Developing High Entropy Alloys for Nuclear Applications

High entropy alloys (HEAs) were developed for their desirability of strength, hardness, and corrosion, wear, and radiation resistance. This makes them ideal for nuclear applications in advanced reactors. High entropy alloys are characterized as alloys containing at least 5 principal elements, each with an atomic percentage between 5 and 35% [1]. A process for fabrication and characterization of these alloys entails ball milling and spark plasma sintering (SPS), then characterization tools such as x-ray diffraction (XRD) and scanning electron microscopy (SEM). [1] D. B. Miracle and O. N. Senkov, “A critical review of high entropy alloys and related concepts,” Acta materialia, vol. 122, pp. 448-511, 2017

Investigation of Deformation Behavior of Additively Manufactured AISI 316L Stainless Steel with in situ Micro-Compression Testing

Additive manufacturing techniques are being used more and more to perform the precise fabrication of engineering components with complex geometries. The heterogeneity of additively manufactured microstructures deteriorates the mechanical integrity of products. In this paper, we printed AISI 316L stainless steel using the additive manufacturing technique of laser metal deposition. Both single-phase and dual-phase substructures were formed in the grain interiors. Electron backscatter diffraction and energy-dispersive X-ray spectroscopy indicate that Si, Mo, S, Cr were enriched, while Fe was depleted along the substructure boundaries. In situ micro-compression testing was performed at room temperature along the [001] orientation. The dual-phase substructures exhibited lower yield strength and higher Young’s modulus compared with single-phase substructures. Our research provides a fundamental understanding of the relationship between the microstructure and mechanical properties of additively manufactured metallic materials. The results suggest that the uneven heat treatment in the printing process could have negative impacts on the mechanical properties due to elemental segregation

Ion Irradiation and Examination of Additive Friction Stir Deposited 316 Stainless Steel

This study explored solid-state additive friction stir deposition (AFSD) as a modular manufacturing technology, with the aim of enabling a more rapid and streamlined on-site fabrication process for large meter-scale nuclear structural components with fully dense parts. Austenitic 316 stainless steel (SS) is an excellent candidate to demonstrate AFSD, as it is a commonly-used structural material for nuclear applications. The microstructural evolution and concomitant changes in mechanical properties after 5 MeV Fe++ ion irradiation were studied comprehensively via transmission electron microscopy and nanoindentation. AFSD-processed 316 SS led to a fine-grained and ultrafine-grained microstructure that resulted in a simultaneous increase in strength, ductility, toughness, irradiation resistance, and corrosion resistance. The AFSD samples did not exhibit voids even at 100 dpa dose at 600 °C. The enhanced radiation tolerance as compared to conventional SS was reasoned to be due to the high density of grain boundaries that act as irradiation-induced defect sinks

High-Entropy Alloys for Nuclear Application

High-entropy alloys (HEAs) have become increasingly popular in recent studies across multiple fields due to their outstanding ductility and performance throughout a large temperature range, as well as characteristics like superparamagnetism and superconductivity. These characteristics make HEAs highly desirable materials in novel and advanced nuclear applications where extreme conditions are prevalent. The fabrication and characterization of HEAs have been approached from various directions, traditionally and innovatively, to explore their capabilities and limitations. In this work, we focused on the study of equimolar refractory alloys fabricated using spark-plasma sintering. Mechanical properties of the samples were accumulated through sample preparation and tensile testing, as well as examining their structures using X-Ray Diffraction and electron microscopy. The findings from this study enabled us to refine and select desirable traits like body-centered cubic phases at equilibrium, mixture homogeneity, and nanometric grain size for their advanced nuclear application as well as alterations to amplify them

Orientation-Selected Micro-Pillar Compression of Additively Manufactured 316L Stainless Steels: Comparison of As-Manufactured, Annealed, and Proton-Irradiated Variants

Irradiation response and deformation mechanisms of additively manufactured (AM) 316 L stainless steel were studied by atomic scale characterization and micro-pillar compression. The AM 316 L stainless steels were fabricated by direct energy deposition, a laser-based additive manufacturing process. Irradiation with 2 MeV protons at 360 °C was performed to create ∼1.8 displacements-per-atom (dpa) damage in AM 316 L. Deformation behaviors of the as-manufactured, annealed, and proton-irradiated variants were studied, focusing on the effects of manufacturing-induced pores, residual stress, and irradiation-introduced defects (dislocation loops and voids). Micro-pillars were prepared from grains of pre-selected orientation, avoiding contributions of grain boundaries and allowing determination of resolved shear stress on {111} glide planes. Transmission electron microscopy was used to characterize the pre- and post-deformation microstructure. It was found that in the as-manufactured alloy variant, moving dislocations were the major deformation carrier, with noticeable blocking by fabrication-induced pores, In the annealed variant, hardness was reduced, and deformation was also accomplished by dislocation gliding. In the proton-irradiated variant, significant twinning was observed. Comparing measured resolved shear stress and predicted critical stress for dislocation dissociation, we conclude that irradiation hardening became high enough to activate twinning. Therefore, the deformation mechanism changes from dislocation gliding to twinning. The study is important for both processing optimization and performance evaluation of AM alloys for reactor applications

