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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 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Evolution of the nanoporous microstructure of sintered Ag at high temperature using in-situ X-ray nanotomography

Silver pastes sintering is a potential candidate for die bonding in power electronic modules. The joints, obtained by sintering, exhibit a significant pore fraction, thus reducing the density of the material compared to bulk silver. The mechanical properties (Young's modulus, yield strength and ultimate tensile stress) are then drastically altered. However, while careful analysis of the nanoporous structure has been reported in 2D, little is known about its quantitative spatial evolution during thermal aging and more specifically during temperature jumps endured by the assembly during operation. High temperature evolutions of the 3D nanoporous structure have been observed in-situ using a heater fitted into the beamline 6-2c at SLAC-SSRL. Segmentation of the porosity and subsequent statistical analysis of the tomographic dataset reveal a complex evolution of the porous nanostructure including growth, separation and coalescence at a constant density. Such an analysis provides insight into the microstructural evolution of sintered nanoporous Ag joints in-service.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Verification and validation of a rapid heat transfer calculation methodology for transient melt pool solidification conditions in powder bed metal additive manufacturing

A fundamental understanding of spatial and temporal thermal distributions is crucial for predicting solidification and solid-state microstructural development in parts made by additive manufacturing. While sophisticated numerical techniques that are based on finite element or finite volume methods are useful for gaining insight into these phenomena at the length scale of the melt pool (100 - 500 µm), they are ill-suited for predicting engineering trends over full part cross-sections (> 10 x 10 cm) or many layers over long process times (> many days) due to the necessity of fully resolving the heat source characteristics. On the other hand, it is extremely difficult to resolve the highly dynamic nature of the process using purely in-situ characterization techniques. This article proposes a pragmatic alternative based on a semi-analytical approach to predicting the transient heat conduction during powder bed metal additive manufacturing process. The model calculations were theoretically verified for selective laser melting of AlSi10Mg and electron beam melting of IN718 powders for simple cross-sectional geometries and the transient results are compared to steady state predictions from the Rosenthal equation. It is shown that the transient effects of the scan strategy create significant variations in the melt pool geometry and solid-liquid interface velocity, especially as the thermal diffusivity of the material decreases and the pre-heat of the process increases. With positive verification of the strategy, the model was then experimentally validated to simulate two point-melt scan strategies during electron beam melting of IN718, one intended to produce a columnar and one an equiaxed grain structure. Lastly, through comparison of the solidification conditions (i.e. transient and spatial variations of thermal gradient and liquid-solid interface velocity) predicted by the model to phenomenological CET theory, the model accurately predicted the experimental grain structures.

36 MATERIALS SCIENCE↗
DOE OSTI2024

A strong fracture-resistant high-entropy alloy with nano-bridged honeycomb microstructure intrinsically toughened by 3D-printing

Strengthening materials via conventional “top-down” processes generally involves restricting dislocation movement by precipitation or grain refinement, which invariably restricts the movement of dislocations away from, or towards, a crack tip, thereby severely compromising their fracture resistance. In the present study, a high-entropy alloy Al0.5CrCoFeNi is produced by the laser powder-bed fusion process, a “bottom-up” additive manufacturing process similar to how nature builds structures, with the microstructure resembling a nano-bridged honeycomb structure consisting of a face-centered cubic (fcc) matrix and an interwoven hexagonal net of an ordered body-centered cubic B2 phase. While the B2 phase, combined with high-dislocation density and solid-solution strengthening, provides strength to the material, the nano-bridges of dislocations connecting the fcc cells, i.e., the channels between the B2 phase on the cell boundaries, provide highways for dislocation movement away from the crack tip. Consequently, the nature-inspired microstructure imparts the material with an excellent combination of strength and toughness.

