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At least 199 records · Page 11
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NASA NTRS2019

Understanding the Space Weathering of Mercury via Simulation of Micrometeorite Impacts

Space weathering alters the surfaces of airless planetary bodies via irradiation from the solar wind and micrometeorite impacts. These processes modify the microstructure, chemical composition, and spectral properties of surface materials, typically resulting in the reddening (increasing reflectance with increasing wavelength), darkening (reducing albedo), and attenuation of characteristic absorption features in reflectance spectra. In lunar samples, these changes in optical properties are driven by the production of reduced nanophase Fe particles (npFe). Our understanding of space weathering has largely been based on data from the Moon and, more recently, near-Earth S-type asteroids. However, the environment at Mercury is significantly different, with the surface experiencing intense solar wind irradiation and higher velocity micrometeorite impacts. Additionally, the composition of Mercury’s surface varies significantly from that of the Moon, including a component with very low albedo known as low reflectance material (LRM) which is enriched with up to 4 wt.% carbon over the local mean. Our understanding of how carbon phases, including graphite, are altered as a result of these processes is limited.

McGlaun, M. L.↗
NASA NTRS2019

Fatigue Behavior of Coated Titanium Alloys

Advanced multi-component TiAlCrTaSiN-based multilayered coatings were developed and processed onto Ti-6Al-4V (Ti-6-4) and GammaMet PX (GMPX or TiAl) alloys by a magnetron-enhanced physical vapor deposition technique. The coatings have recently been in development with the goal of improving advanced titanium alloy, turbine engine component durability and oxidation-erosion resistance. In this work, the performance of the multi-component coating system (including the outer layer nitride based coating and inner TiAlCrSi oxidation barrier bond coat) was studied, and the coating's influence on the fatigue behavior of the titanium-based alloys were compared. Although the multi-component coating was initially optimized to improve the coating's high temperature oxidation and erosion resistance, the lower ductility nitride-based coating needs further work on microstructure or composition to improve its mechanical stress resistance and strain tolerance for use in cyclically loaded, turbine blade operating conditions. This is particularly true for GMPX and other γ-TiAl alloys because these intermetallic titanium alloys have limited ductility and are very sensitive to defects such as coating cracks. However, the inner layer TiAlCrSi-coating had better adhesion and ductility, and may be a suitable choice to improve the oxidation resistance of titanium alloys.

Oxidation Barrier Coatings↗
NASA NTRSDate not supplied

Nanomechanical Characterization of Additive Manufactured GRCop-42 Alloy Developed by Directed Energy Deposition Methods

Propulsion applications such as combustion chambers and nozzles for liquid rocket engines require the use of unique materials with superior mechanical and thermal properties. GRCop-42(Cu-4 wt.% Cr-2 wt.% Nb) is one such candidate material developed by NASA and is now being manufactured using additive manufacturing (AM) techniques. AM offers a unique processing environment different from traditional metal fabrication processes. This study characterized the mechanical and structural properties of as-deposited and heat-treated GRCop-42manufactured with Blown Powder Directed Energy Deposition (BPD)based on preliminary testing. The materials were characterized using several techniques including surface profiling by laser microscopy, mechanical stiffness by instrumented indentation, crystal structure, residual stress measurements using XRD and residual XRD, and chemical composition and microstructural evaluation using SEM with EBSD. The results of the study are compared using the BPD techniques for samples in the as-deposited and heat-treated conditions.

GRCop-42↗
NASA NTRS2021

Microstructural Observations on Advanced Multilayer Protective Coatings for Nuclear Thermal Propulsion

