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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Experiment design for the neutron irradiation of $\mathrm{PM-HIP}$ alloys for nuclear reactors

Here, this article describes the design of an Advanced Test Reactor (ATR) drop-in neutron irradiation experiment aiming to directly compare the performance of nuclear structural alloys fabricated by powder metallurgy with hot isostatic pressing (PM-HIP) against conventional casting or forging. There is growing interest in PM-HIP alloys for nuclear applications because of their microstructural uniformity, superior mechanical properties, and reduced dependence on welding and machining, compared to cast/forged alloys. Nuclear code-qualification of PM-HIP alloys requires neutron irradiation testing to demonstrate performance under relevant conditions. In this experiment, six nuclear structural alloys were irradiated: Ni-based alloys 625 and 690, Grade 91 ferritic steel, SA508 pressure vessel steel, and 304L and 316L austenitic stainless steels. The experiment is assembled into seven capsules in four test trains and irradiated in three ATR inboard A positions. Both the PM-HIP and cast/forged versions of each alloy were irradiated under nearly identical conditions for comparative purposes, to target doses of 1 ± 0.2 and 3 ± 0.2 dpa at temperatures of 300 ± 50 °C and 400 ± 50 °C. A thorough description of the experiment design and thermal, structural and neutronic analyses performed to ensure the targeted irradiation conditions are met is provided. Specimens were configured as small disks, compact tension specimens and tensile bars to facilitate post-irradiation examination (PIE) that will include mechanical testing, microstructure characterization, and fracture toughness testing. Given the considerations for ASTM standardized mechanical testing, comparative fluence and temperature across specimen pairs, and comprehensive PIE planning herein, this work serves as a template for future nuclear materials qualification experiment designs.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2020

Precipitation-site competition in duplex stainless steels: Cu clusters vs spinodal decomposition interfaces as nucleation sites during thermal aging

Competing microstructural evolution mechanisms can exist simultaneously when duplex stainless steels are operating for several decades in a high temperature service environment. Such competition between different microstructural evolution pathways can be difficult to ascertain using simple model alloy systems necessitating detailed structural and compositional analysis of phase transformation mechanisms in complex alloys Thus, in this study duplex stainless steels with complex but well understood chemistries were used to investigate the relative importance of different heterogeneous nucleation sites – specifically spinodal decomposition and Cu clustering – on precipitation of Ni-Si-Mn particles during long term thermal aging. Precipitation of Ni-Si-Mn particles in duplex stainless steels and ferritic steels during thermal aging and irradiation is known to have profound effect on mechanical properties. Using duplex stainless steels with custom modified compositions along with atom probe tomography characterization and first-passage kinetic Monte Carlo (FPKMC) simulations it is revealed that while the interface between Cr and Fe formed during spinodal decomposition can be a pathway for diffusion of Ni, Si, Mn, and Cu, it is not a site that encourages precipitation of Ni-Si-Mn rich particles. Instead, the presence of a higher concentration of Cu in these steels leads to formation of small Cu clusters with high energy interfaces that act as nucleation sites for Ni-Si-Mn particles because of its strong interaction particularly with Ni increasing the chemical driving force for precipitation. These results will help inform predictive models for the use of duplex stainless steels and other precipitation-hardened alloys for extended operation at high temperatures.

Duplex stainless steel, Thermal aging, Precipitati↗
DOE OSTI2023

Influence of Molybdenum on Rapid Solidification Microstructure and Microsegregation in Primary Ferrite Solidified Stainless Steel

