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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

Fatigue Resistance of the Grain Size Transition Zone in a Dual Microstructure Superalloy Disk

Mechanical property requirements vary with location in nickel-based superalloy disks. To maximize the associated mechanical properties, heat treatment methods have been developed for producing tailored microstructures. In this study, a specialized heat treatment method was applied to produce varying grain microstructures from the bore to the rim portions of a powder metallurgy processed nickel-based superalloy disk. The bore of the contoured disk consisted of fine grains to maximize strength and fatigue resistance at lower temperatures. The rim microstructure of the disk consisted of coarse grains for maximum resistance to creep and dwell crack growth at high temperatures up to 704 C. However, the fatigue resistance of the grain size transition zone was unclear, and needed to be evaluated. This zone was located as a band in the disk web between the bore and rim. Specimens were extracted parallel and transverse to the transition zone, and multiple fatigue tests were performed at 427 and 704 C. Mean fatigue lives were lower at 427 C than for 704 C. Specimen failures often initiated at relatively large grains, which failed on crystallographic facets. Grain size distributions were characterized in the specimens, and related to the grains initiating failures as well as location within the transition zone. Fatigue life decreased with increasing maximum grain size. Correspondingly, mean fatigue resistance of the transition zone was slightly higher than that of the rim, but lower than that of the bore. The scatter in limited tests of replicates was comparable for all transition zone locations examined.

Gabb, T. P.↗
NASA NTRS1986

The influence of grain size and composition on slow plastic flow in FeAl between 1100 and 1400 K

The slow plastic flow properties of a series of binary B2 FeAl intermetallics at elevated temperatures, and ranging in aluminum content from 39.8 to 48.7 at. pct, were investigated using constant-velocity compression tests at strain rates from 2 x 10 to the -3rd to 2 x 10 to the -7th. Two deformation mechanisms with the same activation energy are found. For the mode with a stress component of 6, the strength increases with decreasing grain size, probably as a result of Hall-Petch behavior, while for the mode with stress component of 3, the strength increases with increasing grain size, probably due to the action of diffusional creep. Creep in the high stress exponent mode can be described in terms of the initial grain size because the large-angle grain structure is replaced by a small-angle grain microstructure of similar grain diameter during deformation.

Whittenberger, J. D.↗
NASA NTRS1985

The effects of surface finish and grain size on the strength of sintered silicon carbide

The effects of surface treatment and microstructure, especially abnormal grain growth, on the strength of sintered SiC were studied. The surfaces of sintered SiC were treated with 400, 800 and 1200 grit diamond wheels. Grain growth was induced by increasing the sintering times at 2050 C. The beta to alpha transformation occurred during the sintering of beta-phase starting materials and was often accompanied by abnormal grain growth. The overall strength distributions were established using Weibull statistics. The strength of the sintered SiC is limited by extrinsic surface flaws in normal-sintered specimens. The finer the surface finish and grain size, the higher the strength. But the strength of abnormal sintering specimens is limited by the abnormally grown large tabular grains. The Weibull modulus increases with decreasing grain size and decreasing grit size for grinding.

You, Y. H.↗
NASA NTRS2012

Spectral Profiler Probe for In Situ Snow Grain Size and Composition Stratigraphy

An ultimate goal of the climate change, snow science, and hydrology communities is to measure snow water equivalent (SWE) from satellite measurements. Seasonal SWE is highly sensitive to climate change and provides fresh water for much of the world population. Snowmelt from mountainous regions represents the dominant water source for 60 million people in the United States and over one billion people globally. Determination of snow grain sizes comprising mountain snowpack is critical for predicting snow meltwater runoff, understanding physical properties and radiation balance, and providing necessary input for interpreting satellite measurements. Both microwave emission and radar backscatter from the snow are dominated by the snow grain size stratigraphy. As a result, retrieval algorithms for measuring snow water equivalents from orbiting satellites is largely hindered by inadequate knowledge of grain size.