The present research discusses the microstructures of a new multilayered coating concept proposed to solve the “midrange corrosion” problem observed in NbC and ZrC-coated graphite (Gr) nuclear fuels during the NERVA/Rover programs. The concept envisions designing a compliant multilayered coatings architecture to accommodate the thermal expansion mismatch strains between the ZrC outer coating and the Gr/(U,Zr)C fuel matrix along with using a diffusion barrier to minimize carbon and hydrogen diffusion, respectively. Multilayered coatings were deposited on Gr substrates by chemical vapor deposition (CVD). Graphite disks, as well as the inner channels of a 19-hole hexagonal rod 152.4 mm long, were coated by CVD in proof-of concept process demonstration studies. Coated disk specimens were thermally-cycled between ambient and 1900 K for as long as 20 cycles with a 1h hold time at the temperature using both stepped and non-interrupted thermal cycling methods. Detailed cross-sectional microstructural analyses were conducted on the thermally-cycled coated disks. These results showed that the coatings were intact after 20 thermal cycles at 1900 K although the bonding of the overlay Mo coating appeared to be influenced by the oxygen content in the layer. Seven transverse sections were cut along the length from the coated hexagonal rod for microstructural studies. All the nineteen channels were observed to have been coated in all the sections. This paper reports the results of detailed microstructural analyses, compositional line scans and x-ray dot maps conducted on three coated channels from a mid-section of the rod. The measured values of carbon (C) were observed to be exceptionally high in all the specimens, which were attributed to hydrocarbon contamination of the specimen surfaces. It was observed that the oxygen (O) levels were high in all the coating layers even after approximately correcting for the observed high values of C. The source of O contamination of the CVD reactor system was attributed to either the presence of oxychloride impurities in the precursor salts or a pin hole leak in the coating system. It is concluded that high values of O result in poor bonding of the Mo overlay to the ZrC layer. These trials have established that it is possible to deposit multilayered coatings by CVD in 19-hole hexagonal rods 152.4 mm long similar to the NERVA fuel rod design although the quality of the coatings and the uniformity of their thicknesses require further improvement.

Nuclear thermal propulsion, multilayer coatings, m↗
NASA NTRS2024

Grossite-Bearing Refractory Inclusions from Reduced CV Chondrites: Mineralogical and Oxygen Isotopic Constraints on the Parent Body Alteration History

We report the results of coordinated mineralogical, microstructural, and oxygen isotopic analyses of grossite-bearing refractory inclusions from reduced CV (Vigarano type) chondrites to obtain a more complete picture of secondary parent body alteration processes and conditions. Grossite (CaAl 4 O 7 ) occurs in cores of nodules in fine-grained Ca,Al-rich inclusions (CAIs) that likely represent aggregates of nebular condensates. In many occurrences, grossite has been partially replaced by hercynite [(Fe,Mg,Zn)Al 2 O 4 ], which displays complex microstructures and compositions, and magnetite nanoparticles. The alteration of grossite was a crystallographically-controlled, fluid-driven process that occurred via partial dissolution of grossite and subsequent precipitation of hercynite and magnetite during short-lived and low-temperature metasomatic alteration on the CV chondrite parent body. The constituent phases of grossite-bearing CAIs show heterogeneous oxygen isotopic compositions, with grossite and perovskite displaying systematically 16O-depleted compositions (Δ 17 O = −12‰ to −1‰) relative to uniformly 16 O-rich hibonite and spinel (Δ 17 O = −25‰ to −21‰). Melilite is variably 16 O-depleted (Δ 17 O = −25‰ to −2‰). The observed oxygen isotopic distribution is interpreted as a result of mineralogically controlled oxygen isotopic exchange with an 16 O-poor fluid on the CV chondrite parent body. Collectively, the presence of limited fluids played an important role in preferential alteration of grossite to hercynite and magnetite and various degrees of 16O depletion in grossite, perovskite, and melilite during thermal metamorphism. We conclude that, among refractory phases in the inclusions, grossite was the most susceptible to metasomatic reactions with Fe-rich fluids and the second most susceptible, after perovskite, to oxygen isotopic exchange with an 16 O-poor fluid during the thermal history of the CV chondrite parent asteroid.