Due to the widespread use and familiarity of austenitic stainless steels in many industries these alloys are often the first materials used when adapting new processing techniques. The rapid cooling and solidification rates that occur during fusion based additive manufacturing processes like powder bed fusion cause a fundamental shift in the solidification behavior of the material. While the effects that Cr and Ni have on the microstructure have been fairly well documented across a range of solidification conditions, the impact of alloying elements like Mo are not as well understood at rapid solidification rates. For this study, four custom stainless steel alloy feedstocks were made with targeted modification of the Cr and Mo concentrations to maintain a constant Cr/Ni eq ratio of ~1.7. Two-piston splat quenching was used to produce rapidly solidified samples and the solidification mode, phase, and cell size were investigated. Solidification rates were estimated to be between 0.4 and 1.5m/s. A comparison of the microsegregation and partitioning behavior of Cr, Ni and Mo in ferrite solidified material was also performed for the different alloy compositions. Variations of the Mo concentration between each feedstock showed no measurable influence on the solidification/cooling rates, solidification morphology, or the partitioning of Mo. Furthermore, higher concentrations of Mo were found to significantly increase the amount of ferrite phase in the microstructure by suppressing the ferrite to austenite massive transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Evolution of {delta}ferrite in a CF3 cast stainless steel upon neutron irradiation to 3, 5, 10, 20, and 40 dpa

The microstructural evolution of delta ferrite in a CF3 cast stainless steel irradiated to 3, 5, 10, 20, and 40 dpa with two dose rates was studied with atom probe tomography (APT) and transmission electron mi-croscopy. Spinodal decomposition and G-phase precipitates induced by neutron irradiation and thermal aging were quantified systematically. The neutron irradiation significantly enhances the spinodal decom-position in delta ferrite as both the spinodal wavelength (i.e. a characteristic repeat distance) and am-plitude (i.e. magnitude of elemental concentration fluctuation) increase after irradiation. The wavelength only varies slightly when the dose increases from 5 to 40 dpa, while the amplitude increases dramatically from 10 to 20 dpa and starts to saturate before 20 dpa. The dose rate and irradiation temperature also have a notable effect on the spinodal wavelength and amplitude. The study shows that higher dose rate promotes a larger wavelength at given irradiation doses. Regarding the G-phase precipitates, a slightly lower irradiation temperature would result in a smaller mean size of G-phase precipitates, while a lower irradiation dose rate would lead to a larger G-phase precipitates at a given temperature and dose. Over-all, the spinodal decomposition and G-phase precipitates in the delta ferrite continue to evolve with the increasing dose beyond 10 dpa. This study confirms that the formation of G-phase precipitates at the in-terdomain region between alpha and alpha-prime is facilitated by the Si and Mo atoms rejected from Fe rich alpha phase and the Ni and Mn atoms rejected from Cr rich alpha-prime phase. Neutron irradiation plays a dominant role in the ferrite instability, and the effect of prior thermal aging at 400 degrees C for 10,000 hours is negligible as the dose is 3 dpa and above.

duplex stainless steel↗
DOE OSTI2025

Comparative Mechanical Properties Analysis of Triple Ion-Beam Irradiated and Neutron Irradiated Potential Plasma Facing Components

Abstract Several classes of materials are being proposed for use in fusion reactors including oxide dispersion strengthened (ODS) and reduced activation ferritic-martensitic (RAF/M) steels to withstand the severe and harsh conditions. In this work, the mechanical properties of a Fe-16Cr-4Al-2W-0.3Ti-0.3Y 2 O 3 (K3) (ODS) ferritic steel and a Fe-8.9Cr-1.1W-0.47Mn-0.2V-0.14Ta-0.11C (Eurofer 97) (RAF/M) steel) after triple ion beam irradiation were locally evaluated utilizing in-situ micro-pillar compression tests, and continuous stiffness/quasi-static nanoindentation. No change in mechanical properties was observed in the K3 ODS steel. However, the Eurofer 97 RAF/M steel exhibited radiation-induced effects via increases in yield strength. Micro-pillar techniques were expanded to neutron-irradiated materials via an in-situ testing technique employing lift-out methods on Fe-14Cr-0.9Ti-0.3Mo-0.25Y 2 O 3 (MA957) ODS ferritic steel. Both the non-irradiated and irradiated compressive yield stresses of the MA 957 micro-pillars were in good agreement with bulk yield stress values reported in the literature, suggesting that the lift-out micro-pillar compression testing technique is a promising method. The demonstration of these techniques on ion beam and neutron irradiated ODS steels and ion beam RAF/M steels gives information to inform models of the material degradation during use in a fusion device.

