Richard A. Karnesky

4.16 – Tritium Barriers and Tritium Diffusion in Fusion Reactors

Tritium is a radioactive form of hydrogen. Because it is radioactive, its release to the environment must be minimized. Most of the materials used in fusion reactors are metals that have a relatively high permeability for tritium. The fusion community has been working on barrier materials to minimize tritium release by permeation through structural materials. Unfortunately, most barrier materials work very well during laboratory experiments, but fail to meet requirements when placed in radiation environments. This chapter presents tritium permeation characteristics of various materials used in fusion reactors, including plasma-facing, structural, and barrier materials. The parameters necessary for tritium release calculations for various regions of a fusion reactor are given.

Direct measurement of two-dimensional and three-dimensional interprecipitate distance distributions from atom-probe tomographic reconstructions

Edge-to-edge interprecipitate distance distributions are critical for predicting precipitation strengthening of alloys and other physical phenomena. A method to calculate this three-dimensional distance and the two-dimensional interplanar distance from atom-probe tomographic data is presented. It is applied to nanometer-sized Cu-rich precipitates in an Fe-1.7at.% Cu alloy. Experimental interprecipitate distance distributions are discussed.

Review of the synergies between computational modeling and experimental characterization of materials across length scales

With the increasing interplay between experimental and computational approaches at multiple length scales, new research directions are emerging in materials science and computational mechanics. Such cooperative interactions find many applications in the development, characterization and design of complex material systems. This manuscript provides a broad and comprehensive overview of recent trends in which predictive modeling capabilities are developed in conjunction with experiments and advanced characterization to gain a greater insight into structure–property relationships and study various physical phenomena and mechanisms. The focus of this review is on the intersections of multiscale materials experiments and modeling relevant to the materials mechanics community. After a general discussion on the perspective from various communities, the article focuses on the latest experimental and theoretical opportunities. Emphasis is given to the role of experiments in multiscale models, including insights into how computations can be used as discovery tools for materials engineering, rather than to “simply” support experimental work. This is illustrated by examples from several application areas on structural materials. This manuscript ends with a discussion on some problems and open scientific questions that are being explored in order to advance this relatively new field of research.

Imaging and quantification of hydrogen isotope trapping.

The location of hydrogen isotopes is imaged in austenitic stainless steel and model materials using local-electrode atom-probe (LEAP) tomography and trapping energies are measured by thermal desorption spectroscopy. LEAP tomography has sub-nanometer resolution and excellent compositional sensitivity due to pulse counting techniques. Site-specific sample preparation is possible using focused-ion beam, enabling us to show trapping at low density features, such as

Dynamical diffraction peak splitting in time-of-flight neutron diffraction

Time-of-flight neutron diffraction data from 20 and 0.7mm thick perfect Si single crystal samples, which exhibit dynamical diffraction effects associated with finite crystal size, are presented. This effect is caused by constructive interference occurring solely from thin layers bounded by the front (entry) and back (exit) surfaces of the sample with no scattering originating from the layers in between, resulting in two distinct peaks observed for each reflection. If the sample is thin and/or the instrument resolution is insufficient, these two peaks can convolve and cause peak shape aberrations which can lead to significant errors in the strain and peak-broadening parameters obtained from a kinematical diffraction analysis.

Hydrogen Isotope Permeation and Trapping in Additively Manufactured Steels.

