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Wu, Y.

Publications and source records attributed to Wu, Y..

25 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Crystal structure and density of helium to 232 kbar

The properties of helium and hydrogen at high pressure are topics of great interest to the understanding of planetary interiors. These materials constitute 95 percent of the entire solar system. A technique was presented for the measurement of X-ray diffraction from single-crystals of low-Z condenses gases in a diamond-anvil cell at high pressure. The first such single-crystal X-ray diffraction measurements on solid hydrogen to 26.5 GPa were presented. The application of this technique to the problem of the crystal structure, equation of state, and phase diagram of solid helium is reported. Crucial for X-ray diffraction studies of these materials is the use of a synchrotron radiation source which provides high brillance, narrow collimation of the incident and diffracted X-ray beams to reduce the background noise, and energy-dispersive diffraction techniques with polychromatic (white) radiation, which provides high detection efficiency.

Mao, H. K.↗
NASA NTRS1987

Dendritic growth of undercooled nickel-tin. I, II

A comparison is made between high speed cinematography and optical temperature measurements of the solidification of an undercooled Ni-25 wt pct Sn alloy. The first part of this study notes that solidification during the recalescence period at all undercoolings studied occurred in the form of a dendritelike front moving across the sample surface, and that the growth velocities observed agree with calculation results for the dendrite growth model of Lipton et al. (1986); it is concluded that the coarse structure observed comprises an array of much finer, solute-controlled dendrites. In the second part, attention is given to the solidification of levitated metal samples within a transparent glass medium for the cases of two undercooled Ni-Sn alloys, one of which is eutectic and another hypoeutectic. The data obtained suggest a solidification model involving dendrites of very fine structure growing into the melt at temperatures near the bulk undercooling temperature.

Wu, Y.↗
NASA NTRS1987

Solidification of undercooled Ni-Sn eutectic alloy under microgravity conditions in the Space Shuttle

The Space Shuttle Columbia carried an Alloy Undercooling Experiment on its STS 61-C mission in January, 1986. The experiment was performed in an electromagnetic levitator. A sample of Ni-32.5 wt pct Sn eutectic was melted and solidified under microgravity conditions in the Space Shuttle. The specimen achieved only a fairly small undercooling, probably less than 30 K. The specimen was examined by optical and scanning electron microscopy. The surface and cross-sectional microstructures were primarily composed of normal lamellar eutectic, but showed several interesting features, including an apparent surface nucleation site, curved dendrites with nonorthogonal secondary arms, dendrite fragments with extremely fine arm spacing, submicron precipitates, and faceted crystals. The results of the space experiment are presented and compared with ground-based results obtained with the same alloy.

Piccone, T. J.↗
NASA NTRS1986

Comparison of structures of gas atomized and of emulsified highly undercooled Ni-Sn alloy droplets

A comparison is made of microstructures of droplets of Ni-Sn alloys rapidly solidified by gas atomization and in a glass emulsifying medium. Cooling rate of the gas atomized particles ranged from 10 to the 3rd to 10 to the 6th K/s depending on droplet diameter (20-230 microns). In the hypoeutectic alloy studied, /Ni-(25 wt pct Sn)/, most particles showed a dendritic structure. These same particles, melted and resolidified in a glass medium using DTA (Differential Thermal Analysis), showed undercoolings up to 280 K: the structures were dendritic at low undercoolings and nondendritic at undercoolings above 220 K. It is concluded that the gas atomized particles exhibited little or no undercooling before nucleation; the solidification time of the undercooled emulsified droplets is substantially less than that of gas atomized droplets, and the undercooling required to achieve nondendritic structure depends on sample size.

Yamamoto, M.↗
NASA NTRS1976

Magnetic effects of shock and their implications for lunar magnetism. II

Synthetic breccias were produced by hypervelocity impact of projectiles on basalt powders in order to study the magnetization acquired during shock lithification of soil. It was found that some of the lithified products record the field at the point where they land, while others carry the original primary remanence. Even those rocks recording the direction of the ambient field did not always faithfully record the intensity of the field. When rocks were exposed to shock, it was found that the magnetic hardness of a rock has a profound effect on its magnetic response. Shock effects are seen at shock levels of less than 10 kbar, which suggests that lunar samples may have been shock-demagnetized.

Cisowski, S. M.↗