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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.

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®.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.