Growth Mechanism of Dendritic Secondary-Arms and Microsegregation

(1)搭建了熔体流动条件下枝晶生长的实验装置，测量了丁二腈熔体过冷凝固过程中枝晶的生长速率和尖端曲率半径。对于给定的过冷度，生长速率随熔体流量的增加而增大。熔体中的自然对流对生长的影响可达强迫流动速度1 mm。(2)熔体中气泡的迁移，这些气泡会意外地包含在熔体中，并影响枝晶的生长。并对其进行了研究。在微重力条件下观察到了迁移现象。气泡在温度梯度作用下通过Marangoni对流从冷处向热处移动。因此，在过冷熔体凝固过程中，这种迁移将导致枝晶界面的失稳。(3)提出了包含界面边缘稳定性的枝晶生长理论。它考虑了由通过界面的热通量守恒引起的沿界面的生长。预测和实验成功地比较了丁二腈过冷凝固过程中枝晶的生长速度和尖端曲率半径。

The growth mechanism is investigated experimentally and theoretically for dendritic arms.(1) Experimental apparatus for the dendritic growth under melt flow is built up, and the growth rate and tip radius of curvature of dendrite are measured in the undercooled solidification of succinonitrile melt. The growth rate increases with the increase of melt flow for a given undercooling. THe natural convection in the melt is estimated to affect the growth as strong as the forced flow velocity 1 mm.(2) Migration of bubbles in melt, which will be accidentally included in the melt and will affect the growth of dendrite. is studied. Migration is observed in the microgravity condition. A bubble moves under temperature gradient from the cold place to the hot place by Marangoni convection. Therefore, the migration will contribute to destabilize the interface of dendrite in the solidification of undercooled melt.(3)A dendritic growth theory, which includes self-consistently the marginal stability of the interface, is proposed. It takes into account the growth along the interface, which is induced by the conservation of the flux of heat through the interface. The predictions and experiments are successfully compared for the growth rate and the tip radius of curvature for the dendrite in undercooled solidification of succinonitrile.