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At least 91 records · Page 5

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NASA NTRS1980

An ab initio calculation of the zero-field splitting parameters of the 3A-double prime state of formaldehyde

The spin dipole-dipole and spin-orbit contributions to the zero-field splitting of the 3A-double prime state of formaldehyde have been evaluated at the excited state experimental geometry. Ab initio CI wave functions were generated from a Dunning double zeta plus polarization bases set using 3A-double prime rhf orbitals. Twelve states of each symmetry were used to evaluate the second-order spin-orbit effect. The resulting values of D and E were 0.19 and 0.03 kayser with the principal magnetic axes rotated 36 deg from the CO bond. The values of alpha and beta relative to the inertial axes were calculated to be 0.03 and 0.01 kayser compared to the experimental values of 0.05 plus or minus 0.01 and 0.02 plus or minus 0.02 kayser.

Davidson, E. R.↗

NASA NTRS1981

The effect of thermal cycling to 1100 degree C on the alpha (Mo) phase in directionally solidified gamma/gamma prime-alpha alloys

In gamma/gamma prime - alpha eutectic alloys (Ni-Mo-Al), the resistance of the alpha phase to morphological changes during thermal cycling was found to be dependent on the structure formed during directional solidification. Fine, smooth alpha fibers survived up to 1000 five minute cycles to 1100 C with minor microstructural contour changes, while coarser and irregularly shaped alpha fibers tended to spheroidize. A mechanism to explain this phenomenon is proposed. It is suggested that on heating to 1100 C, the alpha phase is likely to undergo morphological changes, until differential thermal expansion creates a stress free interface between the alpha phase and the gamma/gamma prime matrix.

Harf, F. H.↗

NASA NTRS1980

The influence of isothermal annealing on the molybdenum fibers of a directed solid gamma/gamma prime - alpha alloy

The mechanical characteristics of the directed solid eutectic Ni-Al-Mo (gamma/gamma prime)-alpha alloy at high temperatures make it useful in the blades of gas turbines. Structural changes are observed for the alloy under isothermal annealing, particularly the formation of molybdenum plates at the grain boundary and an increase of the gamma phase in relation to the gamma prime phase. Molybdenum lamellas appear at the grain boundary above 1100 C which can have a negative influence on the mechanical properties. The change in the structure near the grain boundary with the increase in temperature is illustrated through a sequence of pictures.

Whittenberger, J. D.↗

NASA NTRS1980

Fatigue properties of MA 6000E, a gamma-prime strengthened ODS alloy

MA 6000E is a corrosion resistant, gamma-prime strengthened ODS alloy under development for advanced turbine blade applications. The high temperature, 1093 C, rupture strength is superior to conventional nickel-base alloys. This paper addresses the fatigue behavior of the alloy. Excellent properties are exhibited in low and high cycle fatigue and also thermal fatigue. This is attributed to a unique combination of microstructural features, i.e., a fine distribution of dispersed oxides and other nonmetallics, and the highly elongated grain structure which advantageously modify the deformation characteristics and crack initiation and propagation modes from that characteristic of conventional gamma-prime hardened superalloys.

Kim, Y. G.↗

NASA NTRS1981

Pole and prime meridian expressions for Phobos and Deimos

Simple trigonometric expressions are derived for the right ascensions and declinations of the spin axes of Phobos and Deimos as well as for their prime meridians. Simple expressions are possible since both satellites are in synchronous rotation about Mars and since the orbits of both satellites are accurately modeled as precessing ellipses. Spin axis expressions reflect the offset and precession of the orbit pole about the Laplacian pole. Prime meridian expressions include orbital mean motion, long-period solar perturbations, secular acceleration, and short-period, tidally induced forced libration. These simple expressions agree with rigorous expressions to + or - 0.2 deg.

Duxbury, T. C.↗

NASA NTRS1982

Effects of heating rate on density, microstructure, and strength of Si3N4-6 wt.% Y2O3 and a beta-prime sialon

The effects of the heating rate during sintering/firing on the final density, microstructure, and strength of Si3N4-6 wt% Y2O3 and beta-prime sialon, sintered for four hours at 1750 C, are examined. In Si3N4-6 wt% Y2O3 increasing the heating rate from 7 C/min to 25 C/min to 90 C/min results in a corresponding decrease in the final density from 3.01 g/cu cm to 2.92 g/cu cm to 2.76 g/cu cm. In the beta-prime sialon composition all three heating rates produce an equivalent final density of 3.13 g/cu cm. All heating rates in both compositions produce nonhomogeneous microstructures. The room-temperature strength of Si3N4-6 wt% Y2O3 increases from 372 to 510 MPa with increased density, while the corresponding strengths for the betaprime sialon at equivalent densities are 345 to 445 MPa.

