Articles
A 3D-Printed SFE-Graded Alloy Mitigates Cryogenic Discontinuous Plastic Flow and Hydrogen Embrittlement
Renhao Wu, Ao Geng, Peihao Geng, Hyojin Park, Eun Seong Kim, Sang-Ho Oh, Stephan Schönecker, Xiaoqing Li, Tianlong Zhang, Shuhui Li, Byeong-Joo Lee, Hyoung Seop Kim
Advanced Science (2026) e77732
2026-10-05
Abstract
The lunar polar permanently shadowed regions, rich in water ice for hydrogen production, impose a trade-off in FCC alloys: lowering stacking-fault energy (SFE) enhances hydrogen embrittlement resistance but exacerbates cryogenic discontinuous plastic flow (DPF). To address this, we designed and fabricated via additive manufacturing a compositionally graded (CoCrMn)100−2xFexNix alloy with a single-phase FCC structure. Guided by thermodynamic calculations and first-principles predictions, the gradient-structured build exhibits spatial SFE and microstructural gradients. Quasi-static tensile tests from 298 K down to 20 K reveal significant strengthening with retained high ductility, while DPF is strongly suppressed, showing only minor stress drops. Mechanistically, the graded architecture partitions deformation: high-SFE regions promote dislocation glide and cellular substructures, whereas low-SFE regions enable planar slip and deformation nanotwinning, delocalizing strain and suppressing avalanche-like dislocation activity. Under hydrogen charging at 298 K, embrittlement resistance is preserved via a dual mechanism combining deformation-induced nanotwins and a Mn concentration gradient. This synergy balances dislocation mobility and hydrogen trapping, offering a new paradigm for simultaneous improvement of cryoplasticity and hydrogen compatibility. Our work establishes a multidisciplinary design strategy for alloys targeting extreme hydrogen and cryogenic environments.
Vacancy formation and migration energies in 15-15Ti and FeCrAl: A study from first principles
Nils Wikström, Ruihuan Li, Stephan Schönecker, Xioqing Li, Pär Olsson
Physical Review Materials (Accepted 2026)
2026-10-04
Abstract
Lead-cooled fast reactors impose harsh environments on the structural materials, including high damage doses, high temperatures and corrosive liquid lead. To withstand these environments, advanced steel concepts, such as a composite of 15-15Ti and FeCrAl, have been proposed as fuel cladding. Microstructural evolution of alloys is governed by fundamental properties, including the vacancy formation and migration energies, yet such data for novel alloy compositions is sparse. In this work, an extensive set of vacancy formation and migration energies in two alloys, 15-15Ti and FeCrAl, was calculated from first-principles, employing density functional theory. Special quasi-random structures were generated for both alloys, and different magnetic states of the alloys were simulated. It was found that the vacancy formation energy is highly influenced by the chemical and magnetic environment in 15-15Ti, while in FeCrAl the size-mismatch between Fe and the alloying elements dominates. The vacancy formation energy was calculated by two different density functional theory formulations, showing excellent agreement. The vacancy migration energy was found to be governed by changes in the magnetic and chemical environment as the atom migrates. Based on these findings, temperature-averaged behaviors of vacancies in the alloys are derived, which can be further used to define effective vacancy formation and migration energies at reactor-relevant temperatures. The effective vacancy formation and migration energies can be used to predict temperature-variable shifts needed to accommodate the higher dose rate when using ion irradiation to emulate neutron irradiation.
Sintering behavior and microstructural evolution of UO2-Gd2O3 fuel processed by spark plasma sintering
Artur Cesar de Freitas, Diogo Ribeiro Costa, Elita Fontenele Urano de Carvalho, Ricardo Mendes Leal Neto, Pär Olsson, Michelangelo Durazzo
Progress in Nuclear Energy 202 (2027) 106614
2026-10-03
Abstract
This study systematically examines the sintering behavior of pure UO2 and UO2 pellets containing 10 wt% Gd2O3 processed via conventional sintering and Spark Plasma Sintering (SPS). The analysis focuses on densification mechanisms, microstructural evolution, and diffusion-driven features related to local Gd redistribution. The results indicate that, under conventional sintering conditions, the incorporation of Gd2O3 promotes the development of Kirkendall-type porosity due to the disparity in diffusion rates between Gd3+ and U4+ cations. This effect leads to increased residual porosity and a consequent reduction in densification of the doped pellets. In contrast, SPS processing yields significantly enhanced densification, with relative densities exceeding 97% of the theoretical density, even in the presence of Gd2O3. The corresponding microstructures are characterized by a higher degree of compaction and a predominance of fine, isolated pores, indicative of improved sintering efficiency. Energy-dispersive X-ray spectroscopy (EDS) mapping reveals a heterogeneous spatial distribution of gadolinium in samples processed by both routes, reflecting limitations in powder mixing and diffusion homogenization. Although forced powder mixing contributes to improved microstructural uniformity, its effect on densification remains comparatively limited. Overall, the findings highlight the superior capability of SPS to mitigate diffusion-induced porosity and to promote enhanced densification and microstructural refinement in UO2-Gd2O3 systems.
