Crystallization of Spherical Single Crystal of Silicon from Undercooled Droplet under Containerless Processing

The images of the growing crystal in the growth process of the undercooled droplets of silicon were lively recorded by using a high-speed camera. The number of crystal that nucleated spontaneously from the undercooled liquid was found to decrease to 1 when the undercooling was higher than 5K. The morphology of the growing single crystal of silicon was a thin plate. A model for predicting the critical undercooling of growing single crystal of silicon from undercooled liquid has been developed. The theoretically predicted value of the undercooling from present model for silicon is in agreement with the experimentally measured result.

Growth Competition During Double Recalescence in Fe-Cr-Ni Alloys

The rapid solidification of a Fe- 12wt%Cr- 16wt%Ni alloy was investigated under containerless processing conditions using both ground-based electromagnetic levitation equipment and aboard the shuttle Columbia using the TEMPUS facility. A high-speed digital video technique was used to image growth of the metastable ferritic phase and the stable austenitic phase into the undercooled melt. Above a critical undercooling, the metastable phase nucleates first. After a delay, a second thermal rise is observed during transformation to the stable phase. Double recalescence events were observed at temperatures consistent with the To, temperature of the bcc phase thus defining a value of the critical undercooling for metastable nucleation which is significantly lower than previously predicted. For a given liquid temperature the velocity of the stable fcc phase is greater than that of the metastable bcc phase. The velocity for growth of the stable phase into the semi-solid which forms during primary metastable recalescence was also measured and found to be independent of the initial undercooling. A model based on competitive growth of the two phases successfully predicts the limit where double recalescence events may be detected.