https://doi.org/10.3365/KJMM.2026.64.5.387

김선진(S. J. Kim) ; 박지니(J. Park) ; 양하영(H. Y. Yang) ; (Raj Narayan Hajra) ; 김정한(J. H. Kim) ; 김지훈(J. Kim) ; 최정묵(J.M. Choi) ; 박준식(J.S. Park)

The increasing demand for lightweight automotive structures has intensified the need for advanced materials and reliable joining technologies for dissimilar hardware components. In this study, four candidate steels (1021B, 5120B, SCM435, and SWCH45K) were investigated to compare their austenite transformation behavior during heating and their tempering response after quenching. For hypoeutectoid steels, Ac1 and Ac3 measured during heating represent the start and finish temperatures of austenite formation under a given heating schedule, whereas Ae1 and Ae3 obtained by Thermo-Calc correspond to equilibrium temperatures. The transformation temperatures (Ac1 and Ac3) were determined by differential scanning calorimetry (DSC, 3 oC/min) and compared with the equilibrium temperatures (Ae1 and Ae3) predicted by Thermo-Calc. The measured Ac1 values were 736?755 oC and the measured Ac3 values were 781?803 oC, while the calculated Ae1 and Ae3 values were 716?736 oC and 770?810 oC, respectively. The differences between measured and calculated temperatures are discussed in terms of equilibrium versus non-equilibrium transformation, heating rate, initial microstructure, and solute redistribution during austenitization. For heat treatment, all steels were austenitized at 910 oC for 1 h, which is sufficiently above the measured Ac3 and calculated Ae3 of all alloys, and then water quenched and tempered at 200?600 oC. After quenching, all materials exhibited predominantly lath martensitic microstructures. With increasing tempering temperature, hardness gradually decreased whereas impact energy generally increased; however, a distinct toughness trough associated with tempered martensite embrittlement (TME) appeared near 350 oC. Because sub-size Charpy specimens were used, the impact data are interpreted mainly for relative comparison among the steels rather than for direct comparison with standard Charpy values. The present work provides an experimentally validated comparison framework linking Ac1/Ac3, Ae1/Ae3, quenchtempered microstructures, and hardness/impact trends for four industrial automotive hardware steels, and offers practical guidance for selecting heat-treatment windows that avoid the TME regime.

https://doi.org/10.3365/KJMM.2026.64.5.400

백하은(Haeun Baik) ; 박지영(Jiyoung Park) ; 임진수랑(Jinsurang Lim) ; 이욱진(Je In Lee) ; 이제인(Wookjin Lee) ; 김도형(Dohyung Kim)

In this study, an Fe-Mn-Al-Ni?based superelastic alloy was fabricated by laser direct energy deposition, and the effect of Cr on its microstructure and superelastic properties was investigated. The Cr addition up to 7 wt.% changed the dominant solidification phase in the as-built state from a face-centered cubic phase to a body-centered cubic (BCC) phase. The elemental Cr powders were not fully melted in the as-built state due to the high melting point of Cr. After solution treatment at 1200 oC for 6 h, all samples, with and without Cr, consisted of BCC-α and showed no evidence of unmolten Cr powder. Before the cyclic compression test to evaluate superelastic properties, further heat treatment was performed at 200 oC for 3 h to ensure high superelasticity through the precipitation of B2?NiAl. In addition to the changes in phase evolution in the as-built state, the Cr addition also stabilized BCC-α in the heat-treated state, resulting in an increase in the critical stress in the phase transformation from BCC-α to FCT (face-centered tetragonal)- γ' from 202.3 MPa for 0Cr to 233.9 MPa for 7Cr. Additionally, the critical stress for slip is also substantially increased by ~74% (560.6 MPa for 0Cr and 976.9 MPa for 7Cr, respectively). Both are favorable for the superelastic property. The former could promote reverse phase transformation upon unloading due to higher BCC-α stability, the latter effectively hinders permanent deformation induced by slip. As a result, the superelastic displacement increased by ~19%, from 3.567 for 0Cr to 4.257 for 7Cr.

