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The formation of helium cavities in coarse-grained materials produces hardening proportional to the number density and size of the cavities and due to the interaction of dislocations with intragranular helium defects. In nanostructured metals containing a high density of interfacial sinks, preferential cavity formation on the grain boundaries instead produces softening and often attributed to enhanced interfacial plasticity. Employing two grades of ultrafine-grained tungsten, we explore this effect using targeted implantation studies to map cavity evolution as a function of the irradiation conditions and quantify its impact on the mechanical response through nanoindentation. Softening is reported at implantation temperatures above the threshold for preferential grain boundary cavity formation but at a sufficiently low fluence prior to the growth of intragranular cavities. Collective changes in the mean cavity size, density, and morphology beneath a residual impression on an implanted surface indicate that cavity coalescence accompanied the reduction in hardness. Complementary atomistic simulations demonstrate that, in tungsten grain structures exhibiting softening, grain boundary bubble coalescence is driven by stress concentrations that further act to localize strain in the grain boundaries through cooperative deformation processes involving local atomic shuffling and sliding, dislocation emission, and even the nucleation of unstable twinning events.

As ductile derivatives of amorphous alloys, metallic glass composites balance high strength with the ability to mitigate catastrophic shear localization, which has long plagued monolithic metallic glasses. In this study, we employ a bonded-interface indentation technique coupled with strain field mapping to identify the governing yield criterion in metallic glass composites as a function of their microstructural length scales. A crossover from the shear plane to maximum shear stress yield criterion is uncovered at a crystalline fraction of approximately 60%, thus signaling an effective transition from incipient to homogeneous flow as the dominant deformation mode of the composite microstructure.

Understanding a material's radiation tolerance requires examining its performance under different irradiation conditions. Here, we investigate the radiation tolerance in terms of helium bubble damage in tungsten irradiated in-situ with 16 keV helium at 1073 K and 1223 K. Damage evolution represented by helium bubble density, size and total change in volume in the grain matrices and the grain boundaries are quantified as a function of fluence. Preferential large bubble formation and a higher change in volume on the grain boundaries occurred at 1223 K, suggesting faster migration of certain helium-vacancy complexes as confirmed by a diffusion-reaction model.

Targeted doping of grain boundaries is widely pursued as a pathway for combating thermal instabilities in nanocrystalline metals. However, certain dopants predicted to produce grain-boundary-segregated nanocrystalline configurations instead form small nanoprecipitates at elevated temperatures that act to kinetically inhibit grain growth. Here, thermodynamic modeling is implemented to select the Mo-Au system for exploring the interplay between thermodynamic and kinetic contributions to nanostructure stability. Using nanoscale multilayers and in situ transmission electron microscopy thermal aging, evolving segregation states and the corresponding phase transitions are mapped with temperature. The microstructure is shown to evolve through a transformation at lower homologous temperatures (600 °C) where solute atoms cluster and segregate to the grain boundaries, consistent with predictions from thermodynamic models. An increase in temperature to 800 °C is accompanied by coarsening of the grain structure via grain boundary migration but with multiple pinning events uncovered between migrating segments of the grain boundary and local solute clustering. Direct comparison between the thermodynamic predictions and experimental observations of microstructure evolution thus demonstrates a transition from thermodynamically preferred to kinetically inhibited nanocrystalline stability and provides a general framework for decoupling contributions to complex stability transitions while simultaneously targeting a dominant thermal stability regime.

This report summarizes the outcome of a workshop, “Multimodal Synchrotron Approach—Research Needs and Scientific Vision,” held during the National Synchrotron Light Source–II (NSLS-II)/Center for F...

Intrinsic size effects in nanoglass plasticity have been connected to the structural length scales imposed by the interfacial network, and control over this behavior is critical to designing amorphous alloys with improved mechanical response. In this paper, atomistic simulations are employed to probe strain delocalization in nanoglasses with explicit correlation to the interfacial characteristics and length scales of the amorphous grain structure. We show that strength is independent of grain size under certain conditions, but scales with the excess free volume and degree of short-range ordering in the interfaces. Structural homogenization upon annealing of the nanoglasses increases their strength toward the value of the bulk metallic glass; however, continued partitioning of strain to the interfacial regions inhibits the formation of a primary shear band. Intrinsic size effects in nanoglass plasticity thus originate from biased plastic strain accumulation within the interfacial regions, which will ultimately govern strain delocalization and homogenous flow in nanoglasses.

