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Abstarct This work focuses on irradiation behaviors of a novel silicon carbide and carbon coated nanostructured ferritic alloy (SiC-C@NFA) composite for potential applications as a cladding and structural material for next generation nuclear reactors. The SiC-C@NFA samples were irradiated with 1 MeV Kr ions up to 10 dpa at 300 and 450 °C. Microstructures and defect evolution were studied in-situ at the IVEM-Tandem facility at Argonne National Laboratory. The effects of ion irradiation on various phases such as α-ferrite matrix, (Fe,Cr)7C3, and (Ti,W)C precipitates were evaluated. The α-ferrite matrix showed a continuous increase in dislocation density along with spatial ordering of dislocation loops (or loop strings) at >5 dpa. The size of the dislocation loops at 450 °C was larger than that at 300 °C. The nucleation and growth of new (Ti,W)C precipitates in α-ferrite grains were enhanced with the ion dose at 450 °C. This study provides new insight into the irradiation resistance of the SiC-C@NFA system. more...

This work focuses on fundamental understanding of microstructure evolution of nanostructured ferritic alloy (NFA) and 25 vol.% Cr3C2 coated SiC(Cr3C2@SiC)-NFA composite during spark plasma sintering at 950 °C and the following thermal treatment at 1000 °C. A unique bi-phase microstructure with distinct Cr-rich and Si-rich phases has been observed for the 25 vol.% Cr3C2@SiC-NFA composite, while for the NFA sample, the traditional large grain microstructure remains. Grain sizes are significantly smaller for the 25 vol.% Cr3C2@SiC-NFA composite compared to those for the pure NFA, which can be attributed to the presence of grain boundary phases in the composite sample. During the thermal treatment, microstructure features can be directly correlated with the dissolution kinetics and phase diagrams calculated using Thermo-Calc/DICTRA/PRISMA®. more...

In this study, bulk silicon oxycarbides (SiOCs) were fabricated from base polysiloxane (PSO) systems with different carbon content by using Ar or Ar + H2O pyrolysis atmosphere. Compared to the Ar pyrolysis condition, the SiOC samples pyrolyzed with water vapor plus Ar generally show lower ceramic yield except for the Tospearl (polymethylsilsesquioxane) sample at 1400 °C. The SiOC ceramics contain significantly less SiC and carbon after pyrolysis under Ar + H2O atmosphere compared to pure Ar atmosphere. The carbon-poor Tospearl sample shows a crystalline SiO2 structure (cristobalite) after pyrolysis at 1400 °C in Ar + H2O, which is also confirmed using TEM diffraction pattern analysis. TEM microstructures indicate little change in microstructures for the carbon-rich samples. The fundamentals, such as total Gibbs free energy, the driving force for crystallization, and phase contents at different pyrolysis temperatures can be calculated based on a Gibbs free energy minimization method. The phase content calculations predict considerable decrease in the amounts of SiC and C and significant increase in the percent of SiO2 after pyrolysis in Ar + H2O compared to Ar. The thermodynamic calculation results match with our experimental observations. This work provides a guided method to synthesize high temperature SiOCs with desired phases. more...

This study is to understand the effect of carbon content on the pyrolysis behaviors and phase contents of silicon oxycarbides (SiOCs). Flash pyrolysis conditions, evolution of different SiOC phases, and free carbon types/amounts are compared for C-rich and less C-rich precursors. The C-rich system experiences the flash event at a much lower pyrolysis temperature with a much higher current density even though the internal temperatures at flash are very similar. SiC formation is more obvious for the C-rich samples along with a much higher carbon content under both flash and traditional pyrolysis conditions. The phase contents of SiO2, SiC, and other SiOC intermediates can be calculated using a Gibbs energy minimization method, showing that the C-rich sample has more C-rich SiOC intermediates while the less C-rich sample has more Si-rich intermediates. This research provides a general framework in assessing the pyrolysis behaviors of different SiOC materials. more...

In the present work, the electrochemical corrosion tests were performed on single-crystal nickel and chromium with different surface orientations having (100), (110), and (111) facets, in a MgCl2-KCl eutectic molten salt at 500oC under an argon atmosphere. The corrosion rate of Ni is lower than that of Cr, indicating that Ni is electrochemically more stable than Cr in the molten salt. The anodic corrosion rates of different single crystals increase in the order of (111) < (100) < (110). This observation shows that the degree of electrochemical stability is related to the coordination of surface atoms and lower density surfaces yield higher corrosion rates. In addition, the corrosion rate of a polycrystalline sample is similar to that of the single-crystal (110) sample, which suggests that the corrosion rate of the polycrystalline surface is dominated by the orientation with the lowest surface density. This work was supported as part of the Molten Salts in Extreme Environments Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science. Keywords: Tafel, Nickel, Chromium, MgCl2-KCl, Single crystal. more...

