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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

High-energy synchrotron x-ray study of deformation-induced martensitic transformation in a neutron-irradiated Type 316 stainless steel

An unusual tensile deformation behaviour in the form of a propagating band along the sample gauge was observed in two neutron-irradiated 316 stainless steel samples during room-temperature tests, leading to a combination of high strength and high ductility. These bands were not observed in an unirradiated counterpart. With the help of in situ high-energy synchrotron x-ray diffraction, the phase-specific crystal information was tracked at different deformation levels in each sample. Post-irradiation and post-deformation samples were examined using electron microscopy to characterize various microstructural features. All samples displayed a deformation-induced martensitic phase transformation, which was identified as a second strain-hardening mechanism accompanying the dislocation hardening. The deformation-induced martensitic transformation was rationalized by the effect of applied stress on the effective martensite start temperature. The results showed that the irradiation did not alter the dislocation hardening and the martensitic transformation mechanisms, but the increased yield strength in irradiated materials facilitated the localized phase transformation at the onset of plastic deformation, in contrast to the unirradiated material which required pre-straining. The hardening effect of the martensitic transformation reduced the tendency towards necking and mitigated the loss of ductility in the irradiated material by carrying the deformation in the form of a propagating band. Despite the beneficial effect from the martensitic transformation, this study indicates that this mechanism cannot not be activated at typical operating temperatures of nuclear reactors.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Understanding the efficacy of concentrated interstitial carbon in enhancing the pitting corrosion resistance of stainless steel

By introducing a high fraction of interstitial carbon through low temperature carburization, the pitting corrosion resistance of austenitic stainless steel can be significantly improved. Previous work attributed this to strengthening of the passive film by carbon. Here, however, we present a paradigm shift by showing that carbon actually weakens the passive film but strengthens the bonding within the alloy by forming localized (covalent) bonds with the metal atoms (Fe, Cr, Ni). Accordingly, the enhancement in pitting resistance is a result of carbon reducing the metal dissolution rate in a local pit environment by many orders of magnitude, which extremely decreases the growth stability of a pit and prevents it from transitioning into stable growth. Electronic structure calculations confirm that carbon bonds to the metal atoms and that the metal–carbon bonds are 1.4 to 2.0 times stronger than the metal–metal bonds. These results suggest a new strategy for the design of corrosion resistant alloys, namely surface engineering by infusion of concentrated interstitials that form strong bonds with the matrix atoms.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2022

Origin, parameters, and underlying deformation mechanisms of propagating deformation bands in irradiated 316L stainless steel

Lüders-type propagating deformation bands were observed in specimens of irradiated 316L stainless steel samples removed from Spallation Neutron Source target vessels after service. Mechanical testing with digital image correlation (DIC) and in-situ tensile testing with scanning electron microscopy electron backscatter diffraction showed that the observed Lüders-type behavior was not related to the known transformation-induced plasticity or twinning-induced plasticity behavior. Instead, the phenomenon occurs at small local strain values before a significant amount of martensite or deformation twins appear in the microstructure. Microstructural analysis and in-situ mechanical test results suggest Lüders-type band formation and propagation were related to the appearance and evolution of defect-free channels—analogous to slip bands. A modified Swift equation with a Ludwigson-like component was offered to rationalize the phenomenon and model the strain-softening processes at small strain values. Finally, the results indicate complex microstructural processes and deformation mechanisms were active at small strain values and underline the benefits of advanced mechanical test approaches such as DIC.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Solidification modes during additive manufacturing of steel revealed by high-speed X-ray diffraction

Solidification during fusion-based additive manufacturing (AM) is characterized by high solidification velocities and large thermal gradients, two factors that control the solidification mode of metals and alloys. Using two synchrotron-based, in situ setups, we perform high-speed X-ray diffraction measurements to investigate the impact of the solidification velocities and thermal gradients on the solidification mode of a hot-work tool steel over a wide range of thermal conditions of relevance to AM of metals. The solidification mode of primary δ-ferrite is observed at a cooling rate of 2.12x10 4 K/s, and at a higher cooling rate of 1.5x10 6 K/s, δ-ferrite is sup-pressed, and primary austenite is observed. The experimental thermal conditions are evaluated and linked to a Kurz-Giovanola-Trivedi (KGT) based solidification model. The modelling results show that the predictions from the multicomponent KGT model agree with the experimental observations. This work highlights the role of in situ XRD measurements for a fundamental understanding of the microstructure evolution during AM and for vali-dation of computational thermodynamics and kinetics models, facilitating parameter and alloy development for AM processes.