36 MATERIALS SCIENCE↗
DOE OSTI2018

High stereographic resolution texture and residual stress evaluation using time-of-flight neutron diffraction

Neutron diffraction texture measurements provide bulk averaged textures with excellent grain orientation statistics, even for large-grained materials, owing to the probed volume being of the order of 1 cm 3 . Furthermore, crystallographic parameters and other valuable microstructure information such as phase fraction, coherent crystallite size, root-mean-square microstrain, macroscopic or intergranular strain and stress, etc. can be derived from neutron diffractograms. A procedure for combined high stereographic resolution texture and residual stress evaluation was established on the pulsed-neutron-source-based engineering materials diffractometer TAKUMI at the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Research Center, through division of the neutron detector panel regions. Pole figure evaluation of a limestone standard sample with a well known texture suggested that the precision obtained for texture measurement is comparable to that of the established neutron beamlines utilized for texture measurement, such as the HIPPO diffractometer at the Los Alamos Neutron Science Center (New Mexico, USA) and the D20 angle-dispersive neutron diffractometer at the Institut Laue–Langevin (Grenoble, France). A high-strength martensite–austenite multilayered steel was employed for further verification of the reliability of simultaneous Rietveld analysis of multiphase textures and macro stress tensors. By using a texture-weighted geometric mean micromechanical (BulkPathGEO) model, a macro stress tensor analysis with a plane stress assumption showed a rolling direction–transverse direction (RD–TD) in-plane compressive stress (about –330 MPa) in the martensite layers and an RD–TD in-plane tensile stress (about 320 MPa) in the austenite layers. The phase stress partitioning was ascribed mainly to the additive effect of the volume expansion during martensite transformation and the linear contraction misfit between austenite layers and newly transformed martensite layers during the water quenching process.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1978

Quantitative Ultrasonic Evaluation of Mechanical Properties of Engineering Materials

Progress in the application of ultrasonic techniques to nondestructive measurement of mechanical strength of engineering materials is reviewed. A dormant concept in nondestructive evaluation (NDE) is invoked. The availability of ultrasonic methods that can be applied to actual parts to assess their potential susceptibility to failure under design conditions is discussed. It was shown that ultrasonic methods yield measurements of elastic moduli, microstructure, hardness, fracture toughness, tensile strength, yield strength, and shear strength for a wide range of materials (including many types of metals, ceramics, and fiber composites). It was also indicated that although most of these methods were shown feasible in laboratory studies, more work is needed before they can be used on actual parts in processing, assembly, inspection, and maintenance lines.

Vary, A.↗
NASA NTRS1995

Thermal conductivity of zirconia thermal barrier coatings

Thermal barrier coatings (TBC's) applied to the hot gas components of turbine engines lead to enhanced fuel efficiency and component reliability. Understanding the mechanisms which control the thermal transport behavior of the TBC's is of primary importance. Physical vapor description (PVD) and plasma spraying (PS) are the two most commonly used coating techniques. These techniques produce coatings with unique microstructures which control their performance and stability. The PS coatings were applied with either standard power or hollow sphere particles. The hollow sphere particles yielded a lower density and lower thermal conductivity coating. The thermal conductivity of both fully and partially stabilized zirconia, before and after thermal aging, will be compared. The thermal conductivity of the coatings permanently increase upon being exposed to high temperatures. These increases are attributed to microstructural changes within the coatings. Sintering of the as fabricated plasma sprayed lamellar structure is observed by scanning electron microscopy of coatings isothermally heat treated at temperatures greater than 1100 C. During this sintering process the planar porosity between lamella is converted to a series of small spherical pores. The change in pore morphology is the primary reason for the observed increase in thermal conductivity. This increase in thermal conductivity can be modeled using a relationship which depends on both the temperature and time of exposure. Although the PVD coatings are less susceptible to thermal aging effects, preliminary results suggest that they have a higher thermal conductivity than PS coatings, both before and after thermal aging. The increases in thermal conductivity due to thermal aging for partially stabilized plasma sprayed zirconia have been found to be less than for fully stabilized plasma sprayed zirconia coatings. The high temperature thermal diffusivity data indicates that if these coatings reach a temperature above 1100 C during operation, they will begin to lose their effectiveness as a thermal barrier.

