Influence of microgravity on the morphology of the directionally solidified front in an alSi alloy
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I.
E X P E R I M E N T S A N D RESULTS
A binary A1Si-alloy (7.0 wt pct Si) was directionally solidified under different crystallization conditions (Table I). The crystallization parameters were selected in order to obtain a dendritic solidification front. As an additional condition the same parameter values for velocity of solidification front VsF and temperature gradient in front of the solidification front GsF were to be obtained for the/xg experiment and the 1 g reference experiments. Figure 1 illustrates the principles of the directional solidification experiments: the samples were solidified from the bottom to the top in the Gradient Furnace with a Quenching device (GFQ). At a constant distance between heat source Qm and heat sink Qo~t, there was an almost constant thermal situation in the specimen during crystallization at constant velocity VNFand temperature gradient G. To stop the solidification front and to quench the residual liquid, heat source and heat sink change their position with high velocity ve, so that the heat sink was then at the point of the solidification front. At this stage, the efficiency of the heat sink was amplified by spraying water against the specimen. After cleating the high pressure water storage basin, the specimen was further cooled with a combination of argon and helium. The solidification parameters were measured by four fixed thermocouples in the center of the specimen. The crystallization parameters velocity Vsr and the temperature gradient Gse ahead of the solidification front were calculated using the four stationary thermocouples. Representatively for all the A1Si experiments, Figure 2 shows that the change in velocity Vse and gradient Gsr during processing was very small and both the values and the curves agreed very well for the/.tg experiments and the corresponding 1 g reference experiments. The values of Table I were obtained HANS M. TENSI is Professor, Technische Universit/it Mfinchen (TUM), 8 Miinchen2, FederalRepublic of Germany. This paper is based on a presentation made in the symposium "Experimental Methods for MicrogravityMaterials Science Research" presented at the 1988TMS-AIMEAnnual Meeting in Phoenix, Arizona, January 25-29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committeeand the Material ProcessingCommittee. METALLURGICALTRANSACTIONSA
immediately before the solidification front was stopped and the residual melt was quenched. The cooling rate of quenching the residual melt should have been better than 15 K / s in the field TL to Te. This means a quenching time tq of less than 3 seconds. It is known that dendritic structures solidified at different cooling rates show different secondary arm spacings, t1'2'31 Unidirectional solidification involving quenching of the residual melt provided the opportunity to observe different dendrite coarsening within a single specimen with high accuracy. Only one directionally solidified specimen had to be produced for all the different cooling rates and hence for different degrees of dendrite coarsening.
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