Engineering topics

Shiohara, Y.

Publications and source records attributed to Shiohara, Y..

13 records

NASA NTRS1988

Dendritic growth and structure of undercooled nickel base alloys

The principal objectives of this overall investigation are to: study means for obtaining high undercooling in levitation melted droplets, and study structures produced upon the solidification of these undercooled specimens. Thermal measurements are made of the undercooling, and of the rapid recalescence, to develop an understanding of the solidification mechanism. Comparison of results is made with the modeling studies. Characterization and metallographic work is done to gain an understanding of the relationship between rapid solidification variables and the structures so produced. In ground based work to date, solidification of undercooled Ni-25 wt percent Sn alloy was observed by high-speed cinematography and the results compared with optical temperature measurements. Also in ground based work, high-speed optical temperature measurements were made of the solidification behavior of levitated metal samples within a transparent glass medium. Two undercooled Ni-Sn alloys were examined. Measurements were carried out on samples at undercoolings up to 330 K. Microstructures of samples produced in ground based work were determined by optical metallography and by SEM, and microsegregation by electron microprobe measurements. A series of flight tests were planned to conduct experiments similar to the ground based experiments. The Space Shuttle Columbia carried an alloy undercooled experiment in the STS 61-C mission in January 1986. A sample of Ni-32.5 wt percent Sn eutectic was melted and solidified under microgravity conditions.

Flemings, M. C.↗

NASA NTRS1988

Dendritic growth of undercooled nickel-tin. III

The paper is concerned with structures and microsegregations in undercooled droplets of Ni-25 wt pct Sn and eutectic Ni-32.5 wt pct Sn alloys. An analysis of experimental results suggests that all samples solidify dendritically and that the final structures are largely the result of ripening. Experimental data on minimum solute composition in the samples produced are bounded by two curves both of which assume equilibrium at all liquid-solid interfaces during recalescence and subsequent cooling. The eutectic alloy samples often show a dendritic structure over the surface in areas where shrinkage is present. This structure appears to be the remnant of an original growth structure in which only limited ripening occurred, due to the rapid removal of interdendritic liquid by shrinkage.

Wu, Y.↗

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 NTRS1985

Undercooling, Remelting, and Coarsening

Solidification of metal alloys with initial high undercoolings occurs with rapid recalescence due to rapid heat release of the latent heat of fusion. The recalescence temperature is usually higher than the solidus and/or the thermodynamic T sub o temperature of the alloy, depending on the initial undercooling and alloy composition. The rapid heating during recalescence alters the initially solidified structures, in ways that must include partial remelting, ripening, coarsening, and decomposition of the rapidly solidified primary metastable or supersaturated phases. New ways to study the solidification and remelting phenomena associated with the rapid heating during recalescence of undercooled alloys were developed. The phenomena are studied by using: (1) emulsions of iron and nickel base alloys in viscous liquid carriers; and (2) rapid heating of metal samples by electric discharge, using a technique similar to the exploding wire process (EWP), except with less power input. Experimental results are combined with theoretical analyses to better understand the solidification of levitated metal droplets with high undercooling, which relates to the solidification process.

Flemings, M. C.↗

NASA NTRS1985

Role of Convection in Grain Refining

A quantitative representation of the heat and fluid flow fields in electromagnetically levitated (positioned) metallic specimens is examined. Experimental studies as well as modelling of those studies on undercooling and solidification of levitated metal droplets are also considered. Thermal measurements will be made of the undercooling and solidification of levitated metal droplets. Thermal measurements will be made of the undercooling and of the details of the ultra-rapid recalescence. This levitation technique combined with quenching will be used to aid in the development of coherent model for the solidification process related to structural studies, including thermodynamics and kinetics for nucleation and metastable solute partition, kinetics for solid/liquid interface velocity with high undercooling, and heat flow consideration.

Szekely, J.↗

NASA NTRS1984

Solidification of highly undercooled Sn-Pb alloy droplets

Experimental work on undercooling and structure of tin-lead droplets emulsified in oil is described. The droplets, predominantly in the size range of 10 to 20 microns, were cooled at rates (just before nucleation) ranging from about 0.1 to 1 million K/s. The higher cooling rates were obtained by a newly developed technique of quenching the emulsified droplets in a cold liquid. Measured undercoolings (at the lower cooling rates) ranged up to about 100 K. Structures obtained depend strongly on undercooling, cooling rate before and after nucleation, and alloy composition. Droplets containing up to 5 wt pct Pb were apparently single phase when undercooled and rapidly quenched. Droplets in the composition range of about 25-90 wt pct Pb solidified dendritically, even at the most rapid quench rates employed, apparently because these alloys undercooled only slightly before nucleation of the primary phase.

