Atomic and Microstructural Evolution of Mixed Spectrum Neutron-Irradiated Tungsten Alloys: Insights From Multimodal X-Ray Spectroscopy and Diffraction Characterization

Atomic and Microstructural Evolution of Mixed Spectrum Neutron-Irradiated Tungsten Alloys: Insights From Multimodal X-Ray Spectroscopy and Diffraction Characterization | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Atomic and Microstructural Evolution of Mixed Spectrum Neutron-Irradiated Tungsten Alloys: Insights From Multimodal X-Ray Spectroscopy and Diffraction Characterization D. J. Sprouster, T. Koyanagi, M. Ouyang, W. Zhong, D. Olds, A. Hasegawa, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8274574/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 11 You are reading this latest preprint version Abstract Tungsten is currently the leading candidate material for plasma-facing applications in fusion devices due to its high melting point and resistance to sputtering. However, its long-term stability is a concern as exposure to neutrons degrades mechanical properties. While high-fluence conditions with large Re and Os transmutation concentrations are known to cause embrittlement, the effects of lower fluence on microstructure and property evolution are less explored. In this study, we use X-ray Absorption Spectroscopy (XAS) and high-energy X-ray Diffraction (XRD) to quantify the atomic and microstructural properties of unirradiated and neutron-irradiated tungsten alloys. The alloys studied include single crystal W, polycrystalline W, K-doped W, W-3%Re, K-doped W-3%Re, and La-doped W-3%Re. Specimens were neutron-irradiated at 850°C and ~ 1100°C to ~ 0.5 displacement per atom. Our XAS results revealed multiple differences in the atomic environment across the W-alloy matrix. These differences were alloy-dependent and varied with irradiation temperature. After 850°C, non-body-centered cubic (BCC) Re and Os components were found with χ phase (Re 3 Os) and hexagonally close packed structures. After 1100°C, the final Re atomic structures were alloy-dependent, with Laves phase (Re 2 W), and χ-phase (Re 3 Os and Re 3 W) intermetallic environments quantified, while Os atoms were found in a body centered cubic and hexagonally close packed environments. XRD results indicated alloy-specific microstructural stability, with Re-La-W and Re-K-W alloys showing the least lattice swelling and lowest two-dimensional defect concentrations, while pure-W specimens show the highest irradiation-induced lattice expansions and two-dimensional defects. Understanding these early-stage evolutions is crucial for developing strategies to mitigate late-stage precipitation effects through alloy design. Physical sciences/Materials science Physical sciences/Physics Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Tungsten (W) and W-based alloys have found numerous technological applications in extreme environments due to their high melting point (~ 3400°C for W), high thermal conductivity in pure W (up to 175 W/m-K at 20°C), high density (~ 19.3 g/cm³), resistance to sputtering, and act as a stable matrix that will retain low tritium [ 1 ]. W and W-alloys have widespread use in nuclear applications, including as spallation targets for neutrinos and neutrons [ 2 – 5 ], as moderators for positron sources [ 6 ], as well being the leading plasma facing material candidate for near-term fusion devices [ 7 , 8 ]. However, W is known to be susceptibility to plasma-induced surface damage (cracking, erosion/exfoliation, and fuzz formation [ 9 ]), and bulk mechanical and thermal properties degrade due to exposure to neutron irradiation [ 10 – 15 ], raising significant concerns about its long term stability [ 7 , 16 ], A major concern in future fusion devices that rely on structural components fabricated with unalloyed-W (such as first wall tiles and diverters) is the impact of neutron irradiation and the formation of radiation-induced defects that can cause hardening and embrittlement. The high fraction of thermal neutrons in mixed-spectrum reactors can be leveraged to emulate some of the environmental extremes expected in fusion reactors that [ 1 , 10 , 17 ]. The neutron capture cross sections within the mixed-spectrum energy range for the naturally occurring isotopes in W and W-Re alloys (specifically 184 W, 186 W 185 Re and 187 Re) lead to the production of high transmutant elements [ 10 , 17 ]. Mixed-spectrum neutron irradiations have shown that the formation of transmutant rhenium (Re) and osmium (Os) elements impacts the mechanical properties, causing dispersoid-like hardening after intermediate fluences (about 1 displacement per atom, dpa) [ 10 , 12 – 14 ]. Beyond ~ 1 dpa, high concentrations of transmutant elements cause excessive hardening, loss of ductility and toughness, reduction in thermal conductivity, and major changes in the ductile-to-brittle transition temperature [ 10 ]. These changes have significant implications, especially as tungsten becomes brittle at temperatures where it would otherwise be ductile, severely limiting its ability to withstand thermal cycles anticipated in reactor operations. Advanced characterization methods capable of quantifying transmutant elements are essential to understand the nucleation and precipitation pathways of transmutation in W and W-based alloys. Advanced electron microscopy methods and X-ray scattering methods, including STEM-EDS and small-angle x-ray scattering, have been heavily used to investigate the precipitation and evolution of neutron-irradiated tungsten [ 10 , 18 – 23 ]. These methods can provide location and correlation of transmutation products to microstructural defects with Re and Os atoms observed to saturate grain boundaries, dislocations, and voids. The local atomic structures of the Re and Os atoms, cluster structures, and their impact on the lattice parameter of W-matrix are however difficult to quantify from microscopy [ 20 ]. The accurate inclusion of transmutation chemistry and impurities, including the amounts and partitioning of Re and Os to substitutional and precipitate environments, into atomic simulations and continuum models is needed [ 24 ]. This is essential to couple defect evolution, structure, and interaction with microstructural defects with mechanical and thermal property degradation and macroscopic performance [ 24 ]. Experiments that can quantify the local structure, such as the nature of stable configurations, intermetallic phase formation within the Re and Os populations, and partitioning as a function of fluence temperature and compositions are needed to aid in model development and validation [ 25 ]. X-ray absorption spectroscopy (XAS) is an element-specific, nondestructive method that can directly probe the local atomic structure of dilute elements within a host matrix [ 26 , 27 ]. XAS has routinely been employed to characterize radiation damage in numerous bulk and nanostructured materials [ 28 – 32 ] and can provide insight into the radiation-induced structural modifications with fluence and temperature. XAS can additionally provide the total amount of diluted elements within a host material by comparing the X-ray absorption edge-jumps of unknown specimens to those from standards with known amounts of diluted elements. When coupled with X-ray diffraction (XRD), the evolution of the bulk microstructural properties of the host W matrix can be directly linked to the Re and Os concentrations and structure. In this work, we have employed complementary XAS and XRD experiments to investigate the effects of neutron irradiation in single crystal W, polycrystalline W, K-doped W, W-3%Re, K-doped W-3%Re, and La-doped W-3%Re alloys. Analysis of the X-ray absorption near edge structure (XANES) provides quantitative insight into the structure of Re and Os atoms while XRD provides quantitative microstructural properties of the host matrix (lattice parameters from XRD peak positions and microstrain from peak broadening) as functions of matrix type and irradiation conditions. XANES also enabled determination of Re + Os concentrations and facilitates correlation of transmutant concentrations with microstructural, mechanical properties. We show that such coupled experiments can shed light on the phase stability of different neutron-irradiated W-alloys and provide evidence-based guidelines on alloy design to mitigate transmutation-based embrittlement. Materials And Methods The materials investigated in this work were high purity W (both single crystal and polycrystal), K-doped W, W-3%Re, K-doped W-3%Re, and La-doped W-3%Re. The specimens investigated here, are identical to those reported in [ 11 ]. The percentages represent the nominal ratio of Re in the alloys, and the K doping and La doping were 30 ppm and 1.07%, respectively. Detailed descriptions of the fabrications process, mechanical and thermal properties are discussed in previous publications [ 11 , 33 – 35 ]. As described in Ref [ 11 ], specimens were neutron irradiated in a mixed-spectrum for 24 days to fluences of 2.08 × 10 25 n/m 2 and 2.36 × 10 25 n/m 2 ( E > 0.1 MeV) in the peripheral target positions of the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. Specimens were irradiated within a rabbit capsule without thermal neutron shielding to promote the production of Re and Os. Two sets of specimens were irradiated at 850°C, and ~ 1100°C in separate capsules. The irradiation temperature of 850°C was experimentally determined using passive SiC thermometry, which was in contact with the tungsten specimens during irradiation. The analysis methodologies for the thermometry can be found elsewhere [ 36 ]. The irradiation temperature of 1100°C is nominal. There were challenges in determining the actual temperature using SiC thermometry, as reported in the reference [ 11 ]. The nominal thermal neutron fluence was 3 × 10 25 n/m 2 . A fluence of 1 × 10 25 n/m 2 ( E > 0.1 MeV) corresponding to 0.2 dpa in tungsten was used to determine the displacing damage levels [ 37 ] resulting in a displacement damage level, of 0.42 dpa for the 850°C capsule, and 0.47 dpa for the 1100°C capsule. Finally, a set of unirradiated specimens were annealed at 850°C for 24 days, to be comparable with one HFIR cycle at elevated temperature. These aged specimens were used as XRD and XAS standards for the elevated temperature irradiation. All specimens were 6mm diameter discs with thicknesses less than 0.3 mm to allow transmission XRD experiments. Characterization of the precipitates in K-doped W was performed using FEI Talos F200X transmission electron microscopy[2] (TEM), which is equipped with Super-X energy dispersive spectrometer system, allowing high resolution chemical analysis. High resolution TEM was also performed to identify the crystallography phase and the orientation relationship of precipitates with the matrix in K-doped W. Images were taken under the [110] zone of the W matrix. XAS and XANES measurements were performed at the BMM beamline at the NSLS-II. Room temperature fluorescence-mode XAS with a four‐element Si‐drift detector (SII Vortex ME4) at the Re and Os L III ‐Edges (10535; 10871 eV, respectively) were collected on as fabricated, as-annealed, and neutron irradiated specimens. XAS were measured with a collimation mirror (paraboloid mirror, 5 nm Rh on 30 nm Pt), Si (111) monochromator, and toroidal focusing mirror (5 nm Rh on 30 nm