Prediction of material performance in fusion reactor environments relies on computational modelling, and will continue to do so until the first generation of fusion power plants come on line and allow long-term behaviour to be observed. In the meantime, the modelling is supported by experiments that attempt to replicate some aspects of the eventual operational conditions. In 2019, a group of leading experts met under the umbrella of the IEA to discuss the current position and ongoing challenges in modelling of fusion materials and how advanced experimental characterisation is aiding model improvement. This review draws from the discussions held during that workshop. Topics covering modelling of irradiation-induced defect production and fundamental properties, gas behaviour, clustering and segregation, defect evolution and interactions are discussed, as well as new and novel multiscale simulation approaches, and the latest efforts to link modelling to experiments through advanced observation and characterisation techniques. MRG, SLD, and DRM acknowledge funding by the RCUK Energy Programme [grant number EP/T012250/1]. Part of this work has been carried out within the framework of the EUROFusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant Agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. JRT acknowledges funding from the US Department of Energy (DOE) through grant DE-SC0017899. ZB, LY,BDW, and SJZ acknowledge funding through the US DOE Fusion Energy Sciences grant DE-SC0006661ZB, LY and BDW also were partially supported from the US DOE Office of Science, Office of Fusion Energy Sciences and Office of Advanced Scientific Computing Research through the Scientific Discovery through Advanced Computing (SciDAC) project on Plasma-Surface Interactions. JMa acknowledges support from the US-DOEs Office of Fusion Energy Sciences (US-DOE), project DE-SC0019157. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the US Department of Energy (DOE) under contract DE-AC05-76RL01830. YO and YZ were supported as part of the Energy Dissipation to Defect Evolution (EDDE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under contract number DE-AC05-00OR22725. TS and TT are supported by JSPS KAKENHI Grant Number 19K05338.

Refining grain size and adding alloying elements are two complementary approaches for enhancing the radiation tolerance of existing nuclear materials. Here, we present detailed in-situ irradiation research on defect evolution behavior and irradiation tolerance of ultrafine W-TiC alloys (thin foils) irradiated with 1 MeV Kr+2 at RT and 1073 K, and compare their overall performance to pure coarse grained tungsten. Loop Burgers vector was studied confirming the presence of loops whose population increased at high temperature. Loop density, average loop area, and overall damage are reported as a function of irradiation dose revealing distinct defect evolution behavior from pure materials. The overall damage generally followed the average loop size trend, which decreased with time for both temperatures, but was higher at 1073 K and attributed to biased vacancy sink behavior of the TiC dispersoids evidenced by large vacancy clusters on their interfaces. By comparison, the overall loop and void damage in pure tungsten was larger by a factor of six and two, respectively. The improved irradiation damage resistance in the alloys is thus attributed to the effect of dispersoids in 1) the enhancement in annihilating defects and mutual defect recombination due to both dispersoids and a higher grain boundary density; 2) decreasing the loop mobility, causing shrinkage and annihilation of loop density, which was confirmed via in-situ video. Several mechanisms are illustrated to describe the performance of the complex alloy system. The results motivate further experimental and modeling research that aims to understand the many different phenomena occurring at different time scales.

The deformation behavior of crystalline-amorphous nanolaminates is controlled by a confluence of mechanisms involving dislocation and grain boundary (GB) plasticity in the crystalline phase, shear transformation zone (STZ) plasticity and its localization into shear bands in the amorphous phase, and their coupling across the amorphous-crystalline interface. Leveraging molecular dynamics simulations, the influence of microstructural length scales on the mechanical behavior is quantified using deformation mechanism maps, which provide mechanistic insights into the scaling of mechanical properties with layer thickness ratio and grain size. We find that flow stress primarily scales with the relative phase fraction as deformation shifts toward STZ dominated plasticity with increasing amorphous layer thickness while the onset of plasticity is also influenced by grain size due to the role of GB plasticity in yielding. Toughness limiting mechanisms involve void formation at GBs and shear localization in the amorphous layer, where the former is attributed to dislocation-GB interactions during mixed mode deformation and the latter to dislocation slip bands biasing the process of shear localization. An Ashby plot representation combining flow stress with void and shear band localization factors demonstrates that configurations with microstructural length scales promoting cooperative strain accommodation through the coupling of dislocation, GB, and STZ plasticity exhibited limited strain localization while retaining a high flow stress, thus providing a mechanistic basis for microstructural design of crystalline-amorphous nanolaminates.