Additively manufactured (AM) austenitic stainless steels are intriguing candidates for the storage of gaseous hydrogen isotopes. Complex vessel geometries can be built more easily than by using conventional machining options. Parts built with AM steel tend to have excellent mechanical properties (with tensile strength, ductility, fatigue crack growth, and fracture toughness comparable to or exceeding that of wrought austenitic stainless steel). However, the solidification microstructures produced by AM processing differ substantially from the microstructures of wrought material. Some features may increase permeability, including both some amount of porosity and a greater amount of ferrite. Because the diffusivity of hydrogen in ferrite is greater than in austenite (six orders of magnitude at ambient temperature), care must be taken to retain the performance that is taken for granted due to the base alloy chemistry. Furthermore, AM parts tend to have greater dislocation densities and greater amounts of carbon, nitrogen, and oxygen. These features, along with the austenite/ferrite interfaces, may contribute to greater hydrogen trapping. We report the results of our studies of deuterium transport in various austenitic (304L, 316, and 316L) steels produced by AM (via either powder bed fusion or blown powder methods). The hydrogen permeability (an equilibrium property) changes negligibly (less than a factor of 2), regardless of chemistry and processing method, when tested between 150 and 500 °C. This is despite increases in ferrite content up to FN=2.7. However, AM materials exhibit greater hydrogen istotope trapping, as measured by permeation transients, thermal desorption spectra, and inert gas fusion measurement. The trapping energies are likely modest (<10 kJ/mol), but may indicate a larger population of trap sites than in conventional 300-series steels. INTRODUCTION Additively-manufactured (AM) metals, particularly those made from austenitic stainless steel powder that is melted with a laser, have been proposed for both building new parts [1] and for repairing damaged parts [2–4] for hydrogen service. While both of these use-cases are proposed to take advantage of AM’s unique abilities to build to complex (and even conformal) geometries, the material selected for the powder is not novel to the process. Conventionally-manufactured austenitic stainless steel is resistant to hydrogen-assisted fracture [5–7] and has a reasonably modest permeability for hydrogen isotopes at typical operating temperatures and pressures [8]. However, the solidification microstructures of AM austenitic steels differ from conventionally-proceesed microstructures. The grain size is fine (d~10-100 μm [9–11]). Although wrought or worked (e.g. rolled) material may also have d<100 μm [12,13], the grain structure of AM materials tends to be different. AM materials tend to have elongated grains (aspect ratios of 2-3:1) and are more highly textured. Due to their rapid solidification, AM austenitic stainless steels tend to retain more δ-ferrite than conventional materials (FN<6, though some still routes lead to levels below detection limits [14]). The initial dislocation density tends to be very high (ρ~2.4x10 14 m -2 [15] vs. ρ~10 12 m -2 for wrought [12]). The chemistry of the AM parts depends not only on the composition of the starting powder (which tends to have greater amounts of N and O) and the build environment (inert Ar, typically), but also on interactions with the laser, which may change the chemistry of the bulk part from the powder due to preferential vaporizing. For example, the level of N in parts built from virgin powder is typically about 0.04 wt.% [9], though building in inert N drives this to about 0.5 wt.% [16]). The distribution of elements within a built part is expected to be different because of the high solidification rate, though limited high resolution studies have been done to date. Finally, the porosity of AM built parts tends to be slightly higher due to closed pores in the starting powder and lack-of-fusion defects. These microstructural differences between AM and conventional materials lead to differences in properties. The literature typically reports greater yield strengths in AM steels, for example [1,9,10,14,15]. The different microstructure may also lead to a difference in how hydrogen isotope transport through AM materials. Porosity and ferrite, for example, may

Investigating Dislocation-Twin Boundary Interactions in Nickel using Diffraction Contrast Scanning Transmission Electron Microscopy

The interaction of dislocations with and near grain boundaries plays an important role in the strain response of metals. Whether a dislocation is blocked or transmitted at a grain boundary is dependent on a number of inter-related factors including the alignment of slip systems, the inclination of the interface, and the nature of the defect transition states within the interface itself. In this presentation we consider the morphology and character of dislocations interacting with {111} and {112} twin boundary facets in Ni. Our work is motivated in part by interest in explaining the influence of twin boundaries on the fracture behavior of metals exposed to hydrogen environments. For instance, Bechtle et al. have reported a reduced susceptibility to intergranular hydrogen embrittlement in Ni processed to have a high fraction of Σ3 twins and related Σ3 special boundaries [1]. However, analyses have also shown that although they are resistant to crack propagation, twins can still be sites of preferential crack initiation during loading in the presence of hydrogen [2], raising questions regarding the precise mechanisms of strain localization at such interfaces and how these are changed with exposure to hydrogen.

Permeation of "Hydromer" Film: An Elastomeric Hydrogen-Capturing Biopolymer.