https://doi.org/10.3365/KJMM.2026.64.5.411

정민섭(Min-Seop Jeong) ; 백관우(Kwan-Woo Paek) ; 오동규(Dong-Kyu Oh) ; 황병철(Byoungchul Hwang)

This study clarifies how microstructural features govern hydrogen embrittlement resistance in high-strength API X65, X70, and X80 linepipe steels produced under different chemical compositions and thermo-mechanical control processing (TMCP) conditions. Microstructures were quantitatively characterized by electron backscatter diffraction (EBSD). Hydrogen embrittlement resistance was evaluated by electrochemically hydrogen-charged slow strain-rate testing (SSRT) using round-notched tensile specimens and the results were expressed as the relative notch tensile strength (RNTS). The hydrogen distribution and trapping behavior were examined by silver decoration and a thermal desorption analysis (TDA). The RNTS values were 0.84 (X65), 0.88 (X70), and 0.86 (X80), indicating that X70 steel offered the highest resistance while X80 steel provided comparable resistance despite having the highest strength. Fractography and crosssectional observations indicated that hydrogen increased cleavage-like fracture near the notch root in all steels. The X65 steel additionally showed quasi-cleavage in the central region, consistent with its large grain size. By contrast, hydrogen-assisted crack propagation was suppressed in the X80 steel. This is attributed to its small grain size and a high fraction of high-angle grain boundaries, together with enhanced trapping at pearlite ferrite?cementite interfaces that reduced diffusible hydrogen. Interrupted SSRT results suggested cracks preferentially initiated at martensite?austenite (M/A) constituents. These findings underscore the roles of microstructural refinement and hydrogen trapping in achieving high strength with improved embrittlement resistance.

https://doi.org/10.3365/KJMM.2026.64.5.422

이승희(Seunghee Lee) ; 박형준(Hyeongjun Park) ; 이진우(Jinwoo Lee) ; 정진영(Jin Young Jung) ; 강신곤(Singon Kang)

The effect of low-temperature annealing on strain-age hardening in severely cold-drawn high-carbon (0.92 wt.% C) pearlitic steel wire was systematically investigated, with particular emphasis on the role of cementite crystallinity. Pearlitic wire subjected to severe cold drawing (true strain ε = 2.41) was annealed at 150 oC for 30 min, and the resulting microstructural and mechanical changes were analyzed using transmission electron microscopy (TEM), X-ray diffraction (XRD) of both electrolytically extracted cementite and bulk ferrite, and mechanical testing. Cold drawing markedly refined the interlamellar spacing (ILS) from ~100 nm to ~20 nm and induced strong colony alignment. Subsequent low-temperature annealing did not cause a statistically significant change in ILS or lamellar morphology. This indicates that the annealing-induced strengthening is not associated with further lamellar refinement. TEM and selected area electron diffraction (SAED) analyses revealed extensive amorphization of cementite during cold drawing, followed by localized nanocrystallization of cementite (?10 nm) during annealing. This crystallinity recovery was corroborated by partial recovery of cementite diffraction peaks in XRD patterns obtained from electrolytically extracted cementite powder. XRD analysis of the bulk ferrite showed lattice expansion after cold drawing due to carbon redistribution into the ferrite, followed by partial contraction after annealing. This is consistent with re-partitioning of carbon back to cementite. Meanwhile, peak broadening analysis indicated that the high dislocation density introduced by cold drawing was largely retained after annealing. Mechanically, low-temperature annealing increased tensile strength by ~4% while further reducing tensile elongation. It also deteriorated torsional ductility, accompanied by longitudinal delamination. These results demonstrate that strain-age hardening at 150 oC is governed primarily by nanocrystallization of amorphized cementite and associated carbon re-partitioning, rather than by ferrite solid-solution strengthening, dislocation density changes, or lamellar refinement.

https://doi.org/10.3365/KJMM.2026.64.5.433

김상엽(Sang-Yeob Kim) ; 송오성(Oh-Sung Song)

