Effect of microgravity on the solidification of aluminum\u2013bismuth\u2013tin immiscible alloys

Hongxiang Jiang,Lili Zhang,Jiuzhou Zhao,Shixin Li,Jie He

doi:10.1038/s41526-019-0086-z

Hongxiang Jiang, Lili Zhang + Show 3 more

Open Access

https://doi.org/10.1038/s41526-019-0086-z

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Abstract

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of the terrestrial sample depends on the solidification direction, it is equiaxed when the sample ampoule was withdrawn against the gravity direction, while it is columnar when the sample ampoule was withdrawn along the gravity direction.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.

Highlights

When a single-phase melt of immiscible alloy is cooled into the miscibility gap, it decomposes into two liquids enriched with different components
It demonstrates that the diameter of the sample solidified in space is almost uniform, while there are differences in the diameters at different positions of the terrestrial samples
There are several large gas cavities in the antigravitational solidification sample and some tiny pinholes exist in the gravitational solidification sample; in order to check the reproducibility of such a phenomena, two additional experiments were carried out on earth by following the same procedure, and similar results were obtained

Summary

Introduction

Immiscible alloys show a phase diagram characterized by the appearance of a miscibility gap in the liquid state.[1,2,3,4,5,6,7,8] They are especially suitable for the manufacturing of either in situ particle composite materials or composite materials with a core/shell structure and have a strong industry application background.[9,10,11,12,13] For instance, Al-Bi alloy can be used as the advanced bearings if the soft Bi phase disperses uniformly in a comparatively hard Al-based matrix,[14,15] and Cu-Fe and Cu-Cr alloys are highstrength, high-electrical-conductivity materials.[16,17,18,19] these alloys have an essential drawback, in that the miscibility gap poses problems during solidification. The droplet motions lead to the formation of a phase-segregated microstructure when these alloys are solidified on earth.

Results

Conclusion