This report analyzes the permeation resistance of a novel and proprietary polymer coating for hydrogen isotope resistance that was developed by New Mexico State University. Thermal gravimetric analysis and thermal desoprtion spectroscopy show the polymer is stable thermally to approximately 250 deg C. Deuterium gas-driven permeation experiments were conducted at Sandia to explore early evidence (obtained using Brunauer - Emmett - Teller) of the polymers strong resistance to hydrogen. With a relatively small amount of the polymer in solution (0.15%), a decrease in diffusion by a factor of 2 is observed at 100 and 150 deg C. While there was very little reduction in permeability, the preliminary findings reported here are meant to demonstrate the sensitivity of Sandias permeation measurements and are intended to motivate the future exploration of thicker barriers with greater polymer coverage.

Understanding H isotope adsorption and absorption of Al-alloys using modeling and experiments (LDRD: #165724)

Current austenitic stainless steel storage reservoirs for hydrogen isotopes (e.g. deuterium and tritium) have performance and operational life-limiting interactions (e.g. embrittlement) with H-isotopes. Aluminum alloys (e.g.AA2219), alternatively, have very low H-isotope solubilities, suggesting high resistance towards aging vulnerabilities. This report summarizes the work performed during the life of the Lab Directed Research and Development in the Nuclear Weapons investment area (165724), and provides invaluable modeling and experimental insights into the interactions of H isotopes with surfaces and bulk AlCu-alloys. The modeling work establishes and builds a multi-scale framework which includes: a density functional theory informed bond-order potential for classical molecular dynamics (MD), and subsequent use of MD simulations to inform defect level dislocation dynamics models. Furthermore, low energy ion scattering and thermal desorption spectroscopy experiments are performed to validate these models and add greater physical understanding to them.

Synergies between computational modeling and experimental characterization of materials across length scales

Materials Science is highly interdisciplinary; greater insight into structure–properties relationships requires the development of multiscale/multiphysics models and comparably advanced experimental design, instruments, and analysis. Such challenges and expanding research scope motivate synergies between experimental and the computational communities. Outcomes of this emerging field within the Materials Science community open up new frontiers and research directions at the crossroads of traditional computational Materials Science, experimental Materials Science, and Integrated Computational Mechanics. Such cooperative interactions find many applications in the development, the characterization, and the design of complex material systems. The manuscripts for this special section included in this issue of the Journal of Materials Science highlight examples of recent advances in coupled computational/experimental approaches in predicting various physical phenomena and mechanisms in materials. The manuscripts can be grouped into several topical areas that include: (i) Materials Science at the atomistic scale; (ii) The development of approaches for heterogeneous microstructures; and (iii) 3-D microstructure analysis, including microstructure evolution and mechanical behavior at the microscale. In ‘‘Review of the Synergies Between Computational Modeling and Experimental Characterization of Materials Across Length Scales,’’ we provide a broad and comprehensive overview of recent trends where predictive modeling capabilities are developed in conjunction with experiments and advanced characterization. This review article highlights recent synergies at various scales both from an experimental perspective and from a modeling perspective, discussing the roles of experiments in multiscale models and vice versa. This review article ends with a discussion on some problems and gaps that have to be addressed in order for this coupled approach to impact research and development in the broad scope of structure– property relations successfully in the future. Yamakov et al.’s ‘‘Multiscale Modeling of Sensory Properties of Co–Ni–Al Shape Memory Particles Embedded in an Al Metal Matrix’’ features an atomistic-to-continuum multiscale model to study the efficacy and variability in the sensory particle transformation to detect damage processes in novel ferromagnetic shape memory alloys. This manuscript exemplifies recent efforts within the modeling community to develop new algorithms and methodologies to not only bridge length scales within heterogeneous microstructures but also to account for the multiphysics dimension associated with such complex materials systems. In ‘‘Experimental and computational studies on the role of surface functional groups in the mechanical behavior of interfaces between single-walled carbon nanotubes (CNTs) and metals’’, Hartmann et al. investigate the structure– property correlation of single-walled CNTs embedded in a noble metal (Pd or Au) through the combination of an experimental and computational approach. The experimental component consists of nanoscale pull-out tests with in situ scanning electron microscope experiments, while & Remi Dingreville rdingre@sandia.gov