Cryogenic-to-ambient evolution of hetero-deformation-induced strengthening in dual-phase medium-entropy alloys

Ha, H.*; Kim, J.*; Gu, G. H.*; Harjo, S.; Gong, W.; Hong, S.-J.*; Kim, H. S.*

Materials Characterization, 234, p.116086_1 - 116086_12, 2026/04

https://doi.org/10.1016/j.matchar.2026.116086

Dual-phase (DP) high- and medium-entropy alloys (H/MEAs), consisting of face-centered cubic (FCC) and body-centered cubic (BCC) phases, have attracted attention because of their excellent strength-ductility balance. However, the temperature dependence of hetero-deformation-induced (HDI) strengthening remains unclear. In this study, an Al(CoNiV) DP MEA was investigated using in-situ neutron diffraction and multiscale electron microscopy. Both intrinsic strength and HDI strengthening increased at cryogenic temperature. This enhancement originated from the increased lattice friction stress of the hard BCC phase and suppressed dislocation mobility at low temperature, which promoted dislocation accumulation at heterogeneous interfaces. These findings provide insights into temperature-dependent HDI strengthening in DP H/MEAs and related alloy systems.

Elastic accommodation and anomalous tetragonality of thin-plate martensite in an Fe-31Ni-10Co-3Ti alloy revealed by neutron diffraction at cryogenic temperatures

山下 享介*; Harjo, S.; Gong, W.; 川崎 卓郎; 森戸 茂一*; 諸岡 聡; 藤井 英俊*; 友田 陽*

Journal of Alloys and Compounds, 1056, p.186668_1 - 186668_11, 2026/02

https://doi.org/10.1016/j.jallcom.2026.186668

In this study, neutron diffraction during cryogenic cooling was performed on Fe-31Ni-10Co-3Ti alloy to investigate changes in the austenite lattice constant and martensite tetragonality during thin-plate martensitic transformation. The austenite lattice constant showed only a slight change because of the Invar effect, but decreased below the Ms temperature due to elastic accommodation of transformation strain. Broadening of austenite diffraction peaks and EBSD analysis indicated localized plastic accommodation near intersecting martensite regions. For martensite, the a-axis decreased while the c-axis increased anomalously with decreasing temperature, leading to enhanced tetragonality below 35 K. During heating, the c-axis decreased and tetragonality was reduced. These behaviors are attributed to Bain strain during twin formation and possible interfacial stress with surrounding austenite.

Hybrid data assimilation methods for nuclear-data-induced uncertainties

丸山 修平; 山本 章夫*; 遠藤 知弘*

Journal of Nuclear Science and Technology, 63(1), p.31 - 44, 2026/01

https://doi.org/10.1080/00223131.2025.2500579

This study proposes new hybrid data assimilation (DA) methods to effectively use experimental data represented in two different ways in the DA process for reducing nuclear-data-induced uncertainties. Conventional DA methods often assume a linear model, where data in a DA database are represented by their differential coefficients, i.e., nuclear data sensitivity coefficients. In contrast, more rigorous and versatile DA methods based on sampling techniques have been proposed recently. These sampling-based DA methods describe data for DA using results derived from direct transport calculations performed with perturbed nuclear data samples. The proposed hybrid DA methods simultaneously use both types of data described by sensitivity coefficients and sampling results and thus are called "hybrid." Two hybrid DA methods are proposed herein: the simple hybrid DA and efficient hybrid DA methods. Among these, the simple method is based on a straightforward concept that uses a random sampling technique. In contrast, the efficient method employs a more technical method that effectively combines a new deterministic sampling technique with sensitivity coefficients to reduce statistical uncertainties associated with the simple method. The efficient method is expected to be a candidate to achieve high precision and rigorous DA with a realistic sample size.

燃料デブリ周辺物質の分析結果に基づく模擬デブリの合成による実機デブリ形成メカニズムの解明と事故進展解析結果の検証によるデブリ特性データベースの高度化(委託研究); 令和5年度英知を結集した原子力科学技術・人材育成推進事業

