PA12 composites containing carbon or glass microfibers were printed by fused deposition modeling (FDM) and selective laser sintering (SLS) and exposed up to 20 MRad (Co-60 gamma irradiation). Specimens were analyzed by modulated differential scanning calorimetry (MDSC), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) to elucidate radiation-induced changes to crystallinity and thermal stability. Mechanical properties were assessed through tensile testing and post-irradiation fractography via scanning electron microscopy (SEM). The FDM melting process delivered more thermodynamically stable parts with improved polymer–polymer contact. DSC thermal cycling did not seem to heal additively manufactured products that suffered minor radiation damage at the highest exposures. The TGA data reveal high gamma irradiation lowers early stage decomposition temperatures (by 27 to 34 °C) of all composites regardless of print method. Neat PA12 SLS had a higher ultimate tensile strength and significantly smaller cross-head displacement at failure compared to FDM. In contrast, radiation-tolerant FDM prints were twice as strong as SLS at 20 MRad. While carbon and glass fibers increased the strength of PA12 FDM composites, this came with increased variation in measured tensile data and fiber pullout at 20 MRad. Together, the data suggest FDM is better than SLS for printing strong, resilient PA12 materials, whose structural integrity can be further enhanced with small (< 10 wt

The movement of heat through amorphous solids on an atomic level remains an outstanding question. Recent studies suggest that the primary thermal carrier in amorphous materials, propagons, essentially behaves like phonons. In this work, we provide experimental evidence that shows the interaction between propagons and phonons by utilizing the phase change chalcogenide germanium telluride. A series of ultra-long time-delay time-domain thermoreflectance measurements are used to analyze the scattering of vibrational thermal carriers at the boundaries of amorphous GeTe thin films relative to scattering across a crystalline-amorphous bilayer. We find that amorphous long wavelength propagons that would otherwise scatter can instead be hosted by a crystalline underlayer and its phonon population. This experimental evidence directly demonstrates propagon–phonon interactions in a clear experimental manner.

Injection molded (IM) and additively manufactured (AM) selective laser sintered (SLS) Nylon-12 was irradiated with gamma irradiation to determine the damage mechanism. Modulated differential scanning calorimetry was used to separate the thermodynamic (reversible) and kinetic (non-reversible) heat flows and evaluate how manufacturing method and radiation dose impacted the thermodynamics of melting, along with thermogravimetric analysis (TGA) to investigate thermal stability. Tensile testing and fractography were utilized to evaluate mechanical behavior. The bulk crystallinity of SLS Nylon-12 was not significantly affected by gamma radiation and was estimated to be 38.2 ± 2.3 α -form to γ -form and results in a lower void volume than SLS.

A plasma spheroidization treatment was applied to stock stainless steel 316L powder for additive manufacturing. The normal and treated powders were compared both in the powder state as well as in the resulting laser powder bed fusion (L-PBF) builds. The plasma spheroidization process slightly increased treated powder aspect ratio and sphericity and shifted the size distribution to larger diameters relative to the normal powder. The normal powder was austenitic in nature whereas the plasma spheroidization process introduced a small fraction (∼3.5 vol %) of ferrite in the treated powder. Ferrite in the powder was not retained in the printed samples and was not shown to negatively affect the build quality. Porosity areal fraction was generally smaller in the treated powder builds. The normal powder builds had a 6% higher yield strength than treated, however the scatter was significantly larger in the 45° and horizontal orientations compared to the treated powder builds.

An outstanding challenge in the manufacturing and joining of oxide dispersion strengthened steels is retaining the nanofeatures in the alloy throughout the fabrication and welding process. MA956 was friction stir welded with two different sets of welding parameters, resulting in a medium and high heat input. After welding, 5 MeV Fe ++ ion irradiations were performed at doses ranging from 50 to 200 dpa in the temperature range of 400 to 500 degrees C. Post-irradiation characterization was performed with scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy to investigate the Y-Al-O dispersoids, voids, and dislocations. After welding, the dispersoid microstructure coarsened, resulting in fewer and larger dispersoids regardless of heat input. After irradiation, the dispersoid behavior in the welded material was sensitive to temperature, exhibiting growth behavior attributed to Ostwald coarsening at 500 degrees C but a mixture of nucleation and more muted growth at 400 and 450 degrees C, attributed to competing mechanisms of radiation-enhanced diffusion and Ostwald coarsening. Void swelling correlated to heat input; being more prevalent in the welded conditions occurring at lower doses and in higher values relative to the base material. The low values of swelling despite microstructure coarsening caused by welding demonstrate the excellent swelling resistance of MA956, even after welding with the highest swelling values of 0.5% noted in the stir zone high heat input condition at 450 degrees C, 200 dpa. The dislocation behavior was inconsistent: the strongest trend was that network density was higher for welded versus base material, and an increase in loop diameter with temperature was observed. A rate theory model based on the observed microstructure suggests at high temperature interstitial loss to sinks was more likely to be dominant compared to mutual annihilation via point defect recombination, because of an increase of the radiation diffusion coefficient with temperature regardless of initial welded microstructure.Published by Elsevier B.V.