Berisford, Daniel F.↗
NASA NTRS1976

Comparative studies of grain size separates of 60009

Five samples from 60009, the lower half of a double drive tube, were analyzed via grain-size methods, with particle types classified and counted in the coarser grain sizes. Studies were undertaken of particle types and distributions by petrographic methods, of magnetic fractions, of the size splits and magnetic splits as analyzed by ferromagnetic resonance (FMR) techniques, of maturity (based on agglutinate content, FMR index Is/FeO, mean size of sub-cm material, magnetic fraction), of possible reworking or mixing in situ, and of depositional history. Maturity indices are in substantial agreement for all of the five samples. Strong positive correlation of percent agglutinates and percent bedrock-derived lithic fragments, combined with negative correlation of those components with percent single crystal plagioclase, argue against in situ reworking of the same soil.

Mckay, D. S.↗
NASA NTRS1993

Lunar soils grain size catalog

This catalog compiles every available grain size distribution for Apollo surface soils, trench samples, cores, and Luna 24 soils. Original laboratory data are tabled, and cumulative weight distribution curves and histograms are plotted. Standard statistical parameters are calculated using the method of moments. Photos and location comments describe the sample environment and geological setting. This catalog can help researchers describe the geotechnical conditions and site variability of the lunar surface essential to the design of a lunar base.

Graf, John C.↗
NASA NTRS1989

1200 to 1400 K slow strain rate compressive behavior of small grain size NiAl/Ni2AlTi alloys and NiAl/Ni2AlTi-TiB2 composites

In order to impart ductility into NiAl-Ni2AlTi alloys, small grain size single (Ni-45Al-5Ti) and two (Ni-40Al-10Ti) phase intermetallics are fabricated by a process which yields fine microstructures in NiAl. The results of a study of elevated temperature compressive properties of two small grain size NiAl-Ni2AlTi alloys are then described. In addition, the behavior of the Ti-modified nickel aluminides with 20 vol pct TiB2 particles of approximately 1 micron in diameter is also investigated, since these compositions have the potential for being the matrix material in high temperature particulate-strengthened composites.

Whittenberger, J. Daniel↗
NASA NTRS1980

Grain size dependence of silicon solar cell parameters

Measurements of the non-uniform diffusion length of the minority carriers near grain boundaries in polycrystalline silicon have been used to develop an analytical model for the calculation of solar cell output as a function of grain size. Experimental results are presented which verify the theoretical analysis. Variation of open circuit voltage and fill factor with grain size is discussed.

Koliwad, K. M.↗
NASA NTRS1975

Noble gas studies on grain size separates of Apollo 15 and 16 deep drill cores

A description is presented of noble gas analyses of grain-size separates of eight Apollo 15 drill-core samples over the depth range 2-80 cm. Noble gas data on grain-size separates of seven Apollo 16 drill-core samples are also reported. On the basis of the obtained results it is suggested that cosmogenic Ne and He in the Apollo 15 core material has been preferentially lost relative to cosmogenic Ar in the Apollo 15 core material in proportion to soil maturity and degree of solar irradiation. A plot of He-4/Ne-20 against Ar-40/Ar-36 demonstrates that material in the lowest 35 cm of the Apollo 16 drill core is distinct from all other analyzed samples in the core, and distinct from nearly all surface soils.

Bogard, D. D.↗
NASA NTRS1994

Grain size of the surface regolith of asteroid 4 Vesta estimated from its reflectance spectrum in comparison with HED meteorites

The grain-size distribution of the regolith of asteroid 4 Vesta has been estimated by comparing its reflectance spectra (0.3-2.6 microns) with those of HED meteorites. The finest grain-size separate (less than 25 micrometers) of a particular howardite has a reflectance spectrum most similar to Vesta's. In order to better simulate Vesta's surface mineralogy, reflectance spectra of those finest HED meteorite powders were linearly combined, and Vesta's spectrum was scaled for the best fit between them. Both the albedo and the shape of reflectance spectrum of Vesta were well reproduced by regional mixtures of the finest (less than 25 micrometers) powders of HED meteorites. The result suggests the heterogeneity of Vesta's surface and provides an estimate of the visible reflectance of Vesta that is close to its Infrared Astronomical Satellite (IRAS) albedo. Thus, this suggests that fine grains can be generated and retained by relatively small bodies (Vesta is approximately 500 km in diameter).