Campbell, S. S.↗

NASA NTRS1984

A simple application of the Bailey-Orowan creep model to Fe-39.8 at. pct Al and gamma/gamma prime - alpha

The results of a study to determine the recovery rates and work-hardening coefficients for creep from constant cross-head speed compressive tests are presented. Stressing and straining rates are computed from measured time-load curves obtained from compression testing between 1200 and 1400 K of several B2 crystal structure Fe-39.8 Al intermetallic materials and the directionally solidified eutectic alloy gamma/gamma prime alpha. These quantities are then fitted to the universal form of the Bailey-Orowan equation for creep. The recovery rates were found to be functions of nominal strain rate, stress, and temperature, while the hardening coefficients were dependent only on temperature. While the work-hardening coefficient for gamma/gamma prime - alpha was about 0.05 of the elastic modulus, the work-hardening coefficients for Fe-39.8 at. pct Al were less than 0.002 of the modulus.

Whittenberger, J. D.↗

NASA NTRS1985

Transition probabilities of the B-prime 1Sigma(u)(+) to X 1Sigma(g)(+) system of molecular hydrogen

From published potential energy curves and transition dipole moments, there are obtained by numerical integration of the equations of nuclear motion the vibrational eigenfunctions of the X 1Sigma(g)(+) and B-prime 1Sigma(u)(+) states of H2. The probabilities of radiative transitions from the discrete vibrational levels of the excited B-prime 1Sigma(u)(+) electronic state of H2 to the discrete and continuum vibrational levels of the ground X 1Sigma(g)(+) electronic state are calculated. The Franck-Condon factors are also presented.

Kwok, T. L.↗

NASA NTRS1985

Fluorescence branching ratios in nitric oxide - Emission from the A2 Sigma(+) (v prime = 1)

The most complete set of branching ratios from the v-prime = 1 level of the A-state in NO measured to data is reported. The assumption that virtually all of the emitted radiation has been sampled is used to extract Franck-Condon factors from the v-prime = 1 manifold. A comparison with previously published results indicates that, except for the (1,0) band, there is reasonably good agreement.

Mcgee, T. J.↗

NASA NTRS1986

Effect of multiple strain-anneal cycles on the 1000 C creep behaviour of gamma/gamma prime-alpha

Various multiple strain-anneal cycles (1000 C) were imposed on specimens of the directionally solidified eutectic (DSE) alloy gamma/gamma prime-alpha to identify thermomechanical processing methods (TMP) which would improve the creep behavior. Specimens of the Ni-32.3Mo-6.3Al wt pct alloy were grown with a modified Bridgeman technique. Some of the cylindrical specimens were alternately heat-treated at 900 C, then strained, or heat-treated only, while other specimens were annealed at 900 C after swaging and then worked at ambient temperature. The specimens were all examined microstructurally using transmission electron microscopy, some before and after being exposed to constant-load compression tests at 1000 C. The creep strain increased for all TMP specimens for strain rates of at least 2 millionths per sec. Strain rates of about 2 ten millionths per sec were only improved with strain annealing with 13 percent work at ambient temperature. A slight improvement, compared to as-grown materials, was observed in the 1000 C creep behavior of materials annealed at 900 C. Strain-annealing was found to introduce three-dimensional dislocation networks into the gamma-prime matrix.

Whittenberger, J. D.↗

NASA NTRS1987

Plume formation in the D-double prime-layer and the roughness of the core-mantle boundary

A series of very-high-resolution finite element calculations of plume formation in the D-double prime-layer has been performed for several plausible rheologies and boundary conditions in order to study both the early and later stages of boundary layer development. The results show that plumes are initiated by coalescence of small-scale convective instabilities within the low-viscosity region immediately above the core-mantle boundary (CMB). These instabilities support topographic roughness on the CMB having horizontal scales of 20-50 km and provide a source for scattered P-waves seen as precursors to the phases PKIKP and PKKP. The calculated structure of fully developed plumes emerging from the D-double prime-layer consists of 5-50 cm/yr flow confined to 50-100 km thick vertical conduits. With strongly temperature-dependent viscosity, plumes exhibit time-dependent behavior, including upward propagating solitary conduit waves, which may contribute to episodicity in hotspot volcanism.

Olson, Peter↗

NASA NTRS1988

A pipeline design of a fast prime factor DFT on a finite field

A conventional prime factor discrete Fourier transform (DFT) algorithm is used to realize a discrete Fourier-like transform on the finite field, GF(q sub n). This algorithm is developed to compute cyclic convolutions of complex numbers and to decode Reed-Solomon codes. Such a pipeline fast prime factor DFT algorithm over GF(q sub n) is regular, simple, expandable, and naturally suitable for VLSI implementation. An example illustrating the pipeline aspect of a 30-point transform over GF(q sub n) is presented.

