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At least 721 records · Page 40

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NASA NTRS1978

Crystallization of pyroxenes in lunar KREEP basalt 15386 and meteoritic basalts

Single crystal X-ray diffraction studies and electron microprobe analyses of pyroxenes from the meteorites Pasamonte, Yamato 74450 and Yamato 74159 and in lunar KREEP basalt 15386 show similarities of pyroxene crystallization history. They each began crystallization with magnesian pyroxene (pigeonite in the meteorites, orthopyroxene in 15386) followed by strong zonation toward iron-rich and calcium-rich compositions. These data appear to be understandable in terms of dynamic crystallization experiments on liquids of similar composition to the meteorites. In contrast, pyroxene compositions in the meteorites Juvinas, Sioux County and Nuevo Laredo lie along an apparent tie line between calcium-poor and calcium-rich end members with no iron-enrichment trend. In Sioux County and Juvinas, the primary pyroxene is calcium-poor, with exsolved augite. In Nuevo Laredo, pigeonite and augite occur as separate grains with exsolution of augite in the pigeonite. The development of prominent exsolution lamellae is consistent with a long thermal annealing after the primary crystallization event, which may also explain the absence of iron/magnesium zonation in the pigeonites.

Takeda, H.↗

NASA NTRS1980

The effect of zirconium on the isothermal oxidation of nominal Ni-14Cr-24Al alloys

The isothermal oxidation of Ni-14Cr-24Al-xZr-type alloys was performed in still air at 1100, 1150, and 1200 C for times up to 200 hr. The zirconium content of the alloys varied from 0-0.63 atom percent (a/o). The oxidized surfaces were studied by optical microscopy, X-ray diffraction, and scanning electron microscopy. The base alloy was an alumina former with the zirconium-containing alloys also developing some ZrO2. The addition of zirconium above 0.066 a/o increased the rate of weight gain relative to the base alloy. Due to oxide penetratio, the weight gain increased with Zr content; however, the scale thickness did not increase. The Zr did increase the adherence of the oxide, particularly at 1200 C. The delta W/A vs. time data fit the parabolic model of oxidation. The specific diffusion mechanism operative could not be identified by analysis of the calculated activation energies. Measurements of the Al2O3 scale lattice constants yielded the same values for all alloys.

Kahn, A. S.↗

NASA NTRS1983

Friction and wear of some ferrous-base metallic glasses

Sliding friction experiments, X-ray photoelectron spectroscopy (XPS) analysis, and electron microscopy and diffraction studies were conducted with ferrous base metallic glasses (amorphous alloys) in contact with aluminum oxide at temperatures to 750 C in a vacuum. Sliding friction experiments were also conducted in argon and air atmospheres. The results of the investigation indicate that the coefficient of friction increases with increasing temperature to 350 C in vacuum. The increase in friction is due to an increase in adhesion resulting from surface segregation of boric oxide and/or silicon oxide to the surface of the foil. Above 500 C the coefficient of friction decreased rapidly. The decrease correlates with the segregation of boron nitride to the surface. Contaminants can come from the bulk of the material to the surface upon heating and impart boric oxide and/or silicon oxide at 350 C and boron nitride above 500 C. The segregation of contaminants is responsible for the friction behavior. The amorphous alloys have superior wear resistance to crystalline 304 stainless steel. The relative concentrations of the various constituents at the surfaces of the amorphous alloys are very different from the nominal bulk compositions.

Miyoshi, K.↗

NASA NTRS1983

Sliding induced crystallization of metallic glass

Sliding friction and wear experiments, electron microscopy, and diffraction studies were conducted with an Fe67Co18B14Si1 ferrous-base metallic glass in sliding contact with aluminum oxide at room temperature in air. The results indicate that the amorphous alloy can be crystallized during the sliding process. Crystallization of the wear surface causes high friction. Plastic flow occurred on the amorphous alloy with sliding, and the flow film of the alloy transferred to the aluminum oxide surface. Two distinct types of wear debris were observed as a result of sliding: an alloy wear debris, and powdery and whiskery oxide debris. Generation of oxide wear debris particles on an alloy can cause transitions in friction behavior.

Miyoshi, K.↗

NASA NTRS1984

Metallic glass as a temperature sensor during ion plating

The temperature of the interface and/or a superficial layer of a substrate during ion plating was investigated using a metallic glass of the composition Fe67Co18B14Si1 as the substrate and as the temperature sensor. Transmission electron microscopy and diffraction studies determined the microstructure of the ion-plated gold film and the substrate. Results indicate that crystallization occurs not only in the film, but also in the substrate. The grain size of crystals formed during ion plating was 6 to 60 nm in the gold film and 8 to 100 nm in the substrate at a depth of 10 to 15 micrometers from the ion-plated interface. The temperature rise of the substrate during ion plating was approximately 500 C. Discontinuous changes in metallurgical microstructure, and physical, chemical, and mechanical properties during the amorphous to crystalline transition in metallic glasses make metallic glasses extremely useful materials for temperature sensor applications in coating processes.