Hiroi, Takahiro↗
NASA NTRS1984

Grain size evolution and fractionation trends in an experimental regolith

The communication of blocky planetary surfaces into fine-grained regoliths was simulated by impacting a fragmental gabbro target 200 times with stainless steel projectiles. It is found that the comminution efficiency of the surfaces changes with time, being highest in the early stages of regolith formation and decreasing gradually. The relationship between mean grain size and cumulative energy is not linear. Individual, fine-grained regolith components can be generated very early from relatively large progenitor fragments without going through intermediate-size fractions. Impact comminution is capable of producing fractionated fines as postulated by Papike et al. (1982). The role of grain-size selective, lateral transport to explain the fractionated nature of lunar regolith fines may have been overestimated in the past.

Horz, F.↗
NASA NTRS1992

Effects of grain size and shape in modeling reflectance spectra of mineral mixtures

An 'isograin model' of reflectance from particulate material is presented. The model treats reflectance as a series of grain interactions, incorporating optical constants of each material and two parameters that model the activity of scattering and absorption in the vertical and the horizontal directions, respectively. Mixing ratios of components in mixture are treated as variables and are calculated using least-squares fit. Grain size and shape have a significant effect on the accuracy of predicting mixing ratios but can be modeled by introducing the concept of effective grain size. The model calculates spectra in good agreement with measured spectra and models reflectance within 1 percent and mixing ratios within 5 wt pct.

Hiroi, T.↗
NASA NTRS1978

Predicted color excess ratios versus interstellar grain size

Various color excess ratios, the ratio (R) of total to selective extinction, and values of the wavelength of maximum interstellar linear polarization (lambda max) are computed and displayed as functions of mean interstellar grain size and absorption coefficients. E(V-K)/E(B-V)-E(u-b)/E(b-y) is shown to be highly sensitive to changes of mean grain size. The shape of the R versus lambda max curve is shown to depend on the amount of absorption initially present in the grains, and on the absorptivity of the material added to the grains. While no simple model of grain growth has been found to fit all the observations, the two stars with the largest values of lambda max (HD 147889 and W67) support the concept of dielectric mantles growing on dielectric grains. It is further shown that more accurate uby and JKL photometry of heavily reddened stars would permit better discrimination between grain growth theories.

Mcmillan, R. S.↗
DOE OSTI2026

Sintering effects on NpO 2 grain size and morphology: The role of precursor Np phase

Neptunium dioxide (NpO 2 ) is a key phase in nuclear material processing as a target material for the production of plutonium-238 ( 238 Pu) and has been historically synthesized via the calcination of a Np oxalate precursor. Alternative synthesis methods for NpO 2 are now more prevalent, necessitating their study and comparison with the more common oxalate route. The purpose of this work was to investigate the microstructural properties of NpO 2 synthesized from a nitrate-based Np precursor phase via the assessment of NpO 2 particle size and morphology as a function of calcination temperature and time. Scanning electron microscopy (SEM) was used to probe the primary grain size and morphology of NpO 2 after calcination at temperatures ranging from 700 to 1100 °C and hold times ranging from 1 to 10 h. Post-image analysis using ImageJ software enabled the quantification of mean particle diameter. This analysis indicated that particle diameter increases with both increasing calcination temperature and hold time. Primary particles were shown to be clumped in irregular patterns into the overall rough, blocky aggregates, but this macroscopic morphology was not affected by calcination time or temperature. Although trends in primary grain size of NpO 2 were consistent with available literature from other Np precursor phases, the macroscopic morphology of the NpO 2 aggregates was quite different than reported for other precursors. Through comparison with historical literature on Np oxalate, this work emphasizes the importance of Np precursor on the physical properties of NpO 2 .

