微重力条件下Ni-Cu合金的快速枝晶生长研究

采用落管方法实现了Ni-50%Cu过冷熔体在微重力和无容器条件下的快速枝晶生长.对微重力条件下的晶体形核和快速生长进行了研究，发现随着过冷度的增大，晶体生长形态由粗大枝晶向规划均匀的等轴晶转变.实验中最大冷却速率达到8×103K/s，获得了218K(014TL)的最大过冷度.理论分析表明，过冷熔体中优先发生异质形核，形核率可达1012m-3s-1以上；Ni-50%Cu过冷熔体中的枝晶生长随过冷度的增大发生由溶质扩散控制向热扩散控制的生长动力学机理转变.在68K过冷度条件下，生长界面前沿的偏析程序最大.关键词：落管微重力深过冷枝晶熔体

Rapid dendritic growth in an undercooled Ni-Cu alloy under the microgravity condition

Droplets of Ni-50%Cu alloy with different sizes are rapidly solidified under the condition of microgravity and containerless state within a 3m drop tube. With the increase of undercooling, the microstructural evolution has shown a transition from coarse dendrites to equiaxed grains. The maximum cooling rate of 8×103K/s and the undercooling up to 218K(014TL) are obtained in experiment. Based on a theoretic analysis, the heterogeneous nucleation occurs preferably in Ni-50%Cu droplets. The nucleation rate exceeds 1012 m-3s-1. The rapid dendritic growth of Ni-50%Cu alloy proceeds from the solutal-diffusion-controlled to the thermal-diffusion-controlled growth with the enhancement of undercooling. Under the undercooling of 68K, the segregation degree of liquid at the growing interface reaches a maximum value.