Pt). Six-to-ten spectra were averaged together to yield high quality XANES spectra for linear combination fitting, and quantification of the composition through edge-step analysis. As the XAS magnitude of the edge step is proportional to the number of absorbing atoms, the edge step of the irradiated specimens was compared to that of known specimens, W-10%Re-5%Os, W-5%Re-3%Os, W-3%Re-0.7%Os, W-3%Re, to facilitate quantification of the Re and Os concentrations[ 38 ]. Background subtraction, edge step quantification, spectra alignment and normalization of all XAS data was performed using ATHENA software [ 39 ]. XANES spectra for hexagonally close packed Re and Os, tetragonal σ-Re 2 W, cubic χ-Re 3 W and χ-Re 3 Os phases [ 40 ], and Laves (HCP) phase Re 2 W were simulated utilizing FEFF 9.0 [ 41 ], and .cif files from The Materials Project [ 42 ]. After energy calibration and normalization, XANES spectra were refined as a linear combination of the unirradiated BCC (Unirradiated W-3%Re, and W-3%Re-0.7%Os) and all intermetallic standards from 15 eV below to 80 eV above the Re edge (25 independent refinements), and Os edge (three independent refinements). χ 2 values were used to gauge the quality of the linear combination refinements. XRD experiments were performed at the Pair Distribution Function (PDF) beamline at the NSLS-II. All measurements were performed in transmission mode with an amorphous silicon-based flat panel detector (Perkin-Elmer) mounted orthogonally to the beam path. The sample-to-detector distance and tilts of the detector relative to the beam were refined using a LaB 6 powder NIST standard (SRM® 660c). The wavelength of the incident X-rays was 0.1665 Å at an energy of 74 keV. The sample-to-detector distance was calculated to be 1245.91 mm. 1200 XRD patterns were collected with detector exposures of 0.1s. Specimens were vertically and horizontally scanned to improve averaging, and to sample a large fraction of the individual specimens. All raw two-dimensional patterns were background corrected by subtracting a dark current image, and the air and Kapton scattering background within IgorPro (Wavemetrics)[ 43 ]. Noticeable artifact regions of the detector (like the beam stop, dead pixels) were masked. The corrected and masked two-dimensional detector images were then radially integrated to obtain powder diffraction patterns. The background subtracted XRD patterns were Rietveld refined within TOPAS (Bruker) software package. The peak profiles were modeled by a modified Thompson-Cox-Hasting pseudo-Voigt (TCHZ) peak function. The instrument contribution to the broadening of the measured profiles was quantified by fitting the LaB 6 NIST powder standard, with a known coherent grain size and negligible microstrain contribution. The Gaussian and Lorentzian-based broadening parameters were subsequently fixed during the analysis of the W alloys under investigation. The BCC W lattice parameter and microstrain components were allowed to be refined. A four-point polynomial background (Chebyshev Polynomial) was included in the refinements to capture the diffuse background. Results And Discussion Figure 1 (a) shows representative XANES spectra of different neutron irradiated specimens at the edges with BCC Re and HCP Re metallic references. The spectra of Re XANES, including the BCC, HCP, and intermetallic phases (FEFF simulated structures are given in supplementary information Figure S1 ) have different features labeled a-d. These features, in general, result from multiple-scattering resonances of the 2p 3/2 core level photoelectron to higher energy unoccupied states, primarily the 5d (5d 3/2 and 5d 5/2 ) states [ 44 , 45 ]. The different XANES fingerprints (heights and positions of features a-d) enable quantification of the different components through a linear combination refinements [ 46 ]. Such combination refinements are routine in dilute systems with XAS [ 27 , 29 , 47 – 49 ]. The linear combination fitting results of the XANES spectra are included in Fig. 1 (a) for the spectra shown (open symbols overlaid on the solid experimental data). Figure 1 (b) shows the separate contributions of the BCC and χ-Re 3 W phases to the spectrum of the 1%La W-3%Re neutron irradiated specimen, with residual intensity included for reference. Both Fig. 1 (a) and (b) demonstrate that while the BCC Re component is dominant in all irradiated specimens, additional non-BCC environments are evident and can be quantified from the spectral features associated with the characteristic phases. The XAS edge step analysis is shown in Fig. 1 (c) with raw X-ray absorption data, χµ(E), for three arc-melted W-Re-Os alloys and three neutron irradiated specimens. The composition of the three W-Re-Os alloys were determined by Wavelength-Dispersive Spectroscopy[ 38 ]. The lower edge steps for the irradiated single crystalline and polycrystalline specimens indicate that these specimens have lower Re concentrations relative to the W-3%Re standard (first yellow point with edge step = 218). The polynomial behavior of the edge step quantified from the standards is shown in Fig. 1 (d), with the calculated Re concentrations overlaid on the calibration curve for irradiated specimens. It is noted that the Re + Os concentrations determined here are close, but not identical, to the values determined from glow discharge optical emission spectroscopy[ 11 ]. Additionally, the trends in the neutron irradiated specimens Re + Os contents determined from the XAS edge step analysis are in agreement with those in Ref [ 11 ]. We in fact anticipated variations in composition when comparing these two methods, due to intrinsic differences in the physical modes associated with each technique, possible artifacts (namely amount of material sampled via each technique) and measurement sensitivity. Given the differences in values are understandable, the values from XAS were used to generate correlations in the specimen set investigated and discussed herein. The non-BCC fractions quantified from the XANES linear combination fitting, as a function of Re + Os concentration determined via the edge step analysis is shown in Fig. 2 . The results show both irradiation temperature and starting composition-dependent atomic structures, with multiple environments apparent in specimens post irradiation. Figure 2 (a) and (b) indicate that at 850°C an apparent threshold of 2 at% Re + Os is required before the intermetallic phases form with χ-Re 3 Os environments at this irradiation temperature. Figure 2 (a) and (b) show that higher starting Re generally leads to a higher intermetallic fractions, that increase with increasing Re + Os content. It is interesting to note that a persistent χ-Re 3 Os structure is quantified from both Re and Os XANES refinements. A notable finding in the alloys studied here is the W alloys containing a starting 3% Re form χ-Re 3 Os and HCP Re (and HCP Os) after irradiation (it is noted that one of the La-doped W-3%Re specimens did not return an HCP fraction). Lower fractions of HCP Os and Re 3 Os are quantified from the Os XANES, as shown in Fig. 2 (b), indicating a preference of Os remain substitutional in the W host. These features at 850°C contrast with the 1100°C irradiation in Fig. 2 (c) where all specimens, regardless of Re + Os content, show evidence of additional and complex environments including Laves Re 2 W, χ-Re 3 Os, and χ-Re 3 W (for three specimens). Unlike 850°C where Os atoms in a BCC environment were dominant, the 1100°C show a specimen-dependent HCP-Os and χ-Re 3 Os environments. At these Re + Os concentrations, the presence of the different phases after neutron irradiation is not unexpected, as irradiation-induced diffusion can promote the formation of high-temperature phases well away from equilibrium [ 13 , 24 , 50 , 51 ]. The dominant BCC environment determined for the Re (and Os) support prior computational studies whereby the precursors to the formation of the intermetallic precipitates were Re-rich coherent clusters with BCC-like structure [ 24 , 51 ]. The precipitate structure in the single crystal and polycrystalline tungsten specimens agree with prior similar irradiations [ 20 , 21 ] where STEM-EDS gave insight that the transmutation precipitates adopted χ-like structures. These results extend upon these previous studies, quantifying the fractions of BCC and non-BCC intermetallic phases, and evidence of complex transmutation precipitate pathways in W-alloys driven by the initial alloy chemistry and irradiation temperature. The Re 3 Os phase after irradiation at 850°C observed from the combined Re and Os XANES analysis, and combination of Laves σ-Re 2 W in the W alloys after 1100°C are additional new information and show the nuanced role starting structure and irradiation temperature have on the final precipitate microstructures. In future fusion reactors, the neutron energy spectrum will differ from that of the present HFIR irradiation environment, leading to different damage levels and transmutation rates [ 38 ]. How these differences influence the appearance and early evolution of transmutation precipitates could be an important issue for future studies. STEM-EDS images for neutron irradiated K-doped W specimens are shown in Fig. 3 for 850°C (a-e) and 1100°C (f-j). The STEM-EDS results show that After 850°C, (i) small nanometer-sized core-shell precipitates with Os rich cores are surrounded by Re-cloud-like shells; (ii) Re-rich loops are scattered throughout the grains; and (iii) larger grain boundary precipitates (~ 300 nm) exhibited a clear mixture of Re and Os. After 1100°C, panels (f-j) show that (i) the core-shell precipitates increase in size; and (ii) the density of small loops decreases. High resolution micrographs of K-doped W specimens irradiated at 1100°C are shown in Fig. 4 (a) and (e) for two different precipitates with corresponding Fast Fourier Transforms (FFTs) of the isolated precipitates given in (b) and (f). The characteristic FFTs for both precipitates are consistent with the cubic χ-phase, identical to the phase quantified from XANES. The simulated FFT patterns for each precipitate are shown in panels (c) and (g) respectively, where reflections from the W matrix are marked in black and those from the χ-phase in blue (panel (d) shows the diffraction pattern of [110] zone axis in the W host for refence). Two orientation relationships of the χ-phase with the W matrix were identified: [110] W || [110] χ , (002) W || (006) χ , and [110] W || [110] χ , (002) W || (4̅42̅) χ . Due to the much smaller precipitate sizes in the specimen irradiated at 850°C, no clear diffraction contrast was detected for the precipitates. This observation is consistent with the XANES results, where a higher fraction of non-BCC Re and Os (χ-phase) are present at the elevated irradiation temperature. Previous STEM-EDS of similar neutron irradiated polycrystalline W have also shown that the loop size, loop density and void density, peak at ~ 850–900°C and subsequently decrease at higher temperatures [20, 21]. The complex segregation behavior of the Re and Os atoms is also visible with STEM-EDS after neutron irradiation at 1200°C, whereby some Re form clouds around loops and voids, while Os tends to segregate into elongated shapes along the dislocation line of loops. For the same neutron fluence, the loop density and void density were found