alloys↗
DOE OSTI2025

Porous Interlayers that Getter Surface-Segregating Species for Improved Silver Wetting, Adhesion, and Electrical Contact on Stainless Steel SOFC Components

Here, porous nickel interlayers or porous platinum interlayers were shown to promote the wetting, spreading, and adhesion of silver on alumina-forming ferritic stainless steel (AFFSS) and chromia-forming ferritic stainless steel (CFFSS). These interlayers resulted in dense, crack-free AFFSS|Ag-Ni|CFFSS and AFFSS|Ag-Pt|CFFSS braze joints that, after 300 h in 650 °C air, displayed shear strengths up to 70 MPa similar to, or larger than, those of AFFSS|Ag-CuO|CFFSS or AFFSS|Ag|CFFSS joints subjected to identical treatment. Similarly, after exposure to 25 cycles of (50 switches between) 12 h of 650 °C air and 12 h of 650 °C 4%H 2 –96%N 2 , AFFSS|Ag-Ni|CFFSS and AFFSS|Ag-Pt|CFFSS braze joints displayed shear strengths significantly larger than those of AFFSS|Ag-CuO|CFFSS or AFFSS|Ag|CFFSS joints subjected to identical treatment. In addition, nickel and platinum were found to chemically getter surface-segregating steel constituents, particularly Al from the AFFSS. As a result, Ag-Ni and Ag-Pt electrical contact resistances on AFFSS and CFFSS substrates were several orders of magnitude lower than those of conventional Ag-CuO reactive air brazes or Heraeus C8710 Ag contact pastes. Together, these results suggest that Ag-Pt and especially Ag-Ni may be useful for the fabrication of durable joints, seals, and/or electrical contacts in electrical/electrochemical devices exposed to high temperatures and/or variable oxygen partial pressure environments.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Fatigue Performance of an Improved Creep Strength 10%Cr Steel

The deployment of 9-12% Cr steels for elevated temperature applications up to 650 °C presents a cost-effective alternative to more expensive nickel-based alloys in steam turbine power generation. To enhance creep resistance at this temperature range, a novel ferritic-martensitic steel, designated CPJ7, was developed and fabricated at the National Energy Technology Laboratory. The alloy design aimed to mitigate the transformation of strengthening carbides into deleterious phases that degrade creep performance. Results have demonstrated that CPJ7 exhibits favorable creep and oxidation resistance at 650 °C. However, its fatigue performance remains unexplored. This study builds upon prior research by evaluating the low cycle fatigue behavior of CPJ7 and verifying that modifications beneficial to creep performance were not detrimental to the fatigue performance. The alloy was tested at both 650 °C and ambient temperature under fully reversed bending conditions (R = − 1) and a load ratio of 0.05. Furthermore, the alloy exhibits cyclic softening, a behavior consistent with other 9-10 wt.% Cr steels. Analysis of the microstructure and hysteresis loops further corroborate cyclic softening mechanisms typical of ferritic-martensitic steels. Overall, the fatigue performance of CPJ7 meets or exceeds that of P91 steel, demonstrating its potential for high-temperature structural applications.

9% Cr ferritic-martensitic steel↗
DOE OSTI2024

Thermomechanical Processing for Improved Mechanical Properties of HT9 Steels

Thermomechanical processing (TMP) of ferritic–martensitic (FM) steels, such as HT9 (Fe–12Cr–1MoWV) steels, involves normalizing, quenching, and tempering to create a microstructure of fine ferritic/martensitic laths with carbide precipitates. HT9 steels are used in fast reactor core components due to their high-temperature strength and resistance to irradiation damage. However, traditional TMP methods for these steels often result in performance limitations under irradiation, including embrittlement at low temperatures (<~430 °C), insufficient strength and toughness at higher temperatures (>500 °C), and void swelling after high-dose irradiation (>200 dpa). This research aimed to enhance both fracture toughness and strength at high temperatures by creating a quenched and tempered martensitic structure with ultrafine laths and precipitates through rapid quenching and unconventional tempering. Mechanical testing revealed significant variations in strength and fracture toughness depending on the processing route, particularly the tempering conditions. Tailored TMP approaches, combining rapid quenching with limited tempering, elevated strength to levels comparable to nano-oxide strengthened ferritic alloys while preserving fracture toughness. For optimal properties in high-Cr steels for future reactor applications, this study recommends a modified tempering treatment, i.e., post-quench annealing at 500 °C or 600 °C for 1 h, possibly followed by a brief tempering at a slightly higher temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Irradiation Creep in Materials