Dinwiddie, R. B.↗
NASA NTRS1995

Thermal conductivity of zirconia thermal barrier coatings

Thermal barrier coatings (TBC's) applied to the hot gas components of turbine engines lead to enhanced fuel efficiency and component reliability. Understanding the mechanisms which control the thermal transport behavior of the TBC's is of primary importance. Physical vapor deposition (PVD) and plasma spraying (PS) are the two most commonly used coating techniques. These techniques produce coatings with unique microstructures which control their performance and stability. The PS coatings were applied with either standard powder or hollow sphere particles. The hollow sphere particles yielded a lower density and lower thermal conductivity coating. The thermal conductivity of both fully and partially stabilized zirconia, before and after thermal aging, will be compared. The thermal conductivity of the coatings permanently increases upon exposed to high temperatures. These increases are attributed to microstructural changes within the coatings. Sintering of the as-fabricated plasma sprayed lamellar structure is observed by scanning electron microscopy of coatings isothermally heat treated at temperatures greater than 1100 C. During this sintering process the planar porosity between lamella is converted to a series of small spherical pores. The change in pore morphology is the primary reason for the observed increase in thermal conductivity. This increase in thermal conductivity can be modeled using a relationship which depends on both the temperature and time of exposure. Although the PVD coatings are less susceptible to thermal aging effects, preliminary results suggest that they have a higher thermal conductivity than PS coatings, both before and after thermal aging. The increases in thermal conductivity due to thermal aging for partially stabilized plasma sprayed zirconia have been found to be less than for fully stabilized plasma sprayed zirconia coatings. The high temperature thermal diffusivity data indicate that if these coatings reach a temperature above 1100 C during operation, they will begin to lose their effectiveness as a thermal barrier.

Dinwiddie, R. B.↗
NASA NTRS2000

Gravitational Effects on Combustion Synthesis of Advanced Porous Materials

Combustion Synthesis (self-Propagating high-temperature synthesis-(SHS)) of porous Ti-TiB(x), composite materials has been studied with respect to the sensitivity to the SHS reaction parameters of stoichiometry, green density, gasifying agents, ambient pressure, diluents and gravity. The main objective of this research program is to engineer the required porosity and mechanical properties into the composite materials to meet the requirements of a consumer, such as for the application of bone replacement materials. Gravity serves to restrict the gas expansion and the liquid movement during SHS reaction. As a result, gravitational forces affect the microstructure and properties of the SHS products. Reacting these SHS systems in low gravity in the KC-135 aircraft has extended the ability to form porous products. This paper will emphasize the effects of gravity (low g, 1g and 2g) on the SHS reaction process, and the microstructure and properties of the porous composite. Some of biomedical results are also discussed.

Zhang, X.↗
NASA NTRS2005

Experiments and Models for Polymeric Microsphere Foams

The current project was performed under the direction of Dr. Byron Pipes as its lead investigator from January 2001 to August 2004. With the permission of the NASA, the project was transferred to Dr. Thein Kyu as the principle investigator for the period of September 2004 - June 2005. There were two major thrust areas in the original proposal; (1) experimental characterization and kinematics of foam structure formation and (2) determination of the mechanical, physical, and thermal properties, although these thrust areas were further sub- divided into 7 tasks. The present project has been directed primarily to elucidate kinematics of micro-foam formation (tasks 1 and 3) and to characterize micro-foam structures, since the control of the micro-structure of these foams is of paramount importance in determining their physical, mechanical and thermal properties. The first thrust area was accomplished in a timely manner; however, the second thrust area of foam properties (tasks 2,4-7) has yet to be completed because the area of kinematics of foam structure formation turned out to be extremely complex and thus consumed more time than what have been anticipated. As will be reported in what follows, the present studies have greatly enhances the in-depth understanding of mechanisms and kinematics of the micro-foam formation from solid powders. However, in order to implement all objectives of the second thrust areas regarding investigations of mechanical, physical, and thermal properties and establishment of the correlation of structure - properties of the foams, the project needs additional time and resources. The technical highlights of the accomplishment are summarized as follows. The present study represents a first approach to understanding the complexities that act together in the powder foaming process to achieve the successful inflation of polyimide microstructures. This type of study is novel as no prior work had dissected the fundamentals that govern the inflation process in this type of systems. The systematic approach to each of the different phenomena (i.e. morphological, diffusive, kinetic and dynamic) brings into context each of them in a way that allows separate understanding and analysis. Of the different phenomena studied, probably the one that gives a higher level of control over the inflation process has been shown to be the morphological aspects of the precursor particles. It is a major contribution of the present work to isolate and identify this phenomenon and highlight the features that with careful control during the synthesis of the precursor material can lead to a highly optimized and specialized final product (neat foam or microstructure). Some of these accomplishments have been presented in various national meetings and some of which are either published in refereed journals or still in various stages of publications. One of the presentations was selected for "Best of ANTEC 2004" Online Presentation Series of the Society of Plastics Engineers (SPE) (September 2004)