We investigated interfacial reactions and structural changes occurring in 15 um -diameter gold bonding wires with varying palladium contents (0, 0.05, 0.1, 0.3, and 1.0%) bonded to 5 um-thick aluminum pads and subjected to accelerated high-temperature testing at 175°C for up to 1,000 h. Compared with conventional 1um aluminum pads, the increased aluminum thickness in 5 um pads caused most of the first ball bond area to transform into intermetallic compounds overtime. For gold bonding wires without palladium addition, rapid gold diffusion resulted in deformation of the first ball bond after 250 h at 175°C . After 1,000 h, all gold at the top of the first ball bond was consumed, forming a thin neck and leading to crack formation. As the palladium content increased, a Pd-rich barrier formed between the first ball bond region and the IMC layer, slowing the diffusion of gold into the aluminum pad. This helped preserve the gold at the top of the first ball bond and maintained a stable bonding interface even after 1,000 h. However, in the case of gold bonding wires containing 1.0% palladium, after 250 h, the Pd-rich barrier further slowed gold diffusion while accelerating aluminum diffusion, resulting in the formation of large voids in the aluminum pad area. Therefore, when applying 15 um fine gold bonding wires to relatively thick 5 um aluminum pads, the addition of an appropriate palladium content (approximately 0.05?0.3%) was found to be advantageous for ensuring high-temperature reliability.

https://doi.org/10.3365/KJMM.2026.64.5.439

정연대(Yeon Dae Jeong) ; 강윤배(Youn-Bae Kang) ; 신동엽(Dongyeop Shin) ; 장정목(Jungmock Jang) ; 윤상현(Sanghyeon Yoon) ; 최현수(Hyeon Soo Choi)

In this study, experiments were conducted using a water model to compare the mixing characteristics within a converter bath according to the number and arrangement of bottom-blowing nozzles (8 and 10 nozzles, linear and zig-zag patterns, etc.). Various nozzle configurations were applied to an acrylic converter model scaled to one-fifth of a 300-ton industrial converter, and the homogeneous mixing time was measured through electrical conductivity variation. As a result, when the same eight nozzles were used, both pattern B (wider nozzle distribution) and pattern F (zig-zag arrangement) exhibited improved mixing efficiency and reduced homogeneous mixing time compared to the conventional pattern A. In contrast, in the case of ten nozzles, only pattern D where both nozzle area and arrangement were optimized showed a notable reduction in mixing time. Additionally, evaluation under partially clogged nozzle conditions revealed that the homogeneous mixing time increased proportionally with the number of blocked nozzles. In conclusion, optimization of the bottom-blowing nozzle arrangement effectively enhanced the mixing characteristicsof the converter bath, thereby maximizing the metallurgical efficiency of top-and-bottom blown converter operations.

https://doi.org/10.3365/KJMM.2026.64.5.448

엄유준(Yoojun Eom) ; 이창우(Changwoo Lee) ; 윤여현(Yeohyun Yoon) ; 서준교(Junkyo Seo) ; 허민수(Minsu Heo) ; 신원호(Weon Ho Shin) ; 김현식(Hyun-Sik Kim)

This study investigates the thermoelectric properties of W-doped p-type Ge0.9Sb0.1Te thermoelectric materials using the Single Parabolic Band (SPB) model and conducts an analysis aimed at optimizing performance at room temperature. GeTe, a type IV-VI chalcogenide semiconductor, has gained attention as a mid-temperature (600 ~ 900 K) thermoelectric material. GeTe-based alloys are particularly utilized as ptype thermoelectric materials at high temperatures. However, pure GeTe contains a high concentration of Ge vacancies, leading to a high hole concentration, a low Seebeck coefficient, and high thermal conductivity, which inherently limit its thermoelectric performance. In this study, the density-of-states effective mass (md *), non-degenerate mobility (μ0), weighted mobility (μW), and B-factor were calculated for Ge0.9-xWxSb0.1Te (x = 0.01-0.05) composites with varying W doping levels (x). Based on the calculations, the theoretical maximum zT and the optimal carrier concentration were derived. Experimental results confirmed that the optimal W doping concentration for maximizing thermoelectric performance at 325 K is x = 0.05. At this composition, due to band convergence, md * reached its peak. Given that the suppression of μ0 was less pronounced relative to the increase in md *, a maximum μW value was obtained. Additionally, the lattice thermal conductivity (κlat) was minimized, and the B-factor reached its peak. The theoretical maximum zT at this composition was approximately 0.91 when nH was optimized to 3.29 × 1019 cm-3, representing a 106.8% increase compared to the pristine phase (Ge0.9Sb0.1Te, ~0.44) and a 295.7% increase compared to the x = 0.05 composition reported by Bayikadi et al. (Ge0.85W0.05Sb0.1Te, ~0.23). And this suggests that this composition has the potential to be utilized not only for conventional high-temperature power generation but also for active cooling applications.