36 MATERIALS SCIENCE↗
DOE OSTI2023

The effect of microstructure on the dynamic shock response of 1045 steel

Herein, the effect of microstructure on the shock response of 1045 steel is investigated via plate impact experiments and postmortem characterization. Three unique microstructures are explored: ferrite-pearlite, martensite, and ferrite with spheroidal cementite (i.e. spheroidized). Two spall recovery experiments, at approximate peak pressures of 3.2 and 3.5 GPa, are conducted to assess the Hugoniot elastic limit (HEL), spall strength, and damage morphology of the various microstructures. The ferrite-pearlite and martensite microstructures exhibit continuous yielding at both quasi-static and dynamic rates, while the spheroidized condition displays discontinuous yielding. Discontinuous yielding of the spheroidized microstructure is attributed to a combined low initial dislocation density coupled with a low dislocation nucleation rate. The spall strength of ferrite-pearlite is consistently lower than the spheroidized microstructure, attributed to elongated cementite that is more susceptible to cracking than more spherical cementite precipitates. Despite a high density of boundaries, martensite exhibits the highest spall strength. A large percentage of the boundaries within the martensite microstructure are found to be low energy (i.e. Σ3 or low angle), and are thus less susceptible to spall damage. Overall, the high spall strength of martensite is likely linked to traditional strengthening mechanisms that limit dislocation motion.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Dislocation channel broadening–A new mechanism to improve irradiation-assisted stress corrosion cracking resistance of additively manufactured 316 L stainless steel

Additively manufactured (AM) 316 L stainless steel (SS) after hot isostatic pressing (HIP) was found to exhibit superior resistance to irradiation-assisted stress corrosion cracking (IASCC) in high-temperature water, as compared to wrought 316 L SS. The well-accepted IASCC factors of radiation-induced segregation (RIS) and radiation hardening are not accurate descriptions of IASCC susceptibility in this case. A decreased strain localization along grain boundaries, caused by dislocation channel broadening, was confirmed to suppress crack initiation. A unique distribution of irradiation-induced defects in HIP AM 316 L SS eased dislocation cross-slip compared to those in the wrought counterpart, thus increasing the channel width near the grain boundaries. Here for the first time, this study highlights the importance of dislocation channel broadening as a potential mechanism to further improve the IASCC resistance of 316 L SS and provides direct experimental evidence based on commercial-grade materials.

316L stainless steel↗
DOE OSTI2025

MX precipitate behavior in an irradiated advanced Fe-9Cr steel: Helium effects on phase stability

As part of an ongoing series aimed at optimizing Fe-9Cr reduced activation ferritic/martensitic (RAFM) alloys for fusion energy systems, this study explores MX precipitate behavior under dual-ion irradiations, specifically examining correlations between helium transmutation and irradiation-induced damage. Utilizing single and dual-beam ion irradiation, the research explores the combined effects of helium (10–25 appm He/dpa), temperature (400–600 °C), and damage levels (15–100 dpa) on the microstructural evolution of CNA9 steel, a variant of Castable Nanostructured Alloys (CNAs). The study demonstrates that helium co-implantation hinders radiation-enhanced coarsening of MX-TiC precipitates at 500 and 600 °C, maintaining MX-TiC precipitate stability at moderate damage levels (15 dpa) but failing to prevent complete precipitate dissolution at higher damage levels (≥50 dpa) when irradiated at 500 °C. Here, a generalized precipitate stability model suggests that helium-induced suppression of diffusion alters the balance between recoil resolution and back diffusion for MX-TiC precipitates, enhancing the current understanding of precipitate behavior under damage and transmutation simulated dual-ion irradiation conditions.