Pipes, R. Byrona↗
DOE OSTI2023

Integrating fiber optic sensors into metallic components for sensing in harsh environments

The integration of fiber optic sensors into high-temperature materials is critical for real-time monitoring and autonomous operation of engineering systems. This study demonstrated a spark plasma sintering (SPS)-assisted embedding process for integrating sapphire fiber optic sensors into stainless steel components during part fabrication. Optical fibers were encapsulated in stainless steel 316L powders which were sintered at different fabrication conditions using SPS to investigate the effects of sintering parameters on the embedment. Measurements of optical transmittance, combined with microstructural analysis (X-ray computed tomography and scanning electron microscopy) and mechanical testing (tensile and microhardness), were conducted to examine the fiber functionality, fiber–matrix bonding quality, and properties of the sintered materials. Here, the results show that under suitable fabrication conditions, intact optical fibers can be encapsulated in highly-densified (>98 % relative density) stainless steel components. These conditions also led to a good bond at the fiber–matrix interface with micron-sized material interdiffusion across the interface. The sintering parameters were observed to affect fiber optical attenuation, where high temperature, pressure, and hold time during SPS enhanced fiber–matrix bonding and adversely affected optical transmission. Tensile testing confirmed the superior tensile strength and ductility of the matrix fabricated by SPS. Furthermore, the materials exhibited limited strength reduction (~70 MPa) upon the integration of fibers. This study demonstrates the effectiveness of SPS for fiber-material integration for high-temperature applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

How Microstructure and Pore Moisture Affect Strength Gain in Portlandite-Enriched Composites That Mineralize CO 2

Binders containing portlandite (Ca(OH) 2 ) can take up carbon dioxide (CO 2 ) from dilute flue gas streams (<15% CO 2 , v/v), thereby forming carbonate compounds with binding attributes. While the carbonation of portlandite particulates is straightforward, it remains unclear how CO 2 transport into monoliths is affected by microstructure and pore moisture content. Therefore, this study elucidates the influences of pore saturation and CO 2 diffusivity on the carbonation kinetics and strength evolution of portlanditeenriched composites (“mortars”). To assess the influences of microstructure, composites hydrated to different extents and conditioned to different pore saturation levels (S w ) were exposed to dilute CO 2 . First, reducing saturation increases the gas diffusivity and carbonation kinetics so long as saturation exceeds a critical value (S w,c ≈ 0.10) independent of microstructural attributes. Second, careful analysis reveals that both traditional cement hydration and carbonation offer similar levels of strengthening, the magnitude of which can be estimated from the extent of each reaction. As a result, portlandite-enriched binders offer cementation performance that is similar to traditional materials while offering an embodied CO 2 footprint that is more than 50 % smaller. Furthermore, these insights are foundational to create new “low-CO 2 ” cementation agents via in situ CO 2 mineralization (utilization) using dilute CO 2 waste streams.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2002

Effect of Adventitious Carbon on the Environmental Degradation of SiC/BN/SiC Composites

Pesting remains a major obstacle to the application of SiC/SiC composites in engine service and selective degradation of the boron nitride interphase at intermediate temperatures is of primary concern. However, significant progress has been made on interphase improvement recently and we now know more about the phenomenon and ways to suppress it. By screening SiC/BN/SiC materials through characterization of strength and microstructures after exposure in a burner rig, some factors that control pesting in these composites have been determined. A key precaution is careful control of elemental carbon presence in the interphase region.