A knowledge of the dimensional stability of reactor structural components, under irradiation conditions, is of major importance in the design of thermal, fast, and fusion reactors. When subjected to simultaneous mechanical loading and irradiation, structural materials exhibit a visco-plastic deformation phenomenon, referred to as irradiation creep, which can be more rapid than the deformation occurring out of irradiation. In this article, the phenomenology of this peculiar behavior is described after a short history of its discovery. Then, the theoretical mechanisms proposed in the literature during these past 60 years are presented, with a special focus on mechanisms based on stress induced preferred absorption of point defects by dislocation loops, dislocations and grain boundaries and on mechanisms based on climb-enhanced glide of dislocations. These mechanisms are discussed in the light of experimental evidences. Finally, irradiation creep in various materials, such as zirconium alloys, austenitic stainless steels, nickel-based alloys, ferritic-martensitic steels and graphite, is described.

Onimus, Fabien↗
DOE OSTI2025

Calibration of RAFM Micromechanical Model for Creep Using Bayesian Optimization for Functional Output

A Bayesian optimization procedure is presented for calibrating a multimechanism micromechanical model for creep to experimental data of F82H steel. Reduced activation ferritic martensitic (RAFM) steels based on Fe(8–9)%Cr are the most promising candidates for some fusion reactor structures. Although there are indications that RAFM steel could be viable for fusion applications at temperatures up to 600°C, the maximum operating temperature will be determined by the creep properties of the structural material and the breeder material compatibility with the structural material. Due to the relative paucity of available creep data on F82H steel compared to other alloys such as Grade 91 steel, micromechanical models are sought for simulating creep based on relevant deformation mechanisms. As a point of departure, this work recalibrates a model form that was previously proposed for Grade 91 steel to match creep curves for F82H steel. Due to the large number of parameters (9) and cost of the nonlinear simulations, an automated approach for tuning the parameters is pursued using a recently developed Bayesian optimization for functional output (BOFO) framework (Huang et al., 2021, “Bayesian optimization of functional output in inverse problems,” Optim. Eng., 22, pp. 2553–2574). Incorporating extensions such as batch sequencing and weighted experimental load cases into BOFO, a reasonably small error between experimental and simulated creep curves at two load levels is achieved in a reasonable number of iterations. In conclusion, validation with an additional creep curve provides confidence in the fitted parameters obtained from the automated calibration procedure to describe the creep behavior of F82H steel.

42 ENGINEERING↗
DOE OSTI2022

Creep behavior of an additively manufactured 9Cr steel in the as-built condition

Limited studies have evaluated the creep behavior of additively manufactured (AM) ferritic-martensitic (FM) steels. This work investigated the creep behavior of a 9Cr FM steel fabricated by powder blown directed energy deposition (DED) technique. Here, the creep testing at 550–650 °C and 150 MPa for the specimens along the deposition direction in the as-built condition, together with corresponding microstructural characterization, revealed a threshold temperature between 600 and 625 °C, below which the steel has creep resistance comparable with Grade 91 cross-welds and noticeably greater than 9Cr-1Mo steel. The threshold temperature distinguishes the creep behavior in two regimes differentiated in creep activation energy, creep deformation, and failure mechanism. Unlike the creep rupture surface ~45° from the loading direction when tested above the threshold temperature, the creep rupture for testing below the threshold temperature resembles type IV failure in the cross-welds of ferritic steels. The DED-induced layer structure in the as-built steel played a significant role on the change of creep behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Creep performance and microstructure of grade 91 steel weldments with integrated welding and thermal processing

