Growth Mechanism of Dendritic Secondary-Arms and Microsegregation

对枝晶臂的生长机理进行了实验和理论研究。(1)建立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。在一定的过冷度下，生长速率随熔体流动的增加而增加。熔体中的自然对流对生长的影响可达强迫流速1mm。(2)熔体中气泡的迁移，这些气泡会意外地包含在熔体中，影响枝晶的生长。研究了在微重力条件下观察到迁移。气泡在温度梯度作用下通过Marangoni对流从冷处向热处运动。因此，在过冷熔体凝固过程中，枝晶的迁移将导致枝晶界面失稳。(3)A提出了包含界面边缘稳定性自洽的枝晶生长理论。它考虑了生长沿着的界面，这是由通过界面的热通量的守恒引起的。模拟结果与实验结果进行了比较，表明模拟结果与实验结果吻合较好。

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.