Thermal diffusivity of ion-irradiated uranium mononitride
Elina Charatsidou, Angus Wylie, Nils Wikström, Maria Giamouridou, Riley Moeykens, Robert J.W. Frost, Eric Jossou, Michael P. Short, Pär Olsson
Journal of Nuclear Materials 634 (2026) 157121
2026-09-30
Abstract
Uranium mononitride is a candidate fuel for advanced fast reactors due to its high fissile density and high thermal conductivity. However, its thermophysical property changes due to both chemical impurities and irradiation-induced damage, have not been extensively studied. In this work, we investigate the effect of selected fission product species on the thermal properties of UN using ion implantation. Dense UN pellets were implanted with elements representative of solid (Zr, Ba) and gaseous (Kr, Xe) fission products at different fluences. Near-surface chemistry, morphology and microstructure were characterised using scanning electron microscopy and time-of-flight elastic recoil detection analysis. Thermal diffusivity was measured using transient grating spectroscopy, probing the top ∼1 μm of the implanted regions, for temperatures up to 670 K. For solid fission product implantation, the thermal diffusivity decreases relative to unimplanted UN and remains stable with temperature, consistent with impurity scattering and implantation-induced damage. In contrast, gaseous fission product implantation leads to a stronger and largely irreversible reduction in diffusivity. Heating and cooling measurements reveal pronounced hysteresis, accompanied by grain-boundary cracking and enhanced surface oxidation after thermal cycling. These observations suggest redistribution and partial release of Kr and Xe to grain boundaries and defects, leading to microstructural degradation that suppresses heat transport. The results highlight clear differences between solid and gaseous fission product effects on thermal properties in uranium mononitride. Additionally, irradiation damage was observed to be the primary contributor to the degradation of thermal diffusivity, while chemical effects are secondary. Lastly, the work demonstrates the usefulness of combining ion implantation and transient grating spectroscopy to study thermophysical behaviour of nuclear fuels, and the capability to separate chemical effects from implanted damage in the degradation of thermal transport.
Atomistic modeling of irradiation-induced microstructure evolution in Ni-based solid-solution alloys
Ebrahim Mansouri, Laurent K. Beland, Pär Olsson, Yanwen Zhang
Physical Review Materials 10 (2026) 093605
2026-09-22
Abstract
Radiation-induced defect evolution in metallic alloys is often interpreted in terms of thermally activated defect diffusion, cascade quenching, and defect recombination. However, athermal structural relaxation can also influence defect evolution, particularly in chemically complex alloys. In this work, we investigate defect evolution in face-centered cubic (fcc) Ni, Ni–Cr, and Ni–Fe alloys using atomistic simulations based on the creation-relaxation algorithm (CRA), in which Frenkel pairs are iteratively introduced into an initially pristine lattice followed by structural relaxation. This approach isolates a nondiffusive, athermal limit by suppressing long-range thermal migration and explicit displacement-cascade effects. By analyzing point defects, defect clusters, athermal recombination probabilities, elastic properties, and dislocation structures, we show that alloying produces modest changes in the total residual defect inventory, despite pronounced differences in bulk elastic properties. Instead, the stronger alloying effects appear in defect morphology, interstitial-cluster statistics, local recombination behavior, and Burgers-vector-resolved dislocation character. Higher Fe concentrations promote fewer but larger interstitial clusters and enhance local athermal recombination with interstitial clusters, whereas pure Ni and Ni–Cr show a higher frequency of smaller clusters. These results indicate that, within the athermal CRA framework, local chemical and elastic environments influence the pathways of defect relaxation and clustering, while their effect on the total surviving defect population remains modest compared with that observed under irradiation conditions where thermally activated processes are active.
Rapid Detection of Dimensional Instabilities in Ion-irradiated Uranium Mononitride
Elina Charatsidou, Janne Wallenius, Pär Olsson
Scripta Materialia 282 (2026) 117400
2026-09-01
Abstract
Quantifying irradiation-induced dimensional changes typically requires destructive preparation or specialized instrumentation. This work presents a non-destructive method using scanning electron microscopy and feature-referenced regression analysis to determine strain from implantation micrographs. Multiple imaging modes establish a detection limit of 0.6 % linear strain. Application on uranium mononitride reveals 10 % volumetric swelling following Ba implantation, 6 % shrinkage after Zr implantation, enhanced contraction in Zr-Ba co-implanted regions, and negligible strain for Xe and Kr implantations. The results demonstrate a rapid, accessible method for surface-sensitive quantification of strain and provide new insight into dimensional stability of uranium mononitride under ion irradiation.