Truong, T. K.↗

NASA NTRS1988

Glasses in ancient and young Apollo 16 regolith breccias - Populations and ultra Mg-prime glass

Electron microprobe analysis was used to determine the major element compositions of glass spheres and fragments in seven possibly ancient (about 4 Gyr) and three young (less than 4 Gyr) Apollo 16 regolith breccias. It is found that mare glasses are extremely rare in the ancient regolith breccias, and silica-poor gabbroic anorthosite and high-alumina silica-poor glasses are not present in ancient breccias. Ultra Mg-prime glasses are unique to the Apollo 16 regolith breccias and are most common in the ancient breccias. It is suggested that the presence of ultra Mg-prime glasses could indicate the presence of previously unsampled crustal or volcanic rock types on the moon.

Wentworth, Susan J.↗

NASA NTRS1989

High temperature creep behavior of single crystal gamma prime and gamma alloys

The creep behavior of single crystals of gamma-prime and gamma alloys were investigated and compared to the response of two-phase superalloys tested previously. High temperature deformation in the gamma alloys was characteristic of a climb-controlled mechanism, whereas the gamma-prime based materials exhibited glide-controlled creep behavior. The superalloys were much more creep resistant than their constituent phases, which indicates the importance of the gamma/gamma-prime interface as a barrier for dislocation motion during creep.

Nathal, M. V.↗

NASA NTRS1990

Template-directed oligomerization of 5-prime-deoxy 5-nucleosideacetic acid derivatives

5-prime-deoxy 5-nucleosideacetic acids H-II are isostructural analogs of nucleotides with a carboxylate group in the place of the 5-prime-phosphate group. In this work, their oligomerization in aqueous solution was studied using a water-soluble carbodiimide as the condensing agent in the presence or absence of an appropriate polynucleotide template. Condensation of adenylic acid analogs IIa, IIIa, and Va in the presence of polyuridylic acid were found to be the most efficient reactions. Cyclization of the activated monomers to lactones and the insolubility of the oligomers in aqueous solution were found to be obstacles to the efficient formation of long oligomers.

Harada, Kazuo↗

NASA NTRS1991

The effect of porosity and gamma-gamma-prime eutectic content on the fatigue behavior of hydrogen charged PWA 1480

The study addresses the effect of systematically varying gamma-gamma-prime eutectic content and porosity level on the fatigue life of a hydrogen-charged single crystal PWA 1480 superalloy. Four microstructural variants are produced, and differences in gamma-gamma-prime eutectic morphology among the four processing variants are analyzed. Single valued tensile test data indicate that the tensile and yield strength of the PWA 1480 are degraded by hydrogen charging, with the exception of the material given a eutectic solution treatment. It is shown that the reduction of the fatigue life can be minimized by a duplex thermomechanical treatment consisting of a eutectic solution followed by hot isostatic pressing.

Gayda, J.↗

NASA NTRS2007

Assessing the Effects of Momentary Priming on Memory Retention During an Interference Task

A memory aid, that used brief (33ms) presentations of previously learned information (target words), was assessed on its ability to reinforce memory for target words while the subject was performing an interference task. The interference task required subjects to learn new words and thus interfered with their memory of the target words. The brief presentation (momentary memory priming) was hypothesized to refresh the subjects memory of the target words. 143 subjects, in a within subject design, were given a 33ms presentation of the target memory words during the interference task in a treatment condition and a blank 33ms presentation in the control condition. The primary dependent measure, memory loss over the interference trial, was not significantly different between the two conditions. The memory prime did not appear to hinder the subjects performance on the interference task. This paper describes the experiment and the results along with suggestions for future research.

Schutte, Paul C.↗

NASA NTRS2008

Cassini Attitude Control Fault Protection Design: Launch to End of Prime Mission Performance

The Cassini Attitude and Articulation Control Subsystem (AACS) Fault Protection (FP) has been successfully supporting operations for over 10 years from launch through the end of the prime mission. Cassini's AACS FP is complex, containing hundreds of error monitors and thousands of tunable parameters. Since launch there have been environmental, hardware, personnel and mission event driven changes which have required AACS FP to adapt and be robust to a variety of scenarios. This paper will discuss the process of monitoring, maintaining and updating the AACS FP during Cassini's lengthy prime mission as well as provide some insight into lessons learned during tour operations.

Meakin, Peter C.↗