Effect of Flow on Dimensions of Dendrite in Undercooled Melt

熔体流动影响过冷熔体凝固过程中枝晶的尺寸。研究熔体流动对尺寸的影响具有重要意义，因为尺寸与溶质的微偏析密切相关。本文研究了熔体流动对尺寸的影响。枝晶的尺寸一般是通过热量和溶质的传递来决定的。过冷熔体中的凝固仅受热传递的控制。从实验和理论两方面研究了纯材料凝固过程中熔体流动对枝晶尺寸的影响。然后研究了熔体流动对合金枝晶尺寸的影响。为了引入熔体内的流动，设计并制造了一个圆形截面的循环槽。熔体流动是由加热熔池部分产生的浮力驱动的。流的速度由施加在加热器上的电力来控制。在一定过冷度下，熔体流动使凝固过程中枝晶的生长速度加快。对于低过冷度的凝固，熔体流动对尺寸的影响是显著的。这些在纯材料和合金的凝固过程中都可以观察到。从理论上解释了较冷的熔体到达枝晶尖端造成了枝晶尖端的温度梯度，生长速度必须加快以保持热/溶质守恒。由于熔体流动与枝晶体的相互作用时间较长，使得熔体流动对过冷熔体凝固的影响显著。

Melt flow affects dimensions of dendrite in the solidiflcation in undercooled melt. It is important to study the effect of melt flow on the dimensions, because the dimensions are very much corelated to the microsegregation of solute. The effect of melt flow on the domensions is studied in this work.Dimensions of dendrite are, in general, determined through the transport of heat and solute. The solidification in undercooled melt is controled by the heat transport alone. The effect of melt flow on the dimensions of dendrite is studied in the solidification for pure material, experimentally and theoretically. Then the effect of melt flow on the dimensions of dendrite is studied for alloy in this work.In order to introduce the flow in the melt, a loop cell with circular cross section is designed and manufectured. The melt flow was drived by the buoyancy force by heating a part of the cell. The velocity of the stream was controled by the electric power applied to the heater.Melt flow makes the growth rate of dendrite increase for the solidification in a given undercooling. The effect of melt flow on the dimensions is significant for the solidification in low undercooling. These are observed in the solidification for the pure material and also for the alloy.It was explained theoretically that the cooler melt coming to the tip of dendrite makes the temperature gradient at the tip, and the growth rate has to increase to maintain the heat/solute conservation. The longer interaction time between the melt flow and dendritic body makes the effect of melt flow significant for the solidification in undercooled melt.