The hierarchical nature of additively manufactured materials necessitates a multimodal approach for quantifying microstructural features and corresponding chemical heterogeneities that ultimately impact their properties and performance. In laser powder-bed fusion (L-PBF) 316L stainless steel, corrosion behavior has been discussed in the context of chemical heterogeneities formed in the presence of these hierarchical microstructures. Here, we employ a suite of advanced synchrotron x-ray techniques and correlative transmission electron microscopy for the analysis of microstructure and chemical heterogeneities in L-PBF 316L as a function of printing speed. Our findings reveal an appreciable dislocation density consistent with the formation of a cellular dislocation microstructure in L-PBF 316L, which is correlated to spatial variations in the local Cr concentration and the formation of complex Mn7C3 nanoinclusions. Cyclic voltammetry experiments reveal that relative to wrought 316L, the printed samples exhibit either a comparable or marginally reduced susceptibility to uniform corrosion but with an increased affinity for pitting particularly in the samples printed at the highest speed with the largest dislocation density. Given the spatial correlations between regions of high dislocation density and the formation of chemical heterogeneities known to degrade corrosion performance, our findings demonstrate the impact of the microstructural defect state and its variation with printing speed on the resistance of L-PBF 316L to uniform and localized corrosion.

Microstructure and phase evolution in magnetron sputtered nanocrystalline tungsten and tungsten alloy thin films are explored through in situ TEM annealing experiments at temperatures up to 1000 °C. Grain growth in unalloyed nanocrystalline tungsten transpires through a discontinuous process at temperatures up to 550 °C, which is coupled to an allotropic phase transformation of metastable β-tungsten with the A-15 cubic structure to stable body centered cubic (BCC) α-tungsten. Complete transformation to the BCC α-phase is accompanied by the convergence to a unimodal nanocrystalline structure at 650 °C, signaling a transition to continuous grain growth. Alloy films synthesized with compositions of W–20 at.% Ti and W–15 at.% Cr exhibit only the BCC α-phase in the as-deposited state, which indicate the addition of solute stabilizes the films against the formation of metastable β-tungsten. Thermal stability of the alloy films is significantly improved over their unalloyed counterpart up to 1000 °C, and grain coarsening occurs solely through a continuous growth process. The contrasting thermal stability between W–Ti and W–Cr is attributed to different grain boundary segregation states, thus demonstrating the critical role of grain boundary chemistry in the design of solute-stabilized nanocrystalline alloys.

The mechanisms of stress-assisted grain growth are explored using molecular dynamics simulations of nanoindentation in nanocrystalline Ni and Ni-1 at.% P as a function of grain size and deformation temperature. Grain coalescence is primarily confined to the high stress region beneath the simulated indentation zone in nanocrystalline Ni with a grain size of 3 nm. Grain orientation and atomic displacement vector mapping demonstrates that coalescence transpires through grain rotation and grain boundary migration, which are manifested in the grain interior and grain boundary components of the average microrotation. A doubling of the grain size to 6 nm and addition of 1 at.% P eliminates stress-assisted grain growth in Ni. In the absence of grain coalescence, deformation is accommodated by grain boundary-mediated dislocation plasticity and thermally activated in pure nanocrystalline Ni. By adding solute to the grain boundaries, the temperature-dependent deformation behavior observed in both the lattice and grain boundaries inverts, indicating that the individual processes of dislocation and grain boundary plasticity will exhibit different activity based on boundary chemistry and deformation temperature.

Nanocrystalline Al-Mg alloys are used to isolate the effect of grain boundary doping on the strength of nanostructured metals. Mg is added during mechanical milling, followed by low homologous temperature annealing treatments to induce segregation without grain growth. Nanocrystalline Al -7 at.% Mg that is annealed for 1 h at 200 {\deg}C is the strongest alloy fabricated, with a hardness of 4.56 GPa or approximately three times that of pure nanocrystalline Al. Micropillar compression experiments indicate a yield strength of 865 MPa and a specific strength of 329 kN m/kg, making this one of the strongest lightweight metals reported to date., Comment: 8 figures

Thermal stability of pulsed laser deposited (PLD) nanocrystalline tantalum was explored through in situ transmission electron microscopy (TEM) annealing over the temperature range of 800–1200 °C. The evolution of the nanostructure was characterized using grain size distributions collectively with electron diffraction analysis and electron energy loss spectroscopy (EELS). Grain growth dynamics were further explored through molecular dynamics (MD) simulations of columnar tantalum nanostructures. The as-deposited grain size of 32 nm increased by only 18% at 1200 °C, i.e., 40% the melting point of tantalum, conflicting with the MD simulations that demonstrated extensive grain coalescence above 1000 °C. Furthermore, the grain size remained stable through the reversible α-to-β phase transition near 800 °C, which is often accompanied by grain growth in nanostructured tantalum. The EELS analysis confirmed the presence of oxygen impurities in the as-deposited films, indicating that impurity stabilization of grain boundaries was responsible for the exceptional thermal stability of PLD nanocrystalline tantalum.