Characterization↗
DOE OSTI2022

High-throughput surface characterization to identify porosity defects in additively manufactured 316L stainless steel

In the present study, a high throughput (HT) methodology is applied to rapidly assess the surface characteristics of 83 additively manufactured 316L stainless steel. The variation in surface roughness (S a ) showed a good correlation with a dimensionless number (π) and provided direct linkages to the internal porosity defects. Lack-of-fusion regime relates to high surface roughness (S a > 5 µm), low dimensionless number (π < 61), and the presence of cavities in between the melt pool tracks. Balling regime correlates to high surface roughness (S a > 5 µm), intermediate dimensionless number (61 < π < 146), and non-uniform melt pool track width. Keyhole regime shows low surface roughness (S a < 5 µm), high dimensionless number (π > 146), and a curved melt pool track. This approach accelerates the process parameter discovery and minimizes the porosity defects for the LPBF process. The impact of the defects on the as-processed tensile mechanical properties demonstrates that samples with porosity exhibit up to 10% less tensile strength and 30% less ductility than optimal samples.

316L stainless steel↗
DOE OSTI2020

In-situ synthesis of oxides by reactive process atmospheres during L-PBF of stainless steel

Traditionally, reactive gases such as oxygen (O 2 ) and carbon dioxide (CO 2 ) have been avoided during laser powder bed fusion (L-PBF) of metals and alloys based on the notion that it may lead to defect formation and poor properties. Here, in this work, we show that instead, these gases can be used to form sub-μm-sized oxide particles in-situ during the L-PBF process in an Fe-Cr-Al-Ti stainless steel and lead to improved room temperature and high-temperature mechanical properties. We manufactured cube samples using pure Ar and various reactive gas atmospheres, namely an O 2 /Argon (Ar) mixture containing 0.2 % O 2 and CO 2 /Ar mixtures containing up to 100 % CO 2 . Co-axial measurements of infrared radiation emitted from the melt pool showed correlation to the presence of O 2 or CO 2 in the gas mixture. Builds produced under CO 2 -containing atmosphere contained complex oxides with an average diameter of ~40 nm, an Al-rich core and a Ti-rich shell. Due to the high cooling rates typical to L-PBF, agglomeration of oxides and slag formation on the surface of the samples could almost be entirely avoided. Compression tests at temperatures up to 800 °C showed that the samples produced in 100 % CO 2 have about 20 % higher yield stress compared to samples produced in Ar. The paper concludes with a discussion of the formation mechanism of the observed oxides. Our results show that in-situ reactions during additive manufacturing processes are a promising pathway to the synthesis of particle-reinforced alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2020

The origin and formation of oxygen inclusions in austenitic stainless steels manufactured by laser powder bed fusion

The origins of nano-scale oxide inclusions in 316L austenitic stainless steel (SS) manufactured by laser powder bed fusion (L-PBF) was investigated by quantifying the possible intrusion pathways of oxygen contained in the precursor powder, extraneous oxygen from the process environment during laser processing, and moisture contamination during powder handling and storage. When processing the fresh, as-received powder in a wellcontrolled environment, the oxide inclusions contained in the precursor powder were the primary contributors to the formation of nano-scale oxides in the final additive manufactured (AM) product. These oxide inclusions were found to be enriched with oxygen getter elements like Si and Mn. By controlling the extraneous oxygen level in the process environment, the oxygen level in AM produced parts was found to increase with the extraneous oxygen level. The intrusion pathway of this extra oxygen was found to be dominated by the incorporation of spatter particles into the build during processing. Moisture induced oxidation during powder storage was also found to result in a higher oxide density in the AM produced parts. SS 316L powder free of Si and Mn oxygen getters was processed in a well-controlled environment and resulted in a similar level of oxygen intrusion. Finally, microhardness testing indicated that the oxide volume fraction increase from extraneous oxygen did not influence hardness values. However, a marked decrease in hardness was found for the humidified and Si-Mn free AM processed parts.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of processing parameters and strut dimensions on the microstructures and hardness of stainless steel 316L lattice-emulating structures made by powder bed fusion