The use of molten salts for large-scale solar concentrated power plants and molten salt reactors has been driving the research to better understand how metals and alloys interact with the molten salt. As the metals may undergo morphological, chemical, and structural change in molten salt environments, it is critical to understand the fundamental mechanisms in these changes. In this work, we will present how we utilized synchrotron X-ray nano-tomography to better understand the 3D morphological evolution of Ni, Cr, and their alloys in molten salt. The effects of temperature and additives in the salt on the morphological evolution will be discussed. At the higher temperature, a characteristic bicontinuous structure can form from molten salt dealloying a binary alloy. [1] This contrasts to the intergranular corrosion found in the same system reacted at a lower temperature. [2] Different additives in the salt were also found to alter the morphological changes of the alloys and can create planar corrosion, percolation dealloying, or redeposition. To complement the morphological studies by X-ray nano-tomography, a suite of X-ray and electron microscopy analyses were also carried out to better understand the chemical and structural (both short-and long-range ordering) evolution. Taking it as a multimodal approach, we will discuss how we couple the analysis from synchrotron operando X-ray absorption spectroscopy, diffraction, and imaging, as well as the multiscale imaging studies from both X-ray and electron microscopy. This work was supported as part of the Molten Salts in Extreme Environments (MSEE) Energy Frontier Research Center (EFRC), funded by the U.S. Department of Energy, Office of Science. References: [1] "Formation of three-dimensional bicontinuous structures via molten salt dealloying studied in real-time by in situ synchrotron X-ray nano-tomography" Xiaoyang Liu, Arthur Ronne*, Lin-Chieh Yu, Yang Liu, Mingyuan Ge, Cheng-Hung Lin, Bobby Layne, Phillip Halstenberg, Dmitry S. Maltsev, Alexander S. Ivanov, Stephen Antonelli, Sheng Dai, Wah-Keat Lee, Shannon M. Mahurin, Anatoly I. Frenkel, James F. Wishart, Xianghui Xiao & Yu-chen Karen Chen-Wiegart* Nature Communications (2021), DOI: 10.1038/s41467-021-23598-8 [2] "Visualizing time-dependent microstructural and chemical evolution during molten salt corrosion of Ni-20Cr model alloy using correlative quasi in situ TEM and in situ synchrotron X-ray nano-tomography" Kaustubh Bawane, Xiaoyang Liu, Ruchi Gakhar, Michael Woods, Mingyuan Ge, Xianghui Xiao, Wah-Keat Lee, Philip Halstenberg, Sheng Dai, Shannon Mahurin, Simon M. Pimblott, James F. Wishart, Yu-chen Karen Chen-Wiegart*, Lingfeng He* Corrosion Science (2021), DOI: 10.1016/j.corsci.2021.109962 more...

In this study, we investigated the type of dislocation loops formed in the proton-irradiated uranium-nitrogen-oxygen (U-N-O) system, which involves uranium mononitride (UN), uranium sesquinitride (α-U2N3), and uranium dioxide (UO2) phases. The dislocation loop formation is examined using specimens irradiated at 400°C and 710°C. Based on the detailed transmission-based electron microscopy characterization with i) the morphology-based on-zone and ii) the invisibility-criterion based two-beam condition imaging techniques, only a single type of dislocation loop in each phase is found: a/2⟨110⟩, a/2⟨111⟩, or a/3⟨111⟩ dislocation loops in UN, α-U2N3, and UO2 phases, respectively. Molecular statics calculations for the formation energy of perfect and faulted dislocation loops in the UN phase indicate a critical loop size of ∼ 6 nm, above which perfect loops are thermodynamically favorable. This could explain the absence of faulted loops in the experimental observation of the irradiated UN phase at two temperatures. This work will enhance the understanding of irradiation induced microstructural evolution for uranium mononitride as an advanced nuclear fuel for the next-generation nuclear reactors. more...

Oxidation behavior of silicon carbide (SiC) - carbon coated nanostructured ferritic alloy (C@NFA) composites was investigated in an air + 45 vol% H2O atmosphere at 500–1000 °C. All the composites show an oxidation structure with an outer Fe-rich layer and an inner Cr-rich layer, as well as internal oxidation along grain boundaries. Oxidation resistance increases with SiC addition. The corresponding fundamental mechanism is proposed. The improved oxidation resistance for the higher SiC content composites is attributed to a delay in ‘breakaway oxidation’ due to improved kinetics for the formation of dense Cr2O3 and SiO2 protective layers. more...