Chu, M. G.↗

NASA NTRS1984

Solidification of undercooled metals

The present investigation is concerned with the subject of undercooling (i.e., supercooling) in the case of metal alloys, taking into account the effects of undercooling on microstructure and microsegregation in alloys which solidify in a crystalline manner. Techniques for obtaining a large degree of undercooling are discussed. These techniques make it possible to eliminate heterogeneous nucleants from the melt and to remove container nucleation effects. The nucleation behavior of small metal droplets is considered along with the formation of a dispersion of fine droplets in a suitable inert medium, the mixing of fine metal powders with glass powder, and the levitation melting technique. Attention is given to solidification with rapid interface velocity, aspects of dendritic morphology, and thermal measurements during solidification of undercooled droplets.

Flemings, M. C.↗

NASA NTRS1984

Electromagnetic containerless undercooling facility and experiments for the Shuttle

An electromagnetic furnace is being prepared for flights aboard the Space Shuttle. This apparatus is capable of melting metals and alloys up to 1400 C melting point by induction heating with subsequent solidification of the freely levitated melt without contact with any container. The solidification can be carried out with greatly reduced fields resulting in minimal heating and stirring of the free melt. Sequential specimens can be processed during flight. Several experiments are planned for a series of flights, beginning in 1985 with an undercooling experiment of NiSn alloys. These will be interspersed with detailed studies of fluid flow caused by low and high field levels in order to quantify the corresponding effect upon the solidification process.

Frost, R. T.↗

NASA NTRS1983

Rapid solidification of highly undercooled alloy droplets

Experimental work is described on undercooling and structure of tin-lead droplets emulsified in oil. The droplets, predominantly in the size range of 10-20 microns, were cooled at rates ranging from about 0.04 K/sec to 1,000,000 K/sec. The higher cooling rates were obtained by a newly developed technique of quenching the emulsified droplets in a cold liquid. Measured undercoolings (at the lower cooling rates) ranged up to about 100 C. Structures obtained depended strongly on undercooling, cooling rate before nucleation, droplet size, alloy composition and the rate of heat extraction after nucleation.

Chu, M. G.↗

NASA NTRS1982

Solidification mechanism of highly undercooled metal alloys

Experiments were conducted on metal droplet undercooling, using Sn-25wt%Pb and Ni-34wt%Sn alloys. To achieve the high degree of undercooling, emulsification treatments were employed. Results show the fraction of supersaturated primary phase is a function of the amount of undercooling, as is the fineness of the structures. The solidification behavior of the tin-lead droplets during recalescence was analyzed using three different hypotheses; (1) solid forming throughout recalescence is of the maximum thermodynamically stable composition; (2) partitionless solidification below the T sub o temperature, and solid forming thereafter is of the maximum thermodynamically stable composition; and (3) partitionless solidification below the T sub o temperature with solid forming thereafter that is of the maximum thermodynamically metastable composition that is possible. The T sub o temperature is calculated from the equal molar free energies of the liquid solid using the regular solution approximation.

Shiohara, Y.↗

NASA NTRS1982

Solidification of undercooled liquids

During rapid solidification processing (RSP) the amount of liquid undercooling is an important factor in determining microstructural development by controlling phase selection during nucleation and morphological evolution during crystal growth. While undercooling is an inherent feature of many techniques of RSP, the deepest undercoolings and most controlled studies have been possible in carefully prepared fine droplet samples. From past work and recent advances in studies of nucleation kinetics it has become clear that the initiation of crystallization during RSP is governed usually by heterogeneous sites located at surfaces. With known nucleant sites, it has been possible to identify specific pathways of metastable phase formation and microstructural development in alloys. These advances have allowed for a clearer assessment of the interplay between undercooling, cooling rate and particle size statistics in structure formation. New approaches to the examination of growth processes have been developed to follow the thermal behavior and morphology in small samples in the period of rapid crystallization and recalescence. Based upon the new experimental information from these studies, useful models can be developed for the overall solidification process to include nucleation behavior, thermodynamic constraints, thermal history, growth kinetics, solute redistribution and resulting structures. From the refinement of knowledge concerning the underlying factors that govern RSP a basis is emerging for an effective alloy design and processing strategy.

Perepezko, J. H.↗