In this study, we present the effects of input processing parameters and strut thickness (in square struts) on microstructure and properties in laser powder bed fusion additively manufactured stainless steel 316L lattice-emulating structures. Lattice-emulating X-structures with square cross-sections of 1.5, 1.0, and 0.5 mm were fabricated using three different parameter sets with varying power, speed, and therefore, linear energy density. Grain size and morphology were shown to be dictated by epitaxial growth, which was dependent on weld pool morphology. Additionally, grain size and morphology were shown to change across the thickness direction of the struts (from the bottom inclined surface to the top inclined surface). The spatial variation in grain size was reflected by changes in hardness through the thickness of each strut. The 0.5 mm struts exhibited more significant grain elongation in the strut direction and larger sub-grain solidification cell diameters than their thicker counterparts. The larger sub-grain solidification cell diameters in the 0.5 mm samples resulted in correspondingly lower hardness values when compared to samples of higher thicknesses.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Ultrafine-grained Fe-TiB 2 high-modulus nanocomposite steel with high strength and isotropic mechanical properties by laser powder bed fusion

Fe-TiB 2 metal matrix composite, also called high-modulus steels (HMSs), are of great interest for applications in fuel-efficient transportation infrastructure, aerospace, and wear industries due to their high specific stiffness and yield strength. However, conventional cast Fe-TiB 2 HMSs often contain coarse and sharp-edged TiB 2 particles which easily trigger premature cracking during loading. Here, we synthesized a Fe-TiB 2 nanocomposite HMS via laser powder bed fusion (LPBF) additive manufacturing of mixed micro-sized powders of Fe, Ti, and Fe 2 B. We investigated the microstructure formation and mechanical behavior of the Fe-TiB 2 HMS. We found that in situ chemical reaction of Ti and Fe 2 B enables the formation of TiB 2 particles at nanoscale during rapid solidification of LPBF. These nanoscale TiB 2 particles can serve as heterogeneous nucleation sites and promote the formation of ultrafine and equiaxed α-Fe grains with random crystallographic textures, which differ from many other additively manufactured (AM) metal alloys characteristic of strong crystallographic textures. As such, isotropic mechanical properties were achieved in the AM Fe-TiB 2 nanocomposite HMS with a high elastic modulus of ~ 240 GPa, an exceptional yield strength of ~ 1450 MPa, and a large plasticity of ~ 20% under compression. Quantitative analysis reveals that the high yield strength primarily originates from strengthening contributions of the ultrafine grains with an average grain size of ~450 nm, the nanoscale TiB 2 reinforcing particles of 20–180 nm, and a high density of printing-induced dislocations of the order of 10 15 m –2 . In situ synchrotron high-energy X-ray diffraction unveils the load partitioning from the softer α-Fe matrix to the stiffer and stronger TiB 2 nanoparticles, contributing to the sustained strain hardening during compression. Our work not only provides a general pathway for achieving high-performance metal matrix nanocomposites by in situ chemical reaction and precipitation of ceramic nanoparticles during additive manufacturing, but also offers mechanistic insights into the deformation mechanism of nanoparticle-reinforced HMS composites.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Effect of thermal treatments on electrochemical behavior of binder jetted 17-4 PH stainless steel

Binder jetted 17–4 PH stainless steel was post-processed to relate heat treatment, microstructure, and corrosion in 3.5 wt% NaCl. Specimens were sintered at 1380 or 1400 °C, solution-annealed at 1055 °C for 1 h, and aged at 482 °C for 1 h. Here, as-sintered parts showed α′-martensitic matrix with a δ-ferrite network and Cu-rich precipitates in ferrite; inclusions (MnS, NbC) promoted localized attack. solutionizing redistributed elements and reduced ferrite, while aging generated coherent Cu nano-precipitates. Corrosion resistance was highly sensitive to post-processing in which aged specimens exhibited the lowest corrosion current density and formed a thicker, stable Cr 2 O 3 -rich passive film, whereas sintered specimens degraded most. Pitting potential depended on sintering temperature and microstructure, with 1400 °C sintering yielding more positive pitting potentials and the best overall performance after aging. Although the 1400-solutionized condition showed a relatively noble pitting response versus 1380-solutionized, it displayed unstable corrosion kinetics attributed to an imperfect passive film linked to higher NbC density. XPS depth profiles corroborated these trends, showing thicker, more continuous Cr 2 O 3 in aged states and discontinuous/thinning oxides in less resistant conditions. Practically, high-temperature sintering (∼1400 °C) followed by solutionizing and aging is recommended, with further gains expected from reducing NbC/MnS populations and porosity via powder and process control.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Monitoring steel fibre orientation in self-compacting cementitious composite slabs during pouring with dynamic X-ray radiography