Ferritic-Martensitic steel welds typically require post weld heat treatment (PWHT) to restore toughness and high temperature performance. This off-line thermal process reduces disparities between weld and base metal, but can cause distortion, cracking, or simply be impractical due to assembly size and joint non-uniformity. Here we show integrated welding and thermal processing applied to modified 9Cr-1Mo (Grade 91) steel, favored for advanced power generation applications, performed in real time through the addition of a secondary heat source near the primary weld head. Optimal integrated processing reduces weld fusion and heat affected zone hardness by 125 HV, approaching performance of conventional 730 °C, 60 min PWHT processing. Microstructures and mechanical performance are compared for mechanized GTAW welds, with equivalent lifetimes noted in cross-weld creep rupture tests up to 234 MPa at 550 °C, and up to 104 MPa at 650 °C. The integrated process was validated on a Grade 91 pressure vessel with multipass cold wire feed GTAW. After 550 °C, 71.4 bar thermomechanical cyclic testing, the maximum weld hardness is <350 HV.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Report on partially complete post-irradiation-examination of the INL samples

Ferritic-martensitic steels (NF616 and T91), and austenitic stainless steel 800H and its Grain Boundary Engineering (GBE)-treated version 800H-TMP (ThermoMechanical Processing) were irradiated in the Advanced Test Reactor (ATR) at Idaho National Laboratory (INL) and their microstructures and mechanical properties were characterized. Ferritic-martensitic steels were irradiated up to 8.16 dpa in the irradiation temperatures ranging from 241 °C to 447.5 °C. Selected 800H and 800H-TMP samples were also irradiated up to 9.12 dpa at 359°C to 431°C. Vickers hardness measurement, fractography, and microstructural characterization were performed on the selected samples in the LAMDA lab. Radiation-induced hardening of NF616 and T91 was observed in the hardness measurements which show ~37% to ~65% increases, depending on irradiation dose and irradiation temperature. Within the irradiation conditions studied in this work, samples with a higher dose had a higher hardness. All the tested alloy 800H and 800H-TMP in this work showed irradiation-hardening by ~96±7% to ~152±10%. Alloy 800HTMP tended to have slightly smaller irradiation-hardening than alloy 800H. The fractography of NF616 samples D2 (2.96 dpa at 291.5°C), D4 (5.91 dpa at 359°C), and D6 (8.16 dpa at 431°C) indicated loss of ductility with negligible necking for the sample D2, yet obvious ductile failure for samples D4 and D6. Fractography of alloy 800H samples N4 (7.27 dpa at 359°C), N5 (3.9 dpa at 451.5°C), and N6 (9.01 dpa at 431°C), and alloy 800H-TMP samples P4 (7.36 dpa at 359°C), P5 (3.95 dpa at 451.5°C), and P6 (9.12 dpa at 431°C) indicated that all samples failed in a ductile mode, with obvious necking. Dimples were observed where large Ti-containing particles were observed for the samples. TEM characterization was performed on alloy 800H (N4, N5, N6) and 800H-TMP (P4, P5 and P6). Accumulation of M23C6 precipitates at grain boundaries was observed in all samples, and the presence of Ti(C,N) precipitates at grain boundaries and in the matrix was observed in alloy 800H-TMP. M23C6 precipitates were found to have the cube-on-cube orientation relationship with the matrix. In addition, nanoscale Si-rich clusters were observed in the matrix of all samples. Atom probe tomography was conducted on the same samples, and their results are being analyzed to be integrated with the TEM results for a confident description of the clustering behavior. Due to the low irradiation temperature (up to 451.5°C), dislocation loops formed in all samples, with the density and the average size of dislocation loops in the order of 1022 – 1023 m-3 and 11.7 – 15.9 nm, respectively. The loop densities in alloy 800H were higher than that in alloy 800H-TMP under the same irradiation conditions. Further systematic data analyses, together with some complementary experiments, will be pursued for these samples to foster peer-reviewed journal article publications.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2021

