Imaging of Structure Evolution in Solidifying Al-Bi Immiscible Alloys by Synchrotron Radiography

Using synchrotron X-ray imaging technique, the segregation evolution in solidifying Al-10 wt% Bi immiscible alloys was investigated at different cooling rates. Irrespective of the cooling rate, most of the Bi solute appeared at the upper part of the sample after solidification. The reason for this Bi enrichment phenomenon is different for different cooling rates. Besides Marangoni motion, positive segregation, which has rarely been noticed before, can also make Bi solute transfer to the hot top zone. It is also found that, bubbles (or pores) appear in solidifying Al-10 wt% Bi alloys, and the number of bubbles (or pores) increases with the increase of the cooling rate, while the size of the bubbles (or pores) decreases.