The joining process for oxide dispersion strengthened (ODS) alloys remains a key challenge facing the nuclear community. The microstructure and mechanical properties were characterized in the base material and friction stir welded ODS MA956 irradiated with 5 MeV Fe2+ ions from 400 to 500 degrees C up to 25 dpa. Nanoindentation was performed to assess changes in hardness and yield stress, and the dispersed barrier hardening (DBH) model was applied to described results. A combination of scanning transmission electron microscopy (STEM) and atom probe tomography (APT) were used to assess evolution of the microstructure including dispersoids, network dislocations and dislocation loops, nanoclusters, and solid solution concentrations. Overall, softening was observed as a result of increased dose, which was exacerbated at 500 degrees C. The formation and coarsening of new dispersoids was noted while nanoclusters tended to dissolve in the base material, and were not observed in the stir zone. Solute nanocluster evolution was identified as a primary driver of the changes in mechanical properties. Published by Elsevier B.V.

High Power Target systems are key elements in future neutrino and other rare particle production in accelerators. These systems transform an intense source of protons into secondary particles of interest to enable new scientific discoveries. As beam intensity and energies increase, target systems face significant challenges. Radiation damages and thermal shocks in target materials were identified as the leading cross-cutting challenges of high-power target facilities. Target material R&D to address these challenges are essential to enable and ensure reliable operation of future-generation accelerators. Irradiation facilities and alternative methods are critical to provide a full support of material R&D and better address these critical challenges.

In this work, we develop a numerical fitting routine to extract multiple thermal parameters using frequency-domain thermoreflectance (FDTR) for materials having non-standard, non-semi-infinite geometries. The numerical fitting routine is predicated on either a 2D or 3D finite element analysis that permits the inclusion of non-semi-infinite boundary conditions, which cannot be considered in the analytical solution to the heat diffusion equation in the frequency domain. We validate the fitting routine by comparing it with the analytical solution to the heat diffusion equation used within the wider literature for FDTR and known values of thermal conductivity for semi-infinite substrates ( SiO 2, Al 2 O 3, and Si). We then demonstrate its capacity to extract the thermal properties of Si when etched into micropillars that have radii on the order of the pump beam. Experimental measurements of Si micropillars with circular and square cross sections are provided and fit using the numerical fitting routine established as part of this work. Likewise, we show that the analytical solution is unsuitable for the extraction of thermal properties when the geometry deviates significantly from the standard semi-infinite case. This work is critical for measuring the thermal properties of materials having arbitrary geometries, including ultra-drawn glass fibers and laser gain media.

Determining the microstructure evolution of oxide dispersion-strengthened (ODS) alloys is important for predicting the safety and structural integrity of fast reactors. In particular, understanding the co-evolution of dispersoids with the dislocation loops and network is critical for a comprehensive understanding of the microstructure response to radiation. Ion irradiations were performed on oxide dispersion strengthened MA956 with 5 MeV Fe++ ions from 400 to 500 °C at doses ranging from 50 to 200 dpa. Characterization was performed primarily with scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy to investigate the Y-Al-O dispersoids, voids and dislocations. Regardless of temperature, the dispersoids increased in diameter and decreased in number density, which was attributed to an Ostwald coarsening mechanism supported by calculations of the radiation enhanced diffusion and ballistic dissolution. MA956 demonstrated excellent void swelling resistance and did not form voids except at 450 °C, 200 dpa where voids nucleated upon dispersoids. The dislocation loop diameter was highest at 500 °C followed by 400 °C then 450 °C while number density tended to decrease with dose. The dislocation behavior was explained as a function of the evolving defect kinetics, utilizing rate theory to calculate point defect concentrations and the increasing diffusivity of vacancies. At 400 °C, the interstitials had high enough diffusivity to nucleate new loops but vacancies remained relatively immobile. At 450 °C, vacancies are able to annihilate interstitials due to non-negligible mutual recombination causing the decreased number density of loops. At 500 °C, vacancy and interstitials are both mobile where the interstitials coalesce to form larger loops and vacancies provide a pathway for solutes diffusing to and from dispersoids.

