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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 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

A two-set order parameters phase-field modeling of crack deflection/penetration in a heterogeneous microstructure

Modeling of crack deflection/penetration or manifested as intergranular/transgranular fracture in polycrystals has long been a challenge in both fracture mechanics and materials science. A phase-field model formulated with two-set order parameters describing the crack field and the microstructure field respectively, is herein established to investigate the competition between crack penetration and deflection at an interface. The advantages of the proposed model are three-fold: (1) the fracture toughness of grain boundaries and/or interfaces, besides single crystal properties (e.g., Young’s modulus, Poisson ratio, and fracture toughness), can be automatically incorporated, (2) cohesive interfaces are treated in one phase-field framework in comparison with the integrated phase-field and cohesive zone model in which cohesive elements have to be manually placed at the interface, and (3) two-set order parameters enable to describe the concurrent crack propagation and microstructure evolution. The proposed model is validated by comparing the simulated critical deflection angles with the results from theoretical analysis of the linear elastic fracture mechanics and the experiments. The model is then applied to a heterogeneous Silicon Nitride polycrystal to determine the effect of interface fracture toughness on the intergranular/transgranular fracture transition and the effective toughness of the materials.

Chen, Hu↗
DOE OSTI2019

The effect of thermal-aging on the microstructure and mechanical properties of 9Cr ferritic/martensitic ODS alloy

In this paper, the thermal-stability of 9Cr F/M ODS alloy was evaluated by the aging treatment in the air at 700 degrees C for 10 h, 100 h, 1000 h, and 2005 h. The variations of mechanical properties with different aging periods were tested in terms of tensile testing and vickers microhardness. And the microstructural evolution of strengthening particles and grain morphologies in the 9Cr F/M ODS alloys were characterized using field emission scanning electron microscopy, transmission electron microscopy and high-energy X-ray diffraction. It was found that the as-rolled 9Cr F/M ODS alloy slightly recovered without recrystallization during the initial stage of less than 10 h aging time, resulting in the slight loss of tensile strength and microhardness and the increase of elongation. With the aging time prolonged to 2005 h, several of nano-sized Y-Ti-O particles coarsened and the composition changed to Y-Ti-Al-O, and the number density of large size nitride precipitations increased and the composition also changed from TiN to (Ti,AI)N. Meanwhile, some of grains tend to grow up in some preferential directions due to the zener pinning effect by the localized uneven distributed particles inside the grains, causing more particles, especially the particles with a size of larger than 40 nm, located at grain boundaries. These microstructural evolutions lead to the increasing of microhardness and a little bit decreasing of eleongation during 10 h-2005 h, and the increasing of tensile strength during 10 h-1000 h with the subsequent slight decreasing during 1000 h-2005 h. In conclusion, 9Cr F/M alloy shows a relatively good thermal-aging stability at 700 degrees C for 2005 h.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Combined Torsion and Tension on the Microstructure and Fracture Behavior of 316L Austenitic Stainless Steel

In this study, the microstructure evolution and fracture behavior of 316L austenitic stainless steel (ASS) deformed by torsion–tension at room temperature was systematically investigated. During combined tension and torsion, the grain size was refined with increasing shear strain. In addition, the microtexture exhibited a preferred orientation with increasing shear stress. The metastable austenite underwent phase transition when the shear stress increased. The shear stress affected the fracture morphology, and dimples with different sizes and depths were observed for different pre-torsions. The dislocation density increased significantly owing to severe shear deformation. In addition, the dislocation structure evolved and subgrains appeared with the accumulation of shear strain during this combined deformation. The result showed that pre-torsion deformation plays an important role in improving the comprehensive performance and controlling the microstructure evolution of the sample subjected to tension deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Radiation Damage in Materials—Helium Effects