Controlling glass-glass interfaces in metallic nanoglasses is essential for tuning their mechanical properties and in particular, the ability to inhibit severe strain localization through distributed shear band formation. In this paper, molecular dynamics are employed to quantify the structural characteristics of interfaces in a scalable nanoglass model produced through hydrostatic compression of amorphous nanoparticles. Using a framework for distinguishing interfaces from amorphous grains based on the correlation between dilatation and atomic volume distributions, we show that the interfaces in our nanoglass model exhibit a volume fraction of 0.36, a width of approximately 2 nm, excess free volume of 1--2%, and full icosahedral (FI) fraction roughly 30% that of a bulk metallic glass counterpart. While these characteristics are quantitatively unique relative to other nanoglass models (e.g., planar interfaces and Poisson-Voronoi constructions), they are consistent with experimental results reported for nanoglasses consolidated from glassy nanoparticles produced through inert gas condensation. Increasing the consolidation temperature enhanced the FI fraction with a strong bias to the interfaces, thus demonstrating a route for tuning interfacial properties in nanoglasses.

High temperature, creep resistant heat sink materials represent a critical need for plasma facing components in future fusion reactors. In this study, we employ direct current sintering (often referred to as spark plasma sintering) to produce a Cu-Cr-Nb-Zr (CCNZ) alloy from gas-atomized feedstock powder with tailored precipitate distributions for enhanced stability and creep resistance. Microstructure was characterized by synchrotron X-ray diffraction, small angle X-ray scattering, and electron microscopy techniques. We report a multi-modal precipitate distribution containing submicron Cr (~493 nm) and Cr2Nb (~90 nm) precipitates at grain boundaries and a high density of nanoscale Cr (~8 nm) precipitates homogeneously distributed through the Cu matrix. By comparing the as-sintered and aged microstructures, precipitation kinetics are discussed in the context of dislocation networks due to the high sintering pressures biasing precipitate formation and the role of subsequent recovery and recrystallization. Due to the presence of the multi-modal precipitate distribution, the sintered CCNZ alloy exhibited a high hardness of 133.2 HV while retaining an appreciable thermal conductivity of 298.4 W/m·K and electrical conductivity of 74.6% relative to the International Annealed Copper Standard.

Deliberately designed nanostructured materials containing a high density of dopant stabilized interfaces provide energetically favorable sites for the annihilation of defects that form within the crystalline matrix due to irradiation. In this study, we explore the role of dopants on the mechanisms governing this behavior in a nanocrystalline W alloy doped with 20 at.% Ti using heavy ion irradiation experiments. Defect evolution is mapped in situ up to the saturation dose and bridged to extreme dose behavior using ex situ measurements with microstructural analysis focused on quantifying both the defect state and the impact of ion irradiation on the nanocrystalline grain structure. Compared with a nominally pure nanocrystalline W film, the W-20 at.% Ti alloy exhibits smaller defect loops and a delayed saturation dose with a period of irradiation induced grain growth occurring during transient damage accumulation. Microstructural evolution is modeled in the context of cascade-induced thermal spikes and reveals that the alloy evolves to a much finer nanocrystalline grain size relative to the predictions for pure W, indicative of Ti stabilizing the nanostructure against irradiation induced grain growth. In situ mapping of defect evolution during the growth of a single grain confirms the correlation between the global defect density trends and evolution of the microstructure, thus providing insights into the effect of Ti on the grain boundary sink strength through the transient defect accumulation and recovery stages.

The deformation behavior of crystalline-amorphous nanolaminates is explored through molecular dynamics simulations using nanolaminate models that contain columnar nanograins in the crystalline layers to more closely resemble experimentally accessible nanolaminate structures. Quantitative analysis of the plastic strain distribution among competing mechanisms and their coupling at the nanoscale is accomplished through the implementation of continuum deformation metrics. The transfer of plastic strain between shear transformation zone (STZ) and dislocation plasticity initially transpires through the emission of dislocations from STZ activity impinging on the amorphous-crystalline interface (ACI). The addition of grain boundaries biases this process to regions near the boundaries at low strains, which reduces the activation barrier for the onset of dislocation plasticity. With increasing strain, dislocations are absorbed into the amorphous layers via slip transfer across the ACI, in turn triggering the activation of new STZs. Cooperative slip transfer between dislocations and STZs suppresses grain boundary microcracking collectively with large-scale shear localization, and provides an explanation for the enhanced toughness of crystalline-amorphous nanolaminates.