The objective of this talk is to describe the state of the research on the testing and qualification of alloys produced by powder metallurgy with hot isostatic pressing (PM-HIP) for nuclear components. PM-HIP processing has attracted interest from the nuclear industry because this method enables complex components to be produced with microstructural uniformity and near-net shape, which simplifies inspection and reduces the need for machining and welding. However, PM-HIP materials must exhibit comparable or better thermal and irradiation stability as conventionally fabricated (i.e. cast, forged) materials. The team is currently in the middle of a multi-year project to evaluate these direct comparisons between PM-HIP and cast/forged material performance in high temperature, high stress, and high irradiation environments. This talk will provide an overview of results to date, and future directions to culminate in qualification of PM-HIP materials for nuclear applications.

Powder metallurgy with hot isostatic pressing (PM-HIP) is an advanced alloy processing method capable of fabricating complex nuclear reactor components near-net shape, reducing the need for machining and welding. For heat exchangers and steam generators, thermal aging of PM-HIP materials must be comparable or superior to conventional castings or forgings. This study compares thermal aging effects in PM-HIP and wrought alloy 625. Isothermal aging is carried out over 400–800°C for 100 h. Both PM-HIP and wrought materials have equiaxed grains with a uniform orientation distribution. The PM-HIP material has finer grains than the wrought material at all aging conditions. Both PM-HIP and wrought materials have a comparable hardness and modulus measured by nanoindentation. Hardness remains unchanged with aging except the wrought material aged at 800°C, which exhibits softening. Overall, PM-HIP alloy 625 responds comparably to wrought alloy 625 and is superior at 800°C. Results are used to calculate a Hall–Petch coefficient.

Interfaces play a critical role in heat dissipation for electronics packaging applications, thermoelectric energy conversion, data center cooling and renewable energy systems architectures. They are particularly influential as device length scales are reduced; in some cases, interfaces are fast becoming the dominant thermal resistors between heat source(s) and heat sink(s). Thus, the need to decrease interfacial thermal resistances (R-T) with the use of novel, highly thermally conductive materials (i.e. thermal interface materials, or TIMs) is paramount for the success of next-generation electronics and energy systems. Despite significant progress in the areas of materials synthesis and development, current thermal characterization techniques are not capable of reliably quantifying the resistance to heat flow across high-performance interfaces (i.e. R-T less than 1 mm(2) K/W). In this work, we develop a new steady-state, miniaturized heat meter bar apparatus capable of measuring R-T between 0.1 mm(2) K/W and 1 mm(2) KM with less than 10% uncertainty using infrared microscopy. Additionally, we demonstrate a related technique that permits the measurement of TIM thermal conductivity and thermal contact resistance (R-C) simultaneously with R-T for bonded TIMs having R-T between 0.1 mm(2) K/W and 1 mm(2) K/W. This work is expected to further the development of next-generation TIMs for use in high power density devices. Published by Elsevier Ltd.

High-Energy Diffraction Microscopy (HEDM) was employed to measure and compare the evolving micromechanical state of two alloys, an austenitic stainless steel (316 L) and nickel-based alloy (Inconel 625) fabricated by both conventional methods and powder metallurgy with hot isostatic pressing (PM-HIP) during in situ uniaxial tensile testing. Each of the four materials was tested through the elastic regime to just beyond yield. HEDM was performed at room temperature in the far-field (ff) configuration at the Cornell High Energy Synchrotron Source to measure grain-average elastic strains and subsequently derive stress tensors. The evolution of the normal stress component along the loading direction in individual grains as a function of macroscopic deformation is presented. Initially, grain-scale stresses in the loading direction are more heterogeneous in the wrought alloys than in the PM-HIP alloys. Notably, many peripheral grains in the wrought specimens are near yield even before load is applied. With increased loading, grain-scale stresses tend to homogenize in all specimens. Orientation fields measured using electron back scatter diffraction (EBSD) are used to determine grain morphologies and interpret the ff-HEDM data. The PM-HIP grains tend to be finer and rounder in shape than the wrought grains, potentially explaining the grain-scale stress distributions. Finally, yield strength and modulus of elasticity are measured for the four alloys and correlated to the resultant grain size and morphology from the fabrication processes.