Despite its scarcity in terrestrial life, helium effects on microstructure evolution and thermo-mechanical properties can have a significant impact on the operation and lifetime of applications, including: advanced structural steels in fast fission reactors, plasma facing and structural materials in fusion devices, spallation neutron target designs, energetic alpha emissions in actinides, helium precipitation in tritium-containing materials, and nuclear waste materials. The small size of a helium atom combined with its near insolubility in almost every solid makes the helium–solid interaction extremely complex over multiple length and time scales. This Special Issue, “Radiation Damage in Materials—Helium Effects”, contains review articles and full-length papers on new irradiation material research activities and novel material ideas using experimental and/or modeling approaches. These studies elucidate the interactions of helium with various extreme environments and tailored nanostructures, as well as their impact on microstructural evolution and material properties.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Thermal stability and irradiation response of nanocrystalline CoCrCuFeNi high-entropy alloy

Grain growth and phase stability of a nanocrystalline face-centered cubic (fcc) Ni 0.2 Fe 0.2 Co 0.2 Cr 0.2 Cu 0.2 high-entropy alloy (HEA), either thermally- or irradiation-induced, are investigated through in situ and post-irradiation transmission electron microscopy (TEM) characterization. Synchrotron and lab x-ray diffraction measurements are carried out to determine the microstructural evolution and phase stability with improved statistics. Under in situ TEM observation, the fcc structure is stable at 300 °C with a small amount of grain growth from 15.8 to ~20 nm being observed after 1800 s. At 500 °C, however, some abnormal growth activities are observed after 1400 s, and secondary phases are formed. Under 3 MeV Ni room temperature ion irradiation up to an extreme dose of nearly 600 displacements per atom, the fcc phase is stable and the average grain size increases from 15.6 to 25.2 nm. Lastly, grain growth mechanisms driven by grain rotation, grain boundary curvature, and disorder are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Elucidating Abnormal Grain Growth in Thermomagnetic Processed Materials with Transfer Learning and Reinforcement Learning

The goal of this research program is to establish the mechanism governing local grain boundary motion, which is needed to design and process desirable microstructures for better performance, by identifying the relative contributions of grain boundary (GB) energy and mobility to grain growth. Classical models for grain growth assume that the primary mechanism for reducing the total interfacial energy is area reduction and that GB restructuring is not significant. This assumption implies that grain growth is locally driven by curvature. However, recent experimental observations using new non-destructive 3D x-ray diffraction microscopy techniques (3D-XRM) reveal that classic descriptors (i.e., curvature, number of neighbors, grain size) do not predict real grain growth. Instead, local GB motion appears to be governed by its energy relative to its neighbors such that low-energy boundaries replace those of higher energy. However, simulations that incorporate GB energy anisotropy still fail to reproduce these observations. These discrepancies suggest that the common assumption for grain growth theory must be re-examined to predict and, thus, control microstructure evolution in real polycrystals. A significant challenge to testing this assumption is due to anisotropic GB mobility. Mobility may cause abnormal grain growth or affect the final grain shapes or growth rate but its true contributions are unknown because it is difficult to measure. For example, observations in Fe have found that grains associated with high energy and high mobility boundaries tend to experience abnormal grain growth, whereas abnormal grain growth is associated with low energy and high mobility boundaries in alumina. As mobility and energy both control GB motion, it is challenging to isolate the local driving forces necessary to test the common assumption that the primary mechanism is area reduction. The novelty of this work is the use of machine learning tools to capture GB mobility and energy from 3D-XRM measurements in polycrystals to test the common assumption used in grain growth models. Machine learning can capture high-order correlations in dynamic systems like those found in the evolving GB topology. The PIs have developed a physics-regularized interpretable machine learning microstructure evolution (PRIMME) model that accurately replicates the grain growth behavior of its trained data set.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Unveiling the nucleation and growth of Zr oxide precipitates in internally oxidized Nb3Sn superconductors

