolled specimen with high dislocation density and deformation energy exhibits the highest shielding effectiveness due to enhanced scattering of incident electromagnetic wave. In contrast, in the high-frequency region, the specimens aged for 250 h and 300 h showed superior shielding performance as the increased amount and size of precipitates and accompanying interfaces to matrix promotes electromagnetic wave reflection. The electrical conductivity decreases after the solution treatment attributed to lattice distortion by solute atoms. It increases again as the amount of the solute atoms decreases with formation of precipitates. The AZ61 specimen aged for 250 h exhibited the highest shielding effectiveness due to the balanced combination of homogeneously distributed precipitates and electrical conductivity. This study demonstrates that the electromagnetic shielding behavior of magnesium alloys is strongly dependent on microstructural evolution during aging, confirming that the AZ61 alloy has high potential as a lightweight and high-performance electromagnetic shielding material.

https://doi.org/10.3365/KJMM.2026.64.6.574

박재형(Jaehyung Park) ; 조진우(Jinwoo Cho) ; 허성준(Seungjun Heo) ; 안효주(Hyoju Ahn) ; 박노근(Nokeun Park)

The automated analysis of metallic microstructures significantly enhances the accuracy and efficiency of lamellar spacing measurement in Ti-6Al-4V compared to traditional manual methods. This study introduces an automated approach that integrates image preprocessing, scale bar detection, and systematic algorithms to ensure consistent and reliable measurements across varying magnifications. By reducing subjectivity and minimizing potential errors inherent in manual measurements, the proposed method achieves high precision and reproducibility. To ensure that this accuracy is maintained when analyzing multiple microstructure images, the proposed approach enables efficient processing while preserving measurement reliability. Image preprocessing techniques, such as noise reduction and contrast enhancement, play a crucial role in extracting clear lamellar patterns, while precise scale bar detection provides accurate dimensional calibration by converting pixel-based measurements into real-world units. Building on these preprocessing and calibration steps, systematic algorithms further streamline the measurement process by automatically identifying and quantifying lamellar spacing with minimal user input. These algorithms are optimized to detect subtle microstructural patterns, thereby supporting detailed quantitative analysis for advanced material research and quality control applications. Overall, by combining accurate preprocessing, reliable calibration, and automated measurement, this approach effectively overcomes the limitations of conventional manual methods. In addition to improved measurement accuracy and reproducibility, the method offers practical advantages in terms of scalability and time efficiency. Its broad applicability positions it as a valuable tool for both research and industrial applications, advancing the quantitative analysis of metallic microstructures.