https://doi.org/10.3365/KJMM.2026.64.5.457

배고운(Go-Woon Bae)

This study proposes a classification for the corrosion characteristics of cast ternary (Cu-Sn-Pb) bronze artifacts excavated from the same archaeological site, based on the structural formation of corrosion layers and the types of corrosion products present in each layer. Five bronze samples were selected for analysis, and the morphological characteristics of the corrosion layer structures and corrosion products were examined. The types and chemical compositions of corrosion products in each layer were identified, and the structural formation of corrosion layers were classified into two main types. The results showed that the excavated bronze artifacts exhibited high tin content corrosion layers caused by decuprification. The migration of Cu and Pb ions, along with environmental factors such as O2, H2O,CO2, and Cl-ions introduced from the burial environment, led to the formation of corrosion products with diverse colors, morphologies, and chemical properties. In particular, the high tin corrosion layer and malachite were found to significantly influence the degree and rate of corrosion. Through this comprehensive investigation of the corrosion layer structures and formation processes of cast Cu-Sn-Pb bronze artifacts, the unique corrosion characteristics of excavated bronzes were identified. The findings of this study are expected to provide fundamental data for understanding the corrosion mechanisms of excavated bronze artifacts and serve as a valuable reference for evaluating bronze artifact authenticity.

https://doi.org/10.3365/KJMM.2026.64.5.469

이병권(Byeong-Kwon Lee) ; 고은찬(Eun-chan Ko) ; 김용호(Yong-Ho Kim) ; 유효상(Hyo-Sang Yoo) ; 손현택(Hyeon-Taek Son) ; 김태훈(Tae-Hoon Kim)

In dual-phase Mg-Li alloy systems, achieving a balance between high strength and superior ductility remains a critical challenge due to the inherent trade-off governed by the α-Mg and β-Li phases. While aluminum (Al) is an effective strengthener, its addition typically stabilizes the hard α-Mg phase, leading to a significant reduction in the ductile β-Li phase and consequent loss of formability. This study proposes a novel alloying strategy utilizing bismuth (Bi) to mitigate Al-induced ductility loss through thermodynamic phase control. The effects of combined Al (4, 6, 8 wt.%) and Bi (2 wt.%) additions on the microstructure and mechanical properties of Mg-Li alloys were systematically investigated. The results demonstrate that Bi exhibits a strong chemical affinity for Mg, preferentially forming thermally stable Mg3Bi2 precipitates. Critically, this reaction preferentially consumes Mg atoms from the matrix, thereby shifting the thermodynamic equilibrium toward the Li-rich side and restoring the volume fraction of the ductile β-Li phase. Consequently, the Bi-added alloys exhibited a simultaneous improvement in yield strength, ultimate tensile strength, and elongation compared to their Bi-free counterparts. Specifically, the optimized Mg-8Li-6Al-2Bi alloy achieved a yield strength of 205.52MPa, an ultimate tensile strength of 250.2MPa, and an elongation of 16.75%, demonstrating an exceptional strength-ductility balance. These findings suggest that modulating the phase constitution via element-specific reactivity offers a promising pathway for designing highperformance ultralight Mg alloys.