Effects of Scan Length and Percent Completion on Porosity and Phase Separation in EBM-PBF Ti-64

Corrosion applications such as nuclear energy and prosthesis demand minimization of manufacturing defects commonly found in additive manufacturing. Porosity is correlated with pit initiation, propagation and SCC (stress corrosion cracking). Grain boundaries between phases are also susceptible to corrosion (1). Printing parameters such as power input, material feed rate, scan velocity, layer thickness and mesh size all affect microstructure in printed metal (3). Holding these constant, scan length is another parameter that can be studied. Scan length describes the distance traveled by the laser or electron beam before it reverses direction, to fuse an area adjacent to its immediate path, or “hatches.” Scan vector length has been found to decrease part density and increase cracking (2). This project investigates the effect of scan length on porosity and phase separation. With greater scan length, porosity increased in the titanium samples. With greater percent completion in the build, porosity decreased. Void size was not necessarily correlated with these parameters, but void prevalence was. Increased density and uniformity as a build progresses is expected, because as more layers are sintered, the temperature gradient between the previous layer and current layer decreases and stabilizes. A similar phenomenon takes place in hatching, on the cross sectional level. As seen in the bar with short scan length, greater uniformity and lesser porosity is achieved when the temperature gradient between adjacent scan vectors is minimized. Long scan lengths increase this gradient and result in lack of fusion defects

Ion irradiation and examination of Additive friction stir deposited 316 stainless steel

This study explored solid-state additive friction stir deposition (AFSD) as a modular manufacturing technology, with the aim of enabling a more rapid and streamlined on-site fabrication process for large meter-scale nuclear structural components with fully dense parts. Austenitic 316 stainless steel (SS) is an excellent candidate to demonstrate AFSD, as it is a commonly-used structural material for nuclear applications. The microstructural evolution and concomitant changes in mechanical properties after 5 MeV Fe++ ion irradiation were studied comprehensively via transmission electron microscopy and nanoindentation. AFSD-processed 316 SS led to a fine-grained and ultrafine-grained microstructure that resulted in a simultaneous increase in strength, ductility, toughness, irradiation resistance, and corrosion resistance. The AFSD samples did not exhibit voids even at 100 dpa dose at 600 °C. The enhanced radiation tolerance as compared to conventional SS was reasoned to be due to the high density of grain boundaries that act as irradiation-induced defect sinks

Microstructural Changes of Proton Irradiated Hastelloy-N and \u3cem\u3ein situ\u3c/em\u3e Micropillar Compression Testing of One Single Grain at Different Local Damage Levels

In situ micropillar compression was used to study the deformation of proton-irradiated Hastelloy-N at different damage levels. Multiple pillars were prepared from a single grain along the cross-section of 2.5 MeV proton-irradiated Hastelloy-N. Depending on the location of micropillars, the critical resolved shear stress was obtained as a function of local damage levels. Such an approach eliminates the variation of yield stress due to the difference in the Schmid factor. Microstructural characterization showed complicated defect structures, including (a) dislocation loops with many in corduroy-like alignments, (2) dislocations pile up, (3) element segregation, and (4) twin boundaries. Silicon atoms are found to segregate at dislocation lines, loops, and twin boundaries and form complicated patterns at nanometer scales. These complexities make it difficult to conclude which hardening mechanism contributes the most to the hardness changes. The critical resolved shear stress, τcrss, and hardening exponents were both extracted as a function of displacements per atom values up to 2.3. There was a 60% increase in τcrss at the highest damage level

Rice <i>GA3ox1</i> modulates pollen starch granule accumulation and pollen wall development

The rice GA biosynthetic gene OsGA3ox1 has been proposed to regulate pollen development through the gametophytic manner, but cellular characterization of its mutant pollen is lacking. In this study, three heterozygotic biallelic variants, “-3/-19”, “-3/-2” and “-3/-10”, each containing one null and one 3bp-deletion allele, were obtained by the CRISPR/Cas9 technique for the functional study of OsGA3ox1. The three homozygotes, “-19/-19”, “-2/-2” and “-10/-10”, derived from heterozygotic variants, did not affect the development of most vegetative and floral organs but showed a significant reduction in seed-setting rate and in pollen viability. Anatomic characterizations of these mutated osga3ox1 pollens revealed defects in starch granule accumulation and pollen wall development. Additional molecular characterization suggests that abnormal pollen development in the osga3ox1 mutants might be linked to the regulation of transcription factors OsGAMYB, OsTDR and OsbHLH142 during late pollen development. In brief, the rice GA3ox1 is a crucial gene that modulates pollen starch granule accumulation and pollen wall development at the gametophytic phase