Miyoshi, K.↗

NASA NTRS1984

Microanalytical study of some cosmic dust discovered in sea-floor sediments in China

The study of cosmic dust can provide useful data in the investigation of the origin of the Earth and the evolution of celestial bodies. Three types of cosmic dust (ferriginous, siliceous, and glassy) were discovered in the seafloor sediments near China. Their chemical composition and microstructure were examined by X-ray diffraction, fractography, and electron microscopy. The major mineral in an iron-containing cosmic dust is magnetite. The silicate spheres contain sundry metals and metal oxides. Glassy microtektites are similar in composition to tektites, and are found in all the major meteorite areas worldwide.

Shijie, Z.↗

NASA NTRS1985

Effects of MAR-M247 substrate (modified) composition on coating oxidation coating/substrate interdiffusion

The effects of gamma+gamma' Mar-M247 substrate composition on gamma+beta Ni-Cr-Al-Zr coating oxidation and coating/substrate interdiffusion were evaluated. These results were also compared to a prior study for a Ni-Cr-Al-Zr coated gamma Ni-Cr-Al substrate with equivalent Al and Cr atomic percentages. Cyclic oxidation behavior at 1130 C was investigated using change in weight curves. Concentration/distance profiles were measured for Al, Cr, Co, W, and Ta. The surface oxides were examined by X-ray diffraction and scanning electron microscopy. The results indicate that variations of Ta and C concentrations in the substrate do not affect oxidation resistance, while additions of grain boundary strengthening elements (Zr, Hf, B) increase oxidation resistance. In addition, the results indicate that oxidation phenomena in gamma+beta/gamma+gamma' Mar-M247 systems have similar characteristics to the l gamma+beta/gamma Ni-Cr-Al system.

Pilsner, B. H.↗

NASA NTRS1985

Effects of wear on structure-sensitive magnetic properties of ceramic ferrite in contact with magnetic tape

Wear experiments and electron microscopy and diffraction studies were conducted to examine the wear and deformed layers in single-crystal Mn-Zn (ceramic) ferrite magnetic head material in contact with magnetic tape and the effects of that contact on magnetic properties. The crystalline state of the single-crystal magnetic head was changed drastically during the sliding process. A nearly amorphous structure was produced on its wear surface. Deformation in the surficial layer of the magnetic head was a critical factor in readback signal loss above 2.5 dB. The signal output level was reduced as applied normal load was increased. Considerable plastic flow occurred on the magnetic tape surface with sliding, and the signal loss due to the tape wear was approximately 1 dB.

Miyoshi, K.↗

NASA NTRS1985

Microstructures in rapidly solidified Ni-Mo alloys

Ni-Mo alloys of compositions ranging from pure Ni to Ni-40 at % Mo were rapidly solidified by Chill Block Melt Spinning in vacuum and were examined by optical metallography, X-ray diffraction and transmission electron microscopy. Rapid solidification resulted in an extension of molybdenum solubility in nickel from 28 to 37.5 at %. A number of different phases and microstructures were seen at different depths (solidification conditions) from the quenched surface of the melt spun ribbons.

Jayaraman, N.↗

NASA NTRS1984

Friction and wear of some ferrous-base metallic glasses

Sliding friction experiments, X-ray photoelectron spectroscopy (XPS) analysis, and electron microscopy and diffraction studies were conducted with ferrous base metallic glasses (amorphous alloys) in contact with aluminium oxide at temperatures to 750 C in a vacuum. Sliding friction experiments were also conducted in argon and air atmospheres. The results of the investigation indicate that the coefficient of friction increases with increasing temperature to 350 C in vacuum. The increase in friction is due to an increase in adhesion resulting from surface segregation of boric oxide and/or silicon oxide to the surface of the foil. Above 500 C the coefficient of friction decreased rapidly. The decrease correlates with the segregation of boron nitride to the surface. Contaminants can come from the bulk of the material to the surface upon heating and impart boric oxide and/or silicon oxide at 350 C and boron nitride above 500 C. The segregation of contaminants is responsible for the friction behavior. The amorphous alloys have superior wear resistance to crystalline 304 stainless steel. The relative concentrations of the various constituents at the surfaces of the amorphous alloys are very different from the nominal bulk compositions.

Miyoshi, K.↗

NASA NTRS1985

Metallic glass as a temperature sensor during ion plating

Miyoshi, K.↗

NASA NTRS1987

Advanced hydrogen electrode for hydrogen-bromide battery

Binary platinum alloys are being developed as hydrogen electrocatalysts for use in a hydrogen bromide battery system. These alloys were varied in terms of alloy component mole ratio and heat treatment temperature. Electrocatalyst evaluation, performed in the absence and presence of bromide ion, includes floating half cell polarization studies, electrochemical surface area measurements, X ray diffraction analysis, scanning electron microscopy analysis and corrosion measurements. Results obtained to date indicate a platinum rich alloy has the best tolerance to bromide ion poisoning.