Engineering stacking fault energy and hierarchical precipitates in a near-fully recrystallized DED Ni-based multi-principal element alloy
Renhao Wu, Hyojin Park, Jae Heung Lee, Shi Woo Lee, Longfei Xu, Do Won Lee, Stephan Schönecker, Jalal Kangazian, Tianle Li, Xiaoqing Li, Hyoung Seop Kim
International Journal of Plasticity 201 (2026) 104682
2026-03-26
Abstract
Laser additive manufacturing involves intrinsic rapid solidification rate and elemental segregation, which induce thermal residual stress and metastable microstructures, potentially leading to mechanical performance degradation. To address this, we tailored stacking fault energy (SFE) and hierarchical precipitation to enable near-full recrystallization in a Ni-based multi-principal element alloy. Guided by phase diagram calculation and density functional theory, a Ni-Cr-Fe-Co matrix with Al/Ti/V additions was designed and fabricated to stabilize a medium-level intrinsic SFE while forming hierarchical precipitates (primary BCC/B2 and secondary nanoscale acicular phases) via direct energy deposition in-situ alloying. Uniformly distributed precipitates nucleate preferentially within the grains, with limited formation at the boundaries. This microstructural change further promotes dynamic recrystallization under inherent severe plastic deformation. Consequently, the as-deposited alloy developed ∼92% recrystallized grains and exhibited a yield strength of 790 MPa, ultimate tensile strength of 1164 MPa, and uniform elongation of 24.6%, while multiscale characterizations confirmed plastic deformation interaction of precipitation within the recrystallized grains. This study demonstrates that engineering SFE and hierarchical precipitations promote dynamic recrystallization and synergetic mechanical properties, offering a generalizable strategy for additively manufactured Ni-based alloys.
Impact of zirconium incorporation on the thermophysical properties of uranium mononitride
Elina Charatsidou, Anita Pazzaglia, Kaitlyn Bullock, Maria Giamouridou, Eleanor Lawrence Bright, Mikael Jolkkonen, Christoph Hennig, Pär Olsson
Journal of Nuclear Materials 623 (2026) 156467
2026-01-17
Abstract
Uranium mononitride (UN) is a promising candidate fuel for next-generation fast reactors due to its high fissile density, superior thermal conductivity, and high melting point compared to conventional oxide fuels. However, scarce experimental data on UN and its thermophysical behaviour under fission product incorporation limits its performance assessment. Zirconium nitride (ZrN) is an efficient thermal conductor and a candidate material for inert matrix fuels. Given its high thermal conductivity, ZrN addition at sufficient concentrations should, in principle, induce percolation conduction and increase thermal conductivity in UN. To decouple chemistry from irradiation-induced porosity, known to dominate thermal degradation at high burnup, this study isolates the intrinsic chemical contribution of Zr incorporation under dense, low-porosity conditions. (U,Zr)N pellets with 6.5 and 20 at. % Zr were fabricated by spark plasma sintering (SPS), using powders produced from arc-melted alloy via the hydride-nitride-denitride route. Synchrotron powder X-ray diffraction confirmed the formation of solid solutions and enhanced Zr solubility after sintering, resulting in improved microstructural homogeneity. Thermal diffusivity was measured between 300 and 1500 K using light flash analysis, and thermal conductivity was derived using heat capacity and density correlations with porosity correction. Despite the intrinsically higher thermal conductivity of ZrN, the incorporation of 6.5 at. % Zr reduced the thermal conductivity relative to UN, consistent with impurity scattering. The 20 at. % Zr composition further decreased conductivity, indicating the microstructure does not meet the conditions required for percolation conduction. Differences in the temperature dependence of thermal diffusivity between UN and Zr-bearing samples highlight a compositional influence on heat transport. The results provide benchmark data for (U,Zr)N and insights into chemical and thermophysical interactions in nitride ceramics.
