Comparison of Structures of Gas Atomized and of Emulsified Highly Undercooled Ni-Sn Alloy Droplets
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COMPARISON OF STRUCTURES OF GAS ATOMIZED AND OF EMULSIFIED HIGHLY UNDERCOOLED Ni-Sn ALLOY DROPLETS MICHIHARU YAMAMOTO*+, YANZHONG WU*, YUH SHIOHARA** AND MERTON C. FLEMINGS* * Dept. of Materials Science and Engineering, MIT, Cambridge, MA02139 ** Materials Processing Center, MIT, Cambridge, MA02139 + now Nippon Mining Co., Ltd., Tokyo, JAPAN ABSTRACT A comparison is made of microstructures of droplets of Ni-Sn alloys rapidly solidified (a) by gas atomization and (b) in a glass emulsifying medium. Cooling rate of the gas atomized particles ranged from 10i to 10K/s depending on droplet diameter (20-230m). In the hypoeutectic alloy studied (Ni-25wt%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 280K: the structures were dendritic at low undercoolings and non-dendritic at undercoolings above about 220K. It is concluded that (1) the gas atomized particles exhibited little or no undercooling before nucleation, (2) the solidification time of the undercooled emulsified droplets is substantially less than that of gas atomized droplets, and (3) the undercooling required to achieve non-dendritic structure depends on sample size. INTRODUCTION The ultrasonic gas atomization technique is a rapid solidification processing (RSP) method of considerable current interest. It is of interest to understand effects on microstructures of undercooling before nucleation. To understand the effects of high undercooling on the solidification process, it is necessary to precisely measure undercoolings and to gain high undercooling. It is, however, almost impossible to measure the exact undercooling in the gas atomization process because of the very rapid quenching process. Therefore the microstructures of the droplets rapidly solidified by gas atomization were compared with those highly undercooled in a glass emulsifying medium whose undercoolings were directly measured by DTA. EXPERIMENTAL PROCEDURE The Ni-25wt%Sn alloy droplets were produced by the ultrasonic gas atomization technique in an argon atmosphere. The droplets obtained (20-230m) were sieved to narrow size ranges and the weight for each size range was measured. Then microstructures of as gas atomized particles were observed by SEM (Scanning Electron Microscopy). In DTA measurements, as gas atomized samples with a mixture of Pyrex* and Vycor* glass were heated to 1620K at a rate of 10K/min and cooled down at the same rate in the DTA furnace, filled with argon gas. The microstructures of the samples solidified in the glass emulsifying medium were examined. RESULTS OF GAS ATOMIZATION The gas atomized particles were almost spherical, and about 40 wt% of the particles were less than 100 microns diameter. The cross-sectioned microstructures in Figure 1 show fine dendrite structures and primary Ni-rich * Trademark of Corning Glass Works,
New York
Mat. Res. Soc. Symp. Proc. Vol. 58. 1986 Materials Research Society
412
aphase surrounded by B phase. Fin
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