Kosek, Jack A.↗

NASA NTRS1987

Interactions of amorphous Ta(x)Cu(1-x) (x = 0.93 and 0.80) alloy films with Au overlayers and GaAs substrates

Amorphous Ta(93)Cu(7) and Ta(80)Cu(20) alloy films are prepared by cosputtering of pure Ta and pure Cu targets with a rotating sample holder table. To investigate the possible application of these materials as diffusion barriers for the Au-GaAs system, vacuum annealings are made in the temperature range from 200 to 800 C. Resistivity change, X-ray diffraction, and Auger electron spectroscopy measurements are performed to find the chemical and metallurgical stabilities of these materials in this system. The reaction temperature for Ta(x)Cu(1-x) in contact with GaAs lies between 500 and 700 C. For Au in contact with Ta(x)Cu(1-x) the reaction occurs at about 600 C. Amorphous Ta(93)Cu(7) shows different interdiffusion characteristics with surrounding elements than does Ta(80)Cu(20).

Oh, Jae E.↗

NASA NTRS1986

Microstructures in rapidly solidified Ni-Mo alloys

Ni-Mo alloys of compositions ranging from pure Ni to Ni-40 at. percent Mo were rapidly solidified by Chill Block Melt Spinning in vacuum and were examined by optical metallography, X-ray diffraction and transmission electron microscopy. Rapid solidification resulted in an extension of molybdenum solubility in nickel from 28 to 37.5 at. percent. A number of different phases and microstructures were seen at different depths (solidification conditions) from the quenched surface of the melt spun ribbons.

Jayaraman, N.↗

NASA NTRS1988

Chemical durability of high-temperature superconductor YBa2Cu3O(7-x) in aqueous environments

The stability of YBa2Cu3O(7-x) in water and 100-percent humidity has been investigated at three temperatures, using pH measurements, X-ray diffraction, and scanning electron microscopy. The oxide-ceramic superconductor is highly unstable; it reacts rapidly with water and degrades in moisture. Dissolution of the oxide perovskite in water is highly incongruent. The corrosion products are found to be BaCO3, CuO, O2, etc. Barium hydroxide is first formed and further reacts with atmospheric CO2 to form needle-shaped crystals of BaCO3. For any practical applications, devices made from these materials would have to be protected with an impermeable coating to prevent deterioration from atmosphere.

Bansal, Narottam P.↗

NASA NTRS1988

Microstructural evolution on crystallizing the glassy phase in a 6 weight percent Y2O3-Si3N4 ceramic

X-ray diffraction and analytical electron microscopy have been used to study the crystallization of the grain-boundary glass in a 6 wt pct Y2O3-Si3N4 ceramic. Upon crystallization, high densities of dislocations formed in the Si3N4 grains and remained after 5 h at temperature. However, prolonged holds at the crystallization temperature effectively annealed out the dislocations. Other features present in the microstructure are characterized.

Lee, W. E.↗

NASA NTRS1989

Synthesis and characterization of high-Tc superconductors in the Tl-Ca-Ba-Cu-O system

Both Tl2Ca2Ba2Cu3O10 and TlCa3BaCu3O8.5 are investigated for superconductivity as a function of the sintering temperature, time, atmosphere, and quench rate in an effort to synthesize the high-Tc superconducting phase in the thallium system. The samples are characterized by electrical resistivity measurements, X-ray diffraction, and scanning electron microscopy. Samples of starting composition Tl2Ca2Ba2Cu3O10 fired in air at 860-900 C and rapidly quenched show a Tc of 96-107 K. In contrast, specimens of starting composition TlCa3BaCu3O8.5 when baked at 900 C and slowly cooled in oxygen superconduct at 116 K and above and consist of Tl2Ca2Ba2Cu3O(10+x) as the dominant phase. The results also show that, in contrast to the case of YBa2Cu3O(7-x), doping with a small concentration of fluorine sharpens the resistive transition and produces a large Tc increase in thallium-based superconductors.

Bansal, Narottam P.↗

NASA NTRS1992

Oxidation characteristics of Beta-21S in air in the temperature range 600 to 800 C

The metastable beta-Ti alloy Beta-21S, Ti-15Mo-2.7Nb-3Al-0.2Si (weight percent), has been proposed as a candidate for use in metal matrix composites in future hypersonic vehicles. The present study investigated the oxidation behavior of Beta-21S over the temperature range 600 C to 800 C. Oxidation weight gain was evaluated using thermogravimetric analysis. Oxidized specimens were evaluated using x ray diffraction techniques, scanning electron microscopy, energy dispersive x ray analysis, and electron microprobe analysis to identify oxidation products and evaluate oxidation damage to the alloy.

Wallace, Terryl A.↗