2-MELT SEPARATION IN SUPERCOOLED CU-CO ALLOYS SOLIDIFYING IN A DROP-TUBE

The microstructure of Cu-Co alloys solidified by a free-fall containerless solidification was investigated using scanning electron microscopy. Spherical Cu-Co drops of about 3 mm diameter were solidified in an evacuated 105 m long drop-tube. The microstructures were compared with those obtained by an electromagnetic levitation technique. It was found that the falling drops were at the "liquid + solid" state at the moment of impact on the floor. Upon bumping on to the floor the drop splits into many fragments, which finally solidify in the form of flakes upon retouching the tube floor. The microstructure depends mainly on the solid fraction in the flake, and on its temperature at the moment of impact. Supercoolings up to 0.2T(M) could be achieved within the drop-tube, causing separation of the liquid into two melts. Under certain conditions the microstructure reveals the occurrence of discarded spheres, suggesting that cooling below a certain temperature (T(misc)) will cause mixing of the two melts into one liquid. Interpretation of the observed microstructures is based on the current understanding of rapid solidification mechanisms.