Jiang Hongxiang, Li Shixin, Zhang Lili, He Jie, Zhao Jiuzhou
1Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016 China.
School of Materials Science and Engineering, University of Science and Technology of China, Shenyang, 110016 PR China.
NPJ Microgravity. 2019 Nov 18;5:26. doi: 10.1038/s41526-019-0086-z. eCollection 2019.
Directional solidification experiment was carried out with Al-Bi-Sn immiscible alloy under microgravity environment onboard the Tiangong 2 space laboratory of China. Sample with a well-dispersed microstructure was obtained by properly designing the experimental scheme, the matrix shows equiaxed morphology, and there is no visible gas cavity or pinhole in the sample. In contrast, the reference samples solidified on earth show phase-segregated structure and contain some gas cavities or pinholes. The grain morphology of the terrestrial sample depends on the solidification direction, it is equiaxed when the sample ampoule was withdrawn against the gravity direction, while it is columnar when the sample ampoule was withdrawn along the gravity direction. The solidification process and affecting mechanisms of microgravity on the microstructure formation are discussed. The results indicate that the microgravity conditions can effectively diminish the convective flow of the melt and the Stokes motions of the minority phase droplets and gas bubbles, which are helpful for suppressing the occurrence of macro-segregation and preventing the formation of porosity. The results also demonstrate that the microgravity conditions favor the detachment between the melt and the wall of crucible, thus increasing the nucleation undercooling of α-Al nuclei and promoting the formation of equiaxed grain.
在中国天宫二号空间实验室的微重力环境下，对Al-Bi-Sn不混溶合金进行了定向凝固实验。通过合理设计实验方案，获得了微观结构均匀分布的样品，基体呈现等轴形态，样品中没有可见的气孔或针孔。相比之下，在地球上凝固的参比样品呈现相分离结构，并且含有一些气孔或针孔。地面样品的晶粒形态取决于凝固方向，当样品安瓿逆重力方向取出时为等轴晶，而当样品安瓿沿重力方向取出时为柱状晶。讨论了凝固过程以及微重力对微观结构形成的影响机制。结果表明，微重力条件能够有效减弱熔体的对流以及少数相液滴和气泡的斯托克斯运动，这有助于抑制宏观偏析的发生并防止气孔的形成。结果还表明，微重力条件有利于熔体与坩埚壁的分离，从而增加α-Al晶核的形核过冷度并促进等轴晶的形成。