廃炉環境国際共同研究センター; 福井大学*

JAEA-Review 2025-007, 120 Pages, 2025/09

JAEA-Review-2025-007.pdf:8.13MB

日本原子力研究開発機構(JAEA)廃炉環境国際共同研究センター(CLADS)では、令和5年度英知を結集した原子力科学技術・人材育成推進事業(以下、「本事業」という。)を実施している。本事業は、東京電力ホールディングス株式会社福島第一原子力発電所の廃炉等をはじめとした原子力分野の課題解決に貢献するため、国内外の英知を結集し、様々な分野の知見や経験を従前の機関や分野の壁を越えて緊密に融合・連携させた基礎的・基盤的研究及び人材育成を推進することを目的としている。平成30年度の新規採択課題から実施主体を文部科学省からJAEAに移行することで、JAEAとアカデミアとの連携を強化し、廃炉に資する中長期的な研究開発・人材育成をより安定的かつ継続的に実施する体制を構築した。本研究は、令和3年度に採択された研究課題のうち、「燃料デブリ周辺物質の分析結果に基づく模擬デブリの合成による実機デブリ形成メカニズムの解明と事故進展解析結果の検証によるデブリ特性データベースの高度化」の令和3年度から令和5年度の研究成果について取りまとめたものである。本研究では、酸化物デブリの形成条件の推定研究として、ガス浮遊法や微小な穴を持つタングステンパイプから溶融・噴出させる方法によりウランを含有する模擬燃料粒子を合成し、その生成条件と性状をまとめた。また、JAEAによりサンプリングデータに基づき作成された凝固パス図を再現し、鉄の挙動が熱力学予測と少し異なる結果などを得た。金属デブリの混合・溶融・凝固状態の評価では、溶融させた金属デブリのステンレスへの落下試験や溶融ステンレスの模擬金属デブリへの落下試験を行いその生成物を分析した。その結果ステンレス鋼温度が1000C程度の場合は溶融金属のZr濃度に関わらず薄い反応相を形成してステンレス鋼損傷が抑制されることがわかり、またBC及びジルカロイのステンレス鋼融体への溶出速度を定量化した。さらに、ステンレス鋼とZrの混合物の各種圧力容器部材や溶接部材との反応速度データを拡充し、大型試験体系での解析可能な簡素化反応速度式を提案した。また、圧力容器下部の材質を参照した大型試験体の実験と反応速度式より、溶融金属と圧力容器構造材との反応による圧力容器下部破損挙動や溶融物流出挙動を評価した。さらに、圧力容器下部におけるデブリ再溶融過程でのウラン化合物とステンレス鋼等の金属物質の反応試験データを拡充し、金属デブリ層へのウラン移行挙動を評価した。また、試験技術の整備として、二酸化ウランとZrと金属との半溶融模擬デブリの合成と分析、CCIM炉とガス浮遊炉を用いた少量のウランの模擬燃料デブリ合成試験の検討を行った。

Prediction of large vapor mass formation under subcooled flow boiling

小野 綾子; 大川 富雄*

Proceedings of 2025 International Congress on Advances in Nuclear Power Plants (ICAPP 2025) (Internet), 12 Pages, 2025/09

次世代炉の設計および安全評価へ資するために、料集合体における限界熱流束(DNB)の新たな予測手法を確立することを目指す。DNBの初期トリガーは伝熱面上に形成される大気泡であると考えられるため、その形成に必要な熱流束の予測に焦点を当てる。本研究では、流数値流体力学(CFD)と大気泡形成モデルを統合し、気泡の発生、成長、運動、接合をシミュレートする簡易な数値解析手法を開発した。さらに、大気泡が形成される際の伝熱面上の蒸気量について分析し、その発生時の条件を予測する。

その場中性子回折を用いたSUS310Sステンレス鋼の水素添加による強さ・伸び向上メカニズムの解明

伊東 達矢; 小川 祐平*; Gong, W.; Mao, W.*; 川崎 卓郎; 岡田 和歩*; 柴田 曉伸*; Harjo, S.

波紋, 35(3), p.129 - 133, 2025/08

Recent studies have shown that the addition of hydrogen to SUS310S stainless steel (Fe-24Cr-19Ni, mass%) simultaneously enhances both strength and ductility, indicating a phenomenon contrary to the conventional understanding of hydrogen embrittlement. In this study, we investigated the underlying mechanism through neutron diffraction experiments during tensile deformation using TAKUMI at the MLF of J-PARC. The results revealed that solid-solution strengthening by hydrogen plays the most significant role in improving the mechanical properties. Solute hydrogen atoms distort the lattice to suppress dislocation motion, thereby increasing the strength. The raised stress in the hydrogen charged sample enables the onset of deformation twinning at a smaller strain compared to the non-hydrogen charged sample. Consequently, the twinning-induced plasticity effect contributes more significantly to work hardening and the improvement of uniform elongation due to the solid-solution strengthening by hydrogen. These findings suggest a new pathway for the effective utilization of hydrogen in austenitic steels.