In situ monitoring of corrosion processes is important to fundamentally understand the kinetics and evolution of materials in harsh environments. A quasi in situ transmission electron microscopy technique was utilized to study microstructural and chemical evolution of a Ni-20Cr disc sample exposed to molten KCl-MgCl2 salt for 60 s in consecutive 20 s iterations. In situ synchrotron X-ray nanotomography was performed to characterize the morphological evolution of a Ni-20Cr microwire exposed to molten KCl-MgCl2. Both techniques captured key corrosion events and revealed mechanisms at different time and length scales, potentially bringing greater insights and deeper understanding beyond conventional analysis. more...

This work focuses on the oxidation resistance of a new class of composites, chromium carbide coated silicon carbide-nanostructured ferritic alloy (Cr3C2@SiC-NFA), in a water vapor containing atmosphere at 500–1000 °C. Oxidation temperature effects on surface morphologies, scale characteristics, and cross-sectional microstructures are investigated and analyzed. The Cr3C2@SiC content in the composites is strongly associated with the oxidation resistance by forming a dense Cr- and Si-rich inner-layer, which can be explained based on the Thermo-Calc simulation. The fundamental understandings offer important guidance for the applications of this class of composites in nuclear reactors and high temperature moist environments. more...

Silicon carbide and nanostructured ferritic alloy (SiC-NFA) composites have the potential to maintain the outstanding irradiation resistance and enhance the mechanical integrity for nuclear cladding. By introducing a carbon reaction barrier on NFA (C@NFA), SiC-C@NFA composites are investigated in order to reduce the reaction between SiC and NFA in this work. The densities of the spark plasma sintered (SPS) SiC-C@NFA composites show an increasing trend with the SiC content to almost 100% dense. Although the SiC phase is absent, the Vickers hardness of the composites reaches 436-638 kgf/mm2. The reaction leads to two types of regions in the composites, the NFA matrix and the micro-sized carbon enrich region, both of which contain extra Si element. The SiC-C@NFA composites show much improved microstructures and phases compared to the SiC-NFA composites without any coating, and the effectiveness of the carbon barrier is further verified based on the phase diagram and Gibbs free energy analysis. The SPS sintered SiC-C@NFA composites offer a new promising system for nuclear cladding. more...

Silicon carbide and nanostructured ferritic alloy (SiC-NFA) composites are expected to be remarkable candidates for nuclear cladding materials. However, SiC and NFA reactions during sintering have been a challenging problem to address. A Cr 3 C 2 coating on SiC particles is introduced as a reaction barrier layer to fabricate novel Cr 3 C 2 @SiC-NFA composites in this work. The composites of 5 vol% Cr 3 C 2 @SiC-95 vol% NFA, 15 vol% Cr 3 C 2 @SiC-85 vol% NFA, and 25 vol% Cr 3 C 2 @SiC-75 vol% NFA achieve > 96% densities under spark plasma sintering (SPS) at 950 °C and have the Vickers hardness of 5–6 GPa. The main phase for the composites maintains the original α-Fe structure of the NFA composition. Although the NFA matrix with silicon diffusion and carbon-rich aggregates are detected, the much improved microstructures in the composites indicate the positive effects of the Cr 3 C 2 coating as the reaction barrier. The Cr 3 C 2 @SiC-NFA composites are expected to be promising cladding materials in harsh nuclear environments. more...

This work focuses on the full characterization of dislocation loops induced by proton irradiation in single-crystal ThO2. Irradiation was performed using 2 MeV H+ ions with sample temperature at 600oC and a dose of up to 0.47 displacements per atom (dpa). Transmission electron microscopy (TEM) characterization was performed on a large number of dislocation loops. Burgers vector ( b → ) analysis using standard g → . b → = 0 invisibility criterion revealed different variants of 1 / 3 〈 111 〉 type dislocation loops. TEM analysis of edge-on dislocation loops was used to determine habit planes as { 111 } type. The nature of dislocation loops was revealed using the inside-outside contrast method as interstitial type. Rel-rod dark field images were obtained by selecting streak at g → = 1 / 2 [ 31 1 ¯ ] in diffraction pattern to identify the Frank loops present in the microstructure. Subsequent analysis of a single dislocation loop using atomic resolution scanning transmission electron microscopy (STEM) confirmed the interstitial nature of the loop with { 111 } habit plane. more...