to decrease by ~ 50% when the irradiation temperature increased from 900 to 1200°C. The preferential segregation and saturation of Re and Os to these microstructural defects was also apparent, as well as at grain boundaries, akin to what we observe in Fig. 3 . The temperature-dependent trends in Figs. 2 – 4 for the single crystal, polycrystalline W and K-W specimens are consistent with past studies [ 12 – 15 , 20 , 21 , 38 ], whereby the Re and Os at 850°C is bound within the dense loop and void networks and unavailable to form precipitates at relatively low damage levels. At elevated temperatures, the Re and Os are less bound due to the lower densities of both loop and voids in addition to their faster diffusivity at higher temperature, thereby allowing the nucleation of intermetallic phases at GBs or regions with less restrictions. While HRTEM could not quantify a crystalline phase at this low fluence [ 20 , 21 ], our XANES and HRTEM results show direct evidence that in addition to the BCC component, the precipitates are consistent with χ-Re 3 Os after 850°C, while higher temperatures lead to χ-Re 3 Os, χ-Re 3 W, and Laves-Re 2 W intermetallic phases as well as HCP Re and HCP Os. The W host microstructure, that is grain size, grain boundary and dislocation densities and starting alloy chemistry clearly have important roles in influencing the subsequent precipitation microstructure, as they could provide sufficient sink sites for transmutation elements to segregate and precipitate. The trends in the W alloy specimens containing Re, La and K are more nuanced, as the higher Re contents generally lead to higher intermetallic fractions. Additionally, these alloys may not have the same microstructural defect evolution with neutron irradiation as high-purity W, thus it is unsurprising the defects that promote the nucleation of higher fractions of transmutation precipitates are unimpeded. Re is in fact known to impact the evolution of irradiation-induced defects, such as restricting the mobility of loops, leading to substantially different sizes and densities of loops and voids. Re changes the mobility of small dislocation loops and self-interstitial atom (SIA) clusters, such that solute Re would suppress void formation. It is thus potentially unsurprising that the Re and Os that would otherwise segregate to the two-dimensional defects is available to nucleate and form precipitates [ 52 , 53 ]. Future studies investigating the structure utilizing XANES in combination with HRTEM after high-fluence irradiation will be able to provide further evidence on the evolution and potential phase transformation of the transmutation precipitates. The XRD analysis described below gives evidence that the irradiation-induced microstructures in the Re-containing W-alloys are indeed different when compared to their pure-W counterparts. XRD patterns for representative baseline (W-3%Re and polycrystalline W) and irradiated specimens (1%La -W-3Re, W-3%Re, polycrystalline W and K-W) are shown in Fig. 5 . The Rietveld refinements are overlaid as solid black symbols for all XRD patterns shown. Panel (b) shows a high-angle region with (310), (222) and (321) hkl’s highlighted from panel (a), demonstrating some of the dominant features observable in the XRD patterns post-irradiation. These include shifts in the peak positions, changes in the XRD peak full widths at half maximum (FWHM), and changes in peak heights relative to the unirradiated baselines, highlighted for the polycrystalline and K-W specimens. These changes are indicative of lattice parameters changes (expansion or contraction, as discussed below) and microstructural changes including changes in the number of two-dimensional defects present in the specimens, namely an increase or decrease in the number of dislocation loops [ 54 – 56 ]. These microstructural changes were highly-dependent upon the starting material and irradiation temperature. It is noted that minor diffuse scattering is apparent in the high Re-containing alloys attributable to the nanocrystalline intermetallic phases evidenced by XANES. Panel (c) shows a highlighted view of a K-W-3%Re specimen irradiated at 850°C, and single crystal W specimen irradiated at 1100°C, with χ and HCP phases overlaying the diffuse background for each specimen (only HCP was observed in the K-W-3%Re specimen). The minor intensity from the transmutation precipitates and broad diffraction signals result from the small precipitate sizes and minor phase fractions [ 57 ]. Nevertheless, these subtle features do aid in the validation of the XANES and TEM above where intermetallic phases were quantified. The quantitative microstructural properties determined from the Rietveld refinements including change in lattice parameters and XRD-based microstructural broadening relative to the corresponding unirradiated counterparts, are shown in Fig. 6 (the full table of results are given in the appendix). Figure 6 (a) - (d) are plotted as functions of material type and Re + Os to highlight trends with different irradiation temperature and composition. Figure 6 (a) and (c) show the change in lattice parameters with open symbols representing the Δa determined for the baseline specimens relative to the polycrystalline W, while Fig. 6 (b) and (d) show the microstrain (XRD-based peak broadening). Figure 6 (a) shows that after irradiation at 850°C both polycrystalline W and W-%3Re alloys BCC lattice parameters expand with increasing Re + Os content. An opposing trend is observable in the W-alloy specimens with an apparent lattice contraction that increases with increasing Re + Os. The expansion, and contraction in the specimens irradiated at 850°C are attributed to both the irradiation induced defects (vacancies and interstitials), as well as the precipitates enabling a complex misfit-strain [ 58 ]. The corresponding microstrain in Fig. 6 (b) shows only an increase in the polycrystalline W specimens is apparent (relative to the unirradiated state), with all other W-alloys showing a decrease or negligible change in microstrain. Such changes are related to the two-dimensional defect networks, such as dislocation densities, and these results show a clear suppression in the number of irradiation-induced defects (at 850°C) in the W-alloy specimens. This suppression is due to the intrinsic number of defect sinks intentionally engineered into their microstructures including K-bubbles and coherent clusters [ 33 ]. A lattice contraction is quantified for all specimens after irradiation at 1100°C, as shown in Fig. 6 (c) which is linearly correlated with Re + Os content. The similarity in the contraction of the baselines (open symbols) and neutron irradiated specimens (closed symbols) indicate that the Re and Os atoms are likely substitutional dopants, with additional minor misfit strain from the transmutation precipitates. The substitutional nature of the Re and Os from both XRD and XANES aids in confirming the reduction in thermal conductivity recently determined for these specimens [ 11 ]. The microstrain after 1100°C in panel (d) shows a similar linear trend to panel (c), albeit with positive values of microstrain and decreasing values with increasing Re + Os. The irradiation-induced two-dimensional defects, as quantified from the microstrain parameters, are clearly compositionally-dependent. Here, the W-alloys containing K-Re and La-Re all appear to be more radiation tolerant (or contain less two-dimensional defects) when compared to the polycrystalline-W and K-W specimens. The microstructural features that likely drive the trends in microstrain are dislocation loops, small defect clusters (< 2nm), changes in grain boundary character, radiation-induced voids, and point defect clusters [ 11 , 59 , 60 ]. To correlate the XRD microstructural and transmutation products results with changes in mechanical properties, we reproduce the hardness for these specimens, leveraged from [ 11 ] and shown in Fig. 7 . At 850°C the polycrystalline W and W-3%Re specimens showed a decrease in hardness after irradiation, while the La-Re and K-Re containing alloys increased in hardness (except for the K-W that showed a decrease in hardness). At 1100°C all specimens show a decrease in hardness, with the notable exception of the single crystal W, with the magnitude approaching zero with increasing Re + Os content. Figures 6 and 7 show that the as-irradiated lattice strain is correlated to hardness. The lattice misfit in these alloys is noted to be a complex function of compressive lattice strain imposed by the La and K grain stabilizers, Re alloy content, and coherent precipitates, that alter the number and nature of point defects. Unlike the thermal conductivity, where depressions were related to substitutional Re and Os, the changes in mechanical properties cannot be accounted for by substitutional doping alone. The changes in hardness are more consistent with complex microstructural states that include the Re and Os substitutional dopants and radiation-induced defects (small clusters, transmutation precipitates, dislocations, voids, point defects). Conclusions In this work, we quantify the microstructural response and metallic transmutation product environments of neutron irradiated tungsten specimens, including single crystal W, polycrystalline W, K-doped W, W-3%Re, K-doped W-3%Re, and La-doped W-3%Re. The alloys were irradiated to 0.42 dpa 850°C and 0.47 dpa at 1100°C in the High Flux Isotope Reactor. The local atomic environment of the Re and Os elements were directly probed by XANES, while the microstructure was probed with high-energy XRD. The intrinsic sensitivity of XANES to the Re and Os elements facilitated uncovering multiple new insights into the alloy- and irradiation temperature-dependent atomic environment evolution of the Re and Os that ensued after neutron irradiation. After 850°C irradiation, non-BCC Re and Os fractions were predominantly found in the χ-Re 3 Os and HCP phases. After 1100°C irradiation, the final Re atomic structures were alloy-dependent, with Laves phase Re 2 W, and χ phase (Re 3 W or Re 3 Os), while Os atoms were found in χ-Re 3 Os and HCP environments. The quantitative XRD results indicated alloy-specific microstructural stability, with Re-La and Re-K containing alloys showing the least lattice swelling and lowest two-dimensional defect concentrations. The Re-free polycrystalline specimens showed the highest irradiation-induced lattice expansions and two-dimensional defect populations, consistent with previous mechanical and thermal property degradation studies. Understanding these early-stage (low-to-intermediate fluence) evolutions is crucial for developing strategies to mitigate late-stage precipitation effects through alloy design. These findings highlight the importance of understanding the effects of neutron irradiation on tungsten alloys to develop more resilient materials for use in extreme environments, such as plasma facing components in fusion reactors. Our approach shown here for transmutation precipitation is by no means limited by the mixed neutron spectrum and Tungsten alloy specimens. Future work, employing combined XANES, TEM and XRD methods, to investigate other relevant specimens irradiated with different neutron energy spectra such as shielded environments to mimic future fusion reactors is currently ongoing. Such studies will