Calcium-aluminum-rich inclusions↗
NASA NTRS2024

A Coordinated Mineralogical and Oxygen Isotopic Study of Refractory Ca-Al-Rich Inclusions

Refractory Ca-Al-rich inclusions (CAIs) in primitive chondritic meteorites represent the oldest solids that formed in our Solar System. Coordinated analyses using multiple ion and electron-beam techniques are vital to fully understand complex nebular and asteroidal processes that CAIs may have experienced in the context of the formation and early evolution of our Solar System. We report the results of a coordinated mineralogical and oxygen isotopic study of unique grossite bearing CAIs from reduced CV (Vigarano type) carbonaceous chondrites using transmission electron microscopy and secondary ion mass spectrometry. Grossite (CaAl 4 O 7 ) occurs in cores of nodules in fine grained CAIs that likely represent aggregates of nebular condensates. In many occurrences, grossite has been partially replaced by hercynite [(Fe,Mg,Zn)Al 2 O 4 ], which displays complex microstructures and compositions, and magnetite nanoparticles. This grossite alteration likely occurred via crystallographically controlled metasomatic reactions with Zn, Fe-rich aqueous fluids on the parent asteroid. The constituent phases of grossite bearing CAIs show heterogeneous oxygen isotopic compositions . That is, grossite and perovskite are 16 O poor and melilite is variably 16 O-depleted in contrast to solar like, 16 O-rich hibonite and spinel. The observed oxygen isotopic distribution is interpreted as a result of mineralogically controlled oxygen isotopic exchange with an 16 O-poor fluid on the parent asteroid. Collectively, we conclude that the presence of limited fluids played an important role in preferential alteration of grossite to hercynite and magnetite and various degrees of 16 O depletion in grossite, perovskite, and melilite during thermal metamorphism on the CV chondrite parent asteroid.

Jangmi Han↗
NASA NTRSDate not supplied

Evaluations of Additively Manufactured Superalloy Lattice Blocks

Cast lattice block structures made up of high-temperature superalloys were previously shown to offer advantages in density, efficiency, and tailored properties. However, inconsistencies in casting die-fill, composition, and microstructure were identified as persistent issues with the casting process path. Three candidate lattice designs were screened through additive manufacturing of a superalloy. Laser powder bed fusion was applied with superalloy powder to produce lattice blocks for three-dimensional octahedral unit-cell designs of three heights. Properties of the structural designs versus additively manufactured lattices were compared, including structural dimensions, densities, defects, grain structures, crystallographic texture, and surface residual stresses. Findings indicate this concept could produce useful superalloy structures for high-temperature-capable static components requiring tailored density, and thermophysical and mechanical properties.

superalloy↗
NASA NTRS1994

Scaling effects of defects in fiber-reinforced composites

Material defects may be introduced willingly or unwillingly during material manufacturing and structural component fabrication stages. Their presence in the material plays a dominant role in determining the material's strength and the associate failure mechanisms. In the sense that the size and the number of defects may increase with the volume of the material, the effect of dimensional scaling may manifest itself in the dependence of material strength on volume. Or, alternatively, there may exist a scaling effect of material defects. In fiber-reinforced composites, manufacturing or fabrication defects may come in several forms: matrix voids, matrix microcracks, fiber misalignment, broken fibers, or interface disbonds, just to mention a few. These are interacting and competing defects in the sense that one type of defect may become dominant under one stress condition and another type of defect may become dominant under a different stress condition. This happens because the fiber reinforcement network, together with the distribution of defects, constitutes the prime microstructure of the composite, and there exist continued interactions between the evolving microstructure and the distribution of defects. In the process, the scaling effects of defects are complicated by this interaction. In this presentation, the scaling effects of defects in fiber-reinforced composites will be briefly discussed with the introduction of the concept of effective defects. It is then shown with the aid of some actual experimental and analysis results that the scaling effects are very much present, but they are regulated by the characteristic dimension of the composite microstructure due to the aforementioned microstructure-defect interaction effect.

Wang, A. S. D.↗
NASA NTRS2010

Role of Microstructure on the Performance of UHTCs

We have investigated a number of methods to control microstructure. We have routes to form: a) in situ "composites" b) Very fine microstructures. Arcjet testing and other characterization of monolithic materials. Control oxidation through microstructure and composition. Beginning to incorporate these materials as matrices for composites. Modeling effort to facilitate material design and characterization.