Advanced nuclear reactor concepts aim to use fuels that must withstand unprecedented temperature and radiation extremes. In these fuels, thermal energy transport under irradiation is directly related to fuel longevity, reactor safety, and is arguably one of the most important performance metrics. Here we provide a comprehensive, first-principles-informed treatment of phonon mediated thermal transport in a defect-bearing actinide oxide with direct comparison to experimental measurements. Pristine and proton irradiated thorium dioxide was chosen as a model system to treat the complexity of thermal transport in the presence of lattice defects. A thermal transport model is implemented using the linearized Boltzmann transport equation (LBTE) with input from first principles calculations and defect evolution models. Density functional theory is used to calculate phonon dispersion in thorium dioxide and used as an input to calculate both intrinsic and extrinsic, defect-induced relaxation times. In addition, a defect evolution model is benchmarked using microstructure characterization of as-irradiated thorium dioxide using a combination of electron microscopy and optical spectroscopy. The output of the LBTE is compared directly to mesoscopic measurements of thermal conductivity on length scales commensurate with defect accumulation. Parametric measurements of conductivity with irradiation dose and temperature suggest a saturation in the reduction of thermal conductivity with increasing defect generation, which is partially captured in our defect evolution model and LBTE framework. This comprehensive, atomistic- to meso-scale treatment provides the necessary basis to investigate thermal transport under irradiation in more complex systems that exhibit strong electron correlation. more...

This work utilizes electron energy loss spectroscopy (EELS) to identify oxidation state of alloying elements in Ni-based alloys after exposure to molten chloride salt systems. Pure Ni and Ni-20Cr model alloy were corroded in molten ZnCl2 and KCl-MgCl2 under argon atmosphere at various temperatures. Oxidation states of Cr (Cr3+) and Ni (Ni2+) in the molten salt after corrosion were determined by monitoring changes in the L2,3 edges of corresponding EELS spectra. Oxidation state mapping technique using principal component analysis and multiple linear least squares fitting in HyperSpy Python package was developed. more...

Molten chloride and fluoride salts possess various properties such as good thermal conductivity and stability, large specific heat and low viscosity which make them important as fuels and/or coolants for molten salt reactors (MSR). Molten chloride salts are also selected as heat transfer and storage fluids to carry thermal energy from a solar concentrator to a steam generator in concentrating solar power (CSP) technologies. In both the applications the nickel-based alloys are used as structural materials. However high operating temperatures and extremely corrosive nature of molten salts promote faster corrosion of the structural materials. Hence, the corrosion behavior of the structural materials needs to be understood and monitored with time when exposed to the corrosive molten salt environment. Electrochemical techniques are useful for understanding the corrosion mechanism of metals and alloys in molten salts. Among them, electrochemical noise (ECN) is a simple in-situ technique to monitor the corrosion in real time without applying any external signals to the metal/alloy in corrosive environments. A unique advantage of ECN technique is the possibility to detect and analyze the early stages of the localized corrosion process during corrosion initiation as well as a type of corrosion. The corrosion of metals and alloys in molten chloride is strongly affected by impurities (O2, H2O) present in the atmosphere, temperature changes, and concentrations of various chemical impurities (metal oxides and metal chlorides). These impurities have been considered as strong oxidizing agents for metals and alloys in molten chloride salts. Chromium is a major alloying element in nickel-based alloys. Cr can be oxidized into metal chlorides CrClx (x=2,3) due to their favorable Gibbs free energies of the formation as compared to NiCl2 at 623 K. Cr3+ and Cr2+ coexist in the molten salt during corrosion of nickel-based alloys, while Cr3+ is the primary valence state. It is important to study the effect of these multi-valent ions on corrosion of metals/alloys in molten salts. To the best of our knowledge, no study on the effect of Cr3+ on the metals/alloys in molten chloride medium has been reported. The effect of CrCl3 on the corrosion behaviour of Ni and Ni-20 wt %Cr (NiCr) alloy in purified molten ZnCl2 at 623 K was investigated for the first time using ECN measurements in an argon-filled glovebox. ECN measurements were carried out in a three-electrode setup using two working electrodes and platinum as a reference electrode in molten CrCl3-ZnCl2 at 623 K. Ni and NiCr corrosion behavior was examined in two different combinations : 1) identically coupled electrodes (Ni-Ni and NiCr-NiCr vs Pt) and 2) galvanically coupled electrodes (Ni-NiCr vs Pt). Different types of current and potential noise fluctuations have been identified for pitting and other forms of corrosion. Analysis of fluctuations in current and potential noise in Ni and NiCr in all the three combinations concluded that Ni and NiCr exhibited localized corrosion. The addition of CrCl3 to ZnCl2 accelerated the dissolution of Cr and Ni at the metal-salt interface. After ECN studies, the working electrodes were examined by scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). STEM/EDS/EELS observations confirmed pit formation, Ni and Cr dissolution, and Cr depletion at the salt-metal interfaces. Ni-rich and Cr-depleted precipitates were observed in the salt with a galvanically coupled NiCr electrode after ECN studies. The in-depth ECN results will be discussed in conjunction with micro- and nano-scale imaging of dissolution of Ni and Cr elements at the metal-salt interface, and plausible corrosion mechanisms will be proposed. The present ECN results on Ni and NiCr are compared with ECN studies in pure ZnCl2 salt. This work was supported as part of the Molten Salts in Extreme Environments Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. more...