We report on atomic-scale analyses of nucleation and growth of Zr oxide precipitates and the microstructural evolution of internally oxidized Nb3Sn wires for high-field superconducting magnet applications, utilizing atom probe tomography (APT), transmission electron microscopy (TEM), and first-principles calculations. APT analyses reveal that oxygen and zirconium are already segregated at grain boundaries (GBs) in the unreacted Nb-1Zr-4Ta (at%) alloy prior to forming Nb3Sn through reacting the Nb alloy with Sn and SnO2. After forming Nb3Sn, Zr oxide precipitates nucleate both at the Nb3Sn/Nb heterophase interfaces and in the Nb3Sn grains, driven by the small solubilities of Zr and O in Nb3Sn compared to their value in Nb. A high number density (Nv) of Zr oxide nanoprecipitates is observed in the Nb3Sn layers, ∼10 23 m −3 , with a mean diameter <10 nm for a heat treatment at 625 °C. Quantitative APT and TEM analyses of the Zr oxide precipitates in the reacted Nb3Sn layers elucidate details of the nucleation, growth, and coarsening processes of the Zr oxide precipitates in Nb3Sn. First-principles calculations and classical nucleation theory are employed to study the nucleation of Zr oxide precipitates in Nb3Sn and to estimate the maximum energy barrier and critical radius for nucleation. Our research unveils the kinetic pathways for nucleation and growth of Zr oxide precipitates and the microstructural evolution of Nb3Sn layers, which helps to understand and improve the superconducting properties of internally oxidized Nb3Sn wires for use in high-field superconducting magnets. •We investigated the nucleation and growth of Zr oxide nanoprecipitates in Nb3Sn superconductors for magnet applications.•APT and TEM analyses reveals the nucleation and growth process of Zr oxide precipitates in Nb3Sn layers.•Classical nucleation theory with DFT is employed to describe the nucleation process of Zr oxide precipitates in Nb3Sn.•We find that the interfacial reactions at Nb/Nb3Sn interface play a critical role in the nucleation of Zr oxide precipitates.•Current study provides a pathway to improve the properties of internally oxidized Nb3Sn superconducting wires.

43 PARTICLE ACCELERATORS↗
DOE OSTI2022

Sample Preparation and Experimental Design for In Situ Multi-Beam Transmission Electron Microscopy Irradiation Experiments

We discuss the need to understand materials exposed to overlapping extreme environments such as high temperature, radiation, or mechanical stress. When these stressors are combined there may be synergistic effects that enable unique microstructural evolution mechanisms to activate. Understanding of these mechanisms is necessary for the input and refinement of predictive models and critical for engineering of next generation materials. The basic physics and underlying mechanisms require advanced tools to be investigated. The in situ ion irradiation transmission electron microscope (I 3 TEM) is designed to explore these principles. To quantitatively probe the complex dynamic interactions in materials, careful preparation of samples and consideration of experimental design is required. Particular handling or preparation of samples can easily introduce damage or features that obfuscate the measurements. There is no one correct way to prepare a sample; however, many mistakes can be made. The most common errors and things to consider are highlighted within. The I 3 TEM has many adjustable variables and a large potential experimental space, therefore it is best to design experiments with a specific scientific question or questions in mind. Experiments have been performed on large number of sample geometries, material classes, and with many irradiation conditions. The following are a subset of examples that demonstrate unique in situ capabilities utilizing the I 3 TEM. Au nanoparticles prepared by drop casting have been used to investigate the effects of single ion strikes. Au thin films have been used in studies on the effects of multibeam irradiation on microstructure evolution. Zr films have been exposed to irradiation and mechanical tension to examine creep. Ag nanopillars were subjected to simultaneous high temperature, mechanical compression, and ion irradiation to study irradiation induced creep as well. These results impact fields including: structural materials, nuclear energy, energy storage, catalysis, and microelectronics in space environments.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Pyrolysis Vacuum-Assisted Plasma Ionization Ion Mobility–Mass Spectrometry for Insoluble Polymer Analysis

Wire-based metal additive manufacturing utilizes the ability of additive manufacturing to fabricate complex geometries with high deposition rates (above 7 kg/h), thus finding applications in the fabrication of large-scale components, such as stamping dies. Traditionally, the workhorse materials for stamping dies have been martensitic steels. However, the complex thermal gyrations induced during additive manufacturing can cause the evolution of an inhomogeneous microstructure, which leads to a significant scatter in the mechanical properties, especially the toughness. Therefore, to understand these phenomena, arc-based additive AISI 410 samples were fabricated using robotic gas metal arc welding (GMAW) and were subjected to a detailed characterization campaign. The results show significant scatter in the tensile properties as well as Charpy V-notch impact toughness data, which was then correlated to the microstructural heterogeneity and delta (δ) ferrite formation. Post-processing (austenitizing and tempering) treatments were developed and an ~70% reduction in the scatter of tensile data and a four-times improvement in the toughness were obtained. The changes in mechanical properties were rationalized based on the microstructure evolution during additive manufacturing. Based on these, an outline to tailor the composition of “printable” steels for tooling with isotropic and uniform mechanical properties is presented and discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Mitigating Scatter in Mechanical Properties in AISI 410 Fabricated via Arc-Based Additive Manufacturing Process