Lders band-assisted high uniform ductility in ultrastrong ferrous medium-entropy alloy via hierarchical microstructure

Kwon, H.*; Lee, J. H.*; Zargaran, A.*; Harjo, S.; Gong, W.; Wang, J.*; Gu, G. H.*; Lee, B.-J.*; Bae, J. W.*; Kim, H. S.*

International Journal of Plasticity, 190, p.104378_1 - 104378_18, 2025/07

https://doi.org/10.1016/j.ijplas.2025.104378

In this work, we harness a hierarchical microstructure to simultaneously tailor strengthening and deformation mechanisms in a CoCrFeNiMo (at%) ferrous medium-entropy alloy (MEA). A simple thermomechanical process (cold rolling and 90 s annealing) produces ultrafine recrystallized grains, non-recrystallized grains with substructures, and intragranular nanoprecipitates. This structure, with high dislocation density and fine grains, yields a high strength of 1.60 GPa but can risk premature fracture. To overcome this, Lders deformation, enabled by ultrafine grain boundaries and stress-induced martensitic transformation at pre-existing nucleation sites, is employed. Stable Lders band propagation delays strain hardening and enables large uniform ductility. As a result, a tensile strength of 1.84 GPa and uniform elongation of 20% are achieved, matching the best tensile properties among reported multi-principal element alloys.

Unique deformation behavior of ultrafine-grained 304 stainless steel at 20 K

Mao, W.*; Gong, W.; 川崎 卓郎; Gao, S.*; 伊東 達矢; 山下 享介*; Harjo, S.; Zhao, L.*; Wang, Q.*

Scripta Materialia, 264, p.116726_1 - 116726_6, 2025/07

https://doi.org/10.1016/j.scriptamat.2025.116726

An ultrafine-grained 304 austenitic stainless steel exhibited pronounced serrated Luders deformation at 20 K, with stress and temperature oscillations reaching 200 MPa and 20 K. neutron diffraction and digital image correlation revealed discontinuous Luders band propagation and burst martensite formation. During deformation, austenite phase stress remained lower than at upper yielding, indicating elastic behavior. Notably, martensite phase stress stayed lower than austenite until fracture, likely due to stress relaxation from burst martensitic transformation at 20 K. The low martensite stress delayed brittle fracture until austenite plastically yielded during uniform deformation.

Fundamental mechanisms of discontinuous deformation in metals for cryogenic-environment applications

Kim, Y. S.*; Kang, T.*; Hong, S.-K.*; Brechtl, J.*; Lebyodkin, M.*; Cheng, Y.-H.*; Huang, E.-W.*; Liaw, P. K.*; Harjo, S.; Gong, W.; et al.

Acta Materialia, 292, p.120970_1 - 120970_16, 2025/06

https://doi.org/10.1016/j.actamat.2025.120970

Metallic materials can exhibit low-temperature serrated deformation (LTSD) at cryogenic temperatures, potentially causing sudden failures. Understanding LTSD is thus crucial for ensuring material reliability in such environments. LTSD has been explained by two main mechanisms: (i) dislocation-based mechanical instability and (ii) thermomechanical instability, but each has limitations when considered alone. To address this, we propose a new LTSD mechanism, a thermally induced dislocation dynamics model, based on cryogenic experimental evidence. This model accounts for dislocation avalanches and localized heating, leading to hierarchical dislocation networks and transitions in deformation modes. A modified deformation-mechanism map for SS316L is also presented. Our findings highlight the rate-dependent nature of LTSD and negative strain-rate sensitivity, including the first observation of links between small stress fluctuations and large serrations.

Martensitic transformation and shape memory effect in Mn-rich Cu-Mn-Al alloys

伊東 達矢; Xu, S.*; Xu, X.*; 大森 俊洋*; 貝沼 亮介*

Shape Memory and Superelasticity, 11(2), p.167 - 175, 2025/06

https://doi.org/10.1007/s40830-025-00519-y

The microstructure, crystal structures, cold workability, and martensitic transformation temperatures of several CuMnAl (40 55; 10 13; at%) alloys were analyzed. For the first time, martensitic transformation was revealed Mn-rich Cu-Mn-Al alloys. The surface relief of the martensite phase reversibly appeared and disappeared during cooling and heating, along with small thermal hysteresis, implying a thermoelastic transformation. The crystal structures of the parent and martensite phases are B2 and 2M(BCT), respectively, with cMnAl (at%) sample. This alloy demonstrates moderate cold workability and cost-effectiveness, exhibiting potential as an alternative for conventional shape memory alloys in various applications.