This paper presents a new technique based on dynamic X-ray radiography that can be used to assess fibre orientation during pouring of steel fibre reinforced cementitious composites. Synthetic examples were used to assess the suitability and robustness of the technique, which was shown to provide reliable measurements of fibre orientation even when the signal-to-noise ratio is relatively high. A study was then carried out on the effect of formwork aspect ratio, time/duration of pouring, and rebar placement on the fibre orientation while pouring self-compacting cementitious composite slabs. Results demonstrated the ability of the technique in monitoring the movements of fibres while pouring, and the strong effect of the flow in inducing preferential fibre alignment within the slabs. Fibre orientation was found to vary progressively over time and could take about half of the duration of pouring to fully stabilise.

36 MATERIALS SCIENCE↗
DOE OSTI2021

A closer look at corrosion of steel reinforcement bars in concrete using 3D neutron and X-ray computed tomography

Corrosion of reinforcing bars constitutes the largest threat to the durability of concrete structures. Thus, several studies have investigated the nature of the corrosion products, most using post-mortem analyses. However, corrosion products evolve when in contact with oxygen, hindering result interpretation. This work presents instead a state-of-the-art, non-destructive 3D method for the assessment of corrosion of embedded reinforcements. Multimodal neutron and X-ray tomography was used to observe, non-destructively, the characteristics of the corrosion products in two concrete samples, with the aim of investigating possible benefits of the use of this technique for reinforced concrete structures. One sample was naturally corroded, extracted from an 81-year-old bridge, the other was corroded via the galvanostatic method, resulting in corrosion-induced cracks. Quantitative and qualitative data was acquired, including the iron-to-rust volumetric ratio in macroscopic interfacial voids and the thickness of the corrosion layer at the steel concrete interface. The iron-to-rust volumetric ratio corresponded to large, soluble, corrosion products, forming in environments with low availability of oxygen for both samples.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Oxide dispersion strengthened 304 L stainless steel produced by ink jetting and laser powder bed fusion

This paper discusses the fundamentals of a novel hybrid method to synthesize oxide dispersion strengthened (ODS) 304 L stainless steel (SS) alloy using a modified laser powder bed fusion (LPBF) machine. Previously, ODS metal matrix composites have been produced by LPBF via ball-milling, which is expensive to scale. Here, we selectively dope yttria nanoparticles into a SS matrix by jetting a precursor chemistry onto the SS substrate prior to laser conversion and consolidation. Here, the new alloy shows good room temperature mechanical properties. Microstructures are studied using electron microscopy, energy dispersive spectroscopy and electron backscatter diffraction.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Carburization resistance of cu-coated stainless steel in supercritical carbon dioxide environments

This study investigates the effectiveness of Cu as a corrosion barrier in supercritical carbon dioxide (s-CO 2 ) by coating 316 stainless steel (316) with various thicknesses of Cu. In this work, 316 exposed to s-CO 2 with 50 ppm CO showed a reduction in oxidation corresponding to the thickness of its Cu barrier coating. Additionally, a continuous Cu layer between the environment and the alloy was found to correlate to an elimination of carburization and corrosion-related mechanical degradation. This Cu coating technique could be applied over a variety of temperatures to improve the corrosion resistance of alloys that are susceptible to carburization.

36 MATERIALS SCIENCE↗
DOE OSTI2020

In situ synchrotron X-ray tomography of 304 stainless steels undergoing chlorine-induced stress corrosion cracking

To understand the mechanisms behind chlorine-induced stress corrosion cracking (CISCC) in 304 stainless steel, synchrotron high energy x-rays were used to perform in-situ x-ray microtomography on a pre-cracked sample under load in a corrosive simulated marine environment. The tomography scans showed the crack morphology evolving into a branching crack around the surface of the sample. Finite element analysis and stress intensity analysis are conducted to explain the observed crack branching.

304 Stainless Steel↗