provide deeper insight into the evolution of transmutation precipitation. Declarations Acknowledgements: DJS and TK thank K. Wehunt and B. Heneveld of Brookhaven National Laboratory for their help with handling radioactive specimens at NSLS-II. Patricia Tedder, at ORNL, coordinated the shipment of radioactive materials for the beamline experiments. We thank Dr. Bruce Ravel from the National Institute of Standards and Technology for his support of these experiments at BMM, and in spurring scientific discussions and rewarding analysis directions. Funding: These experiments and analysis were supported by the DOE Office of Fusion Energy Sciences under contract DE-SC0018322 with the Research Foundation for the State University of New York at Stony Brook and DE-AC05-00OR22725 with UT-Battelle LLC. This research used resources at the BMM and PDF beamlines of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. The XRD and XAS analysis were supported by the Research Foundation for the State University of New York at Stony Brook. A portion of this research used resources at the HFIR, a US Department of Energy Office of Science User Facility operated by ORNL. This work was also partially supported by the US-Japan PHENIX and FRONTIER Collaboration Project supported by the US Department of Energy, Office of Science, Fusion Energy Sciences and Ministry of Education, Culture, Sports, Science and Technology, Japan. Author Contributions: DJS, TK and LLS conceived and designed the experiments. DJS, MO, TK and DO performed the synchrotron experiments. DJS analyzed the XAS and XRD data. WZ performed the electron microscopy experiments and analysis tasks. AH, TK, and YK oversaw the neutron irradiation campaign. DJS and TK wrote the manuscript. All authors discussed and commented on the manuscript. Competing Interests: No, I declare that the authors have no competing interests as defined by Nature Portfolio, or other interests that might be perceived to influence the results and/or discussion reported in this paper. Data Availability: Correspondence and requests for materials should be addressed to DJS References S.J. Zinkle, L.L. 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09:47:56","extension":"png","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":75963,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/d7b11f95d355e7b21ef31de4.png"},{"id":100667299,"identity":"52330750-33c2-485f-aabe-eff8d0fdf0b5","added_by":"auto","created_at":"2026-01-20 09:46:20","extension":"png","order_by":16,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":62916,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/b9b8e5bd3fe2f44db0f2b111.png"},{"id":100667570,"identity":"726cda83-e21f-4c91-86bf-a8cead225253","added_by":"auto","created_at":"2026-01-20 09:48:08","extension":"png","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":33327,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/340bf48de4d5537ad4640fb1.png"},{"id":100667172,"identity":"8548eda8-9400-43b8-bfd8-bd44db7e552b","added_by":"auto","created_at":"2026-01-20 09:45:53","extension":"xml","order_by":18,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":149101,"visible":true,"origin":"","legend":"","description":"","filename":"a6903c3851b84c56942a9b9c59c34ccf1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/7d5bcfedc2c035ec1bba3ae6.xml"},{"id":100667540,"identity":"94a70efa-1f2b-42d8-9e6f-4b802c4a14df","added_by":"auto","created_at":"2026-01-20 09:48:04","extension":"html","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":159247,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/3ad360224856d4a423c44154.html"},{"id":100667259,"identity":"803e5c0d-b9b9-4aaf-9224-652cad5eed99","added_by":"auto","created_at":"2026-01-20 09:46:09","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":580782,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Representative Re L\u003csub\u003eIII\u003c/sub\u003e XANES spectra of BCC (W-3%Re) and HCP (metallic Re) standards and neutron irradiated W specimens. The open black symbols are the linear combination fitting results. (b) Example of the linear combination fitting results showing the BCC and intermetallic components and residual from linear combination fitting. (c) Raw XANES absorption showing the effect of Re content on the edge step. (d) Calibration curve used to quantify Re content from known W-Re standards with open yellow symbols, open blue symbols are Re contents determined for the irradiated specimens.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/a3b360faa4609c75517621e3.png"},{"id":100667249,"identity":"cb5a8d5a-b1ea-4721-9d7c-f0dc81c6c35d","added_by":"auto","created_at":"2026-01-20 09:46:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":357657,"visible":true,"origin":"","legend":"\u003cp\u003enon-BCC fractions of Re and Os atoms quantified from the XANES linear combination fitting for all neutron irradiated W specimens. Data is represented as a function of Re+Os content determined from the XANES edge-step analysis. Panel (a) and (b) show the specimens irradiated to 0.42 dpa at 850 °C, and panel (c) and (d) show the specimens irradiated to 0.47 dpa at 1100 °C. Colored\u003cstrong\u003e \u003c/strong\u003ecircles are used to highlight phases of the non-BCC components. Nested circles of two colors represent two phases\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/232f2fc388c862c617de54d7.png"},{"id":100667609,"identity":"d19c14f9-04be-4f78-8d05-2803319e6146","added_by":"auto","created_at":"2026-01-20 09:49:03","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1606780,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003ebright filed STEM micrographs (a) and (f), with corresponding EDS maps of W, Re, Os, and Re+Os elements and (b-e) and (g-j) for K-doped W irradiated at 850 °C and 1100 °C, respectively.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/d74d8c6ba86c2f4e0793d05d.png"},{"id":100667167,"identity":"d0ee2088-19df-4dbf-aeb6-faa7b3e7b6bd","added_by":"auto","created_at":"2026-01-20 09:45:49","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1482642,"visible":true,"origin":"","legend":"\u003cp\u003e(a) (e) HRTEM, (b) (f) Fast Fourier transform and (c) (g) simulated patterns and indexing of X phase precipitates in K-doped W irradiated at 1100°C. The images were taken under [110] zone of the W matrix, with the diffraction pattern shown in (d).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/63c5e15ee8edd32bb69a79fe.png"},{"id":100667537,"identity":"15b93088-4a05-4985-8c71-f80fb4b6bed1","added_by":"auto","created_at":"2026-01-20 09:47:59","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":473187,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of baseline and neuron irradiated W specimens. (b) shows a highlighted region of (a) with (310), (222) and (321) hkl’s identified. Open symbols are Rietveld refinements to the experimental data. The microstructural features are discussed in the body of the article. (c) XRD patterns highlighting the diffuse and weak signal from additional nanocrystalline transmutation product phases in neutron irradiated K-W-3%Re and single crystal W.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/de6b7faa2795f61be8fef03e.png"},{"id":100667432,"identity":"dc99f554-fb07-4e7c-b50b-6468d1766cb4","added_by":"auto","created_at":"2026-01-20 09:46:47","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":302864,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eQuantitative microstructural properties- (a) and (c) lattice parameters, and (b) (d) microstrain parameters determined from the XRD analysis for the BCC host matrix. Panels (a)-(b) correspond to 850 °C and (c)-(d) correspond to 1100 °C irradiation. The changes in the lattice parameters and microstrain parameters were determined relative to their unirradiated and unirradiated-aged baselines. Error bars from the quantitative refinements fall behind the symbols (in most cases). Dashed lines are guides only.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/e33200b077cbc1fa31876d4f.png"},{"id":100667396,"identity":"d836538b-658a-4214-8973-6b74630c9241","added_by":"auto","created_at":"2026-01-20 09:46:36","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":145283,"visible":true,"origin":"","legend":"\u003cp\u003eChange in Hardness for all W specimens (relative to unirradiated baselines), as a function of Re+Os content. Mechanical data was extracted from [11]. Dashed lines show the microstructure-specific increase and decrease in hardness discussed in the main body.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/efb0c15366d3b96007a1b5b5.png"},{"id":100804067,"identity":"a0b50c02-2105-404e-9078-8c12bb7f0a87","added_by":"auto","created_at":"2026-01-21 14:35:50","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6231802,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/61d4a3ec-5f8e-4b50-8dc1-fdeb138b93d4.pdf"},{"id":100667302,"identity":"dc2af555-e26e-457c-9a8d-d982b55bc5ee","added_by":"auto","created_at":"2026-01-20 09:46:22","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":98209,"visible":true,"origin":"","legend":"","description":"","filename":"SuppdataWMXC2025.docx","url":"https://assets-eu.researchsquare.com/files/rs-8274574/v1/d67b7c0679bd3d61c4190f80.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Atomic and Microstructural Evolution of Mixed Spectrum Neutron-Irradiated Tungsten Alloys: Insights From Multimodal X-Ray Spectroscopy and Diffraction Characterization","fulltext":[{"header":"Introduction","content":"\u003cp\u003eTungsten (W) and W-based alloys have found numerous technological applications in extreme environments due to their high melting point (~\u0026thinsp;3400\u0026deg;C for W), high thermal conductivity in pure W (up to 175 W/m-K at 20\u0026deg;C), high density (~\u0026thinsp;19.3 g/cm\u0026sup3;), resistance to sputtering, and act as a stable matrix that will retain low tritium [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. W and W-alloys have widespread use in nuclear applications, including as spallation targets for neutrinos and neutrons [\u003cspan additionalcitationids=\"CR3 CR4\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], as moderators for positron sources [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], as well being the leading plasma facing material candidate for near-term fusion devices [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. However, W is known to be susceptibility to plasma-induced surface damage (cracking, erosion/exfoliation, and fuzz formation [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]), and bulk mechanical and thermal properties degrade due to exposure to neutron irradiation [\u003cspan additionalcitationids=\"CR11 CR12 CR13 CR14\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], raising significant concerns about its long term stability [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e],\u003c/p\u003e \u003cp\u003eA major concern in future fusion devices that rely on structural components fabricated with unalloyed-W (such as first wall tiles and diverters) is the impact of neutron irradiation and the formation of radiation-induced defects that can cause hardening and embrittlement. The high fraction of thermal neutrons in mixed-spectrum reactors can be leveraged to emulate some of the environmental extremes expected in fusion reactors that [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. The neutron capture cross sections within the mixed-spectrum energy range for the naturally occurring isotopes in W and W-Re alloys (specifically \u003csup\u003e184\u003c/sup\u003eW, \u003csup\u003e186\u003c/sup\u003eW \u003csup\u003e185\u003c/sup\u003eRe and \u003csup\u003e187\u003c/sup\u003eRe) lead to the production of high transmutant elements [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Mixed-spectrum neutron irradiations have shown that the formation of transmutant rhenium (Re) and osmium (Os) elements impacts the mechanical properties, causing dispersoid-like hardening after intermediate fluences (about 1 displacement per atom, dpa) [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Beyond ~\u0026thinsp;1 dpa, high concentrations of transmutant elements cause excessive hardening, loss of ductility and toughness, reduction in thermal conductivity, and major changes in the ductile-to-brittle transition temperature [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. These changes have significant implications, especially as tungsten becomes brittle at temperatures where it would otherwise be ductile, severely limiting its ability to withstand thermal cycles anticipated in reactor operations.