Wire-based metal additive manufacturing utilizes the ability of additive manufacturing to fabricate complex geometries with high deposition rates (above 7 kg/h), thus finding applications in the fabrication of large-scale components, such as stamping dies. Traditionally, the workhorse materials for stamping dies have been martensitic steels. However, the complex thermal gyrations induced during additive manufacturing can cause the evolution of an inhomogeneous microstructure, which leads to a significant scatter in the mechanical properties, especially the toughness. Therefore, to understand these phenomena, arc-based additive AISI 410 samples were fabricated using robotic gas metal arc welding (GMAW) and were subjected to a detailed characterization campaign. The results show significant scatter in the tensile properties as well as Charpy V-notch impact toughness data, which was then correlated to the microstructural heterogeneity and delta (δ) ferrite formation. Post-processing (austenitizing and tempering) treatments were developed and an ~70% reduction in the scatter of tensile data and a four-times improvement in the toughness were obtained. The changes in mechanical properties were rationalized based on the microstructure evolution during additive manufacturing. Based on these, an outline to tailor the composition of “printable” steels for tooling with isotropic and uniform mechanical properties is presented and discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Aluminum Addition on the Evolution of Microstructure, Crystallographic Texture and Mechanical Properties of Single Phase Hexagonal Close Packed Mg-Li Alloys

In the present investigation, an effort has been made to understand the effect of aluminum addition to α Mg-Li alloys. The corresponding composition Mg-4Li-xAl (x = 0, 2, 4 and 6 wt.%) alloys have been prepared by stir casting route under an argon environment. Extrusion was carried out at 300 °C with the extrusion ratio of 15:1. Significant grain refinement was observed after extrusion. X-ray diffraction-based investigation of the cast and extruded alloys showed the presence of intermetallic compounds such as Mg{sub 17}Al{sub 12} and AlLi in the Al-rich alloys namely, Mg-4Li-xAl (x = 4 and 6 wt.%). These precipitates were also present in the extruded plus annealed samples, indicating the stability of the precipitates at high temperature. The bulk x-ray texture measurement revealed a crystallographic texture where the c-axis of the h.c.p crystals was perpendicular to the extrusion direction (ED) for extruded sample. A texture transition was observed on annealing. The c-axis was oriented parallel to the ED. Mechanical properties of the cast, extruded and extruded plus annealed material illustrate that the addition of Al led to enhancement in hardness, yield strength and ultimate tensile strength.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Evolution of Microstructure, Residual Stress, and Tensile Properties of Additively Manufactured Stainless Steel Under Heat Treatments

This study investigated the microstructure, residual stress and tensile properties of the directed energy deposited stainless steel 316L (SS316L) under thermal annealing. Microstructure characterization shows the as-printed sample has laser-generated patterns where dendritic structure is observed at the edge of the patterns and cellular structure dominates the interior region. Here, the thermal annealing at 983 and 1093°C effectively removes the dendritic/cellular structures. Synchrotron X-ray diffraction reveals that the as-fabricated SS316L exhibits compressive residual stress of -197.4 MPa. The stress is greatly relieved to -53.8 MPa after annealing at 1093°C. The room temperature tensile testing indicates that the yield strength and ultimate tensile strength drop from 378 and 502 MPa in the as-printed sample to 258 and 446 MPa in the annealed samples (1093°C), respectively. Our study provides insights into the relationship between microstructure, residual stress, and tensile properties of laser additive manufactured SS316L.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Phase structures and morphologies of tempered CA6NM stainless steel welded by hybrid laser-arc process

