Structural materials for molten-salt fast neutron reactors (MSRs) need to exhibit excellent mechanical properties at high temperatures and be able to resist corrosion by chloride salts and neutron irradiation. This study presents design considerations for a new class of nickel-based alloys, exemplified by Nimolloy-202. This alloy is defined its high molybdenum content (20 wt%), the absence of chromium and iron, and its hardening by gamma' phase (Ni3(Al, Mo, Ti)) nano-precipitation which improves both mechanical properties and irradiation resistance. Nimolloy-202 demonstrates excellent corrosion resistance in purified binary NaCl-MgCl2 salt at 600 degrees C for one week (168 h) under argon flow. Microstructural characterizations confirm the precipitation of nano-gamma' with a density approaching 1023 m-3. Their strengthening effect on the material is quantified and modeled. Collectively, these results highlight the potential of chromium-free Ni-Mo alloys for fast neutron chloride salt MSRs. Furthermore, these alloys, which can also be used as coatings, could be of interest to solar power plants that operate with chloride salts.

This study investigates the microstructure after quenching and the precipitation behavior during subsequent tempering in Fe-2.5Ni-0.5Al, Fe-2.5Cu, and Fe-2.5Cu-2.5Ni-0.5Al (wt%) steels, with and without Mo additions. All alloys were solution-treated at 900 °C for 60 min, followed by quenching and tempering at 550 °C for up to 40 min. Microstructure and precipitation characteristics were analyzed using microscopy, atom probe tomography, and in situ small-angle X-ray scattering, supported by thermodynamic calculations and continuous cooling transformation diagram simulations. The Fe-Ni-Al steels (with or without Mo) exhibited a ferritic–bainitic microstructure. The Fe-Cu steel was primarily ferritic, while Mo addition promoted a ferritic-bainitic structure. The Fe-Cu-Ni-Al steel displayed a ferritic–martensitic microstructure, which transformed into a fully martensitic structure with Mo addition. During tempering, no precipitates were detected in the Fe-2.5Ni-0.5Al steel, whereas Cu-rich precipitates formed in both Fe-2.5Cu and Fe-2.5Cu-2.5Ni-0.5Al steels. The enhanced bainitic/martensitic transformation induced by Mo addition resulted in a higher dislocation density after quenching, which facilitated Cu precipitate nucleation during tempering. Hybrid Monte Carlo/Molecular Dynamics simulation confirm that Mo alters the matrix distortion in Fe-2.5Cu-2.5Ni-0.5Al steel, a key factor influencing nucleation and precipitation kinetics. Moreover, the addition of Mo reduced precipitate growth and coarsening, contributing to the retention of high hardness after tempering.

A high-speed synchrotron radiography system has been developed to facilitate in situ imaging of dynamic processes in electron beam powder bed fusion (PBF-EB). Using the P61A White Beam Engineering Materials Science beamline at PETRA III, this system achieves high temporal resolution and a spatial resolution of approximately 10 mu m. The scintillator screens are coupled to a diamond plate and housed within a specialized nitrogen gas cooling system, effectively mitigating thermal stress caused by the intense synchrotron beam. These innovative components ensure stable imaging performance and enhance the system's ability to operate under extreme conditions. By resolving fringes at short propagation distances for the partially coherent beam, the imaging system has enabled the efficient visualization of crack formation and pore evolution in high-Z materials, such as nickel-based superalloys, during the PBF-EB process. These advances not only optimize imaging in extreme environments but also open new avenues for high-energy synchrotron applications, including dynamic phase imaging and laser welding studies of dense samples.

In this study, we investigate the precipitation kinetics of Cu in 15–5 PH maraging stainless steel during high-temperature thermal treatments in the fully austenitic state. This provides direct evidence that Cu precipitation can occur in the austenite phase of martensitic or ferritic steels. The kinetics of Cu precipitation in austenite are examined at 700 and 800 °C using in situ synchrotron small-angle and wide-angle X-ray scattering, complemented by atom probe tomography investigations to analyze the precipitates, particularly their chemistry, following heat treatment. The resulting experimental data, which include the evolution of size, volume fraction, number density and chemical composition, are used to inform precipitation kinetics modelling using the Langer-Schwartz-Kampmann-Wagner (LSKW) approach coupled with CALPHAD thermodynamic and kinetic databases. The simulations accurately capture the experimental data by adjusting the interfacial energy in an inverse modelling approach. The insight that Cu precipitation occurs in austenite and subsequently in martensite paves the way for design of hierarchical structures with a bi-modal particle size distribution of Cu precipitates with varying crystal structures and compositions. Additionally, the validated LSKW modelling approach establishes a foundation for designing Cu-alloyed high-performance steels, taking into account various manufacturing routes.