\u003c/p\u003e \u003cp\u003eAdvanced characterization methods capable of quantifying transmutant elements are essential to understand the nucleation and precipitation pathways of transmutation in W and W-based alloys. Advanced electron microscopy methods and X-ray scattering methods, including STEM-EDS and small-angle x-ray scattering, have been heavily used to investigate the precipitation and evolution of neutron-irradiated tungsten [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan additionalcitationids=\"CR19 CR20 CR21 CR22\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. These methods can provide location and correlation of transmutation products to microstructural defects with Re and Os atoms observed to saturate grain boundaries, dislocations, and voids. The local atomic structures of the Re and Os atoms, cluster structures, and their impact on the lattice parameter of W-matrix are however difficult to quantify from microscopy [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The accurate inclusion of transmutation chemistry and impurities, including the amounts and partitioning of Re and Os to substitutional and precipitate environments, into atomic simulations and continuum models is needed [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. This is essential to couple defect evolution, structure, and interaction with microstructural defects with mechanical and thermal property degradation and macroscopic performance [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eExperiments that can quantify the local structure, such as the nature of stable configurations, intermetallic phase formation within the Re and Os populations, and partitioning as a function of fluence temperature and compositions are needed to aid in model development and validation [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. X-ray absorption spectroscopy (XAS) is an element-specific, nondestructive method that can directly probe the local atomic structure of dilute elements within a host matrix [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. XAS has routinely been employed to characterize radiation damage in numerous bulk and nanostructured materials [\u003cspan additionalcitationids=\"CR29 CR30 CR31\" citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e] and can provide insight into the radiation-induced structural modifications with fluence and temperature. XAS can additionally provide the total amount of diluted elements within a host material by comparing the X-ray absorption edge-jumps of unknown specimens to those from standards with known amounts of diluted elements. When coupled with X-ray diffraction (XRD), the evolution of the bulk microstructural properties of the host W matrix can be directly linked to the Re and Os concentrations and structure.\u003c/p\u003e \u003cp\u003eIn this work, we have employed complementary XAS and XRD experiments to investigate the effects of neutron irradiation in single crystal W, polycrystalline W, K-doped W, W-3%Re, K-doped W-3%Re, and La-doped W-3%Re alloys. Analysis of the X-ray absorption near edge structure (XANES) provides quantitative insight into the structure of Re and Os atoms while XRD provides quantitative microstructural properties of the host matrix (lattice parameters from XRD peak positions and microstrain from peak broadening) as functions of matrix type and irradiation conditions. XANES also enabled determination of Re\u0026thinsp;+\u0026thinsp;Os concentrations and facilitates correlation of transmutant concentrations with microstructural, mechanical properties. We show that such coupled experiments can shed light on the phase stability of different neutron-irradiated W-alloys and provide evidence-based guidelines on alloy design to mitigate transmutation-based embrittlement.\u003c/p\u003e"},{"header":"Materials And Methods","content":"\u003cp\u003eThe materials investigated in this work were high purity W (both single crystal and polycrystal), K-doped W, W-3%Re, K-doped W-3%Re, and La-doped W-3%Re. The specimens investigated here, are identical to those reported in [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The percentages represent the nominal ratio of Re in the alloys, and the K doping and La doping were 30 ppm and 1.07%, respectively. Detailed descriptions of the fabrications process, mechanical and thermal properties are discussed in previous publications [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan additionalcitationids=\"CR34\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. As described in Ref [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], specimens were neutron irradiated in a mixed-spectrum for 24 days to fluences of 2.08 \u0026times; 10\u003csup\u003e25\u003c/sup\u003e n/m\u003csup\u003e2\u003c/sup\u003e and 2.36 \u0026times; 10\u003csup\u003e25\u003c/sup\u003e n/m\u003csup\u003e2\u003c/sup\u003e (\u003cem\u003eE\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.1 MeV) in the peripheral target positions of the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. Specimens were irradiated within a rabbit capsule without thermal neutron shielding to promote the production of Re and Os. Two sets of specimens were irradiated at 850\u0026deg;C, and ~\u0026thinsp;1100\u0026deg;C in separate capsules. The irradiation temperature of 850\u0026deg;C was experimentally determined using passive SiC thermometry, which was in contact with the tungsten specimens during irradiation. The analysis methodologies for the thermometry can be found elsewhere [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The irradiation temperature of 1100\u0026deg;C is nominal. There were challenges in determining the actual temperature using SiC thermometry, as reported in the reference [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The nominal thermal neutron fluence was 3 \u0026times; 10\u003csup\u003e25\u003c/sup\u003e n/m\u003csup\u003e2\u003c/sup\u003e. A fluence of 1 \u0026times; 10\u003csup\u003e25\u003c/sup\u003e n/m\u003csup\u003e2\u003c/sup\u003e (\u003cem\u003eE\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.1 MeV) corresponding to 0.2 dpa in tungsten was used to determine the displacing damage levels [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e] resulting in a displacement damage level, of 0.42 dpa for the 850\u0026deg;C capsule, and 0.47 dpa for the 1100\u0026deg;C capsule. Finally, a set of unirradiated specimens were annealed at 850\u0026deg;C for 24 days, to be comparable with one HFIR cycle at elevated temperature. These aged specimens were used as XRD and XAS standards for the elevated temperature irradiation. All specimens were 6mm diameter discs with thicknesses less than 0.3 mm to allow transmission XRD experiments. Characterization of the precipitates in K-doped W was performed using FEI Talos F200X transmission electron microscopy[2]\u003ca class=\"FNLink\" href=\"#Fn1\" id=\"#FNLinkFn1\"\u003e\u003c/a\u003e (TEM), which is equipped with Super-X energy dispersive spectrometer system, allowing high resolution chemical analysis. High resolution TEM was also performed to identify the crystallography phase and the orientation relationship of precipitates with the matrix in K-doped W. Images were taken under the [110] zone of the W matrix.\u003c/p\u003e \u003cp\u003eXAS and XANES measurements were performed at the BMM beamline at the NSLS-II. Room temperature fluorescence-mode XAS with a four‐element Si‐drift detector (SII Vortex ME4) at the Re and Os L\u003csub\u003eIII\u003c/sub\u003e‐Edges (10535; 10871 eV, respectively) were collected on as fabricated, as-annealed, and neutron irradiated specimens. XAS were measured with a collimation mirror (paraboloid mirror, 5 nm Rh on 30 nm Pt), Si (111) monochromator, and toroidal focusing mirror (5 nm Rh on 30 nm Pt). Six-to-ten spectra were averaged together to yield high quality XANES spectra for linear combination fitting, and quantification of the composition through edge-step analysis. As the XAS magnitude of the edge step is proportional to the number of absorbing atoms, the edge step of the irradiated specimens was compared to that of known specimens, W-10%Re-5%Os, W-5%Re-3%Os, W-3%Re-0.7%Os, W-3%Re, to facilitate quantification of the Re and Os concentrations[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Background subtraction, edge step quantification, spectra alignment and normalization of all XAS data was performed using ATHENA software [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. XANES spectra for hexagonally close packed Re and Os, tetragonal σ-Re\u003csub\u003e2\u003c/sub\u003eW, cubic χ-Re\u003csub\u003e3\u003c/sub\u003eW and χ-Re\u003csub\u003e3\u003c/sub\u003eOs phases [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e], and Laves (HCP) phase Re\u003csub\u003e2\u003c/sub\u003eW were simulated utilizing FEFF 9.0 [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e], and .cif files from The Materials Project [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. After energy calibration and normalization, XANES spectra were refined as a linear combination of the unirradiated BCC (Unirradiated W-3%Re, and W-3%Re-0.7%Os) and all intermetallic standards from 15 eV below to 80 eV above the Re edge (25 independent refinements), and Os edge (three independent refinements). χ\u003csup\u003e2\u003c/sup\u003e values were used to gauge the quality of the linear combination refinements.