Characterization methods capable of providing critical information across multiple structural length scales are essential in materials exposed to the extreme environments such as anticipated fusion power systems. Complementary techniques capable of uncovering the complicated microstructural irradiation-induced evolution are also important to verify and validate advanced computational models. To date, the primary microstructural tools informing such lower-length scale models have included analytical electron microscopy, positron annihilation spectroscopy, atom probe tomography, and small-angle neutron scattering. In this paper, we discuss the application of state-of-the-art synchrotron-based x-ray characterization methods in fusion material research. Specifically highlighted are opportunities in leveraging synchrotron-based techniques to address fundamental and applied materials science challenges at various length scales and in support of modeling efforts. Examples presented in this article include: a combined small angle x-ray scattering and x-ray diffraction study of transmutation-induced precipitation in neutron irradiated tungsten, and the identification of size and structure of nm-scale transmutation precipitates and voids; quantitative characterization of thermodynamically predicted minor precipitate populations in advanced reduced activation ferritic-martensitic steels through high energy x-ray diffraction; and a review of recent synchrotron-based studies dedicated to quantifying the radiation response of fusion relevant materials. The latter includes a pair distribution function analysis investigation of neutron irradiated SiC with insights into the different radiation response of the silicon and carbon sublattices, and a dose dependent decrease in the size of defect free material.

Additive manufacturing (AM) is a disruptive technology that has the potential to dramatically improve the US Department of Defense supply chain. The corrosion performance of these AM materials is not well understood and contradicts equivalent conventionally-manufactured materials. AM316L steel frequently has defects and pores that are 25-µm or less, and these are implicated as the cause for localized corrosion including crevice and pitting. The 316L feedstock used in manufacturing AM316L contains up to 0.03% wt carbon, and the depletion of chromium due to sensitization and formation of Cr26C3 is yet another cited reason for localized corrosion. We demonstrate that mixing the feedstock with small amounts of certain ceramic materials could might improve these shortcomings. The surface of this new material - AM316L-MMC (metal matrix composite) - is hydrophobic, in contrast to the hydrophilicity of AM316L, and shows little or no evidence of pitting and crevice corrosion in aqueous solutions of 5%, 30% and 60% FeCl3. It also remains free of pitting and crevice corrosion under anodic polarization in aqueous 3.5% sodium chloride electrolyte.

Graphite moderators have an extensive historical performance record, but also feature inherent challenges for modular High Temperature Gas-Cooled Reactors (mHTGRs). Challenges with graphite include non-uniform expansion and contraction under irradiation and build-up of potential energy during the bombardment of high energy neutrons that results in a large energy release under annealing. These challenges have led to the investigation and development of alternative moderators to be utilized in mHTGRs, including beryllium- and hydride-based concepts with compositions selected for favorable moderating power and the potential for improved in-service lifetime as compared to graphite. The proposed moderators are fabricated as two-phase composites with magnesium oxide, MgO, as the radiation-stable host matrix and beryllium metal, Be, beryllium oxide, BeO, or zirconium hydride, ZrHx=1 (to account for hydrogen loss from the hydride phase during processing), as the entrained moderating phase. Here, we evaluate the reactor performance and safety characteristics of these moderator concepts relative to a graphite reference using a Ft. Saint Vrain-style fuel block. We assessed the cycle length, discharge burnup, natural resource utilization, neutron flux spectra, moderating power, moderating ratio, critical size, moderator and fuel temperature feedback, fuel cycle cost, spent nuclear fuel and high level waste radioactivity per unit energy generated, and environmental impact per unit energy generated. The results demonstrate that the advanced moderators have the potential for comparable or enhanced cycle performance to that of the graphite reference case with significantly improved performance for an optimized moderator-to-fuel ratio design. These advanced moderators are also assessed from a reactor safety standpoint for Design Basis Accidents (DBAs) including Pressurized Loss of Forced Cooling and Depressurized Loss of Forced Cooling accidents for a 350 megawatt thermal prismatic-type mHTGR. The full core thermohydraulic analysis of DBAs show that the high volumetric heat capacity of the beryllium-based moderator grants them a greater margin to fuel failure in these analyses than a conventional graphite moderated system, but the lower thermal conductivity of the beryllium-based moderators leads to longer times at elevated temperatures.

Elucidating the synergistic effects of different energetic beams on the radiation response of nuclear materials is critical for developing an improved methodology for their evaluation when exposed to extreme environments. This article describes in-situ sequential (He implantation followed by Kr irradiation and vice-versa) and simultaneous (heavy ion Kr irradiation and He implantation) dual beam irradiations performed on tungsten at 1223 K. Dislocation loop density, average area, and total loop damage as a function of irradiation history and dose/fluence are quantified. The loop Burgers vectors and cavity damage (cavity density, size and total change in volume) are also determined at the final maximum dose for each condition. The loop damage evolution was different in all cases, with the smallest loop damage observed in the simultaneous experiment. Annihilation of Kr generated dislocation loops during He implantation in the Kr + He experiment was an unanticipated observation that may be explained by the dynamic evolution of dislocation loop sink strengths and time-dependent defect fluxes. Dislocation loop raft formation, denuded zones near extended defects, and cavity damage are compared across the different conditions. The phenomena observed and discussed in this work will stimulate further experimental and computational modeling activities leading to improved fundamental understanding of the irradiation response of nuclear materials under reactor-similar environments.

