In situ observation of freckle formation in Ga - In alloys under the influence of melt convection

The directional bottom-up solidification experiments were carried out using a Ga–25wt%In alloy. A visualization of the solidification process and the main convection pattern in solidifying alloys was obtained by using the X-ray imaging technique [1, 2]. Variations of the temperature gradients applied over the solidification cell induce modifications of the melt flow pattern, which lead to different segregation structures.
Solidifications carried out at low temperature gradients (up to 1 K/mm) revealed a considerable number of perturbations of the growing dendritic network. Stable chimneys occur mainly at positions with initial growth defects or grain boundaries; however, not every initial segregation channel evolves into a stable chimney. The long-term stability of these segregation channels is strongly influenced by the transient nature of the melt convection.
The situation at higher temperature gradients up to 2 K/mm is characterized by a converging flow ahead of the mushy zone coming from the side walls. This leads to a continuous accumulation of solute in the central part of the mushy zone followed by a remelting of the solid fraction and the occurrence of a sustaining chimney. This mechanism of chimney formation is different as compared to the case where the development of the segregation channel is linked with any initial growth defect.