\u003c/p\u003e \u003cp\u003eXRD experiments were performed at the Pair Distribution Function (PDF) beamline at the NSLS-II. All measurements were performed in transmission mode with an amorphous silicon-based flat panel detector (Perkin-Elmer) mounted orthogonally to the beam path. The sample-to-detector distance and tilts of the detector relative to the beam were refined using a LaB\u003csub\u003e6\u003c/sub\u003e powder NIST standard (SRM\u0026reg; 660c). The wavelength of the incident X-rays was 0.1665 \u0026Aring; at an energy of 74 keV. The sample-to-detector distance was calculated to be 1245.91 mm. 1200 XRD patterns were collected with detector exposures of 0.1s. Specimens were vertically and horizontally scanned to improve averaging, and to sample a large fraction of the individual specimens. All raw two-dimensional patterns were background corrected by subtracting a dark current image, and the air and Kapton scattering background within IgorPro (Wavemetrics)[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Noticeable artifact regions of the detector (like the beam stop, dead pixels) were masked. The corrected and masked two-dimensional detector images were then radially integrated to obtain powder diffraction patterns. The background subtracted XRD patterns were Rietveld refined within TOPAS (Bruker) software package. The peak profiles were modeled by a modified Thompson-Cox-Hasting pseudo-Voigt (TCHZ) peak function. The instrument contribution to the broadening of the measured profiles was quantified by fitting the LaB\u003csub\u003e6\u003c/sub\u003e NIST powder standard, with a known coherent grain size and negligible microstrain contribution. The Gaussian and Lorentzian-based broadening parameters were subsequently fixed during the analysis of the W alloys under investigation. The BCC W lattice parameter and microstrain components were allowed to be refined. A four-point polynomial background (Chebyshev Polynomial) was included in the refinements to capture the diffuse background.\u003c/p\u003e"},{"header":"Results And Discussion","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (a) shows representative XANES spectra of different neutron irradiated specimens at the edges with BCC Re and HCP Re metallic references. The spectra of Re XANES, including the BCC, HCP, and intermetallic phases (FEFF simulated structures are given in supplementary information Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) have different features labeled a-d. These features, in general, result from multiple-scattering resonances of the 2p\u003csub\u003e3/2\u003c/sub\u003e core level photoelectron to higher energy unoccupied states, primarily the 5d (5d\u003csub\u003e3/2\u003c/sub\u003e and 5d\u003csub\u003e5/2\u003c/sub\u003e) states [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. The different XANES fingerprints (heights and positions of features a-d) enable quantification of the different components through a linear combination refinements [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. Such combination refinements are routine in dilute systems with XAS [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan additionalcitationids=\"CR48\" citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. The linear combination fitting results of the XANES spectra are included in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (a) for the spectra shown (open symbols overlaid on the solid experimental data). Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (b) shows the separate contributions of the BCC and χ-Re\u003csub\u003e3\u003c/sub\u003eW phases to the spectrum of the 1%La W-3%Re neutron irradiated specimen, with residual intensity included for reference. Both Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (a) and (b) demonstrate that while the BCC Re component is dominant in all irradiated specimens, additional non-BCC environments are evident and can be quantified from the spectral features associated with the characteristic phases. The XAS edge step analysis is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (c) with raw X-ray absorption data, χ\u0026micro;(E), for three arc-melted W-Re-Os alloys and three neutron irradiated specimens. The composition of the three W-Re-Os alloys were determined by Wavelength-Dispersive Spectroscopy[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. The lower edge steps for the irradiated single crystalline and polycrystalline specimens indicate that these specimens have lower Re concentrations relative to the W-3%Re standard (first yellow point with edge step\u0026thinsp;=\u0026thinsp;218). The polynomial behavior of the edge step quantified from the standards is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (d), with the calculated Re concentrations overlaid on the calibration curve for irradiated specimens. It is noted that the Re\u0026thinsp;+\u0026thinsp;Os concentrations determined here are close, but not identical, to the values determined from glow discharge optical emission spectroscopy[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Additionally, the trends in the neutron irradiated specimens Re\u0026thinsp;+\u0026thinsp;Os contents determined from the XAS edge step analysis are in agreement with those in Ref [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. We in fact anticipated variations in composition when comparing these two methods, due to intrinsic differences in the physical modes associated with each technique, possible artifacts (namely amount of material sampled via each technique) and measurement sensitivity. Given the differences in values are understandable, the values from XAS were used to generate correlations in the specimen set investigated and discussed herein.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe non-BCC fractions quantified from the XANES linear combination fitting, as a function of Re\u0026thinsp;+\u0026thinsp;Os concentration determined via the edge step analysis is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The results show both irradiation temperature and starting composition-dependent atomic structures, with multiple environments apparent in specimens post irradiation. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (a) and (b) indicate that at 850\u0026deg;C an apparent threshold of 2 at% Re\u0026thinsp;+\u0026thinsp;Os is required before the intermetallic phases form with χ-Re\u003csub\u003e3\u003c/sub\u003eOs environments at this irradiation temperature. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (a) and (b) show that higher starting Re generally leads to a higher intermetallic fractions, that increase with increasing Re\u0026thinsp;+\u0026thinsp;Os content. It is interesting to note that a persistent χ-Re\u003csub\u003e3\u003c/sub\u003eOs structure is quantified from both Re and Os XANES refinements. A notable finding in the alloys studied here is the W alloys containing a starting 3% Re form χ-Re\u003csub\u003e3\u003c/sub\u003eOs \u003cb\u003eand\u003c/b\u003e HCP Re (and HCP Os) after irradiation (it is noted that one of the La-doped W-3%Re specimens did not return an HCP fraction). Lower fractions of HCP Os and Re\u003csub\u003e3\u003c/sub\u003eOs are quantified from the Os XANES, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b), indicating a preference of Os remain substitutional in the W host. These features at 850\u0026deg;C contrast with the 1100\u0026deg;C irradiation in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (c) where all specimens, regardless of Re\u0026thinsp;+\u0026thinsp;Os content, show evidence of additional and complex environments including Laves Re\u003csub\u003e2\u003c/sub\u003eW, χ-Re\u003csub\u003e3\u003c/sub\u003eOs, and χ-Re\u003csub\u003e3\u003c/sub\u003eW (for three specimens). Unlike 850\u0026deg;C where Os atoms in a BCC environment were dominant, the 1100\u0026deg;C show a specimen-dependent HCP-Os and χ-Re\u003csub\u003e3\u003c/sub\u003eOs environments. At these Re\u0026thinsp;+\u0026thinsp;Os concentrations, the presence of the different phases after neutron irradiation is not unexpected, as irradiation-induced diffusion can promote the formation of high-temperature phases well away from equilibrium [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. The dominant BCC environment determined for the Re (and Os) support prior computational studies whereby the precursors to the formation of the intermetallic precipitates were Re-rich coherent clusters with BCC-like structure [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. The precipitate structure in the single crystal and polycrystalline tungsten specimens agree with prior similar irradiations [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] where STEM-EDS gave insight that the transmutation precipitates adopted χ-like structures. These results extend upon these previous studies, quantifying the fractions of BCC and non-BCC intermetallic phases, and evidence of complex transmutation precipitate pathways in W-alloys driven by the initial alloy chemistry and irradiation temperature. The Re\u003csub\u003e3\u003c/sub\u003eOs phase after irradiation at 850\u0026deg;C observed from the combined Re and Os XANES analysis, and combination of Laves σ-Re\u003csub\u003e2\u003c/sub\u003eW in the W alloys after 1100\u0026deg;C are additional new information and show the nuanced role starting structure and irradiation temperature have on the final precipitate microstructures. In future fusion reactors, the neutron energy spectrum will differ from that of the present HFIR irradiation environment, leading to different damage levels and transmutation rates [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. How these differences influence the appearance and early evolution of transmutation precipitates could be an important issue for future studies.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSTEM-EDS images for neutron irradiated K-doped W specimens are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e for 850\u0026deg;C (a-e) and 1100\u0026deg;C (f-j). The STEM-EDS results show that After 850\u0026deg;C, (i) small nanometer-sized core-shell precipitates with Os rich cores are surrounded by Re-cloud-like shells; (ii) Re-rich loops are scattered throughout the grains; and (iii) larger grain boundary precipitates (~\u0026thinsp;300 nm) exhibited a clear mixture of Re and Os. After 1100\u0026deg;C, panels (f-j) show that (i) the core-shell precipitates increase in size; and (ii) the density of small loops decreases. High resolution micrographs of K-doped W specimens irradiated at 1100\u0026deg;C are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (a) and (e) for two different precipitates with corresponding Fast Fourier Transforms (FFTs) of the isolated precipitates given in (b) and (f). The characteristic FFTs for both precipitates are consistent with the cubic χ-phase, identical to the phase quantified from XANES. The simulated FFT patterns for each precipitate are shown in panels (c) and (g) respectively, where reflections from the W matrix are marked in black and those from the χ-phase in blue (panel (d) shows the diffraction pattern of [110] zone axis in the W host for refence). Two orientation relationships of the χ-phase with the W matrix were identified: [110]\u003csub\u003eW\u003c/sub\u003e || [110]\u003csub\u003eχ\u003c/sub\u003e, (002)\u003csub\u003eW\u003c/sub\u003e || (006)\u003csub\u003eχ\u003c/sub\u003e, and [110]\u003csub\u003eW\u003c/sub\u003e || [110]\u003csub\u003eχ\u003c/sub\u003e, (002)\u003csub\u003eW\u003c/sub\u003e || (4̅42̅)\u003csub\u003eχ\u003c/sub\u003e. Due to the much smaller precipitate sizes in the specimen irradiated at 850\u0026deg;C, no clear diffraction contrast was detected for the precipitates. This observation is consistent with the XANES results, where a higher fraction of non-BCC Re and Os (χ-phase) are present at the elevated irradiation temperature. Previous STEM-EDS of similar neutron irradiated polycrystalline W have