Mechanistic insight into the ferritization of austenite in Pb via a discontinuous reaction governed by a migrating liquid film
Corrosion Science 258 (2026) 113398
Kin Wing Wong, Peter Szakálos, Christopher Petersson, Dmitry Grishchenko, Pavel Kudinov
2025-10-15
Abstract
The dissolution of austenitic steel in liquid lead-based alloys can induce a phase transformation characterized by a sharp dissolution front separating ferrite and austenite grains, a process commonly referred to as ferritization. Although widely reported, the mechanism driving this transformation remains under debate. This study re-examines ferritization as a discontinuous reaction via a migrating liquid film and proposes a thermodynamically consistent model for the initiation and propagation of the dissolution front. The proposed mechanism is supported by experiments at 500–550°C, literature evidence, and diffusion calculations. Under low oxygen conditions, Cr transport through liquid Pb channels is identified as the rate-limiting step, setting the theoretical corrosion rate in stagnant environments. High-speed erosion-corrosion tests show enhanced corrosion rates, driven by erosion-limited channel lengths that locally boost mass transport. In contrast, under moderate oxygen concentrations relevant for lead-cooled fast reactor (LFR) operation, the rate-limiting step shifts to metal transport across a nanometer-scale amorphous oxide layer at the reaction front. Other Ni-containing austenitic steels, including alumina-forming austenitic (AFA) alloys and Ni-based high-entropy alloys (HEAs) can also be susceptible to discontinuous reactions under direct contact with liquid Pb-based alloys, lacking the self-healing oxide protection as observed in alumina-forming ferritic steels. This limitation may present a concern for the long-term use of bare austenitic steel in liquid Pb environments.
Irradiation-induced polymorphism in Fe–Cr alloys
Scientific Reports 15 (2025) 35050
Ebrahim Mansouri, Xiaoqing Li, Pär Olsson
2025-10-08
Abstract
Direct damage evolution simulations based on electronic structure physics show a significant correlation between Cr concentration and polymorphism in the form of localized formation of C15 Laves phase structures in Fe–Cr alloys under irradiation. We elucidate the role of Cr content in the formation and stabilization of the C15 Laves phase structure, which is crucial to understanding the behavior of materials under extreme conditions. This study also reveals a connection between non-linear magnetic behavior and irradiation-induced swelling in Fe–Cr alloys. These results advance the comprehension of radiation-induced changes in magnetization and suggest a novel experimental approach for detecting C15 clusters in irradiated Fe–Cr alloys.
The mechanistic effects of martensitic resultants on dynamic strain aging in medium-Mn steel
Shuren Fu, Changwei Lian, Dengpeng Ji, Renhao Wu, Xiaoqing Li, Haiming Zhang
Materials Science & Engineering A 940 (2025) 148470
September 2025
Abstract
This study explores the dynamic strain aging mechanism in medium-Mn steel, emphasizing its interplay with the kinetics of strain-induced martensite transformation (SIMT). By tracking the migration of each Portevine-Le Châtelier band, we investigated the complex dynamics of SIMT and its impact on the mechanical behavior. Characteristics of stress serrations are closely related to the dynamic propagation behavior of PLC bands. The epsilon martensite, being softer than austenite, contributes negligibly to the work hardening rate, while plays a key role in coordinating plastic deformation. The alfa' martensite with the hardest value of 6.08 GPa, locally pins the mobile dislocations, triggering the stress serrations in the stress-strain curve. However, as alfa' martensite grows, the pinning effect on dislocations diminishes substantially. We delimited a critical size of alfa' martensite, about 350 nm, beyond which it loses the ability to pin dislocations and instead begins to undergo plastic deformation. Therefore, the serrations intensity is closely governed by the nucleation and growth rates of alfa' martensite: a nucleation-dominant regime intensifies serrations, while a growth-dominant regime reduces them. The deep learning of the effect of SIMT kinetics on DSA mechanism is pivotal for understanding the mechanical stability and ductility of medium-Mn steels under strain.
Assessing the near-surface diffusion of Xe and Kr in Zirconia by time-of-flight elastic recoil detection analysis
Nuclear Instruments and Methods in Physics Research B 566 (2025) 1657736
Nils Wikström, Maria Giamouridou, Elina Charatsidou, Pär Olsson, Johan Oscarsson, Daniel Primetzhofer, Robert J.W. Frost
2025-06-06
Abstract
The diffusion of two volatile fission products, xenon (Xe) and krypton (Kr), in zirconia (ZrO2) is investigated. Samples of Yttria (Y2O3)-stabilised tetragonal ZrO2 were implanted with either Xe or Kr, at 300 keV, with a fluence of 10¹⁷ at./cm2, and subsequently analysed with time-of-flight elastic recoil detection analysis (ToF-ERDA) to obtain elemental composition depth profiles. Samples were then annealed at 1200 ◦C for 9 h, and the effect of the annealing was assessed by ToF-ERDA measurements. From these measurements, first-order approximations of diffusion coefficients for Xe and Kr in ZrO2 were derived, using a model based on Fick’s second law, these being (1.36 ± 0.87) × 10⁻¹⁹ m²/s and (2.94 ± 1.96) × 10⁻¹⁹ m²/s at 1200 ◦C for Kr and Xe respectively. It was shown that ToF-ERDA can provide data to analyse the diffusion of elements in solid sample matrices and that a model based on Fick’s Law can predict the diffusion of the implanted ions.