The post-weld tempered microstructure of hybrid laser-arc welded CA6NM, a cast low carbon martensitic stainless steel, was investigated. The microstructural evolutions from the fusion zone to the base metal were characterized in detail using optical microscopy, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and microhardness techniques. The fusion zone, in its post-weld tempered condition, consisted of tempered lath martensite, residual delta-ferrite with various morphologies, reversed austenite and chromium carbides. The reversed austenite, which can be detected through both EBSD and XRD techniques, was found to be finely dispersed along the martensite lath boundaries, particularly at triple junctions. Based on the EBSD analysis, the orientation relationship between the reversed austenite and the adjacent martensite laths seemed to follow the Kurdjumov-Sachs (K-S) model. The results also revealed the presence of the reversed austenite in the different regions of the heat affected zone after post-weld tempering. The microindentation hardness distribution was measured, and correlated to the evolution of the corresponding microstructure across the welds. - Highlights: •The EBSD analysis was performed on hybrid laser-arc welded CA6NM. •The FZ consisted of tempered lath martensite, reversed austenite, carbides and δ ferrite after tempering. •The reversed γ was formed along the α′ lath boundaries, particularly at triple junctions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Evolution of microstructures and mechanical properties during solution treatment of a Ti–V–Mo-containing high‑manganese cryogenic steel

Highlights: • High-Mn cryogenic steels are getting the spotlight in the cryogenic market. • The grain size grew slowly and the yield strength remained almost stabilized with an increase of solutionizing temperature. • The Hall–Petch strengthening coefficient for yield strength was small, estimated to be 153 MPa μm{sup 0.5}. • The weakening effect of solutionizing temperature on grain size was attributed to the nano-sized (Ti, V, Mo)C precipitate. - Abstract: The effect of solution treatment on the microstructure and mechanical properties of a high‑manganese (Mn) twin-induced plastic (TWIP) cryogenic steel that was alloyed from elemental titanium (Ti), vanadium (V) and molybdenum (Mo) was investigated by a variety of techniques such as microstructural characterization, room tensile testing and − 196 °C impact testing. The austenite grain size grew slowly with an increase of the solutionizing temperature (ST). The relatively weak effect of the ST on the austenite grain size is attributed to the superior thermal stability and coarsening resistance of the (Ti, V, Mo)C precipitate, which hindered austenite grain growth. An increase in ST improved the low-temperature toughness (LTT) significantly and decreased the strain hardening rate (SHR), but had no significant effect on the yield strength. The Hall–Petch strengthening coefficient for yield strength was small, and was estimated to be 153 MPa μm{sup 0.5}. Undissolved large-sized chromium (Cr) carbide can deteriorate the LTT significantly. Because of its excellent mechanical properties, high-Mn steel that was alloyed by elemental Ti, V and Mo can be used to manufacture storage tanks for liquefied natural gas.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of W on creep behaviors of novel Nb-bearing high nitrogen austenitic heat-resistant cast steels at 1000 °C

Highlights: • The synergistic CALPHAD and experimental approaches are adopted. • Creep behaviors of high nitrogen austenitic alloys are investigated at 1000 °C. • Creep properties are improved through the W solid solution strengthening. • W additions promote the formation of (Cr,Fe,W){sub 23}C{sub 6} instead of (Cr,Fe,W){sub 7}C{sub 3}. • (Cr, W)-rich precipitates severely degrade the creep properties. - Abstract: Recently, there is an urgent demand from automotive industries to develop new austenitic heat-resistant cast steels that withstand exhaust gas temperatures as high as 1000 °C. In this study, the effect of W additions on the creep behavior of a series of Nb-bearing high nitrogen austenitic heat-resistant cast steels was investigated at 1000 °C and 50 MPa. Microstructures before and after creep rupture tests were carefully characterized to illustrate the microstructural evolution during creep deformation. Microstructural analyses revealed that the W addition did not affect the formation of primary Nb(C,N), whereas significantly promoted the precipitation of primary (Cr, W)-rich phases, particularly those cellular structures along grain boundaries. The W solid solution strengthening and the secondary precipitation strengthening of submicron-scale (Cr, W)-rich phase improved the creep resistance of the alloys through increasing the hardness of γ-austenite and pinning the dislocations. However, the primary cellular (Cr, W)-rich structures significantly accelerated the nucleation and propagation of creep cracks along grain boundaries, thereby increasing the creep rate and decreasing the creep life.

36 MATERIALS SCIENCE↗