also shown that the loop size, loop density and void density, peak at ~\u0026thinsp;850\u0026ndash;900\u0026deg;C and subsequently decrease at higher temperatures [20, 21]. The complex segregation behavior of the Re and Os atoms is also visible with STEM-EDS after neutron irradiation at 1200\u0026deg;C, whereby some Re form clouds around loops and voids, while Os tends to segregate into elongated shapes along the dislocation line of loops. For the same neutron fluence, the loop density and void density were found to decrease by ~\u0026thinsp;50% when the irradiation temperature increased from 900 to 1200\u0026deg;C. The preferential segregation and saturation of Re and Os to these microstructural defects was also apparent, as well as at grain boundaries, akin to what we observe in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe temperature-dependent trends in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e for the single crystal, polycrystalline W and K-W specimens are consistent with past studies [\u003cspan additionalcitationids=\"CR13 CR14\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], whereby the Re and Os at 850\u0026deg;C is bound within the dense loop and void networks and unavailable to form precipitates at relatively low damage levels. At elevated temperatures, the Re and Os are less bound due to the lower densities of both loop and voids in addition to their faster diffusivity at higher temperature, thereby allowing the nucleation of intermetallic phases at GBs or regions with less restrictions. While HRTEM could not quantify a crystalline phase at this low fluence [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], our XANES and HRTEM results show direct evidence that in addition to the BCC component, the precipitates are consistent with χ-Re\u003csub\u003e3\u003c/sub\u003eOs after 850\u0026deg;C, while higher temperatures lead to χ-Re\u003csub\u003e3\u003c/sub\u003eOs, χ-Re\u003csub\u003e3\u003c/sub\u003eW, and Laves-Re\u003csub\u003e2\u003c/sub\u003eW intermetallic phases as well as HCP Re and HCP Os. The W host microstructure, that is grain size, grain boundary and dislocation densities and starting alloy chemistry clearly have important roles in influencing the subsequent precipitation microstructure, as they could provide sufficient sink sites for transmutation elements to segregate and precipitate. The trends in the W alloy specimens containing Re, La and K are more nuanced, as the higher Re contents generally lead to higher intermetallic fractions. Additionally, these alloys may not have the same microstructural defect evolution with neutron irradiation as high-purity W, thus it is unsurprising the defects that promote the nucleation of higher fractions of transmutation precipitates are unimpeded. Re is in fact known to impact the evolution of irradiation-induced defects, such as restricting the mobility of loops, leading to substantially different sizes and densities of loops and voids. Re changes the mobility of small dislocation loops and self-interstitial atom (SIA) clusters, such that solute Re would suppress void formation. It is thus potentially unsurprising that the Re and Os that would otherwise segregate to the two-dimensional defects is available to nucleate and form precipitates [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. Future studies investigating the structure utilizing XANES in combination with HRTEM after high-fluence irradiation will be able to provide further evidence on the evolution and potential phase transformation of the transmutation precipitates. The XRD analysis described below gives evidence that the irradiation-induced microstructures in the Re-containing W-alloys are indeed different when compared to their pure-W counterparts.\u003c/p\u003e \u003cp\u003eXRD patterns for representative baseline (W-3%Re and polycrystalline W) and irradiated specimens (1%La -W-3Re, W-3%Re, polycrystalline W and K-W) are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The Rietveld refinements are overlaid as solid black symbols for all XRD patterns shown. Panel (b) shows a high-angle region with (310), (222) and (321) hkl\u0026rsquo;s highlighted from panel (a), demonstrating some of the dominant features observable in the XRD patterns post-irradiation. These include shifts in the peak positions, changes in the XRD peak full widths at half maximum (FWHM), and changes in peak heights relative to the unirradiated baselines, highlighted for the polycrystalline and K-W specimens. These changes are indicative of lattice parameters changes (expansion or contraction, as discussed below) and microstructural changes including changes in the number of two-dimensional defects present in the specimens, namely an increase or decrease in the number of dislocation loops [\u003cspan additionalcitationids=\"CR55\" citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. These microstructural changes were highly-dependent upon the starting material and irradiation temperature. It is noted that minor diffuse scattering is apparent in the high Re-containing alloys attributable to the nanocrystalline intermetallic phases evidenced by XANES. Panel (c) shows a highlighted view of a K-W-3%Re specimen irradiated at 850\u0026deg;C, and single crystal W specimen irradiated at 1100\u0026deg;C, with χ and HCP phases overlaying the diffuse background for each specimen (only HCP was observed in the K-W-3%Re specimen). The minor intensity from the transmutation precipitates and broad diffraction signals result from the small precipitate sizes and minor phase fractions [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]. Nevertheless, these subtle features do aid in the validation of the XANES and TEM above where intermetallic phases were quantified.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe quantitative microstructural properties determined from the Rietveld refinements including change in lattice parameters and XRD-based microstructural broadening relative to the corresponding unirradiated counterparts, are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (the full table of results are given in the appendix). Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (a) - (d) are plotted as functions of material type and Re\u0026thinsp;+\u0026thinsp;Os to highlight trends with different irradiation temperature and composition. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (a) and (c) show the change in lattice parameters with open symbols representing the Δa determined for the baseline specimens relative to the polycrystalline W, while Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (b) and (d) show the microstrain (XRD-based peak broadening). Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (a) shows that after irradiation at 850\u0026deg;C both polycrystalline W and W-%3Re alloys BCC lattice parameters expand with increasing Re\u0026thinsp;+\u0026thinsp;Os content. An opposing trend is observable in the W-alloy specimens with an apparent lattice contraction that increases with increasing Re\u0026thinsp;+\u0026thinsp;Os. The expansion, and contraction in the specimens irradiated at 850\u0026deg;C are attributed to both the irradiation induced defects (vacancies and interstitials), as well as the precipitates enabling a complex misfit-strain [\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]. The corresponding microstrain in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (b) shows only an increase in the polycrystalline W specimens is apparent (relative to the unirradiated state), with all other W-alloys showing a decrease or negligible change in microstrain. Such changes are related to the two-dimensional defect networks, such as dislocation densities, and these results show a clear suppression in the number of irradiation-induced defects (at 850\u0026deg;C) in the W-alloy specimens. This suppression is due to the intrinsic number of defect sinks intentionally engineered into their microstructures including K-bubbles and coherent clusters [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA lattice contraction is quantified for all specimens after irradiation at 1100\u0026deg;C, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (c) which is linearly correlated with Re\u0026thinsp;+\u0026thinsp;Os content. The similarity in the contraction of the baselines (open symbols) and neutron irradiated specimens (closed symbols) indicate that the Re and Os atoms are likely substitutional dopants, with additional minor misfit strain from the transmutation precipitates. The substitutional nature of the Re and Os from both XRD and XANES aids in confirming the reduction in thermal conductivity recently determined for these specimens [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The microstrain after 1100\u0026deg;C in panel (d) shows a similar linear trend to panel (c), albeit with positive values of microstrain and decreasing values with increasing Re\u0026thinsp;+\u0026thinsp;Os. The irradiation-induced two-dimensional defects, as quantified from the microstrain parameters, are clearly compositionally-dependent. Here, the W-alloys containing K-Re and La-Re all appear to be more radiation tolerant (or contain less two-dimensional defects) when compared to the polycrystalline-W and K-W specimens. The microstructural features that likely drive the trends in microstrain are dislocation loops, small defect clusters (\u0026lt;\u0026thinsp;2nm), changes in grain boundary character, radiation-induced voids, and point defect clusters [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo correlate the XRD microstructural and transmutation products results with changes in mechanical properties, we reproduce the hardness for these specimens, leveraged from [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] and shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. At 850\u0026deg;C the polycrystalline W and W-3%Re specimens showed a decrease in hardness after irradiation, while the La-Re and K-Re containing alloys increased in hardness (except for the K-W that showed a decrease in hardness). At 1100\u0026deg;C all specimens show a decrease in hardness, with the notable exception of the single crystal W, with the magnitude approaching zero with increasing Re\u0026thinsp;+\u0026thinsp;Os content. Figures\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e show that the as-irradiated lattice strain is correlated to hardness. The lattice misfit in these alloys is noted to be a complex function of compressive lattice strain imposed by the La and K grain stabilizers, Re alloy content, and coherent precipitates, that alter the number and nature of point defects. Unlike the thermal conductivity, where depressions were related to substitutional Re and Os, the changes in mechanical properties cannot be accounted for by substitutional doping alone. The changes in hardness are more consistent with complex microstructural states that include the Re and Os substitutional dopants \u003cb\u003eand\u003c/b\u003e radiation-induced defects (small clusters, transmutation precipitates, dislocations, voids, point defects).