Johnson, Sylvia M.↗
NASA NTRS1991

Advanced composite applications for sub-micron biologically derived microstructures

A major thrust of advanced material development is in the area of self-assembled ultra-fine particulate based composites (micro-composites). The application of biologically derived, self-assembled microstructures to form advanced composite materials is discussed. Hollow 0.5 micron diameter cylindrical shaped microcylinders self-assemble from diacetylenic lipids. These microstructures have a multiplicity of potential applications in the material sciences. Exploratory development is proceeding in application areas such as controlled release for drug delivery, wound repair, and biofouling as well as composites for electronic and magnetic applications, and high power microwave cathodes.

Schnur, J. M.↗
NASA NTRS2001

Mechanical Properties and Microstructural Characterization of Particulate Reinforced Diboride Composites for High Temperature Leading Edge Applications

Previous work on refractory diboride composites has shown that these systems have the potential for use in high temperature leading edge applications for reusable reentry vehicles. Experiments in reentry environments have shown that these materials have multiple use temperatures greater than 1900 C. The work to be discussed focuses on three compositions: HfB2/SiC, ZrB2/SiC, and ZrB2/C/SiC. These composites have been hot pressed and their mechanical properties measured at room and elevated temperatures. Extensive microstructural characterization has been conducted on polished cross sections and the fracture surfaces have been examined to determine their failure origins.

Ellerby, Donald T.↗
NASA NTRS2001

Mechanical Properties and Microstructural Characterization of Particulate Reinforced Diboride Composites for High Temperature Leading Edge Applications

Previous work on refractory diboride composites has shown that these systems have the potential for use in high temperature leading edge applications for reusable reentry vehicles. Experiments in reentry environments have shown that these materials have multiple use temperatures greater than 1900 C. The work to be discussed focuses on three compositions: HfB2/SiC, ZrB2/SiC, and ZrB2/C/SiC. These composites have been hot pressed and their mechanical properties measured at room and elevated temperatures. Extensive microstructural characterization has been conducted on polished cross sections and the fracture surfaces have been examined to determine their failure origins.

Ellerby, Donald T.↗
NASA NTRS1985

The influence of cobalt, tantalum, and tungsten on the microstructure of single crystal nickel-base superalloys

The influence of composition on the microstructure of single crystal nickel-base superalloys was investigated. Co was replaced by Ni, and Ta was replaced by either Ni or W, according to a matrix of compositions based on MAR-M247. Substitution of Ni for Co caused an increase in gamma-prime solvus temperature, an increase in gamma-gamma-prime lattice mismatch, and the precipitation of W-rich phases in the alloys with high refractory metal levels. Substitution of Ni for Ta caused large decreases in gamma-prime solvus temperature, gamma-prime volume fraction, and gamma-gamma-prime lattice mismatch, whereas substitution of W for Ta resulted in smaller decreases in these features. For the alloys with gamma-prime particles that remained coherent, substitution of Ni for Co caused an increase in gamma-prime coarsening rate. The two alloys with the largest magnitude of lattice mismatch possessed gamma-prime particles which lost coherency during unstressed aging and exhibited anomalously low coarsening rates. Creep exposure at 1000 C resulted in the formation of gamma-prime lamellae oriented perpendicular to the applied stress axis in all alloys.

Nathal, M. V.↗
NASA NTRS1990

The influence of microstructure on the tensile behavior of an aluminum metal matrix composite

The relationship between tensile properties and microstructure of a powder metallurgy aluminum alloy, 2009 was examined. The alloy was investigated both unreinforced and reinforced with 15 v/o SiC whiskers or 15 v/o SiC particulate to form a discontinuous metal matrix composite (MMC). The materials were investigated in the as-fabricated condition and in three different hot-rolled sheet thicknesses of 6.35, 3.18, and 1.8 mm. Image analysis was used to characterize the morphology of the reinforcements and their distributions within the matrix alloy. Fractographic examinations revealed that failure was associated with the presence of microstructural inhomogeneities which were related to both the matrix alloy and to the reinforcement. The results from these observations together with the matrix tensile data were used to predict the strengths and moduli of the MMC's using relatively simple models. The whisker MMC could be modeled as a short fiber composite and an attempt was made to model the particulate MMC as a dispersion/dislocation hardened alloy.

