Concentration and fluid flow effects on kinetics, dendrite remelting and stress accumulation upon rapid solidification of deeply undercooled alloys
Concentration and fluid flow effects on kinetics, dendrite remelting and stress accumulation upon rapid solidification of deeply undercooled alloys
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DOI：
10.1016/j.jallcom.2018.02.065
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发表时间：
2018-05
影响因子：
6.2
通讯作者：
Xiaolong Xu;Yu-hong Zhao;H. Hou;Feng Liu
中科院分区：
文献类型：
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作者：
Xiaolong Xu;Yu-hong Zhao;H. Hou;Feng Liu
在深度过冷熔体自由凝固时，界面和本体液体中的溶质扩散都远未达到平衡，并且浓度和流体流动也可能在凝固中发挥重要作用。因此，在这种条件下，关于局部平衡、理想稀溶液凝固和无流体流动凝固的假设不再有效。在本工作中，首先，为了揭示浓度效应，我们比较了描述稀和非稀过冷熔体凝固的非平衡枝晶生长模型的结果。研究发现，在局部非平衡条件下，在中间过冷度范围内，非稀溶液模型和稀溶液模型的预测结果存在一定程度的差异。还发现浓度效应会显着降低松弛效应。其次，我们考虑了流体流动对过冷熔体快速凝固的影响。研究发现，流体流动不仅会影响枝晶尖端半径的大小（主要在小过冷范围内），还会影响中过冷范围内的枝晶生长速度。特别是，流体流动使得高过冷时枝晶尖端半径的尺寸接近于稀释模型预测的尺寸。因此，流体流动可以使非稀溶液表现出稀溶液凝固行为。研究还发现，流体流动在一定程度上降低了松弛效果。考虑到流体流动效应，我们使用扩展的化学过热模型来预测非稀熔体的枝晶重熔现象。该模型预测，一旦枝晶生长速度超过整体过冷熔体中的溶质扩散速度，枝晶重熔现象就会突然消失。第三，考虑流体流动效应，我们使用等效过冷的概念和最近开发的物理模型来计算快速凝固过程中的应力积累。这个新模型的结果可以很好地解释高过冷时晶粒细化的应力诱导枝晶破碎机制。
Upon free solidification of a deeply undercooled melt, the solute diffusion in both the interface and the bulk liquid is far from equilibrium, and the concentration and the fluid flow may also play an important role in the solidification. Thus, under such conditions, assumptions about local equilibrium, ideal dilute solution solidification and solidification free of fluid flow can no longer be valid. In the present work, first, in order to reveal the concentration effect, we compared the results of the non-equilibrium dendrite growth models describing the solidification of dilute and non-dilute undercooled melts. It was found that under local non-equilibrium conditions, the predicted results of the non-dilute solution model and the dilute solution model are to a certain extent different from each other at the intermediate undercooling range. It was also found that the concentration effect could substantially decrease the relaxation effect. Second, we considered the effect of fluid flow on the rapid solidification of an undercooled melt. It was found that the fluid flow would affect not only the size of the dendrite tip radius, mainly at the small undercooling range, but also the dendrite growth velocity at the intermediate undercooling range. In particular, fluid flow makes the sizes of the dendrite tip radius at high undercooling close to those predicted by the dilute model. Thus, fluid flow could make the non-dilute solution exhibit dilute solution solidification behaviors. It was also found that fluid flow reduces the relaxation effect to some extent. Considering the fluid flow effect, we used an extended chemical superheating model to predict the dendrite remelting phenomenon of the non-dilute melt. The model predicted that the dendrite remelting phenomenon would abruptly disappear once the dendrite growth velocity exceeded the solute diffusion velocity in a bulk undercooled melt. Third, considering the fluid flow effect, we used the concept of equivalent undercooling and a recently developed physical model to calculate the stress accumulation during rapid solidification. The results of this new model could explain well the stress-induced dendrite breakup mechanism of grain refinement at high undercooling.