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn this work, we quantify the microstructural response and metallic transmutation product environments of neutron irradiated tungsten specimens, including single crystal W, polycrystalline W, K-doped W, W-3%Re, K-doped W-3%Re, and La-doped W-3%Re. The alloys were irradiated to 0.42 dpa 850\u0026deg;C and 0.47 dpa at 1100\u0026deg;C in the High Flux Isotope Reactor. The local atomic environment of the Re and Os elements were directly probed by XANES, while the microstructure was probed with high-energy XRD. The intrinsic sensitivity of XANES to the Re and Os elements facilitated uncovering multiple new insights into the alloy- and irradiation temperature-dependent atomic environment evolution of the Re and Os that ensued after neutron irradiation. After 850\u0026deg;C irradiation, non-BCC Re and Os fractions were predominantly found in the χ-Re\u003csub\u003e3\u003c/sub\u003eOs and HCP phases. After 1100\u0026deg;C irradiation, the final Re atomic structures were alloy-dependent, with Laves phase Re\u003csub\u003e2\u003c/sub\u003eW, and χ phase (Re\u003csub\u003e3\u003c/sub\u003eW or Re\u003csub\u003e3\u003c/sub\u003eOs), while Os atoms were found in χ-Re\u003csub\u003e3\u003c/sub\u003eOs and HCP environments. The quantitative XRD results indicated alloy-specific microstructural stability, with Re-La and Re-K containing alloys showing the least lattice swelling \u003cem\u003eand\u003c/em\u003e lowest two-dimensional defect concentrations. The Re-free polycrystalline specimens showed the highest irradiation-induced lattice expansions and two-dimensional defect populations, consistent with previous mechanical and thermal property degradation studies. Understanding these early-stage (low-to-intermediate fluence) evolutions is crucial for developing strategies to mitigate late-stage precipitation effects through alloy design. These findings highlight the importance of understanding the effects of neutron irradiation on tungsten alloys to develop more resilient materials for use in extreme environments, such as plasma facing components in fusion reactors. Our approach shown here for transmutation precipitation is by no means limited by the mixed neutron spectrum and Tungsten alloy specimens. Future work, employing combined XANES, TEM and XRD methods, to investigate other relevant specimens irradiated with different neutron energy spectra such as shielded environments to mimic future fusion reactors is currently ongoing. Such studies will provide deeper insight into the evolution of transmutation precipitation.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003e\u003cem\u003e\u003cu\u003eAcknowledgements: \u0026nbsp;\u003c/u\u003e\u003c/em\u003e\u003c/strong\u003eDJS and TK thank K. Wehunt and B. Heneveld of Brookhaven National Laboratory for their help with handling radioactive specimens at NSLS-II. Patricia Tedder, at ORNL, coordinated the shipment of radioactive materials for the beamline experiments. We thank Dr. Bruce Ravel from the National Institute of Standards and Technology for his support of these experiments at BMM, and in spurring scientific discussions and rewarding analysis directions. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003e\u003cu\u003eFunding:\u003c/u\u003e\u003c/em\u003e\u003c/strong\u003e These experiments and analysis were supported by the DOE Office of Fusion Energy Sciences under contract DE-SC0018322 with the Research Foundation for the State University of New York at Stony Brook and DE-AC05-00OR22725 with UT-Battelle LLC. This research used resources at the BMM and PDF beamlines of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. The XRD and XAS analysis were supported by the Research Foundation for the State University of New York at Stony Brook. A portion of this research used resources at the HFIR, a US Department of Energy Office of Science User Facility operated by ORNL. This work was also partially supported by the US-Japan PHENIX and FRONTIER Collaboration Project supported by the US Department of Energy, Office of Science, Fusion Energy Sciences and Ministry of Education, Culture, Sports, Science and Technology, Japan.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003e\u003cu\u003eAuthor Contributions:\u0026nbsp;\u003c/u\u003e\u003c/em\u003e\u003c/strong\u003eDJS, TK and LLS conceived and designed the experiments. DJS, MO, TK and DO performed the synchrotron experiments. DJS analyzed the XAS and XRD \u0026nbsp;data. WZ performed the electron microscopy experiments and analysis tasks. \u0026nbsp;AH, TK, and YK oversaw the neutron irradiation campaign. DJS and TK wrote the manuscript. All authors discussed and commented on the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003e\u003cu\u003eCompeting Interests:\u003c/u\u003e\u003c/em\u003e\u003c/strong\u003e No, I declare that the authors have no competing interests as defined by Nature Portfolio, or other interests that might be perceived to influence the results and/or discussion reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003e\u003cu\u003eData Availability:\u0026nbsp;\u003c/u\u003e\u003c/em\u003e\u003c/strong\u003eCorrespondence and requests for materials should be addressed to DJS\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eS.J. Zinkle, L.L. Snead, Designing Radiation Resistance in Materials for Fusion Energy, Annual Review of Materials Research 44 (2014) 241\u0026ndash;267.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eG.S. 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Ecker, Structural characterization of nanoscale intermetallic precipitates in highly neutron irradiated reactor pressure vessel steels, Scripta Materialia 113 (2016) 18\u0026ndash;22.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Akhlaghi, T. Steiner, S.R. Meka, E.J. Mittemeijer, Misfit-induced changes of lattice parameters in two-phase systems: coherent/incoherent precipitates in a matrix, Journal of Applied Crystallography 49(1) (2016) 69\u0026ndash;77.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Fukuda, A. Hasegawa, T. Tanno, S. Nogami, H. Kurishita, Property change of advanced tungsten alloys due to neutron irradiation, Journal of Nuclear Materials 442(1, Supplement 1) (2013) S273-S276.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eP. Wen, X. Hu, C. Fan, Y. Tan, W. Qiu, T. Zhao, S. Li, C. Wu, J. Du, J. Tang, Irradiation hardening of WK-based PFM exposed to 14 MeV protons, Fusion Engineering and Design 187 (2023) 113379.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Footnotes","content":"\u003cp\u003e\u003cspan\u003e\u003cspan type=\"BoldItalic\" class=\"BoldItalic\" name=\"Emphasis\"\u003e[2] \u003c/span\u003e\u003cem\u003eCertain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.\u003c/em\u003e\u003c/span\u003e\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-materials-degradation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmatdeg","sideBox":"Learn more about [npj Materials Degradation](http://www.nature.com/npjmatdeg/)","snPcode":"41529","submissionUrl":"https://submission.springernature.com/new-submission/41529/3","title":"npj Materials Degradation","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8274574/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8274574/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTungsten is currently the leading candidate material for plasma-facing applications in fusion devices due to its high melting point and resistance to sputtering. However, its long-term stability is a concern as exposure to neutrons degrades mechanical properties. While high-fluence conditions with large Re and Os transmutation concentrations are known to cause embrittlement, the effects of lower fluence on microstructure and property evolution are less explored. In this study, we use X-ray Absorption Spectroscopy (XAS) and high-energy X-ray Diffraction (XRD) to quantify the atomic and microstructural properties of unirradiated and neutron-irradiated tungsten alloys. The alloys studied include single crystal W, polycrystalline W, K-doped W, W-3%Re, K-doped W-3%Re, and La-doped W-3%Re. Specimens were neutron-irradiated at 850\u0026deg;C and ~\u0026thinsp;1100\u0026deg;C to ~\u0026thinsp;0.5 displacement per atom. Our XAS results revealed multiple differences in the atomic environment across the W-alloy matrix. These differences were alloy-dependent and varied with irradiation temperature. After 850\u0026deg;C, non-body-centered cubic (BCC) Re and Os components were found with χ phase (Re\u003csub\u003e3\u003c/sub\u003eOs) and hexagonally close packed structures. After 1100\u0026deg;C, the final Re atomic structures were alloy-dependent, with Laves phase (Re\u003csub\u003e2\u003c/sub\u003eW), and χ-phase (Re\u003csub\u003e3\u003c/sub\u003eOs and Re\u003csub\u003e3\u003c/sub\u003eW) intermetallic environments quantified, while Os atoms were found in a body centered cubic and hexagonally close packed environments. XRD results indicated alloy-specific microstructural stability, with Re-La-W and Re-K-W alloys showing the least lattice swelling and lowest two-dimensional defect concentrations, while pure-W specimens show the highest irradiation-induced lattice expansions and two-dimensional defects. Understanding these early-stage evolutions is crucial for developing strategies to mitigate late-stage precipitation effects through alloy design.\u003c/p\u003e","manuscriptTitle":"Atomic and Microstructural Evolution of Mixed Spectrum Neutron-Irradiated Tungsten Alloys: Insights From Multimodal X-Ray Spectroscopy and Diffraction Characterization","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-20 08:40:33","doi":"10.21203/rs.3.rs-8274574/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-04-23T07:27:25+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-15T12:12:53+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-26T10:04:33+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-18T11:48:30+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"269090899366736690628107410750082230808","date":"2026-03-11T15:29:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"336028697677367584543355230102926278881","date":"2026-03-09T04:27:31+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"101245803256200660482182134768179306882","date":"2026-02-18T14:04:52+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-13T06:45:53+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-07T23:51:48+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-07T15:27:05+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Materials Degradation","date":"2025-12-04T02:16:45+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-materials-degradation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmatdeg","sideBox":"Learn more about [npj Materials Degradation](http://www.nature.com/npjmatdeg/)","snPcode":"41529","submissionUrl":"https://submission.springernature.com/new-submission/41529/3","title":"npj Materials Degradation","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"95ec0a7e-c930-4020-a73d-16f162aeb8c0","owner":[],"postedDate":"January 20th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[{"id":60971261,"name":"Physical sciences/Materials science"},{"id":60971262,"name":"Physical sciences/Physics"}],"tags":[],"updatedAt":"2026-04-23T07:39:48+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-20 08:40:33","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8274574","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8274574","identity":"rs-8274574","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}