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
NASA NTRS1988

The influence of grain size and composition on 1000 to 1400 K slow plastic flow properties of NiAl

The compressive slow plastic flow behavior of several B2 crystal structure NiAl intermetallics has been studied in air between 1000 and 1400 K. Small grain-sized Ni-48.25 at. pct Al (of about 10 microns) was found to be stronger than the previously studied 17 microns diameter material. While grain refinement improved the strength at all test temperatures, the exact mechanism is not clear. Experiments at lower temperature revealed that composition as well as grain size can be an important factor, since Ni-49.2Al was weaker than Ni-48.25Al. Pronounced yield points were found during slow strain-rate testing at 1000 K; however, continued deformation appears to take place by the same mechanism(s) as found at high temperatures. Small changes in thermomechanical processing (TMP) schedules to fabricate Ni-49.2Al indicated that basic deformation characteristics (stress exponent and activation energy) are not affected; however, the preexponential term could be modified if TMP alters the grain structure.

Whittenberger, J. Daniel↗
NASA NTRS2013

The Importance of Physical Models for Deriving Dust Masses and Grain Size Distributions in Supernova Ejecta. I. Radiatively Heated Dust in the Crab Nebula

Recent far-infrared (IR) observations of supernova remnants (SNRs) have revealed significantly large amounts of newly condensed dust in their ejecta, comparable to the total mass of available refractory elements. The dust masses derived from these observations assume that all the grains of a given species radiate at the same temperature, regardless of the dust heating mechanism or grain radius. In this paper, we derive the dust mass in the ejecta of the Crab Nebula, using a physical model for the heating and radiation from the dust. We adopt a power-law distribution of grain sizes and two different dust compositions (silicates and amorphous carbon), and calculate the heating rate of each dust grain by the radiation from the pulsar wind nebula. We find that the grains attain a continuous range of temperatures, depending on their size and composition. The total mass derived from the best-fit models to the observed IR spectrum is 0.019-0.13 Solar Mass, depending on the assumed grain composition. We find that the power-law size distribution of dust grains is characterized by a power-law index of 3.5-4.0 and a maximum grain size larger than 0.1 micron. The grain sizes and composition are consistent with what is expected for dust grains formed in a Type IIP supernova (SN). Our derived dust mass is at least a factor of two less than the mass reported in previous studies of the Crab Nebula that assumed more simplified two-temperature models. These models also require a larger mass of refractory elements to be locked up in dust than was likely available in the ejecta. The results of this study show that a physical model resulting in a realistic distribution of dust temperatures can constrain the dust properties and affect the derived dust masses. Our study may also have important implications for deriving grain properties and mass estimates in other SNRs and for the ultimate question of whether SNe are major sources of dust in the Galactic interstellar medium and in external galaxies.

Importance↗
NASA NTRS1977

Does application of the Rosiwal Principle to lunar soils require that concentrations of solar-wind-implanted species be grain-size independent

A reconsideration of the application of the Rosiwal Principle to lunar soils indicates a flaw in arguments put forth previously by Criswell (1975). Specifically, by introducing a boundary condition which must exist at the lunar surface, it is shown that concentrations of solar-wind-implanted species showing a dependence on grain size may be able to develop in soils at concentration levels below those required for saturation of grain surfaces. As a result, observed grain-size-dependent concentrations of solar-wind species in lunar soils do not necessarily require the exposure time scales or solar-wind fluxes deduced from the arguments of Criswell.

Becker, R. H.↗
NASA NTRS2003

The Effect of Film Composition on the Texture and Grain Size of CuInS2 Prepared by Spray Pyrolysis

Ternary single-source precursors were used to deposit CuInS2 thin films using chemical spray pyrolysis. We investigated the effect of the film composition on texture, secondary phase formation, and grain size. As-grown films were most often In-rich. They became more (204/220)-oriented as indium concentration increased, and always contained a yet unidentified secondary phase. The (112)-prefened orientation became more pronounced as the film composition became more Cu-rich. The secondary phase was determined to be an In-rich compound based on composition analysis and Raman spectroscopy. In addition, as-grown Cu-rich (112)-oriented films did not exhibit the In-rich compound. Depositing a thin Cu layer prior to the growth of CuInS2 increased the maximum grain size from - 0.5 micron to - 1 micron, and prevented the formation of the In-rich secondary phase.

Jin, Michael H.↗