Microstructure and mechanical properties of high Mn-containing ferritic-martensitic alloys exposed to cyclical thermal treatment

Substantial residual tensile stress tends to accumulate in currently available high-Cr ferritic martensitic steels that are subjected to cyclical heat treatment, which leads to premature brittle fracture. By tailoring the alloy composition, this thermal cycling can be exploited to induce a high number density of nanoprecipitates and phase transformations countering residual tensile stresses. In this work, three new variants of ferritic-martensitic steels have been designed with computational thermodynamics to meet the goals of mitigating residual tensile stresses by lowering martensite start temperatures and of enhancing mechanical strength and irradiation sink strength by increasing the number density of nanoprecipitates. Furthermore, ctenast materials were subjected to cyclical heat treatment. The thermally cycled samples were evaluated with mechanical testing and microstructural analysis to identify the optimal composition in which figures of merit include low residual stress and a high density of nanoscale MX (M = metal, X = C/N) precipitates, leading to high yield strength with reasonable ductility. The noticeably higher density of nanoprecipitates in the optimal alloy favor its higher yield strength, which is supported by the microstructure-derived yield strength calculation and precipitation kinetics simulation.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Impact of Hydrogen Combustion on the Oxidation-induced Degradation of Heavy-duty Diesel Engine Piston Materials

High temperature ferritic-martensitic steels are candidate materials for heavy-duty diesel engine pistons. The envisioned transition to hydrogen blended fuels is expected to alter the post-combustion atmosphere in the engines, primarily resulting in a higher water vapor content (> 20 vol%) and potentially higher exhaust gas temperatures. The oxidation resistance of existing and newly developed alloys will be a critical life-limiting mechanism under these conditions. In the present work, the oxidation behavior of candidate piston alloys was evaluated in air+10 vol.% H2O and air+30 vol.% H2O at 700°C. Thermal cyclic (1h cycle) exposures were conducted for two variants of commercial UNSS42200 ferritic-martensitic steel and two developmental alloy steels for up to 300h. The developmental alloys each have similar compositions but with one containing elevated Cu levels of 3 wt.%, A significant reduction in resistance to breakaway oxidation was observed for the commercial alloys in the higher water vapor atmosphere. Microstructural characterization (optical metallography, scanning electron microscopy and electron microprobe analysis) revealed the formation of thick Fe-rich oxides even for the high Cr (~12 wt.%) steels after an initial stage of protective oxidation with MnCr-rich spinels. The impact of the evaporation-induced loss of Cr on the time and temperature dependent compositional changes in the alloys was correlated with experimental findings. For the developmental alloys, Cu additions appear to play a role in significantly reducing oxidation kinetics in air+30 vol.% H2O at 700°C.

Pillai, Rishi [ORNL] (ORCID:0000000243688197)↗
DOE OSTI2020

Microstructure and fracture toughness characterization of three 9Cr ODS EUROFER steels with different thermo-mechanical treatments

Ferritic martensitic ODS steels are one of the candidate structural materials for future Gen-IV nuclear fission and fusion reactors. Here, the dependence of fracture toughness on microstructure was investigated by comparing three 9Cr ODS EUROFER steels manufactured through different thermo-mechanical processing routes. Quasi-static fracture toughness testing was performed with sub-sized C(T) specimens and microstructural characterization was carried out using scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. It was found that at lower test temperatures (-100 – 22 °C), the fracture toughness was primarily controlled by crack initiation at sub-micron particles and by production of secondary cracks during fracture. At higher temperatures (above 100 °C), fracture toughness was predominantly controlled by the matrix ductility and the grain boundary strength with a relatively ductile coarse-grained alloy demonstrating higher fracture toughness compared to high-strength fine-grained alloys. These results and discussion show that variations in thermomechanical treatments can produce significant differences in microstructure and fracture toughness behavior of ferritic martensitic ODS steels.

36 MATERIALS SCIENCE↗