Silicon oxynitride (SiON) coated AISI 441 was prepared by pyrolysis of a dip-coated perhydropolysilazane (PHPS) precursor polymer. The high temperature oxidation behaviors of SiON coated AISI 441 substrates were studied in Ar + O2, Ar+H2O, and Ar + CO2 atmospheres at 800 °C for 100 h. The SiON coated AISI substrates showed better performance in all three atmospheres compared to the uncoated AISI 441 substrate. This improvement was attributed to the dense, crack-free SiON coatings, which provided complete separation between oxidizing species and the steel substrate. A thin oxide scale was observed on top of the SiON coating for all the atmospheres, which was attributed to the reactions between the oxidizing species in the atmosphere and the metallic species (Cr, Mn, and Fe) diffused through the SiON coating from the AISI 441 substrate. more...

Microstructures and tensile properties of Direct Metal Laser-Sintered (DMLS) CoCrMo were investigated in the as-printed condition and after heat treatment. A dense (> 99.5 pct) as-printed DMLS CoCrMo was obtained in the as-printed condition eliminating the need for any hot isostatic pressing. Solution heat treatment carried out at 1150 degrees C revealed complete recrystallization resulting in an equiaxed grain structure with an average grain size of 40 mu m. The microstructure after solution heat treatment and aging at 980 degrees C revealed inter and intragranular precipitations, enriched in Mo and Si. Solution treatment resulted in the decrease of the room-temperature tensile strength from 1378 MPa (as-printed) to 1114 MPa, which was attributed to the increasing grain size from 0.6 to 1 mu m (column width) to similar to 40 mu m (grain size). The decrease in yield strength was accompanied by the increasing ductility from 5.7 to 15 pct. An enhancement in ductility to nearly 25 pct was observed in tensile tests at 925 degrees C. This paper comprises a detailed microstructural evaluation of DMLS CoCrMo alloy to determine its suitability for high-temperature structural applications involving repair and refurbishment of components, including an evaluation of microstructural and tensile properties after welding the DMLS CoCrMo to cast FSX414. more...

Hot work tool steels like AISI H11 is used mainly for high temperature metal forming operations. Many operating difficulties and loss of production arise through die wear resulting in die failure. This work mainly focuses on the study of cryogenic treatment and its effect on wear performance of H11. Specimens of H11 were hardened (1293 K), double tempered (773 K each for 2 h), cryotreated (88 K) for varying periods viz., 8, 16, 24, 32 h and then soft-tempered at varying temperatures viz., 323 K, 373 K, 423 K, 473 K. Characterization techniques like SEM, hardness and Pin-On-Disc wear test were employed. Standardized parameters were analyzed for shift in wear regimes and finally a correlation between soft tempering temperature (STT) and the conventional tempering temperature (CTT) has been established. more...

In this work, the ion irradiation responses of a Fe-based nanostructured ferritic alloy or ‘NFA’ (Fe–9Cr–2W–0.2V–0.4Ti–0.3Y2O3) and a Cr3C2@SiC-NFA composite were assessed. In-situ ion irradiation with TEM observation was carried out by using 1 MeV Kr++ ions at doses of 0, 1, 3, 5, 10 dpa and temperatures of 300 °C and 450 °C. Both the NFA and Cr3C2@SiC-NFA samples showed significant dislocation density after 10 dpa at 300 °C. However, the Cr3C2@SiC-NFA composite showed a significantly lower dislocation loop density and a smaller average loop size during the irradiation at 450 °C as opposed to the NFA. At 300 °C, 1/2 type dislocation loops were observed in both the NFA and Cr3C2@SiC-NFA samples. Interestingly, at 450 °C, type loops were dominant in the NFA sample while 1/2 type loops were still dominant in the Cr3C2@SiC-NFA sample. The results were discussed based on the large surface sink effects and enhanced interstitial-vacancy recombination at higher temperatures. The additional Si element in the Cr3C2@SiC-NFA sample might have played a significant role in determining the dominant loop types. more...