Additive manufacturing (AM) of alloys such as stainless steels has the potential to be a disruptive technology for design of complex structures which eliminates the potentially damaging effects of mechanical failure and localized corrosion associated with interfaces between separate parts (which can now be printed as part of a single assembly). In addition, increasingly complex geometries can be manufactured with nearly as much ease as simple ones, and new mechanisms for optimization and generative design can allow manufacturers to achieve significant material and weight savings. However, challenges remain, in particular due to the potential for material variabilities both between parts made with the same nominal build parameters and even within a single build, as the consistency in properties inherent in the commercial-scale forging of alloys to be machined into components is exchanged for the considerable advantages of on-site, distributed custom parts production. Our studies and related work by other groups indicates that this is clearly true in the case of corrosion susceptibility in AM alloys. By studying the relationship between build parameters and electrochemical properties, we propose that it will be possible to tailor alloys for enhanced corrosion properties. AM processes produce materials with hierarchical microstructures containing fusion boundaries at the macroscale, irregular grains and grain boundaries at the microscale, and subgrain dislocation structures at the nanoscale. In laser powder bed fusion (LPBF) formed 316L stainless steel structures, corrosion performance has been discussed in the context of chemical heterogeneities formed in the presence of these hierarchical microstructures. However, the large variability in reported measurements underscores the need for statistically significant microstructural data, which is often difficult to access via electron microscopy alone. In this presentation, we explore multi-modal synchrotron X-ray techniques for quantifying hierarchical microstructures and their connection to the underlying chemical distribution in 316L stainless steel. Our results show that the dislocation density depends on the printing conditions with implications for the chemical distribution at the nanoscale, which in turn may play a key role in inconsistent corrosion behavior. LPDF 316L samples formed at varying speeds using pulsed and continuous laser deposition are compared via cyclic polarization in 3.5% NaCl and 0.1M HCl solutions, as well as using standard methods for characterizing sensitization. Surface corrosion layers are characterized using laboratory-based X-ray photoelectron spectroscopy to determine the impact on passivity of the aforementioned microsegregation associated with process-induced microstructures. Corrosion rates and pit densities are then discussed to build a connection between printing conditions and corrosion performance vis-à-vis microstructural and chemical information from synchrotron measurements performed at Brookhaven National Laboratory (BNL), including high energy X-ray diffraction using the X-ray Powder Diffraction (XPD) beamline at the National Synchrotron Light Source-II (NSLS-II) which provided data on minor precipitate population(s), atomic structure and dislocation microstructures critical to corrosion behavior. In addition, heterogeneities in elemental composition data is provided by 2D X-ray fluorescence (XRF) and 2D X-ray Absorption Spectroscopy (XAS) nanometer resolution-mapping obtained at the Hard X-ray Nanoprobe (HXN) beamline at the NSLS-II and correlated with process-induced hierarchal structures via use of the microscopy facilities at the Center for Functional Nanomaterials (CFN). By employing a combination of multi-modal synchrotron characterization techniques, electron microscopy, and baseline testing protocol combined with electrochemical polarization experiments, we are able to study the the role of microstructure across multiple length scales on electrochemical properties and corrosion performance. Understanding the impact of microstructure and chemical heterogeneities on the susceptibility to pitting and intergranular attack will enable microstructurally-informed process optimization and materials design for enhancing the corrosion resistance of LPBF 316L stainless steel, with implications for AM and subsequent durability of alloys in general.

Extrinsic approaches for the pursuit of ductility in metallic glasses have involved the introduction of a ductile crystalline phase to inhibit the propagation of a dominant shear front through the amorphous matrix material. Using nanoindentation, we explore the role of crystalline inclusions in metallic glass matrix composites with a focus on the onset of shear banding in the amorphous matrix and the nature of shear band propagation using the propensity for localization and its dependence on indentation strain rate. When indentation length scales are favorable to distribute strain to both the crystalline dendrites and amorphous matrix, we reveal a reduction in the number of detectable displacement bursts and an accompanying decrease in the magnitude of individual depth excursions from shear banding. A decrease in the stress at the onset of shear banding is also correlated with shear band trajectories interacting with the amorphous-crystalline interfaces (ACIs). Our results thus demonstrate that ACIs reducing the activation barrier for shear banding combined with dendrites arresting propagating shear fronts act to enhance the nucleation rate and in turn, promote a more homogeneous plastic response.