Inconel 625 is a nickel-based superalloy of critical interest to both civilian and defense nuclear reactor community due to its excellent high temperature properties including corrosion resistance as well as creep resistance, making it ideal for internal reactor components such as steam generator tubes [1]. Traditionally, Inconel 625 has been wrought or cast during the manufacturing process but recently, powder metallurgy and hot isostatic pressing (PM-HIP) has been considered as a low cost alternative to fabricate components near-net shape. Thus, an analysis of the microstructure is required to determine if PM-HIP is suitable for use in reactor applications.

The effect of ion irradiation on the microstructure of oxide dispersion strengthened (ODS) MA956 steel, before and after friction stir welding (FSW), was studied. Both the base material (BM) and welded stir zone (SZ) were irradiated with 5 MeV Fe++ ions at 450 degrees C up to 25 displacements per atom (dpa). Characterization was performed using scanning transmission electron microscopy (STEM) and atom probe tomography (APT), with particular emphasis on the Y-Al-O dispersoid characteristics and dislocation microstructures. After irradiation, the dispersoids in the BM increased in diameter and decreased in number density, which was explained by an Ostwald ripening mechanism. FSW caused significant coarsening and agglomeration of the dispersoids. After irradiation, both the diameter and number density of the SZ dispersoids increased, which was explained by an irradiation-enhanced diffusion mechanism. Dislocation loop and network behavior was also characterized and large dislocation loops of approximate to 20 nm diameter formed by 1 dpa in both the BM and SZ samples, whereas the network density remained nearly constant with irradiation. Published by Elsevier B.V.

Understanding the void swelling and phase evolution of reactor structural materials at very high damage levels is essential to maintaining safety and longevity of components in Gen IV fast reactors. A combination of ion irradiation and modeling was utilized to understand the microstructure evolution of ferritic-martensitic alloy HT9 at high dpa. Self-ion irradiation experiments were performed on alloy HT9 to determine the co-evolution of voids, dislocations and precipitates up to 650 dpa at 460 degrees C. Modeling of microstructure evolution was conducted using the modified Radiation Induced Microstructure Evolution (RIME) model, which utilizes a mean field rate theory approach with grouped cluster dynamics. Irradiations were performed with 5 MeV raster-scanned Fe2+ ions on samples pre-implanted with 10 atom parts per million He. The swelling, dislocation and precipitate evolution at very high dpa was determined using Analytical Electron Microscopy in Scanning Transmission Electron Microscopy (STEM) mode. Experimental results were then interpreted using the RIME model. A microstructure consisting only of dislocations and voids is insufficient to account for the swelling evolution observed experimentally at high damage levels in a complicated microstructure such as irradiated alloy HT9. G phase was found to have a minimal effect on either void or dislocation evolution. M2X played two roles; a variable biased sink for defects, and as a vehicle for removal of carbon froth solution, thus promoting void growth. When accounting for all microstructure interactions, swelling at high damage levels is a dynamic process that continues to respond to other changes in the microstructure as long as they occur. Published by Elsevier B.V.

The effects of transmutation produced helium and hydrogen must be included in ion irradiation experiments to emulate the microstructure of reactor irradiated materials. Descriptions of the criteria and systems necessary for multiple ion beam irradiation are presented and validated experimentally. A calculation methodology was developed to quantify the spatial distribution, implantation depth and amount of energy-degraded and implanted light ions when using a thin foil rotating energy degrader during multi-ion beam irradiation. A dual ion implantation using 1.34 MeV Fe+ ions and energy-degraded D+ ions was conducted on single crystal silicon to benchmark the dosimetry used for multi-ion beam irradiations. Secondary Ion Mass Spectroscopy (SIMS) analysis showed good agreement with calculations of the peak implantation depth and the total amount of iron and deuterium implanted. The results establish the capability to quantify the ion fluence from both heavy ion beams and energy-degraded light ion beams for the purpose of using multi-ion beam irradiations to emulate reactor irradiated microstructures.

The growth of M2X phase in HT9 irradiated to high dpa was explored using self-ion irradiation. HT9 was pre-implanted with 10 appm He and irradiated with a raster-scanned Fe2+ beam with a damage rate of similar to 1 x 10(-3) dpa/s at 460 degrees C. The precipitation of M2X was observed and a combination of high resolution transmission electron microscopy (HRTEM), energy filtered transmission electron microscopy (EFTEM) and diffraction analysis was used to characterize the Cr-rich carbide observed at 250 dpa and above. Cr2C was determined to be semi-coherent with the matrix such that [100](Cr2C)//[100](alpha) and [001](Cr2C)//[10 (1) over bar](alpha).with a = 2.71 angstrom and c = 2.82 angstrom. Published by Elsevier B.V.