Morphological evolution of the interface microstructure in the presence of bubbles during directional solidification

The evolution of morphology during directional solidification is investigated in terms of the interaction between bubbles and the solid–liquid interface. The results reveal that the solid phase grows along the bubble boundary to form solid envelopes and a liquid gap. As the interface velocity increases, the expansion coefficients of bubbles increase continually, and then decrease. The solidification microstructures of bubbles transform in the sequence water-drop→elongated→irregular with increasing interface velocity.