中文摘要：

讨论了定向凝固下合金初始过渡区的溶质分布，说明合金可以在初始过渡区内发生平界面失稳现象，给出了界面失稳的临界速率表达式，获得了两种定向凝固过程中初始凝固的平界面距离与凝固速率之间的关系。对Cu-Cr合金初始凝固的平界面距离进行了分析，发现Cu-Cr合金中由于存在密度差，使得合金初始凝固的平界面距离增加，理论获得的规律与实验测量的结果基本吻合。

英文摘要：

The liquid solute distribution in an initial transient zone has been investigated at different growth rates in directional solidification. The results show that the planar interface instability can take place in the transient zone and the critical growth rate calculated for the occurrence of the planar interface instability is presented. The solidification distances for the growth of the planar interface with various growth rates are obtained at two types of directional solidification processes. The calculation results based on the theoretical analysis are in agreement with the experiment results of Cu-0.5%Cr alloy, in which the liquid density difference between Cu and Cr elements enlarges the value of the solidification distance for the growth of the planar interface during directional solidification.