We report the development, testing, and demonstration of a setup for modulation excitation spectroscopy experiments at the Inner Shell Spectroscopy beamline of National Synchrotron Light Source - II. A computer algorithm and dedicated software were developed for asynchronous data processing and analysis. We demonstrate the reconstruction of X-ray absorption spectra for different time points within the modulation pulse using a model system. This setup and the software are intended for a broad range of functional materials which exhibit structural and/or electronic responses to the external stimulation, such as catalysts, energy and battery materials, and electromechanical devices.

Additive manufacturing (AM) of metal alloys to produce complex part designs via powder bed fusion methods such as laser melting promises to be a transformative technology for advanced materials processing. However, effective implementation of AM processes requires a clear understanding of the processing–structure–properties–performance relationships in fabricated components. In this study, we report on the formation of micro and nanoscale structures in 316L stainless steel samples printed by laser AM and their implications for general corrosion resistance. A variety of techniques including x-ray diffraction, optical, scanning and transmission electron microscopy, x-ray fluorescence, and energy dispersive x-ray spectroscopy were employed to characterize the microstructure and chemistry of the laser additively manufactured 316L stainless steel, which are compared with wrought 316L coupons via electrochemical polarization. Apparent segregation of Mo has been found to contribute to a loss of passivity and an increased anodic current density. While porosity will also likely impact the environmental performance (e.g., facilitating crevice corrosion) of AM alloys, this work demonstrates the critical influence of microstructure and heterogeneous solute distributions on the corrosion resistance of laser additively manufactured 316L stainless steel.

Embedded heat flux sensors fabricated using direct-write technology have been developed and tested within a direct-connect gaseous hydrocarbon-fueled scramjet combustor under Mach 5 flight conditions. The sensors employed a multilayer thermopile architecture fabricated via the additive direct-write thermal spray process, which enables their integration directly onto the components to be tested. The sensors were embedded within a 0.50-mm-thick (0.020-in.-thick) thermal barrier coating on test plugs that were inserted into the tunnel for testing. Fuel equivalence ratios and dynamic pressures were varied from φ=0.6 to 1.0 and Q=24 and 48 kPa (500 and 1000 psf), respectively, with increases in the global heat flux found to depend on the tunnel location. Heat flux values ranging from 30 to 75 W/cm2 (26 to 66 Btu/ft2·s) were measured downstream of the cavity flame holder, 20 to 150 W/cm2 (18 to 132 Btu/ft2·s) in the cavity flame holder, and 20 to 40 W/cm2 (18 to 35 Btu/ft2·s) upstream of the flame holder....

Heat flux sensors fabricated using Direct Write Thermal Spray are thin, surface-based devices that can operate at high temperatures. In this paper, Direct Write heat flux sensors are calibrated over a temperature range of 25 °C-860 °C. A substitution-based quartz lamp configuration is used to measure the steady-state sensitivity and transient response of Direct Write sensors at ambient temperatures, with repeatability confirmed over a 10-month period and after thermal aging. A matched heat flow approach is used to characterize the sensors at operating temperatures up to 860 °C. The sensitivity is found to increase by a factor of two from 25 °C to 650 °C, after which it plateaus up to the maximum tested temperature of 860 °C. A cubic polynomial calibration function captures the temperature dependence of the sensitivity and provides a good agreement for the measured data.

The unique ability of grain boundaries to act as effective sinks for radiation damage plays a significant role in nanocrystalline materials due to their large interfacial area per unit volume. Leveraging this mechanism in the design of tungsten as a plasma-facing material provides a potential pathway for enhancing its radiation tolerance under fusion-relevant conditions. In this study, we explore the impact of defect microstructures on the mechanical behavior of helium ion implanted nanocrystalline tungsten through nanoindentation. Softening was apparent across all implantation temperatures and attributed to bubble/cavity loaded grain boundaries suppressing the activation barrier for the onset of plasticity via grain boundary mediated dislocation nucleation. An increase in fluence placed cavity induced grain boundary softening in competition with hardening from intragranular defect loop damage, thus signaling a new transition in the mechanical behavior of helium implanted nanocrystalline tungsten.