Ogbuji, L. U. J. T.↗
NASA NTRS2003

Lower-Conductivity Thermal-Barrier Coatings

Thermal-barrier coatings (TBCs) that have both initial and post-exposure thermal conductivities lower than those of yttria-stabilized zirconia TBCs have been developed. TBCs are thin ceramic layers, generally applied by plasma spraying or physical vapor deposition, that are used to insulate air-cooled metallic components from hot gases in gas turbine and other heat engines. Heretofore, yttria-stabilized zirconia (nominally comprising 95.4 atomic percent ZrO2 + 4.6 atomic percent Y2O3) has been the TBC material of choice. The lower-thermal-conductivity TBCs are modified versions of yttria-stabilized zirconia, the modifications consisting primarily in the addition of other oxides that impart microstructural and defect properties that favor lower thermal conductivity.

Miller, Robert A.↗
NASA NTRSDate not supplied

Next Generation Durability and Damage Tolerance to Support Certification of Flight Hardware

Durability and Damage Tolerance (D&DT), as currently applied to flight hardware throughout the Agency, is based on continuum and similitude assumptions and does not consider local material properties, environments and responses. These limitations impact our ability to support certification of new component designs, material systems and manufacturing approaches. The primary concern in this Engineering R&A plan is related to the imminent proliferation of parts produced by additive manufacturing (AM). These parts are being advocated by NASA’s suppliers for use on a myriad of flight hardware because of their design flexibility and cost advantages. AM provides opportunities to reduce part counts through complex geometry and reduce manufacturing costs of low volume parts. However, AM materials have some notable metallurgical and microstructural differences compared to traditionally fabricated materials. One of the challenges for the acceptance of AM is the greater tendency for a deleterious defect state, most commonly in the form of porosity, to exist in AM parts. Though this defect state is typically reduced through a hot isostatic pressing (HIP) processing step, the structural performance risks associated with the remaining defects and HIP-healed features is not known. Potential fracture control issues that must be resolved stem from the real possibility that an unhealed defect or a closed defect with less than pristine strength remains at a fracture critical location after HIP. As a result, NASA and the entire aerospace community have been confronted with the need to develop a robust and relevant certification methodology to enable safe implementation of these components. The present work is an important step toward positioning NASA to credibly respond to vendors’ push to implement this new AM materials technology by improving our understanding of AM processing and performance and transitioning that research-based understanding to next-generation engineering capabilities.

Edward H Glaessgen↗
DOE OSTI2018

High-temperature deformation behavior and microstructural characterization of high-Mn bearing titanium-based alloy

Highlights: • The hot deformation behavior of a biomedical low-cost Ti-10Mn alloys was studied. • The flow curves of the alloy display periodic serrations attributed to dynamic strain aging. • Mn alloying increases the activation energy to 243 kJ/mol compared to pure Ti. • The dynamic recovery is more effective than dynamic recrystallization at hot deformation. - Abstract: Ti-Mn alloys exhibit an excellent potential for biomedical applications as well as structural engineering applications, especially in the aerospace industry. In order to control and enhance grain structure during the manufacturing of Ti-Mn alloys and thereby help to enhance mechanical properties such as strength and toughness, we studied the hot-deformation behavior of βTi-10Mn alloys. Isothermal compression tests were conducted in the strain rate range of 0.01–10 s{sup −1} and temperatures in the range of 850–1000 °C using a Gleeble thermomechanical simulator. High-temperature flow stress curves exhibited discontinuous yielding and pronounced periodic serrations without any strain hardening during compression straining of these alloys. Such peculiar behavior of this alloy is due to active dynamic strain aging in its β-bcc structure. Metallographic observations by electron-backscattered diffraction (EBSD) analysis revealed that dynamic recovery (DRV) is more active than continuous dynamic recrystallization (CDRX) when the alloy is deformed at high strain rates, i.e. higher than 1 s{sup −1}. Furthermore, the constitutive behavior of the alloy was modeled and the apparent hot-deformation activation energy of the alloy was estimated to be 243 kJ/mol, which is ~60% higher than the self-diffusion energy in pure titanium.

36 MATERIALS SCIENCE↗