Birt, Michael J.↗
NASA NTRS1985

Non-equilibrium phase distribution in an Al-SiC composite

The phase distribution in an Al-SiC composite has been investigated using high resolution analytical electron microscopy. Particular attention was focused on Al-SiC interfaces, matrix boundaries and impurity phases which would impede the easy glide of dislocations. Small crystallites of MgO were distributed singly and in clusters along Al-SiC interfaces in all specimens. Interfacial segregation and precipitation involving alloy species apparently affected precipitation in the matrix, where the distribution of phases was found to be very heterogeneous. Matrix phases also included unusually large constituent particles and dispersoids, a consequence of the composite processing methods. The relationship between the observed microstructure and the composite mechanical behavior reported by others is discussed. The heterogeneous distribution of matrix phases is expected to result in a wide variaiton in local yield stress and local work-hardening rate within the composite.

Nutt, S. R.↗
NASA NTRS1996

Microstructural Preparation and Examination of Polymer-Matrix Composites

Adapting procedures widely used in the metallographic characterization of metals and alloys, the microstructural preparation and examination of three polymer-matrix composites (PMC's) is described. The materials investigated contained either hollow ceramic filler particles or woven, continuous carbon/graphite fibers. Since the two particulate composites were considered to be isotropic, only one sample orientation was prepared. For the fiber composite, both longitudinal and planar orientations were studied. Once prepared, the samples were examined using reflected light microscopy. A number of microstructural features were evaluated qualitatively, including porosity and cracks, filler-matrix interfacial bonding, filler particle characteristics (shape, size, size distribution, and loading variation) and fiber characteristics (orientation, packing variation, and discontinuities).

Elban, Wayne L.↗
NASA NTRS2012

Compositional Effects on Nickel-Base Superalloy Single Crystal Microstructures

Fourteen nickel-base superalloy single crystals containing 0 to 5 wt% chromium (Cr), 0 to 11 wt% cobalt (Co), 6 to 12 wt% molybdenum (Mo), 0 to 4 wt% rhenium (Re), and fixed amounts of aluminum (Al) and tantalum (Ta) were examined to determine the effect of bulk composition on basic microstructural parameters, including gamma' solvus, gamma' volume fraction, volume fraction of topologically close-packed (TCP) phases, phase chemistries, and gamma - gamma'. lattice mismatch. Regression models were developed to describe the influence of bulk alloy composition on the microstructural parameters and were compared to predictions by a commercially available software tool that used computational thermodynamics. Co produced the largest change in gamma' solvus over the wide compositional range used in this study, and Mo produced the largest effect on the gamma lattice parameter and the gamma - gamma' lattice mismatch over its compositional range, although Re had a very potent influence on all microstructural parameters investigated. Changing the Cr, Co, Mo, and Re contents in the bulk alloy had a significant impact on their concentrations in the gamma matrix and, to a smaller extent, in the gamma' phase. The gamma phase chemistries exhibited strong temperature dependencies that were influenced by the gamma and gamma' volume fractions. A computational thermodynamic modeling tool significantly underpredicted gamma' solvus temperatures and grossly overpredicted the amount of TCP phase at 982 C. Furthermore, the predictions by the software tool for the gamma - gamma' lattice mismatch were typically of the wrong sign and magnitude, but predictions could be improved if TCP formation was suspended within the software program. However, the statistical regression models provided excellent estimations of the microstructural parameters based on bulk alloy composition, thereby demonstrating their usefulness.

MacKay, Rebecca A.↗
NASA NTRS1991

Continuous fiber-reinforced titanium aluminide composites

An account is given of the fabrication techniques, microstructural characteristics, and mechanical behavior of a lightweight, high service temperature SiC-reinforced alpha-2 Ti-14Al-21Nb intermetallic-matrix composite. Fabrication techniques under investigation to improve the low-temperature ductility and environmental resistance of this material system, while reducing manufacturing costs to competitive levels, encompass powder-cloth processing, foil-fiber-foil processing, and thermal-spray processing. Attention is given to composite microstructure problems associated with fiber distribution and fiber-matrix interfaces, as well as with mismatches of thermal-expansion coefficient; major improvements are noted to be required in tensile properties, thermal cycling effects, mechanical damage, creep, and environmental effects.

Mackay, R. A.↗