Tungsten (W), a primary candidate for the plasma-facing components of nuclear fusion reactors (e.g. the divertor region in ITER) is susceptible to cracks, blisters, bubbles, and other morphological changes when irradiated with energetic particles. This work investigated two new materials, nanograined W and a nanograined W–Ti alloy, for potential use as plasma-facing materials. Their retention properties and morphological changes after exposure to deuterium (D) and helium (He) plasma at 50 eV and surface temperatures of 500 and 1000 K were analyzed. Nanograined W was found to have smaller blisters and be less prone to fuzz formation than commonly-utilized micro-grain polycrystalline W. Additionally, the nanograined W–Ti alloy exhibited a lower concentration of blisters on its surface than pure W, including nanograined W. Keywords: Tungsten, Plasma-facing materials, Deuterium, Helium

The deformation behavior of nanocrystalline Ni–W alloys is evaluated by nanoindentation techniques for grain sizes of 3–150 nm, spanning both the range of classical Hall–Petch behavior as well as the regime where deviations from the Hall–Petch trend are observed. The breakdown in strength scaling, observed at a grain size of 10–20 nm, is accompanied by a marked transition to inhomogeneous, glass-like flow (i.e. shear banding) at the finest grain sizes approaching the amorphous limit. As a consequence of this mechanistic crossover, additional inflections arise in the mechanical properties; maxima are observed in both the rate and pressure dependence of deformation at approximately the same grain size as the onset of the Hall–Petch breakdown. These data experimentally connect the mechanical properties of nanocrystalline alloys to the well-known behavior of amorphous metals.

In this experimental study, heat flux measurements were made at four locations within the isolator/combustor/nozzle region of a direct-connect gaseous hydrocarbon-fueled scramjet combustor. Combustor conditions were fixed and representative of a Mach-5 flight condition. The heat flux measurements were made using sensors deposited directly on the engine thermal barrier coating (TBC) using Direct Write Technology. Flush-wall fuel injection locations were fixed and a cavity on the body side wall provided the necessary flame holder. Overall equivalence ratios were varied from 0.6 to 1.0, but were held constant for a given run. Preliminary results show heat flux values rising with combustor equivalence ratio at all measurement locations. Heat flux values ranging from 400 to 1000 kW/m 2 (35 to 88 BTU/ft 2 -s) were measured in the location downstream of the cavity flame holder. Heat flux values ranging from 600 to 2000 kW/m 2 (53 to 176 BTU/ft 2 -s) were measured in the cavity flame holder location. Heat flux values ranging from 200 to 600 kW/m 2 (18 to 53 BTU/ft 2 -s) were measured in the locations upstream of the cavity flame holder. Measured local heat flux values were consistent with area-averaged heat flux measurements over a section of the combustor.

High-temperature nanoindentation experiments are conducted to assess the activation enthalpy for deformation of nanocrystalline Ni–W alloys, for grain sizes between 3 and 80 nm. Thermal softening becomes less pronounced at finer grain sizes, and the activation enthalpy has an apparent inflection at a grain size near ∼10–20 nm, in the vicinity of the Hall–Petch breakdown. This inflection is related to that observed in the activation volume for deformation, and is associated with a shift to grain boundary-mediated deformation at the finest grain sizes., United States. Army Research Office (Institute for Soldier Nanotechnologies at MIT), United States. Army Research Office (contract DAAD19-03-1-0235)

The breakdown of Hall–Petch strength scaling is investigated in nanocrystalline Ni–W alloys at high strain rates, using dynamic hardness testing at grain sizes ranging from 3to150nm. Whereas quasistatic tests show a strength plateau below about 15nm, high-rate tests (indentation strain rate of ∼103s−1) exhibit a pronounced strength peak and a regime of “inverse Hall–Petch” weakening. This effect is shown to be the result of a grain size-dependent rate sensitivity that exhibits a maximum at a grain size near 10–20nm. High strain rates are also shown to promote shear banding at nanocrystalline grain sizes.

Lightweighting of structural materials has proven indispensable in the energy economy, predicated on alloy design with high strength-to-weight ratios. Modern aluminum alloys have made great strides in ambient temperature performance and are amenable to advanced manufacturing routes such as additive manufacturing, but lack elevated temperature robustness where gains in efficiency can be obtained. Here, we demonstrate the intentional design of disorder at interfaces, a notion generally associated with thermal runaway in traditional materials, in a segregation-engineered ternary nanocrystalline Al–Ni–Ce alloy that exhibits exceptional thermal stability and elevated temperature strength. In-situ transmission electron microscopy in concert with ultrafast calorimetry and X-ray total scattering point to synergistic co-segregation of Ce and Ni driving the evolution of amorphous intergranular films separating sub-10 nm Al-rich grains, which gives rise to emergent thermal stability. We ascribe this intriguing behavior to near-equilibrium interface conditions followed by kinetically sluggish intermetallic precipitation in the confined disordered region. The resulting outstanding mechanical performance at high homologous temperatures lends credence to the efficacy of promoting disorder in alloy design and discovery.