In duplex stainless steels (DSSs), phase separation of iron and chromium is a well-known phenomenon causing low-temperature embrittlement, which greatly limits the lifetime of components in service conditions at temperatures above 250 °C–300 °C. Hence, means of mitigating the underlying phase separation causing this embrittlement is highly interesting to extend the service life of DSSs in certain applications. In this work, we investigate the effect of intermediate heat treatments (5 minutes annealing at temperatures between 700 °C and 900 °C), performed after the conventional solution treatment, on the kinetics of phase separation super duplex stainless steel 2507. Using in situ small-angle neutron scattering at accelerated aging conditions (i.e., aging at 475 °C), we show that the application of intermediate heat treatments, which change the “initial state” of the material, can slow down development of the concentration fluctuation amplitude by up to 65 pct during aging inside the miscibility gap. This indicates great potential to delay the embrittlement process of duplex stainless steel. All intermediate heat treatments, conducted prior to aging, change the phase separation kinetics but to different extent. The 800 °C intermediate heat treatment shows the largest reduction in phase separation kinetics as compared to the reference sample. This sample also correspondingly shows the lowest hardness increase after aging. These findings show that intermediate heat treatments can be effective to reduce phase separation kinetics in duplex stainless steel and thus mitigate low-temperature embrittlement during service. The origin of the intermediate temperature treatment effect on phase separation kinetics is discussed in relation to the short-range atomic order introduced during intermediate heat treatment, prior to accelerated low-temperature aging.

Thermochemical treatments like nitrocarburizing and gas nitriding form hardened surface layers of iron nitrides and carbides, improving wear, fatigue, and corrosion resistance in loaded components made of steel. This study employs micro-focused X-ray diffraction (µXRD) imaging at a synchrotron facility to characterize the microstructure of nitrocarburized and gas-nitrided steel surfaces in three steel grades (46MnVS3, 34CrNiMo6, 16CrMnNiMo9–5–2). Through line profile analysis with fine-step mesh grid scanning, we spatially resolve phase distributions and elastic strains in the compound layer. The ε-phase exhibits isotropic residual strain, transitioning from expansion to compression with depth, while the γ’-phase displays anisotropic strain, expanding perpendicular to the surface and compressing parallel to it. These findings highlight µXRD's potential for detailed structural analysis, enabling optimization of surface hardening processes.

The parent austenite grain size (PAGS) plays a critical role in governing the austenite to martensite transformation in steels, as it directly influences the stability of the austenite phase. Therefore, a thorough understanding of the PAGS effect is essential for the design of advanced steels with optimized performance. In this study, we investigate the influence of PAGS on both athermal and deformation-induced martensitic transformation (AMT and DIMT) in an Fe-18Cr-12Ni (wt.%) alloy. In situ and ex situ high-energy synchrotron X-ray diffraction (HEXRD) measurements were conducted during tensile loading and after cooling, respectively. These measurements were complemented by electron microscopy to elucidate the underlying mechanisms governing the PAGS effect on martensitic transformation. The results reveal PAGS exerts opposite effects on AMT and DIMT. Grain refinement increases the barrier for martensitic transformation onset, and in the case of AMT, this leads to a lower martensite start temperature (Ms) and suppressed transformation. Conversely, although grain refinement delays DIMT and raises the critical stress required for its initiation, the associated Hall-Petch strengthening enhances the flow stress, thereby promoting DIMT at higher strain levels. These findings offer valuable insights for the microstructural design of austenitic steels and contribute to the modeling of martensitic transformation behavior under varying mechanical and thermal conditions.

Small-angle neutron scattering (SANS) is a valuable method for the analysis of phase decomposition in Fe-Cr alloys; however, quantification of the decomposition requires careful modeling of the scattering data considering factors such as interface character and short-range order. Here, we quantify the phase decomposition in a high-performance super duplex stainless steel in situ during accelerated aging in the early stage of decomposition by modifying a previously suggested quantitative SANS data modeling method. The proposed revised method can accurately model the SANS data and paves the way for revisiting the detailed phase decomposition kinetics in situ during aging in various Fe-Cr-based alloys.

The microstructure evolution associated with the cold forming sequence of an Fe-14Cr-1W-0.3Ti-0.3Y2O3 grade ferritic stainless steel strengthened by dispersion of nano oxides (ODS) was investigated. The material, initially hot extruded at 1100 °C and then shaped into cladding tube geometry via HPTR cold pilgering, shows a high microstructure stability that affects stress release heat treatment efficiency. Each step of the process was analyzed to better understand the microstructure stability of the material. Despite high levels of stored energy, heat treatments, up to 1350 °C, do not allow for recrystallization of the material. The Vickers hardness shows significant variations along the manufacturing steps. Thanks to a combination of EBSD and X-ray diffraction measurements, this study gives a new insight into the contribution of statistically stored dislocation (SSD) recovery on the hardness evolution during an ODS steel cold forming sequence. SSD density, close to 4.1015 m−2 after cold rolling, drops by only an order of magnitude during heat treatment, while geometrically necessary dislocation (GND) density, close to 1.1015 m−2, remains stable. Hardness decrease during heat treatments appears to be controlled only by the evolution of SSD.

ODS steels are candidate materials for the future generation of nuclear power plants. Ferritic / Martensitic (F/M) ODS steels display better formability thanks to high temperature austenitic transformation. The precipitation kinetics of a F/M Fe‑9Cr ODS steel during powder consolidation up to 1100 °C has been characterized by in‑situ Small Angle X-ray Scattering (SAXS). The influence of the matrix phase transformation has been established, showing an increase of the growth rate of the nano‑oxides in austenite, leading to nano‑oxides ∼ 2x larger in diameter than in Fe‑14Cr ferritic ODS grades at the end of the thermal treatment. These results are further supported by local atom probe tomography (APT) performed across grains showing contrasted microstructure and composition. Anomalous SAXS as well as comparison between APT and SAXS provide evidence that the nano‑oxides stabilize with a Y2Ti2O7 or Y2TiO5 stoichiometry around 1100 °C.